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Geochemical Approach Mclennan

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  • 5/13/2018 Geochemical Approach Mclennan


    Geological Society or AmericaS pe cia l P ap er 2 84


    Geochemical approaches to sedimentation, provenance,and tectonics

    S. M. McLennan, S. Hemming, D. K .. McDaniel,and G..N. HansonDepa 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 42 10 0

    ABSTRACfGeochemical and isotopic approaches to consinining provenance of sedimentary

    rocks complement the information inferred frompetr.ography. Geochemi.cal approacheshave several advantages, including applicability to both matrix-rich sandstones andshales and ability to constrain provenance age and geochemical history. Five provenancec.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 variousExotic Components, such as ophiolites, Among the most important geochemical charac-teristic. s that define these provenance types are Nd isotopic composition (reflectingaverage provenance age), eur.opium anomalies (reOecting intracrostal igneous differentiation 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) andaccessory (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 smallpopulations and, f.or favorable circumstances, individual sand-sized grains of detritalquartz; 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 theage of the Archean Superior Province, from which this formation is mainly derived. Theinitial Pb isotopic composition may be approximated by the Pb isotopie corapcshion ofleached feldspars, due to their low V /Pb and Tb/Pb ratios. Pb isotopic compositions ofdetrital feldspars may also provide informati.on about sediment.ary provenance. K~feldspars from thePokegama Quartzite and Early Proterozoic Chelmsford FormationfaU within the field of Archean Superior Province igneous K-feldspars. They are distinctfrQm K-feldspars found in other potential provenanceterranes, including the PenokeanOr.ogen or, in the ease of the Pokegama, the Minnesota. River Valley gneisses.

    INTRODUCIION andisotopic techniques. A preliminary evaluation of some ofthese geochemical and isotopic approaches is the primary focus ofthis work. To date, there are exceedingly few studies that havecombined the various petrographical, geochemical, and isotopictechniques. Nor have many studies applied such approaches 10the study of individual components. Although we have drawn onthe available integrated studies wherever possible, many of ourinferences come from studies using far more limited data bases.

    The notion that there is a record of tectonic history withinthe composition of sedimentary rocks and their components isnow weUestablished (e.g., Blatt, 1967; Dickinson, 1970) ..Mucheffort has been expended in discriminating the tectonic setting ofthe provenance of sedimentary rocks, mainly using sandstonepetrography 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

  • 5/13/2018 Geochemical Approach Mclennan


    22 S. M Mel.ennan a nd G lh er sA ccordingly, ou r approach and m any o f o ur co nclu sio ns arec 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 sm 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 tso 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 eth 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 ataw ith m ajo r and trace elem ent geo ch em ical data, again w ith anem 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 andK 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 sh av e a ls o a pp lied is oto pic d ata , m ain ly n eo dymium is oto pe s, tos 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 ghs 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 onand 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 andHemm 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 ryp ro ven an ce stu dies is esp ecially aIlu rin g becau se it p ro vid es af 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 fn 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 esSm 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 tofo 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 sis 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 dW 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 ine 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 ldre 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 op 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 andT 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 hth 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 oallo 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 hin 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 picm easu rem en ts an d d evelo pm en t o f io n m icro pro be tech niqu esa 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 ndth 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 minget 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 ndiso to pic data to th e evalu atio n o f th e pro venance and tecto nich 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 top 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 hc 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 urstu dies h as been th at geo ch em ical and is oto pic ap pro ach es bestp 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 ryro ck s. Instead, o ur intent is to sh ow that g eo ch em ic al d ata c anp 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 orcertain 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 rya 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 oretr 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 sfo r fine-grained as w ell as very co arse grained sedim ents aredifficult, A n im po rtant caveat is th at fine-grained sedim entsty 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 isc om po sitio na l v aria bility th at c omm o nly le ad s to in sig hts a bo u tpro venance and attendant sedim entary pro cesses. Th ere isgro wing evidence th at even intim ately asso ciated sands andm 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 ntsedim 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 tom 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 es 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 ins om e cas es m ay be u sed to qu an tify th e o ccu rrence an d lor extento f s om e seco ndary p ro cesses, su ch a s we at he rin g, s o rt in g, a ndd 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 nso m e cases , reco gn izin g ev en triv ial am o un ts o f certain exo ticc 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 importancein 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 fp 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 ofu 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 aso 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 lfo 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 restim ating th e cry stallizatio n o r res ettin g (co olin g) age o f th e

  • 5/13/2018 Geochemical Approach Mclennan


    Geochemical approaches to sedimentation 23TABLE 1. PETROGRAPHICAL AND GEOCHEMICAL PERSPECTIVES



    Sedimentary Processes1. Weathering Ambiguous. Some mineralogi

    cal effects (e.g . . feldspar/cl .ayratios)

    2. Diagenesis Relative chronology and min-erai react ions. Overgrowth,albi tization, smecti te/il li tetransformationTextural maturit y, heavy min-eral content and variety

    3. Sorting

    4. Sedimentary recycling Quartz content. sedimentaryrock 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), redoxelements (e.g., Fe:K/Fe2+ ), var-ious isotopes (Ab/Sr, UfPb)Major elements (Si/AI). Quanti-fy heavy mineral f ract ionationsuch as zircon (Zr, H I) andmonazite (AEE)Various geochemical approach-es (CIA. Zr, HI, ThJU); Nd-iso-topes

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

    Combined trace elamenUNd-isotopes (O:d Upper Crus!,Young Differentiated Arc,Young Undifferentiated Arc)Pb isotopic composition offeldspars 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 /mantlesources (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 asprovenance components using Pb isotopic data for feldspars (asan estimate of the initial Pb isotopic composition) and further toevaluate the geochemical history of U, Th, and Pb in thosecomponents.SEDIMENTARY PROCESSES: PERSPECTIVESFROM 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 processesfrom chose due to provenance. In addition, sedimentary rocksassociated with certain tectonic regimes in some cases appearmore or less affected by such processes. inTable 1 a comparison

    of the types of information that may be obtained from bothpetrographic 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 bestWeathering 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 offeldspar can be usefu l , attempts at quantification have not beenforthcoming. Similarly, diagenesis also commonly involves thealteration 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 andvan de Kamp, 1984).

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

  • 5/13/2018 Geochemical Approach Mclennan


    24 S. M. Mclennan and Othersa subs tantial increase in the Rb/Sr ratio o f mo st ro ck s. Th is isbecause Rb", a largealkali trace element (1.72 A f or 1 2-fo ldcoordination), is m o re re ad ily retained o n exch ange sites o f daysth an th e sm aller 5,r2+ (1.26 A fo r eigh t fo ld co ordinatio n); T hismajo r ch ange in Rb/Sr provides oppo rtu nities fo r applying Srisotopes 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 earthelem 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 sso 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 isl 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 toredistribute 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 ando thers, 1991; altho ugh see the alternate view o f Anwiller andM 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 byexam ining th e iso to pic sys tem atics o f co -exis ting diageneticphases (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 fs 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). TheChem ical Index o f A lteratio n (CIA ) has been established as ag 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 ru nco nso lidated sedim ents , it is so metim es also necessary to m ak ea 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 orbelow , res idual c lays have values o f near 100,. and typical shalesaverage 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 stratedin 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 gdegrees o f weath ering fo r a m afic and fe ls ic igneous ro ck areillu strated. On the A -C N~K diagram , these path s are co nfirm edbo 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 eringpro files alone. There are several co mplexities that m ust be kept inm ind when employing such an approach . The firs t is th at m anym 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 icalcom 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 icm 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 inFigure 1 B. In some cases t h e t ra je ct o ry of th e pa th way will h elpin dis tingu ish ing m ixing from weathering trends (H . W . N esbitt,personal communicatio n). Fo r example, a very flat trend thatin tersects the A -C N bo undary is inc on sis ten t w ith w eath eringbeing the so le contro l and m ixing rna y be i nd ic at ed ..Mass balancerequires that w eath ering (o r m ast w ater-ro ck interactio n) trendsshould 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 materialcomposition,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 [0evalu ate th e te xtu ral m atu rity o f th e sed im ent, u sin g ch aracteris ticgrain sizes and sh apes, and m ineralogy (e . .g., Fo lk , 1974). Thech em ical com po sitio n o f a sedim entary ro ck m ayaIso be helpfulin 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 izedmaterial, resu lting in an elevatio n o f the Si021 A l203 ratio and adecrease in t h e a bu n da nc es of m ost trace elem ents . O f specialinterest 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 em ajo r fram ew ork m ineral plagio clase du e to sedim entary so rtingprocesses . Th is m ay have serio us co nsequ ences bo th fo r tecto nicinterpreta 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 isph 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 ngenrichm ent in Eu ., a rare earth elem ent typically enrich ed Infeldspars , fo r som e sam ples . These enrichm ents co rrelate wellw 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 fs 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 sandsfro m active tec to nic settings h ad elevated Eu/Eu* (where Eu * is

  • 5/13/2018 Geochemical Approach Mclennan


    Geochemical approaches to sedimentation

    AA Kaolinite, Glbbs/re, Chlorite

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

  • 5/13/2018 Geochemical Approach Mclennan


    . '26 S. M Mcl.ennan and Others

    Q)0)~ELLC.~'0rtiCDE0.. .10

    -0-- - -- M284M285MK14 Framewor i lP l a g i o d a . s eC o n t e n t33,8%36.6%68 .8%


    .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 eDevonian Baldwin Formation, normalized to the Baldwin Formationaverage (of non-En-enriched samples). The Baldwin Formation is avolcanogenic 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 typicalREE pattern for plagioclase from an andesite. The Eu enrichment insome of the Baldwin Formation sandstones is likely due to plagioclaseenrichments formed during sedimentary sorting. Data from Schnetzlerand Pbilpous (1970), Chappell (1968), Nance and Taylor (1977), Bhalia(1981, 1985).

    other REE (Mclennan, I989a). In contrast, Th/Sc is a goodoverall indicator of igneous chemical differentiation processessince Th is typically an incompatible element, whereas Sc istypically compatible in igneous systems. It can be seen that bothTb/Sc and ZrlSc vary sympathetically for turbidite sands fromactive margin settings, following a trend consistent with igneousdifferentiation being the primary control (i,e., provenance). Incontrast, turbidite sands from passive margins show ZrlSc in-creasingsubstantially, with Th/Sc increasing fa r less, consist.entwith zircon enrichment In the case of monazite, an REE-enriched heavy mineral, it is also possible to estimate enrichmentsfrom REE patterns (Mclennan, 1989a; Fig. 4). Since monazitehas 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 sedimentaryREE patterns and most upper crustal igneous rocks rarely haveGdN/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 Recyclingo (Zircon Addition) ..0ot! . 0 . . . . , -

    ()(f) .1J::I-

    .01 _ Trailing Edge Turololteso Active Margin Turbidites


    .1 10 100 '000Zrl ScFigure 3. Plot ofTh/Sc versus Zr/Sc for modem turbidites from varioustectonic settings showing the e nr ic hmen ts o f 'z ir con (high ZrlSc ratio) inlTailing 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 isinterpreted to be due to composi t ional variations of th e provenance.Data from Mclennan et al, (1990).



    Monazite Addition Monaz.ite: Gd=14.700ppmVb" 540ppm

    15/30/,20Shale 12(Th~10ppm)


    1.0 2.0 3.0 4.0 5.0Gd N IYb N


    Figure 4. Plot of GdN versus GdN/YbN to illustrate the effect on RE Epallems ofconcentrating monazite in sedimentary rocks. Calculations fortwo 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. anREE pauemparallel to typical post-Archean shales (Taylor and Mclen-nan, 1985) is assumed. Both are assumed to have 10 ppm Th. MonaziteREE data are from Lee and Bastron (1967), and a typical monazite Thabundance of 6% is assumed. Adding even small amount of monazite {aslittle as 0.005% to the quartzite) can result in significant increases in theGd/Yb ratio of the sediment. The GdN/YbN ratio of sedimentary rocksis usually rather constant and in the range of 1.0 to 2.0.

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    Geochemica l approaches t o sedimen ta ti on 270.4 Ga

    10 Mantle EVOlution 2.0Ga 2.7Ga5 Crust, 0 ----- --_ .",0.0 "".Nd -5 60\ ::;...~o "".".-10 / R e c Y c i e d Recycled-15 Sediment 1-20 Sedimenl2

    -25 SedimentaryRock

    1.0 3.0.0Geologic 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 fs 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 ra 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 ah 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 talm 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 1.8Ga. 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 1.2Ga 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 newcru 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 basiso 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 trecycled 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 totectonic history (Veizer and Janson, 1985). In Figure 5, the Nd-isotopic evolution of a sedimentary rock is shown in terms of ahighly simplified sedimentary recycling model. The model age ofthe sediment (in this example, 1.7 Ga) has no bearing on anyparticular provenance component Instead, it represents a com-plex mix of differing ages of crust that has gone through variouscycles of sedimentary processes.SEDrMENTARY PROVENANCE: PERSPECfIVESFROM GEOCHEMISTRYTerrane types

    It is recognized that sediment compositions are characteristicof certain assemblages of igneous/metamorphic/sedimentarysource 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 tectonicconditions can bemade. In this section, we describe terrane types

    that can be identified from geochemical data, mainly combiningmajor and trace elements with Nd-isotopic compositions ofwhole rocks. Of special importance is the observation that theseterrane types do not precisely correspond to those recognizedfrom petrographical approaches but instead provide a differentperspective of the plate tectonic association. It is also important tonote that, although a given terrane type may dominate the prove-nance of certain sedimentary rocks (and some types are morelikely than others), it i s j us t as likely that the provenance will be amix. Unravelling the contributions of each terrane type is , ineffect, a major part of the task at hand. Some of the chemical andisotopic characteristics of these terrane types are summarized inTable 2.

    These concepts are largely based on combined geochemicaland Nd isotope studies of the provenance of young continentalmargin sediments (e.g., White et at, 1985; Ben Othman et 31.,1989; Mclennan et al., 1990; Basu etal., 1990). Similar studiesof Phanerozoic sequences are rare but the general approachappears 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 presentin the Archean upper crust. These are discussed in greater de-tail below.

    Two of the major factors used incharacterizing terrane types

  • 5/13/2018 Geochemical Approach Mclennan



    OF TERRANE (PROVENANCE) TYPESTerrane Type E,..d1 Eu/Eu' ThJSc Th/U OtherOld Upper Conti - S-10 ",0.60-0.70 =1.0nental Crust (OUG)

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

    Young Undifferen- ~'l-5t iated Arc (YUA)



    Evolved major element composi-tion (e.g . high Si/AI. CIA); highLlLE abundances; uniform compo-sitionsEvidence of heavy mineral con-centration from trace elements(e.g . . Zr, HI for z ircon, REE formonazite)

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    Geochemical approaches 10 sedimentation 29

    ArcMORBo 0 Andesiteo Felsic0



    ..... 0Mafic ComponentCNd 0

    Quartzoseo Nonquartzose+ 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 bulkcomposition 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 turbiditestend to have variable but higher tNd (reflecting a younger provenance)and more variable but commonly lower Th/Sc. ratio. The quartzoseturbidites are of more uniform composition and generally from an olderprovenance ..Also plotted are modem sands from the Andean Foreland(Basu et at, 1990) considered to be derived from old recycled uppercrust, In this case, high Th/Sc is not due to derivation from more felsicsources but isrelated to concentration of heavy mineral during sedimen-tary recycling and sorting.

    Ouartzoseo Non'quar1zose+ Andean Forela~ LREE-depleled

    LREEenriched Arc ' R i i C k s - : : : : : : :: :~ ~ ...__ .'0Z.._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 Figure6 . T he d eg re e of LREE enrichment in a sedimentary rock is also sensitiveto bulk compositions; such a diagram conveys similar information to thatin Figure 6. Note that this diagram is essentially a slightly recast "iso-chron" diagram (see Shirey and Hanson, 1986, for an explanation).


    ----0-- Bay BiscaySohm BasinU)Q). . . . . .~"0co_coEQQ__


    10E0...Q --.0-- Demerara Plain (543-26-2)1----1.__- Andean Foreland (25)

    LaCePrNd SmEuGdTbDyHoEr YbFigure 8. Chondrite-normalized REE plot of selected modern sedimentsthat have an old upper continental crustal. (OUC) provenance. The tNdfor each of these samples is less than -10. Note the similarity of thepatterns with LREE enrichment, fairly flat HREE (i.e., GdN/YbN = 10to 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 dBa su e r a !. ( 19 90 )_

    The geochemistry of sediments derived from this COmponentis normally characterized by relatively uniform compositions, re-fleeting a broad, well-mixed provenance (Potter, ]978) and thegreater level of recycling expected for such environments (Veizerand Jansen, 1 979, 1 98 5). In addition to a substantial negative Euanomaly (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 adominance of upper crustal granitic sources and a relatively se-vere weathering (and recycling) history. Trace element featuresinclude enrichments of normally incompatible over compatibleelements (e.g., LREE enrichment, high ThlSc, La/Sc) reflectingrelatively felsic average provenance compositions, and highRb/Sr (>0.5) and Th/U (>3.8 for shales) reflecting weatheringand sedimentary recycling.

    Examples of sedimentary rocks dominated by OUC includeabundant cratonic shales (Nanceand Taylor, 1976), most passivemargin turbidites (Mclennan et aL, 1990), and foreland' 6asinsediments 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 thisterrane type in Figure 8. Each displays the negative Eu anomalyand LREE enrichment characteristic of the upper continentalcrust and each 'has EN d

  • 5/13/2018 Geochemical Approach Mclennan


    30 S. M. Mcl.ennan and Others10 .


    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 lyz 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 sar e seen fo r Ihe aeolian loess (silt), due 10 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 dBasu 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 fromtectonically 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 ldupper con tinen tal cru s t (OU C) inclu des recycled sedim entaryand 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 iss traig htfo rw ard eith er by identifyin g s ed im en tary ro ck fragm entso r abraded quartz overgrow th s and m ay be sugges ted by thequartz-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 fractionationand enrichm ent o f heavy m inerals , no tably zircon. A s no tedabo ve, it may be po ss ible to identify su ch enrich men ts u sing tracee 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 lyrew ork ed f irs t-cycle sedim ents cou ld also po ssess enrichm ents inh ea vy m in era ls (J obn ss on et aI., 1988).Th e latter effect m ay beseen in data fo r lo ess, where Z r and Hf are stro ngly enrich ed o verupper 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 ret al., 1983; B rim hall et a t, 1991), presum ably due to someoptim 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 aybe iden tified com es from the Andean fo reland bas in sands o fBo livia and Peru (Basu et al., 1990). In these sands , a m ajo r

    100(/)ill.-e._"0C0.s:0 10Eo,o,-EQQ


    --0--. MarianasN. 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 andP 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 rcno 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 mN 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 identifiedfrom the petrography. Su ch a com ponent is also o bservedth rou gh tbe trace elem ent relationsh ips where zircon and o thertrace m inerals a.re co ncentrated du e bo th to th e sedim entaryprovenance and fu rth er enrichm ents related to sedim entarytrans po rt (F ig. 9).

    3. Young undifferentiated arc (YUA). T his C omp on en tco 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 ss 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 inarc terranes are com plex, th ey are typically less evo lved. w ithbasaltic and andesitic rock s (and p lu to nic equ ivalents ) being larm o re common than in s tabilized upper continental cru s t. Ayo ung provenance is dem onstrated by radiogenic neodym iumiso to pic com pos itions su ch th at fN d is typicaUy above abou t +5fo 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 variablem 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 ndless 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 nregarding th e natu re o f m antle sou rces m ay be extracted from th eg eo ch em ic al a nd is oto pic data, Fo r exam ple, Ph iso topic data fo rpo tassium feldspar (o r plagioclase) shou ld co nstrain the U /Pb

  • 5/13/2018 Geochemical Approach Mclennan


    Geochemica l approaches t o sedimen ta ti on

    60 WCal.he,ingTr:w>d

    Active Margin Mudso Passive Margin Mudso Australian Shales


    o 0::J Uppec Csus!--; 4.0 .cr c e 0&[ .

    } ~ W d.0 Mantle Sources0.0 .1 10Th (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 lyo 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 thata 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 wTh 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 rm antle-derived igneou s rock s th em selves (e.g., Sun, 1 .980). Fo rm os t upper cru s tal ro ck s, Th/U is typically abou t 3.5 to 4.0. Inmost 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 elatter 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 entarypro 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 theg re at er m o bility of th is elem ent. M any sedim ents from activem 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 dby low Th and U contents (F ig . 1 I) and th is is interpreted asdo 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 arerath er com mon in m antle-derived vo lcanic rock s and refleci th egeochem icaU y depleted natu re o f such reservo irs (e.g ., N ew manel at, 1984).Th is provenance com ponent is lik ely to dom inate in fo rearcbas ins. It is also lik ely to be found in m ost o th er vo lcanicallyactive tecto nic s ettin gs; h ow ever, data fo r m odem tu rbid itesin 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 whereY UA do minates is th e D ev onian-C arbo nifero us vo lcano genicsedim 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-


    e nC D..-10-""0Ca.c;0 10Eo,o,- -Eo,CL


    S. Aleut ians(Fore arc)

    LaCePrNd SmEuGdTbDyHoEr YbFigure 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 malys 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 byintracrustal 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 ecom ponent represents young (mantle derived) vo lcanic and plu -tonic igneo u s rock s from is land and continental arcs th at h avebeen affected by intracrustal d iffe re ntia tio n s uc h th at th ey possesss 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 thearc 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 isdistingu ished from o ld upper cru s tal sou rces (QU C a nd R SR ) byits relatively h igh 143Nd/l44Nd ratio (~Nd>+5 fo r modernsediment), In effect, th is com ponent represents "young" upperc 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 llyrem oves h eat-pro du cing elem ents to th e u pper cru st and s tabilizesth 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 ofm uch greater variability th an o ld upper cru stal provenance dueto th e m ore lo calized s ou rces . B ecau se o f th e firs t-cycle ch aracterand rapid eros ion rates o f h igh -s tanding arc terranes , in mos tcases th is com ponent is lik ely to be less affected by weatheringprocesses (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 isdissected (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 isu nlik ely th at it w ou ld co mm only be dom inant, compared toQUC o r YUA. Instead, m ixtu res with QUC and YUA are to beexpected. F lo yd and Leveridge (1987) have sugges ted a dissectedcon tinental arc p rovenance fo r m uch o f th e Devonian Grarnsca-

  • 5/13/2018 Geochemical Approach Mclennan


    S. M. Mcl.ennan and a/hers0.20 Passive Marg"in Sequence

    0 Volcanogenic Sequence

    ~~~~1.7Ga~~~~ .

    ~ II-, . . __ 0~ MixingCIUst /W#&0.00 R2-3 "0zR6-3 - - EPL-1 (f) '+-- 0.20-0.40321000

    Passive Margin Sequence(/)Q)._"0 100co..coE0....0....- 10

    Volcanogenic Sequence-0-- 81----0-- 132

    0 1351 LaCePrNd SmEuGdTbDyHoEr Vb


    Figure 13. Chondr i te-normal ized REE plot showing selected Early Prot-erozoic sedimentary rocks from northern New Mexico and southernColorado. Examples from two sequences are shown. One is a lowervolcanogenic 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 Euanomalies, characteristic of the upper continental crust, Unpublisheddata.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 ithEu/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 isn 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 gth 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 whereth is is no t th e case. Early Pro terozo ic rock s o f so u thwes ternUnited S tates appear to reco rd a m ajo r cru s t-fo rm ing episo de atabou 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 youngerqu artzite-sh ale s equ en ce, representing depo sitio n o n a s tabilizedpass ive m argin (Soegaard and Erik sso n, 1986). Trace elem entdata from th e fine grained sed im entary rock s are cons istent w ith ad 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 elower graywackes and a reaso nably well-m ixed (i.e ., un ifo rmcom 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 EarlyProterozoic of New Mexico and Colorado. The data are recalculated forthe time of sedimentation, a t c a. I. 7 Ga. Although there is a difference inthe 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 settingand subsequently stabilized, with deposition of a passive marginquartzite-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 edfrom 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 sp 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 no f th is reg io n. A lth ough a firs t-cycle o rigin fo r th ese quartzites isapparent 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 provenancecomponen t s .

    5. Exotic components. In som e cases, id en tific atio n o feven trivial am ounts o f certain exo tic com ponents can have veryim 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 picallyd is tinct allo ch th ono us terrane w ou ld pro vide im po rtan t evid en cere 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 Taconicflysch 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 ianscou 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-slipm 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 tcomponen 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 icrelations.

  • 5/13/2018 Geochemical Approach Mclennan


    Geochemical approaches 10 sedimentationARCHEAN PROVENANCE

    I0 Recent Q(\j < : \ iII 0 Phanerozoic ~~ Proterozoic z+ '--~ . ..~ + 0 ~ ." < !J. .\\ J'~.: .,. ,~- - - . .~ -EulEu'=O,85 ')..


    8 . 0

    > 6.0- --() 4.02. 0

    0. 00. 0 2. 0

    Taconic Flysch Autochthonouso Allochthonous

    Mixing (10 and 2)

    1 1.0 Y IN i

    Figure 15. Plo t o f Cr/V versu s Y /N i for early Paleo zo ic Taco nic flyschfrom eastern N orth A merica illu s trating (he greater im po rtance o f ano p hi ol it ic p ro ve na nc e a s one m oves no rth . H igher num bers and leitersindicate m ore no rth erly o utcro ps . E ach data po in t r epr esen ts t h e a ve ra geo 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 Crabundances . The C r/V ratio is an index o f th e enrichment o f Cr o ver th eo 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 llevel o f f er romagn es ia n t ra ce elem ents (N i) co mpared 10 a proxy fo rH REE (Y ). M afic-u ltram afic so urces tend to h ave h igh ferro magnesiana bu nd an ce s; s uc h a p ro venance w ou ld resultin a decrease in Y /Ni. Am ineral su ch as ch ro mite, im po rtant in o ph io litic sequ ences , tends toco ncentrate C r p referentially o ver o th er ferro rnagnesian elem ents , D atafrom Hiscott( 1984) .


    . - 1.0:JW- - -JW 0.8+0.6 o

    +1.0 1.5 2.0 2.5Gd N/YhN 3.0 3.5


    G eochem ical characteris tics o f A rchean terranes are lik elyto be su bstantially different th an in yo unger enviro nm ents. T hereis abundant evidence to indicate th at th e composition o f manyplu to nic and vo lcanic igneo us rocks differed d urin g th e A rc hea ncom 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 Archeanare 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 eytended to be more K- and K -felds par-rich grano dio rites-monzoni tes-grani tes. Implications of th is fo r petrograph ical andm 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, Archeansedim entary rock s from vo lcanically active settings , when com -pared to Pro terozo ic and Phanerozo ic sedim entary rock s froms im ilar settings, lend to have (M clennan and Taylo r,1991):(I) les s depletion in Eu , reflecting an absence o f intracru stald iff ere nt ia tio n in the s ou rc e ro ck ; (2) greater depletio n in h eavyREE (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 ticso r having had garnet (HREE enrich ed) as a fractio nating phasedu ring fo rm atio n; and (3) higher Th/U r at io s , p er ha ps in di ca tin gle 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




    A B


    1.0 1.5 2.0 2.5GdN/YbN3.0 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 lyactive tectonic settings . A rchean tu rbidites tend (0 have h igher EuIEu , suggesting th at. th eigneo us/m etam orph ic pro venance, o n average, w as less affected by intracru stal differentiatio n. Th eyalso 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 eanactive m argin sedim ents . Su ch differences indicate th at cau tio n is w arranted in u sing p ost-A rch eanterrane types in interpreti ng A rch ean sedim entary rock s or sedim entary sequ ences derived prim aril yfrom A rchean sou rces . F rom M clennan (1989a), where data sou rces are lis ted.

  • 5/13/2018 Geochemical Approach Mclennan



    34 S. M. Mciennan and Othersprovenance concepts to A rchean sed im entary rock s. By th e endo f th e A rch ean, th e upper cru st a ppea rs to ha v e s tabil ized to astate 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 beinf luenced by th ese d ifferences. An example com es from theEarly 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 issequence appear to be dom inated by pro venance components o fs tabilized upper con tinental cru s t, bu t m ost samples have REEpatterns 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 evaluatingbo 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 anum 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, 238UI2 06.P b, 2 3S U/2 07P b, a nd 232ThI20 8Pb, commonly allows fo rsuch processes to be ev alu ated and fo r co ns iderable info rm atio nregarding 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 ereader 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 alpebbles o r co bbles and exam ine them in detail fo r geochem istryand age. A lthough o f great u se, such sedim entary rock s typicallyrenee! only localized sou rces (e.g., Ingerso ll, 1990). A t th e o therextrem 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 ncentrateon 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 asatech 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 ixedpo 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 sedsuccess 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 eearth (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 thesedimentary 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 500 ppmand shales rarely have Zr>250 ppm ): (2) th ey are m ore lik ely toh 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 ldrecycled 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 ithrespect to zircon and acco rding ly w ill no t be rep resented in th es 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 gicalstud y bu t su ffer fro m m il by o f th e sam e sho rtco rn i ngs as zircon, o rth ey have additional complexities (e.g., sphene m ay fo rmdiagenetically).

    G eoch rono lo gical studies o f m ajo r m ineral phases providean 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 datingdue to extrem ely low U conten t, although K I Ar and Rb/Srdating 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, especiallyfOT R b/Sr geo ch ro no lo gy, bu t th ese m in erals are s tro ngly affectedby diagenesis and sedim entation ages o r m ixed ages are typicallyobtained (e.g., C lauer, 1982; Oh r et ai., 1991). There is lik elyp oten tial fo r K/Ar and Rb/Sr dating o f detrita l m ica (H eller etal., 1985; Heller and Fro s t, 1988 ; Kelly and B luck , 1989) andperhaps iso top ic dating o f individual sand-s ized ro ck fragm entgrains; h ow ever, s uch appro ach es . h av e yet to beinves tigatedina 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 allquartz popu lations and under favo rable circumstances fo rind 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 inm ineral and/o r flu id inclu s io ns and .th at, under a w ide variety o fgeo 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 andvariable 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 -Pbgeochronology.

    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 minget al . . 1990, J 991). In Figu re 17, Pb- Pb isoch rons a re displayedfortwo A rchean plu tons from the Superio r Pro vince o f no rth ernM inneso ta. In bo th cases, quartz-when combined w ith who le-rock .and feldspar analyses -provides Pb-Pb ages in agreem entw ith co nventio nal iso topic approaches. In bo th cases , quartz ism ore radiogenic (h igher 207Pbli04Pb and 206Pb/20 4Pb) thanthe who le-rock analyses; however, th e radiogenic natu re o f th equartz 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 interm s o f 207Pb/2(J4Pb versu s 206Pb/204Pb. Th is fo rm ation isgenerally co nsidered to be derived from the A rchean Superio r

  • 5/13/2018 Geochemical Approach Mclennan


    Geochemical approaches 10 sedimentation 3528 Range granite

    (W T Rb~Sr age = 2620+/-63 Ma)(sphene Pb-Pb age := 2640+/-' 6M

  • 5/13/2018 Geochemical Approach Mclennan


    3 6 S. M. Mcl.ennan and Others

    Pokegomo Quartzite clear detrital quartzo detrital K-feldsporo whole rock


    120.00... 100. . , .aN

  • 5/13/2018 Geochemical Approach Mclennan


    rGeochemica l approaches 10 sedimentation

    pas s. In m any cases. it is th e ratio 2 07P b/20 4Pb th at separates th ePb i so topic cornpo i tion of many terrane . The 207Pb parentisotope, 23SU, is relatively sh ort-lived co mp ared to ;138U (parentto 2 06P b) a nd was fa r moreimportam in th e ea rlie r h is to ry of th ee 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 no f continental cru st) th at to ok place early in earth h isto ry resu lt ine 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 r10 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 ckP.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. 7.8,8.014 ~~~~~~~ww~~~~~~~~~~~13 14 15 16 17 18 19

    206Pb/204PbF 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 rsfrom the Early Proterozoic C helm sfo rd F orm atio n and Po kegam aQuartzite an d w ho le rocks fro m th e C helm sf ord F orm atio n. G ro wthc 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 srocks f rom the Superio r P rovince and M inneso ta R iver Valley andP 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 manys 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 gleg 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 fPenokean-aged fe ldspars. F or th e Pokegarna, n eith er th e P en ok ea n n orM 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 rdc 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, andrn 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) .


    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 rK -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 rdK -feld spars clearly h ave been derived alm os t exclu sively fro m th eSuperio r Province, th us prov id ing an im po rtant co nstraint on theo rig in o f th ese sedim entary rock s . W ho le-rock data are alsoshow 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 wthbetween 2.7 and 0.0 Ga, u sing the range o f Superio r Prov inceK-feldspars as po tential in itia l ratio s. T here is no evidence in thesedata 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 to1. 8 Ga, which m igh t be expected if Penokean-aged rock s were as 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 arrayco 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 , Ifsu 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 ticalteratio 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) area ls o s ho w n in Figu re 19. In addition to th e Superio r P rovince andPenokean O rogen, p lau s ible sou rce rock s include the M inneso taR 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 arsfrom MR V igneou s rock s indicate a com plex h isto ry and are alsoshown in Figu re 19 . The Pokegam a K-feldspars plo t w ith in o rvery clo se to th e f ield o f th e Superio r province. Bo th th e MRVand the Penok ean O rogen can be excluded as s i gn if ic ant s o u rc e sfo r th e d etrita l K -feldsp ars .

    A pplying such an approach to yo unger terranes in manycases 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 d207Pb/20 4Pb ratio s fo r Kfeldspars th at are derived directly fro mth es e o lder ro ck s and fo r K -feld spars fo rm ed du ring later m eltingofthese 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 esedim 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.


    CONCLUSIONSOur 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 thepro 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 atap ro v id e c o ns tr ain ts on th e m ean age o f th e pro venance. Thesetechn iqu es perm it an evalu ation o f sedim ent h is to ry, wh ich intu m af fords insigh ts into th e prevailing tectonic conditions . To a

  • 5/13/2018 Geochemical Approach Mclennan


    ,38 S. M. Mcl.ennan Gild Olherslarge degree, the information gleaned from geochemistry com-plemenlthose of petrography and accordingly provide a differentperspective on these problems.

    Pb isotopic techniques have evolved to the stage where verysmall populations or single grains o f mO~1minerals can be ana-lyzed for their Pb isotopic compsition. Interpretation of suchanalytical 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. Interpretationsdepend 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 theapplication of geochemical and isotopic data to understanding theorigin 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 helpfulreviews.

    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 theAmerican Chemical Society (0 S.M.M.).

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