Contrib Mineral Petrol (1985) 89:215-225 Contributions to Mineralogy and Petrology �9 Springer-Verlag 1985

Ternary feldspar thermometry in granulites from the Oaxacan Complex, Mexico C.I. Mora and J.W. Valley Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA

Abstract. Coexisting feldspars from across 2,000 km 2 of the granulite facies Oaxacan Complex, southern Mexico exhibit variable amounts of solid solution from nearly binary ( A b - An and A b - Or) to substantially ternary ( A b - A n - Or). Reintegrated analyses of 21 coarsely exsolved perthite (AF)-plagioclase (PL) pairs yield AF = 0% 0 - 6 a Ab3o - 56An2 - 1 s and PL = Or 1 _ 2AbTo_ 84Anl 1 - 28. These data have been used to test existing two feldspar geother- mometers for this extended composition range.

For all compositions, temperature estimates show rela- tively little spread in value (660 ~ to 795 ~ C, 7 kbar) using the Haselton et al. (1983) calibration (HHHR). These tem- peratures are in fair agreement with estimates of 750_+ 40 ~ C for feldspar pairs with nearly binary compositions using the Stormer (1975) thermometer (STO). However, STO temperatures increase significantly (to 990 ~ C) with increas- ing ternary solid solution in AF, suggesting that thermome- ters derived for binary systems are inaccurate for ternary compositions. Isotherms drawn from HHHR which take into account variable anorthite solution in alkali feldspar show that estimated temperature decreased by 50-100~ C for each 5 mole percent anorthite in alkali feldspar.

Experimentally determined solvus relations (Seek 1971) require feldspars with significant ternary solid solution to have crystallized or to have equilibrated at higher tempera- ture than feldspars with more binary compositions. How- ever, petrographic and field relations of ternary and binary feldspars in the Oaxacan Complex suggest they were all equilibrated at similar metamorphic pressures and tempera- tures and do not support a model where ternary feldspars have preserved higher premetamorphic temperatures. The composition of coexisting feldspars from other Precambrian granulite-facies terranes are also inconsistent with Seck's (1971) results. Hence, thermometers which fit Seck's solvus relations may not yield accurate temperatures in high grade metamorphic terranes. Parallel tie-lines for ternary and bi- nary feldspars in the Oaxacan Complex and the consistency of inferred temperatures (HHHR) for many granulite ter- ranes suggest that estimation of temperature using tie-line slopes rather than solvus width may yield more accurate results for these samples.

Peak metamorphic conditions in the Oaxacan Complex are inferred to have been 730_+ 50 ~ C, 7 + 1 kbar. Pressure estimates from four garnet-plagioclase barometers show good agreement. Results of feldspar thermometry are con-

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sistent with diopside-forsterite equilibria in marbles which restrict T= 720-765 ~ C at P = 7 kbar.

Introduction

Two-feldspar geothermometry (Stormer 1975) has been found to yield estimates of peak metamorphic temperatures for high-grade rocks that are consistent with results from mineral phase equilibria and other geothermometers (Boh- len and Essene 1977; Stormer and Whitney 1977; Dahl 1979; Bohlen et al. 1980, 1985; Stoddard 1980; Perkins et al. 1982). For granitic terranes, this system offers the only useful mineral thermometer for accurate estimation of peak metamorphic temperatures.

A principal assumption of the Stormer thermometer (STO) is that ternary solid solution in feldspars is minor and can be closely approximated by a binary solution model such that ):Ab, ternaryfeldspar=TAb, binaryfeldspar. Successful use of this thermometer, therefore, depends in part on how closely the composition of naturally occurring feldspar pairs ap- proach the double binary system (albite-orthoclase and al- bite-anorthite). This was not a significant problem in most previous studies, which report only minor ternary solid so- lution with XAn, A F or J(O~,PL less than 0.05.

For feldspars in which ternary solid solutions are sub- stantial, feldspar thermometry is more problematic. Graph- ical thermometers derived for binary systems are inaccurate for ternary compositions. Lack of ternary activities and conflicting experimental data on the ternary system (Seck 1971 ; Johannes 1979) make derivation of a ternary thermo- dynamic model difficult. Several thermometers have been proposed which account for ternary solid solution. The PowelI and Powell (1977) thermometer accounts for ternary solution in alkali feldspar, but does not account for ternary solid solution in plagioclase. Brown and Parsons proposed a graphical two-feldspar thermometer (BP) with sharply curved isotherms which terminate on the KD = 1 line at a finite value of XAb. AF<I.0 , reflecting the intersection of the ternary isotherms with the consolute curve. The Ghiorso (1984) model, based on the Seck (1971) data, yields consis- tent temperatures for many volcanic feldspar assemblages, but noticeably high estimates for feldspar pairs in granulite- facies rocks. A formulation by Haselton etal. (1983) (HHHR) combines recent calorimetric data on high plagio- clase (Newton et al. 1980) and alkali feldspar (Haselton

216

et al. 1983), partially accounting for ternary solution in both feldspars. The graphical form of HHHR (their Fig. 8), derived for strictly binary endmembers, results in tempera- ture estimates 0.100~ C higher than estimates calculated using STO. However, their numerical expression (HHHR, Eq. 5) partially accounts for ternary solution and, depend- ing on composition, can give significantly different results than the graphical form. This thermometer has not yet been widely applied to natural systems.

Coexisting feldspars from the granulite facies Oaxacan Complex in southern Mexico exhibit both nearly binary and substantial ternary solid solution. These rocks provide a good opportunity to evaluate existing two feldspar ther- mometers for an extended range of compositions.

The Oaxacan Complex

The Oaxacan Complex is a lithologically diverse, 10,000 km 2, high-grade metamorphic terrane in southern Mexico. It is the largest and southernmost exposure of Grenville-age (1,080_+ 10 my; Anderson and Silver 1971), high-grade metamorphic rock in North America (Fig. 1). There is no petrologic or structural evidence of polymeta- morphism (Kesler and Heath 1970; Ortega-Guti6rrez 1981). Concordant U - P b dates of 975+ 10 my for a post-defor- mational pegmatite give a minimum age for the last signifi- cant penetrative deformation (Anderson and Silver 1971). Rock types exposed include charnockite, sillimanite-garnet gneiss, anorthosite, quartzofeldspathic and biotite-horn- blende banded gneiss, wollastonite- and scapolite-bearing calc-silicate gneiss, and mafic orthogneiss (Bloomfield and Ortega-Guti~rrez.1975; Ortega-Guti~rrez 1984; Mora et al. 1985). The lithologies, metamorphic P - T conditions, and the timing of metamorphism of the Oaxacan Complex are similar to those of Grenville-age metamorphites of S.W. Ontario and the Adirondacks, New York.

A distinctive feature of the complex is an andesine (An45_5o) anorthosite massif with an inferred area of 400 km 2 exposed in the north-central part of the complex (Ortega-Guti6rrez 1981 ; Mora et al. 1985). Common acces- sory minerals are orthopyroxene, ilmenite and magnetite. Premetamorphic intrusion of the massif is suggested by the parallel contact of the body and the overlying mafic gneiss sequence and by the lack of a contact metamorphic aureole (Ortega-Guti~rrez 1981). The better-known Pluma Hidalgo anorthosite (Paulson 1964; Anderson 1969; Heath and Fairbairn 1969) is exposed in the extreme southern end of the complex and differs from the Oaxacan Valley massif by its more alkalic bulk chemistry and abundant rutile.

Mineralogy and textures of two-feldspar rocks

Coexisting feldspars are common in most of the rock types exposed in the study area. Charnockites contain quartz, perthite, plagioclase, and orthopyroxene with variable amounts of clinopyroxene, hornblende, biotite, magnetite, ilmenite, and sphene. Biotite-hornblende banded gneisses contain no pyroxenes but may have minor garnet, apatite and zircon. Magnetite and ilmenite occur as small, discrete, homogeneous grains which have apparently exsolved and reequilibrated after the peak of metamorphism, precluding use of the magnetite-ilmenite geothermometer (Buddington and Lindsley 1964; Spencer and Lindsley 1981). Sample 81-OC-136 (Table 1) is a silicate-rich marble containing cal-

, ; ,jo: ,.oo Fig. 1. Location of the Oaxacan Complex and other exposures of Grenville-age high grade metamorphic rocks : the Llano Uplift and Van Horn, Texas; Chihuahua, Ciudad Victoria, and Hidalgo, M6xico (Fries and Rincdn-Orta 1965; Garrison 1979; Mauger and McDowell 1982; Ortega-Guti~rrez 1978, 1981). The box shows the area studied, shown in more detail in Figure 6

cite and clinopyroxene with minor quartz, perthite, plagio- clase, scapolite, sphene and apatite. The petrography of all samples analyzed here has been previously reported (Mora et al. 1985).

No single mineral assemblage distinguishes rocks con- taining ternary feldspars from those with binary feldspars. Hypersthene and clinopyroxene occur in samples with ter- nary feldspars, but are not restricted to those rocks. Biotite is found more commonly, but not exclusively, in samples with binary feldspars. Quartzofeldspathic gneiss, with only trace amounts of mafic phases, may have either ternary or binary feldspars.

Feldspar textures were closely examined in all samples. Perthitic exsolution is common in slowly cooled, high-grade metamorphic rocks and is sometimes accompanied by diffu- sion of the albite component to crystal boundaries, forming discrete grains. For accurate thermometry, it is important to recognize the scale at which such diffusion has occurred. In the absence of significant grain boundary diffusion, exso- lution lamellae can sometimes be reintegrated to yield peak metamorphic compositions of originally homogeneous al- kali feldspar (Bohlen and Essene 1977). However, if some of the albite component has been lost by diffusion then reintegration will underestimate feldspar miscibility and in- ferred temperatures will be low.

Textural relationships in the Oaxacan samples have been discussed in detail by Mora et al. (1985). For samples reported in this study, alkali feldspars exhibit coarse (10-25 gm), regular, perthitic exsolution lamellae such as shown in Fig. 2. Plagioclase is texturally homogeneous, pre- sumably because of the sluggishness of the tetrahedral A1- Si diffusion that must accompany N a - Ca migration. There is no apparent textural difference between perthites with nearly binary compositions and those with substantial (up to 15 mol%) ternary solid solution in the reintegrated com- position. As will be shown below, reintegrated electron mi- croprobe analyses of feldspars with this texture yields con- sistent, geologically reasonable temperatures. Samples with homogeneous alkali feldspar or perthite exhibiting finer and more sparse exsolution lamellae give systematically low temperatures of < 600 ~ C. Several charnockite samples con-

217

and Gasparrini 1969) or a modified Bence-Albee scheme (Bence and Albee 1968) and a comparison of these results shows no signifi- cant differences. Mineral formulae were normalized about cations. The accuracy of reported analyses is believed to be _+_3% of the amount present for major elements and + 5-10% of the amount present for elements with < 1 weight percent abundance. A 1 cr standard deviation is reported for average analyses.

Fig. 2. Photomicrograph of typical perthitic exsolution texture seen in both binary and ternary alkali feldspar in Oaxacan granulites (81-OC-38, Table 1). Field of view is 1.3 mm across; crossed nicols

tain very coarse (100 gm) irregular intergrowths of plagio- clase and alkali feldspar. These feldspars are similar in ap- pearance to alkali feldspar-plagioclase "symplectite" de- scribed in mesoperthite-bearing syenogabbro (Parsons and Brown 1983) and perthites described in the Scourie Com- plex (Rollinson 1981). It is uncertain if these feldspars ex- solved from a homogeneous phase or result from low tem- perature simultaneous growth (Parsons and Brown 1983). Therefore, feldspars with this texture were not investigated for thermometry.

The correlation of coarsely exsolved perthites and reten- tion of peak metamorphic temperatures was also noted by Bohlen and Essene (1977) in granulite facies rocks from the Adirondacks. However, high-grade rocks with different post-metamorphic histories may show either more or less extensive retrogradation. For example, rapidly quenched granulite facies xenoliths in diatremes (Padovani and Carter 1978) and in lavas (Matty et al. 1982) yield concordant tem- perature estimates using homogeneous feldspars. Valley and Essene (1980) note retrograde resetting of temperatures for texturally homogeneous feldspar pairs in slowly cooled, high-grade marbles.

Analytical Procedure

Quantitative chemical analysis of coexisting feldspars and other selected mineral assemblages was made on the ETEC autoprobe at Rice University and the ARL-SEMQ at the University of Wisconsin-Madison. Samples were first analyzed for 100 s by ener- gy dispersive analysis to detect minor elements (Z>Na) present in abundance >0.2 element weight percent. Six to eight elements were then analyzed by wavelength dispersive analysis (WDA) using an accelerating potential of 15 kV and a sample current of 0.015 ~ta. Counting time for WDA ranged from 10-30 s per spot. Several mineral grains were analyzed in each thin-section. For homoge- neous grains, 5-10 spots were analyzed per grain. There was no apparent zonation in garnet, pyroxene or plagioclase and each phase was homogeneous over the scale of a thin-section. Perthitic feldspar was reintegrated by averaging analyses of 50-150 spots per grain (5 s/spot) using a slightly defocused (20 g) beam, similar to the method of Bohlen and Essene (1977). Comparison of reinte- grated perthite compositions from mutually perpendicular thin- sections (81-OC-149 and -149A, Table 1) indicates that there was no significant orientation bias in these analyses. Well-analyzed nat- ural and synthetic crystalline standards were used. Matrix correc- tions were made using the program EMPADR VII (Rucklidge

Feldspar compositions

Microprobe analyses of coexisting feldspar pairs are listed in Table 1. Reintegrated perthites show a range of composi- tions from XAU ' AF = 0.30--0.56 and XA~ ' AF = 0.02--0.15. Pla- gioclase compositions are less variable with XAb, pL=0.70-- 0.84 and Xof, PL_<_0.03. These alkali feldspars show more extensive ternary solid solution than has been reported in most previous studies on granulite-facies samples (Bohlen and Essene 1977; Stormer and Whitney 1977; Padovani and Carter 1978; Dahl 1979; Bohlen et al. 1980; Stoddard 1980; Matty et al. 1982; Perkins et al. 1982; Matty 1984) (Table 4). Metamorphic ternary feldspars (XAn ,AF> 0.07) are reported in the Scourie Complex (O'Hara and Yarwood 1977; Rollinson 1982) and the Adirondack Highlands (Fuhrman and Lindsley 1983). Ternary feldspars are more commonly reported in igneous rocks (Fuhrman and Linds- ley 1983; Parsons and Brown 1983).

Feldspar pairs from this study are plotted in Figure 3. Many samples lie close to or outside Tuttle and Bowen's (1958) limit of feldspar miscibility. Recent work by Parsons and Brown (1983) extends the limit of feldspar solid solu- tion considerably (to Ora6Ab4~An2o at 1,060 ~ C, 1 bar), encompassing all our samples. With only one exception, tie-lines are nearly parallel, suggesting equilibrium. Sample 81-OC-147, which has the discordant tie-line, is a fine- grained biotite gneiss with secondary albite at perthite grain intersections and may be in disequilibrium.

Regardless of the amount of ternary solution in alkali feldspar, all plagioclases in the study area contain little or- thoclase (Xor, eL<0.03). Similarly, Xor, p L is less than or equal to 0.03 in other published studies of slowly cooled granulite rocks, except for several samples in the Scourie Complex (O'Hara and Yarwood 1978). Experimental work (Seek 1971; Johannes 1979) predicts up to 10 tool% misci- bility of orthoclase in plagioclase, yet only in rapidly-cooled granulite xenoliths are values of Xor ' eL close to those deter- mined experimentally (Padovani and Carter 1978; Matty et al. 1982; Matty 1984). Hence, the discrepancy between the experimental and field data suggests retrograde diffu- sion of K from plagioclase during slow cooling and uplift.

Thermometry results

The STO thermometer is based on the partitioning of albite between plagioclase and alkali feldspar assuming ideal, one- site mixing in plagioclase and a regular solution in alkali feldspar. The simplifying assumption that anorthite has no effect on alkali feldspar activity and orthoclase has no effect on plagioclase activity limits the thermometer's applicabili- ty to feldspar pairs with nearly binary compositions. With more extensive ternary solution, activity coefficients derived for binary components are no longer accurate.

In Figure 4, feldspar pairs are plotted against the deter- minative curves for STO at P = 7 kbar. The isotherms have been dashed where we have extrapolated Stormer's equa-

218

Table 1. Microprobe analyses of coexisting alkali feldspar and plagioelase. Temperatures calculated using Stormer (1975) (STO), Brown and Parsons (1981) (BP), and Haselton et al. (1983) (HHHR) for P = 7 kbar

81-OC-4 a 81-OC-6 a 81-OC-36 a 81-OC-37 81-OC-38 81-OC-48 a 81-OC-55

kfs plag kfs plag kfs plag kfs plag kfs plag kfs plag kfs plag

qtz-fsp gns qtz-fsp gns charnockite qtz-fsp gns charnockite charnockite biot-gar gns

SiO 2 64.21 63.98 63.52 61.80 65.02 64.82 65.55 64.68 64.24 63.44 65.57 62.81 64.06 62.27 A120 3 21.59 23.66 20.72 23.97 19.29 22.04 19.99 23.22 20.96 22.92 21.69 23.56 21.60 24.59 BaO 0.65 0.51 <0,05 <0.05 0.64 0,52 0.71 0.61 0.53 0.55 0.18 <0.05 0.66 0.46 CaO 2.25 4.47 2.18 5.47 0.85 2.70 0.90 3.90 2.31 4.15 2.78 4.65 2.54 5.66 N a 2 0 5.62 8.57 3.75 7.56 5.15 9.03 4.62 8.73 5.76 8.25 5.99 8.22 4.23 7.84 K20 6.58 0.25 9.11 0.17 9.22 0.37 10.16 0.27 5.90 0.22 4.80 0.15 8.58 0.24

Total 100.90 101.44 99.28 98.97

Formulae normalized to 5 cations

Si A1 Na Ca K Ba O

%An %Ab %Or

T(STO) T(BP) T(HHHR)

100.17 99.48 101.93 101.41 99.70 99.53 101.01 99.39 101.67 101.06

2.877 2.811 2.910 2.789 2.939 2.898 2.920 2.843 2.912 2.849 2.930 2.813 2.868 2.755 1.140 1.226 1.118 1.275 1.027 1.161 1.049 1.203 1.120 1.213 1.142 1,243 1.141 1.282 0.488 0.730 0.333 0.661 0.451 0.782 0.399 0.744 0.506 0.718 0.519 0.713 0.367 0.673 0.108 0.210 0.107 0.265 0.041 0.129 0.043 0.184 0.112 0.200 0.133 0.223 0.122 0,268 0.376 0.014 0.532 0.010 0.531 0.021 0.577 0.015 0.341 0.012 0.273 0.008 0.490 0.014 0.011 0.009 0.001 0.001 0.011 0.009 0.012 0.011 0.009 0.010 0.003<0.001 0.012 0.008 8.015 8.052 8.037 8.091 7.962 8.077 7.957 8.065 8.049 8.089 8.105 8.074 8.010 8.053

11.1 22.0 11.0 28.3 4.0 13.8 4.2 19.5 11.7 21.5 14.4 23.6 12.4 28.1 50.2 76.5 34.3 70.6 44.1 83.9 39.2 78.9 52.7 77.2 56.1 75.6 37.5 70.5 38.7 1.5 54.7 1.1 51.9 2.3 56,6 1.6 35.6 1.3 29.5 0.8 50.1 1.4

895 805 755 770 910 990 840 815 790 725 760 805 805 845 730 700 725 765 710 700 705

SiO2 AlzOa BaO CaO Na /O K20

81-OC-91 81-OC-109 81-OC-119 a 81-OC-124 81-OC-135" 81-OC-136" 81-OC-143 a

kN plag kN plag kN plag k~ plag kN plag k ~ plag k~ plag

mangerite mangerite charnockite biot-gar gns mangerite marble hbde gns

63.99 63.12 65.08 63.12 62.98 62.21 63.83 64.12 67.72 67.39 65.38 65.00 65.40 64.61 20.73 22.93 20.16 22.80 20.74 23.13 19.50 21.64 19.73 21.85 20.63 22.15 18.96 22.00

0.29 <0.05 0.49 0.43 0.43 0.20 0.44 0.49 <0.05 <0.05 0.13 <0.05 <0.05 <0.05 3.06 5.20 1.53 4.02 2.64 5.08 0.88 2,19 0.73 2.57 1.63 4.52 0.84 3.48 4.75 7.52 5.06 8.33 4.43 7.68 3.66 9.24 5.82 9.22 4.34 8.20 4.33 8.70 6.54 0.14 8.38 0.31 7.60 0.19 10.74 0.34 7,17 0.12 8.96 0.18 9.61 0. t4

Total 99.37 98.91 100.70 99.01

Formulae normalized to 5 cations

Si A1 Na Ca K Ba O

% A n %Ab %Or

T(STO) T(BP) T(HHHR)

98.82 98.49

2.925 2.857 2.926 2.842 2.896 2.822 1.117 1.223 1.069 1.211 1.125 1.238 0.421 0.660 0,441 0.727 0.395 0.678 0.150 0.252 0.074 0.194 0.130 0.247 0.382 0.008 0.481 0.018 0.446 0.011 0.005 0.001 0.009 0.008 0.008 0.004 8.082 8.135 8,000 8.075 8.038 8.097

15.7 27.4 7.4 20.7 13.4 26.4 44.2 71.7 44.3 77.4 40.6 72.4 40.1 0.9 48.3 1.9 46.0 1.2

905 830 855 795 795 820 660 760 675

99.05 98.02 101.17 101.15

2.938 2.900 3.018 2.957 1.057 1.154 1.036 1.131 0.326 0.811 0.503 0.785 0.043 0.106 0.035 0.121 0.630 0.020 0.408 0.006 0.006 0.009 <0.001 <0.001 7.989 8.062 8.081 8.127

4.3 11.2 3.7 13.3 32.4 85.7 53.2 86.0 62.6 2.1 43.1 0.7

675 775 670 690 615 730

101.07 100.05 99.14 98.93

2.936 2.901 2.992 2.903 1.092 1,164 1.022 1.165 0.378 0.709 0.384 0.757 0.078 0.216 0.041 0.167 0.513 0.010 0.561 0.008 0.003<0.001 <0.001<0.001 8.037 8.124 8.031 8.103

8.1 23.1 4.2 17.9 39.0 75.8 38.9 81.2 52.9 1.1 56.9 0.9

800 750 775 730 730 730

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Table 1 (continued)

81-OC-144 a 81-OC-147 81-OC-149"

kfs plag kfs plag kfs plag

biot-hbde gns biot gns qtz-fsp gns

81_OC.149A a 81-OC-152" 81-OC-t58 a 81-OC-160

kfs plag kfs plag kfs plag kfs plag

qtz-fsp gns qtz-fsp gas qtz-fsp gas mangerite

SiO z 65.06 64.47 67.41 64.81 65.44 64.16 A120 3 20.40 22.20 19.58 23.37 19.86 22.45 BaO 1 �9 0.54 0.60 0.50 0.40 0.36 CaO 1.29 3.49 0.46 4.50 1.03 3.38 NazO 5.48 8.72 5.42 8.38 4.26 9.26 K20 7.45 0.24 8.51 0.25 10.07 0.27

65.34 64.75 63.92 62.32 65.01 63.58 63.57 62.39 19.67 22.05 19.74 22.66 20.29 22.96 22.13 23.59 0.22 0.15 0.57 0.57 0.90 0.57 0.52 0.21 1.34 3.58 1.27 4.39 1.05 3.27 2.32 4.67 4.49 8.69 4.01 10.39 4.15 8.96 5.67 8.18 8.92 0.16 9.68 0.20 10.36 0.30 5.53 0.29

Total 100.74 99 .66 101.98 101.81 101.06

Formulae normalized to 5 cations

Si 2.930 2.884 2.994 2.843 A1 1.083 1.170 1.025 1.209 Na 0.478 0.756 0.467 0.713 Ca 0.062 0.167 0.022 0.212 K 0.428 0.014 0.482 0.014 Ba 0.019 0.009 0.010 0.009 O 8.019 8.084 8.032 8.084

%An 6.4 17.8 2.3 22.6 %Ab 49.4 80.7 48.1 75.9 %Or 44.2 1.5 49.6 1.5

r(STO) 825 885 T(BP) 760 83O T(HHHR) 755 990

99.88 99.98 99.38 99.19 100.53 101.76 99.64 99.74 99.33

2.942 2.847 2.966 2.898 2.936 2.724 2.911 2.835 2.880 2.799 1.052 1.174 1.053 1.164 1.068 1.168 1.071 1.207 1.181 1.247 0.371 0.797 0.395 0.754 0.357 0.881 0.360 0.775 0.498 0.711 0.050 0.161 0.065 0.172 0.062 0.206 0.050 0.156 0.113 0.224 0.578 0.015 0.517 0.009 0.567 0.011 0.592 0.017 0.319 0.016 0.007 0.006 0.004 0.003 0.010 0.010 0.016 0.010 0.009 0.003 7.949 8.028 8.037 8.103 8.008 7.862 7.971 8.043 8.062 8.059

5.0 16.5 6.7 18.4 6.3 18.8 5.0 16.5 12.1 23.6 37.1 82.0 40.4 80.6 36.2 80.2 35.9 81.8 53.6 74.7 57.9 1.5 52.9 1.0 57.5 1.0 59.1 1.8 34.3 1.7

730 765 740 725 975 715 745 720 720 835 685 695 680 690 795

" Average analysis of 2 4 grains in same thin section.

Rock abbreviations: qtz-fsp gns= quartzofeldspathic gneiss; biot-gar gas = biotite-garnet gneiss; hbde gas = hornblende gneiss; biot-hbde gns= biotite-hornblende gneiss; biot gas = biotite gneiss

! AN

60 . . . . . . . . . . .

AB 80 60 40 20 OR --)

Fig. 3. Microprobe analyses of coexisting feldspar pairs. All sam- ples fall within the proposed limit of feldspar solid solution (Par- sons and Brown 1983). The sample with the discordant tie-line appears texturally not to be in equilibrium

tion beyond his explicit limit of X A b ' AF = 0.40. The samples have been divided into two groups: those with nearly binary compositions (XAn,AF < 0.07), plotted as circles, and those with more ternary compositions (XA, ,AF> 0.07), plotted as crosses. The data form two distinct groups which show virtually no overlap. Only the sample already discussed, that texturally appears to be in disequilibrium, falls outside its respective group. Binary and ternary alkali feldspars have approximately the same range of XAb ' AF but the terna- ry feldspars coexist with more calcic plagioclases (Fig. 4), resulting in much higher temperature estimates (800-995 ~ C). The average temperature estimate for binary pairs is 750___ 40 ~ C. Four of these samples lie outside the suggested limit of STO and yield estimates of 780_+ 30 ~ C, slightly higher than estimates for other samples in the

group. Temperature estimates for all thermometers are ta- bulated in Table 1.

The BP thermometer is shown in Figure 5 for P = 1 kbar, with samples plotted as in Figure 4. Increased pres- sure will shift the estimated temperatures approximately 18 ~ C/kbar (Brown and Parsons 1981) yielding an average T = 7 3 5 _ 4 0 ~ C for binary samples and T=810-t-20 ~ C for ternary samples. For all samples, T=770-1-55~ C with a range of 670-845 ~ C (Table 1).

Using the H H H R (1983) equation:

T(K) = ((Xor ' AF) 2 ( l 8 , 8 1 0 -~- 1 7,030 X A b ' AF -t- 0 . 364 Pbars) -- (XAn ' pL) 2 (28,230--39,520 XAb ' PL)) /

(10.3 (Xor AF)2 + 8.3143 �9 in ( (Xab ' m) ~ (2 - - .gAb ' PL)/XAb, AF)) (1)

Oaxacan samples with binary compositions yield an average T = 705-t-45 ~ C, approximately 50 ~ C lower than the STO estimate. For pairs with ternary compositions, T=710_+ 40 ~ C, up to 300 ~ C lower than the STO estimate.

Errors of _ 3% of the amount present for major ele- ments may be expected from microprobe analysis and rein- tegration of perthites. Hence, + 50 ~ C is a reasonable esti- mate of analytical error for all the above thermometers.

Calc-sil icate equilibria

Mineral equilibria in marbles can be used to independently estimate the temperature of peak metamorphism and thus provide a check on the accuracy of the various feldspar thermometers. The isobari- cally univariant assemblage dolomite + diopside + forsterite + cal- cite was analyzed in four samples. The reaction

220

/ O / O - - . . . .

600 ~ / ,/ + ." .- I .o'l" + ~ / " ,oo" / / , - / / 800"/.," /

.6 / / ooo/ / - . / / / /1ooo" . /

g .4 / +

x P = 7 k b .2 /

O0 I I I I I I .2 .4 .6

XAB IN A L K A L I F E L D S P A R

Fig. 4. Oaxacan feldspar compositions plotted against the deter- minative curves of Stormer (1975) for P = 7 kbar. Isotherms are dashed where they have been extrapolated beyond XAb, AF=0.40. Circles are samples with XAn,AF~0.07 , crosses are those with XAn,A F > O. 0 7

3 CaMg(CO3) 2 + CaMgSizO 6

= 4 CaCO a + 2 Mg2SiO4+2 CO 2 (2)

has been experimentally reversed by K/ise and Metz (1980) and Jacobs (Eggert and Kerrick 1981). The results of K/ise and Metz at 1, 3, 5, and 10 kbar are self-consistent and have been used to calculate equilibria at 7 khar.

Ideal, ionic activities were calculated for the silicate phases assuming complete disorder on M1 and M2 for forsterite such that C~Vo,OL=(XMg) 2 and ~Di, CPX=(J[ZCa, M2)(XMg, M1). Solid solution is minor in silicates with Xvo,oL=0.95 and XDi,cPx=0.98 (Mora et al. 1985). The carbonate phases contain no solid solution outside the system C a O - M g O - CO z and were assumed to be solvus equi- librated in the experiments on reaction (2), as indicated by the results of Goldsmith and Newton (1969). Hence, carbonate activi- ties are defined as unity.

At Pco2 = 7 kbar, the equilibrium temperature of the reaction is 785_+ i0 ~ C. Solid solution shifts the temperature down approxi- mately 20 ~ C. At P = 7 kbar, reaction (2) is stable over the entire range 0 .2<Xco2<1.0 and the equilibrium temperature is in the range 720-765 ~ C, but cannot be uniquely fixed unless Xco 2 can be estimated.

The occurrence nearby of marbles containing the assemblage wollastonite + calcite + quartz, which under the same pressure and temperature conditions buffers the fluid composition to still lower Xco2, indicating that fluid compositions were buffered locally at the peak of metamorphism, as has been reported in the Adiron- dacks (Valley and O'Neil 1984).

Garnet Barometry

Metamorphic pressure must be known to apply any feldspar ther- mometer because of the significant pressure dependence of the feld-

i I

OtJ t I

�9 8 ~ l ~

I /

~p~. 1 kb z .4 ; ; ,

X i l/I

. t~t ?o,.

U O 0 I I I I I I I I l

.2 ,4 .6 .8 1.C XAB IN ALKALI FELDSPAR

Fig. 5. Oaxacan feldspar compositions plotted against the Brown and Parsons (1981) thermometer at P = 1 kbar. Symbols are the same as in Figure 4. A pressure correction of 18 ~ C/kbar is applied to temperature estimates

spar solvus (18 ~ C/kbar, Brown and Parsons 1981). We have esti- mated P = 7_+ 1 kbar in the Oaxacan Complex from four indepen- dent garnet barometers:

3anorthite + 3ferrosilite = grossular zalmandine 2 + 3quartz (3)

3anorthite + 3enstatite = grossularzpyrope 2 + 3quartz (4)

anorthi te + diopside = grossular2pyrope I + quartz (5)

anorthite = grossular + A12SiO s + quartz. (6)

Sources of experimental or calorimetric data for these barometers are: Bohlen et al. (1983) for reaction (3), Newton and Perkins (1982) for reactions (4) and (5), and Newton and Haselton (1981) for reaction (6). Mineral analyses are given in Table 2.

Activities .of impure phases were calculated using the models suggested by the respective authors. Spessartine is a minor garnet component and its mixing properties are poorly known. Inclusion of spessartine as a fourth garnet component shifts the calculated pressures in Oaxaca only 0.1 khar, assuming ideal mixing of Mn.

Calculation of plagioclase activity using the results of Orville (1972) or the Al-avoidance model (Newton and Perkins 1982) pre- dicts approximately the same activity for all samples for which XA,,PL < 0.7. However, for more anorthitic plagioclase (sample 25277, Table 2) the Orville model predicts a positive deviation from ideality whereas the Al-avoidance model predicts a negative devia- tion. In the range XA,,p L > 0.8, the latter model deviates substan- tially from experimental measurement of activity. This may be due to an ordering transformation in intermediate plagioclase (Carpen- ter and McConnell 1984). For calculations involving anorthite, the error introduced by choice of plagioclase activity model is 0.3 kbar.

Pressure calculations are summarized in Table 3. Each esti- mate is the average calculated pressure for the 3-4 assemblages analyzed in each sample. The average pressures are: P = 7 . 2 + I kbar for reaction (3), P=7.7_+1 kbar for reaction (4) and P = 5.0+1 kbar for reaction (5). Newton and Perkins (1982) noted reaction (5) yields pressures approximately 2.5 kbar lower than reaction (4) and suggested the source of the discrepancy may be errors in the thermodynamic data used to derive the expression for reaction (5). A correction of --0.5 kbar for reaction (4) and +1.5 kbar for reaction (5) is recommended (R. Newton, pers. comm., 1984). Estimates for reaction (6) are less precise (6.0,

Table 2. Microprobe analyses ofminerals used in garnet barometry

16076 81-OC-116 25277 16976 9377 26577 16076 81-OC-116 25277 16976 gar gar gar gar gar gar opx opx opx opx

221

SiO 2 37.68 38.40 37.87 38.46 38.27 38.40 50.48 50.67 52.02 50.10 TiO 2 0.03 0.04 0.03 0.03 <0.03 <0.03 0.09 0.08 0.09 0.10 AI20 3 21.67 22.55 21.70 21.14 21.91 22.03 1.19 2.36 1.49 1.15 FezO 3 1.61 0.00 1.30 0.14 2.40 2.51 1.21 1.46 0.00 1.13 FeO 26.44 29.91 25.51 28.04 26.31 23.77 30.12 28.86 27.55 30.10 MnO 1.14 1.36 0.83 1.09 0.51 0.64 0.43 0.40 0.29 0.37 MgO 4.93 5.70 5.76 4.59 9.44 11.05 16.13 17.38 18.05 15.84 CaO 6.79 2.49 6.76 6.87 1.35 1.21 0.86 0.21 0.76 0.88

Total 100.29 100.45 99.76 100.36 100.19 99.61 100.51 101.42 100.25 99.67

Formulae normalized to 8 cations (gar), 4 cations (pyx), or 5 cations (plag)

Si 2.950 3.002 2.961 3.011 2.951 2.942 1.953 1.927 1.985 1.955 A1 Iv 0.050 0.000 0.039 0.000 0.049 0.058 0.047 0.073 0.015 0.045 A1 vl 1.951 2.078 1.962 1.954 1.940 1.931 0.007 0.033 0.052 0.009 Ti 0.002 0.002 0.002 0.002 < 0.002 0.001 0.003 0.002 0.003 0.002 Fe 3+ 0.095 0.000 0.073 0.200 0.109 0.125 0.034 0.036 0.000 0.032 Mg 0.575 0.664 0.671 0.537 1.083 1.261 0.930 0.985 1.026 0.921 Fe 2+ 1.731 1.955 1.671 1.826 1.724 1.542 0.976 0.923 0.879 0.986 Mn 0.076 0.090 0.055 0.071 0.033 0.041 0.014 0.013 0.009 0.012 Ca 0.570 0.209 0.566 0.579 0.111 0.099 0.036 0.008 0.031 0.038 O 12.000 12.043 12.000 12.000 12.000 12.000 6.000 6.000 6.022 6.000

16076 25277 16976 16076 81-OC-116 25277 16976 9377 26577 cpx cpx cpx plag plag plag plag plag plag

SiO z 52.84 51.91 51.31 SiO 2 55.71 63.13 47.12 57.44 59.87 63.80 TiO z 0.14 0.25 0.24 AlzO 3 27.97 23.72 33.68 27.77 26.15 23.34 A120 3 1.29 1.87 1.92 Fe20 3 0.25 0.26 0.25 0.23 0.22 0.24 F%O3 0.00 1.08 1.31 CaO 9.13 4.81 16.29 9.07 6.63 3.55 FeO 11.48 10.87 13.29 Na /O 5.89 7.94 1.94 5.67 7.19 9.09 MgO 12.45 13.02 11.45 K20 0.22 0.18 0.09 0.20 0.16 0.15 CaO 21.74 20.40 21.02 Na20 0.35 0.34 0.44 Total 99.17 100.04 99.46 100.38 100.22 100.17

Total 100.29 99.74 100.98

Formulae normalized to 8 cations (gar), 4 cations (pyx), or 5 cations (plag)

Si 1.984 1.951 1.947 Si 2.525 2.817 2.177 2.580 2.669 2.821 AI Iv 0.016 0.049 0.053 A1 1.494 1.248 1.831 1.471 1.375 1.216 AI vj 0.041 0.034 0.033 Fe 3 + 0.008 0.008 0.009 0.008 0.008 0.008 Ti 0.004 0.007 0.007 Na 0.517 0.687 0.173 0.494 0.622 0.779 Fe 3+ 0.000 0.031 0.038 Ca 0.443 0.230 0.805 0.436 0.317 0.168 Mg 0.696 0.729 0.648 K 0.013 0.010 0.005 0.011 0.009 0.008 Fe 2+ 0.360 0.350 0.387 O 8.011 8.097 8.006 8.067 8.045 8.040 Ca 0.874 0.821 0.855 Na 0.025 0.028 0.032 O 6.004 6.001 6.000

Fe 3 + calculated by charge balance of normalized formula assuming no site vacancies

9.3 kbar). The garnet mixing model of Ganguly and Saxena (1984) does not significantly change these estimates. An estimate of 9 kbar must be at least 1 khar too high because at 730 ~ C, kyanite would be the stable aluminosilicate polymorph (Holdaway 1971), while only sillimanite has been reported in the Oaxacan Complex (Or- tega-Guti6rrez 1981; Mora et al. 1985). The error for pressures determined by reaction (6) is large ( + 1.5 kbar) because of uncer- tainties in the activity data for grossular in extreme dilutions and in the AV at P, T for the endmember reaction. Still, the overall agreement between barometers is very good indicating 7 kbar pres- sures for the peak of granulite facies metamorphism.

Comparison of feldspar thermometers

A c o m m o n cha rac t e r i s t i c o f m a n y P r e c a m b r i a n g r anu l i t e t e r r a n e s is t h a t p e a k m e t a m o r p h i c t e m p e r a t u r e shows l i t t le c h a n g e over d i s t ances o f > 50 k m ( S t o r m e r a n d W h i t n e y 1977; B o h l e n et al. 1985). N o m e t a m o r p h i c i sog rads can be de l i nea t ed in the O a x a c a n C o m p l e x a n d the m i n e r a l o g i e s n o t e d in m o s t l i tho log ies in the s t u d y a rea a re c o n s i s t e n t w i th a p p r o x i m a t e l y i s o t h e r m a l c o n d i t i o n s t h r o u g h o u t the t e r r a n e (B loomf i e ld a n d O r t e g a - G u t i ~ r r e z 1975; M o r a et al.

222

Table 3. Metamorphic pressure estimates from garnet-forming re- actions of the Oaxacan Complex (P in kbar)

Sample Reaction Reaction Reaction Reaction (3) (4) (5) (6)

16076 7.0 8.2 a 5.1" - 81-0C-116 8.2 7.5 - - 25277 6.3 7.5 4.6 - 16976 7.3 7.8 5.2 - - 9377 -- -- -- 6.0 26577 - - - 9.3

These pressures are calculated from Newton and Perkins (1982) and should be corrected approximately -0.5 kbar for reaction (4) and + 1.5 kbar for reaction (5) (R. Newton, pers. comm., 1984)

1985). Thus it is not expected that the feldspar thermometry will yield results that vary greatly on a regional or local basis. However, the STO estimates (Table 1) are inconsis- tent with this observation. Coexisting feldspar pairs (Fig. 4) suggest a 300~ range of equilibrium temperatures (675-990 ~ C) in the study area. These very high tempera- tures and the steep gradient in metamorphic grade they suggest are geologically unreasonable. For example, sample 81-OC-158 (XA, ' AF = 0.05), a quartzofeldspathic gneiss, and 8]-OC-160 (XAn,AF=0.12), a two pyroxene mangerite, oc- cur approximately 800 m apart in the field. There is no mineralogic evidence for a contact metamorphic zone and no major structural discontinuity is noted between these two samples, yet T(STO)=725~ for 81-OC-158 and 975 ~ C for 81-OC-160.

The shape of the BP isotherms greatly extend the range over which XAb,A F can vary in an approximately isothermal manner for a given plagioclase composition (Fig. 5) and yield temperatures that do not vary as much as an extrapo- lation of STO would indicate (Table 1). The total range of the temperature estimates using BP is 200 ~ C, 100~ less than the range indicated by STO, but still greater than the maximum gradient that is consistent with the petrologic evidence. The inferred temperature variation may in part be the result of reduced accuracy for the more sodic feldspar pairs due to convergence of the isotherms at higher -gAb, AF" Ternary feldspars yield uniformly higher temperatures than binary feldspars, as they did using STO. Brown and Parsons (1981) note that the isotherms should be contoured for ter- nary components. The importance of such contouring is demonstrated below for the H H H R thermometer, and sug- gests that the BP graphical thermometer, as published (BP Fig. 2), will not be accurate for ternary compositions.

H H H R estimates for ternary feldspars are not substan- tially different than estimates for binary pairs (Table 1). Using HHHR, the discrepancy in estimated temperature for samples 81-OC-158 and -160 is reduced from 250~ (STO) to 110 ~ C. For feldspars with nearly binary composi- tions, the STO temperatures (750 +__40 ~ C) are slightly high- er than the H H H R estimates (710___40 ~ and may be more consistent with temperatures estimated by marble equilibria and with the relative scarcity of garnet in the Oaxacan Com- plex. However, the H H H R temperatures are the most con- sistent of the three thermometers over the entire composi- tional range and are in the best overall agreement with the marble equilibria. H H H R feldspar temperature data are

\

17000 ,

"~ 1 7 : 5 0 7 0 0 ,

i r - - _ . - - ,

6 8 5 = * 7 9 5 . . . . . . ,

N

1' 0 i , , , , i

km

10 i

660 ri '., 7 6 5 e e "

",,~ f71 7

9 7 0 0 0 , \,,,=...

Z I M A T L A N

..~ ;)5 k m .) ,o EJUTLA

700

Fig. 6. Feldspar temperature data (circles) from the Haselton et al. (1983) thermometer for the Oaxacan Complex. Squares show the location of marne assemblages that restrict T=720-765~ at 7 kbar. The Oaxacan Valley Anorthosite Massif is shown by hat- chures and crosses designate a Triassic granite

plotted in Figure 6 on a simplified geologic map of the Oaxacan Complex. Although the BP model also reduces the total range of temperatures in Oaxaca, greater precision in the estimates results from using the H H H R equation.

The effect of ternary solution on HHHR is illustrated in Figure 7. Isopleths of XA, ' AF were derived at 700 ~ C for a ternary ( A n - A b - O r ) alkali feldspar coexisting with bi- nary ( A b - A n ) plagioclase. For scale, the 600 ~ C isotherm is also plotted. The H H H R 700~ binary isopleth lies above the STO 700 ~ C isotherm (dashed), which results in higher temperature estimates. However, the addition of only 5 mol% An in alkali feldspar shifts the isopleth signifi- cantly downward, corresponding to a decrease in estimated temperature of 50-100 ~ C. Likewise, the isopleths for 10 and 15 mol% anorthite in alkali feldspar show a downward displacement resulting in a dramatic decrease in estimated temperature. Sample 81-OC-48, with XAn, AF=0.14, yields T= 990 ~ C using the binary STO isotherms and T--700 ~ C using HHHR. Isopleths similarly derived by varying Xo~,p L in plagioclase in equilibrium with a binary alkali feldspar show only slight (10 ~ C) displacement relative the binary isopleth. The displacement is in the same direction as the shift due to anorthite solution in alkali feldspar.

Feldspar thermometry in other terranes

Application of HHHR to previously published data on high-grade rocks is summarized in Table 4. In rocks with nearly binary feldspars, the average temperature estimates are not significantly changed, although the standard devia-

1.0 ~ I ),- i f t .oo / .oo-~ -- -- - - ~

///f"L--- 600~ .05

/ f _

ua r 09 < ..J , 6 o t O

_.1 13_

Z . 4 oo%/ X

.z / PfTkb

O 0 I I I I t 1 .2 .4 .6

XAB IN ALKALI FELDSPAR Fig. 7. Isopleths of X A n , A F for the 700 ~ C isotherm of Haselton et al. (1983), solid line. The binary 600 ~ C isotherm (HHHR) and the Stormer (1975) 700 ~ C isotherm (dashed) are shown for compar- ion. Plagioclase compositions are K-free

tion (1 a) is increased. For 27 pairs (XAn, AV<0.05 ) from the Adirondack Mountains (Bohlen and Essene 1977; Boh- len etal . 1980), T (STO)=645-780~ with an average T(STO) = 720 + 40 ~ C. Using H H H R , the range of estimates is extended to 650-830~ but the average temperature shifts only slightly to 725 + 45 ~ C.

Previous interpretations of ternary feldspar composi- tions in the Scourie Complex suggest very high tempera- tures: approximately 1,000 ~ C, interpreted as the premeta- morphic igneous temperature of granite sheets preserved through granulite facies metamorphism (Rollinson 1982) and 1,145 ~ C and 850 ~ C, interpreted as representing ultra- high metamorphic temperatures (O'Hara and Yarwood 1978). Conditions of metamorphism in the Scourie Com- plex have been estimated by garnet-pyroxene thermometry and barometry and magnetite-ilmenite thermometry at T = 820_+ 50 ~ C at P = 11 kbar (Rollinson 1979, 1981). Recalcu-

223

lation of the feldspar temperatures using H H H R ( P = 11 kbar) yields an average T = 825 + 35 ~ C for Rollinson's (1982) samples and T=765+_65~ for O 'Hara and Yar- wood's (1978) samples. These estimates are much more con- sistent with metamorphic conditions estimated by other thermometers and require no unusual geological interpreta- tion.

It is notable that although the large downward shifts in estimated temperature are proportional to the extent of ternary solution in alkali feldspar for Oaxacan and Scour- ian samples, the smaller shifts in the Adirondack samples are not as systematic. This may be due to variations in tie-line slopes in the Adirondack samples.

Origin of ternary feldspars

In light of the experimentally determined ternary solvus relationships (Seek 1971), which require ternary feldspars to have crystallized or to have been equilibrated at higher temperatures than binary feldspars, it is necessary to exam- ine the meaning of results whereby both ternary and binary feldspars yield the same temperature.

The agreement between the STO thermometer and Seck's experimental data at 650~ is very good. Hence, the STO estimates may be used to examine the Oaxacan data in light of the experimentally determined solvus rela- tionships. I f real, the 300 ~ C range of estimated temperature (from STO) might suggest gross disequilibrium in the feld- spar system, with ternary compositions being preserved from pre-metamorphic igneous temperatures in some sam- ples while binary feldspar compositions represent equilibra- tion at peak metamorphic conditions in others.

However, we think that the consistency of the Oaxacan data and the mineral textures do not support such disequi- librium. More than 20 coexisting feldspar pairs, from four different rock types, have approximately parallel tie-lines (Fig. 3). These samples represent rocks with marie and gran- itic compositions and which often also contain hydrous phases such as biotite or hornblende. Coexisting Oaxacan pyroxenes do not show the extensive exsolution noted i n high temperature metamorphosed igneous pyroxenes from other granulite gneisses (Bohlen and Essene 1978; O 'Hara and Yarwood 1978; Powers and Bohlen 1984). Thus, reten- tion of premetamorphic igneous temperatures in feldspars through granulite facies metamorphism, such as suggested

Table 4. Comparison of temperatures estimated from Haselton et al. (1983) with previous estimates in other granulite facies terranes

Locality # XAb. AV XA,.AF XO~XL XA~.p L P Previous HHHR samples (kb) ]P ~ 5 ~ ~

Oaxacan Complex, Mexico 20 ~ 0.30-0.56 0.02-0.15 0.01-0.02 0.1J-0.28 7 675-990 (815) 6t5 795 (710) Adirondack Mtns, NY 13b 0.20--0.47 0.01--0.07 0.01--0.03 0.05--0.43 8 645--780 (720) 650 830 (725) Adirondack Mtns, NY 14 r 0.25-0.38 0.01-0.05 0.01-0.03 0.14-0.32 8 660-780 (720) 660-795 (730) Salvador, Bahia, Brazil 9 d 0.16-0.38 0.01-0.06 0.00-0.01 0.20-0.34 7 600-740 (695) 610-760 (700) Scourie Complex, Scotland 5 * 0.37-0.54 0.11-0.19 0.02-0.09 0.23-0.25 11 850-1,145(910) 720-830 (795) Otter Lake, Quebec 4 f 0.30-0.55 0.01-0.04 0.00-0.01 0.12-0.22 6 670-715 (695) 64~765 (715) Xenoliths, Kilbourne 6 g 0.24--0.33 0.02-0.07 0.08-0.10 0.28-0.40 7 785-1,050 (885) 880-1,445 (1,075)

Hole, NM Xenoliths, E. Snake 17 ~ 0.20-0.44 0.01 0.04 0.03-0.08 0.17-0.38 5 702-997 (880) 728-1,180(875)

River Plain, ID

" this study b Bohlen and Essene (1977) O'Hara and Yarwood (1978), Rollinson (1982)

h Matty (1984)

c Bohlen et al. (1980) d Stormer and Whitney (1977) Perkins et al. (1982) g Padovani and Carter (1977)

224

AN

AB so OR

Fig. 8. Composition of coexisting feldspars in slowly cooled granu- ]ite terranes (vertical shading) and granulite xenoliths (horizontal shading), from data summarized in Table 4. Compositions from the granulile-facies Oaxacan Complex and Scourie Complex are shown by the stippled areas. Many of these assemblages are incon- sistent with the position of the Seek (1971) solvus. The 650~ Seck isotherm, interpolated for P = 7 kbar is shown. PB is a possi- ble high temperature (950-1,050 ~ C), low pressure isotherm pro- posed by Parsons and Brown (1983) which has a markedly different shape than the Seek counterpart. The limit of feldspar solution consistent with PB is also shown (from Parsons and Brown 1983)

by Rollinson (1982), seems unlikely when other, typically more refractory, systems such as garnets, pyroxenes, and Fe - -T i oxides have equilibrated during metamorphism.

Several recent studies (Brown and Parsons 1981; Par- sons and Brown 1983) have demonstrated inconsistencies between Seck's solvus and natural igneous feldspar compo- sitions. These studies suggest that the limits of feldspar solid solution are considerably greater than indicated by Seck's data, particularly at the Ab apex (Fig. 8). Compositions of coexisting feldspars from several high grade metamor- phic terranes are also shown in Figure 8. Feldspar composi- tions reported for slowly cooled, granulite facies rocks (Bohlen and Essene 1977; Stormer and Whitney 1977; Boh- len et al. 1980; Perkins et al. 1982) are shown by areas of vertical shading. Areas o f horizontal shading are composi- tions of rapidly cooled feldspars from granulite facies xeno- liths (Padovani and Carter 1977; Matty 1984). The stippled area includes data from the Oaxacan and Scourie Com- plexes (this paper, Rollinson 1983). The figure demonstrates that many granulite facies feldspar assemblages may also be inconsistent with the Seek solvus. Most of the alkali feldspar data fall inside this solvus and many of the plagio- clase data fall outside the solvus. Hence, thermometers which fit Seck's experimental solvus relations may not yield accurate results in high grade metamorphic terranes, partic- ularly for Ab- or An-rich feldspar compositions. Thermom- eters fit to tie-line relations, such as H H H R , may be more suitable for these rocks.

For many samples, the reintegration of perthitic alkali feldspar suggests retention of peak compositions. As pre- viously noted, plagioclase compositions in high grade, slow- ly cooled rocks show considerably less Or solid solution

than compositions reported in rapidly cooled xenoliths (Fig. 8), suggesting retrogressive loss of K from plagioclase. The effect of retrogression on feldspar thermometry must also be considered. If it is accomplished by exchange of Na and K in plagioclase, the amount of albite in plagioclase may be overestimated, resulting in a significant decrease in estimated temperature. As suggested by Figure 8, a rea- sonable estimate of the extent of such diffusion is approxi- mately 5 mol%. However, it is possible that some other mechanism for retrogression would have less effect on the KD.

Correction for a 3-5 mol% retrograde N a - K exchange in plagioclase results in small increases in the Oaxacan tem- perature estimates (HHHR) from 710~ to 760~ (7 kbar), increasing the concordance of the feldspar thermo- metry and temperature estimated by phase equilibria in marbles. Slightly higher temperature estimates are also more consistent with the relative scarcity of garnet in the Oaxacan Complex.

Acknowledgements. This research was supported by a CONOCO Fellowship, AGI Minority Scholarship, Geological Society of America Penrose Grant 2885-81 (to CIM), National Science Foun- dation Grant EAR 81-21214, and The Gas Research Institute Grant 5083-260-0852 (to JWV). We thank EJ Essene and HT Ha- selton for helpful reviews of the manuscript and E Glover and DJ Matty for discussion and aid in microprobe analysis. We partic- ularly thank F. Ortega-Guti6rrez for introducing us to this area, helping with logistical support and providing samples 9377, 24277, 16076, 16976 and 26577. We thank PE Brown for many helpful conversations.

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Received January 5, 1985