Geochemistry of the shawmere anorthosite complex, kapuskasing structural zone, Ontario

Precambrian Research~ 1~[ (1980) 43--71 © Elsevier Scientific P~bllshing Company, Amsterdam -- Printed in The Nethe~land~

GEOCHEMISTRY OF THE SHAWMERE ANORTHOSITE KAPUSKASING STRUCTURAL ZONE, ONTARIO

43

COMPLEX,

E. C R A I G SIMMONS*, G I L B E R T N. H A N S O N Department of Earth & Space Sciences, State University of New York, S~ony Brook~ N Y 11794 (U.S.A.) S.B. LUMBERS

Department of Mineralogy and Ceology, Royal Ontario Museum, 100 Queen's Par~ Toronto, Ont. M5S 2C6 (Canada) (Received and accepted August 9, 1979) ABSTRACT

Simmons, E.C., Hanson, G.N. and Lumbers, S.B., 1980. Geochemistry of the Shawmere Anorthosite Complex, Kapuskasing Structural Zone, Ontario. Precambrian Res.~ 11: 43--71. The Archean Shawmere Anorthosite Complex, at the southern end of the K a p u s k ~ t ~ Structural Zone, consists dominantly of anorthosite (An~s_es) with minor gabb~oi¢ ~ d ultramafic units, which are completely enclosed and cut by tortalite~ Both the ~tno!'tha,~,~ and the tonalites are themselves cut by narrow dikes of gabbroic anorthosite. All of the ~ $ have undergone high grade metamorphism and are recrystaillzed so that few igneous tt,~. tures remain. The anorthosites, gabbros and ultramafic rocks of this complex are cumtda~es which contain calcic plagioclase (An6s_gs) and have atomic Mg/(Mg ÷ Fe ~*} ratios ( M ~ ~¢~|~r than 0.6; less than 3 ppm Rb; 150~210 ppm t~r; and less than 60 ppm l b . REE a b u s c t ~ range from 0.2 to 10 times chondritic and exhibit both llght-enriched and tight~dep|¢t~d REE patterns. The lower Mg~ for the samples having more enriched light REE tndt¢~t~ ~ut~ stantial fractions of ferromagnesian minerals crystallized in addition to p[agioclase d~ri~tg fractional crystallization, suggesting that the parent magma was basaltic, and not a n o ~ h ~ sitic. The ranges in St, Ba and REE abundances required for the magmas a ~ t ~ p i ~ of those for tholeiitic basalts from Archean greenstone belts. Thus the Shawmere A n o r t h t ~ t e Complex may represent cumulates of a crtmtal-level magma chamber which could h ~ been the immediate source of basic Arche~m volcanics. One gabbroic anorthositic dike sample has a steeply fractionated R EE pa~ter~t w~th b#~,~v)~ REE abundances less than chondrites and a large positive Eu anomaly~ The p r v p ~ ~pretation is that this rock formed by partisl melting of mafic eumulatt~s, p e r h ~ ~ h ~ ~f the Shawmere Anorthosite Complex itself. INTRODUCTION T h i s s t u d y is c o n c e r n e d w i t h t h e genesis o f t h e S h a w m e r e A n o r t h o s i t e C ~ t n p l e x ( F i g . l ) as r e v e a l e d b y r e c o n n a i s s a n c e g e o l o g i c a l s t u d i e s , m a j o r elemeJ~t *Present address: Department of Chemistry and Geochemistry, Colorado ~ h o ~ t of M ~ t ~ Golden, CO 80401, U.S.A.

44 whole-rock and mineral chemistry, and trace element m~d St-isotope analyses. Preliminary geochronological data by Watkinson et al. (1972) and data reported herein suggest that the complex is itself Archean in age and is among the oldest anorthositic intrusions thus far recorded. Anorthosites are particularly relevant to tectonic interpretations of the Precambrian because of their limited occurrence, both spatially and temporally within the geologic record. Three major types of anorthosite are current,t~, ~ecognized, each belonging to a distinct association of plutonic rocks" (1) the anorthosite suite proper (charnockite--anorthosite series), typified by the massif anorthosites of the Grenville Province, where predominantly sodic anorthosite occurs as a major to dominant constituent in small sheet-like to large batholitic bodies accompanied by leucogabbro and minor gabbroic to ultramafic units in spatial association with cross-cutting dioritic, tonalitic, syenitic, or quartz monzonitic to gTea~itephases that commonly contain orthopyroxene; (2) the stratifor~u Bushveld type where bytownite anorthosite occurs as a minor constituent within stratiform mafic sheets and lopoliths; and (3) calcic anox'thositic complexes, older than 3.0 Ga, and forminA~ layered stratiform sheets accompanied by major units of gabbro, pyroxenite, and peridotitc (Windley, 1970; Windley et al., 1973; Windley and Smith, 1974). The first type can be considered to constitute an anorthosite suite without prejudgment as to genetic implications (Emslie, 1978), and few well documented complexes of this type are older than 2.0 Ga. The second type is found throughout the geological column and is readily distinguished from the other two types. The Archean calcic anorthositic complexes differ from anorthosite suite complexes chiefly by: (1) containing a much larger proportion of associated mafic to ultramafic rocks; (2) the presence of chromite and a high chromiura content in ferromagnesium minerals; (3) a general lack of spatially associated quartz-bearing and syenitic intrusive rocks; and (4) their old radiometric ages. The Shawmere Anorthosite Complex is anomalous with respect to these types of anorthosite. While these rocks contain calcic plagioclase (An> 65) and have atomic Mg/(Mg + Fe2+):> 0.6 like other Archean Anorthosites, the overall bulk composition of the exposed complex is plagioclase.rich like the massiftype. G EOLOGICAL~ETTING OF THE $HAWMERE COMPLEX The Shawmere A:orthosite Complex (Fig.l) intrudes gneisses within the southern part of the Kapuskasing Structural Zone, a south-southwest trending feature that cuts across the Superior Province for more than 400 km, from James Bay to near the eastern end of Lake Superior (Ayres et al., 1971). The Kapuskasing Structural Zone has received reconnaissance geophysical and geological study (Innes, 1960; Bennet et al., 1967; Innes et al., 1967; Gittins et al., 1967; Bennet, 1969; McGlynn, 1970; Thurston et al., 1977). The tault bounded

45 structural zone consists of high grade gneisses. Its anomalously high gravity and magnetic fields may reflect deep crustal displacements involving the mantle. Alkalic rock-carbonatite intrusive complexes of 1700 and 1050 Ma are scattered along the structural zone. Supracrustal and plutonic rocks of the adjacent parts of the Superior Province locally extend into it. Boundaries and major features of the southern part of the structure where the Shawmere intrusion was emplaced are difficult to define because of poor exposure and indistinct geophysical expression. Figure 1 shows the general geology of the Shawmere intrusion and its surrounding rocks. A major zone, as broad as 2 km, of extensive cataclasis, mylo. nitization and a concentration of late dikes of diabase and lamprophyre marks the eastern boundary of the structural zone in the map-area. Rocks of the Abitibi Belt (Goodwin et al., 1972), which consist of greenschist-facies metavolcanics and minor associated metasediments intruded by massive granodiorite and trondhjemite, are in fault contact with gueissic rocks of the Structural Zone. These Superior Province rocks can be traced into the Structural Zone in the southern part of the area where they were deformed and coarsely recrystallized into gneisses. Gneissic granitic rocks shown in Fig.1 at the western edge of the complex also may be equivalents of the Superior Province granitic rocks, but data are insufficient to prove this relationship. The remainder of the rocks surrounding the complex are gueissic metasediments, which locally contain small bodies of gneissic granitic rocks (shown undifferentiated in Fig.l). In the southwestern and eastern parts of the area, these metasediments are dominated by coarsely recrystallized, gneissic arkose and imptre sandstone, but elsewhere the metasediments appear to be gneissic equivalents of poorly sorted sandy and silty deposits, possibly a greywacke sequence. In the southwest corner of the area between the alkalic rock-carbonatite complexes (Fig.l), gneissic metaconglomerate units containing a variety of granitic boulders and pebbles are present in the gneissic metasediments. At the western boundary of the map-area, gueissic meta-arkose appears to lie unconformably upon the gneissic metavolcanics which are an extension of the Abitibi Belt into the Structural Zone, suggesting that these gneissic metasediments are younger than rocks of the Abitibi Belt. The Shawmere Anorthosite Complex intruded both the metavolcanic rocks and the gneissic metasediments. Following emplacement, all of the rocks were intensely deformed and coarsely recrystallized by high rank regional metamorphism, during the waning stages of which, late granite pegmatite dikes were emplaced in all of the rocks of the structural zone. Later the structural zone was subjected to major faulting and intrusion of alkalic rock-carbonatite complexes, and diabase and lamprophyre dikes. Some of the faults near the eastern boundary most likely represent reactivation of faults initially formed early in the geological history of the area. Lamprophyre dikes are confined mainly to the eastern boundary of the Structural Zone, but diabase dikes are widespread throughout the zone. The timing of these late structural and in-

46 trusive events has not been worked out in detail. Most of the alkalic rockcarbonate complexes are cut by faults, and some of the d i a b ~ e dikes are themselves faulted, whereas others are undeformed. Thins, the dikes appear to represent multiple ages relative to the faulting, and some most likely postdate the complexes, whereas others probably are older.

LEGEND m

AlkOh~,

rock" C~I~IOnOI*t~ ¢On~pl~Yl

ll;:!;Ci G. . . . . . . . . . . , ~ . , , . , ~ o . .... o°o,,,,o,,,,., .... ~..., ond rntnor OnOrt~o$~hc ~obb~o L.LLL

~ . . . . . . . . . ~. . . . . . . . . . . . . . . . . . . . .

E

I

Fig. 1. Generalized geologic map of the Shawmere Anorthosite Complex, Ontario on which are shown the locations of samples analyzed.

47

Thus, the geological s e t t i ~ o1"the Shawmere Anorthosite Complex is similar to that of most massif..type anorthosite complexes elsewhere throughout the world. It is in a setting marked by high rank regional metamorphism and is itself regionally metamorphosea In this study we describe the geology, the petrography and geochemistry of the complex and suggest possible models for tt~e composition, source and evolution of the melts making up the anorthositic complex and anorthositic dikes cutting the complex. ANALYTICALMETHODS

Samples of approximately 10 kg were taken from blocks ranging from 20 to 100 kg. Smaller samples of the tonalite, AN3, anorthosite dike, AN25, and mafic gabbro, AN1, were taken from blocks of 5--10 kg. Total iron as FeO, MgO, MnO, CaO, Na:O and K20 were analyzed by atomic absorption by K.R. Simmons. All samples and standards were run in duplicate. The precision is better than one percent of the amount present, except for elements with abundances less than 1--2 wt.%, which agree within the limits specified by Wright et al, (1975). Silicon, aluminum and titanium were analyzed by atomic absorption by N.H. Suhr, Pennsylvania State University. The concentrations of K, Rb, Sr, Ba and the rare earth elements (REE) (Table III) were determined by isotope dilution following the chemical procedures of Arth (1973), Arth (pets. commun., 1973), and Hanson (1976). The elements were analyzed on a 6-inch, 60°-sector or a 12-inch, 90 °-sector automated Shields-type (NBS) mass spectrometer. The precision is generally 1--2% except for Lu, for which the precision is better than 5%. Larger differences between duplicate analyses of AN 1 and AN 2 (Table III) are attributed to sample inTABLE I Average mode (vol.%) of gneissic tonalite

Quartz Plagioclase % An K-feldspar Biotite Amphibole Epidote Carbonate Garnet Opaque minerals Apatite Zircon Total

Average of 8 modes

Estimated range

23,5 48.0 35 trace 7.8 15.2 trace 0.1 3.1 1,2 0.8 0.3

(12--31) (37--56)

100.0

(32-40) (1--14) (6--32) (0--7)

(0.5-2.5) (o.5--1) (O.l-O.8)

48 homogeneity, as the duplicate chondrite-normalized REE patterns are smooth and subparallel. STSr/86Sr ratios were determined using the chemical procedure of Hanson et al. (1971). Eleven runs of standard SRM 987 yield a mean for 8~Sr/S*Sr of 0.71020 -+0.00005 (20). The ~TSr/S6Sr ratios for the whole.rock samples in Table I are the averages of two or more separate runs and have an uncertainty of approximately -+0.0001 (2 ~ ), The uncertainty for the biotite, AN 3, is approximately ±0.1%. Selected phases from samples of the Shawmere Complex were analyzed on an ARL-EMX electron microprobe, operated at an accelerating voltage of 15 kV and a sample current of 0.15 u A (standardized on brass}. Data was reduced on-line, using the program of Bence and Albee (1968). GENERAL DESCRIPTION OF THE SHAWMERE ANORTHOS1TECOMPLEX The complex is an elongate intrusion underlying about 1700 km 2 (Fig.l). Much of the central and western parts of the complex are poorly exposed, so that lithologic contacts shown in these parts of the complex are necessarily somewhat subjective. In a few places, particularly along the northwestern contact, gneissic tonalitic dikes and apophyses cut across the metasediments, and inclusion~ of the metasediments are present in the tonalite. Regional metamorphism caused all rocks of the coml:lex to be at least partly recrystallized and most are gneissic, especially at the margins. Phases containing well preserved primary mineralogy, textures, and structures can be identified only within the interior of the main body more than about 2.5 km from the margins. In general, the metamorphic features closely parallel those of anorthositic intrusions described by Lumbers (1975) in the Grenville Province. Gneissic foliation and relic primary layering within the complex generally trend northeastward, but near the margins the trend approximates that of the contact of the complex. Foliation within the main body typically dips northwestward at angles rarely more than 50 °. Locally, particularly i~ the central part of the main body, the foliation dips subvertically and local reversals in dip occur accompanied by closures in the foliation suggestive of recumbent folding. For the most part, foliation in the two southerly appendages on the southeastern side of the main body dips subverticlaly to steeply northwestward. Lithologically, the complex is dominated by gneissic anorthosite* and gabbroic anorthosite, and minor relic layers of anorthositic gabbro, gabbro, and minor ultramafic rocks (Fig.2). The mafic layers, which generally are less than one meter wide, increase in abundance near the margins. The anorthositic

*The anorthositic rocks are classified according to Buddington's (1939) scheme into: (1) anorthosite, less than 10%; (2) gabbroic anorthosite, 22.5%; (3) anorthositie gabbro, 22.5-35%; and (4) gabbro, more than 35% ferromagnesian minerals.

49

Fig.2. Typical exposure of interlayered gneissic anorthosite and gabbroic anorthosite containing a partly dismembered layer of gabbro (bottom of photo). Catty Lake, about 5 km south of locality 25 (Fig.l).

rocks are surrounded by an envelope of gneiss~c tonalite, and locally within the main body, dikes and irregularly shaped masses of gneissic tonalite cut across the anorthositic rocks. In a few places, particularly in the central and northeastern parts of the main body, small dikes of gneissic anorthosite and gabbroic anorthosite generally less than one meter wide, cut the older anorthositic and associated mafic rocks, and gneissic tonalite. At locality 25, a breccia composed of angular blocks of anorthosite as much as 40 cm across, set in a matrix of partly altered peridotite cuts anorthosite and gabbro~,c anorthosite dikes (Fig.3). The order of emplacement of the main phases of the complex from oldest to youngest is as follows: (1) anorthositic rocks containing minor layers of mafic to ultramafic rocks; (2) tonalite and minor gabbro; (3) localized intrusions of gabbro and ultramafic rocks; and (4) dikes of anorthosite and gabbroic anorthosite. Widespread dikes of late granite pegmatite and diabase, and rare dikes o f alkalic rocks were emplaced either during the waning of stages of regional metamorphism, or following the regional metanlorphism. Thus, they are not genetically related to the complex.

50

Fig.3. Breccia at locality 25 (Fig.l) composed of angular blocks of anorthosite in a matrix of metamorphosed peridotite and marie gabbro that intrudes anorthosite and gabbroic

anorthosite (not visible in photo). Note the late dikes of anorthosite and gabbroic anorthosite ne.~r the hammer.

MODAL AND PETROGRAPHIC DATA

Most of the medium- to coarse-grained anorthositic rocks as well as the late fine- to medium-grained anorthositic dikes are completely recrystallized, and consist mainly of plagioclase, amphibole, garnet and rarely biotite, and epidote. Plagioclase commonly is partly altered to various mixtures of finegrained sericite, epidote, carbonate, and scapolite. Amphibole, which replaces primary pyroxene, locally contains intergrown quartz and biotite. Most of the amphibole shows very pale pleochroism and is optically positive, and may be pargasite. Traces of quartz are common as fine-grained blebs in plagioclase. A few of the gneissic anorthositic rocks contain megascopic lenses of quartz a few centimeters long and only a few millimeters wide. Pale greenish alteration veinlets only a few millimeters wide composed of quartz, epidote, [and scapiolite are common in the gneissic anorthositic rocks. Microprobe analyses of the major minerals in a mafic gabbro (AN 1), ~abbro (AN 24), gabbroic anorthosite (AN 2) and a gabbroic anorthosite dike (AN 25) are given in Table IV.

51 TABLE 11 Major element analyses (wt.%) and CIPW normsa of whole-rock samples of the Shawmere Complex AN24

AN1

SiO 2 AI30 a TiO~ "FeO ''b MgO MnO CaO Na20 K20

51.3 15.3 0.21 5.40 10.61 0.12 15.0 1.30 0.10

44.0 21.3 0.06 7.60 12,6 0.15 11.4 1,32 0,08

Z

99.3

98.5

Quartz Orthoelase AIbite Anorthite nmenite Magnetite Diopside Hypersthene Olivine

. 0.61 11.1 35.8 0.39 0.89 31.2 19.9 0.06

.

Mg/(Mg+Fe~*)a % AN (normative} % Normative feldspar

0.80 0.75 47.5

AN22 47.9 24.8 0.13 4.55 7.19 0.07 13.6 1.89 0.17 100.3

AN2 47.6 29.7 0,06 2.66 3.50 0.03 14.7 1.94 0.09

AN11

50.4 22.3 0.35 5.54 5.34 0.09 12.1 2.75 0.27

49.3 28.8 0.14 2.50 1.98 0.03 14.2 2.80 0.11

AN20

AN25

48.9 52.5 26.0 26.3 0.40 0.16 5.33 2.37 3.05 3.06 0.08 0.04 14.0 10.8 2.38 4.44 0.16 0.19 100.3

99.9

AN3 67.5 15.7 0.41 4.30 1,73 0.07 4.83 4.0~ 0,67

99.1

99.9

. 0.45 16.4 71.9 0.12 0.44 0.65 3.87 6,21

. 1.62 23.5 48.1 0.68 0.91 10.1 10.7 4.42

.

0.46 11.3 52.7 0.12 1.25 3.74 -30.4

. 1.00 15.9 58.5 0.24 0.74 7.03 3.92 12.7

0.67 23.7 65.7 0.27 0.42 3.90 1.28 3.93

0.95 1.11 20.1 37.6 59.6 51.3 0.76 0.30 0.86 0,37 7.94 :t 84 5.44 -4.39 7.43

26.0 3.98 34.8 22.7 0.78 0.69 1.t8 9,97 -

0.77 0.82 64.5

0.76 0.78 75.4

0.72 0.81 88.8

0.66 0.66 73.2

0.61 0.73 90.1

0,53 0.72 0.74 0,57 80.7 90.0

0,44 0.37 61,5

.

100.3

AN9

.

99.3

aCalculated assuming atomic Fe 2*/(Fe 2+ + Fe 3÷) = 0.9. b"FeO" means total iron as FeO.

Legend for Tables L H and III Sample l o c a t i o n s and d e s c r i p t i o n s of w h o l e - r o c k samples of t he S ha w me re C o m p l e x AN 24

AN 1

( L e m o i n e T o w n s h i p ) : L e m o i n e Lake on east shore, south-central p o r t i o n of lake, d i r e c t l y across lake from cabin o n west shore. gabbro: 40% plagioclase, 35% h o r n b l e n d e , 20% c l i n o p y r o x e n e , 5% o r t h o p y r o x e n e , plus m i n o r b i o t i t e and o p a q u e s ( F o l e y e t T o w n s h i p ) : 5 k m n o r t h of h i g h w a y 101 along road t o Ranger Tower.

ultramafic: 25% plagioclase (Ang0---gs), 50% hornbl e nde , 25% garnet,plus m i n o r (1--2%) ortho- and c l i n o p y r o x e n e . AN 22

( L e m o i n e T o w n s h i p ) : L e m o i n e Lake on we s t e rn shore, 3 k m s o u t h o f n o r t h end of lake, 200 m n o r t h of narrow e m b a y m e n t on west side. gab broic anorthosite: 80% plagioc!ase (AN~0), 20% hornbl e nde , m i n o r (1%) o p a q u e s and very m i n o r a l t e r a t i o n products.

AN 2

( F o l e y e t T o w n s h i p ) : 50 meters s o u t h w e s t o f Ranger Tower, at w e s t e r n margin of F o l e y e t Township. anorthosite: 85% plagioclase (AnTs~--so)2 12% h o r n b l e n d e , 3% garnet.

52

AN 9

(Lemoine Township): Sample from north end of island in east-central edge of small lake in southeast corner of Lemolne ~ownship. gabbroic anorthosite: 80% plagloclase (An~0), hornblende, 5% orthopyroxene, clinopyroxene and quartz.

AN 11 (Lemolne Township): Sample from western shore, 200 m north of south end of small lake in southeast corner of Lemoine Township. gabbroic anorthositv: 85% plagloclase (An~s), 15% hornblende, plus w~-iable(1%) amounts of biotite, garnet and opaques. AN 20

(Lemoine Township): Lemoine Lake along western shore. 200 m south of creek going northwest to Shawmere Lake. gabbroic anortkosite: 80% plagioclase (An~0), 15% hornblende, 5% garnet, biotite, quartz and opaques.

AN 25

(Lemoine Township): Lemoine Lake cm east shore, 1 km north of southern end of Lemoine Lake, 200 meters no~-~hof where creek empties into lake from southeast. gabbroic anorthosite dike: 80% plagioclase (Ans2), 10% hornblende, 5% garnet. plus 5% biotite and minor alteration products.

AN

(Lemoine Township): Samples from south edge of island near northern end of small lake in southeast corner of Lemolne Township. tonolite: 15% quartz, 50% plagioclase (An3s), 20% hornblende, 5% garnet, 7% biotite, ~% opaques.

3

Anorlhosite to gabbroic anorthosite

Where least metamorphosed, anorthosite, anorthositic gabbro and gabbroic anorthosite c o n t a i n relic subophitic t e x t u r e and relic megacrysts of plagioclase u p to 45 cm long (Figs. 4 and 5). Some of these rocks c o n t a i n relic, slightly zoned clinopyroxene and o r t h o p y r o x e n e r i m m e d corona fashion by amphibole and garnet (see sample AN 24, Table IV). Clinopyroxene free of corona structure (AN 24, Table IV) also is present, and locally may c o n t a i n t i n y inclusions of green spinel (probably hercynite). These clinopyroxenes exhibit no exsolution lamellae, and texturally appear to coexist in equilibrium with amphibole and matrix plagioclase. O r t h o p y r o x e n e , o n the other hand, is confined to the cores of corona structures, and exhibits,exsolution lamellae of clinopyroxene parallel to (100). Plagioclase is a d o m i n a n t mineral and ranges in c o m p o s i t i o n from An6s to An~s. The most calcic Compositions are confined to the gabbroic rocks. Relic primary plagioclase shows less range in c o m p o s i t i o n (AnTs--Ans0) and is confined to the cores of megacrysts, where it appears as dark grey, t r a n s l u c e n t masses surrounded by a white to light grey, fine- to coarse-grained mosaic of recrystallized plagioclase. The primary plagioclase contains t i n y amphibole needles similar to those described b y Smith and Steele (1974). Although the rocks of the complex are normatively silica undersaturated (Table II), relic quartz occurs as a trace c o n s t i t u e n t interstitial to plagioclase megacrysts. Recrystallized plagioclase rarely contains fine-grained blebs of quartz.

53

Fig.4. Relic subophitic texture in slightly foliated anorthosite at locality 2 (Fig.l). Light grey veinlets and concentrations of veinlets cutting anorthosite arc alteration veinlets composed o f quartz, epidote, and scapolite.,

Fig. 5. Relic megacrysts o f plagioelase developed in gneissic anorthosite and gabbroic an ~ orthosite near locality 20 (Fig. 1).

54

Gabbro to ultramafic rocks

Gabbroic layers differ from the anorthosites chiefly by their higher amphibole content. Ultramafic layers are composed essentially of amphibole and most have concentrations of garnet either at their margins, or within the layers. Gabbro and ultramafic rocks of the late, minor intrusions in the main body generally are only partly recrystallized and they contain relic, primary pyroxene. Rare primary olivine is present in some of the ultramafic rocks. Tonalites and gabbroic anorthosite dikes

Regional metamorphism effectively destroyed primary textural and minerslogical features of the gneissic tonalite. T~,.erock is composed essentially of sodic andesine (An32-40), quartz, amphibole, biotite and minor garnet, but is relatively rich in accessory zircon (Table I). Potassic feldspar is absent in most samples of the tonalite, but trace amounts occur locally as separate grains interstitial to plagioclase. The gabbroic anorthosite dikes are strongly recrystallized. Plagioclase is dominantly generally close to Anss. Hornblende, biotite, and garnet (see An 25, Table IV) are minor phases. RESULTS

The samples amalyzed in this study, typical of the various phases of the inT A B L E III Trace e l e m e n t a b u n d a n c e s (ppm) in whole-rock samples o f the S h a w m e r e C o m p l e x AN24

Rb Sr Ba Ce Nd Sm Eu Gd Dy Er Yb Lu K/Rb Rb/Sr Sr/Ba (Ce/Yb)~, (Eu/Eu) 8TRb/a~Sr aTSr/S%r

1.74 81.1 13.5 0,919 0.913 0,386 0.245 0.657 0.925 0.625 0.595 0.0889 465. .0215 6.01 ,40 1.5 .0619 .70309

AN1

2.50 31.5 37.2 0,592 0,319 0.0765 0.0987 0.0715 0.0750 0.0470 0.0482 0.0082 353. .0794 .847 3.2 4.1

AN22

0.374 0.0996 0.114 0.103 0.0659 0.0733 0.0122

3.4

1.02 158. 52.4 2,76 1.71 0.340 0.280 0.405 0.388 0.239 0,229 0.0342 1370. 0.0065 3.02 3.1 2.1 0.0188 0.70150

AN2

AN9

.583 160. 25.6 1.24 0,608 0.120 0.250 -0.101 0.0564 0.0509 0.0080

3.53 186. 48.4 4.15 2.70 0.789 0.463 1.03 1.17 0.765 0.735 0,111

1.22 0.475 0.106 0.232 -0.0919 0.0521 0.0490 0.0077

719. 0.0037 6.25 6.2 6.4 6.6 6.8 0.0106 0.70132

583. 0.0190 3.84 1.5 1.6 0.0548 0.70321

aAverage of t w o analyses. bThe R E E values for the Leedey C h o n d r i t e (Masuda el al., 1 9 7 3 ) are the m o s t accurate values yet reported for a c h o n d r i t i e meteorite. As they arc 20% higher than o t h e r average c h o n d r i t e values, Leedey values d i v i d e d by 1.2 are used for normalizing p u r p o s e d in this study.

55

trusion, were collected only from the northeastern part of the anorthositic complex (Fig. 1) because the best exposures are there. Trace element abundances and 87Sr/8~Sr ratios of these samples and of one biotite separate are given in Table III. Rb-Sr data are plotted on a strontium evolution diagram in Fig.6. K-Rb and Sr-Ba data are shown in Figs 7 and 8, respectively. Chondrite-normalized REE patterns for anorthositic rocks and gneissic tonalite are shown in Figs. 9--11.

Sr-isotope data The anorthositic rocks show small range~ in Rb/Sr (0.01-0.06), and consequently in STSr/a6Sr (0.7013--O.7032). A least-squares fit of the data using the methods of York (1966) and McIntire et al. (1966), yields an isochron with an imprecise age of 2444 + 500 (20) Ma and an initial ratio of approximately 0.701 (Fig. 6). Most of the data do not fall within analytical uncertainty of the isochron, suggesting that these rocks are not all the same age; that they do not have the same initial ratio; or that the St-isotopic compositions of these rocks have been distrubed. The Rb-Sr results and the 2519 Ma K-Ar apparent age for hornblende (Watkinson et al., 1972 ) suggest that these rocks probably are Archean and that the ages approximate the time of last deformation and metamorphism. The initial aTSr/8~Sr ratio of about 0.701 is similar to those reported for basaltic rocks of Archean greenstone terranes (e.g., Peterman et al., 1972). The data point for the gabbroic anorthosite dike lies close to the isochron

AN11

0.287 210. 50.7 2.89 1.27 0.289 0.340 0.325 0.340 0.203 0,195 0.0299 3180. 0.0014 4.14 3.8 3.4 0.0039 0.70146

AN20

AN25

AN3

0.868 180. 60.0 7.66 4.23 1.14 0.626 1.34 1.39 0.793 0.676 0.0952

1.28 469. 73.9 6.81 3.84 0.771 0.420 0.592 0.387 0.187 0.156 0.0231

5.30 345. 352. 22.4 8.12 1.29 1.04 1.02 0.676 0.354 0.336 --

1530. 0.0048 3.00 2.9 1.6 0.0139 0.70160

1120. 0.0027 6.35 11. 1.8 0.0078 0.70142

1000. 0.0154 0.980 17. 2.7 0.0444 0.70366

AN3(Bt) BCR1a

107. 14.6

7.33

21.2 1.253

47.5 329. 681. 56.1 29.4 6.69 1,99 6.86 6.40 3.73 3.39 0.485

Chondritesb

0.813 0,597 0.192 0.0722 0.259 0.325 0.213 0,208 0.0323

56 T A B L E IV Shawmere mineral data

SiC, AI~O 3 TiO~ "FeO" MnO MgO CaO Na:O K20 Cr:O s

Z (Oxides)

1

2

3

4

5

44,8 36.1 n.a. 0.12 n.a. n.d. 19.5 0.56 n.d. n.a.

47.6 33.0 n.a. 0.10 n.a. n.d. 16.6 2.16 0.01 n.a.

48.6 32.9 n.a. 0.10 n.a. 0.01 15.3 2.70 0.01 n.a.

49,3 32,7 n.a. 0.10 n.a. n.d. 15.0 2.81 0.08 n.a.

55.0 28,8 n.a. 0.18 n.a. n.d. 10.9 5.37 0 e3 n.a.

101.1

99.5

99.6

10~).0

100.3

Oxygens

8

8

8

8

8

Si AI Ti "Fe" Mn Mg Ca Na K

2.05 1.95 . 0.01

2.20 1.79

2.23 2.25 1.78 1.76 . . ----. . . 0.75 0.73 0.24 0.25 -0.01

.

. --

. . 0.95 0.05 --

Cr

.

:~ (Carlo.s)

.

5.01

An Ab Or

95.0 5.0 --

0.702

I

°

. . 0.82 0.19 -.

.

0.01 -0.53 0.47 --

.

5.0--'--~ ;.00 80.9 19.1 --

2.47 1.53

5.00

75.7 24.3 --

74.4 ~5.1 0.5

;.00 52.7 47.1 0.2

]

~

J2 2

0 701

~ ( 8 7 S ¢ / 8 6 S , ) = 070109

o:o,

o'o3

0'05

87Rb/B6sr

Fig.6. Rb and St-isotope data for eight whole-rock samples from the Shawmere Complex plotted on a St-evolution diagram.

57

TABLF IV (continued) 6

7

8

9

SiO~ AI203 TiO~ "FeO" MnO MgO CaO Na=O K20 Cr20 ~

53.9 4.09 0.01 12.0 0.11 29.5 0.19 n.d. n.a. n.d.

52.7 1.40 0.01 19.2 0.64 25.1 0.46 n.d. n.a. 0.06

53.1 1.58 n.d. 5.62 0.22 15.7 22.9 0.30 n.a. 0.19

52.9 1.62 0.01 5.66 0.23 15.6 23.1 0.31 n.a. 0.18

(Oxides) Oxygens

99.8 6

99.6 6

99.6 6

99.6 6

Si Al Ti "Fe"

1.91 0.17 . 0.36

1.95 0.06 . . 0.59

1.96 0.07 . 0.17

0.18

Mn Mg

-1.56

0 02 1.38

0.01 0.86

0.01 0.86

Ca

--

0.02

0.91

0.92

N~

--

--

0.02

K

.

Cr

--

2; (Cations)

4.00

.

.

-4.02

1.96 0.07

0.02 .

0.01 4.02

0.01 4.03

Wo En

0.37 80.9

0.9 68.6

46.5 44.2

46.8 43.9

Fs

18.7

30.5

9.3

9.3

in Fig. 6, suggesting that this rock could have an age and initial ratio simil~x to that of the anorthositic rocks. The data p o i n t for the tonalite lies well above the reference line, suggesting t h a t it is older, has been c o n t a m i n a t e d , or its p a r e n t had a significant history in a relatively high Rb/Sr ratio e n v i r o n m e n t . I n any case, the tonalite appears to be unrelated to the rocks of the anorthosite suite. The whole-rock-biotite isochron for the tonalite, AN3, yields an age of 1854 Ma, which may reflect events associated with uplift of the Kapuskasing Feature.

Anorthositic rocks The K and Rb c o n c e n t r a t i o n s and K / R b ratios for the anorthositic rocks show significant ranges, b u t little correlation with either the mineralogy or chemistry of these samples (Fig.7). The higher c o n c e n t r a t i o n s of K a n d Rb tend

58 TABLE IV (continued)

SiO~ AI20~ TiO~ MnO MgO CaO Na:O K~O

Cr203 (Oxides) Oxygens

10

11

12

13

14

15

16

44.5 17.4 0.05 7.30 0.03 15.6 11.3 2.37 0.17 n.d.

46.9 13.5 0.09 14.4 0.18 20.3 0.54 1.31 n.d. n.d.

46.1 13.7 0.21 9.47 0.03 15.6 10.6 1.43 0.22 0.01

45.5 t3,3 0.91 9,92 0.10 15.1 11.2 1.29 0.46 n.d.

50.0 8,40 0.39 8.36 0.14 17.2 11.7 0.84 0,49 0.30

41.7 23.2 n.d. 18.0 0.66 14.1 4.18 n.d. n.d. 0.03

40.1 23.0 0.03 22,8 0.71 10.1 610 n.d, n.d. n.d.

98.7

97.2

97.4

97,8

97.8

23

23

23

23

23

6.26 2.88 -0.86

Si Al Ti "Fe" Mn Mg Ca Na K C~

Ca/(Ca+Mg+Fe) Mg/(Ca+Mg+Fe) Fc/(Ca+Mg+Fe)

f

/

102.8

12

12

3.26 1.71 0.64 0.03 .

6.66 2.26 0.01 1.71 0.03 4.29 0.08 0.36 -. .

6.60 2.31 0.02 1.13 -3.32 1.62 0.40 0.04 .

6,53 2,25 0.I0 1.19 0.01 3.22 1.72 0.36 0.09

7.09 1.41 0.04 0.99 0.02 3.64 1.77 0.23 0.09 0.03

--

15.65

15.40

15.44

15.47

15.28

7.97

8.03

0.29 0.56 0.15

0.01 0.71 028

0.27 0.55 0.19

0.28 0.53 0.19

0.2~ 0.57 0.15

0.11 0.52 0.37

0.16 0.37 0.47

-

r_ (Cations)

101.9

-

3OO0

2000

3.02 1.98 -1.09 0.04 1.52 0.32 ---

I000

/

y." oo

AN2~o IAN22 )

/

~

N2 /

2N24

I0

20

~'~/

30

4-0

50

Rb (ppm)

Fig.7. K vs. Rb plot for samples o f the 8hawmere Anorthosite Complex.

2.96 2.02 -1.41 0.05 1.11 0.48 ----

59 Legend to Table IV Feldspars

1 AN 1: 2 3 4 5

Representative analysis of plagioclase~ coexisting in equilibrium with metamorphic hornblende and garnet. AN2: Analysis of recrystallized plagioclase from core of remnant megacryst. AN2: Analysis of plagioclase adjacent to hornblende of corona structure. AN 24: ~epresentative analysis of recrystallized plagioclase from central portion of plagioclase augen. AN25: Representative plagioclase analysis.

Pyroxenes

Fine grained (<0.1 ram) orthopyroxene contained within metamorphic amphibole, pyroxene is optically continuous with nearby grains of similar size and composition. 7 AN 24: Orthopyro:~ene enclosed in corona of hornblende (analysis 13) and clinopyroxene (analysis 9). Optically continuous wi~h nearby grains of similar size and composition. 8 AN 24: Host of augite-low Ca pyroxene intergrowth interpreted as a possible primary igneous phase. 9 AN 24: Metamorphic clinopyroxene, part of corona around orthopyroxene (analysis 7) coexisting in equilibrium with metamorphic hornblende (analysis 13). 6 ANI:

Horn blendes 10 A N I : 11 AN 2:

Hornblende coexisting in equilibrium with garnet (analysis 15). Cummingtonite from core of corona structure, surrounded by hornblende (analysis 12) separating it from plagioclase (analysis 3). 12 AN 2: Hornblende part of corona structure around cummingtonite (analysis 11 ) separating it from plagioclase (analysis 3). 13 AN 24: Hornblende forming part of corona structure, along with clinopyroxene (analysis 9) around orthopyroxene (analysis 7). 14 AN 25: Hornblende coexisting in equilibrium with garnet (analysis 16).

Garnets 15 AN 1" Garnet coexisting in equilibrium with hornblende (analysis 10 ). 16 AN 25: Garnet coexisting in equilibrium with hornblende (analysis 14 )

t o o ccu r in rocks with th e m o s t h o r n b l e n d e , suggesting t h a t K and Rb m ay have been mobile during t h e m e t a m o r p h i s m , and t h a t t h e abundances o f these elements m ay reflect m e t a m o r p h i c as well as igneous processes. If Rb was m o b i l e during m e t a m o r p h i s m , t h e distribution o f points along t h e Rb-Sr reference line (Fig.6) w o u l d reflect a m e t a m o r p h i c , n o t an igneous, event, Th e Sr c o n t e n t s o f th e anorthositic rocks are relatively u n i f o r m ( 1 5 8 ~ 2 ! 0 p p m ) , and those o f the m o r e mafic samples are a factor o f t w o t o five lower. Th e Ba c o n t e n t s o f t h e anorthositic rocks are less than 60 ppm, and correlate a p p r o x i m a t e l y with t h e light REE. With t h e e x c e p t i o n o f t h e mafic gabbro (AN 1), all samples have Sr/Ba ratios greater than three (Fig.8), and t h e a b u n dances o f Sr and Ba, as well as t h e Sr/Ba ratios o f these rocks are proportio~ nal to the fraction o f plagioclase in these rocks.

60

~0

,25 3

20C

29" •

~22

IOC

/

~o

'bo

3;0

Ba (ppm) Fig,8. Sr and Ba data for nine whole-rock samples of the Shawmere Complex. AN 20~ •

AN

91 Gobbroic Anor thosile AN 22~ o AN I I J x AN 2' Anorthosite t.

•

/

rO 1

0.2

] Ce

1 Nc~

I EU I Gd I Sm

I Oy

ErI

Yfb Lu I

Fig.9. Chondrite-normalized REE data for five plagioclase-rich (> 80%) rocks from the Shawmere C~~mplex.

61 The REE abundances ~or the anorthositic rocks (Figs. 9 and 10) vary by more than a factor of ten. The patterns are both enriched and depleted in the light REE, and all exhibit positive Eu anomalies. The (Ce/Yb)N and (Eu/Eu*) ratios for the plagioclase-rich rocks (Fig.9) are inversely proportional to the total REE abundances. [] ANI • AN24 I0--

~..~ ~.....~..~. ~ .2.'.'....2.-.~

~

Ultramafic Gabbto

,;, ,.::~ /.'...~

,..//

,ao,oo,oss ,,o, Cumulates

[.:.::::::.:.::::::.

i':':.:.::':':.:.::" I:.:.:.:.':-:.:.:.': ~:::.:.::::::.:~

".~ ~:.'....:.:.'.'..:.:.:.-...-:.:.'....:.:.'....:.:.I .'. e:~ .'.':.:.:.'.'..:.:.'.':':..'.'."I.'./ D:.:~ ~.:.:.:<..:.:.:.:.-..:<.:.:..:.:.:...:.:.:.:..:~ :.:.I ~.:.:.'..:.:.:.'..:.:.:.'...-:-:.'.'..:-:.'.':.:-j v..:.:.'.'. ~ !...' .'.'I v ".'-':':".'.':':".'.':':"-'.':<"-'.':':'"'~ i:;:i~ :•:,J /\, , ~ ~':.':'::':.'.':':':':.':':':':.':':'::.':':':~ : : :::: :::: : : : : ' : : : ::~ I "~ "~::::::' :.! :::::::::::::::::::::::::::::::::::::::::::::::: y ~'..:.:.:.'..:.:-:. "..:.:-:. ".-:.:-:. "."..:-:.', ~...... .:........:.:......:.:.... .:.:..,...: o.~ ::::::::::::::::::::::::::::::::::::::::::::::

0 2I

Ce

""': N~d

~':;.-....:.:........:.:...-..:.:.....~ .: ..: .-....:> -_.~ .,/\ ~:.::.:.:.:.:..:.:.:.:..:.:.:.:..:.:.:.:.:..:.:.~ I Eiu I Sm Gfd Dy Fir YbJ Lui

Fig.10. Chondrite-normalizedREE data for two mafic cumulates from the Shawmere Complex, compared to the field for the plagioclase-rich cumulates.

Tonalite and gabbroic anorthosite dike Both the tonalite and gabbroic anorthosite dike have K and Rb abundances and K/Rb ratios similar to those of the anorthositic rocks, but higher Sr and Ba abundances. The REE patterns for the gabbroic anorthosite dike and tonalite are light-enriched, and have positive Eu anomalies as large as those of some of the anorthositic rocks of the anorthosite suite (Fig.11). If either of these rocks were parent or residual magmas for the cumulates of the anorthositic rocks, then they should have REE abundances uniformly higher than those of the cumulate anorthosites, but lower (Ce/Yh)N and (Eu/Eu*) ratios and Sr abundances. As these rocks have higher (Ce/Yb)N ratios, large positive Eu anomalies, Sr abundances as much as three times higher than the anorthositic rocks, and REE patterns which cross the field of the anorthositic rocks, the data preclude either of these rocks as being parent or residual magmas for the anorthosites.

62

'\

AN 3 Tonolite AN25 Gobbroic Ano~thos=te Dike

0

30 -~ a

• k

,o

Camulott t

q.:.: i =..'..'.:..'., ~:....2......... ql f":':'""::'"":'"" r'. ":". ", "" ".:. ":. ~-:.-.:.:-.-......: r. .'.:.:.'.'..::':.: r.'.:..':.'..".. E. ". ".".'.-,-:--.'. L'. ":'" .'.','-:-""

I i

:....~.~..:......:......:.,

~...'::,:.,','..:.:: r.-'.'.'.",'.':," ;.:. :.:.:.::.:.:.:'.'.:" >,..'..'.:..:.:.-..:.:,,.'.:.,

1 0,2 L

I

CB

Nd

I

I

Srn Eu Gtd"

I ~y

I Er

:.:,:.'.:,j , ,:,

I Lu Yb

Fig.11. Chondrite-normalizedREE data for the tonalite (AN 3) and the gabbroic anorthosite dike (AN 25) compared to field for the anorthosite series cumulates.

PETROGENESIS Quantitative trace element models were calculated to set limits on the composition of parent magmas and on the modes of origin for rocks of the Shawmere Complex. These models have been constructed to be consistent with the geologic relationships, inferred primary mineralogy, appropriate phase equilibria relations, and major and trace element chemistry. Construction of models such as these assume that the REE patterns and abundances reflect the original igneous event and have not been changed by metamorphism. Indirect evidence supporting this assumption has been sum. marized by Kay and Senechel (1976) and Arth and Hanson (1975). The results of this study also support such an assumption. In particular, the mafic gabbro, AN 1, has a metamorphic mineral assemblage dominated by garnet and hornblende, both of which have particularly high affinities for the REE, but the lowest REE abundances of any of the samples analyzed.

Petrogenesis of anorthositic rocks The gtadational relationships among the anorthositic rocks suggest that they may be either cumulates from basaltic or plagioclase-rich melts. The lack of a known exposed chilled margin or associated volcanic rocks make impossible a direct observation of the parent magma.

63 The plagioclase-rich nature of the intrusion, the occurrence of subophitic textures in the least deformed rocks, the high Mg~ (:>0.69) of the whole-rocks and primary pyroxenes, the factor of ten variation in REE abundances, and the ubiquitous occurrence positive Eu anomalies are consistent with a cumulate origin. If so, the REE patterns, which mimic those of mineral.melt K a ' s would suggest a parent melt having a relatively flat chondrite-normalized REE pattern. The occurrence in the Shawmere Complex of gabbroic anorthosite dikes such as AN25 shows that anorthositic magma was present in the region after intrusion of the complex. If such a magma also was present prior to intrusion of the complex, as discussed before (see Analytical Results, Tonalite and G~bbroic Anorthosite Dike, p. 61), it could not be the parent of the anorthosites. In order to calculate models for the parent magmas for these rocks, some estimate of the igneous mineralogy of the anorthositic rocks is required. Rocks of the Shawmere Complex, however, are metamorphosed and recrystallized to assemblages including hornblende and garnet, precluding direct observation of the original assemblages. Because pyroxenes are common within coronas of hornblende + garnet, the original ferromagnesian minerals were probably dominated by anhydrous phases, such as pyroxenes and olivines. Therefore, we have used the CIPW norms of these rocks (Table II) as guides to their original mineralogy. The trace element abundances in the melt in equilibrium with the cumulate phases were calculated in the following manner. For rocks interpreted to be adcumulates (i.e., cumulates with little intercumulus liquid) either on the basis of their texture or low REE abundances with l~ge positive Eu anomalies, we used their normative mineralogy to calculate a bulk distribution coefficient, D. The abundances of the individual trace elements in the parent magmas, CL, are calculated from CL = Cs/D, where Cs is the concentration in the cumulate. Rocks with higher REE abundances and smaller Eu anomalies were assumed to represent mixtures of plagioclase-rich cumulates plus liquid. The Kd'S used in this study are from basaltic or andesitic liquids and are given in Appendix 1. The values for the Kd'S for plagioclase are similar to the experimentally determined values of Drake and Well {1975), who found that the Kd'S for the REE are relatively insensitive to temperature. A high plagioclase content, low total REE abundances, light REE.enriched pattern and large positive Eu anomaly of the anorthosite, AN 2, suggest an origin as a plagioclase-rich adcumulate. An original cumulate of 90% plagio. clase yields a calculated melt for this sample having ~ 1 0 0 p p m St, ,~80 p p m Ba, and a flat chondrite-normalized REE pattern, 10--15 times chondrites. This melt is geochemically similar to Archean tholeiitic basalts analyzed by Arth and Hanson (1975). If the melt had no Eu anomaly, the Eu almmaly in AN 2 suggests a plagioclase-melt Kd for Eu of approximately 0.3--0.4, which is within the range reported by Schnetzler and Philpotts (1970) for basal~s. The low total REE abundances for the mafic gabbro, AN 1, suggest that it also was an adcumulate. In addition to plagioclase, the rest of the cumulate assemblage had to be phases whose presence would reduce the light REE by a

64 factor of two, but hold the heavy REE constant, relative to the anorthosite, AN 2. The only phase consistent with the normative mineralogy and satisfying these conditions is olivine (Table II). Gabbro AN 24 has a light-depleted REE pattern and low heavy-REE abundances implying large fractions of cumulate orthopyroxene, clinopyroxene or hornblende and little intercumulus liquid. This is consistent with its normative and modal mineralogy. The similar abundances of middle to heavy REE suggest that the original cumulate assemblage was dominated by clino- and orthopyroxene, in approximately subequal proportions. Gabbroic anorthosite AN 9 is enriched in the light REE, This suggests that the original cumulate contained less than 40% ferromagnesian phases such as orthopyroxene, clinopyroxene or hornblende. The high heavy-REE abundances, and the plagioclase-rich mineralogy suggest incorporation of as much as 30% intercumulus magma similar in abundances and pattern to the melt for anorthosite AN 2. The gabbroic anorthosites, AN i1 and AN 22, have very similar REE patterns which are enriched in the light REE and have heavy REE abundances five times those observed in the anorthosite, AN 2. The melt from which AN 11 and AN 22 accumulated had to be light-REE enriched relative to the melt for AN 2. The shape of the REE pattern and size of the Eu anomaly of AN 20, another gabbroic anorthosite, is similar to those of AN 9, but the light REE are even more enriched relative to the heavy REE than gabbroic anorthosites AN ! 1 and AN 22, suggesting that the melt from which AN 20 accumulated was also more enriched in.the light REE. The Ce abundances would have to be at least three times more enriched than in the magma proposed as the parent for anorthosite, AN 2.

10-

i

c~

[3

Pereni Mogrno ~--c3--c3 c] AN9

•

ANI

--

02 I

./\

•

]

c~

063 p~og O07 opx o o,o

037 p~ag

o 09 op,

"

\

" ~ .

O 4 o1 I Ce

t []

l Nd

~--~| I i Gld Sm Eu

r Dy

I _ Er

I Yb

Fig.12. Chondrite-non~alized R E E plot of trace element models for generation of A n 1,

An 24 and two plagioclase-richcumulates (AN 2 and AN 9).

65

The results of quantitative model calculations for the cumulate rocks of the Shawmere Complex are shown in Figs. 12 and 13. AN 1, 2, 9 and 24 can be derived from a magma (Fig. 12), having a fiat chondrite-normalized REE pattern and abundances of twelve times chondrites. The calculated mineralogies of the adcumulates (AN 1, 2 and 24) agree reasonably well with the CIPW norms, except for AN 1, for which the calculated cumulate contains too much olivine and too little plagioclase. The plagioclase-rich cumulates (AN 2, 11, and 20) cannot all be derived from the calculated melt shown in Fig.8 (A in Fig.13). A magma with REE abundances like those of C in Fig.13 represents the minimum enrichment of the REE required to produce the gabbroic anorthosite, AN 20. The relative enrichment in tee light REE for the melt in going from AN 2 to AN 20 requires fractionation of at least 30--40% ferromagnesian phases, such as clinopyroxene. Fractional crystallization of pyroxene alone would result in an increase in the Sr content of the magma, since pyroxenes have relatively low Kd'S for Sr (e.g. Philpotts and Schnetzler, 1970). The nearly constant Sr content of the anorthositic cumulate rocks indicates that plagioclase accompanied the precipitation of pyroxene. As Arth and Hanson (1975) pointed out, changing a fiat REE pattern with ten times chondrites by differentiation of a basaltic magma to a light enriched pattern with C e ~ 3 0 and Yb ~ 1 5 times chondrites requires approximately 60% solidification of approximately subequal proportions of clinopyroxene and plagioclase. Thus the parent magma for the Shawmere anorthosites was most probably basaltic and not anorthositie. Therefore,

30

C o

Parent

A o

o

o .....

Magmas

o

o

o

[]

I0--

-11

/

/.\

o2o.0,c, o

"

// A \

~

ANII/AN22:0.90 pi.g

{__

e --'---'--I

AN~:

0 . 9 0 plo(:J 0 10 opx 0.00

o ~ ~ .

~

liq (A)

O; { Ce

{ NcI

{ { I $rn Eu Gd

I Dy

'

( Er

I Yb

Fig. 13. Chondrlte-normalized REE plot of trace element models for generation of the plagioclase-rich cumulates of the Shawmere Complex.

66 at least as much gabbro as anorthosite must be present within the complex. The gabbros may be present but covered due to the limited exposure. Figure 14 compares the range of REE patterns for the calculated parent liquids needed to explain the REE patterns of the cumulate rocks of the anorthosite series to REE patterns of six Arehe~m tholeiitic basalts and basaltic andesites from ArCh and Hanson (1975), The similarity suggests that the rock~ of the anorthosite series may represent cumulates from a tho!eiitic magma. The labradorite.bytownite composition of the plagioclases, the high Mg¢/oi the primary ferromagnesian minerals, and the initial S~Sr/S~Sr ratio of 0.701 also are consistent with a basaltic parent magma. ~.... • --~,

.. .. .. .. .. .. ..

S hawmere hrcheen

C otcuic~led P o r e n i s BOSQII

.• . ' , ' . ' ..'...' .. .' .. -. .. .. . . . . ~ r .

and Honson (~975)

A~th

: : r : ~ • .. .~. . 4. . ., ~

.--,----~ ~• ;

I

IO---

i

i

f L

©; l

Ce

l .....

Nd

l

I __J- .....

$m Eu Gd

[ ......

Dy

~

£r

......

1. . . . .

Yb

Fig. 14. Cbondrite-normalized REE plot comparing the range ot: REE abundances necessary to explain the cumulate rocks of the anorthosite series to analy:~esof six Archean tholeiitic basalts and basaltic andesites from Arth and Hanson (1975).

Petrogenesis of the gabbroic anorthosite dike The occurrence of gabbroic anorthosite as a dikes, combined with the lack of any observed cumulate textures (which are preserved in other rocks of the complex), indicate that AN 25 probably represents a liquid composition. Simmons {1976), and Simmons and Hanson (1978) summarized the evidence for the existence of aluminous magmas such as this, and proposed a model for their origin, involving partial melting of tholeiitic compositions, leaving a pyroxene-dominated residue. However, the large positive Eu anomaly, low K, Rb, Ba and Ce contents, along with high Mg # and Sr/Ba ratios of the dike

67 relative to Archean basalts (Arth and Hanson, 1975) suggest a cumulate gabbro rather than a basaltic parent for AN 25. For small to moderate de~ees of partial melting, the Ce content of the dike indicates a parent having a Ce content between one to two times chondrites, based on the Kd data for pyroxenes (e.g. Schnetzler and Philpotts, 1970). A potential parent rock would be a gabbro such as AN 24. Figure 15 shows a calculated REE model for partial melting of gabbro AN 24 leaving a residue of 17% plagioclase, 8% garnet, 25% clinopyroxene and 50% orthopyroxene, using the Kd'S given in Appendix 2. Ten percent melting nearly duplicates the REE pattern of the dike, AN 25, except for the light REE. The size of the Eu anomaly in the dike requires a plagioclase-melt Kd for Eu of 0.75, consistent with that deduced for the pla~oclase of massiftype anorthosites (Simmons and Hanson, 1978). Calculated K (0.5--1.0%) and Rb (10--20 ppm) abundances are a factor of ten too high. Perhaps as was mentioned earlier these elements were mobile during metamorphism. Calculated Sr abundances are extremely sensitive to the plagioclase-melt Kd for St; Partial

~0 ~

.{

Melting

Model

Dike

&

AN 25 AN24

~ "~l~I~' ~

~ z ~ "~ I e/o" " - . - , ~ ~ o

0

* ~, ~.

,oo,o.... '~

"<:.'.,

for

Anorthosite

o\

Gabbroic

Dike Gebbro

--

Catculoled Melt Residue 8°/° gt 17% plag sO'~o

o~,

/",,,2 i

" /°

'%

o/° 0 5' i___k Ce

,

Nd

J- _ I ~-Sm Eu Gd

£ . . . . . . . . . ]___ Dy Er

yIb

Fig.15. Chondrite-normalized REE plot of the trace element model for generation of the gabbroic anorthosite dike, AN 25, by partial melting of the gabbro, AN 24.

use of a Kd as low as one still results in calculated Sr abundances (300--400 ppm) which are 20--30% too low. Also, AN 24, is too aluminous to have been the parent rock for the dike. Removal of 10% melt, having the composition of the dike, AN 25, from AN 24 still leaves 14% A1203 • Thus the A1203 and Sr data indicate that the dike could be a result of partial melting of a gabbro similar to AN 24, but containing one-half the plagioclase.

68 CONCLUSIONS

We deduce that cumulate rocks of the Shawmere Complex were derived from an original tholeiitic magma. This conclusion is based on: (1) The calcic plagioclase compositions (AN~o_ 80 ); (2) the high atomic Mg/(Mg + Fe :÷) ratios of the whole-rocks and primary ferromagnesian phases; (3) trace-element data and models which imply that the parent magmas for these cumulate rocks had St, Ba and REE abundances remarkably similar to basalts in Archean greenstone belts; and (4) the increasing enrichment of the light REE relative to the heavy REE in the magma, which accompanies a decrease in the Mg/(Mg + Fe2÷). This would suggest crystallization of significant fractions of both plagioclase and clinopyroxene. Thus, the Shawmere Complex probably represents the cumulates of a crustal-level magma chamber that may have been the source of basalts like those of Archean greenstone belts (e.g. Arth and Hanson, 1975). The later gabbroic anorthosite dike is best explained by partial melting at crustal levels of the mafic cumulates of a basaltic magma, such as those of the Shawmere Complex itself. APPENDIX 1 Mineral-melt distribution c,Jefficient data used in calculations for cumulate models for the Shawmere Anorthosite Complex As the g~vatest sil~gle source of uncertainty in trace elemer.~ model calculations, the choice of mineral-mel~ distribution coefficients is critical to any trace element study. The distribution coefficienls listed below and in Appendix 2 w~re selected for two reasons. First, on the basis of the data in the literature (see compilation of Arth, 1973~, the values used here are believed to best approximate these required for given (or inferred) solid and liquid compositions, temperature and fo conditions. Second, to avoid introduction of additional bias, wherever possible the dlstrlbutivn coefficients determined from solid phases from the s a m e rock were used. •

•

2

Plagioc'asea

Orth~pyroxene b

Clinopyroxeneb

Olivinec

K Rb Sr Ba

0.13 0.04 1.84 0.16

0.014 0.022 0.017 0.013

0.011 0.015 0.12 0.013

0.0068 0.0098 0.014 0.0099

Ce Nd Sm Eu Gd Dy Er Yb

0.13 0.10 0.07 0.34 0.05 0.042 0.034 0.028

0.0026 0.0083* 0.014 0.023 0.03* 0.055* 0.078* 0.11

0.17 0.38 0.74 0.75 0.82 1.05" 1.10" 1.01

0.0069 0.0066 0.0066 0.0068 0.0077 0.0096 0.011 0.014

*By interpolation. aAll data are the average of GSFC 184 (Ans0) and GSFC 271 (An~7) (Philpotts and Schnetzler, 1970; Schnetzler and Philpotts, 1970), except Eu, taken from Arth (1973). bREE for both are from the same alkali olivine basalt (Onuma et ai., 1968). K, Rb, St, Ba are average values compiled by Arth (1973). CAverage of six values compiled by Arth (1973).

69

APPENDIX 2 Mineral-melt, distribution coefficient data used in model calculations for the gabbroic anorthosite dike of the Shawmere Complex Plagioclasea

Orthopyroxene b

Clinopyroxeneb Garnet c

K Rb Sr Ba

0.36 0.14 1.~3 0.59

0.019 0.015 0.024 0.012

0.019 0.013 0.065 0.041

Ce Nd Sm Eu Gd Dy Er Yb

0.20 0.14 0.11 0.73 0.066 0.055 0.041 0.031

0.038 0.058 O.10 0.079 0.17 0.29 0.46 0.67

0.30* 0.65 0.95 0.68 1.35 1.46 1.33 1.30

0.020 0.0086 0.015 0.017 0.35 0.53 2.66 1.50 10.5 28.6 42.8 39.9

*By extrapolation.

aK, Rb, and Ba data from GSFC 225 (Philpotts and Schnetzler, 1970); Sr and REE data from GSFC 271 (AnT~) (PhUpotts and Schnetzler, 1970; Schnetzler and Philpotts, 1970)~ bBoth from same andesite GSFC 271 (Philpotts and Schnetzler, 1970; Schnetzlcr and Phiipotts, 1970). CFrom Japanese dacite, GSFC 218 (Philpotts and Schnetzler~ 1970; Schnetzler and Philpotts, 1970). ACKNOWLEDGEMENTS We wish to t h a n k P.C. T h u r s t o n f o r supplying unpublished i n f o r m a t i o n , P.W. Weiblen f o r supplying o n e o f t h e standards used f o r the :major elemenf; analyses, and L. Wasserzug and M. Convery f o r assistance in smnple prepar~tion. We are also grateful t o K.R. S i m m o n s f o r partial m aj o r e l e m e n t analyses, and J.W. Delano f o r advice and help in c o n n e c t i o n with t h e m i c r o p r o b e amdyses. A.E. Bence, D.H. Lindsley, and S.A. Morse reviewed the thesis u p o n which this paper is based. This w o r k was s u p p o r t e d by N S F grant ~ DES 7100471AO2. REFERENCES Arth, J.G., 1973. Geochemistry of Early Precambrian Igneous Rocks. Minnesota--Ontario. Ph.D. Thesis, State Univ. New York at Stony Brook, 153 pp. Arth, J.G. and Hanson, G.N., 1972. Quartz diorites derived by partial melting of eclogi~e or amphibolite at mantle depths. Contrib. Mineral. Petrol., 37: 61--74. Arth, J.G. and Hanson, G.N., 1975. Geochemistry and origin of the early Precambrian crust of northeastern Minnesota. Geochim. Cosmochim. Acta, 39: 325--362. Bence, A.E. and Albee, A.L., 1968. Empirical correction factors for the electron micro~ analysis of silicates and oxides. J. Geol., 76: 382--403. Bennett, G., 1969. Geology of the Belford-Strachan area, District of Cochrane, Ontario. Ont. Dep. Mines, Geol. Rep., 78:30 pp. (wi~h Map 2181).

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