Petrology of syenites from center I11 of the Coldwell alkaline complex, northwestern Ontario, Canada

ROGER H. MITCHELL, R. GARTH PLATT, JURATE LUKOSIUS-SANDERS, MAUREEN ARTIST-DOWNEY, AND SHELLEY MOOGK-PICKARD

Department of Geology, Lakehead University, Thunder Bay, Ont., Canada P7B 5E1

Received April 15, 1992 Revision accepted September 3, 1992

Center I11 of the Coldwell alkaline complex consists of metaluminous hypersolvus syenites, which in order of intrusion are magnesiohornblende syenite, contaminated ferro-edenite syenite, ferroedenite syenite, and quartz syenite. Contaminated syenites were formed by the assimilation of coeval basaltic volcanic rocks. The suite as a whole is characterized by the presence of a wide variety of amphiboles ranging in composition from magnesiohornblende through ferroedenite and ferro- richterite to arfvedsonite. Pyroxenes are rare and hedenbergite is present in significant amounts only in quartz syenite. Whole- rock major element data indicate that the majority of the syenites do not represent liquid compositions. The syenites have high contents of Nb, Zr, Th, U, Y, and Ga and have the geochemical character of A-type granitoids. Rare earth and other trace element abundances suggest that the quartz syenites cannot be differentiates of the magma that formed the ferroedenite syenites. All syenites are considered to have originated by the extensive fractional crystallization of mantle-derived basalt magma within the plutonic infrastructure of the complex. The syenite suite does not represent the differentiation products of a single batch of magma. Multiple intrusion, contamination, and brecciation of preexisting syenite plutons have resulted in the complex geological relationships characteristic of center 111.

Le centre I11 du complexe alcalin de Coldwell est forme de syenites mCtalumineuses de type hypersolvus qui placCes par ordre d'intrusion sont une sytnite A hornblende magnksienne, une sytnite A tdtnite ferrifkre contaminke, une syCnite i edknite ferriEre et une syenite quartzique. Les syenites contaminkes ont ete formCes par l'assimilation de roches basaltiques volcani- ques coexistantes. La suite, considCree dans son ensemble, est caracterisee par la presence d'une grande variCtC d'amphiboles dont la composition varie de hornblende magnksienne A CdCnite ferrifire et de richterite ferrifkre A arfvedsonite. Les pyroxenes sont rares, et il n'y a que la syCnite quartzique qui contient de I'hCdenbergite en quantitCs significatives. Les rksultats des analyses des ClCments majeurs sur roches totales indiquent que ces syenites en gCnCral ne representent pas les compositions initiales des magmas. Ces syCnites contiennent des teneurs Clevtes de Nb, Zr, Th, U, Y et Ga et elles prosskdent les caracteres gCochimiques des granitoides de type-A. Les concentrations de terres-rares et de d'autres ClCments traces suggkrent que les syenites quartziques ne sont pas les produits de la differenciation du magma dont derivent les syCnites i CdCnite ferrifkre. L'origine de toutes ces syCnites est interprCtCe cornme le resultat de la cristallisation fractionnee etendue, au sein de l'infra- structure plutonique du complexe, d'un magma basaltique derive du manteau. Cette suite de syenites ne reprksente pas les produits d'une differenciation d'un seul et unique volume de magma. La multiplicite des intrusions, la contamination et la brechification des plutons de syenite prkexistants ont creC les relations geologiques complexes qui caracterisent le centre 111.

[Traduit par la rCdaction] Can. J. Earth Sci. 30, 145- 158 (1993)

Introduction The Proterozoic Coldwell alkaline complex is a large intru-

sion located on the north shore of Lake Superior. Mitchell and Platt (1978, 1982) have shown that the formation of the com- plex involved at least three distinct episodes of magmatism, designated as intrusive centers I, 11, and 111. Each intrusive center is characterized by multiple magmatic events.

Center I, at the present level of erosion, consists principally of ferroaugite syenite. The magma from which these rocks formed was characterized by a differentiation trend that led to the formation of oversaturated peralkaline residua (Mitchell and Platt 1978). A heterogeneous suite of subalkaline gabbros forms a border phase to the ferroaugite syenite intrusions and marks the eastern boundary of the complex. Several discrete layered or massive intrusions, together with megaxenoliths, can be recognized within this unit, which is considered to be a multiple-ring dike. The gabbros are cut by pegmatites of ferroaugite syenite and, on this evidence, are considered to have been emplaced prior to the ferroaugite syenites.

Center I1 magmatic activity formed nepheline-bearing alkali biotite gabbros and hastingsite-nepheline syenites. The bulk of the latter were emplaced subsequent to the formation of the gabbros and have been subjected to high-temperature shearing and recrystallization (Mitchell and Platt 1982). A suite of carnp- tonite, monchiquite, sannaite, and analcite-tinguaite dikes is coeval with this activity (Mitchell et al. 1991). All rocks in Pr~nted in Canada I Imprimt au Canada

center I1 are considered to have formed by the fractional crys- tallization of alkali-basaltic parental magmas.

Center I11 syenites and quartz syenites form the western portion of the complex. These rocks commonly carry xeno- liths of Archean country rock (hornfels metasediments), meta- morphosed - metasomatized contemporaneous basaltic rocks, consanguineous and earlier-formed syenites, and gabbros. Reac- tion ofbasaltic xenoliths with center I11 magmas has led to the formation of a diverse suite of contaminated syenites (Jago 1980; Lukosius-Sanders 1988; Nicol 1990). Mitchell and Platt (1978, 1982) have noted that center I11 rocks are saturated to over- saturated alkaline syenites and quartz syenites. Unlike center I syenites, they do not differentiate to peralkaline aenigmatite- bearing residua, nor is their emplacement preceded by an intru- sion of gabbro.

The object of the present study is to document the character of the center I11 magmatism and place it within the context of the evolution of the complex as a whole. To this end, new data on the mineralogy and geochemistry of the center I11 syenites and contaminated syenites are presented.

Center I11 syenites: field relations and petrography

Study area This paper is concerned with center I11 syenites occurring

along the western contact of the complex, along Ontario High- way 17 north of Ashburton Bay, and in the Guse Point area

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CAN. 1. EARTH SCI. VOL. 30, 1993

Nepheline syenite

Basaltic xenoliths Ferroaugite syenite

FIG. 1. Simplified geological map of the southwestern margin of the Coldwell alkaline complex, after Mitchell et al. (1991), showing the location of all samples referred to in the text.

(Fig. 1). Detailed geological maps of these areas are given in Jago (1980), Lukosius-Sanders (1988), and Nicol(1990). These portions of centre I11 were selected for investigation because of the excellent exposures provide by the highway road cuts and glaciated shorelines. The exposures reveal extraordinarily complex interrelationships between several varieties of syenite and their xenolith suites. Typically, lithologies are observed to change abruptly on a decametre to metre scale. This variation, coupled with differences in the degree of deuteric alteration and (or) amount of assimilation of xenolithic material, results in an apparent plethora of syenites. Individual syenite types can only be recognized by a combination of macroscopic and petrographic observations. Specimens that appear to be identical in the field are commonly petrographically distinct and vice versa. Hence, a simple field mapping approach to the study of these syenites is inappropriate.

Away from these exposures, outcrops are scarce owing to

the dense vegetation, and the exposures are commonly strongly and deeply weathered. This alteration produces, on flat-lying glaciated surfaces, featureless outcrops, masking the internal complexity of the rocks. The combination of these factors, together with the common presence of megaxenoliths up to 200 m in length, renders conventional geological mapping of center 111 futile. However, material from poorly exposed areas is not significantly mineralogically or texturally different from the well-exposed syenites. Accordingly, we consider that the syenites discussed in this paper are representative of the majority of the rocks belonging to center 111.

Petrographic and mineralogical studies show that center 111 consists of four varieties of syenite, which in order of intrusion, as deduced from crosscutting relationships, are magnesiohorn- blende syenite, contaminated ferroedenite syenite, ferroedenite syenite, and quartz syenite. Detailed petrographic descriptions of each type are provided by Jago (1980) and Lukosius-Sanders

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a~ ET AL. 147

(1988). Magnesiohornblende syenites are the least abundant variety and ferroedenite syenites are the most abundant, in the areas examined.

Magnesiohornblende syenites Macroscopically, these syenites are pink or red with promi-

nent aggregates of mafic minerals. They consist essentially of alkali feldspar (45 - 85 modal%) and amphibole (9 -40 modal %), together with minor biotite (1 - 5 modal %) and quartz (trace to 2 modal%). The accessory minerals include magnetite, ilmenite, apatite, pyroxene, zircon, titanite, fluorite, pyrite, and olivine. The rocks consist predominantly of synneusis-textured amphibole set in a hypidiomorphic-granular to porphyritic- hypidiomorphic-granular matrix. Synneusis clusters are rounded (3 mm in diameter) aggregates of subhedral, olive-green to brown-green amphiboles, which poikilitically enclose magne- tite, biotite, apatite, and rare alkali feldspar. Triple junctions are commonly observed between interlocked amphiboles. The margins of the clusters are irregular, with interstices between the amphiboles occupied by matrix alkali feldspar. Isolated sub- hedral crystals of amphibole, identical to those of the clusters, are spread sparsely throughout the feldspar groundmass. Alkali feldspars are complex, commonly mantled, patch perthites, which are typically strongly sericitized and saussuritized. Sub- hedral to anhedral perthites are characteristic of the hypidio- morphic syenites. In the porphyritic types, the phenocrysts, of patch perthite alkali feldspar, grade into microphenocrystal subhedral and randomly oriented laths. Minor plagioclase is present in the groundmass as albite-twinned subhedral to anhedral crystals. Quartz and fluorite occur as late-stage inter- stitial phases. The alkali-feldspar-rich groundmass contains euhedral apatite crystals, late-stage-poikilitic light-green to bluish green amphibole, and yellow-brown biotite. Altered, colorless to green pyroxenes occur in trace amounts in the amphibole clusters and the felsic groundmass. Biotite forms reaction rims around these pyroxenes and magnetite.

Ferroedenite svenites Fresh ferroedenite syenite is white, yellow, or buff in color.

Altered varieties are light pink, owing to the hematization of the feldspars. Typically, the syenite is fine- to medium-grained, with tabular (5 mm in length) alkali feldspar phenocrysts. The rocks are amphibole poor ( < 5 modal%) and consist principally of alkali feldspar (65 - 80 modal %) and quartz (4 - 25 modal %). Minor phases are biotite (4-5 modal%) and magnetite- ilmenite (trace to 7 modal%). Accessory minerals include apa- tite, titanite, pyroxene, fluorite, zircon, chevkinite, monazite, rare-earth fluorocarbonates, pyrochlore, columbite, and allanite (McLaughlin and Mitchell 1989).

The ferroedenite syenites contain phenocrysts of alkali feld- spar set in a matrix of perthitic alkali feldspar prisms: Late-stage quartz, sodic amphiboles, and fluorite occur in the interstices between the feldspar prisms. Phenocrysts are commonly com- plex, with an unexsolved core mantled by several (4- 10) antiperthitic overgrowths. Matrix prisms are tabular single- phase Carlsbad-twinned braid antiperthites. Quartz is typically strain free and coexists with purple fluorite. Wide variations in the modal amounts of amphibole and quartz may be found as the ferroedenite syenites differentiate from relatively mafic- rich types to quartz-rich pegmatitic varieties.

Contaminated ferroedenite syenite Macroscopically, these rocks may be recognized by their

purple color, high content of xenolithic material, and the pres-

ence of oval clusters of metasomatic biotite. They consist prin- cipally of alkali feldspar (55 -70 modal%), amphibole (15 -20 modal % ) , and biotite (7 - 10 modal %) . They are relatively poor in quartz compared with the contamination-free ferroedenite syenites and may contain significant quantities of plagioclase (up to 10 modal %). Accessory minerals include apatite, magne- tite, ilmenite, zircon, fluorite, pyroxene, rare-earth fluorocar- bonates, chevkinite, zircon, pyrochlore, columbite, zirconolite, allanite, and titanite (McLaughlin and Mitchell 1989). The syenites contain mantled phenocrysts of vein and braid per- thitic feldspar set in an allotriomorphic-granular groundmass of alkali feldspar (vein and patch perthites), minor plagioclase, and quartz. The groundmass plagioclase occurs as discrete laths or as anhedral grains forming a consertal texture with alkali feldspar and quartz. These feldspars are typically strongly hematized in contrast with those in the contamination-free ferroedenite syenites. The amphiboles are subhedral to euhedral, brown or greenish brown to green varieties that commonly form mantles on cores of green pyroxene.

Oval to spherical clusters of biotite (up to 3 mm in diameter) are a characteristic feature of these syenites. They are distributed uniformly throughout the groundmass and consist principally of randomly oriented interlocking plates of brown-green biotite. Some contain minor amounts of amphibole, apatite, and opaque minerals. Similar ovoids also occur within mafic xenoliths found in these syenites.

Quartz syenites These rocks are olive green, black, yellow, or buff where

fresh, and red when hematized. They consist principally of alkali feldspar (80 - 90 modal %) and amphibole (10 - 15 modal %), with minor quartz (trace to 5 modal%). Accessory minerals include pyroxene, apatite, zircon, chevkinite, columbite, pyro- chlore, rare-earth fluorocarbonates, fluorite, magnetite, Nb- rutile, allanite, monazite, fersmite, and fergusonite (McLaughlin and Mitchell 1989). The syenites are typically hypidiomorphic granular in texture and rarely porphyritic. Amphiboles and pyroxenes are subhedral and distributed uniformly throughout the feldspar matrix. Quartz occurs as a late-stage interstitial phase with minor fluorite. Alkali feldspars are vein and braid antiperthites, which are typically Carlsbad-twinned.

Quartz syenites differ from ferroedenite syenites with respect to the texture and type of feldspars, the quartz content, the presence of relatively greater amounts of pyroxene, and the absence of titanite. In addition, all syenites differ from each other with respect to the composition of their amphibole and biotite (see below). Quartz syenites crosscut all other center 111 syenites.

Xenolith suite Ferroedenite syenite and contaminated ferroedenite syenite

locally contain abundant xenoliths of basaltic material. In the ferroedenite syenites, the xenoliths show all stages of assimila- tion, from relatively fresh oligoclase basalt to ghosted xenoliths, recognizable in their host syenite only by an increased concen- tration of amphibole and biotite. The xenoliths range from angular types to varieties with irregular convoluted or lobate margins. Ferroedenite syenite containing almost completely assimilated xenoliths takes on the character of contaminated ferroedenite syenite but lacks the purple color and biotite ovoids typical of that facies. Xenolith-bearing, pink to white ferroedenite syenites always intrude and brecciate older, purple contaminated syenites.

Contaminated ferroedenite syenites contain abundant meta-

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148 CAN. J. EARTH SCI. VOL. 30, 1993

volcanic xenoliths in all stages of assimilation. The xenolith spar assemblage has been rapidly quenched subsequent to assemblage is chaotic, and relatively fresh clasts may be juxta- exsolution. The characteristic presence of braid and (or) patch posed against ghosted clasts. Xenoliths typically contain ovoid antiperthite sets the center I11 syenites petrographically apart clusters of biotite, interpreted by Lukosius-Sanders (1988) and from center I and I1 syenites. In these latter rocks, the feld- Nicol (1990) to be of metasomatic origin. spars are typically cryptoperthitic (Mitchell and Platt 1978,

The xenolith suite is considered to represent metamorphosed 1982; Waldron and Parsons 1992). and metasomatized, contemporaneous; basaltic volcanic rocks (Nicol 1990).

Other xenoliths found in the ferroedenite and contaminated ferroedenite syenites include centre I1 nepheline syenite and alkali gabbro and Archean metasedimentary rocks. They con- stitute a relatively small proportion ( < 5 % ) of the xenolith suite. Quartz syenite and magnesiohornblende syenite lack meta- volcanic xenoliths. Quartz syenite rarely contains xenoliths of center I syenite and center I1 alkali gabbro.

Mineralogy

The compositions of minerals in the syenites were determined by conventional wavelength and energy dispersive methods, using electron microprobes located at Dalhousie, Cambridge, and Purdue universities.

Feldspars Because of the complexity of the feldspar assemblage, a

detailed study of feldspar compositional variation is considered to be beyond the scope of this paper and only the results of a preliminary investigation (Lukosius-Sanders 1988) are presented here. Further studies of the feldspars utilizing X-ray diffrac- tion (XRD), cathodoluminescence (CL), transmission electron microscopy (TEM), and scanning electron microscopy - energy dispersive spectrometry (SEM - EDS) techniques are in pro- gress.

Magnesiohornblende syenites contain patch antiperthites con- sisting of orthoclase and albite of near end-member composi- tion, single nonperthitic crystals, which may be either sodic (Ar1~-~~,0r~ -2 mol%) or potassic (Or,,- loo,Ano- mol%), and late-stage groundmass albite (An3 -20 mol %).

Ferroedenite syenites contain patch and braid antiperthites. The sodic phase ranges in composition from Abs9An90rl to Ab,oo. he-exsolved potassic phase ranges from Org4 to OrlOO mol%, with less than 3 mol% An. Mantled feldspars have either plagioclase cores, with compositions ranging from Ab95 to Ab76, or perthitic cores. Both have perthitic mantles.

Contaminated ferroedenite syenites contain feldspars in which it is not possible to distinguish between the host and the exsolved phase. odic phases range from Ab9,3 to Abg5, with 1 -3 mo1% Or. The potassic phase ranges from Or53 to Org1 and has less than 5 mol% An.

Quartz syenites contain braid antiperthites. Host albitic feld- spars range from Ab9, to Abloo and have less than 5 mol% An. The potassic phase ranges from Or9, to Or, and has less than 1 mol% An.

BaO contents of all feldspars are low, ranging from 0 to 0.4 wt. % . Fe203 contents range from 0.1 to 1.5 wt. % . There are no significant differences in the Ba or Fe content of feld- spars in the different syenite types.

In all of the center 111 syenites, no trends in feldspar compo- sition are evident, although each syenite differs with respect to the complexity of the feldspar assemblage present. Feldspars in many of the ferroedenite syenites appear to be derived from several sources, as evidenced by the presence of several com- positionally distinct varieties of mantled phenocrysts. Microcline is notably absent in all of the syenites, indicating that the feld-

Amphiboles Figures 2 and 3 show that the amphiboles in the suite, as a

whole, exhibit an exceptionally wide range of composition. However, the amphibole assemblage of each syenite type is dominated by a particular variety of amphibole.

All of the amphiboles in the magnesiohornblende syenite are calcic, with the compositional trend, as determined from zoned amphiboles, ranging from brown magnesiohornblende (dominant) and magnesian hastingsitic hornblende to green ferrohornblende and-ferroedenitic hornblende (Fig. 2). Amphi- boles that exhibit synneusis textures are relatively evolved hornblendes, whereas discrete amphiboles in the groundmass are magnesiohastingsites zoned to ferroedenitic margins. The predominant compositional trend is one of Si enrichment and Na depletion. The Mg# of the amphiboles ranges from 0.18 to 0.70. Within a single specimen the range in Mg# may be large, e.g., 0.2-0.55.

Primary magmatic amphiboles in the ferroedenite syenites are predominantly calcic, ranging in composition from brown ferrohornblende through ferroactinolitic hornblende, green ferroedenite (dominant), and blue calcic ferroedenite to blue- green ferrorichterite, katophorite, ferrowinchite, and ferro- barroisite (Figs. 2 and 3). The compositional trend is one of Si-, Na and Fe enrichment and A1 and Ca depletion. The total range in Mg# is 0.3 -0.60, but the range within any one speci- men is less than 0.1. Acicular, blue and blue-green riebeckites and arfvedsonites occur marginal to, or replace, earlier-formed calcic amphiboles. These amphiboles are considered to be of subsolidus or deuteric origin (Mitchell 1990).

All amphiboles in the contaminated ferroedenite syenite are calcic. They range from hastingsitic hornblende through ferro- hornblende, ferroedenitic hornblende to ferroedenite (dominant). Individual crystals are not zoned and there is only a limited compositional range with respect to Si (6.45 -7.87) and Mg# (0.13 -0.48). The amphibole compositions are similar to those of the least evolved amphiboles in the ferroedenite syenites (Fig. 2).

Amphiboles in the quartz syenites show a very wide varia- tion in composition from calcic to sodic varieties, ranging from brown ferrohornblende through olive-green ferroedenite to blue-green ferrowinchite, katophorite, ferrorichterite, and blue riebeckite. Figures 2 and 3 show that the amphiboles in the quartz syenite are similar to those of the ferroedenite syenite but in general are more evolved, having higher Si contents and lower Mg# (0.05-0.35). In the majority of samples, isolated subhedral amphibole grains of one compositional type are present. Zoned crystals are rarely found and, where present, show a variety of trends, e.g., from silicic ferroedenite to ferro- richterite or riebeckite, or from katophorite to ferrowinchite. Pyroxenes may be rimmed by ferroactinolitic hornblende, fer- roactinolite, or subsilicic subcalcic ferroedenite.

Mitchell (1990) has shown that the compositional trends of amphiboles in alkaline plutonic complexes may be used to deter- mine the relative degree of evolution of rocks formed from different batches of magma within a given complex. Amphi- bole compositional data for center I11 syenites are interpreted to indicate that the trend of evolution is from magnesian horn-

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MITCHELL ET AL.

( Na + K 1, c 0.50; Ti c 0.50 1.0 1 1 MAGNESIOHORNBLENDE 1

ACTlNOLlTlC I HORNBLENDE - I

' 0 0 0 0 B O ~ ~ O 0

FERROACTINOLITIC- HORNBLENDE

0 0 0 08 O

FERRO- . i i&p+;. .Q0: ACTIN~LITE kt + I FERROHORNBLENDE

MAGNESIO - HORNBLENDE SYENITE

CONTAMINATED FERRO - EDENITE SYENITE

FERROEDENITE ( Na + K 1, 3 0.50 ; Ti c 0.50; ~ e ~ + ~ A I ~

1.0 SYENITE

SlLlClC EDENITE + QUARTZ SYENITE 0

$ * 0'5 1 SlLlClC FERRO- 0 FERROEDENITE . . EDENITE . • + .

+ Q+. ": *. 4.. 8 FERROEDENITIC

HORNBLENDE 8.00 7.50 6.75 6 .50

MAGNESIAN HASTlNGSlTlC

1.0 HORNBLENDE

EDENITE

0.5 0 0 MAGNESIAN FERROEDENITE * .8 00 HASTINGSITE

SlLlClC + . + + +;. .

FERROEDENITE %%*:J(;**. 0 0 . -I+ ++++ HASTINGSITE

0

8.00 7.50 FERRO - 6 . 7 5 1 6.50 \ 6.25 5 . 7 5 EDENlTlC HORNBLENDE HASTlNGSlTlC HORNBLENDE

FIG. 2. Compositional variation and classification (Leake 1978) of the calcic amphiboles occurring in center I11 syenites of the Coldwell complex.

blende syenite through ferroedenite syenite to quartz syenite. The amphibole compositions follow the "primary magmatic trend," as defined by Mitchell (1990), and are thus similar to amphiboles found in many other oversaturated and under- saturated alkaline complexes. Amphiboles in the ferroedenite and quartz syenites evolve along similar compositional paths, from ferroedenite towards ferrorichterite. They differ in that (i) strong enrichment of the residual liquids in Na occurs in the ferroedenite syenites, resulting in the formation of late- stage (subsolidus - deuteric) riebeckite and arfvedsonite, and (ii) quartz syenites lack early-formed unevolved Ca-, Al-rich ferrohornblendes.

Center III amphiboles are similar in their compositional trends to the primary magmatic amphiboles found in center I ferro- augite syenites, but are different in that the ferroactinolitic subtrend is not well developed (Mitchell and Platt 1978; Mitchell 1990). Amphiboles in the center I1 miascitic nepheline syenites are ferropargasites and magnesian hastingsitic hornblendes (Mitchell and Platt 1982), and are totally unlike center 111 amphiboles.

found in the magnesiohornblende syenites, ferroedenite syenites, and their contaminated equivalents. These are diopside - heden- bergites (W045F~16En39 - with 1.4 wt. % Ti02, 0.6-1.1 wt.% Na20, and 1.2-2.2 wt.% Al2O3, and are similar in composition to pyroxenes found in the quartz syenites.

Pyroxenes in the quartz syenites are principally hedenbergites with compositions ranging from Wo49F~21En30 to (Fig. 4), with 0.1 - 1.4 wt. % Ti02, 0.5 - 1.36 wt. % Na20, and 0.2- 1.2 wt. % Al2O3. Manganese contents reach up to 1 wt. % MnO in the most evolved pyroxenes. The evolution- ary trend of composition is from diopside to hedenbergite (Fig. 4). Aegirine contents are typically low (4- 10 mol%), and only a very limited trend toward Na-enrichment (maximum, 17 mol%) is observed before pyroxenes are replaced by amphi- boles as liquidus phases. Figure 4 shows that the trend of pyroxene compositions in the center 111 quartz syenites is similar to the initial portions of compositional trends of pyroxenes from center I and I1 syenites. The pyroxenes differ in that they do not evolve to the hedenbergitic-aegirine solid solutions characteristic of the latter rocks.

Pyroxene Mica Pyroxene is a trace accessory mineral in all syenites and is Micas in the entire suite of center 111 syenites exhibit a wide

only relatively abundant in the quartz syenites. It is commonly compositional range with respect to their Mg and Fe2+ con- strongly altered, and only four grains suitable for analysis were tents, although individual varieties of syenite are characterized

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150 CAN. J. EARTH SCI. VOL. 30. 1993

( NO + K 1, > 0 - 5 0

FERRO- KATO - 1 RlCHTERlTE PHORITE

+ +.+*+* #+

(NO + K I A ~ 0 . 5 0

FERRO- FERRO- WlNCHlTE BARROISITE

: +

I

08 . *. NoB>1.34 ;

( N o + K ) ~ > O . ~

ARFVEDSONITE

FIG. 3. Compositional variation and classification (Leake 1978) of the sodic -calcic and sodic amphiboles occurring in center 111 syenites. Ferric iron estimated on a stoichiometric basis by the method of Droop (1987). Symbols as in Fig. 2.

FIG. 4. Compositions of pyroxenes from centre I11 quartz syenites compared with the compositional trends of pyroxenes from center I and I1 syenites (Mitchell and Platt 1978, 1982).

by micas of restricted composition (Fig. 5). Magnesiohorn- blende syenites contain biotites with Mg# ranging from 0.51 to 0.20 and Ti02 from 2.6 to 4.6 wt. % , whereas ferroedenite syenites have biotites with Mg# 0.23 to 0.10 and Ti02 from 2.3 to 3.8 wt. % Ti02. Quartz syenites contain annites with Mg# ranging from 0.1 to 0.02 and TiOZ from 0.4 to 3.2 wt. % . MnO contents in all micas ranees from 0.2 to 0.6 wt. %. with

u

no correlation between Mn and Fe content. Enrichment in Fe is coupled with decreasing A1 contents; thus micas in the

FIG. 5. Compositions of micas in center I11 syenites. Symbols as in Fig. 2.

FIG. 6. Compositional variation of iron- titanium oxides in center III syenites.

FIG. 7. Calculated (Stormer 1983) temperatures and oxygen fugaci- ties of coexisting ilmenite and magnetite from center 111 syenites. QFM, quartz - fayalite - magnetite oxygen buffer. Data points enclosed in squares represent samples whose compositions are considered to lie outside of the accuracy range of the method. Symbols as in Fig. 2.

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TABLE 1. Representative whole-rock compositions (wt. %) of center 111 syenites

MHS FES CFES Qs C2120 C178 C180 C2077 C2053 C2133 C2222 C2129 C2038 C2026 C2028 C2060 C2091 C2198

SiO, 52.12 55.56 60.54 56.94 61.68 65.22 72.65 76.14 58.69 61.03 66.40 57.67 60.92 64.37 TiO, 0.58 0.66 0.74 0.55 0.41 0.37 0.13 0.13 0.79 0.84 0.24 0.73 0.60 0.38 A1203 15.16 15.58 16.29 15.86 16.92 15.41 13.21 10.75 17.10 17.69 15.79 14.35 16.42 16.47 FqO, 2.27 4.44 6.28 3.74 2.31 0.99 1.40 2.91 2.29 2.70 2.50 7.17 3.13 2.18 FeO 5.96 4.48 1.80 3.84 2.84 3.32 0.96 0.28 4.04 0.72 0.96 4.24 3.40 1.72 MnO 0.16 0.17 0.17 0.17 0.16 0.12 0.07 0.03 0.18 0.14 0.09 0.33 0.19 0.09 MgO 6.50 4.02 1.27 3.34 0.48 0.59 0.01 0.41 1.19 1.28 0.21 0.78 0.81 0.57 CaO 9.12 5.66 3.07 5.84 1.62 1.58 1.17 0.65 2.99 3.02 1.23 4.03 1.86 1.00 Na,O 3.80 4.18 5.08 5.28 5.14 4.37 4.62 3.18 5.12 6.08 5.62 4.74 5.29 5.20 K20 1.91 3.50 3.83 2.72 6.03 6.26 4.22 4.47 4.84 3.94 4.93 5.18 5.60 5.96 P205 0.34 0.33 0.26 0.35 0.09 0.08 n.d. n.d. 0.30 0.27 0.0 0.10 0.16 0.01 H20 1.07 2.32 1.61 1.43 1.07 0.89 2.68 0.18 1.07 1.96 0.71 1.07 1.08 1.25 C02 0.48 0.77 0.29 0.15 0.29 0.29 0.11 0.04 0.33 0.04 0.29 0.26 0.15 0.15 Total 99.47 101.67 101.23 100.21 99.04 99.49 101.23 99.17 98.93 100.71 98.55 100.65 99.60 99.35

NOTES: MHS, magnesiohornblende syenite; FES, ferroedenite syenite; CFES, contaminated ferroedenite syenite; QS, quartz syenite; n.d.. not determined. Sample locations shown in Fig. 1.

magnesiohornblende syenites contain 12.0- 12.7 wt. % A1203, given that the oxide pairs have undergone reequilibration and whereas annites in the quartz syenites contain 7.1 -9.7 wt. % are not in equilibrium with late-stage amphiboles. Al7O1. Center 111 micas are, as expected, not aluminous (1 1.4- 16.3 wt. % A1203) as those found in the miascitic center I1 nepheline syenites (Mitchell and Platt 1982). Mica composi- Whole-rock major and trace element composition

tions suggest that quartz svenites are the most evolved rocks Major elements were determined, with an accuracy of f 2 %, of center-111. by xIray fluorescence (XRF) spectrometry using methods

Iron -titanium oxides Discrete crystals of magnetite and ilmenite are present in all

syenites as inclusions within perthite, biotite, amphibole, and late-stage fluorite. In general, each Fe-Ti oxide appears to be optically and chemically homogeneous. Ilmenites contain 2.2-17.8 wt.% MnO, but have generally < 5 wt.% MnO and < 0.1 wt. % MgO. Magnetites contain 0.6- 13.5 wt. % Ti02, 0-1.2wt.% MnO, and <0.1 wt.% MgO. Fe-Tioxidecom- positions are illustrated in Fig. 6. ~lmenites do not plot on the ilmenite- hematite tie line because of their high MnO contents.

Equilibration temperatures and oxygen fugacities for coexist- ing magnetite -ilmenite pairs were calculated as described by Stormer (1983). Figure 7 shows that the oxides equilibrated at oxygen fugacities slightly above those defined by the quartz- fayalite - magnetite (QFM) buffer. There is no correlation between equilibration parameters and syenite type. Most samples record oxygen fugacities between approximately 10-l7 bar (1 bar = 100 kPa) at 680°C and lop2' bar at 560°C. The cal- culated equilibration parameters are lower than expected for the initial stages of crystallization of syenitic magmas and are considered to record late-stage reequilibration events. Other samples (not plotted) record the effects of disequilibrium or are considered to have compositions that lie outside the accuracy limits defined by Stormer (1983) for this geothermobarometer.

Oxygen fugacities during the final stages of crystallization may be estimated from amphibole stabilities. The late-stage amphiboles in the center 111 syenites are similar to those formed during the final stages of c&stallization of center I syenites. Mitchell and Platt (1978) and Mitchell (1990) have suggested that these formed at oxygen fugacities of bar at temperatures of 500 - 550°C. These oxygen fugacities are similar to those defined by QFM but lower than those defined by the earlier forming oxides. This disparity is to be expected

described by Norrish and Hutton (1969). The trace elements, Ni, Cu, Zn, Pb, Zr, Y, Sr, Rb, Ba, Nb, and Ga, were deter- mined by XRF spectrometry using pressed powder pellets. The rare earth elements, Cr, Co, Hf, Ta, Sc, and Th, were determined by radiochemical and instrumental neutron activa- tion analysis at Lakehead University. U was determined by delayed neutron activation analysis. Accuracy for all trace ele- ment determinations is 2 -5 % .

Major elements Representative whole-rock major element compositions for

the four syenite types are given in Table 1. The majority of the samples are quartz normative and metaluminous. None are peralkaline.

Magnesiohornblende syenites are the least Si-rich syenites (52.1 -60.5 wt. % Si02; differentiation index = 43 -75) and are chemically classified (Streckeisen and LeMaitre 1979) as quartz monzonites and monzodiorites. Ferroedenite syenites exhibit the widest compositional range (54.7-76.1 wt. % SO2; differentiation index = 53 -95) of the center I11 rocks. The majority of the ferroedenite syenites classify chemically as alkali feldspar - quartz syenites. Samples that are modally enriched in quartz or amphibole classify as alkali feldspar granite or monzonite, respectively. Contaminated ferroedenite syenites have reduced Si contents (59.3 -66.4 wt. % Si05 differentiation index = 68-90) and higher Mg and Ca con- tents (Table I), relative to those of ferroedenite syenites, as a consequence of the assimilation of basaltic xenoliths. Quartz syenites have very similar compositions (52.3 -64.3 wt. % SO2; differentiation index = 64 - 88) to those of the majority of the ferroedenite syenites. They classify chemically as alkali feldspar - quartz syenites and alkali-feldspar syenites.

Figure 8 shows that the majority of the syenites have compo- sitions that plot near the Ab-Or boundary in the haplogranite

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CAN. J . EARTH SCI. VOL. 30, 1993

3 kbar, 670'C 1 I , \ , ,.I= % \

FIG. 8. Normative compositions of center 111 syenites plotted in the system Qtz-Ab-Or. Water-saturated ternary minima and eutectics are from Luth et al. (1964). F, , F3, and F, are the positions of the minima in the water-saturated system containing 1, 3, and 4 wt. % F, respec- tively (Manning 1981). BM, binary minimum on the Ab-Or join. Symbols as in Fig. 2. 1 kbar = 100 MPa.

system. Some plot close to the binary minimum, whereas others straddle the cotectic leading to the low-pressure ternary minima and (or) eutectics. No compositional trends are evident. Because the modes of the rocks are dominated by alkali feldspar, it is expected that whole-rock compositions will fall near the albite - potassium feldspar join. Consequently, the syenites are best regarded as an accumulation or aggregation of feldspar crystals that have been concentrated during the fractional crystalliza- tion of a parental magma. Residual liquids may have been trapped in the pore spaces within this crystal aggregate (see below), but we consider that most of the syenite whole-rock compositions are far removed from those of their parent magmas and cannot represent liquid compositions.

Figure 8 shows that only the ferrwdenite syenites richest in quartz plot near the low-pressure water-saturated and fluorine- bearing ternary minima and (or) eutectics (Luth et al. 1964: Manning 198 1) in the haplogranite system. These are the only whole rocks that may have bulk compositions dose to liquid compositions. Rocks of this composition are very rare in the suite of center 111 syenites outcropping at the present level of exposure (see below). These rocks contain amphiboles and micas that are less evolved in terms of their composition than those found in the quartz-rich ferroedenite syenites. This obser- vation suggests that the quartz syenites cannot be regarded as differentiates of the magma that was parental to these ferro- edenite syenites.

Trace elements The trace element content of representative samples of the

four syenite types (given in Table 2) demonstrates that the rocks are enriched in Zr, Nb, Y, Th, U, Cu, Zn, and Ga, rela- tive to I- and S-type granites (Eby 1990). The abundances of these incompatible elements are similar to those found in many anorogenic or A-type granites (Whalen et al. 1987; Eby 1990), which are much richer in silica and which have compositions that plot near ternary eutectics in the haplogranite system. As most of these elements are not sequestered in alkali feldspar, the observation implies that the residual liquids in the Coldwell syenites must have been extremely enriched in incompatible elements.

Plots of GaIAl-Zr (Fig. 9) and Nb-Y (Fig. 10) confirm the geochemical affinities of the center I11 syenites, as all of the data fall within the fields defined for A-type, or within- plate, granitoids. Whalen et al. (1987) have shown that in many A-type granitoids there is a positive correlation between the GaIA1 ratio and Zr content. However, the Ga/A1 ratios of the center 111 syenites do not vary significantly, and the com- positional trend evident in Fig. 9 results from differences in Zr content. The absence of a GaIAl-Zr correlation may be related to the lack of peralkaline differentiates of center 111 magmas or simply to heterogeneous zircon distribution.

Plots of Ce/Nb, Yb/Ta, and Ba/La versus Yb/Nb (Fig. 11) also show that Coldwell center 111 syenites have affinities with A-type ,pnites originating from mantle-derived magmas. rather than rhose produced by the partial melting of continental crust. In particular. the clustering of the data within the oceanic island basalt field is thought to indicate that continental crust was not involved in magma genesis (Eby 1990). The low Ba/La ratios (Fig. l l c ) of many of the syenites suggests that alkali feldspar fractionation has played a significant role in the genesis of the syenites. Alternatively, low BaILa ratios may result from enrichment in La due to the late-stage hydrothermal overprint that led to the formation of rare earth fluorocarbonates in these syenites (McLaughlin and Mitchell 1989).

Plots (not illustrated) of Nb-Zr. Ba-Ce. Sr-Ce, Th-Ce. Rb-Sr, and Ba-Sr do not show any significant interelement correlations within or between syenites. Only geochemically similar elements exhibit significant positive correlations, e.g., Hf-Zr, Th-Zr, Nb-Ta. The variations are related to the abundances of zircon and pyrochlore in the rocks analyzed, and therefore they have no petrogenetic significance.

Table 2 demonstrates that, relative to the other syenites, the magnesiohornblende syenites have the highest Sr and Ba con- tents and the lowest content of high-field-strength elements. 1 This observation is in keeping with the hypothesis. based on amphibole and mica compositions, that these rocks are rela- tively unevolved. Many of the ferroedenite syenites are enriched in incompatible elements relative to the mineralngically more- evolved quartz syenites. These data are interpreted to indicate h a t the qua* symites are not likely to be differentiates of the 1

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TABLE 2. Representative trace element abundances (ppm) in center I11 syenites

MHS FES CFES Qs C180 C178 C2135 C2101 C2129 C2340 C2329 C2091 C2923 C2442

NOTES: MHS, magnesiohornblende syenite; FES, ferroedenite syenite; CFES, contaminated ferroedenite syenite; QS, quartz syenite; n.d., not detected, n.a. , not analyzed. Sample locations shown in Fig. 1.

FIG. 9. Plot of Zr versus 10 000 (GaIAl) for center 111 syenites. I-S, compositional field of I- and S-type granites. Arrow indicates main differentiation compositional trend observed in A-type granites (Whalen et al. 1987; Eby 1990). Symbols as in Fig. 2.

same magma that formed the ferroedenite syenites. Table 3 presents the rare earth element (REE) contents of

representative syenites. All are enriched in REE relative to I- and S-type granites (Whalen et al. 1987; Eby 1990). Chondrite- normalized REE distribution patterns (Fig. 12) are characterized

FIG. 10. Plot of Nb versus Y for center 111 syenites. WPG, within plate granite; VAG, volcanic arc granite; ORG, oceanic ridge granite. Compositional boundaries from Pearce et al. (1984). Symbols as in Fig. 2.

by the presence of a significant negative Eu anomaly. Similar REE distribution patterns are typical of A-type granitoids (Bowden and Whitley 1974; Drysdall et al . 1984; Turner et al. 1992).

Magnesiohornblende syenites exhibit no, or small, Eu anoma- lies (Fig. 12a) and the least enrichment in REE. Ferroedenite syenites show a very wide range in REE abundances and dis-

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154 CAN. J. EARTH SCI. VOL. 30. 1993

tribution patterns (Fig. 12b). Unevolved (SO2-poor) rocks are characterized by light REE enrichment, while quartz-rich late differentiates have almost flat distribution patterns owing to light REE depletion and heavy REE enrichment. Within the suite of ferroedenite syenites, the magnitude of the Eu anomaly increases as the absolute abundances of the REE increase, e.g., EuIEu* decreases from 0.26 to 0.07 in samples 2223 and 2135, respec- tively (Fig. 12b, Table 3). In the most evolved ferroedenite syenites, e.g., sample 2129, the light REE depletion is accom- panied by an increase in EuIEu* to 0.13. Contaminated ferro- edenite syenites have light-REE-enriched distribution patterns (Fig. 12c) that are very similar to those of the contamination- free types, although Eu anomalies are smaller (EuIEu* = 0.57-0.1).

The REE distribution patterns of the quartz syenites are similar to those of the majority of the relatively unevolved ferro- edenite syenites in exhibiting light REE enrichment (Table 3, Fig. 12d) and significant negative Eu anomalies (EuIEu* = 0.21 -0.11). The REE data suggest that the quartz syenites cannot be differentiates of the same magma that formed the ferroedenite syenites, as the derivatives of that magma are light-REE-depleted, e.g., sample 2 129.

0- I

Discussion

"I+

i

A petrogenetic model for the center I11 syenites must account for the following observations:

(1) Center III syenites are metaluminous hypersolvus syenites and quartz syenites. They have geochemical affinities to A-type granitoids and their REE distribution patterns are character-

0.1 I 10

Y / Nb

FIG. 11. Plot of (a) CeINb, (b) YblTa, and (c) BalL versus YlNb for center III syenites. Compositional fields for Oslo granites (OG), oceanic island basalts (OIB), crustal granites (CG), fold belt granites (FB) and island arc basalts (IAB) after Eby (1990). Symbols as in Fig. 2.

ized by significantly negative Eu anomalies. (2) There are no simple geochemical or mineralogical rela-

tionships between the various syenite types; that is, they do not form differentiation series.

(3) The syenites are rocks whose major element composi- tions have been determined by crystal accumulation and none have whole-rock compositions representative of their parental magmas.

(4) The trace element geochemistry of the syenites reflects the proportions of alkali feldspar to interstitial liquids. Most high-field-strength elements are concentrated in the latter. These liquids are extraordinarily enriched in rare elements and must represent extreme products of fractionation.

(5) Ferroedenite syenites are commonly contaminated by the assimilation of coeval basaltic xenoliths.

(6) Field relations demonstrated that each batch of syenite solidified before intrusion of the next batch.

(7) Syenitic magmatism is not associated with contemporane- ous gabbroic magmatism, and at the current level of exposure granites are very rare.

(8) Initial 87Sr/86Sr ratios (Platt and Mitchell 1982) and CeINb, YbITa, and YINb ratios (Fig. 11) of the syenites sug- gested that continental crust has not been involved in the genesis of the magmas.

Eby (1990) and Turner et al. (1992) have reviewed the petrogenesis of A-type granitoids and concluded that no single petrogenetic model satisfactorily explains the origins of these rocks. Current petrogenetic opinion is divided principally between models based on fractionation crystallization of mantle- derived basic magma and those suggesting anatexsis of the continental crust.

Following Eby (1990) and Turner et al. (1992), we consider

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MITCHELL ET AL.

La Ce Nd SmEu Tb Yb Lu La Ce Nd Sm Eu Tb Y b

FIG. 12. Chondrite-normalized REE distribution patterns of representative (a) magnesiohornblende syenites, (b) ferroedenite syenites, (c) contaminated ferroedenite syenites, and (d) quartz syenites.

TABLE 3. Abundances of rare earth elements (ppm) in representative center I11 syenites

MHS FES CFES Qs C178 C180 C2223 C2025 C182 C2135 C2129 C2026 C2308 C2334 C2153 C2138 C2923 C2442 -

La 111 113 194 293 361 6 1 70 175 218 366 91 256 308 462 Ce 208 195 371 503 578 170 225 325 361 606 155 469 521 959 Nd 97 78 131 143 196 59 113 137 174 256 71 153 108 252 Sm 14.8 11.8 21.1 19.5 27.3 17.1 30.3 17.9 24.9 30.8 10.8 19.6 16.3 31.8 Eu 1.74 2.94 1.41 0.78 0.73 0.55 1.35 2.85 1.31 0.81 0.85 0.69 0.93 0.79 Tb 1.50 1.26 2.37 1.82 2.46 2.87 5.79 1.72 2.50 2.69 1.07 1.56 3.27 3.09 Yb 4.67 0.36 6.53 9.17 8.66 13.6 33.6 5.16 9.31 8.76 3.40 5.71 11.0 9.70 Lu 0.79 0.56 0.80 1.23 0.62 1.44 1.89 0.67 n.d. n.d. 0.62 0.67 0.88 1.29

NOTES: MHS, magnesiohornblende syenite; FES, ferroedenite syenite; CFES, contaminated ferroedenite syenite; QS, quartz syenite; n.d., not determined. Sample locations shown in Fig. 1.

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156 CAN. J. EARTH

that involvement of the upper crust in the formation of the center 111 magmas is unlikely given their low 87Sr/86Sr, YbINb, CeINb, and YbITa ratios, and high GaIA1 and ZrINi ratios. This conclusion is supported by the limited Pb, Sr, and Nd iso- topic data for zircons and other minerals from the Coldwell complex presented by Heaman and Machado (1992). The zir- cons have nonradiogenic Pb isotopic compositions, which indi- cate insignificant Archean upper crustal contributions to all of the Coldwell magmas. The Sr, Nd, and Pb isotopic data for a titanite from a center 111 ferroedenite syenite is interpreted as indicating only minor contamination of a mantle-derived magma with lower crustal granulitic material. The isotopic data clearly rule out derivation of the syenitic magmas by extensive partial melting of the lower crust or assimilation of upper crust in basaltic magma.

The isotopic data, trace element abundances, and ratios (Figs. 9 - 11) suggest that the Coldwell syenites were derived from a basaltic source. Although gabbroic rocks are not found associated with center I11 at the present level of exposure, the presence of contemporaneous basaltic magmatism is supported by the occurrence of xenoliths of coeval basalt in the syenites (Nicol 1990). It may be argued that an origin by fractionation of basalt is implausible, as the formation of granitic residua requires 90% crystallization of large volumes of basaltic material and that this is not typically associated with granite plutons (Hess 1989). However, at Coldwell there is direct geophysical and geological evidence (Mitchell et al. 1983; Mitchell and Platt 1982) for the existence of a large differentiated basic intrusion underlying the syenites. Modelling of the infrastructure of the complex indicates that the felsic rocks form a veneer 3 -5 km in thickness over a 3 - 5 km thick gabbro, that is in turn under- lain by 3 km of peridotite and pyroxenite (Mitchell et al. 1983). Turner et al. (1992) have noted that many other occurrences of A-type granitic rocks are associated with positive gravity anomalies. These anomalies are also attributed to subsurface mafic plutons.

Accordingly, we suggest that the center 111 syenites result from the extended differentiation of large volumes of basaltic magma occurring in a series of magma chambers within the infra- structure of the complex. This source accounts for the geochem- ical features of the syenites and in particular the negative Eu anomalies, which may be ascribed to extensive plagioclase frac- tionation (Turner et al. 1992). Because of the large range in trace element concentrations. the absence of any well-defined fractionation trends, and lack of knowledge of the composition of the parent magma, we consider that numerical modelling of the magmatic fractionation process is inappropriate. pooling of unerupted basic magma could provide a heat source for the minor anatexsis and (or) assimilation of lower crustal granu- lites suggested by the isotopic data.

The Coldwell complex is considered to represent the plu- tonic root of a large, rift-related volcanic center (Mitchell and Platt 1982). This center was created during the earliest stages of the development of the Mid-Continental Rift system (Heaman and Machado 1992). The center is characterized by repeated . .

eruption of mantle-derived basaltic magmas and associated syenites. Mitchell and Platt (1982) have noted that the focus of magmatic activity was not stationary and migrated with time in a northwesterlv direction.

The basaltic magmatism associated with center 111 resulted in the formation of contemporaneous basaltic volcanic rocks. These are now found as xenoliths within the contaminated ferro- edenite syenites. The basalts were probably erupted from magma

SCI, VOL, 30, 1993 I1 chambers located within the infrastructure of the volcanic center. Migration of the source of the basalts would result in isola- tion of a magma chamber. Once the chamber was no longer replenished, fractionation of the magma would generate mon- zonitic to syenite liquids. We consider that these derivative magmas were subsequently emplaced at high levels in the crust, perhaps as a consequence of the cauldron subsidence and faulting that has played a major role in the formation of this complex (Mitchell and Platt 1982), aided by density con- trasts. Segregation is required to account for the paucity of plagioclase and earlier-formed ferromagnesian minerals in the syenites. Mixing of several batches of syenitic liquids is also required to explain the complex assemblage of feldspars within any given syenite.

Further differentiation of syenitic magma would lead to the formation of granitic residua, which would migrate towards the roof of the pluton. A feldspar-rich accumulate would occupy the lower portions of the pluton. The accumulate of feldspar prisms would be porous, and residual liquids highly enriched in incompatible elements would be free to circulate throughout the crystal pile. Crystallization of such liquids can account for the extreme enrichment of incompatible elements in the relatively unevolved basal portions of the syenite plutons. Circulation of fluids can also account for the reequilibration of iron - titanium oxides noted above, for the hydrothermal overprint recorded by the presence of REE-bearing fluorocarbonates, and for the replacement of preexisting REE silicates (McLaughlin and Mitchell 1989). Circulation of fluids at low temperatures has occurred within the syenites in center I (Mitchell and Platt 1978; Waldron and Parsons 1992), and there is no reason to suggest that this process did not occur in the similar syenites of center 111.

Mitchell and Platt (1982) have previously estimated that the Coldwell syenites were emplaced at shallow depths (3 -6 krn) equivalent to 1-2 kbar pressure. Erosion of the upper portions of the intrusion to the present level of exposure would remove the volcanic carapace and much of the high-level residual granitic fractions. The paucity of pegmatites and miarolitic cavi- ties in the syenites support the hypothesis that only the deeper parts of the syenite-granite complex have been preserved.

Our hypothesis, and the geological, geochemical, and miner- alogical relationships of the syenites, requires at least four discrete episodes of magmatism. These were of broadly similar character; being formed from similar parental magmas. The resulting syenites are currently preserved at different stages of evolution and (or) structural level. One batch of syenite obvi- ously interacted with extrusive rocks and was strongly con- taminated. To account for the presence of the xenoliths in their present setting it is necessary to propose, following Didier (1973) and Bonin (1986), that initial high-level assimilation was followed by convective transport of xenoliths down the margins of the pluton. Intrusion of later batches of syenite into these xenolith-bearing contaminated syenites gave rise to the complex breccia zones that are so characteristic of center I11 1 magmatism. Subsequent block faulting produced the currently observed complex geology of the center 111 (and 11) syenite I series (Mitchell and Platt 1982).

In conclusion we consider that the center ITI syenites have the I

characteristics of anorogenic granitoids formed by the differen- tiation of basaltic magmas. We have no direct evidence as to the composition of the basalt precursor. Basic xenoliths within the Coldwell syenites are strongly metasomatized by their host syenites (Nicol 1992). Whole-rock compositional data for the

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MITCHELL ET AL. 157

least-altered xenoliths suggest that tholeiitic basalts are com- moner than alkaline basalts in the xenolith suite. W e believe an origin for the syenites by contamination of an alkaline basalt parent is extremely unlikely on the basis of the isotopic data and the trace element composition of the syenites. Accordingly w e suggest a tholeiitic parent for the syenites.

The center 111 syenites are similar in many respects to the center I ferroaugite syenites in their geochemistry and miner- alogy and were undoubtedly derived from similar parental magmas. The center I and I11 syenites differ principally in their volatile contents, center I syenites being relatively water- and fluorine-poor. Center I magmas have differentiated to per- alkaline residua, while such rocks are apparently absent from center 111. They could have been present in the now-eroded roof zones of the center I11 plutons, however, we consider that peralkaline differentiates were never formed, as the trapped liquids in the feldspar accumulates are believed to represent late-stage center I11 fluids. The differences in evolutionary trends a re probably related to compositional differences between the two parental basaltic magmas. In addition, those parental to center I11 probably had the opportunity to melt and assimilate differentiates of center I and I1 magmas during their emplace- ment. Lacking tangible evidence regarding the composition of the parental basalts, we are reluctant to speculate upon the nature of such processes. Differences in the evolutionary trends a re unlikely to stem from changes in redox conditions, as w e have noted above that these were similar during the final stages of crystallization of both varieties of syenites.

Mitchell and Platt (1978, 1982) have discussed the tectonic setting of the Coldwell complex and noted that it, and the nearby Killala Lake complex, may be considered as typical rift-related complexes similar in character to those associated with the Oslo Graben, the Gregory and Kavirondo rifts of East Africa, and the Kangerdlugssuaq area of East Greenland. The Coldwell complex is more deeply eroded, accounting for the paucity of contemporaneous volcanic rocks. The origin of the basaltic magmas that were parental to center I11 syenites must be sought in the broader framework of tectonism and magmatism associated with the development of the Proterozoic Midcontinen- tal Rift System (Hutchinson et al. 1990). Unfortunately such discussion is beyond the scope of this paper, and is dependent upon further isotopic studies of the Coldwell complex.

In summary, we consider that all of the center 111 syenite rocks of the Coldwell complex are derived by the extensive fractionation of mantle-derived basaltic magma.

Acknowledgments Our research on the petrogenesis of alkaline rocks is sup-

ported by the Natural Sciences and Engineering Research Council of Canada and Lakehead University. Henry Meyer and Barry Clarke are thanked for the use of electron micro- probes at Purdue and Dalhousie universities, respectively. Nelson Eby, Else Ragnhild Neumann, and Paul Henderson a re thanked for constructive critical comments on an earlier version of this paper.

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