Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the Adirondack...

Geochimica et Cosm~himica Aaa, Vol. 58, No. 24, pp. 5525-5535. 1994

Pergamon Copyright 0 1994 Elsevier Science Ltd Printed in the USA. All rights reserved

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0016”7037(94)00345-9

Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the Adirondack Mountains, New York, USA

JOHN M. EILER* and JOHN W. VALLEY

~pa~rnent of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA

(Received March 2 1, 1994; acrepied in revisedjbrm July 18, 1994)

Abstract-The Adirondack Mountains, New York, expose a diverse group of Proterozoic igneous rocks that were metamorphosed to granulite facies conditions during the Ottawan phase of the Grenville orogeny. Oxygen isotope data for seventy whole rock samples of gabbroic to granitic meta-igneous rocks, primarily from the chamockite suites from the Tupper and Saranac sheets in the central Adirondacks, demonstrate a correlation between 6 180w, and major element composition within continuous, mappable, meta-plutonic units. No such relationship is seen among nonconsanguineous granitoids. Variations in mineral 6180 values and large differences in 6180 between rocks with non~onsanguineous protoliths but similar bulk composition demonstrate that these rocks were not infiltrated by, or isotopically ~uilibrat~ through, a pervasive metamo~hic fluid. Values of S “0 for mineral separates preserve generally high temperature fractionations indicative of dominantly closed-system retrograde exchange.

Values of d”O,, in Adirondack orthogneisses were not significantly shifted during granulite facies metamorphism and were dominantly controlled by processes active during premetamorphic magmatism. Values of 6 ‘*Owr may thus serve as a petrogenetic indicator, allow discrimination of rock units, and serve as a source constraint for meta-igneous rocks. Fayalite meta-granites with low 6’*0 values can be dis- criminated from surrounding granitoids having high S “0 values. A record of assimilation in the evolution of differentiated granitic units is preserved. The preservation of primary igneous d ‘*O values in granulite facies orthogneiss imposes constraints on the synmetamorphic and postmetamorphic fluid history of the Adirondack Highlands. Oxygen isotopic compositions at the hand-sample scale have been preserved through granulite-facies metamo~hism. Fluid absence or low ~uid/rock ratios on a regional scale are indicated.

INTRODIJCI’ION

THE ADIRONDACK MOUNTAINS, New York, are a classic and well-studied example of both granulite facies metamorphism and Proterozoic massif-type anorthosites, and their associated monzonitic to granitic rocks. This study reports the oxygen isotopic compositions of various meta-igneous lithologies from this terrain, principally focusing on the monzonitic to granitic members of the AMCG (ano~hosite-mange~te- charno~kite-granite) suite (MC LELLAND, 199 i ) that are spa- tially and temporally associated with the Marcy anorthosite massif. As such, this is both the first intensive study of the oxygen isotope geochemistry of the granitic members of the anorogenic trinity, and a test of the effect of high-grade metamorphism in altering the stable isotope ratios of rocks.

The AMCG complexes found throughout eastern North America uniquely combine issues central to the igneous and metamorphic geochemistry of the deep crust. These complexes represent major addition to, and remobilization of, the crust during the middle Proterozoic ( EMSLIE, 1978). The relative importance of melts from the mantle or the lower crust in the parental magma(s) for the various members of the AMCG suite has been, and continues to be, open to debate (MORSE, 1982; DUCHESNE, 1983; OWENS et al., 1993). Re- solving this issue must include a detailed understanding of the geochemistry of the granitic members of this association.

* Current address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 9 I 125, USA.

Gaining such an understanding is complicated in many AMCG complexes by a metamorphic overprint. The granulite facies conditions reached in the Adirondack Mountains in particular represent an extreme condition in the earth’s crust during which temperatures near the solidus of many common rock types were maintained over a long period of time. Study of the geochemistry ofgranulites must invariably address the impact ofsuch an event on chemical and isotopic dis~bution, a central question being whether or not the ge~hernist~ at the hand-sample scale (tens of centimeters) has been sub- stantially changed.

Regional Geology

The Adirondack Highlands are dominated by granulite facies massif-type anorthosite-granite complexes (the AMCG suite, following MCLELLAND, 1991) surrounded by lesser amounts of gabbroic to granitic orthogneiss and Proterozoic metasediments of the Grenville Supergroup (Fig. la). Mem- bers of the AMCG suite intruded the central and southern Highlands between Ii.57 and 1125 Ma ( MCLELLAND et al., 1988; CHIARENZELLI and MCLELLAND, 1993). The largest of these is the 2500 km2 Marcy anorthosite in the central Highlands, with the monzonitic to granitic Tupper and Sar- anac sheets on its western and northern margins, respectively (Fig. lb,c). Anorthosite and related gabbroic rocks are interpreted as differentiates of a mantle-derived melt, while monzonitic to granitic rocks are considered to be remobilized lower crust which underwent anatexis during anorthosite genesis or intrusion (EMSLIE, 1978; ASHWAL and SEIFERT, 1980: MORSE, 1982; DUCHESNE, 1983).

5525

I . 5km ’ 8 Hornblende &anile

FK;. 1, (a) Generalized geologic map of the Adirondack Mountains. NY. Moditied after Mc L.r I I ZUD et al. ( 198X i, Boxes mark areas enlarged in figures I b to Id. Sample localities of rocks of the Saranac sheet ( h ). the Tupper sheet (c). and the Snowy Mountain dome ((I). Sample localities of assorted other samples arc shown err la. Sample lahcls corresponding to numbers can be found in Table 1.

Model Sm/Nd ages of 1.35- I.50 Ga for the granitic mem-

bers of the AMCG suite, as well as younger granitic gneisses of the Highlands, are similar to ages of older tonalites. which suggests that monzonitic to granitic rocks may have resulted from a significant component of crustal recycling with the tonalites themselves as a plausible source (DAI )’ and M~LEI.L.AND, I99 I ). Approximately contemporaneous with AMCG magmatism was intrusion of a variety of hornblende and orthopyroxene bearing granites throughout the Adiron- dack Highlands ( MCLELLAND et al., 1988; MCLELL?\ND and CHIARENZELLI, 1989 ). Gabbros from the AMCG suite are often seen in mutually crosscutting relations with anorthosite. and are considered to represent remobilized cumulates ofthe

anorthosite-producing magma. or cogenetic but noncomag-

matic mantle derived melts (EMSIX, 1978: BOIII..IW et al.,

1992). The last major episode of magmatic activity recognized in

the Adirondacks was the intrusion of a series of younger granites ( BLJDDIN(;TON, 1939). These include fayalite granites, hornblende granites, highly leucocratic hornblende and biotite-bearing granitic rocks and magnetite-bearing alaskites. These units crystallized between 1 I IO Ma and the peak of granulite metamorphism at ca. 1050 Ma ( M<.LEK.LAND et al., 1988; MCLL:I.I.AND. 199 I ). Relatively little is known about the magmatic history of these rocks, and the plutonic origin and synmetamo~hic formation ages of some of these

Oxygen isotope com~sition of granite gneiss, Adirondack ~oun~ins 5527

units are still debated (e.g., WHITNEY and OLMSTEAD, 1988; CHIARENZELLI and MCLELLAND, 1991).

The Adirondack Highlands experienced several periods of metamorphism, the most significant of which were an early contact metamorphism of metasediments associated with the intrusion of the Marcy anorthosite massif ( 1150- 1130 Ma) (VALLEY and ESSENE, 1980; VALLEY and O’NEIL, 1982; VALLEY, 1985; MCLELLAND and HUSAIN, 1986; MORRISON and VALLEY, 1988a; MCLEL~ND and CHIARENZELLI, 1989; VALLEY et al., 1990), and a later regional granulite facies event which reached its peak (675800°C 7-8 kb; BOHLEN et al., I985 ) 1080- 1050 Ma; 50-80 Ma after the last date of emplacement of AMCG intrusives (SILVER, 1969; MCLEL- LAND et al., 1988).

Evidence from phase equilibria has demonstrated that granulite facies metamorphism in the Adirondacks occurred under heterogeneous fluid conditions, in cases fluid-absent (VALLEY et al., 1990). In addition, sharp stable isotopic gradients between adjacent lithologies are prohibitive of large fluid fluxes during high temperature metamorphism (VALLEY and O'NEIL, 1984; CARTWRIGHT and VALLEY, 199 1). This is in contrast to theories of granulite metamorphism involving dehydration by regional scale flooding of the middle to lower crust by a CO* rich fluid (NEWTON et al., 1980; NEWTON and HANSEN, 1983; TOURET, 1985).

Previous stable isotopic studies of orthogneisses interpreted elevated 6 I80 values as indicators of pervasive fluid infiltration (TAYLOR, 1969). TAYLOR f 1969) discovered the elevated S’*O values in Adirondack anorthosite, and concluded that they were the result of regional scale, synmetamo~hic exchange through a pervasive fluid with higher 6 “0 metasediments. Values of 6 “0 of approximately + 10% found in syenitic rocks around the Marcy anorthosite (corresponding to the monzonite to granite members of the AMCG suite of this study) were also attributed to isotopic exchange with a pervasive metamorphic fluid.

Considering evidence against a pervasive fluid phase from studies of the metamo~hism, and the low d I80 values found at anorthosite contacts, MORRISON and VALLEY (1988a) proposed that high 6 “0 anorthosite is the result of an anom- alous source region or crustal assimilation, the latter hypoth- esis being supported by strontium isotope data from the Marcy Massif (ASHWAL and WOODEN, 1983). Therefore, one of the objectives of this study is the reevaluation of the influence of fluids and granulite facies metamorphism on the stable isotopic signatures of granitic orthogneisses.

Several recent petrologic, fluid inclusion, and stable isotope studies have identified a widespread fluid infiltration event after the peak ofgranuiite facies metamo~hism (MORRISON and VALLEY, 1988b, 1991; LAMBED al,, 1987, 1991; VALLEY and GRAHAM, 199 I. 1993 ). Retrograde fluid infiltration occurred at greenschist or subgreenschist facies conditions, at low fluid/rock ratios, and had little if any effect on the stable isotope characteristics of most meta-igneous rocks of the Marcy massif ( MORRISON and VAUEY, 1988a, 1988b).

SAMPLING

Samples of orthogneiss were collected from assorted gabbroic, SYenitic, and granitic gneisses throughout the central Adirondack Mtns. (Fig. la), focusing on the Tupper and Saranac sheets on the

northwest margin of the Marcy anorthosite (Fig. Ib,c). A traverse of the Snowy Mountain dome was also conducted (Id). Samples chosen for analysis were classified into one of twelve rock types based on the mineralogy of thin sections and stained slabs, and published maps (summarized from BUDDINGTON, 1939; ISACHSEN and FISHER, 1970). The compositional subdivisions of the AMCG suite used in this study are monzonite, monzo-syenite, syenite, quartz syenite, granite, anorthosite, related gabbroic gneiss, and hybrid granitic gneiss (samples of the monzonite to granite series that contain a significant fraction of andesine plagioclase megacrysts: an indication of mixing with ano~hositic magma). The com~sitional names are based on the standard IUGS classification of plutonic rocks ( STRECKEISEN, 1973). Samples not clearly part of anorthosite-granite AMCG complexes were subdivided into fayalite granite, leucocratic gneiss (in- cluding the alaskites of CHIARENZELLI and MCLELLAND, 1991), hornblende granite, and pyroxene granite gneiss (compositionally and texturally similar to many members of the AMCG suite, but distinct in field occurrence: syenite to granite in composition).

ANALYTICAL TECHNIQUES

Portions of each sample (0.5-i .O kg) were trimmed of any visible alteration rinds and crushed. Splits were powdered in an alumina shatterbox for whole rock analysis or sieved to a HI-200 mesh fraction for mineral separation.

All measurements of 6’*0 were made at the University of Wis- consin-Madison stable isotope laboratory. Whole rock and silicate mineral separates ( 10-I 5 mg each) were reacted with BrFS at 650°C for 8-10 hours, after the method of CLAYTON and MAVEDA ( 1963). Fe-Ti oxide mineral separates were reacted with BrFS at 65O”C, but run times were 60 hours to assure quantitative reaction. Extracted Or was converted to COZ with a graphite rod/platinum filament con- verter at -800°C. Collected COz was analyzed on a Finn&m MAT 25 I gas-source mass-spectrometer.

Ninety percent of analyses were duplicated or t~plicated, with a resultant mean deviation of rtO.16%0. NBS-28 standard run concur- rently with samples was found to have a c?‘~O~,,,,,~ value of 9.45 It 0.24%0 ( IO, n = Is), over a period of I8 months.

Splits of whole rock powders were fused into glass beads with a lithium tetrahorate flux and analyzed for major element contents by X-ray fluorescence at Carleton College. The average of analytical standard deviation for internal laboratory granite standard was 1.4% of the total abundance of each element. Loss on ignition values av- eraged 0.23 wt%. These LOI values may be slightly in error due to weight gain by iron oxidation during heating. Stable isotope and wt% SiOz data are given in Table 1. A complete table of major element data from this study can be found in EILER ( 1991).

OXYGEN ISOTOPES

Whole Rocks

The 6”O values were determined for seventy whole rock samples of orthogneisses, principally from the AMCG complexes of the Tupper and Saranac sheets adjacent to the Marcy anorthosite and the Snowy Mountain Dome. Additional samples included a survey of assorted gabbroic rocks from the Oregon dome and broadly granitic rocks for which there is no evidence of a genetic relation to the AMCG suite (Figs. 2a,b; 3a,b; 4; Table 1) .

Orthogneisses with no established igneous relationship to the AMCG suite exhibit a broad range of 6 r80w, values, both within and between protolith rock types (Fig. 2a). These data reveal large differences in cS’~O, values between samples with similar bulk compositions. Particularly striking are the large range found in hornblende granites (3.4%0) and the difference between the fayalite granites and undifferentiated leucocratic granite gneisses (3.2L), units with comparable abundances of major minerals.

I-Aft! I’ I. I.ithology. oxygen isot~pc data. and SiQ i wt"i ) mlucs for- samplt~s from the central Adirondack Mountains. Identification numbers r&r to samplr locations on maps la-d. Rock type abhrcvi~tions art‘: P‘cCirCin -. unditlcrcntiatetl p\;roxene granitiv gneiss. flbCirC;n y: hornblende granilic gneiss. F:lyC;r I$alitc granite. LeucC;rl;n ~~u~(~~r~n~t~~ gneiss: AMCCi suite rocks are suhdix idcd inIt>

mr imowonite), ms (monro-svcnitei. s?/ I svmite). qs lquartr svenitc 1. gr (grzmite). hy ( hybrid), an (xwrthositc 1. and C;h(;n gahhroicgnciss. Add&on;ll abbreviations arc: Qtr (quart/). Fsp (feldspar). IIh (hornhlendc). I’xn (p~rcrwncl. .md hlt (magnetite), Wr whole rocki;.

Sample RockType Map# 6’s0,,.r si02 bvt%) Oregon Dome Meta-Gabhro

AD89-33 GhGll AD89-34 GbGn RD89-35 GhGn

Assorted Orthognew

AD8Y- I PxGrCn AD89-28 PXGrCrtl

AD89-29 PxGrGn

IL90-6 P,XGffin TL90- I3 HbCrGn TL90.14 HbGrGn TL90- 15 HbGrGn TL90- 16 rrbGrGn AD89- IS FayGr AD89-26 FayGr WKYO-la FayGr

WK9O- I h FayGr

AD89-22 LeucGrCn AD89.24 I>eucCrGn AD89-25 LeucCrGn AD89-2-f LeucGrGn

I

2

i

4

5

6

7 8 9 IO II I2 13 14 IS

I6 17 IX 19

20 21 22 23 24 25 26 27 78 29 30

31 32 33 34

74

71 1.5

Y.3 9.2 x.1 9.4 8.7 lh 11.0 9.Y 6.5 8.8 8.9 Y.0 L I.7 12.1 11.6 IO.6

9.7 9.2 IO.3 IO.0 9.6 Y4 9.4 9.2 8.7 X.8 9.5

9.2 10.0 8.9 10.2

Fsp: 8.5; Rb: 6.7 Qtz: 9.7: Fsp: 9 5

Qtz: 10.2; Fsp: Y.8

Snowy Mountain Dome IL90-7 AMCG &r) LL90-8 AMCG (45)

IL90-9a AMCG (4s) lL90-9b AMCG iqs)

IL90- IOa AMCG (hy) IL90-IOh AMCG (hy)

IL90- I I AMCG (hy) IL90- 12 AMCG (hyi

fL90- 13a AMCG (ill)) lL90- 1% AMCG ( im) IL9O- I4 AMCG (gr)

60.4 60.1 69.6 61.5 57.0 54.3 55.4 56. I 53.0 51.7 61.1

Saranac Sheet AD894 AMCG (sy) AD89-5 AMCG (ysf AD89-6 AMCG (mz) ADX9-7 AMCG (gr)

55.1

66.1 62.9 66.6

The range of 6’*0 values found for rocks of the AMCG

suite (+8.0 to + 10.9%) is in agreement with the findings of TAYLOR ( 1969) and MORRISON and VALLEY ( 1988a), con- firming that all members of the Adirondack anorthosite- granite suite have 6 I80 values above those of primary mantle-

derived melts (Fig. 2b). However. the more focused sampling of this study reveals a distinct pattern to these elevated 6 ‘“0 values. The subdivision of these samples by rock type (see Sampling) reveals a strong correlation between bulk composition and S “Ow, in the monzonitic to granitic sheets surrounding anorthosite. Averages for each rock type increase steadily from +8.7 in monzonites. to f9.1 in mon~osyenites~ +9.3 in syenites, +9.6 in quartz syenites. and + 10.1 in granites (Fig. 2b). Where anorthosite and hybrid granitic rocks have

been sampled. hybrid rocks have b”O,, values that are jn- termediate between those of adjacent granitic rocks and anorthosite, consistent with mixing between anorthosite and the granitic rocks which envelope them (the details of this relationship will be treated below). Oxygen isotopic ratio is, therefore, correlated with composition and varies continu- ously throughout the monzonitic to granitic portions of the AMCG suite.

This relationship can be seen quantitatively in a comparison of 6180,, with major element composition for the monzonitic to granitic members of the AMCG suite. A plot of 6 “Owr vs. SiOa (wt%) (Fig. 3a.b) shows the consistency of the relationship between oxygen isotope ratio and bulk chemical composition. Data for both the Tupper and Saranac

Oxygen isotope composition of granite gneiss, Adirondack Mountains 5529

TABLE 1. (Continued)

Sample RackType Map# t?O,,, sia, (MB) B1nO,,,-,n

ADa9-a Ah4CG w 35 10.1 63.8

AD89-9 AMCG k-N 36 9.3 63.7

AD89-10 AMCG W 37 10.4 67.1

ADa9-11 AMCG (ms) 38 9.1 54.8

AD89-12 AMCG (sy) 39 9.5 66.2

AD89-13 AMCG (qs) 40 9.6 62.3

AD89-14 AMCG (mz) 41 9.0 49.2

WI-3a AMCG (sy) 42 9.3 59.6

S90-3b AMCG (sy) 43 9.5 61.1

s90-5 AMCG (mz) 44 8.8 45.8

s90-7 AMCG (ar) 45 10.2 67.9

No-9 AMCG (mz) 46 8.8 49.2

s90-10 AMcG (sy) 47 9.6 52.7

s90-11 AMCG (ms) 48 9.3 53.3

s90-13 Ah,fCG (a0 49 10.6 69.5

WO-14 AMCG (ms) 50 9.5 55.1

s90-1.5 AMCG (ms) 51 9.0 60.6

Tupper Sheet AD89-2 TL90-1

TL90-2a TL90-2b TL90-3a TL90-3b

TL90-4 TL90-5

TL90-6a TL90-6b TL90-6c TL90-7

TL90-8a TL90-8b TL90-9a TL90-9b TL90-10 TL90- 11 TL90-17

AMCG (mz) 52 9.0

AMCG (an) 53 9.3

AMCG (ar) 54 10.3

AMCG (a0 55 9.6

AMCG (qs) 56 9.9

AMCG (sy) 57 9.1

AMCG (ar) 58 9.9

AMCG (ar) 59 10.1

AMCG (ar) 60 10.0

AMCG (ar) 61 9.9

AMCG (sy) 62 8.7

AMCG (sy) 63 9.2

AMCG (qs) 64 9.5

MCG (ar) 65 10.6

AMCG (sy) 66 9.7

AMCG (a0 67 10.0 AMCG (ms) 68 a.7 AMCG (mz) 69 8.0 AMCG (qs) 70 8.8

62.2 Fsp: 9.4; Pxn: 7.1

70.9 62.7 65.9 66.1 70.2 69.1 78.4 64.2 63.4 61.4 63.0 70.2 62.5

62.5 54.3 62.0

Qtz: 10.4; Hb: 8.0

Fsp: 10.4; Hb: 6.4

Qtz: 10.3; Fsp: 10.3

Fsp: 10.3

Fsp: 10.8; Mt: 3.6

Qtz: 11.0; Fsp: 9.2; Mt: 1.6

Qtz: 10.6; Fsp: 9.1; Mt: 1.6

Qtz: 10.0; Fsp: 9.4; Mt: 1.9

Qtz: 10.5; Fsp: 9.8; Mt: 1.3 Qtz: 10.4; Fsp: 9.0; Mt: 1 .O Fsp: 8.9; Mt: 1.9

sheets show strong correlations, with 6 180W, higher in more evolved, SiO+ch members of the suite. Correlations within each continuous sheet are better than for the entire suite as a whole, and the more complete dataset for the Saranac sheet suggest either a steepening of the 6 ‘80,,-Si02 trend with increasing SiOZ, or a break in slope at approximately 60 wt%.

These trends are comparable to trends of major and trace element covariations seen in previous studies of these and related rocks in the Adirondack highlands (BUDDINGTON,

1972; MCLELLAND, 1991; WHITNEY, 1991), in which changes in FeO, MgO, Ti02, CaO, K20, and compatible/ incompatible trace element ratios have been shown to be both correlated with Si02 and permissive of crystal fractionation as a major control of differentiation. Major element analyses of samples analyzed in this study confirm the ob- servations of these previous studies (available in EILER, 199 1 ), indicating that the increases in 6 IBOW, seen in this study are part of a general trend of increasing differentiation with Si02 in the AMCG suite.

In comparison to AMCG rocks, the bulk composition of assorted other granitic rocks is a very poor predictor of 6 ‘*O,,,, (Fig. 2a). Leucocratic and fayalite granites make up the most felsic rocks collected in this study, and are fairly uniform in

.Non-AMCG orll10gnf4ur0 (4

MonzonIta

MOllZoSyenItO Syanlta

Quatlz-SyanIta GmnIta

Hybf’d AnotioaIta

Gabbrolc gnelr

FIG. 2. Relationship between oxygen isotopic ratios ofwhole rocks and rock type in: (a) granitoids from the central Adirondacks not belonging to the AMCG suite (open circles), and (b) members of the AMCG suite (filled circles). Additional analyses of the northwest portion of the Marcy massif anorthosite are shown as the 1 o range of fourteen values from MORRISON and VALLEY, 1988a (9.3 + 0.2).

J. M. Eiler and J. W. Valley

I 3a: Saranac Sheet

11

3b: Tupper Sheet

a

11

IO

UP -m

9

8 ‘r

45

80

W

:: Snowy Mountain Dome

l

l

2 0 00

e

l

1 l Granitoids

FK. 3. Plot of whole rock oxygen isotopic ratios vs. whole rock SO2 (wt%) for the differentiated monzonite-granite rocks of the AMCG suite: (a) from the Saranac sheet north of the Marcy massif (Fig. 1 b), (b) from the Tupper sheet north-west of the Marcy massif (Fig. Ic), and (c)from the Snowy Mountain Dome (Fig. Id). Lines indicate inferred lines of isotopic evolution due to magmatic processes (see discussion). In Fig. 3a and 3b these trends are thought to represent I80 enrichment due to AFC processes, Lines in Fig. 3c represent mixing between granitic and anorthositic members of the AMCG suite to form hybrid rocks.

their high alkali feldspar and quartz content, yet they exhibit a range of 6 r80w, values that define the highest and lowest values in this study (+6.5 to +I 2. I %I). The hornblende granites of the central Highlands show a similarly wide range of b “Ow, values within a single Ethology of mesocratic quartz- feldspar-hornblende-biotite gneiss. Of nonAMCG rocks, only

the gabbroic and pyroxene granite gneiss units have consistent

d”O,, values within individual units of fairly homogeneous

bulk composition. The correlation of variations in 6’“0,,

with bulk composition is, therefore. restricted to the mcta- morphosed comagmatic suite of orthogneiss.

Mineral Separates

Analyses of 6”O for mineral separates have been made

from representative samples of the whole rock specimens

f Fig. 4). These data show several consistent features over the

range of rock types sampled: quartz (Qtz}-magnetite (Mt)

and feldspar (Fsp)-magnetite separates exhibit fmctionations of 9.02 + 0.46 ( n = 5 ) and 7.49 i 0.32 ( n = 7 ), respectively; Qtz-Fsp fractionations are less regular, averaging 0.83 C 0.64

(n = 8); pyroxene (Pxn) and hornblende (Hb) separates record broadly high-temperature fractionations with quartz

and feldspar: and no disequilibrium isotopic reversals are seen in any of the samples, The Qtz-Mt and Fsp-Mt frac-

tionations seen in this study are typical of values found in high-grade regional metamorphic rocks (e.g., WILSON et al.. 1970: SHARPet at.. 1988: Sttt~ti. 1985: SHIEH and SCHWAR(‘%.

1974; Swttr et al., 1976: GAKLK’K and EPSTEIN, 1967 ). These averages contrast somewhat with those found within similar rock types in the Diana complex in the western Adirondacks. CAR I’wRIUI’I et at. ( 1993) found fractionations comparable to those found in this study in rocks which had been sheared

during retrogression, but those same samples had much more scattered fractionations between coexisting phases than in the present study. Unsheared samples analyzed by CAKl- wRlC;trl et al. ( 1993) showed more regular fmctionations, but much higher temperature magnetite-silicate fractionations (near peak-metamorphic conditions).

When experimental calibrations are applied to the data from this study. it is apparent that the fractionations, though consistent, are partially reset from the peak metamorphic tem~rat~lre of appr(~ximately 750°C in the Adirondack Highlands (BU~LI:N et al.. 1985). Application of the CLA\,-

TON et al. ( 1989 f and CHINA et al. f 1989) fractionations yields

FIG. 4. Oxygen isotopic ratios of mineral separates from representative samples of orthogneiss. along with corresponding whole rock values. The equilibrium distribution between coexisting minerals at 750°C (the average peak metamorphic temperature) is shown in the side box. This distribution makes use ofthe calibrations Of CLAYTON et al. ( 1989), CHIBA et al. ( 1989). and BOTTINGA and JnVOv ( 1975; for Hb compositions only).


Qtz-Mt temperatures of 56 I + 2 1 “C, and similar Fsp-Mt temperatures of 558 f 18°C; both 200°C below the peak metamorphic conditions experienced by these rocks.

Fractionations among the various pairs in the Qtz-Fsp- Hb-Pxn subsystem record generally high temperature fractionations, averaging somewhat above the peak metamorphic temperatures of 750°C (see reference fractionations in Fig. 4); in marked contrast with the silicate-magnetite thermometry. Mineral separate data seem to reflect consistent, mineral- specific resetting from peak metamorphic distributions of I80 between coexisting phases.

Fractionations are independent of absolute 6 “O,.,, values of the constituent minerals. This is most clear in a 6-6 plot of coexisting quartz, feldspar, and magnetite (Fig. 5). Vari- ations in the oxygen isotope ratios of each of those minerals span ranges greater than 1 %o, yet most of the fractionations are near the lines of slope I on this figure that mark the 550°C isotherms.

DISCUSSION

The principle observation made in this study is that within comagmatic, differentiated meta-igneous units, 6 ‘8O,.,, cor- relates with major element chemical composition, whereas among unrelated rocks there is no such relationship. At least three events in the history of these rocks may have produced or affected whole rock isotopic trends. From the most recent to oldest events, these are: retrograde exchange or fluid infiltration, granulite facies metamorphism, and premetamorphic igneous processes. In some parts of the central Adi- rondacks the premetamorphic processes have been shown to include contact metamorphism and exchange with hydro- thermal fluids (VALLEY and O’NEIL, 1982; MORRISON and

VALLEY, 1988a).

Retrogression

Stable isotopic fractionations between coexisting minerals provide useful tests for the preservation of igneous or metamorphic isotopic ratios or for the magnitude and mechanisms of retrograde alteration ( DEINES, 1977; GREGORY and CRISS,

550X isotherm

FIG. 5. Plot of oxygen isotope ratios of coexisting quartz and magnetite (filled circles) and feldspar and magnetite (unfilled circles). The solid lines represent 550°C using the calibration Of CHlBA et al., 1989.

TABLE 2. Input for and results of calculations of diffusion-controlled retrogression in granitic gneiss. Modal abundances and grain sizes are representative of values measured in granitic members of the AMCG suite (see EI- LER, I99 I ). Diffusion coefficients used were, for quartz: DENNIS ( 1984), for the feldspars: GILETTI et al. ( 1978), for pyroxene: FARVER ( 1990). for amphibole: FARVER and GILETTI (1985), and for magnetite: SHARP (1991). Fractionation factors were taken from CHIBA et al. (1989), CLAYTON et al. (1989), and BOTUNGA and JAVOY ( 1975: for amphibole only). The cooling rate was taken to average 2”C/Ma, with a cooling function such that I / T (“K) increased linearly with time ( MEZGER et al., 1990).

Input for diffusmn modehng

Mineral Abundance Length/Diameter (vol. %) (mm)

Quartz 10 0.6 K-feldspar 30 1.0

Plagioclase (An25) 35 1.0 Hornblende 11 1.0

Pymxene 11 0.6 Magnetite 3 0.4

Comparison of thermometry and modeling

Mineral Pair Qtz-Kspar

Observed Predicted 0.4

Qtz-Plag 1.4 Qtz-Bulk Fsp 0.8M.6 0.9

Qtz-Mt 9.0i0.s 8.6 Fsp-Mt 7.6i0.3 7.7

Fsp-Pxn 2.3 2.1 QtZ-Hb 2.4 3.9 Fsp-Hb 2.8 3.0

1986). In particular, modeling of the results of diffusive re- equilibration of coexisting minerals during cooling provides a prediction of the endmember case of retrogression as a closed-system with respect to stable isotopic ratios ( D~DSON, 1973; GILETTI, 1986; EILER et al., 1992, 1993). We have used the Fast Grain Boundary model for interdiffusion ( EILER

et al., 1992) to calculate the expected pattern of stable isotope fractionations in the granitic rock types studied, based on existing calibrations of the diffusion coefficients and fractionation factors for the minerals of interest, and the cooling rate constrained by geochronology ( MEZGER et al., 1990). The modal abundances and grain sizes for the rock modeled are representative of measured values for the granitic rock types that make up the bulk of the samples ( EILER, 199 1). Table 2 presents the input for and results of this calculation, respectively.

The expected pattern for closed-system retrogression (magnetite-silicate temperatures near 55O”C, minor resetting of other fractionations) is in good agreement with the general results of stable isotope thermometry (above; Fig. 4), and is consistent with the interpretation that any retrograde alteration in these rocks had little effect on bulk-rock stable isotope ratios. Only one fractionation, AQfz_Hb, differs significantly from the model prediction, and this represents only one measurement. Where multiple measurements have been made, observed fractionations vary from sample to sample

5532 J. M. Eiler and J. W. Valley

by more than analytical uncertainty, but a variety of factors

make this expected. Quartz-magnetite and feldspar-magnetite fractionations are very temperature sensitive, enhancing the

effect of small variations in grain size and modal abundance.

The scatter in quartz-feldspar fractionations is attributable to either or both of two factors: calculations show that closed-

system retrogression will fractionate oxygen isotopes between

potassium feldspar and plagioclase on the order of I%,

(Table 2). No attempt was made to separate the feldspars

when bulk separates were picked by hand, and the observed

variations m Aou.,, may reflect a variable ratio of potassium-

and plagioclase-feldspar in the separates. Variations may also reflect minor open-system alteration, most likely of feldspars.

Nevertheless, there has been no systematic shift away from the average value expected from diffusion modeling, indi-

cating that alteration has not seriously biased the d 180w1 val-

ues. The trends seen in Fig. 5 are also commonly interpreted as representing equilibrium or closed-system retrogression, as opposed to the vertical or horizontal arrays typical of fluid-

hosted alteration. This conclusion is in agreement with the

results of previous studies of the alteration history and stable isotope geochemistry of Adirondack anorthosites. Previous studies have found large variations in the 6 “0 of secondary

carbonates in anorthosites with no significant variation in host-rock 6 “0, indicating a lack of equilibrium between primary and secondary minerals and suggesting that alteration

occurred with little fluid-rock isotopic exchange ( M~KRIS~N and V.&L.I_EY, 1988b. 1991).

The pattern of fractionations interpreted to have been formed from closed-system retrogression in this study differs from that found in similar rocks in the Diana complex in the western Adirondacks also interpreted as having closed-

system retrograde histories ( CARTWRKH I et al.. 1993 ). The reasons for these differences are not immediately clear. If due to variations in the cooling rate, those differences would have to be very large (several orders of magnitude in dT/dt ). As also noted above, the pattern of resetting seen in this study is very common in high-grade metamorphic rocks, and we

propose the differences may lie in some unsuspected aspect of the history of the Diana complex.

Synmetamorphic Fluid Infiltration

Whatever process led to high 6’“0,, values within the granitic suites surrounding anorthosite, it must be consistent with the correlation between major element composition and 6 ‘*O,,,, (Fig. 3). and the lack of such a correlation between unrelated units (Fig. 2a). The regular compositional corre-

lation seen for almost fifty samples (Figs. 2b, 3) requires a homogeneous process by which 6 ‘“0 values became elevated over at least the 3000 km* region of the Tupper and Saranac sheets. Such a homogeneous process is also suggested by the nearly constant 6 I80 values from adjacent anorthosite of the

northwestern Marcy massif (MORRISON and VALLEY, 1988a). As discussed in MORRISON and VALLEY ( 1988a) and VALLEY et al. ( 1990) for the Marcy massif, if the high 6 I80 values in the Tupper and Saranac sheets were caused by peak-metamorphic fluid infiltration, the oxygen isotopic data require that this infiltration occurred pervasively over a regional extent.

The whole rock data from hornblende granites, fayalite

granites, and other leucogranitic gneisses indicate that such an event cannot have occurred (Fig. 2a). As described above, these data preserve a 6%0 range of 6 ‘*O,.,, for samples of similar

bulk composition. Such variations could hypothetically be attributed to channellization of fluids if some property of

each rock unit controlled the isotopic composition of infil-

trating fluids or the degree of exchange. Given that the various

units being discussed are petrographically very similar (all

are quartzo-feldspathic orthogneisses) such a process would

require the most fortuitous of circumstances to produce the regular variations of 6”O with bulk composition in some

rocks (Figs. 2b, 3 ), accompanied by large and irregular differences in others (Fig. ?a).

Additional evidence against regional equilibration through

a pervasive fluid is found in the analyses of mineral separates, If pervasive fluids circulated through these rocks at or near peak metamorphic temperatures, causing the “0 enrich-

ments, then filxO values of minerals (particularly quartz. which is resistant to retrograde exchange under anhydrous conditions; DENNIS, 1984) should be equilibrated and 6 “O,,

should be a simple function of sample mineralogy. Analysis

of mineral separates (Table I, Figs. 4, 5) shows that this is not the case. Ranges of 6 “0 seen in minerals analyzed live or more times are: quartz: 9.7-l 1 .O%O; feldspar (nearly all

An 20-30): 8.5-10.3%~; and magnetite: I.O-3.6%-/oo. These ranges are only slightly more restricted than the range of whole rock values in the AMCG suite (8.0-10.6%0), and only for magnetite can a substantial portion of the range in 6’“O values be attributed to closed-system retrogression.

We conclude. therefore, that 6’“0,, values seen in Adi- rondack orthogneiss preserve pregranulite facies values. just as large premetamorphic gradients are preserved in skarns elsewhere in the Adirondacks (VALLEY and O’N~IL, 1982). The dependence of 6 ‘*Our values on major element chemistry that is seen in rocks of the AMCG suite seem. therefore. to

be the result of magmatic processes.

Premetamorphic History

The foregoing discussion suggests that the patterns of fi “0 values seen in Adirondack orthogneisses are best explained as being derived from premetamorphic magmatic processes. Due to the range of d”O,, values seen (+6.5-l2.1%0) and the pronounced trends of fi “0 vs. composition. this conclusion restricts theories about the sources and magmatic cvo- lution of Adirondack granitic orthogneisses.

Oxygen isotope systematics discriminate between members of the AMCG suite and other units. The classification of

members of various lithologic units has varied somewhat over time. with some authors having included the leucogranitic gneisses and fayalite granites with the AMCG suite. Stable isotopic results from this study suggest that these are unrelated to one another, given their distinct 6 “0 values. This difference is confirmed by evidence from geochronologic studies of these units. U-Pb geochronology of zircons in all three units in- dicates that nearly 100 Ma. separate the younger fayalite granites and the alaskites from the older AMCG rocks (MCLELLAND et al., 1988; CHIARENZELLI and MCLELL.AND, I99 1: MC LEI.I.AND. 199 I ). The stable isotope data support

Oxygen isotope composition of granite gneiss. Adirondack Mountains 5533

the conclusion that the rock types are unrelated and are a

useful petrogenetic marker for subtle discrimination among

orthogneisses. The 6%0 range in 15’~0,, values found in the fayalite, horn-

blende, and leucogranitic gneisses suggests a diversity of

source material for evolved granites. The relatively low 6 I80 values of fayalite granites (+6.5 to +8.9%0; Fig. 2) are con-

sistent with the A-type setting generally associated with flu-

orite bearing fayalite granites. Leucogranitic samples of the

northern and western Adirondacks provide the most consis-

tently high d I80 material identified in this study ( + 10.6 to

+ 12.1 %o; Fig. 2). The origin of some members of this group

is in question. The chemistry of some alaskites belonging to this group has been interpreted as suggesting a protohth that

experienced extensive surface alteration before metamorphism (WHITNEY and OLMSTEAD, 1988), while the syn-

metamorphic ages of igneous zircons from these rocks have been taken to indicate that they are synmetamorphic granites

( CHIARENZELLI and MCLELLAND, 199 1). High 6 ‘*Ow, values are consistent with these rocks either being metasediments

or partial melts of metasediments, and, therefore, do not

clearly discriminate between the existing hypotheses. The contrast with lower 6’*0 fayalite granites and some hornblende granites does, however, argue for a protohth source

that differs from that of some of the other 1100-1050 Ma

old granites. Hornblende granites collected from the vicinity of the

Tupper sheet in the central Highlands show both low and high 6 ‘*Ow, values (+7.6 to + I 1 .O%O; Fig. 2). These rocks may record isotopic variability of crustal sources. However, detailed geochemical study of these rocks has not been done, and the possibility must be entertained that, as with the alaskites, the isotopic signature may be due to a metasedimentary origin. Values of 6 ‘*Ow, for the pyroxene granite gneisses of the southern Adirondacks cannot conclusively discriminate them from or align them with the monzonitic to granitic sheets. However, until further study is made, data from other rock units demonstrates that caution is required in inferring

comagmatic relationships among meta-igneous rocks.

Estimates of source 6 “0 values for the monzonite to granite rocks of the AMCG suite are accessible through consid- eration of 6 ‘80,,composition trends. The most mafic samples of the Saranac and Tupper sheets converge toward 6 “0 values of +8.0 to +8.8%0 (Fig. 3). This is consistent with derivation of initial monzonitic liquids from relatively low 6 “0 material,

such as older tonalitic intrusives already proposed as a plausible source on the basis of Nd-isotope data (DALY and MCLELLAND, 199 1). This range of 6 I80 values is consistent with the I-type source previously proposed for this suite ( BASU and PETTINGILL, 1983; MCLELLAND, 1991). Higher 6’*0, evolved members of this suite could have been produced either through mixing with melts of metasedimentary rocks or by direct assimilation of metasediments into which these sheets intruded. Previous studies of major and trace element variations in the AMCG suite have led several authors to conclude that the monzonitic to granitic members of this suite are related by crystal fractionation ( BUDDINGTON, 1972; MCLELLAND, 1991; WHITNEY, 1991). Because crystal fractionation alone is not capable of producing significant shifts in 6’*0 in syenitic rocks over most of their crystallization

history (TAYLOR, 1980; KALAMARIDES, 1986), elevation of

6 ‘*O with magmatic evolution may indicate that this differ-

entiation history included a significant component of assim-

ilation, and that the magmatic evolution of this unit may be better described as an assimilation/fractional crystallization

( AFC) process. A similar conclusion was reached in studies

of granitic rocks related to the Rogaland anorthosite that

have 6 “0 values higher than those of mantle derived material

(DEMAIFFE and JAVOY, 1980). Xenoliths within the Adi-

rondack AMCG suite indicate that assimilation at the current

level of exposure did take place, at least to some degree

(BUDDINGTON, 1939; MORRISON and VALLEY, 1988a;

MCLELLAND, 199 1). The 6’*0 values of igneous zircons from

granitoids of this unit are relatively homogenous at +8.1

f 0.4%0 throughout the Adirondack Mountains (VALLEY et al.. 1994). but are more scattered in rocks from other suites.

This supports the results of this study indicating differences

between the various meta-igneous units. It further suggests

that the parental magmas of granitoids from this suite were homogenous in 6 “0. The assimilation event that led to large

variations in 6 “0 values seen in this study was, therefore, a local phenomenon occurring after zircon crystallization in

the primary magmas. This result is corroborated by the ob-

servation from this study that d ‘*O-wt% SiOz trends are more tightly restricted within individual sheets than in the entire

suite. Textural evidence for local magma-mixing at the borders

between anorthosite and monzonite/granite sheets is found

in andesine megacrysts (identical to those found in anorthosite) within the monzonitic border rocks ( DEWAARD and ROMEY, 1969a,b). A sampling traverse was made across the Snowy Mountain Dome in order to characterize the stable isotopic effects of this mixing. Values of 6180,, in hybrid granitic rocks (IL90- 1 Oa to 12) are found to lie between those of adjacent anorthosite and granite, consistent with hybrid

rocks being mechanical mixtures of the two (DAVIS, 1969; REYNOLDS et al., 1969; MCLELLAND, 1991). The samples are listed in Table 1 in order of their spatial relationship (Fig.

Id), showing that the values of d 180,,,, within hybrid granitic gneiss also decrease toward lower 6 “0 values of adjacent anorthosite.

SUMMARY

The oxygen isotopic characteristics of Adirondack anorthosite and surrounding orthogneisses have been a focus of longstanding debate relating to premetamorphic history vs. peak metamorphic fluid environment. This debate is part of the broader question of the scale at which high-grade metamorphism affects geochemistry, and the importance of fluids in granulite facies metamorphism. Granitic orthogneisses provide evidence for the dominance of premetamorphic pro-

cess in producing whole-rock oxygen isotopic ratios. This study illustrates the role of assimilation in the magmatic evolution of granitic sheets related to anorthosite, the wide diversity of source 6 “0 signatures found in orthogneisses, and the preservation of these primary features through deep crustal, granulite facies metamorphism. The preservation of these geochemical features restricts models of granulite facies metamorphism, prohibiting both large amounts of pervasive fluid infiltration and large scale isotopic equilibration during

5534 .I. M. Eiler and J. W. Valley

deep-crustal metamorphism. These results provide a picture of the deep crust as a region of considerable geochemical

heterogeneity, where even long lasting and intense meta-

morphism does not erase the evidence of pre-metamorphic

events.

ilcknowl~dgmenrs-We gratefully acknowledge Shelby Boardman and Cathy Manduca for use and assistance in the X-ray fluorescence laboratory at Carlton College. We also wish to thank Jim McLelland and Lukas Baumgartner for stimulating and helpful discussion; Kevin Baker and Mike Spicuzza for assistance in the stable isotope laboratory, and Clark Johnson for use of specimen preparation facilities and a helpful review of an earlier version of this work. We thank S. Hoernes, U. Lichtenstein. J. Morrison, and Z. Sharp for thorough and helpful reviews. This work was supported by the National Science Foundation (grant EAR89-05 101 and a graduate research fellowship), and a Geological Society of America Penrose research grant.

&iitoriu/ bundling: K. Mezger

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Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the Adirondack...

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