Geochemical Approach Mclennan

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Geological Society or America

S pe cia l P ap er 2 84

1993

Geochemical approaches to sedimentation, provenance,

and tectonics

S. M. McLennan, S. Hemming, D. K .. McDaniel,and G..N. Hanson

Depa rtm ent o f E arth a nd S pa ce S cie nc es , S late U niv ersity o f N ew Y ork a t S to ny B ro ok , S IO Il)!B ro ok , N ew Y or k /1 79 4·2 10 0

ABSTRACf

Geochemical and isotopic approaches to consinining provenance of sedimentary

rocks complement the information inferred frompetr.ography. Geochemi.cal approaches

have several advantages, including applicability to both matrix-rich sandstones and

shales and ability to constrain provenance age and geochemical history. Five provenance

c.omponents,.or terrane types, have been defined on the basis.of whole-rock chemicaland Nd-isot.opiccQmpQsitiQn, including Old Upper Continental Crust, Recycled Sedi-

mentary Rocks, YOl1ng Undifferentiated Arc, Y.oung Differentiated Arc, and various

Exotic Components, such as ophiolites, Among the most important geochemical charac-

teristic. s that define these provenance types are Nd isotopic composition (reflecting

average provenance age), eur.opium anomalies (reOecting intracrostal igneous differenti·

ation processes), large-ion lithophile element enrichments (provenance composition ),

alkali and alkaline earth depletions (weathering' and alteration) , Zr and Hf enrichments

(heavy mineralenrichments),and high Cr abundances (ultramafic sources).

Pblsotoplc compositions of whole rocks and framework (quartz, feldspar) and

accessory (e.g., zircon, monazite) grains constrain the age and crustal history .of sedi-

ment sources, The V-Pb system may be used to date the time of crystallization of small

populations and, f.or favorable circumstances, individual sand-sized grains of detrital

quartz; The Early Proterozoic Pokegama Quartzite contains detrital quartz populationsthat give a Pb-Pb age of 2647 ± 16 Ma,c.onsistenl witb detrital zircon ages and with the

age of the Archean Superior Province, from which this formation is mainly derived. The

initial Pb isotopic composition may be approximated by the Pb isotopie corapcshion of

leached feldspars, due to their low V /Pb and Tb/Pb ratios. Pb isotopic compositions of

detrital feldspars may also provide informati.on about sediment.ary provenance. K~

feldspars from thePokegama Quartzite and Early Proterozoic Chelmsford Formation

faU within the field of Archean Superior Province igneous K-feldspars. They are distinct

frQm K-feldspars found in other potential provenanceterranes, including the Penokean

Or.ogen or, in the ease of the Pokegama, the Minnesota. River Valley gneisses.

INTRODUCIION andisotopic techniques. A preliminary evaluation of some of

these geochemical and isotopic approaches is the primary focus of

this work. To date, there are exceedingly few studies that have

combined the various petrographical, geochemical, and isotopic

techniques. Nor have many studies applied such approaches 10

the study of individual components. Although we have drawn on

the available integrated studies wherever possible, many of our

inferences come from studies using far more limited data bases.

The notion that there is a record of tectonic history within

the composition of sedimentary rocks and their components is

now weUestablished (e.g., Blatt, 1967; Dickinson, 1970) ..Much

effort has been expended in discriminating the tectonic setting of

the provenance of sedimentary rocks, mainly using sandstone

petrography but more r ecent ly employ ing various geochemica l

Mc .L e nn an , S . M . , H emm in g, R , McDan ie l, . D, K_ ,a nd H an so n, G . N ., 1 99 3, G eo ch em ic al a pp ro ac he s 10 s ed im e nt at i on ,p ro v en an ce , a nd t ec to n ic s. in .Johnsson,

M_J~a nd B as u, A~ eds, P ro c es se s C o nt ro ll in g t he C ompo s it io n o fCl as ti c S ed im e nt s: B o u ld er , C o lo ra do , Ge o lo g ic a! Society o f America S pe ci al P ap er 2 84 _

21 ~ '"-{("l



22 S. M Mel.ennan a nd G lh er s

A ccordingly, ou r approach and m any o f o ur co nclu sio ns are

c ert ain ly t en ta tiv e a nd w ill n o d o ub t r eq uir e e xte ns iv e r ev is io n a s

m o re d at a b ec om e a va ila ble .

U s in g r ela tiv e a bu n da nc es o f m a jo r f ram ewo rk c ompon en ts

o f s an ds to n es , s ed im e nto lo g is ts h a v e e nd ea vo re d t o d is cr im in at e

th e te cto nic s ellin g o f th e p ro ve na nc e (e .g ., D ic kin so n a nd S ue -

z ek , 1 979 ; V aJlo ni a nd M a yn ard , 1 98 1; D ic kin so n, 1 98 5, 1 98 8).

T here h as been s om e in terest in co mbinin g p etro grap hical d ata

w ith m ajo r and trace elem ent geo ch em ical data, again w ith an

em ph asis o n d is crim in atio n o f tecto nic settin g (e.g., S ch wab,

1971; B hatia, 1983, 1985; B hatia and C ro ok , 1986; R oser and

K ers ch , 1 98 6; F lo yd a nd L ev erid ge , 1 98 7). A n um be r o f s tu die s

h av e a ls o a pp lied is oto pic d ata , m ain ly n eo dymium is oto pe s, to

s ed im en tary ro ck s. S tu die s o (N d is oto pe s h av e c en te re d p rim ar-

ily o n is su es o f cru stal ev olu tio n (e.g ., M cC ullo ch an d W as ser-

bu rg, 1978; Taylo r et al., 1983; M iller et al., 1986), alth ou gh

s om e h av e a ls o a tte mp te d to a dd re ss te cto nic is su es (e .g ., N els on

and D eP ao lo , [988; B aro vich et al., 1989; Fro st and C oo mbs,

1989; M clennan et al., 1990; L inn et al., 1991; M clennan and

Hemm in g, 1 99 2).

A pp ly in g ra dio ge nic iSO IO pe t ec hn iq ue s to s ed im en ta ry

p ro ven an ce stu dies is esp ecially aIlu rin g becau se it p ro vid es a

f un damen ta lly d iff ere nt p ers pe ctiv e, th at o f lim e. In th e c as e o f

n eo d ym ium is o to p es , t he m o st im p o rt an t p ro ce ss t ha t f ra ct io n at es

Sm from N d, th u s p ro vid in g th e c on ditio ns n ec es sa ry f or s ig nif i-

cant iso to pic variatio ns o ver tim e, is m elting o f th e m antle to

fo rm the m ore evo lved plu to nic and vo lcanic ro cks o f th e ex-

p os ed c on tin en ta l c ru st. T he N d m o de l a ge o f s ed im en ta ry ro ck s

is g en era lly in te rp re te d to re fle ct th e m ea n a ge o f m an tle ex tra c-

lio n fo r th e v ario us pro ven a.n ce co mp on en ts (M cC ullo ch an d

W a ss erb urg , 1 97 8). In p ra ctic e, s uc h d ata a pp ea r m o st u se fu l in

e va lu a tin g t he . re la tiv e ro le o f n ew (ma nt le d eriv ed ) c ru s t a nd o ld

re cy cle d u p pe r c ru st a t t he t im e o f d ep o sitio n . O th e r c on ve nt io n alis oto pic a pp ro ac hes , in clu din g R b-S r a nd K -A r d atin g, h av e a ls o

p ro v en u s ef ul ( e.g . . P et erm a n e t a l., 1 98 1; H e lle r e t a l., 1 98 5) .

A n aly se s o f in div id u al c om pon en ts a ls o b o ld great promise.

K -feldspar, and often plagioclase, h ave very lo w U/Pb and

T h IP b ra tio s . C o ns eq ue nt ly , P b is o to p e ra tio s t yp ic ally a pp ro ac h

th e in itia l c om po sitio n, w h ic h in m an y c as es ap pe ars to be char-

a cte ris tic o f s pe cif ic te cto nic a ge te rra ne s. P b is oto pe d ata als o

allo w an evalu atio n o f th e g eo ch em ical h is to ry o f U , T h, an d P h

in th e s ou rc e ro ck s. Im pro veme nts in c on ve ntio na l U /P b is oto pic

m easu rem en ts an d d evelo pm en t o f io n m icro pro be tech niqu es

a llow f o r p re cis e d at in g o n p o rt io n s o f s in gle g ra in s o f z ir co n , a nd

th u s it is p os sible to e sta blis h a ge d is tribu tio ns f or th is m in era l.

Im p ro v em e nts in c on v en tio n a 1 t ec h niq u e s f urt he r p erm it analysis .o f th e is oto pic c om po sitio n a nd , u nd er f av ora ble c irc um sta nc es ,

th e age o f o th er m ajo r co mpo nents , no tably qu artz (H em ming

et aI., 1 99 0, 1 99 1) a nd m an y o th er m in or m in era l c om p on en ts .

In th is w o rk w e e xamin e th e a pp lic atio n o f g eo ch em ic al a nd

iso to pic data to th e evalu atio n o f th e pro venance and tecto nic

h is to ry o f s ed im e nta ry r oc ks . It is no t o ur pu rpose to attem pt to

p ro v id e n ew me th o d s f or d is crim in at io n o f te ct on ic s ett in gs ; w it h

c urre ntly av aila ble d ata it is no t po ssible to o ffer s ignif icant

fu rth er ad van ces in th is area. 1 n fact, a m ajo r co nclu sio n o f o ur

stu dies h as been th at geo ch em ical and is oto pic ap pro ach es best

p rov id e a c omp lemen ta ry p er sp e ct iv e (0 t h e o r ig in o f s ed imen ta ry

ro ck s. Instead, o ur intent is to sh ow that g eo ch em ic al d ata c an

p ro v id e in sig h t in to t he s ed im e nta ry p ro ce ss es a ff ec tin g t he ro c ks ,

th e natu re o f the pro venance, and th e geo ch em ical h is to ry or

certain p ro ven an ce co mp onen ts . In tu rn , su ch in sigh t m ay p ro -

v id e co ns tra in ts o n th e te cto nic c on ditio ns a nd te cto nic h is to ry

a ff ec ti ng t h e s ed imen ta ry r o ck s .

ADVANTAGES OF GEOCHEMICAL APPROACHES

T here are a n um ber o f in trin sic ad van tages th at g eo ch em -

ical app ro ach es to sed im entary pro ven an ce h av e o ver th e m ore

tr ad it io n al p et ro g ra ph ic al a pp ro ac h es . S om e o f th e se a re s umma -

r iz ed b elow .

I. In g en eral, g eo ch em ical app ro ach es are equ ally ap pli-

cable to co ars e- an d fin e-g rain ed s edim entary ro ck s. T his co n-

tr as ts w ith p et ro g ra ph ic al a pp ro a ch e s wh er e p ro v en an ce s tu d ie s

fo r fine-grained as w ell as very co arse grained sedim ents are

difficult, A n im po rtant caveat is th at fine-grained sedim ents

ty pic ally re pre se nt m o re h omo ge ne ou sly m ix ed s ou rc es , b ut it is

c om po sitio na l v aria bility th at c omm o nly le ad s to in sig hts a bo u t

pro venance and attendant sedim entary pro cesses. Th ere is

gro wing evidence th at even intim ately asso ciated sands and

m u ds m ay b e d eriv ed from qu ite d if fe re nt s ou rc es w ith d if fe re nt

sedim entary h is to ries and so bo th requ ire evaluatio n (e.g.,

M clenn an et al., 1 9.90).

2. In princip le, it is po ss ible to apply su ch tech niqu es to

m i ne ra log ic al ly a lt er ed s ampl es i n c as es whe re a lt er at io n doe s no !

a ff ec t b uLk c hem is try o r b y c o nc en tra tin g o n e lem en ts a nd is o to p e

s y st em s Do t s ub st an tia ll y a ff ec te d b y s u ch p ro c es s es . A c co r di ng ly ,

t he re is g re at p o te nt ia l f o r e va lu a tin g t he o rig in o f s an ds to n es w it hs u bs ta nt ia l am ou n ts o f s ec o nd ar y m a trix . In t um , g e ochem is tr y in

s om e cas es m ay be u sed to qu an tify th e o ccu rrence an d lor extent

o f s om e seco ndary p ro cesses, su ch a s we at he rin g, s o rt in g, a nd

d ia ge ne sis ( e.g ., M cDan ie l a nd Mc le nn an , 1 99 1).

3 . K ey trace elem en ts o r iso to pe sy stem s m ay be v ery sen si-

t iv e in id en tif yin g m in o r c omp one nt s n o t re ad ily re co g niz ed p et-

ro g ra ph ic ally ( e.g ., H is co tt , 1 98 4; N e ls o n a nd De Pa olo , 1 98 8) . I n

so m e cases , reco gn izin g ev en triv ial am o un ts o f certain exo tic

c omp one nt s, e .g ., o p hio lit es , m a y b e o f c o ns id er ab le importance

in u n de rs ta nd in g t he t ec to n ic h is to ry .

4 . In cases w here certain geo ch em ical ch aracteris tics o f

p ro ve na nc e c om po ne nts c an be in fe rr ed , th e re is th e p o te ntia l t o

fu rth er e va lu ate th e o rig in a nd g eo ch em ic al h is to ry o f th at c om -p o ne nt . F o r e xam ple , re co g nit io n that a v olc an ic c om po ne nt h as

o rig in at ed f rom . e it he r a la rg e-io n lith o p hile e lem en t (L ILE )-d e-

p le te d o r e nric h ed m a nt le s o ur ce m a y h a ve t ec to n ic im p lic at io n s.

5 . U s in g v ar io u s is o to p ic a pp ro a ch es , th e re is g re at p o te ntia l

fo r co nstrain in g th e ag e o f th e p ro ven ance an d its tem po ral h is-

to ry . It is po ssible to evalu ate bo th th e m ean age o f th e pro ve-

nance, u sing techniques su ch as Sm -N d w ho le-rock data, o r

estim ating th e cry stallizatio n o r res ettin g (co olin g) age o f th e



Geochemical approaches to sedimentation 23

TABLE 1. PETROGRAPHICAL AND GEOCHEMICAL PERSPECTIVES

OF SEDIMENTARY PROCESSES AND PROVENANCE

ProcesS/Provenance Petrographic

Perspective

Geochemical

Perspective

Sedimentary Processes

1. Weathering Ambiguous. Some mineralogi·

cal effects (e.g . . feldspar/cl .ay

ratios)

2. Diagenesis Relative chronology and min-

erai react ions. Overgrowth,

albi tization, smecti te/il li te

transformation

Textural maturit y, heavy min-

eral content and variety

3. Sorting

4. Sedimentary recycling Quartz content. sedimentary

rock fragments

Sedimentary Provenance1_Rock types

Possible to quantify in tavor-

able conditions. Major ele ~

ments (CIA). Rb/Sr (also Sr-isotopes)

Potential but few studies.

Major element.s (CIA), redox

elements (e.g., Fe:K/Fe2+ ), var-

ious isotopes (Ab/Sr, UfPb)

Major elements (Si/AI). Quanti-

fy heavy mineral f ract ionation

such as zircon (Zr, H I) and

monazite (AEE)

Various geochemical approach-

es (CIA. Zr, HI, ThJU); Nd-iso-

topes •

Rock f ragments; quartz rnor- UWe or no direct information

phology

Combined trace elamenUNd-

isotopes (O:d Upper Crus!,

Young Differentiated Arc,

Young Undifferentiated Arc)

Pb isotopic composition of

feldspars and whole rocks

Relative age of components in Mean age (Nd-model ages);

favorable circumstances component ages (UJPb ages

of zircon, quartz, etc.)

2. Terrane type Q-F-L (continental block. mag-

matic arc, recycfsd orogen)

3. Terrane identification Uttie or no direct information

4. Provenance age

5. Crust/mantle character UttJe or no information Nature of crust/mantle melting

(Eu-anornalias, HREE-deple-lion). Nature of crust /mantle

sources (Th/U, ! A i l

individual components (e.g., U-Pb dating of zircon, monazite,

sphene, and quartz; Rb-Sr dating of detrital muscovite). In addi-

tion. it may be possible 10 identify specific tectonic terranes as

provenance components using Pb isotopic data for feldspars (as

an estimate of the initial Pb isotopic composition) and further to

evaluate the geochemical history of U, Th, and Pb in those

components.

SEDIMENTARY PROCESSES: PERSPECTIVES

FROM GEOCHEMISTRY

To identify geochemical approaches relevant to characteriz-

ing provenance, it is first important to separate as much as possi-

ble the geochemical "signals' due predominately to processes

from chose due to provenance. In addition, sedimentary rocks

associated with certain tectonic regimes in some cases appear

more or less affected by such processes. inTable 1 a comparison

of the types of information that may be obtained from both

petrographic and geochemical/isotopic data. is summarized.

Weathering and diagenesis

Using petrography to evaluate the weathering history pre-

ceeding the deposition of a sediment is ambiguous at best

Weathering involves the conversion of volcanic glass and unsta-

ble minerals, mainly feldspars and mica, to clay. Although miner-

alogical data and textural informatiea regarding the alteration of

feldspar can be usefu l , attempts at quantification have not been

forthcoming. Similarly, diagenesis also commonly involves the

alteration of feldspars and other unstable minerals to clays; ac-

cordingly, it may be difficult to distinguish these processes geo-

chemically (e.g., Hower et at, 1976; Land, 1984; Helmhold and

van de Kamp, 1984).

During weathering (and in many cases, diagenesis), there is



24 S. M. Mclennan and Others

a subs tantial increase in the Rb/Sr ratio o f mo st ro ck s. Th is is

because Rb", a largealkali trace element (1.72 A f or 1 2-fo ld

coordination), is m o re re ad ily retained o n exch ange sites o f days

th an th e sm aller 5,r2+ (1.26 A fo r eigh t fo ld co ordinatio n); T his

majo r ch ange in Rb/Sr provides oppo rtu nities fo r applying Sr

isotopes to the eval u ation o f such h is to ry (e.g ., Oh T er al., 1991).

There is evidence. tba t u nder certain conditio ns Ih e rare earth

elem ents (R EE ) m ay be fractio nated su bs tantially in th e w eath er-i ng p r of il e- (Nes b it t, 1979.; B anfield and Eggleto n, 1 98 9).. H ow -

ever, il ap pears th at there is o nly rarely any signif icant gain o r lo ss

o f Sm or Nd from the weath ering pro file du ring subsequent ero -

s io n a rid sedimentat ion and, acco rdingly, th e N d isotopesystem is

l es s l ik e ly to p ro vid e in sig ht s in to w ea th er in g history o f s e dimen -

tary ro cks . D iagenetic reac tio ns in some cases also appear to

redistribute REE am ong vario us diagenetic m inerals, w ith ou t sig-

nificant m ovement o f REE into o r o u t o f th e sys tem (Oh r and

o thers, 1991; altho ugh see the alternate view o f Anwiller and

M ack , 1 99 1), A cco rdingly, diagenesis do es no t ap pear to s ignifi-

can tly a ffect. bulk-rock REE patterns (also see M cLennan,

I 989a). It m ay be po ss ible to evalu ate th e tim in g o f d ia gen esis by

exam ining th e iso to pic sys tem atics o f co -exis ting diagenetic

phases (Ohret al., 1991).

The bu lk ch em ical changes th at take place du ring weather-

ing h ave also been u sed to quantify the weathering h is to ry o f

s ed im en ta ry ro ck s, p rim arily 1 .0 u nderstand pas t clim atic co ndi-

tions (Nesbittet al., 1980; Nesbitt and Young, 1982, 1984). The

Chem ical Index o f A lteratio n (CIA ) has been established as a

g en er al g uid e 1.0 the degree o f weathering (Nesbitt and Young,

1 98 2),. a lt ho u gh c he mic al c ha ng es re su lt in g from o t he r p ro c es se s,

such as diagenesis and m etam orph ism , h ave no t beel] f u ll y e va lu -

a te d. U sin g m o le cu la r p ro po rt io ns :

o r , abbrev ia t ing ,

CIA ~ AlfA + K + N + C] * 100,

w here C aO * rep resents th at residing o nly in s ilicate m inerals (i,e .,

co rrected fo r calciu m res iding in carbo nates and ph osph ates). Fo r

u nco nso lidated sedim ents , it is so metim es also necessary to m ak e

a correction on Na20 fo r seawater sah (M cL ennan et al., 19 90).

On such a.scale, u nweath ered igneou s ro cks have values o f 50 or

below , res idual c lays have values o f near 100,. and typical shales

average abou t 70 to 75.

Th e p etro lo gical ratio nale beh ind this index, and fo r there la tio nsh ip betw een th ese elem ents and Fe and M g, is illu strated

in Figure I A. T he b ulk -ro ck ch em is try is dom inated by th e con-

versio n o f feldsp ars (and fo r vo lcanic ro ck s, glass) to clay m iner-

als du ring weathering. The expected pathways f o r i nc re as in g

degrees o f weath ering fo r a m afic and fe ls ic igneous ro ck are

illu strated. On the A -C N~K diagram , these path s are co nfirm ed

bo th fro m w eath ering p ro files and th erm odynam ic/k inetic calcu -

lations. In the case o f the A -CNK-FM diagram , appropriate ki-

netic data are u navailable and trends are derived fro m w eath ering

pro files alone. There are several co mplexities that m ust be kept in

m ind when employing such an approach . The firs t is th at m any

m ineral reactio ns th at tak e p lace du ring diagenesis invo lve th ese

.sam e m inera ls , and fo r a sys tem where any of these chem ical

com ponents are added or lo s t th ere will be comparable effects.

S tudies are no t ye t available to ev alu ate th is in a ny s ys tem at ic

m an ne r (a lt ho u gh s e e N esbitt and Yo ung, 1989). A n additionalcom plexity com es from m ixing provenance components o f dif-

ferent com po sitio ns and/o r weathering h is to ries , illu strated in

Figure 1 B. In some cases t h e t ra je ct o ry of th e pa th way will h elp

in dis tingu ish ing m ixing from weathering trends (H . W . N esbitt,

personal communicatio n). Fo r example, a very flat trend that

in tersects the A -C N bo undary is inc on sis ten t w ith w eath ering

being the so le contro l and m ixing rna y be i nd ic at ed ..Mass balance

requires that w eath ering (o r m ast w ater-ro ck interactio n) trends

should lieon a line ex trap olated . fro m gro und w ater co mp os itio ns

(tha t lie on the CN-K jo in) and th ro ugh th e parent material

composition,

Sedimentary sorting

Th ere are several petro graph ic appro aches to exam ine so n-

i ng p ro ce ss es i n s ed im e nt ar y ro ck s. T he mo st . s tra ig ht fo rw ard is [0

evalu ate th e te xtu ral m atu rity o f th e sed im ent, u sin g ch aracteris tic

grain sizes and sh apes, and m ineralogy (e . .g., Fo lk , 1974). The

ch em ical com po sitio n o f a sedim entary ro ck m ayaIso be helpful

in evalu ating th e inf lu ence o f sedim entary so rting.

W ith increas ing textu ral m atu rity in sands to nes , th ere is typ-

ically an increase in qu artz at th e expense o f prim ary day-s ized

material, resu lting in an elevatio n o f the Si021 A l203 ratio and a

decrease in t h e a bu n da nc es of m ost trace elem ents . O f special

interest is t h e r ec ogn it io n that, on a lo cal scale, in som e mineral-

ogically im matu re sedim ents , th ere can be an enrichment in th e

m ajo r fram ew ork m ineral plagio clase du e to sedim entary so rting

processes . Th is m ay have serio us co nsequ ences bo th fo r tecto nic

interpreta tio ns o f sandsto ne petro grap hy and ch em ical co mpo si-

tio n in sam ples w here plagio clase is abu ndant (N ance and Taylo r,

1977; M clennan, 1989a)_ A well-documented example o f th is

ph enomenon com es from the Devonian Baldwin Fo rmatio n

(N ance and Taylo r, 19 77). In th is vo lcano genic tu rbidite o f pro b-

able fo rearc o rigin , Nance and Taylo r (1977) no ted a s tro ng

enrichm ent in Eu ., a rare earth elem ent typically enrich ed In

feldspars , fo r som e sam ples . These enrichm ents co rrelate well

w ith framewo rk plagio clase content (Chappell, 1968; Bhatia,

1 9 81 ) ,. a nd a cc o rd in gl y have been interp reted. as resu lting fro mplagio clase enrich ments du e to sedim entary so rting pro cesses

(F ig. 2 ). A ny va riatio ns in R EE abu ndance patterns of sedimen-

tary ro ck s th at can be ass igned to sedim entary pro cesses are o f

s ig ni fi ca nc e b ec au s e REE p atterns are used w idely fO I p ro ve-

nance ch aracterizatio n (T aylo r and M cL ennan, 19 85; M cl.ennan,

1989a). 10 a s tu dy o f deep-sea tu rbidites from a variety o f tec-

t on ic s et tin gs , M c l.e nn an etal, (1990) no ted that m any sands

fro m active tec to nic settings h ad elevated Eu/Eu* (where Eu * is



Geochemical approaches to sedimentation

AA Kaolinite, Glbbs/re, Chlorite

PIBg;cx:IB5e 1I'--'--4-___,<-----__.

o 0Granodiorite

'Andesite

CNNatural Waters

A

CN

25

A

K CNKClinopyroxene

FM

A

K CNK FMFigure I. A, Ternary plots of A-CN-K and A-CNK-FM where, in mole fraction, A :; A1203, C:; Cao(silicate fraction only), N = Na20, K - = K20, F :; total Fe as FeO, and M = MgO (after Nesbit t and

Young, 1 9 84 , 1 98 9) . Ploned are simplified compositions of major minerals, typical rock types, and

natural waters. Arrows indicate the general trends of weathering exhibited by v ario us ro ck types.

B, Ternary plots of A-CN-K and A-CNK-FM showing sandstones and argillites. from the Proterozoic

Chelmsford Formation, Sudbury Basin, Ontario (McDaniel, \992). Two argillite samples a re the two

most evolved samples in terms of CIA. These samples show a linear trend that is inconsistent with

simple weathering being the sale control of the composition (compare with Fig, IA). For example, on

the A-CN-K diagram the predicted weathering trend isshown for a hypothetical composition derived

from extrapolating the sediment trend to an unweathered composition. Possib le explanations include

mixing of a relatively weathered source with an unweathered source of differing primary composition or

perhaps some influence from secondary sedimentary process thai resulted in redistribution (gains or

losses depending on composition) o C Ca, Na, and/or K in the silicate fraction (note that plagioclasein

these sandstones is albitized).

a theoretical value forEu, assuming no chondrite-normalized Eu

anomaly), by as mucb as 0.10 over coexisting muds. This-was

also ascribed as being possibly due to sorting processes, and sug-

gests that such a process may be more important than commonly

thought. In tectonic settings where abundant plagioclase is not

found there is no evidence of systematic Eu enrichment in sands

over coexisting muds (Nathan, 1976; Bhatia, 1985).

Since a number of heavy minerals are dominated by ele-

merits that are trace elements in most sedimentary rocks (e.g., Zr

inzircon, REE in monazite and allanite), it is possible to evaluate

the role of heavy mineral concentration during sedimentary sort-

ing (Mclennan, 1989a). An example of this can be illustrated for

modern deep--sea turbidites. In Figure 3, Th/Sc is plotted against

ZrlSc. The ratio ZrlSc is a useful index of zircon enrichment

since Zr is strongly enriched in zircon, whereas Sc is not enriched

but generally preserves a signature of the provenance similar to



. '

26 S. M Mcl.ennan and Others

Q)

0)

~

ELL

C.~'0

rtiCD

E0.. .10

-0-

- - -- M284

M285

MK14

Framewor i l

P l a g i o d a . s e

C o n t e n t

33,8%

36.6%

68 .8%

E0Q_

.01 ~L~a-C~e~P~r~N~d~-'S~m~E~u-'G~d~'=T~b-'Dy~H~o~E-r~-Y~b~~~~

Figure 2. REE diagram for En-enriched turbidite sandstones from th e

Devonian Baldwin Formation, normalized to the Baldwin Formation

average (of non-En-enriched samples). The Baldwin Formation is a

volcanogenic turbidite thought to have been deposited in a forearc set-

ling. The amount of framework plagioclase in these samples isshown onthe right. The formation average is about 16%.Also shown isa typical

REE pattern for plagioclase from an andesite. The Eu enrichment in

some of the Baldwin Formation sandstones is likely due to plagioclase

enrichments formed during sedimentary sorting. Data from Schnetzler

and Pbilpous (1970), Chappell (1968), Nance and Taylor (1977), Bhalia

(1981, 1985).

other REE (Mclennan, I989a). In contrast, Th/Sc is a good

overall indicator of igneous chemical differentiation processes

since Th is typically an incompatible element, whereas Sc is

typically compatible in igneous systems. It can be seen that both

Tb/Sc and ZrlSc vary sympathetically for turbidite sands from

active margin settings, following a trend consistent with igneousdifferentiation being the primary control (i,e., provenance). In

contrast, turbidite sands from passive margins show ZrlSc in-

creasingsubstantially, with Th/Sc increasing fa r less, consist.ent

with zircon enrichment In the case of monazite, an REE-

enriched heavy mineral, it is also possible to estimate enrichments

from REE patterns (Mclennan, 1989a; Fig. 4). Since monazite

has very high REE abundances and a very steep chondrite-

normalized heavy REE pattern, even small amounts «0.01%)

result in significant increases in the GdN/YbN ratio (where sub-

script N refers to chondrite-normalized values). Monazite en-

richments may be recognizable since post-Archean sedimentary

REE patterns and most upper crustal igneous rocks rarely have

GdN/YbN outside of the range of 1.0 to 2.0 (Mcl.ennan, 1989a;Mclennan and Taylor, 1991).

Sedimentary recycling

Quantifying sedimentary recycling processes, or the canni-

balistic turnover of the sedimentary mass, is becoming increas-

ingly important in understanding issues such as crustal evolution

(e.g., Veizer and Jansen, 1979; Mclennan, 1988) and the teeton-

sediment Recycling

o (Zircon Addition) ..

0ot! • •. 0 . . . . , -()

(f) .1

J::

I-

.01_ Trailing Edge Turololtes

o Active Margin Turbidites

o

.001L_~~~~~~~W_~~~~~~~~

.1 10 100 '000

Zrl Sc

Figure 3. Plot ofTh/Sc versus Zr/Sc for modem turbidites from various

tectonic settings showing the e nr ic hmen ts o f 'z ir con (high ZrlSc ratio) in

lTailing edge settings resulting from sedimentary sorting and recycling.Active margin samples, which are less affected by this sedimentary proc-

ess , show a simple correlation fOTthese rat ios , and. this relationship is

interpreted to be due to composi t ional variations of th e provenance.Data from Mclennan et al, (1990).

10

Quartzile(Th~10ppm)

Monazite AdditionMonaz.ite: Gd=14.700ppm

Vb" 540ppm

15/30

/,20Shale 12(Th~10ppm)

.033%

1.0 2.0 3.0 4.0 5.0

Gd N IYb N

6.0

Figure 4. Plot of GdN versus GdN/YbN to illustrate the effect on RE E

pallems ofconcentrating monazite in sedimentary rocks. Calculations for

two hypothetical sediments are shown, one a low REE sediment (e.g..quartzi te) and the other a high REE sediment (e.g., shale). For both. an

REE pauemparallel to typical post-Archean shales (Taylor and Mclen-

nan, 1985) is assumed. Both are assumed to have 10 ppm Th. Monazite

REE data are from Lee and Bastron (1967), and a typical monazite Th

abundance of 6% is assumed. Adding even small amount of monazite {as

little as 0.005% to the quartzite) can result in significant increases in the

Gd/Yb ratio of the sediment. The GdN/YbN ratio of sedimentary rocks

is usually rather constant and in the range of 1.0 to 2.0.



Geochemica l approaches t o sedimen ta ti on 27

0.4 Ga

10Mantle EVOlution 2.0Ga

2.7Ga5 Crust

£,0 ----- --_ .

",0.0 "".Nd -5 60\ ::;...

~o "".

".

-10

/ R e c Y c i e dRecycled

-15 Sediment 1

-20 Sedimenl2

-25Sedimentary

Rock

1.0 3.0.0

Geologic Time (Ga)

Figure 5. P lo t o f ENd versus lim e to illu s tra te the interpre ta tion o f sedim entary N d m odel ages in term s o f

s ed im e n ta ry r ec yc li ng . A sedim ent w ith a 250-M a s tra tigraph ic age h as a present day ENd o f -14 a nd, fo r

a ty pical sed im en tary Sm /N d ratio, gives a deple ted mantle mood age (TDM) o f 1.7 Ga. Shown is a

h igh ly s implif ied m odel o f the sedim ent's o rig in. Tbe 2 .7-Oa upper crus t is m ixed with new cru s tal

m aterial o f 2 .0-G a age and inco rpo ra ted in to a sedim entary rock (Recycled Sedim ent I) at abo u t 1.8

Ga. T h is s e dim en t is t he n re cy cle d in to a no th er s ed im en ta ry basin (R ec yc le d S ed im e nt 2) a t a bo ut 1.2

Ga with no additio n o f new cru st material, A t th e time o f sedim enta tio n, R ecycied Sedim ent 2 and new

cru s t, fo rm ed during th e Early Paleozo ic , ac t as th e pro venance fo r the analyzed sediment . In pract ice ,

th e h is to ry is likely 10 be e ve n m o re complicated, Also , it is n ot p os sib le to d is tin gu is h, solely on the basis

o f Nd isotopes,betw een o ld ig neo us /m etam orph ic cru st ac tin g directly as the pro venance and o ld crus t

recycled th ro ug h th e s edim en tary reco rd .

its of sedimentary basins (e.g., Ingersoll, 1988; Johnsson et aI.,

1988; Johnsson and Meade, 1990; Steidtmann and Schmitt,

1988). Recycled sediment is likely the dominant source of pre-

served sedimentary rocks, but in detail the extent of any recycledcomponent is highly variable and in many cases is related to

tectonic history (Veizer and Janson, 1985). In Figure 5, the Nd-

isotopic evolution of a sedimentary rock is shown in terms of a

highly simplified sedimentary recycling model. The model age of

the sediment (in this example, 1.7 Ga) has no bearing on any

particular provenance component Instead, it represents a com-

plex mix of differing ages of crust that has gone through various

cycles of sedimentary processes.

SEDrMENTARY PROVENANCE: PERSPECfIVES

FROM GEOCHEMISTRY

Terrane types

It is recognized that sediment compositions are characteristic

of certain assemblages of igneous/metamorphic/sedimentary

source rocks that have specific styles of sedimentary history (e.g.,

Dickinson, 1985, 1988), or what is here termed terrane (or prove-

nance) types. In tum, some inferences about the tectonic

conditions can bemade. In this section, we describe terrane types

that can be identified from geochemical data, mainly combining

major and trace elements with Nd-isotopic compositions of

whole rocks. Of special importance is the observation that these

terrane types do not precisely correspond to those recognizedfrom petrographical approaches but instead provide a different

perspective of the plate tectonic association. It is also important to

note that, although a given terrane type may dominate the prove-

nance of certain sedimentary rocks (and some types are more

likely than others), it i s j us t as likely that the provenance will be a

mix. Unravelling the contributions of each terrane type is , in

effect, a major part of the task at hand. Some of the chemical and

isotopic characteristics of these terrane types are summarized in

Table 2.

These concepts are largely based on combined geochemical

and Nd isotope studies of the provenance of young continental

margin sediments (e.g., White et at, 1985; Ben Othman et 31.,

1989; Mclennan et al., 1990; Basu etal., 1990). Similar studies

of Phanerozoic sequences are rare but the general approach

appears to apply (Andre et al., 1986; Frost and Coombs, 1989;

Chen etal., 1990). In Precambrian terranes, additional complex-

ities arise, largely related to different lithologies being present

in the Archean upper crust. These are discussed in greater de-

tail below.

Two of the major factors used incharacterizing terrane types



S. M. Mcl.ennan and Others

TABLE 2. SUMMARY OF GEOCHEMICAL AND Nd-ISOTOPIC CHARACTERISTICS

OF TERRANE (PROVENANCE) TYPES

Terrane Type E,..d1 Eu/Eu' ThJSc Th/U Other

Old Upper Conti - S-10 ",0.60-0.70 =1.0

nental Crust (OUG)

Recycled Sedimen- S-10 o;().60-O.70 ~1.0

tary Hocks (ASR)

Young Undifferen- ~'l-5

t iated Arc (YUA)

=1.0

>3.8

(shales)

Evolved major element composi-

tion (e.g .• high Si/AI. CIA); high

LlLE abundances; uniform compo-

sitions

Evidence of heavy mineral con-

centration from trace elements

(e.g . . Zr, HI for z ircon, REE for

monazite)

<1,0 <3.0 Unevolved major element compo-

sitions (e.g., low Si/AI. CIA); low

UL E abundances; variable compo-

sitions

Young Differen-

t iated Arc (YDA)

...0,50-0.90 Variable Variable Evolved major' element composi-

tion (e.g., high SilAl. CIA): high

ULE abundances; var iable compo-

sitions

Exotic components Chemical and/or isotopic signature depends on the nature of the component;

e.g .. very high Mg. Cr. Ni, V, and erN would be distinctive of ophiolite

sourc-es.

1~d values are for modem sediments only. Distinctions between tanane types would persist for older

sedimentary rocks but would be increasingly lass pronounced with age. Exact values would depend

on sedimentation age.

are bulk chemical composition and Nd-isotopic composition at

the time of sedimentation. The influence of these factors can be

summarized in Figure 6 where modern turbidites (Mclennan

et al., j990) are plotted on a diagram of tNd versus Th!Sc ratio,

where ENdrepresents deviations of the 143NdJ I44Nd ratio, in

parts per 104, from average chondritic meteorites (representative

of the bulk earth). The Th!Sc ratio is a convenient and sensitive

index of bulk composition (Taylor and McLennan, 1985). Th is a

relatively incompatible element in most igneous processes where-

as Sc behaves compatibly; both appear to be transported in the

terrigenous component during sedimentary processes. The sam-

ples are divided into quartzose (Recycled Orogen, Craton, Tran-

sitional Continental in tenus of QFL) and nonquartzose.

Quartzose samples include those from trailing edge margins and

continental collisions with an additional few from active settings

in Japan and Java where old continental Crust is present within or

near the arcs. The main features to note are that nonquartzose

turbidites (mainly active settings) have higher fNd (reflectingyoung provenance components) and highly variable and com-

monly low Th/Sc ratios (reflecting heterogeneous and generally

less differentiated provenance). ln contrast, quartzose turbidites

have generally lower fNd (reflecting older provenance) and very

uniform Th!Sc ratios of about 1.0, the value of the upper crust

(Taylor and McLennan, 1985). Also shown are data for the

Andean foreland (Basu er al., 1990) interpreted to be derived

from Paleozoic-Mesozoic upper crust The extension of these data

toward high Th/Sc is likely related to heavy mineral enrichments

since samples with highest Th/Sc have exceptionally high Zr

content (up to 2,500 ppm).

An entirely comparable approach, using only Nd-isotopic

data, is shown in Figure 7, anjSm/Nd - €Nddiagram. The value

jSm/Nd is the fractional deviation of the 147Sm/I44Nd ratio from

that in chondritic meteorites (fSm/Nd = ( 14 7Sml I44Nd)sam_

ple/(147SmJl44Nd)chondrile - 1). Since REE patterns tend to

become more light REE (LREE; La-Sm)-enriched (i,e., lower

Sm!Nd) during igneous differentiation processes,fSm/Nd moni-

tors the general extent of differentiation, albeit in a fa r less sensi-

tive manner than Th/Sc (Mclennan and Hemming, 1992) .

1. Old upper conlinRntal crust (ODe). This provenance

component constitutes old igneous/metamorphic/sedimentary

terranes that have been influenced by intracrustal geochemical

differentiation. Old stable cratons and the old continental founda-

tions of active tectonic settings are examples. Intracrustal frac-

tionation includes processes of both intracrustal partial meltingand/or crystal fractionation, but in either case involves consider-

able separation of plagioclase (stable essentially only at crustal

depths). It is this intracrustal fractionation that imparts the nega-

tive Eu anomaly to the upper crust (Taylor and Mclennan,

1985). The relatively great age of this component is reflected in

the nonradiogenic Nd isotopic character (i.e., low 143Nd/144Nd

ratio), with ENdtypically being less than about -10 for modem

sediments and appropriately low for older sedimentary rocks.



Geochemical approaches 10 sedimentation 29

ArcMORB

o 0Andesite

oFelsic0

-10

Componento00

..... 0

Mafic Component

CNd 0

• Quartzose

o Non·quartzose

+ Andean Foreland

Upper Crust

IOld Crust

" t ·

-20

.01 .1

Th/ScFigure 6. Plot of tNd versus Th/Sc ratio for various modern sediments.

The Nd isotopic composition isstrongly controlled by th e mean prove-

nance age whereas Th/Sc ratio is a sensitive index of the bulk

composition of the provenance. Plotted are modern turbidites (Mel.en-

nan et a l., 1 99 0) divided into quartzose (mainly trailing edge) and non-

quartzose (mainly active margins). Note that the nonquartzose turbidites

tend to have variable but higher tNd (reflecting a younger provenance)

and more variable but commonly lower Th/Sc. ratio. The quartzose

turbidites are of more uniform composition and generally from an older

provenance ..Also plotted are modem sands from the Andean Foreland

(Basu et at, 1990) considered to be derived from old recycled upper

crust, In this case, high Th/Sc is not due to derivation from more felsic

sources but isrelated to concentration of heavy mineral during sedimen-

tary recycling and sorting.

• Ouartzose

o Non'quar1zose+ Andean Forela~ LREE-depleled

LREE·enriched Arc ' R i i C k s - : : : : : : :: :~ ~ ...__ .'0

Z.._

EU) -0.2

'+-

' : . : ' : - : - : - : - : - : 0 - : ' : " :. . . . . . . . . . . . . ...............

: : : : : : : i i : : : : : : : : : : : : :' : : : : J t : : 8 , : : :...-----------

: , p . : . : . : . : . : . : . :

-0.4

-0.6 1.L..£.£....L~~;"c..L,LL....:...a.: ':..L.£~E..£.",L---L _ _L__.__--L_...J

-40 -30 ·20 o 1010

E N dFigure 7. Plo t o ffm/Nd versus tNd fo r the-s am e s amp le s s h own in Figure

6 . T he d eg re e of LREE enrichment in a sedimentary rock is also sensitive

to bulk compositions; such a diagram conveys similar information to that

in Figure 6. Note that this diagram is essentially a slightly recast "iso-

chron" diagram (see Shirey and Hanson, 1986, for an explanation).

1000

----0--

•Bay Biscay

Sohm BasinU)

Q). . . . . .~

"0co_c

oEQ

Q__

100

10

E0...Q --.0-- Demerara Plain (543-26-2)

1----1.__- Andean Foreland (25)

LaCePrNd SmEuGdTbDyHoEr Yb

Figure 8. Chondrite-normalized REE plot of selected modern sediments

that have an old upper continental crustal. (OUC) provenance. The tNd

for each of these samples is less than -10. Note the similarity of the

patterns with LREE enrichment, fairly flat HREE (i.e., GdN/YbN = 10

to 2.0) and the negative Eu anomaly, characteristic of the upper conti-

nental crust, Data from White et al, (1985), Mclennan ei al, (1990), an d

Ba su e r a !. ( 19 90 )_

The geochemistry of sediments derived from this COmponent

is normally characterized by relatively uniform compositions, re-

fleeting a broad, well-mixed provenance (Potter, ]978) and the

greater level of recycling expected for such environments (Veizer

and Jansen, 1 979, 1 98 5). In addition to a substantial negative Eu

anomaly (Eu/Eu" about 0.60 to 0.70; Taylor and Mclennan,

1985), sediments typically have evolved major element composi-

lions (e.g., high Si021 A 1 203, K20/Na20 CIA), reflecting a

dominance of upper crustal granitic sources and a relatively se-

vere weathering (and recycling) history. Trace element features

include enrichments of normally incompatible over compatible

elements (e.g., LREE enrichment, high ThlSc, La/Sc) reflecting

relatively felsic average provenance compositions, and high

Rb/Sr (>0.5) and Th/U (>3.8 for shales) reflecting weathering

and sedimentary recycling.

Examples of sedimentary rocks dominated by OUC include

abundant cratonic shales (Nanceand Taylor, 1976), most passive

margin turbidites (Mclennan et aL, 1990), and foreland' 6asin

sediments derived from the older exposed continental crust (in-

c lu din g, to r e xamp le , the . ex. -p o se .d c o nt in enuU bu lg e ). Ch a r1 l.Ue I\ S-

tic REE patterns are shown for sediments dominated by this

terrane type in Figure 8. Each displays the negative Eu anomaly

and LREE enrichment characteristic of the upper continental

crust and each 'has EN d <-10. Most sediments derived from tec-

tonically active settings also contain variable amounts of such



30 S. M. Mcl.ennan and Others

10 .

•• •• «7

:~••••

Compositional

Variations

o(/)

- - -.c :I--

.1 t. -

Figure 9. Plo t of Th ISc v ers us Z rlS c fo r m od em s an ds fro m th e A nd ea nfo re la nd a nd fo r loess . . S an ds aIe e nric he d in h ea vy m in era ls , n otab ly

z ir co n . d u e to s e dim en ta ry s o rt in g a nd r ec yc lin g . S im i la r e n ri chm e nt s

ar e seen fo r Ihe aeolian loess (silt), due10

t h e e f fi ci en t c o ncen tr at io no f zirc on d urin g w in d transport, Data f ro m T ay lo r et al, (1983) an d

Basu ct .a t (1990).

material. In general, can nibalis tic s edim entary recy cling is co n-

siderably m ore efficien t at preserving sedim entary ro ck s asso -

ciated with stable craton ic tectonic settings th an tho se from

tectonically active settings (V eizer and Jansen, 1985). A cco rd-

ingly, sedim entary rock s do minated by th is provenance co mpo-

nen t are preferentially p reserv ed in th e g eo lo gic reco rd .

2. Recycled sedimentary rocks (RSR). A lthough o ld

upper con tinen tal cru s t (OU C) inclu des recycled sedim entary

and m etasedim entary ro ck s, in favo rable cases it m ay be po ssibleto dis tin gu is h recycled s ed im en t as a s ep arate co mpo nen t. P etro -

g raph ically . identificatio n o f recycled s ed im en tary co mpo nents is

s traig htfo rw ard eith er by identifyin g s ed im en tary ro ck fragm ents

o r abraded quartz overgrow th s and m ay be sugges ted by the

quartz-rich natu re o f sands (althou gh no te Joh nsson et aI., 1988).

S ed im en ta ry re cy clin g m ay a ls o be ac co m pa nie d by fractionation

and enrichm ent o f heavy m inerals , no tably zircon. A s no ted

abo ve, it may be po ss ible to identify su ch enrich men ts u sing trace

e lemen t r el at io n sh ip s . It is lik ely inapp ro priate to u se geo ch em i-

cal data alo ne becau se m os t well-so rted sed im ent and h igh ly

rew ork ed f irs t-cycle sedim ents cou ld also po ssess enrichm ents in

h ea vy m in era ls (J obn ss on et aI., 1988).Th e latter effect m ay be

seen in data fo r lo ess, where Z r and Hf are stro ngly enrich ed o ver

upper cru stal abundances (Fig. 9). It appears th at aeo lian proc-

ess es are particu larly efficien t at trans po rting zirco n (e.g., T aylo r

et al., 1983; B rim hall et a t, 1991), presum ably due to some

optim um com bination o f specif ic gravity and typical g rain s ize

(a nd p erh ap s s ha pe ).

An exam ple where a recycled sedim entary com ponent m ay

be iden tified com es from the Andean fo reland bas in sands o f

Bo livia and Peru (Basu et al., 1990). In these sands , a m ajo r

100

(/)

ill.-e._"0C

0.s:

0 10Eo,o,-EQQ

1

--0--. Marianas

N. Solomon.

•

-0-- Baldwin (M283)

• Brook Street

Figu re 10. C ho ndrite-no rm alized R EE plo t o f selected m od ern and

P h an e ro zo ic t ur bi di te s t ha t a re d om in at ed b y y o un g u n di ff er en ti at ed a rc

(YUA) p r ovenance . Except fo r th e Ba ldw in s amp le , fo r wh ic h t he re a rc

no N d is oto pic data, each sam ple h as ~Nd> +6 a t th e lim e o f d ep os it io n,

C om pa re d to s am ple s w itb Q UC p ro ve na nc e (F ig . 8), these h a ve l ow e rR EE abu ndan ces . v ariable bu t les s LR EE en rich ment (in s om e cas es ,

LREE d ep le tio n ) a nd la ck s ig n if ic an t negative Eu anoma li es . Data f ro m

N an ce an d T ay lo r (1977). F ro st and C oo mbs (1989), a nd Mc le nn anet al, (1990).

recycled m etas edim entary co mpo nent m ay be directly identified

from the petrography. Su ch a com ponent is also o bserved

th rou gh tbe trace elem ent relationsh ips where zircon and o ther

trace m inerals a.re co ncentrated du e bo th to th e sedim entary

provenance and fu rth er enrichm ents related to sedim entary

trans po rt (F ig. 9).

3. Young undifferentiated arc (YUA). T his C omp on en t

co nstitu tes yo ung (dom inan tly m antle derived) vo lcanic and plu -

tonic igneou s rock s from is land and continental arcs th at h ave no tu nd erg on e in tra cru stal d if fe re ntia tio n to th e d eg re e th at s ep ara te s

s ig nific an t a mo un ts o f p lag io clas e. A cc ord in gly , s ig nific an t n eg a-

tive Eu anom alies are absent. A lthough litho logical relations in

arc terranes are com plex, th ey are typically less evo lved. w ith

basaltic and andesitic rock s (and p lu to nic equ ivalents ) being lar

m o re common than in s tabilized upper continental cru s t. A

yo ung provenance is dem onstrated by radiogenic neodym ium

iso to pic com pos itions su ch th at fN d is typicaUy above abou t +5

fo r m odem sedim ents . In additio n to lack ing negative Eu ano ma-

lie s ( Fig . 10), such sedim ents typically po ssess rath er variable

m ajo r and trace elem ent co mpo sitio ns reflecting relatively lo cal-

ized pro venance an d general lack o f s edim entary recycling. T hu s.

m ajo r elem en ts ty pically h ave relativ ely lo w, bu t v ariable. SiO:/

AI20), K20/Na20, and CIA, re fle ctin g le ss d iff ere ntia te d a nd

less w eathered sou rces , and low er ratio s o f incom patib le to com -

patible elem ents, su ch as Th /Sc « l.O ), and less LR EE enrich -

m ent th an o ld upper cru stal sou rces (see Figs. 3 , 6 , 7 , 10).

Fo r Phanerozo ic sedim entary rock s, at least, info rm atio n

regarding th e natu re o f m antle sou rces m ay be extracted from th e

g eo ch em ic al a nd is oto pic data, Fo r exam ple, Ph iso topic data fo r

po tassium feldspar (o r plagioclase) shou ld co nstrain the U /Pb



Geochemica l approaches t o sedimen ta ti on

60WCal.he,ing

Tr:w>d

• Active Margin Muds

o Passive Margin Muds

o Australian Shales

o

o 0::J

Uppec Csus!--;4.0

•.cr • c e 0&

•

[ .

•

} ~ W d.0 • •

• •Mantle Sources

0.0

.1 10

Th (ppm)

Figure II. Plot of Th /U versu s Th fo r modem tu rbidite m uds Cromva ri o us t ec t on ic settings a nd f or A u st ra lia n p os t-A rc he an s ha le s m a in ly

o f c rato nic p ro ve na nce . D urin g w ea th erin g, th ere is a t en de nc y f or ane le va tio n o f Th/U a bo ve u pp er c ru st al ig ne ou s v alu es o f 3.5 to 4.0 that

a rc re fle ct ed in s ha le s. S ed im e nt ary p ro ce ss es th at lo we r Th/U usual lyar e the r es u lt o f U enr ichments. It is s u gg es te d t ha t low Th/U, commonlys ee n in ac tiv e m arg in s ed im en ts , e sp ec ia lly w he n ac co mp an ie d by lo w

Th an d U abu ndances , r ef le c ts g eo ch em ic a ll y depl et e d man tl e s o u rc e s o ft he a rc p ro v en an ce .

ra tio an d Pbisotopic h isto ry o f th e mantle sources as it does fo r

m antle-derived igneou s rock s th em selves (e.g., Sun, 1 .980). Fo r

m os t upper cru s tal ro ck s, Th/U is typically abou t 3.5 to 4.0. In

most cases, w eath ering and sedim entary recycling u nder o xidiz-

ing conditions typically resu lts in oxidation o f U4+ to U6+, th e

latter being m ore so lu ble. D isso lu tio n and lo ss du ring sedirnenta-

lio n fo llo ws, th us elevating th e ThlU r at io , e sp ec ia lly f or s ha le s,

wh ere h eavy m inerals are less lik ely to be a co mplicating facto r

(e.g., M clennan and Taylo r, 1980; Taylo r and M clennan,

1 98 5). In o th er cases wh ere low Th /U exists due to sedim entary

pro cesses , it is lik ely th at it resu lts fro m U enrich ment and acco rd-

ingly th is w ill be accom panied by h igh U content ref lecting the

g re at er m o bility of th is elem ent. M any sedim ents from active

m argin tecto nic settings, with m ajo r com ponents o f young undif-

fe re ntia te d c ru st, h av e T h/U significantly b elo w 3 .5 a cc om p an ie d

by low Th and U contents (F ig . 1 I) and th is is interpreted as

do minantly reflecting a lo w ratio in th e s ou rce ro ck s (M cL en nan ,

1989b, M cLennan and Taylo r, 1991). Low Th/U ratios are

rath er com mon in m antle-derived vo lcanic rock s and refleci th e

geochem icaU y depleted natu re o f such reservo irs (e.g ., N ew man

el at, 1984).Th is provenance com ponent is lik ely to dom inate in fo rearc

bas ins. It is also lik ely to be found in m ost o th er vo lcanically

active tecto nic s ettin gs; h ow ever, data fo r m odem tu rbid ites

in dicate th at m ost o f th es e s ettings also nav e significant bu t varia-

ble amounts o f o ld upper con tinental cru st. Examples where

Y UA do minates is th e D ev onian-C arbo nifero us vo lcano genic

sedim ents o f eas tern A ustralia (C happell, 1968; N ance and Tay-

lo r, 1977; Bhatia, 198 I, 1985' Hensel et al. 1985) and the Per-

31

100e nC D..-10-

""0C

a.c;

010

Eo,o,

- -Eo,CL

1100

S. Aleut ians

(Fore arc)

LaCePrNd SmEuGdTbDyHoEr Yb

Figure 12. Chondr i te-normalized REE plo t o f a t u rb id it e s amp le from

( he A l eu ti an s f or ea rc . This sam ple has an tNd of +7.7, suggest ive of ap ro v en an ce d om in at ed by th e island aT C. Th e negative E u ano maly

s ug ge sts th at ig neo us ro ck s in th e y ou ng a rc p ro ven an ce w ere fo rm ed by

intracrustal d if fe re nt ia ti on t ha t f ra ct io n at ed p la gi oc la se . T h is i s a n e xam -

p le o f a y ou ng d if fe re ntia te d a TC(YDA) p ro ve na nc e. D a ta f ro m M c le n-nan el al. (1990).

m ian Brook S treet terrane o f souihwestemmost N ew Z ealand

(Fro st and Coombs , 1989) (Fig. 10).

4. Young differenLial.edarc (YDA). T his p ro ve na nc e

com ponent represents young (mantle derived) vo lcanic and plu -

tonic igneo u s rock s from is land and continental arcs th at h ave

been affected by intracrustal d iffe re ntia tio n s uc h th at th ey possess

s ig nif ic an t n eg at iv e Eu anomalies. It is dom inated by differen-

tiated fels ic vo lcanics and ashes and plu tons tbat fo rm with in the

arc and are expo sed as th e arc becom es dissected. Th is com po -

nent is distingu ished from young undifferentia ted arc (YUA )

s ou rces by th e presence o f significant negative Eu anomalies. It is

distingu ished from o ld upper cru s tal sou rces (QU C a nd R SR ) by

its relatively h igh 143Nd/l44Nd ratio (~Nd>+5 fo r modern

sediment), In effect, th is com ponent represents "young" upper

c on tin en ta l c ru st s in ce th e in ira cru stal d iffe re ntia tio n es se ntia lly

rem oves h eat-pro du cing elem ents to th e u pper cru st and s tabilizes

th e continental cru st. In term s o f m ajo r and trace elem ent com po -

sitio ns, th is component appears to be generally sim ilar bu t of

m uch greater variability th an o ld upper cru stal provenance due

to th e m ore lo calized s ou rces . B ecau se o f th e firs t-cycle ch aracter

and rapid eros ion rates o f h igh -s tanding arc terranes , in mos t

cases th is com ponent is lik ely to be less affected by weathering

processes (i.e ., low C IA , Rb/S r) th an is o ld upper cru st (OUC).Su ch a component sh ou ld be im po rtan t in matu re arc envi-

ro nments, su ch as th e A leu tians (Fig. 12), where the arc is

dissected (M cLennan et al., 1990) o r where abundant fels ic vo l-

canism occu rs. The o rigin o f th is component suggests th at it is

u nlik ely th at it w ou ld co mm only be dom inant, compared to

QUC o r YUA. Instead, m ixtu res with QUC and YUA are to be

expected. F lo yd and Leveridge (1987) have sugges ted a dissected

con tinental arc p rovenance fo r m uch o f th e Devonian Grarnsca-



S. M. Mcl.ennan and a/hers

0.20

• Passive Marg"in Sequence

0 Volcanogenic Sequence

~~~~1.7Ga~

~~~ .

~ II-, . .__ 0~ Mixing

CIUst /

W#&

0.00

• R2-3 "0

zR6-3 - -• E

PL-1

(f)

• '+-- ·0.20

-0.40

32

1000

Passive Margin Sequence

(/)

Q)

._

"0 100co..coE0....0....- 10

Volcanogenic Sequence

-0-- 81

----0-- 132

0 135

1LaCePrNd SmEuGdTbDyHoEr Vb

E0....0....

Figure 13. Chondr i te-normal ized REE plot showing selected Early Prot-

erozoic sedimentary rocks from northern New Mexico and southern

Colorado. Examples from two sequences are shown. One is a lower

volcanogenic turbidite sequence, lacking substantial negative Eu anorna-

lies. typical of an undifferentiated arc provenance. The second, younger,

sequence is characterized by LREE-enriched patterns with negative Eu

anomalies, characteristic of the upper continental crust, Unpublished

data.

tno tu rbid ite sequence from C ornwall, and m ajo r and trace ele-

m ent data are co nsis ten t w ith characteris tics described above.

S im i la rly , D a ba rd (1990) s ug ges te d a fe ls ic v olc an ic p ro ve na ncefo r Late P ro tero zo ic tu rbidites fro m tbe A rm orican M assif, w ith

Eu/Eu* in t he ra ng e 0.7 to 0.9.

D rawing a line between o ld and y ou ng differentia ted cru st is

n ot n ece ss arily s tra ig htfo rw ard . U nd er m os t c irc um stan ce s d urin g

th e P hanero zo ic, th e intracru stally differentia ted co mpo nent ap-

pears to be subs tantially o lder th an new m antle-derived cru st.

The average Nd-model age o f most Ph an ero zo ic crato nic sedi-

m ents is abou t 1.5 to 2.0 Ga, altho ugh th ere are exam ples where

th is is no t th e case. Early Pro terozo ic rock s o f so u thwes tern

United S tates appear to reco rd a m ajo r cru s t-fo rm ing episo de at

abou t 1.8 to L7 Ga (Nelson and D ePao lo , 1985). The sedim en-

ta ry s equ en ce rec ord s a lo we r v olca no ge nic g ra yw ac ke s equ en ce ,

rep resenting derivation from an arc terrane, and a younger

qu artzite-sh ale s equ en ce, representing depo sitio n o n a s tabilized

pass ive m argin (Soegaard and Erik sso n, 1986). Trace elem ent

data from th e fine grained sed im entary rock s are cons istent w ith a

d om in an tly y ou ng , u nd iffe re ntia te d a rc te rra ne as a sou rce fo r th e

lower graywackes and a reaso nably well-m ixed (i.e ., un ifo rm

com pos itio ns) upper cru stal sou rce fo r th e pass ive margin sedi-

m ents (M cLennan and Taylo r, 1989 ; Fig. 1 3). N d-is oto pic d ata

(Fig" 14), however, ind icate th at th e dom inant sou rce o f th ese

·0.60-15 ·10 ·5 0 5

Figure 1 4. P Ia l of{>m/Nd versus ENdfor sedimentary rocks from the Early

Proterozoic of New Mexico and Colorado. The data are recalculated for

the time of sedimentation, a t c a. I. 7 Ga. Although there is a difference in

the Sm/Nd ratio, al l samples have ENdconsistent with a dominant prove-nance of 1.8 to 1.7 Ga with little or no older Archean crust This.

indicates that substantial crust was generated in an active tec tonic setting

and subsequently stabilized, with deposition of a passive margin

quartzite-shale sequence. over a very short period of time at 1.7 Ga ..

Unpublished data,

sedim ents , inclu ding th e qu artzite-sh ale s equ en ce, w ere d eriv ed

from the m antle at abou t J.7 to 1.8 G a, just p rio r to s ed im en ta -

tion . Th is sugges ts an eno rm ous vo lum e o f new cru st was gener-

ated and stabilized over a very sh o rt period o f tim e and th u s

p la ce s im po rta nt c on stra in ts o n th e P ro te ro zo ic te cto nic e vo lu tio n

o f th is reg io n. A lth ough a firs t-cycle o rigin fo r th ese quartzites is

apparent from sedim ento lo gical dati (Soegaard an d Eriksson,1989), co mbined geo ch em ical an d iso to pic data penuit quanti ta-

tive constraints to be placed on the po tentiaJ provenance

componen t s .

5. Exotic components. In som e cases, id en tific atio n o f

even trivial am ounts o f certain exo tic com ponents can have very

im po rtan t tecton ic im plications . Fo r exam ple, th e firs t appear-

ance o f any components from a geo chem ically o r iso to pically

d is tinct allo ch th ono us terrane w ou ld pro vide im po rtan t evid en ce

re ga rd in g it s p ro xim ity (e.g., D ick inson, 1988). H isco tt (1984)

related variations in ferrom agnesian trace elem ents in Taconic

flysch to pro xim ity o f an obdu cted oph io litic so urce terrane (Fig.

15). T he lack o f o ph io litic detritu s in th e so uth ern A ppalach ians

cou ld be related to complexities in th e natu re o f th e Taco nico rogeny, such as a change in regim es from mo re direct conver-

gence in the no rth to eith er o blique subduction o r s trik e-slip

m o tion in the sou th (Ho scitt, 1984). In a different setting,

M clennan et at. (1990)" were able to identify an ocean cru s t

componen t in m odem fo rearc tu rb id ites from the M arianas re-

gion , based o n com bined m ajo r and t ra ce e lem en t g eo ch em is try

(e.g., lo w Balla, Th/U; h igb Mg, C r) and Nd-Sr iso top ic

relations.



Geochemical approaches 10 sedimentation

ARCHEAN PROVENANCE

•I

0 •Recent Q

(\j • • • < : \ iII

0 Phanerozoic ~~

Proterozoic • •z

+ '--

~ • . .

.~• •+ 0 ~

." • • < !J

. .\\ • J'~.:• .,. ,~••- - - . .~ -

EulEu'=O,85 • ')..•

10.0

I'op.hiolite'(Chromite)

8 . 0

> 6.0- --()

4.0

2. 0

0. 0

0. 0 2. 0

Taconic Flysch

• Autochthonous

o Allochthonous

Mixing (10 and 2)

1 •

1.0

Y IN iFigure 15. Plo t o f Cr/V versu s Y /N i for early Paleo zo ic Taco nic flysch

from eastern N orth A merica illu s trating (he greater im po rtance o f an

o p hi ol it ic p ro ve na nc e a s one m oves no rth . H igher num bers and leiters

indicate m ore no rth erly o utcro ps . E ach data po in t r epr esen ts t h e a ve ra ge

o f several analyses fro m o ne formation/area, A lso sh ow n is a m ixing linegenerated from th e two data po ints w ith th e lowest and highest Cr

abundances . The C r/V ratio is an index o f th e enrichment o f Cr o ver th e

o ther ferromagnesian t ra ce e le m en ts , w h er ea s Y/N i m o nito rs th e g ene ra l

level o f f er romagn es ia n t ra ce elem ents (N i) co mpared 10 a proxy fo r

H REE (Y ). M afic-u ltram afic so urces tend to h ave h igh ferro magnesian

a bu nd an ce s; s uc h a p ro venance w ou ld resultin a decrease in Y /Ni. A

m ineral su ch as ch ro mite, im po rtant in o ph io litic sequ ences , tends to

co ncentrate C r p referentially o ver o th er ferro rnagnesian elem ents , D ata

from Hiscott( 1984) .

1.2

. - 1.0:J

W

- - -JW 0.8

+

0.6 o ••+

1.0 1.5 2.0 2.5Gd N/YhN

3.0 3.5

33

G eochem ical characteris tics o f A rchean terranes are lik ely

to be su bstantially different th an in yo unger enviro nm ents. T here

is abundant evidence to indicate th at th e composition o f many

plu to nic and vo lcanic igneo us rocks differed d urin g th e A rc hea n

com pared to th e Phanerozo ic, mos t probably in response to dif-

fering th erm al regim es (Taylo r and M clennan, 1985; Goodwin,1991). F or exa mp le , g ran itic rocks fo rm ed d uring the Archean

are more commonly Na- a nd p la gio cla se -ric h g ra no dio rit es -

tonalites -sanuk ito ids, whereas du ring the Phanerozo ic th ey

tended to be more K- and K -felds par-rich grano dio rites-

monzoni tes-grani tes. Implications of th is fo r petrograph ical and

m ajor elem ent relationsh ips have been discu ssed by M clennan

(1984) and Taylor and M clennan (1985). D iffe ren ce s a ls o ap -

pear to exis t fo r trace elem ents (e.g., F ig. 16). Thus, Archean

sedim entary rock s from vo lcanically active settings , when com -

pared to Pro terozo ic and Phanerozo ic sedim entary rock s from

s im ilar settings, lend to have (M clennan and Taylo r,1991):

(I) les s depletion in Eu , reflecting an absence o f intracru stal

d iff ere nt ia tio n in the s ou rc e ro ck ; (2) greater depletio n in h eavy

REE (HREE; Gd- Vb) ( i. e. , h ig h GdlYb ratios), reflecting igne-

o us pro venance w ith m antle so urces h aving su ch ch aracteris tics

o r having had garnet (HREE enrich ed) as a fractio nating phase

du ring fo rm atio n; and (3) higher Th/U r at io s , p er ha ps in di ca tin g

le ss d ep le te d m an tle SOUfCC5 fo r th e igneo us pro venance o r differ-

in g m ech an is ms o f fo rm atio n.

A cco rdingly, care m ust be taken when applying the above

3.0

Archean

•• ••c. •

• ••

A B

EuIEu-=O,85 •

••

1.0 1.5 2.0 2.5

GdN/YbN

3.0 3,5

Figu re 16. P lo t o f Eu /Eu - versu s GdN /YbN for tu rbidites o f vario us ages depo sited in volcanical ly

active tectonic settings . A rchean tu rbidites tend (0 have h igher EuIEu ·, suggesting th at. th e

igneo us/m etam orph ic pro venance, o n average, w as less affected by intracru stal differentiatio n. Th ey

also co mm only h ave G dN /Y bN >2.0, w hereas su ch h igh ratio s are virtu ally absent fro m po st-A rch ean

active m argin sedim ents . Su ch differences indicate th at cau tio n is w arranted in u sing p ost-A rch ean

terrane types in interpreti ng A rch ean sedim entary rock s or sedim entary sequ ences derived prim aril y

from A rchean sou rces . F rom M clennan (1989a), where data sou rces are lis ted.



rI

34 S. M. Mciennan and Others

provenance concepts to A rchean sed im entary rock s. By th e end

o f th e A rch ean, th e upper cru st a ppea rs to ha v e s tabil ized to a

state comparable to th at seen in you nger lim es. H owever, sedi-

m entary rock s o f any age, derived prim arily from A rchean ter-

ranes, especially from relatively localized so u rces , m ay be

inf luenced by th ese d ifferences. An example com es from the

Early Pro terozo ic Chelm sfo rd Fo rm atio n derived from the Ar-chean Superio r Pro vince (M cDaniel and M clennan , 1991;

M elraniel, 1992). Tu rbidites from the lowerm ost part o f th is

sequence appear to be dom inated by pro venance components o f

s tabilized upper con tinental cru s t, bu t m ost samples have REE

patterns w ith ou t n eg ative E u ano malies .

PB ISOTOPES AND COMPONENT ANALYSIS

The lead iso tope sys tem has great po tential fo r evaluating

bo th the o rigin and h isto ry o f sedim entary rock s and their indi-

vidu al m ineral constitu ents. U and Pb m ay be transpo rted under a

num ber o f lo w-tem peratu re geo lo gical processes relevant to sed-

im entary rock s. H ow ever, th e occu rrence o f th ree independent,

bu t geochem ically related , Ph -iso tope decay schem es, 238UI

2 06.P b, 2 3S U/2 07P b, a nd 232ThI20 8Pb, commonly allows fo r

such processes to be ev alu ated and fo r co ns iderable info rm atio n

regarding age and geo ch em ical evo lu tio n o f th e ro ck s o r m in erals .

H is no t w ith in the scope o f th is s tu dy to fu lly review the funda-

m entals o f U -Th -Pbiso tope.system atics ; fo r such a rev iew .th e

reader is referred to S tacey and K ram ers (1975), D oe and Zan-

m an (1979), and Fau re (1986).

F or co nglo meratic ro ck s it is p oss ib le to rem ov e individu al

pebbles o r co bbles and exam ine them in detail fo r geochem istry

and age. A lthough o f great u se, such sedim entary rock s typically

renee! only localized sou rces (e.g., Ingerso ll, 1990). A t th e o ther

extrem e, s ingle-grain com po nents o f s ilts tones and shales are d if-

ficult (0 s eparate an d analyze. In th is discu ssio n, w e co ncentrate

on the evaluatio n o f th e. provenance o f sands and sandstones.

Dating components

U/Pb geo ch rono logy o f zircons in sedimentary ro ck s asa

tech niqu e fo r estim ating th e ageof th e provenance is well es tab-

lish ed (e.g ., Tatsum o to and Patterson , 1964; G audette et al.,

1981). A nalyzing single zircon grains. h as been an especially at-

tractive approach becau se it elim inates any am bigu ity o f m ixed

po pu lations from different sou rces. Conventional m ass spec-

trom etry, u s ing abraded zirco n grains, and ion m icroprobe iso -topic analyses o f individual grains have bo th been u sed

success fu lly. A lthough much o f tile wo rk to dale has been in-

vo lved w ith identify ing theearlies t cru s tal components o f th e

earth (e.g. M aas et at, 1992) som e stud ies have also demon-

strated th e value o f sedim entary zircon geoch rono lo gy fo r tee-

tonic problem s (e.g., R oss et al., 1991; Ireland , 1992).

Z Ircons are o f great value in unders tand ing pro venance h is-

lo ry , no t least due to th eir robu s tness and pers istence in the

sedimentary reco rd. On th e o ther h and, u sed in iso lation fo r

characterizatio n o f p rovenance. zirco ns have several sho rtco rn-

ings: (I ) th ey rep resent o n Iy a very sm all fracti on o f any sedim en-

tary rock , typically <0.1% (sandstones rarely have U>500 ppm

and shales rarely have Zr>250 ppm ): (2) th ey are m ore lik ely to

h e recycled in sedim entary ro ck s th an arc less resistant m inerals

(see abo ve). and acco rdingly are lik ely to p re fe re nt ia lly re fle ct o ld

recycled com ponen ts ; and (3) th ey are absen t in igneou s ro ck sth at fo rm in m agm aiic.environm ents th ai are undersatu rated w ith

respect to zircon and acco rding ly w ill no t be rep resented in th e

s edim en laryzirco n reco rd. O th er trace m inerals , in clu ding m ona-

zite an d sp hene, are als o ap pro priate fo r U -Pb geo ch ro no lo gical

stud y bu t su ffer fro m m il by o f th e sam e sho rtco rn i ngs as zircon, o r

th ey have additional complexities (e.g., sphene m ay fo rm

diagenetically).

G eoch rono lo gical studies o f m ajo r m ineral phases provide

an im po rtant new perspective on the age o f sed im entary pro ve-

nance. Feldspars, on their own, are no t su itable fo r U/Pb dating

due to extrem ely low U conten t, although K I Ar and Rb/Sr

dating tech niques ho ld prom ise (e.g., Harr ison an d B e, 198 3).

C lay m inerals h ave been the cen ter o f m uch attention, especially

fOT R b/Sr geo ch ro no lo gy, bu t th ese m in erals are s tro ngly affected

by diagenesis and sedim entation ages o r m ixed ages are typically

obtained (e.g., C lauer, 1982; Oh r et ai., 1991). There is lik ely

p oten tial fo r K/Ar and Rb/Sr dating o f detrita l m ica (H eller et

al., 1985; Heller and Fro s t, 1988 ; Kelly and B luck , 1989) and

perhaps iso top ic dating o f individual sand-s ized ro ck fragm ent

grains; h ow ever, s uch appro ach es . h av e yet to beinves tigatedin

a ny d et ail.

Quartz. is th e m os t abundant m ineral in m o st sandstones .

R ecent advances in low -level analytical techniques have per-

m iued U/Th /Pb iso topic data to be co llected fo r very sm all

quartz popu lations and under favo rable circumstances fo r

ind i v id ua l sand-s ized qu artz grai ns (H em ming et aI., 1990,. 19 91).

Early resu lts suggest th at U and Th are contained m ainly in

m ineral and/o r flu id inclu s io ns and .th at, under a w ide variety o f

geo logical conditio ns, quam retains th e o riginal U /Th /Pb iso -

top ic relationsh ips . In addition, quartz com monly has h igh and

variable U/Pb ratio s , m ak ing th e m ineral su itable fo r Pb-Pb dat-

ing and, in cases o fextrem ely h igh U /Pb ratio s.su itable fo r U -Pb

geochronology.

Th is appro ach is cu rrently being inves tigated in Precam -

brian ro ck s from no rthern M inneso ta and elsewhere (H em ming

et al . . 1990, J 991). In Figu re 17, Pb- Pb isoch rons a re displayed

fortwo A rchean plu tons from the Superio r Pro vince o f no rth ern

M inneso ta. In bo th cases, quartz-when combined w ith who le-rock .and feldspar analyses -provides Pb-Pb ages in agreem ent

w ith co nventio nal iso topic approaches. In bo th cases , quartz is

m ore radiogenic (h igher 207Pbli04Pb and 206Pb/20 4Pb) than

the who le-rock analyses; however, th e radiogenic natu re o f th e

quartz is h igh ly variable. In Figu re 18, th ePb iso topic com pos i-

ticn o f clear qu artz. fro m th e early P ro tero zo ic P ok egarna Q uartz-

ite, represen ting the dom inant qu artz popu lation, is plo tted in

term s o f 207Pb/2(J4Pb versu s 206Pb/204Pb. Th is fo rm ation is

generally co nsidered to be derived from the A rchean Superio r



Geochemical approaches 10 sedimentation 35

28

Range granite(W T Rb~Sr age = 2620+/-63 Ma)

(sphene Pb-Pb age := 2640+/-' 6M<:J)

26

22

2.4

20

18

16"

• quartz

• K-feldspar14

14

15 20 25 30 35 40 45 50 55 60 65 70 75 80

206pb

/204pb

16

..cn,~a('I,j

<, 15

..cn,

r - - . .a('I,j

14

Saganaga tonalite(sphene Pb-Pb age = 2694+/-18 Ma)

(wr Rb-Sr oge = 2548+/-180 Ma)

• quartz

• plag io clase

• who le rock

13 14 15 16 17 18 19 20 21

206pb/204Pb

22 23

Figu re 17. P lo ts o f 207Pbl 2 04 Pb v ers us 2 06 Pb /20 4Pb fo r samples from Archean plu tons from Superio r

Pro vince in no rth ern M inneso ta. G ro wth cu rves are fo r 1 . 1 . = 8.0 w ith 500-M a intervals . N ote th at qu artz

analyses are relatively radio genic (i.e . . h igh 206Pb/20 4Pb) and are collinear with who le-ro ck and

feldspar analyses . W hen co mbined, th ese data define ages co nsis tent w ith ages derived fro m co nven-

tio nal radio metric tech niqu es . Th e radio genic natu re o f th e qU3TIZ is variable betw een th es e p lu to ns ,

Th is can be appreciated by no ting th at th e inset fo r th e G iants Range granite (A ), wh ich show (he

sam ples w ith lo wer 2 06P b/20 4Pb, has th e sam e scale as th e lower diagram , wh ich shows data from the

S aganaga to nalite (B ). U np ublish ed data o f S . H em m ing. C onventio nal g eo ch ro no lo gical data co mp iled

from s ev er al s o u rc es .



3 6 S. M. Mcl.ennan and Others

Pokegomo Quartzite• clear detrital quartz

o detrital K-feldspor

o whole rock

,40

120

.0

0... 100. . , .aN

<,80

.0

0.... . . . . .a

60

40

20

2647+/-16MO

20llpb 0;2311U

0.5Z5 ,.

."./

2_/ •.2657'/-66 Ijo

23 -:0. 4

:/ 207Pb ./2J~U

9 10 1 1 12 13 14

200 600 70000 400 500

206pb/20""Pb

Figure 18. Plot of 207Pb/20 4Pb versus 206Pb/20 4Pb for samples from the Earl y Proterozoic Pokegama

Quartzite from northern Minnesota. Whole rocks, feldspars, and quartz define a line of 2647 ± 16 Ma,

consistent with a dominant provenance from the Superior Province. Inset, Concordia-discordia plot of

radiogenic quartz samples from the Pokegama Quartzite. Increments are at O.I-Ga intervals. These

quartz samples display variable discordance and a range in 206Pb/20 4Pb (61 to 748) that cannot be

correlated to the discordance. Together they define an upper intercept of 2657 ± 86 and lower intercept

of 660 ± 661 Ma, with the scatter about the line being greater than would be expected simply from

analytical uncertainty, Unpublished data of S. Hemming.

100

Province to th e north . An age o f 2647 ± ]6 M a is o btained,

co nsis ten t w ith th e ag e o f th e S up erio r P ro vin ce an d co nsis tent

with an age o f 2673 ± 4 M a fo r detrital zirco ns fro m tbe sam eu nit (H emm in g e t al., 1991 ). I n t h is c as e, t h e r ad iog en ic c h ar ac te r

o f th e qu artz p en n its U -P b g eo ch ro no lo gy , a nd fo u r cle ar qu artz

sam ples p ro vid e an age o f 2 657 ± 8 6 M a (low er in te rc ep t 6 60 ±

6 61 Ma ) w ith v ar ia ble d is co rd an ce (in se t, F ig . 1 8) .

Jdentification oj terranes

T he in itial P b is oto pic co mp ositio n o f a ro ck pro vides im -

po rtant in fo rm atio n abo ut th e o rig in an d g eo ch em ical h is to ry

becaus e i t is s en sit iv e t o t he ra tio o f th e p are nt -d au g ht er is o to p es o f

th e so urce. W ho le-ro ck P b iso to pic data, e sp ec ia lly f or s h ale s

(w h ic h e ffic ie ntly m ix p ro ve nan ce ), m ay f urt.h er c on stra in th e

tim in g o f an y U /P b fractio natio n. Fo r P b, th e in itial U /P b ratio ,jJ.j (jJ. = 2 38U /2 04 Pb ) m a y b e d ete rm in ed e it he r f rom t he in te rs ec -

t io n o f 207Pb/ 20 4Pb -206Pb/ 20 4Pb i so c h ron s w it h a n a ppr opr ia te

P b iso to pic gro wth cu rve o r by an aly zin g m inerals th at co ntain

little o r n o U , and th us p res erve th e P b iso to pic ratio s at th e tim e

o f fo rm atio n, F or sed im entary stu dies bo th ap pro ach es m ay be

u se fu l; fo r e xamp le , th e e stim ate d l J . i fo r P ok eg am a F orm atio n

q ua rt z d ata is about 7.9 (F ig . 18 ),wll ich is en ti re ly cons is t en t

w ith th e K -feld spar d ata (see belo w) and w ith an o rig in fro m th e

S up erio r P ro vince to th e n orth (e.g ., T ilto n, 19 83; G ariep y and

A l leg re , ]985 ) .

A na ly se s o f ac id -le ac he d d etrital f eld sp ar, p artic ula rly K -f el ds p ar , a ls o ho ld s g re at p rom i se f o r c ons tr ai ni ng t ec ton ic h is to r y

o f s ed im e nt ar y b as in s b y id en tif yin g d is tin ct p ro v en an ce c o nt ri-

bu t ions . Fe ldspar is susceptible L O a lt e ra t ion and r ec rys ta ll iza ti on

d u rin g d ia ge ne sis , a nd th e p o te nt ia l f or t he se p ro ce ss es t o d is tu rb

th e P b iso to pe sy stem w ith in clas tic sed im en tary ro ck s h as n ot

been fully evaluated. It sh ou ld be no ted, h owever, th at m os t

low -t em p er atu re f lu id s h a ve e xt rem ely low Pb c on te nt ( e.g ., s ea -

water Pb < 0.] ppb; river water Pb < ]0 ppb; Taylo r and

M cl.enn an, 1 985), and co ns id erable in teractio n is lik ely to be

necessary fo r sig nificant d is tu rbance o f th e P b iso to pic sys tem

un de r s uc h c on dit io n s. F u rt he rm o re , a lte ra tio n t ha t is re st ric te d t o

th e o u te r s u rf ac es o f tb e f eld sp ar g ra in s is r em ov ed d u rin g s am p le

preparatio n. W here dis tu rbance o f th e Pb iso to pic sys tem h aso cc u rr ed , it may be p os sible to e valu ate th e tim in g o f d ia ge ne sis

a nd th e g eo ch em ica l n atu re o f th e flu id s.

A number o f s tudies o f igneous and m etamo rph ic ro cks

h a ve s h own t ha t d if fe rin g t ec to n os tra tig ra ph ic t err an es a re o fte n

ch ara cte riz ed by d is tin ct in itia l P b is oto pic c om po sitio ns (e .g .

K ro g st ad e t a I., ] 9 8 9; A y u so a nd B ev ie r, 1 99 1) . T h es e d if fe re nc es

re la te bo th to d if fe rin g m an tle c om po sitio ns (a nd a ge s o f m an tle

d epletio n) an d th e m ean ag e o f th e cru st th ro ug h w hich m agmas



r

Geochemica l approaches 10 sedimentation

pas s. In m any cases. it is th e ratio 2 07P b/20 4Pb th at separates th e

Pb i so topic cornpo i tion of many terrane . The 207Pb parent

isotope, 23SU, is relatively sh ort-lived co mp ared to ;138U (parent

to 2 06P b) a nd was fa r moreimportam in th e ea rlie r h is to ry of th e

e art h. A c co rd in gly , in cre as es in U IPb ( e .g . . d urin g th e f orm atio n

o f continental cru st) th at to ok place early in earth h isto ry resu lt in

e le va te d 2 07 Pb /204Pb th at pers is ts th ro ugh geo lo gical tim e. In -

creases in U/Pb that look place more recently re su lt in e le va te d206Pb/20 4Pb over L im e.

Tw o ex a m pies o f us i ng P b iso to pic co m po si t ions o f fe ld sp a r

10 d is cr im in at e p ro ve na nc e, both from early Pro tero zo ic se-

quences , arc discu ssed below . Figu re 19 shows the Pb iso topic

Chelmsford Formation.. feldspar. • w ho le ro ck

P.okegama Quartzite

'. feldspar

16.0Q_

- < : t "

o(\J-0Q_

, . . . .aC\J

.. ,'. . . . . . . . . . . . .

15.._ .... Superio r evo lu tion

-~ .. ,.. . . . . . . -

. . . . . .S u pe ri or f el ds p ar 11'"7.4. 7.8,8.0

14 ~~~~~~~ww~~~~~~~~~~~13 14 15 16 17 18 19

206Pb/204Pb

F ig ure 1 9. P lo t o f 2 07 Pb /104Pb VI.:fSUS ~06Pb /~~Pb f o r l ea c h ed f el dsp a rs

from the Early Proterozoic C helm sfo rd F orm atio n and Po kegam a

Quartzite an d w ho le rocks fro m th e C helm sf ord F orm atio n. G ro wth

c urv es a rc fo r Ii"7.4, 7.8 , and 8.0, w ith increm ents at 500 M a. A lsoploued a re f ie ld s r ep re s en t i ng l ea ch ed K-feldspars f or A rc he an ig ne ou s

rocks f rom the Superio r P rovince and M inneso ta R iver Valley and

P ro te ro zo ic ig ne ou s ro ck s o f P en ok ea n a ff in it y (c om p ile d from many

s ou rc es ). F eld sp ars (ro m th e C he lm sfo rd a nd P ok eg arn a, bo th s in gle

g ra in s a nd sm all p op ula tio ns , p lo t v ery close to t he S u pe rio r P ro v in ce .Fo r th e C helm sfo rd, th is indicates v ery lillie o r no invo lvem ent o f

Penokean-aged fe ldspars. F or th e Pokegarna, n eith er th e P en ok ea n n or

M in ne so ta R iv er Valley g ne is s es a re l ik e ly so urces, T he ChelmsfordF o rm a ti on w h o le rocks ( sa nd s to n es a nd a rg il lit es ) c o in ci de w it h a r ef er -e nc e l in e d ef in in g Pb g ro wt h f ro m S up erio r P ro vin ce K -f eld sp a.rs , be -

tween 2.7 a nd 0 .0 Ga ( d ot te d l in e s) ; t h is s u gge s ts t h at , i f P enok e an- agedrocks re pre se nt m o re t ha n a t riv ia l c om p on en t t o t he p ro ve na nc e, s uc h ac om p on en t d id n ot c on trib ut e f eld sp ar. F eld sp ar s f ro m t he C h elm s fo rd

c oin cid e w ith a lin e d efin in g P b g ro wth be tw ee n 2.7 and 1.8 Ga, and

rn ay i nd ic a te t h at t h e f e ld sp ar s we re i nf lu enced by Penok e an- aged f lu i ds ,

p erh ap s d urin g d ia ge ne sis (p la gio cla se in t he se s am p le s a re a lb uiz ed ).D ata from M cDaniel (1992) and S Hemm in g ( un pu b li sh e d) .

37

co mpo sitio n o f felds pars fro m th e P ro tero zo ic Chelmsford For-

m ation, Sudbu ry B asin. Plo tted fo r co mparison are th e fields fo r

K -feldspar from the A rchean Superio r P rovince and the Prater-

oic Penokean Orogen, bo th po tential so urces . The C helm sfo rd

K -feld spars clearly h ave been derived alm os t exclu sively fro m th e

Superio r Province, th us prov id ing an im po rtant co nstraint on the

o rig in o f th ese sedim entary rock s . W ho le-rock data are also

show n, and the trend betw een these sam ples and the leas t evo lvedfeldspar co incides w ith is och ro ns defining Pb iso top ic gro wth

between 2.7 and 0.0 Ga, u sing the range o f Superio r Prov ince

K-feldspars as po tential in itia l ratio s. T here is no evidence in these

data f or s ig ni fi ca nt d is tu rb an ce of th e Pb i so tope system.at 2.0 to

1. 8 Ga, which m igh t be expected if Penokean-aged rock s were a

s ig nif ic an t c om po nen t to th e p ro ve na nc e.

Th ere is a po ss ibility th at th e feldspars fo rm a linear array

co inciding to an isochron defining a 2.7- to J .8 -G a is oc hro n,

altho ugh such an interpretation is no t dem anded statis tically , If

su ch a tren d exists , it could be interpreted as resu lting fro m m ix-

in g o f S up erio r P ro vin ce Pb w ith 1 .8 -G a f lu id s d urin g d ia ge ne tic

alteratio n o f th e feld spars (M cD an iel, 1992).

K -felds pars fro m th e Po kegam a Q uartzite (see Fig. 18) are

a ls o s ho w n in Figu re 19. In addition to th e Superio r P rovince and

Penokean O rogen, p lau s ible sou rce rock s include the M inneso ta

R iv er V alle y (MRV) g ne is se s. T he e xte nd ed fie ld fo r K -fe ld sp ars

from MR V igneou s rock s indicate a com plex h isto ry and are also

shown in Figu re 19 . The Pokegam a K-feldspars plo t w ith in o r

very clo se to th e f ield o f th e Superio r province. Bo th th e MRV

and the Penok ean O rogen can be excluded as s i gn if ic ant s o u rc e s

fo r th e d etrita l K -feldsp ars .

A pplying such an approach to yo unger terranes in many

cases is u nlik ely to be straigh tfo rw ard . T erranes th at h ave u nder-

gone long and/o r com plex geo logical h isto ries are lik ely to in-

clude som e components o f o ld cru st. Th is will re su lt in e le va te d

207Pb/20 4Pb ratio s fo r Kfeldspars th at are derived directly fro m

th es e o lder ro ck s and fo r K -feld spars fo rm ed du ring later m elting

ofthese terranes. In ef fect, th e co mplexity o f an extended geo lo gi-

cal h is to ry is lik ely to average ou t m any o f th e very dear distinc-

tio ns in Pb is oto pic c om po sitio n s ee n in o ld er te rra ne s. N ev erth e-

les s, w here distinct P b iso to pic co mpo sitio ns do exis t betw een ter-

ran es th at are po tential s ou rces, th is is lik ely t o be re fle cte d in th e

sedim entary reco rd (e.g., K rogstad and H anson , 1991). A cco rd-

ingly, P b is oto pes p ro vide an important a dd itio na l to ol in evaluat-

ing th e tecto nic h is to ry reco rded in s edim entary ro ck s.

20

CONCLUSIONS

Our unders tand ing o f th e con tro ls on th e ch em ical and iso-

to pic c om po sitio n o f s ed im en ta ry rocks has a dv an ce d c on sid era -

bly. M ajo r and trace elem ent data pro vide constrain ts on bo th the

pro ven an ce co mpo sitio n and th e effects ofsedimentary processes,

such as w eath erin g and s edim entary so rting. Nd is oto pic d ata

p ro v id e c o ns tr ain ts on th e m ean age o f th e pro venance. These

techn iqu es perm it an evalu ation o f sedim ent h is to ry, wh ich in

tu m af fords insigh ts into th e prevailing tectonic conditions . To a



,

38 S. M. Mcl.ennan Gild Olhers

large degree, the information gleaned from geochemistry com-

plemenlthose of petrography and accordingly provide a different

perspective on these problems.

Pb isotopic techniques have evolved to the stage where very

small populations or single grains o f mO~1minerals can be ana-

lyzed for their Pb isotopic compsition. Interpretation of such

analytical data mayor may not have provenance-age significance,

depending on questions such as diagenetic and metamorphic his-

tory, conditions of isotopic closure, and so forth. Interpretations

depend on the geological context of th e samples being analyzed,

which is best done with the widest range of petrographic, geo-

chemical, and isotopic data available.

ACKNOWLEDGMENTS

Many people have influenced our thinking about the

application of geochemical and isotopic data to understanding the

origin and history of sedimentary rocks, notably B . Bock, K. A.

Eriksson, E. Krogstad, M. T. McCulloch, K. Mezger H. W.

Nesbitt, W. D. Sharp S. R. Taylor, and G. M .. Young. We arealso grateful to Ross Taylor for commenting on an earlier draft,

and to Mark Johnsson, Anne Linn, and Zell Peterman for helpful

reviews.

This research was supported by National Science Founda-

lion Grants EAR-88 16386 and EAR-8957784 and by the Do-

nors of the Petroleum Research Fund, administered by the

American Chemical Society «(0 S.M.M.).

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