Significance of Chloritoid-Bearing and Staurolite-Bearing Rocks in the Picuris Range, New Mexico

Significance of chloritoid-bearing and staurolite-bearing rocks in the Picuris Range, New Mexico

M. J. HOLD AW AY Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275

ABSTRACT INTRODUCTION

In the Picuris Range near Taos, New Mexico, the Precambrian The Picuris Range is a small segment of the southern Sangre de Ortega Quartzite and Rinconada Formation are folded together Cristo Range which projects westward into the Rio Grande de- and share a common history. The Ortega contains chloritoid-Al pression a few kilometres southwest of Taos, New Mexico (Fig. 1). silicate and essentially no staurolite, and the Rinconada contains Folded Precambrian metasedimentary rocks trend east-west, are lo- staurolite-almandine—bearing assemblages with minor graphite. Al cally intruded by Embudo granite, and are surrounded on all sides silicate-bearing Rinconada rocks locally contain Mn-rich garnet. by younger rocks (Montgomery, 1953). All the rocks of the area crystallized at temperatures a little above The metasedimentary rocks include a lower series: from oldest to the Al silicate triple point, as indicated by the paragenetic sequence youngest, Ortega Quartzite, schist and quartzite of the Rinconada kyanite —» andalusite —* sillimanite. Analysis of the assemblages in Formation, and slate and phyllite of the Pilar Formation; and an relation to chloritoid, staurolite, and Al silicate stability data, tak- upper series: Vadito Formation schist, amphibolite, metaconglom- ing into account the mineral compositions and the effect of graphite erate, and metafelsite (Nielsen, 1972). An unconformity separates on fluid composition, shows that all the rocks crystallized at 532 ± the two series, but the significance of this break is not clear. The 20 °C and about 3,700 b Pal, but PHJPminerals Rinconada rocks and near 1 in the Ortega rocks. This small differ- are less abundant than in the lower series, but the differences may ence in PHío appears to be the most important factor in explaining only reflect the less mature sediment typical of the Vadito. The the juxtaposition of chloritoid and staurolite in the two units. study described here involves only the Ortega Quartzite and the

105° 45'

Figure 1. Generalized geologic map of part of Picuris Mountains, based on map of Nielsen (1972). Localities men- tioned in Table 4 and text: 1, Rattlesnake Gulch; 2, La Sierrita; 3, Hondo Canyon; and 4, Agua Caliente Canyon. Faults are not shown. Triangles = pelitic enclaves in Ortega Quart- zite.

Geological Society of America Bulletin, v. 89, p. 1404-1414, 5 figs., 12 tables, September 1978, Doc. no. 80912.

1404

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overlying schist and quartzite of the Rinconada Formation. The Structural formulae of silicates were determined on the basis of pilar rocks are principally graphite-muscovite phyllite and slate. the numbers of oxygens in the ideal anhydrous formulae. Ratios of Their fine grain size and lack of porphyroblasts appear to relate to divalent ions in the octahedral or eightfold positions were deter- rock composition and abundance of graphite and quartz. They con- mined by correcting Fe for estimated Fe+3 content. Chloritoid that tain no chlorite or other minerals that are restricted to low grade. crystallized with hematite was assumed to have 9% of its Fe in the Montgomery (1953) described the complex folding about east- Fe+3 state, whereas the chloritoid that grew in the absence of west axes. He also noted sillimanite in the Ortega and staurolite in hematite or reducing oxides was assumed to have 7% Fe+3. These the Rinconada and concluded that the Ortega was metamorphosed corrections are based on Halferdahl's (1961) new analyses. Stauro- to higher grade. In about 1965 Charles Naeser discovered lite, biotite, and almandine all grew with graphite and ilmenite. chloritoid in the Ortega Quartzite, indicating that it could not be These were given minimum corrections of 5%, 3%, and no correc- higher in grade, but could, in fact, be lower in grade than the Rin- tion, respectively, in accord with analyses given by Deer and others conada schists. (1962) for these minerals found in reducing assemblages. Inspec- Detailed structural analysis by Nielsen (1972), combined with tion of the structural formulae (Tables 5, 8, 9) shows that site oc- age dating by Fullagar and Shiver (1973) and Gresens (1975), cupancies are slightly improved if the above proportions of the Fe permits reconstruction of a tentative tectonic-metamorphic se- are added to the Al sites and subtracted from the Fe+2, Mg, Mn quence. During low-grade metamorphism there were two penetra- sites. The correction for ferric Fe produces a 1 to 2 mole percent tive events, one before and one after emplacement of the Embudo reduction in ratios involving Fe in the numerator. granite (1,673 m.y. ago). The first event produced a schistosity that nearly parallels bedding planes; the second event imposed an axial PETROLOGY AND MINERAL CHEMISTRY plane schistosity that locally transposes the earlier surface. In thin- section analysis of samples, evidence of these two events cannot be For the purpose of this report, quartzite and aluminous musco- clearly separated. Combined, the events produced the observed pre- vite quartzite are grouped as Ortega Quartzite. With rare ex- ferred orientation in most of the muscovite and in some of the bio- ceptions, the first staurolite-bearing or biotite-bearing rocks mark tite, kyanite, and chloritoid. One or, perhaps, two later events the bottom of the Rinconada Formation, which contains more (1,325 and 1,257 m.y. ago) were primarily thermal. Some of the pelitic schists than quartzites. This contact is easily seen in the field kyanite, chloritoid, and muscovite probably recrystallized during and has considerable petrologic significance. In two localities en- the later event(s), and staurolite, almandine, andalusite, sillimanite, claves of pelitic composition occur within the Ortega Quartzite and additional biotite grew at this stage. The thermal event(s) pro- (Fig. 1). Mineral abbreviations and formulae are given in Table 1, duced coarse, undeformed mineral growth in all units, and and mineral assemblages are given in Tables 2 and 3. presumably all minerals with the exception of kyanite approached equilibrium at this stage. Two later mild events occurred, the first Ortega Quartzite associated with limited retrogression of biotite and staurolite to chlorite and almandine to iron oxides. Quartzite. Most of the Ortega Quartzite is massive blue-gray This report summarizes the petrology and mineral chemistry of cross-bedded quartzite. In places it is red from hematite. Locally the the Ortega and Rinconada Formations and explains the immediate quartzite is rich in Al, K, and Fe, leading to development of miner- juxtaposition of chloritoid-Al silicate in the Ortega and als useful as metamorphic grade indicators. Aluminous layers may staurolite-almandine in the Rinconada. Ganguly (1969) and

Richardson (1968) showed that in the simple system Al203-Fe0-

Si02 these two assemblages are separated by two chemical reac- TABLE 1. MINERAL ABBREVIATIONS AND FORMULAE tions. Albee (1972, Fig. 14) showed that in pelitic compositions the Mineral Abbreviation Formula assemblage Fe chloritoid-Al silicate-muscovite (representative of

Ortega) must pass through five successive topology changes before Quartz Qz Si02 staurolite-almandine-biotite-muscovite (representative of Rin- Kyanite Ky Al2Si05 conada) becomes stable. Clearly, if the two units crystallized at the Andalusite And Al2Si05 Sillimanite Sill Al Si0 same pressure and temperature, factors such as fluid composition 2 5 Muscovite Mus KAl3Si3O10(OH)2 and bulk-rock composition are responsible for the close proximity Paragonite Par NaAl3Si3O10(OH)2 of these assemblages in the Picuris Mountains. Biotite Biot K0.84(Fe, Mg)2.52Al1.72Si2.74O10(OH)2 Specimens have been collected from the Hondo Canyon area, Almandine Aim (Fe, Mg)3Al2Si3Oi2 Spessartite- Agua Caliente Canyon, Rattlesnake Gulch, and the area north of almandine Spes (Fe, Mn, Mg)3Al2Si3012 t La Sierrita (Fig. 1). About 110 thin sections have been studied in Staurolite St (Fe, Mg)2Al9Si4022.5(0H)2 detail. Minerals in 16 specimens have been analyzed by electron Chloritoid Ctd (Fe, Mg)Al2Si05(0H)2 microprobe (ARL-EMX, University of Texas at Dallas), using Plagioclase Plag (Na, Ca)(Al, Si)4Os Mn ferrogedrite Ged (Fe, Mn, Mg) Al Si 0 (0H) natural silicate standards. For each mineral except muscovite, two 5 4 6 22 2 Rutile Rut Ti02 different grains were analyzed at six closely spaced points on each. Hematite Hem Fe203 For muscovite, one analysis of six points was done. Analytical data Ilmenite Urn FeTi03 were converted to chemical analyses using the data reduction pro- Magnetite Mag Fe304 gram of J. Rucklidge and E. L. Gasparrini (unpub. data). Most Graphite Gra C analyses total to between 98% and 102% when water of hydration Guidotti and others' (1975) average. is taken into account. Accuracy of major elements is estimated to be + Griffin and Ribbe (1973). Oxygen has been increased by 0.5 to produce ±2% to 3% of the amount present, with A1 and Si the least accu- electrostatic balance. Note that this formula is 47/46 times the structural rate. formula.

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be traced locally; there are kyanite-rich rocks in Hondo Canyon Minerals were analyzed from four specimens of quartzite (Table and andalusite-rich layers in Rattlesnake Gulch. 4). Chloritoid (Table 5) is Fe-rich and has a higher Fe+2/(Fe+2 + The aluminous quartzite contains muscovite and various combi- Mg) ratio when it occurs with hematite than when hematite is ab- nations of A1 silicate, chloritoid, and hematite (Table 2). Some of sent. Muscovite analyses (Table 6) show low phengite content, as the kyanite, chloritoid, and muscovite show parallel alignment, indicated by low Fe + Mg and/or structural formulae with close to suggesting growth during the early penetrative deformations. An- the ideal 6 Si. Other analyses not shown in the tables are as follows: dalusite occurs as unoriented porphyroblasts enclosing kyanite sillimanite in PR-8B contains 0.65% by weight Fe203; hematite- and/or chloritoid or replacing kyanite. Sillimanite occurs as radiat- ilmenite in PR-8B is very inhomogeneous on a small scale and aver- ing fibrolite or, less commonly, as unoriented larger crystals. The ages 0.60% A1203 and 20.9% Ti02 (hematite59 ilmenite41); oc- paragenetic sequences for A1 silicate are kyanite —> andalusite, tahedra of magnetite in PR-8B contain 0.36% A1203 and 0.03% kyanite —> andalusite —» sillimanite, kyanite —> sillimanite, and Ti02; hematite in PR-21 contains 0.57% A1203 and 0.37% TiOz. rarely andalusite —» sillimanite. Kyanite is the most abundant A1 Ortega Quartzite AFM projections for analyzed assemblages are silicate, and sillimanite is the rarest. On the basis of the textures given in Figure 2. These quartzites show a great deal of similarity to and relative abundances of A1 silicates, the later static metamor- the Clough Quartzite in New Hampshire described by Rumble phism is believed to have begun in the kyanite stability field and (1971). The Clough in the locality described by Rumble also crys- moved through the andalusite field and, at least locally, into the sil- tallized near the A1 silicate triple point, but it contains more limanite field. However, the presence of kyanite or kyanite- aluminous layers and much more staurolite than the Ortega. chloritoid without andalusite or sillimanite in many rocks suggests that neither the kyanite-andalusite nor the kyanite-sillimanite phase TABLE 3. RINCONADA MINERAL ASSEMBLAGES boundary was surpassed by very much during the static metamorphism. Assemblage No. of thin Chloritoid occurs with each of the A1 silicates. In two specimens sections chloritoid, quartz, and coarsely crystalline sillimanite coexist with no staurolite or other sign of instability. None of the samples show Rinconada schists (+ Qz-Mus-Ilm-Gra-Tourmaline) chloritoid-A1 silicate reacting to staurolite. a. Biot 2 7 Hematite apparently behaves as an excess mineral representing b. Biot-Alm +3 c. St 1 Fe in the parent sandstone of some quartzites. When present, d. St-Biot 11 hematite exists with chloritoid, with chloritoid and A1 silicate, and e. St-Biot-Alm 17 with A1 silicate only, and there is no reaction relationship such as f. St-Biot-Alm-Plag 5 sillimanite or hematite replacing chloritoid. Thus kyanite-hematite represents a bulk composition with no Fe+2, and chloritoid could Rinconada Al-Mn schists (+ Qz-Mus-Ilm-Gra-Tourmaline) g. Biot-Spes-And 2 not form in such rocks. In rocks with hematite, the silicates are ex- h. Biot-Spes-And-Sill 1 pected to contain significant amounts of Fe+3. i. St-Biot-Spes-And-Par 1 Small amounts of muscovite represent the only mica in the j. St-Biot-Spes-Ky-Rut 1 Ortega Quartzite. One specimen contains staurolite (Table 2, h); Rinconada quartzites (+ Qz) rutile is the principal Ti mineral. Paragonite, pyrophyllite, and k. Biot 1 feldspars were sought but not found. The only spatial mineralogic 1. Mus-Gra 1 pattern seen in the quartzite is a greater abundance of sillimanite in m. Mus-Biot 1 the northern part of the area. Other assemblages are developed n. Mus-Biot-Spes 2 3 randomly, with several possible in the same outcrop. o. Ged-Spes

TABLE 2. ORTEGA MINERAL ASSEMBLAGES TABLE 4. SPECIMENS ANALYZED

Assemblage No. of thin sections Number Unit* Locality Assemblage

Ortega Quartzite (+ Qz-Mus-Rut-Tourmaline) P-1C Or Q Rattlesnake Gulch 2,e a. Ky 14 P-3A Or Q Rattlesnake Gulch 2,e b. Ky-And 4 PR-21 OrQ North Hondo Canyon 2, 1 c. Ctd 1 PR-8B OrQ North Hondo Canyon 2,o d. Ctd-Ky 4 P-26C Or P North of La Sierrita 2,Q e. Ctd-Ky-And 7 PR-53A R, S Hondo Canyon 3, b, e f. Ctd-Ky-And-Sill 1 P-23 R2 S North of La Sierrita 3, d g. Ctd-And 1 PR-53C R< S Hondo Canyon 3,e h. St-Ky-Sill 1 PR-62 R< S Hondo Canyon 3,e i. Hem-Ky 3 PR-25A R2S Hondo Canyon 3,f j. Hem-Ky-And-SUl 1 PR-74D R2 S Hondo Canyon 3,f k. Hem-Ky-Sill 2 P-78A R2s Hondo Canyon 3,f 1. Hem-Ctd 1 PR-67C1 R, Al-Mn S North Hondo Canyon 3, h, i m. Hem-Ctd-Ky 1 P-50B RA Al-Mn S South of Hondo Canyon 3, i n. Hem-Ctd-Ky-Sill 4 P-50A RE Al-Mi S South of Hondo Canyon 3, j o. Hem-Mag-Ctd-Sill 1 P-25B R Q North of La Sierrita 3,o Ortega pelitic enclaves (+ Qz-Mus-Tourmaline) 3 p. St 1 * Or = Ortega, R = Rinconada, Q = quartzite, P = pelitic enclaves, S = q. St-Ctd-Bio-Dm 1 schist. r. St-Ctd-And-Ky-Rut 1 Table number, assemblage designation letter.

Pelitic Enclaves. Two outcrops of pelitic rock were observed porphyroblasts, with the larger staurolite (~1 cm) giving the rock a entirely enclosed in quartzite (Fig. 1). Size and field relations could knobby appearance. Plagioclase is seen rarely as gray crystals on not be determined due to limited outcrop. The enclaves are inter- the weathered surface, appearing much like fine-grained andalusite

mediate in composition and mineralogy between the quartzites and or cordierite. R3 is friable tan to blue quartzite containing small

the Rinconada schists (Tables 2, 3). They contain chloritoid and amounts of micas and rare garnetiferous layers. R4 is fine- to staurolite with biotite or andalusite. Chloritoid and staurolite in medium-grained gray to silver muscovite-biotite-garnet schist with +2 +2 specimen P-26C (Tables 5, 9) are Fe

Mg)) and Fe93, respectively. cally more massive. R6 is layered silver-gray muscovite-biotite- garnet phyllite. Staurolite is a minor component in many rocks Rinconada Schist from Rg, and kyanite and andalusite were found at one locality (P-50; Table 4). Apart from the schist-quartzite fluctuations, the

Nielsen (1972) has subdivided the Rinconada into six members, general trend of decreasing grain size from R2 to R4 to R« seems to

Ri to R6 from bottom to top. R! is muscovite-biotite-andalusite relate to increasing graphite and decreasing staurolite, garnet, and (very coarse) schist, well developed in northern Hondo Canyon but biotite. The Pilar Formation represents an extreme of this trend.

absent in many other areas. R2 is medium-grained muscovite- For petrologic purposes I have grouped Rinconada into normal staurolite-biotite-garnet schist. The latter three minerals occur as schists (discussed below), Al-Mn schists (R] and one locality in R«), and quartzites (R3 and R5). TABLE 5. CHLORITOID CATIONS ON THE BASIS Normal Rinconada Schist. For most of the normal schists the OF 12 OXYGENS mineral assemblage is staurolite-biotite-almandine-muscovite- quartz-ilmenite-graphite with possible plagioclase (Table 3). Mus- P-1C P-3A PR-21 PR-8B P-26C covite and locally some biotite show preferred orientation and are IV SI 1.993 2.044 2.098 2.020 2.067 cut by porphyroblasts of staurolite, almandine, and biotite which Al 0.007 must have crystallized during the thermal event(s). Staurolite is sieved with quartz but locally contains rectangular inclusion-free VI Al 4.042 3.937 3.839 3.955 3.916 areas that may have once been chloritoid. Plagioclase occurs rarely Ti 0.001 0.001 0.000 0.001 0.001 4.043 3.938 3.839 3.956 3.917 as small untwinned porphyroblasts sieved with quartz. Sulfides and alkali feldspar are absent. Fe 1.712 1.618 1.623 1.866 1.874 Mineral analyses for normal schists (Tables 4, 7, 8, 9) show Mg 0.198 0.263 0.142 0.095 0.113 staurolite Fe to Fe 6> almandine Fe 7 to Fe with an average of 12 Mn 0.029 0.125 0.281 0.064 0.003 81 8 8 90 2 1.939 2.006 2.046 2.025 1.990 mole percent combined spessartite and grossularite, and biotite Fe52 to Fe62- AFM diagrams (Fig. 2) and Kn plots for coexisting mineral Fe+2/(Fe+2 + Mg + Mn)" 87.5 79.5 77.1 91.4 93.8 pairs (Fig. 3) indicate a good approach to equilibrium and suggest Mn/(Fe+2 + Mg + Mn) 1.6 6.6 14.8 3.4 0.2 +2 +2 that Mn has no effect on Mg/Fe KD values. The almandine-biotite Fe /(Fe + Mg) 89.6 85.1 91.2 94.7 93.9 average K of 0.154 indicates a temperature of 530 °C by the Assemblage* 2,e 2, e 2,1 2,o D 2,q Thompson (1976) geothermometer and 550 °C by the modification * Corrected for Fe+3 by subtracting 7% of total Fe in hematite-absent proposed by Holdaway and Lee (1977). Errors for these tempera- specimens and 9% in hematite-bearing specimens (see text); in mole percent. ture estimates are about ±75°. + Table number, assemblage designation letter. Garnet and staurolite crystals are reasonably homogeneous.

TABLE 6. MUSCOVITE CATIONS ON THE BASIS OF 22 OXYGENS

P-1C P-3A PR-8B PR-53A PR-53C PR-62 PR-25A PR-74D PR-78A PR-67C1 P-50B P-50A IV Si 6.125 5.976 6.151 6.117 6.227 6.147 6.153 6.233 6.153 6.073 6.159 6.054 Al 1.875 2.024 1.849 1.883 1.773 1.853 1.847 1,767 1.847 1.927 1.841 1.946

VI Al 3.856 3.920 3.681 3.828 3.775 3.805 3.725 3.638 3.733 3.575 3.846 4.008 Ti 0.030 0.044 0.042 0.048 0.040 0.040 0.048 0.046 0.050 0.076 0.040 0.034 Fe 0.066 0.060 0.295 0.105 0.088 0.088 0.113 0.157 0.109 0.253 0.060 0.062 Mg 0.056 0.056 0.086 0.094 0.072 0.086 0.149 0.149 0.125 0.185 0.062 0.094 Mn 0.000 0.002 0.002 0.002 0.002 0.002 0.003 0.003 0.004 0.002 0.001 0.002 1 4.008 4.082 4.106 4.077 3.977 4.021 4.038 3.993 4.021 4.091 4.009 4.200

XII K 1.552 1.513 1.621 1.414 1.492 1.568 1.646 1.729 1.618 1.630 1.333 1.361 Na 0.376 0.318 0.251 0.370 0.456 0.358 0.310 0.344 0.354 0.384 0.535 0.499 Ca 0.004 0.002 0.001 0.002 0.002 0.004 0.004 0.005 0.002 0.002 0.001 0.003 2 1.932 1.833 1.873 1.786 1.950 1.930 1.960 2.078 1.974 2.016 1.869 1.863

Par9 19.5 17.4 13.4 20.7 23.4 18.6 15.8 16.6 18.0 19.1 28.6 26.8 Phengt 6.1 5.8 18.7 9.9 8.1 8.8 13.1 15.5 11.8 21.5 6.1 7.5 Assemblage 2, e 2, e 2,o 3, b, e 3,e 3,f 3,f 3,f 3,f 3, h, i 3, i 3, i * Na/(Na + K), in mole percent. + 2 (Fe+Mg+Mnffi1*') — phengitic component for muscovite with low Fe+3; in mole percent. * Table number, assemblage designation letter.

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Garnet rims average 2.5 mole percent less spessartite and 1.5 mole more Na, Ca, and Mn. Of greater significance is the presence of percent less grossularite than cores, whereas the Fe/Mg ratio graphite and ilmenite in the schist, indicating conditions more re- changes little. Staurolite rims in two rocks average about 1.5 mole ducing than those for the Ortega Quartzite. Except for the low percent more Fe-rich than cores. plagioclase and the lack of sulfides, Rinconada schist is similar to Muscovite in the normal schists (Table 6) contains 16 to 23 mole most Paleozoic pelitic schist. percent paragonite and 8 to 15 mole percent phengite. Plagioclase Rinconada Al-Mn Schists. A1 silicate-bearing schists locally de- in specimen PR-25A is An^. Ilmenite in specimen PR-53C contains veloped in the Rinconada contain A1 silicate (andalusite, 0.39% by weight MnO as its greatest impurity and no hematite porphyroblasts as large as 10 cm in R,, or kyanite), biotite, stauro- solid solution. lite, and varying amounts of Mn-rich garnet (Table 3). Staurolite is

Compositionally, the normal Rinconada schist differs from Fe80 to Fe82, garnet is Fe75 to Fe88 with 31 to 39 mole percent spes-

Ortega quartzite in having higher K/Al, higher Mg/Fe, and slightly sartite, and biotite in one specimen is Fe32 (Figs. 2, 3). Staurolite in

Figure 2. AFM projections of analyzed mineral assemblages. Only the most Fe-rich and the most Mg-rich assemblage is shown for

each lithology. Closed symbols = analyses; open symbols = estimated composition based on KD values (Fig. 3) for altered or trace minerals. A, Ortega Quartzite: h = hematite present, dashed lines = pelitic enclave; B, Rinconada schist; C, Rinconada Al-Mn schist: number refers to mole percent spessartite. See Table 1 for mineral abbreviations; AlSil = aluminum silicate.

TABLE 7. BIOTITE CATIONS ON THE BASIS OF 22 OXYGENS

PR-53A P-23 PR-53C PR-62 PR-2.5A PR-74D PR-78A PR-67C1 IV Si 5.203 5.235 5.350 5.336 5.315 5.458 5.315 5.410 Al 2.797 2.765 2.650 2.664 2.685 2.542 2.685 2.590

VI Al 0.944 1.030 1.085 0.998 0.877 0.752 0.979 0.845 Ti 0.259 0.221 0.237 0.205 0.197 0.213 0.205 0.175 Fe 2.712 2.813 2.716 2.596 2.513 2.546 2.511 1.533 Mg 1.862 1.711 1.603 1.888 2.232 2.250 2.047 3.210 Mn 0.001 0.004 0.002 0.002 0.002 0.002 0.004 0.021 X 5.779 5.779 5.643 5.689 5.821 5.763 5.746 5.784

XII K 1.698 1.634 1.726 1.788 1.769 1.765 1.742 1.765 Na 0.078 0.093 0.082 0.078 0.020 0.072 0.070 0.070 Ca 0.001 0.001 0.000 0.007 0.000 0.003 0.003 0.000 2 1.777 1.728 1.808 1.873 1.789 1.840 1.815 1.835

Fe+2/(Fe+2 + Mg + Mn)* 58.5 61.4 62.1 57.1 52.2 52.3 54.3 31.5 Mn/(Fe+2 + Mg + Mn) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.5 Fe+2/(Fe+2 + Mg) 58.6 61.5 62.2 57.1 52.2 52.3 54.3 31.7 Assemblage+ 3, b, e 3, d 3,e 3,e 3. f 3, f 3, f 3, h, i 1 Corrected for Fe+3 by subtracting 3% of total Fe (see text); in mole percent. Table number, assemblage designation letter.

these rocks contains 4.5 mole percent Mn, whereas coexisting an- (Cao.04Nao.01Fe3.20Mn2.08Mgo.09Al1.70) (Al2.22Si5.7s)O22 (OH)2. The dalusite is free of Mn. Apparently Mn+2, stable under reducing amphibole is an Mn ferrogedrite, which plots above garnet on an conditions, cannot substitute in andalusite. The viridine occurrence AFM diagram and has about the same Mn/Fe ratio as the coexisting described by Stensrud (1972) from the Picuris Mountains is in garnet. The Rinconada quartzites may be in part metachert. On the hematite-bearing Ortega Quartzite near the town of Pilar. Speci- whole, these quartzites are less aluminous and crystallized under men P-50B contains traces of paragonite, suggesting that the limit more reducing conditions than the Ortega Quartzites.

of paragonite solid solution in muscovite is about Mus7iPar29 (Ta- ble 6) in rocks of this grade. The schists of this category differ from PETROLOGIC INTERPRETATION the normal schists in having more Al, less Fe, and in some cases more Mn. If the Ortega Quartzite and Rinconada schists were seen in different areas, one might conclude that they were metamorphosed at Rinconada Quartzite different grades. However, the two units were folded together, share the same history, share a unique sequence of Al silicates, and Thin sections of Rinconada quartzite (Table 3) have minor everywhere exhibit the same lithologic transition from chloritoid- amounts of biotite and muscovite, garnet, or garnet and amphibole. bearing Ortega Quartzite to staurolite-bearing Rinconada schist. In the analyzed specimen (P-25B, Table 8), the garnet contains 38 This transition takes place over at most 5 to 10 m and may be seen mole percent spessartite, and the amphibole has the formula in localities separated by as much as 10 km. There is no alternative

TABLE 8. GARNET CATIONS ON THE BASIS OF 12 OXYGENS

PR-53A Pr-53C PR-62 PR-25A PR-74D P-78A PR-67C1 P-50B P-50A P-25B

IV Si 3.006 3.033 3.029 2.990 2.999 2.986 2.974 3.004 2.983 2.970 Al 0.010 0.001 0.014 0.026 0.017 0.030

VI Al 2.059 2.025 1.998 2.047 2.028 2.066 2.010 2.020 2.048 2.015 Ti 0.000 0.001 0.002 0.001 0.000 0.000 0.006 0.004 0.002 0.010 2 2.059 2.026 2.000 2.048 2.028 2.066 2.016 2.024 2.050 2.025

VIII Fe 2.427 2.426 2.263 2.248 2.196 2.198 1.439 1.558 1.550 1.641 Mg 0.257 0.278 0.269 0.259 0.291 0.281 0.483 0.220 0.237 0.050 Mn 1.119 0.104 0.264 0.200 0.227 0.256 0.935 1.146 1.157 1.142 Ca 0.096 0.083 0.145 0.223 0.241 0.172 0.127 0.124 0.124* 0.140 2 2.889 2.891 2.941 2.930 2.955 2.907 2.984 2.948 2.968 2.973

Alm+ 83.7 83.9 76.9 76.7 74.3 75.6 48.2 52.8 52.2 55.2 Pyrf 8.9 9.6 9.1 8.8 9.8 9.7 16.2 7.5 8.0 1.7 Spes 4.1 3.6 9.0 6.8 7.7 8.8 31.3 38.9 39.0 38.4 Gro 3.3 2.9 4.9 7.6 8.2 5.9 4.3 0.8 0.8 4.7 Fe+2/Fe+2 + Mg) 90.4 89.7 89.4 89.7 88.3 88.6 74.8 87.6 86.7 97.0 Assemblage* 3, b, e 3,e 3,e 3, f 3,f 3,f 3, h, i 3, i 3, i 3, 0 * Estimated value. Aim = almandine, Pyr = pyrope, Spes = spessarite, Gro = grossularite; in mole percent. Table number, assemblage designation letter.

TABLE 9. STAUROLITE CATIONS ON THE BASIS OF 23.5 OXYGENS

P-26C PR-53A P-23 PR-53C PR-62 PR-25A PR-74D P-78A P-50B P-50A

IV Si 4.102 4.032 3.993 4.132 4.021 4.088 4.208 4.011 4.091 3.988 Al 0.007 0.012

VI Al 8.883 8.838 8.863 8.679 8.900 8.652 8.521 8.809 8.876 9.024 Ti 0.046 0.070 0.058 0.061 0.074 0.065 0.059 0.063 0.054 0.072 2 8.929 8.908 8.921 8.740 8.974 8.717 8.580 8.872 8.930 9.096

Fe 1.753 1.752 1.779 1.797 1.665 1.806 1.774 1.758 1.497 1.413 Mg 0.128 0.282 0.308 0.290 0.283 0.392 0.398 0.368 0.310 0.336 Mn 0.002 0.004 0.009 0.013 0.014 0.014 0.013 0.015 0.086 0.081 2 1.883 2.038 2.096 2.100 1.962 2.212 2.185 2.141 1.893 1.830

Fe+2/(Fe+2 + Mg + Mn)» 92.8 85.3 84.2 84.9 84.2 80.9 80.4 81.3 78.2 76.3 Mn/(Fe+2 + Mg + Mn) 0.1 0.2 0.4 0.6 0.7 0.7 0.6 0.7 4.7 4.6 Fe+2/(Fe+2 + Mg) 92.9 85.5 84.6 85.5 84.8 81.4 80.9 81.9 82.1 80.0 AssemblageT 2,q 3, b, e 3, d 3,e 3,e 3,f 3, f 3,f 3, i 3, i * Corrected for Fe+3 by subtracting 5% of total Fe (see text). Table number, assemblage designation letter.

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but to conclude that the Ortega and Rinconada crystallized under sentially the same composition as staurolite (Fe93). In any case, the

the same conditions of Ptot and T. Furthermore, as will be seen be- compositional similarity of these two minerals when they coexist low, the assemblages restrict the conditions sufficiently that these suggests that reactions involving them will not be strongly affected Ptot-T conditions may be applied over the whole Picuris Range. by Fe/Mg ratio. In the remainder of this report, we will consider mainly Ortega As a result of differences in fluid composition and bulk-rock Quartzite, Rinconada schist, and Rinconada Al-Mn schist. The composition, several topologies are represented at a single grade in Ortega pelitic enclaves are transitional in nature, and the Rin- the Picuris Mountains. The Ortega Quartzite occurrences, with one conada quartzites give no useful information concerning metamor- exception, did not undergo reaction of chloritoid—Al silicate to phic conditions. staurolite. Ortega is represented by stage a or b (Fig. 4). The Ortega It is probably safe to conclude that all the rocks in the area crys- pelitic enclaves (Table 2) containing chloritoid and staurolite are tallized at slightly higher temperatures and slightly lower pressures represented by stages c and e. In the Rinconada schists, not only did than the Al silicate triple point during the thermal event(s). If none chloritoid react with Al silicate to allow initial formation of stauro- of the boundaries were overstepped by more than about 25 °C, any lite, but also chloritoid disappeared completely by reaction with Al silicate could remain in individual specimens, depending on local muscovite to produce staurolite, almandine, and biotite. The variations in access to water the other kinetic factors. Ideally all presense of the pair bioi:ite-Al silicate and the absence of rocks should show the sequence kyanite —» andalusite —» silliman- staurolite-chlorite (Table 3) indicate that stageg was reached in the ite, but in some rocks kinetics prevented reaction, and in others the Rinconada rocks. In the Al-Mn schists, staurolite-muscovite began andalusite field was crossed and sillimanite began to grow before reacting to garnet—Al silicate-biotite by a divariant reaction in andalusite could get started. The transition andalusite —» sillimanite which the products were stabilized by high Mn in garnet (stages h has the lowest entropy change and therefore requires the greatest and i). In some of these rocks staurolite disappeared entirely — if it degree of overstepping (Holdaway, 1971). This transition is ob- ever was present.1. served least often in the Picuris Range (Tables 2, 3). In order to interpret the mineral assemblages of the Picuris 1 Kepezhinskas and Khelstov (1977) suggested the possibility that Mountains, I assume the validity of Figure 14 of Albee (1972), part staurolite could be slightly richer in Fe than coexisting chloritoid. If this of which is reproduced here as Figure 4. Albee's analytical results were the case, the Picuris rocks would have resulted from a smaller range of show that chloritoid is slightly more Mg-rich than coexisting topologies than indicated by Albee's data. However, the main conclusions staurolite, leading to the sequence of topologies with increasing of this report would be unaffected, because normal Rinconada staurolite and the Ortega chloritoid overlap in Fe+2/(Fe+2 + Mg) and differ by only 7 temperature as shown. The present data neither confirm nor negate mole percent, on the average. K0 values are close enough to 1 that a reversal Albee's results, because minerals were analyzed in only one in preference for Fe would not explain the differences between the quartzite chloritoid-staurolite rock (P26-C) in which chloritoid (Fe^) has es- and the schist.

BIOTITE STAUROLITE 0.5 1.0 1.5 2.0 0.02 0.04 0.06 0.8 i r 'Mn ' Mn

Mg/Fe Mn / Fe / - - 0.6 ÜJ z o 0.4 | 2 <_/ / \/ M n - Fe = 12.6 / ^ Alm - St 0.2

Figure 3. KD diagrams for coexisting -1 L_ / 0 mineral pairs in Rinconada Formation. r Q 4 Mn-rich garnets are so identified. See Table 1 for mineral abbreviations.

Mg/Fe Mg/Fe 0.3 ijj z M F o - KDA r- sV06l 0.2 §

Mn * Mn K M9~Fe =0 154 DAIm - Biot 0.1

-J L_ 0.5 1.0 1.5 2.0 0.1 0.2 0.3 BIOTITE STAUROLITE

We now examine what characteristics of fluid and mineral com- must be adjusted for the composition or expected composition of position would allow the various topologies described above to the minerals and fluid in the reaction. (2) If the assemblage on one

exist all at the same P„)t and T. side of an experimentally determined reaction is present, then the rock crystallized within the stability field of that assemblage (ad- CONDITIONS OF METAMORPHISM justed for mineral and fluid composition). For example, Rinconada staurolite grew from reaction of chloritoid-muscovite to Unfortunately, the experimentally determined equilibria relating staurolite-almandine-biotite. Even though reaction 2, below, may to chloritoid and staurolite are largely Fe end-member reactions never have occurred in these rocks, they must still have crystallized with no micas present. The reactions may be applied to the present on the high-temperature side of this reaction, because staurolite- problem using the following principles. (1) Each reaction condition almandine is present and chloritoid-quartz is absent. Reference to Albee (1972) shows that Fe-Mg divariant reaction 2 terminates in the univariant chloritoid-muscovite reaction. By choosing the ter- minal chloritoid composition, one can model the chloritoid- muscovite breakdown conditions using reaction 2. (3) In similar fashion, if all the phases on both sides of an experimentally deter- Aim Aim mined reaction are present, conditions were on the equilibrium boundary for that reaction (assuming appropriate adjustment for mineral and fluid composition). This is the general procedure used Biot by Carmichael and his coworkers (Carmichael and others, 1974) on igneous rocks and by Ghent (1975) on metamorphic rocks. The pertinent experimental equilibria are shown in Figure 5. The Al silicate diagram of Holdaway (1971) was chosen on the basis of supporting arguments given in that paper and because it allows the Aim Aim stable coexistence of chloritoid, sillimanite, and quartz, as seen in the Ortega Quartzite. On the basis of this diagram, we assume Ptot = 3,700 b, and we further assume that during progressive 2 metamorphism Pnuw = Ptot- The Fe end member reactions are

4 Ctd + 5 Sill ^ 2 St + Qz + 2 HzO, (1)

23 Ctd + 8 Qz ^ 4 St + 5 Aim + 19 HaO, (2)

Aim and

6 St + 11 Qz^ 4 Aim + 23 Sill + 6 H20. (3)

Biot Biot Blot These reactions have been determined in the presence of a nearly

Figure 4. AFM projections illustrating sequence of topology pure H20 fluid phase by Ganguly (1969) and Richardson (1968) changes when almandine-chlorite tie line breaks early in sequence. and are probably accurate to ±15 °C. Because of the coupled reac- Letters indicate relative temperatures. After Albee (1972, Fig. 14). See Table 1 for mineral abbreviations; Ch = chlorite; Cd = 2 If for any rock Pnuid < Ptot — that is, a fluid phase did not exist — then cordierite. the calculated fugacities and partial pressures must be upper limits.

Figure 5. PH.l0-T diagram of experimentally determined Al silicate and Fe end- member equilibria for reactions 1,2, and 3. Data are from Ganguly (1969), Richardson (1968), and Holdaway (1971). See Table 1 for mineral abbreviations.

500 550 600 650 700 T °C

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tion principle (Fyfe and others, 1958), reaction 1 must proceed at combination of minerals or oxides as close as possible to the struc- lower temperatures than reaction 2. ture of the mineral was used, and a volume correction was included In the calculations that follow, we assume that the Fe end mem- according to the method of Fyfe and others (1958). The main con- bers of chloritoid, almandine, and staurolite participate in ideal tribution to AS is the dehydration of water, equal to 9 to 11 cal/ ionic solid solutions. This is only an approximation, especially in degree mole of water. the case of staurolite (Griffin and Ribbe, 1973). However, stauro- Whenever possible, the analyzed mineral compositions were used

lite and chloritoid have a nearly constant KD for Mg/Fe with vari- to evaluate Ka, but for reactions 1 and 2 the compositions of pos- able composition (Albee, 1972), indicating that ideality may give sible staurolite or chloritoid had to be determined independently. I reasonable estimates at Fe-rich compositions. assumed that the first staurolite to form by reaction 1 in Ortega The calculation includes a term for dilution of the Fe end mem- Quartzite would be in exchange equilibrium with the present bers by solid-solution components Mg, Mn, and Ca, and a second chloritoid and that the last chloritoid to disappear in the Rinconada

term for dilution of the pure H20 in the fluid phase. Let T0 be the Formation, modeled by reaction 2, would have been in exchange initial temperature for each reaction, 533 °C for reaction 1, 548 °C equilibrium with the staurolite that formed. I further assumed

for reaction 2, and 680 °C for reaction 3; let Ta be the temperature that this staurolite did not: change significantly in composition

after mineral composition adjustment and Tb be the final tempera- once it formed. The average value of Klf cta-st determined by Al- ture after mineral composition and fluid composition adjustment. bee (1972) is 1.2, allowing calculation of the appropriate com- The standard relationship for ideal solid solution in a reaction at positions. The Mn site occupancy of coexisting staurolite and

constant Ptot = PH2o is chloritoid was assumed to be equal (Albee, 1972). The composition of staurolite in PR-67C1 (present in small amounts but not analyzed) was approximated using Mg/Fe and Mn/Fe values for alman- K T„ - T„ = AT„ = " , (4) dine-staurolite (Fig. 3). A Sa For the Ortega Quartzite, XH20 must have been near 1 in the ab-

where ASa is reaction entropy at the midpoint of the interval AT„ sence of graphite and carbonates, assuming that the fluid phase

and at the pressure of interest. The equilibrium constant Ka in- contained no other fluid components to dilute it. The graphite- volves mole fraction of Fe+2 in each mineral raised to the number of bearing Rinconada schists and Al-Mn schists must have contained times Fe+2 appears in that mineral for the balanced reaction. For other components in the fluid phase. The following reactions may st 8 Alm 15 ctd 23 reaction 2, Ka is (XFe ) (XKt ) /(XFt, ) . be written between possible fluid components and graphite (Eugs- ter and Skippen, 1967). RT K T, - T„ = AT, = "h_ " . (5) AS 6 2H20 + 2Gra ^ CH4 + C02 (6) The same relation adjusts equilibrium temperature for dilution of 2H20 ^ 2H2 + 02 (7)

the fluid phase at constant Ptol and mineral compositions. Here ASb 02 + 2Gra ^ 2CO (8) is reaction entropy involving the designated solid solutions taken at 02 + Gra ^ C02 (9) the midpoint of ATb. At compositions not far from the end-mem- 2 Pnutd = 3,700 b. (10) ber compositions, AS„ ~ ASb at a given temperature because exchange reactions normally have low entropies (Thompson, 1976). The standard free energies for reactions 6 through 9 were evaluated Assuming ideal mixing at high mole fractions of H20, and for the using data of Robie and Waldbaum (1968), including a correction present reactions involving H20 as the only volatile, Kb is X"I)20 for volume of graphite at 3,700 b. Equilibrium constants were cal- where XHa0 = PH2O/Pt0t and n is the number of moles of H20 culated from the free energies at 800 °K and equated to the fugacity in the reaction (Kerrick, 1974). Each step of the calculations was ratios for each reaction. Using the Lewis-Randall rule, fugacity done twice so that rough T values could be used to improve AS coefficients were evaluated from data of Ryzhenko and Volkov and Tb. (1971) and Burnham and others (1969) at 3,700 b and 800 °K Entropy values used for the calculations are given in Table 10. (Table 11). Where entropies were estimated from known minerals or oxides, a With five equations for fluid component pressures and six un- known pressures, one additional relation is required. Two pos- TABLE 10. ENTROPY VALUES FOR sibilities were investigated. The first is that no C0 or CH enters Fe END MEMBERS, REACTIONS 2 4 the schists and that water produced by dehydration reacts with S800 °K S900 °K Source

Quartz 23.76 25.75 Robie and Waldbaum (1968) TABLE 11. FUGACITY COEFFICIENTS AND Sillimanite 60.68 66.01 Robie and Waldbaum (1968) PARTIAL PRESSURES FOR GASES, f Almandine 178.05 192.15 Gro + FeO - CaO 800 °K, 3,700 B, WITH GRAPHITE Staurolite 329.33 356.65 Ky + A12Os + FeO + Ice" — Chloritoid 106.63 114.58 Oxides* Gas P CO2 Pc H< fo. = QFM Water (3,700 b) 32.48 34.40 Burnham and others (1969)

H2O 0.41 3,255 b 3,133 b AS, 17.46 19.48 co2 3.44 218 b 466 b

AS2 182.12 199.61 CH, 4.82 218 b 94 b

AS3 65.38 70.08 H2 2.69 9 b 6 b CO 5.13 0.6 b 1 b -2308 22 75 Note: Entropy (S) in cal/degree mole. 02 fo, = 10 atm fo, = 10~ atm * Corrected for disorder (Holdaway, 1971). XhîO 0.88 0.85 + Volume correction included (Fyfe and others, 1958). * Configurational term included (Ulbrich and Waldbaum, 1976). * Ryzhenko and Volkov (1971), Burnham and others (1969).

graphite, reaction 6, to produce equal amounts of C02 and CH4 in posite directions. Additional possibilities are (1) error in Albee's

the absence of other C-bearing solid phases. Thus, Pco, = Pch« (1972) Kd value for chloritoid-staurolite; (2) small amounts of Zn leading to partial pressures as given in Table 11 (first column). in the pelitic schists might have stabilized staurolite slightly; (3) Another possible relation suggested by the high content of Fe+2 in Fe+3 in chloritoid may have had a small stabilizing effect in the = the silicates is that /o2 is approximately that of the quartz-fayalite- quartzites; and (4) the value of XH2o 0-88 is clearly an upper limit magnetite (QFM) oxygen buffer (Table 11, second column). The for Rinconada schists, and any of several factors could have low-

equal C02, CH4 assumption leads to slightly lower f02 and XHl0 at a ered it, thus promoting the complete destruction of chloritoid. maximum value of 0.88. It is possible that more rapid diffusion of These factors are the dilution of fluid by external C02, the more

smaller CH4 molecules leads to a distribution closer to the QFM rapid diffusion of CH4 than C02 from the rocks, and the nonexis- = buffer, and XH2o 0.85. For the calculations, 0.88 was used, lead- tence of a fluid phase promoted by more rapid diffusion of fluid

ing to a minimum value for ATb. The fluid phase composition is components through the pelites than through the quartzites. A similar to that calculated by Ghent (1975) for similar rocks with value of XH2O = 0.78 would double ATb (Table 12) and give an graphite but no sulfides. upper limit of chloritoid stability of 510 °C. The final calculations (Table 12) show that on the average, Ortega Quartzite crystallized below 539 °C, Rinconada schists CONCLUSIONS crystallized above 526 °C, and Rinconada Al-Mn schists formed at about 550 °C with a larger error. Mg content increased the tem- In rocks of the Picuris Mountains, mineral assemblages repre- perature of reaction 1 in Ortega Quartzite, and Mn and Ca in gar- senting several topologies in Albee's (1972) AFM projections may

net reduced the average temperature of reaction 2 in Rinconada be explained as crystallizing at 532 ± 20 °C and 3,700 b Ptot if dif- schist. However, the main effect on the normal schists was reduc- ferences in fluid and bulk-rock composition are taken into account. tion of temperature of reaction 2, caused by reaction of graphite The results underscore the importance of making corrections for with water, producing C02 and CH4. High Mn in garnets of the mineral and fluid compositions. For equilibria involved in pelitic Al-Mn suite caused local drastic lowering of the equilibrium tem- systems, Al-rich quartzites containing neither graphite nor car- perature for reaction 3. PR-67C1 is the most Mg-rich rock studied, bonates come closest to representing the ideal dehydration equilib- and presumably the anomalous temperature for this rock results ria. from the breakdown of the staurolite ideality assumption. Calcu- The presence of chloritoid and sillimanite together in Ortega lated staurolite composition for this rock was X$t = 0.624. The Quartzite with no sign of reaction attests to the presence of a best estimate of temperature for these rocks is 532 ± 20 °C, consis- chloritoid-sillimanite-quartz stability field, as suggested by the tent with all analytical data except that for PR-67C1. chloritoid stability data of Ganguly (1969) and Richardson (1968) Although the calculations indicate that a temperature of 530 °C and the Al silicate data of Holdaway (1971). A stability field for is plausible for all the Picuris rocks, one would expect some tem- paragonite-sillimanite-quartz may also exist, and the assemblage perature fluctuations, perhaps between 520 and 540 °C in an area might be found in the Picuris Mountains if endugh sillimanite- as large as the Picuris Mountains. (Temperatures cannot have lo- bearing rocks were examined. cally varied above about 540 °C, or chloritoid-Al silicate would have begun to react in the quartzite.) Yet none of the Rinconada schists contain even small remnants of chloritoid. If we exclude the ACKNOWLEDGMENTS possibility of large nonrandom errors in the entropy estimates and ideal mixing assumptions, it would appear that additional factors I thank Harold Dailey and John Futch for assistance in the field were operative in driving the equilibria for reactions 1 and 2 in op- and in pétrographie work. Elaine Padovani, James Toni, and James Carter were helpful with microprobe analyses. I acknowledge with thanks University of Texas at Dallas for making their electron mi- TABLE 12. EQUILIBRIUM TEMPERATURES croprobe available. I am indebted to Edward Grew for noting the CALCULATED FROM REACTIONS 1, 2, AND 3 presence of two oxides in PR-8B. This project was supported by Rock Reaction AT/ A T? Ti b* National Science Foundation Grant GA-35644. Facilities of South- (°C) ern Methodist University's Fort Burgwin Research Center, New P-1C 1 +7 0 <540 ± 2 Mexico, were used during the field work for this research. P-3A 1 +9 0 <542 ± 2 PR-8B 1 +3 0 <536 ± 1 REFERENCES CITED PR-53A 2 +3 -21 >530 ± 6 PR-53C 2 + 1 -21 >528 ± 6 Albee, A. L., 1972, Metamorphism of pelitic schists: Reaction relations of PR-62 2 -6 -21 >525 ± 7 chloritoid and staurolite: Geological Society of America Bulletin, v. PR-25A 2 0 -21 >527 ± 6 83, p. 3249-3268. PR-74D 2 -3 -21 >524 ± 6 Burnham, C. W., Holloway, J. R., and Davis, N. F., 1969, Thermodynamic P-78A 2 -3 -21 >524 ± 6 properties of water to 1000°C and 10,000 bars: Geological Society of PR-67C1 3 -76 -19 585 ± 24 America Special Paper 132, 96 p. P-50B 3 -112 -19 549 ± 33 Carmichael, I.S.E., Turner, F. J., and Verhoogen, J., 1974, Igneous petrolo- P-50A 3 -108 -19 553 ± 32 gy: New York, McGraw-Hill Book Co., 714 p. Best value 532 ± 20 Deer, W. A., Howie, R. A., and Zussman, J., 1962, Rock forming minerals. Vol. 1, Ortho- and ring silicates: New York, John Wiley 8t Sons, 333 *T,- T0, effect of mineral compositions. P- T — T , effect of dilution of water by C0 , CH4. fJ 3 2 Eugster, H. P., and Skippen, G. B., 1967, Igneous and metamorphic reac- * Error is estimated at 25% of AT. Error for the best value includes ±5° from analysis of all but PR-67C1 and ±15° error in the experiments tions involving gas equilibria, in Abelson, P. H., ed., Researches in for reactions 1, 2, and 3. geochemistry, II: New York, John Wiley &c Sons, p. 492-520. Fullagar, P. D., and Shiver, W. S., 1973, Geochronology and petrochemis-

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