Systematic Retrograde Metamorphism of Sillimanite-Staurolite Schists, New Salem Area, Massachusetts

Systematic retrograde metamorphism of sillimanite-staurolite schists, New Salem area, Massachusetts

KURT HOLLOCHER Geology Department, Union College, Schenectady, New York 12308

ABSTRACT domes extending from Long Island Sound to the Maine-New Hampshire The New Salem retrograde metamorphic zone occupies an ap- border (Thompson and others, 1968; Robinson, 1979). The retrograde proximately triangular area covering -26 km2 in sillimanite- rocks lie entirely within Paleozoic cover rocks in the east-dipping isoclinal staurolite-grade rocks, largely pelitic schists of the Devonian Littleton Prescott syncline that lies between the Monson dome to the east and the Formation. Acadian prograde metamorphism at ~6 kbar and -600 °C gneiss-cored Kempfield anticline and Pelham dome to the west (Fig. 2). produced the common assemblage quartz-muscovite-biotite-garnet- The New Salem area was first mapped by Emerson (1898, and 1917, staurolite-ilmenite-albite-graphite±sillimanite±pyrrhotite. Systematic p. 76) and subsequently by Robinson (1963). These and more recent

hydration of the prograde assemblages by an influx of HzO produced studies have led to a relatively detailed, if still evolving, understanding of the retrograde assemblages chlorite-K-feldspar-celadonitic musco- the regional geology (Robinson, 1979,1983; Thompson and others, 1968; vite-sphene-anatase±pyrite (common) and chlorite-chloritoid-musco- Field, 1975; Tucker, 1977; Robinson and others, 1979, 1982a, 1982b; vite-anatase-sphene (rare). Retrograde sillimanite-out, staurolite-out, Hollocher, 1980,1981a, 1981b; Michener, 1983; Thompson, 1985). and garnet-out isograds have been mapped, representing the completion of important retrograde hydration reactions. These isograds plus Rock Units the thicknesses of chlorite rims on garnets (0 and 0.2 mm) can be used to subdivide the area into six concentric retrograde metamorphic There are four mapped rock units within the retrograde zone zones, designated R1 to R6. (Fig. 3A). The Partridge Formation is largely composed of rusty- During retrograde metamorphism, the exchange reactions K ^ weathering pyrrhotite-muscovite-biotite schist, locally bearing garnet, Na, Mn ^ Fe+Mg, and Fe ^ Mg proceeded while more or less staurolite, and sillimanite. Minor amphibolite is also present. The Clough consistent tie lines between the sheet silicates and ilmenite were main- Quartzite is a massive stretched quartz-pebble conglomerate, with thin, tained. Biotite and chlorite became more Fe rich owing to staurolite, discontinuous pelitic horizons. The Fitch Formation is composed of calc- garnet, and ilmenite breakdown. Biotite, chlorite, and ilmenite became silicate rocks and schists and is poorly exposed. The Littleton Formation is more Mn rich owing to garnet breakdown. Coexisting biotite and a gray-weathering muscovite-biotite-garnet-graphite schist. Staurolite (or muscovite became more K rich during biotite breakdown. Muscovite its pseudomorphs) is locally abundant, and sillimanite is rare. Minor also became progressively more celadonite rich. Chlorite, biotite, and quartzite and rare calc-silicate horizons are also present. ilmenite are homogeneous in single thin sections. Muscovite is zoned +2 +2 /F with respect to M /(M +A1 ) and K/(K+Na) ratios, which are Prograde Metamorphism higher in muscovite rims. There is no evidence for re-equilibration of staurolite, garnet, or albite during retrograde metamorphism. The rocks in the study area underwent deformation and prograde Temperature and pressure estimates for retrograde metamor- metamorphism during the Devonian Acadian orogeny. Acadian prograde phism are poorly constrained to -3.5 kbar and -280 °C. Each kilome- metamorphism reached chlorite grade in the Connecticut Valley-Gaspe tre of vertical extent of the retrograde zone required an influx of -0.26 synclinorium to the northwest of the New Salem area and reached the 3 km of H20 (at STP). Water may have been derived from a large granulite facies in central and south-central Massachusetts to the southeast volume of surrounding rock and concentrated in the study area by a (Tracy and others, 1976; Robinson, 1979; Robinson and others, 1982a). pressure shadow effect around the north end of the Prescott intrusive The boundary between the kyanite-staurolite and sillimanite-staurolite complex. zones (prograde zones I and II, respectively, of Tracy and others, 1976) is shown in Figure 3A. The location of this metamorphic boundary is inter- INTRODUCTION polated on the basis of aluminosilicate localities off the map to the north and south. The retrograded rocks occur entirely within the sillimanite- Geologic Setting staurolite zone, although sillimanite actually occurs in local bulk compositions only to the east of the prograde sillimanite-in isograd (Fig. 3A). The The retrograde metamorphic rocks occur in the Bronson Hill anti- prograde assemblages for pelitic schists in the study area are given in clinorium (Fig. 1), a north-northeast-trending belt of mantled gneiss Table 1A.

Additional material for this article (appendix tables) may be obtained free of charge by requesting Supplementary Data 87-18 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 98, p. 621-634, 10 figs., 6 tables, June 1987.

621

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/6/621/3434577/i0016-7606-98-6-621.pdf by guest on 01 October 2021 Figure 1. Regional geologic map of west-central Massachu- setts, simplified after Zen (1983). The retrograde metamorphic zone is within the small rectangle in the center of the map. A-B is the line of section for Figure 2.

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0 2 4 6 km Lower limit of interpretation

Figure 2. Cross section through central Massachusetts along the line A-B shown in Figure 1, simplified after Zen (1983). Patterns explained in Figure 1.

Retrograde Metamorphism which contain lower greenschist facies assemblages. Zone R1 occurs only to the east of the prograde sillimanite-in isograd. Table IB lists the assem- Local, incomplete retrograde effects are extremely common in meta- blages in the retrograde metamorphic zones. morphic rocks. These can include local retrograde ion exchange between adjacent minerals (for example, Tracy and others, 1976; Tracy and METHODS OF INVESTIGATION Dietsch, 1982) and minor local retrograde hydration reactions that produce such minerals as chlorite and sericite (for example, Spear and Selver- Two seasons of field work were conducted, and a total of ~80 thin stone, 1983). The pervasive and extensive retrograde hydration of rocks, sections were examined. A 174-m diamond drill core from the retrograde resulting in the recasting of prograde assemblages into retrograde assemblages in a broad region, is typically described as "polymetamorphism"

(for example, Eusden and others, 1984; Kato, 1985). In some cases, perva- TABLE 1. A. PROGRADE METAMORPHIC ASSEMBLAGES IN PELITIC SCHISTS IN THE STUDY sive retrograde metamorphism occurred systematically, and retrograde AREA AND B. ASSEMBLAGES OBSERVED IN RETROGRADE METAMORPHIC ZONES Rl TO R6 metamorphic isograds can be mapped (for example, van Reenen, 1986). A. Garnet (rare) This is the case in the New Salem retrograde zone. Biolite Biotite-gamet The New Salem retrograde zone (Figs. 3B and 3C) is an approxi- Biotite-garnet-staurolite mately triangular area covering ~26 km2 that straddles the axial surface of Biotite-garnet-staurolite-sillimanite the isoclinal Prescott syncline between flanking bodies of gneiss (Fig. 1). B. Retrograde Assemblages and isograds Plus or The southern base of the triangle is apparently in contact with the Prescott metamorphic minus intrusive complex of Acadian age, which is not significantly retrograded, zone

and the northern apex of the triangle is not exposed. The most severely Ri AH prograde assemblages Chlorite retrograded rocks occur in the Littleton Formation ~3 km north of the Prescott complex. R2 Chlorite Biotite-chlorite Retrograde hydration reactions have superimposed retrograde iso- Biotite-garnet-chlorite «•feldspar grads on the Acadian prograde metamorphic terrane. The three isograds Biotite-gamet-staurolite-chlorite Anatase

represent the completion of sillimanite-out, staurolite-out, and garnet-out R3 to R5 Chlorite Biotite-chlorite K-feldspar retrograde hydration reactions. The abundance of garnet in the Littleton Biotiie-gamet-chlorite Anatase Formation and the regular style of replacement of garnet by chlorite have Gamet-chlorite (rare) Sphene Garnet-chlorite-chloritoid (rare) Pyrite allowed for the mapping of two additional isograds, the first appearance of chlorite rims on garnets and the occurrence of 0.2-mm-thick chlorite rims R6 Chlorite K-feldspar Chlorite-biotite Anatase on garnets. The three retrograde metamorphic isograds plus the lines of 0- Sphene and 0.2-mm-thick chlorite rims separate the retrograded area into six Pyrite concentric retrograde metamorphic zones, designated R1 to R6, from the Note: the sillimanite-bearing assemblage occurs only east of the prograde sillimanite-in isograd (Fig. 3A). least retrograded sillimanite-bearing rocks to the most retrograded rocks,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/6/621/3434577/i0016-7606-98-6-621.pdf by guest on 01 October 2021 Devonian R2// I Sillimanite- Littleton Staurolite-out^j out isoqrad Formation

J RI Silurian

1 I Formation Staurolite- out Clough sograd Quartzite Ordovician Partridge Formation Ammonoosuc Volcanics Ordovician or older AAA < >1 Fourmile Gneiss svvv Monson Gneiss

•First chlorite Faults rims on garnets ' Roads -Chlorite rims on garnets »0.2mm / R3

/ : A V/ U =

vx " \\

///KO^ ^ |, ^ - '* * " // M v\ „ B.

Figure 3. Detailed maps of the retrograde zone. (A) Geo- logic map showing rock units and prograde metamorphic isograds. (B) Retrograde metamorphic isograds and retrograde metamorphic zones R1-R6, shown without geologic contacts for clarity. (C) Geologic contacts from Figure 2A with superimposed retrograde isograds and selected sample locations.

zone (station 6A) was also examined in detail. Nine polished thin sections Ray (1970). Standards were natural and synthetic crystals and a synthetic were selected for electron microprobe analysis, for which sample localities diopside glass. All iron in the mineral analyses is expressed as FeO, except are shown in Figure 3C, and estimated modes are given in Table 2. for ilmenite in which FeO and Fe203 were calculated on a basis of three Analyses were performed on the wavelength-dispersive ETEC autoprobe oxygens and two cations per formula unit. at the Department of Geology and Geography, University of Massachu- setts, Amherst. Operating parameters were beam current, 0.02 ¡xa; acceler- MINERALOGY AND CRYSTAL CHEMISTRY ating potential, 15 kev; and beam spot diameter, 2 /urn for oxides and garnet and 10 ¿im for all other minerals. Analyses were corrected using the Sillimanite occurs as fibrolite and as small discrete rods. The freshest procedures of Bence and Albee (1968) with alpha factors of Albee and sillimanite in the study area occurs at station 131 (Rl) as partially altered

TABLE 2. ESTIMATED MODES OF SAMPLES FOR WHICH MINERAL ANALYSES ARE AVAILABLE

Zone R2 R2 R2 R3 R4 R5 R5 R6 R5

Sample UOIA NSII8 6A-273 6A-438 NS18 NS87 NS85 NS90 NS7I

Quartz 55 50 45 12 45 35 50 55 30 Plagioclase 1 1 3 4 5 3 K-feldspar tr (r I tr 2 Slaurolite 3 5 5 P P P P

Garnet 5 2 3 35 2 1 Ir P 1 Biotite 9 10 8 6 3 1 2 P Muscovite 25 30 30 4 35 45 35 32 55 Chlorite tr I 5 35 9 12 9 10 10

Chloritoid 4 Tourmaline tr tr tr tr tr tr tr tr Allanite ? tr tr tr tr tr Apatite tr tr tr 2 tr tr tr tr tr

Zircon tr tr tr tr tr tr tr tr tr Spitene 7 ? 7 7 tr ? Ilmenite 1 tr 1 1 I 1 1 tr tr Anatase 1 1 tr tr tr ? I tr

Graphite tr tr tr tr 1 tr tr tr tr Pyrite tr 3 tr tr Siderite 1

Note: P = only pseudomorphs after the mineral are present; tr = trace constituent, < 1%; 7 = probably present, but not positively identified.

TABLE 3. REPRESENTATIVE SAMPLE AVERAGE ANALYSES OF STAUROLITE, ANATASE, ILMENITE, CHLORITOID, AND SPHENE

Mineral Staurolite Anatase Ilmenite Chloritoid Sphene Sample UOIA several UOIA NS7I NS90

Si02 27.32 0.15 24.05 29.84 Ti02 0.63 91.02 51.49 0.03 37.09

AI2O3 54.06 0.04 0 41.29 3.24

Cr203 0.05 0.01 0.02 0.06

Fe203 2.21 FeO 14.60 0.16 45.92 25.52 0.13 MnO 0.01 0 0.35 0.41 0 MgO 1.52 0 0 1.65 0.06 ZnO 0.29 0.05 0 0.09 0.08 CaO 0.16 0 28.45

Na20 0 0 0.03

K20 0 0

Total 98.48 91.58 99.98 93.06 98.98

Cation proportions

Si 3.785 0.002 Ti site Ti 0.979 Si site Si 1.985 Si site Si 0.968 Ti 0.066 0.995 Fe*3 0.021 Al 0.015 Al 0.032 Al 8.828 0.001 Sum 1.000 Sum 2.000 Sum 1.000 Cr 0.005 Fe 1.692 0.002 Fe site Fe*3 0.021 Al sites Al 4.000 Ti site Ti 0.905 Mn 0.001 0 Fe*2 0.971 Al 0.092 Mg 0.314 0 Mn 0.008 M sites Al 0.005 Cr 0.002 Zn 0.030 0.001 Mg 0 Ti 0.002 Zn 0.002 Ca 0.002 Zn 0 Cr 0.001 Mg 0.004 Sum 1.000 Fe 1.763 Fe 0.003 Total 14.721 1.003 Mn 0.029 Sum 1.008 Total 2.000 Mg 0.203 Zn 0.005 Ca site Ca 0.990 Sum 2.008 Na 0.002 Sum 0.992 Total 8.008 Total 3.000

(OH") 0.132

Note: staurolite formula calculated to 23 oxygens, anatase formula calculated to 2 oxygens, ilmenite formula calculated to 3 oxygens and 2 cations, chloritoid formula calculated to 12 oxygens, and sphene formula calculated to 5 oxygens and 3 cations.

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Figure 4. Sketches of garnets in thin section, showing various stages of replacement by chlorite in zones R2 to RS.

relics embedded in patches of randomly oriented fine-grained muscovite of grade metamorphism, Zn typically concentrates in relict staurolite. retrograde origin. In zones R2 to R6, sillimanite occurs only as inclusions Apparently this did not occur during retrograde metamorphism in the within quartz. New Salem area. For example, staurolite in sample 6A-273 has been 80% Staurolite1 occurs around the northern periphery of the retrograde replaced by sheet silicates, but there is no evidence of a Zn-enriched rim or area in zones R1 and R2 as subhedral prisms. Slightly retrograded stauro- of bulk Zn enrichment. The slightly higher Zn content of staurolite in lite in zone R2 is associated with plates and sprays of optically positive sample 6A-273 (0.41%) as compared to samples UOIA and NS118 Mg-rich chlorite. Staurolite becomes progressively replaced by fine- (0.29% and 0.31% Zn, respectively) is probably well within the typical grained muscovite and chlorite as the staurolite-out isograd is approached. variation of prograde compositions. Zn released during staurolite break- The resulting pseudomorphs that occur within the staurolite-out isograd down may have been taken up by chlorite and relict biotite, as indicated weather out in raised relief over the surrounding quartz-rich matrix. Pseu- by the generally higher Zn contents of these minerals in zones R3 to R6 as domorphs after staurolite commonly retain the euhedral pattern of graph- compared with R2. Staurolite breakdown was apparently a disequilibrium ite dust inclusions that were present in the parent staurolite. The process. fine-grained chlorite-muscovite pseudomorphs after staurolite are in con- In the drill core at station 6A, large staurolite crystals are progres- trast to pseudomorphs of coarse-grained muscovite after staurolite in the sively replaced with increasing depth by sheet silicates. In sample 6A-273 Partridge Formation on the east limb of the Prescott syncline. The coarse- (83 m depth), -80% of staurolite has been replaced, and only pseudo- grained pseudomorphs are probably of prograde origin, as are those de- morphs occur below 94 m. The drill core therefore passes through the scribed in other areas by Guidotti (1970) and Foster (1983). staurolite-out isograd, suggesting that the isograd surfaces dip east, proba- Staurolite was found to be homogeneous and to vary little in compo- bly parallel to the foliation in the Prescott syncline. sition between samples (representative analysis in Table 3). During pro- Garnet occurs as equant crystals as much as 1 cm across. The first direct association of chlorite with garnet is visible in hand specimen as a thin green veneer (zone R4, Fig. 4B). The chlorite veneer thickens with more severe rétrogradation (Figs. 4C and 4D), and chlorite completely 1 Tables A-E contain selected individual analyses and complete sample average composition data for staurolite, muscovite, chlorite, biotite, and ilmenite. These replaces garnet in zone R6. Fractured garnets have the same type of tables may be obtained free of charge by requesting Supplementary Data 87-18 chlorite replacement along cracks as on rims, indicating that both rims and from the GSA Documents Secretary. cores were in disequilibrium with respect to the retrograde assemblages.

TABLE 4. GARNET CHEMICAL COMPOSITIONS EXPRESSED AS PROPORTIONS OF THE END-MEMBER COMPONENTS The average rim analyses (squares and hexagons, Figs. 6C and 6F) are from rocks with staurolite-bearing prograde assemblages similar to Sample Analysis Location Aim.' Spess. Pyrope Gross. those of sample UOl A. As a result, the compositions are all close to or within the compositional field of garnet in sample UOl A, with no ten- UOIA 1 Rim .861 .013 .105 .021 2 .827 .019 .118 .036 dency to shift composition toward higher Fe/(Fe+Mg) ratios or Mn 3 .791 .055 .105 .049 contents, as would be expected if chemical equilibrium were approached 4 Core .740 .090 .078 .092 during rétrogradation. The increase in Mn and Ca contents of garnet rims NS85 5 Rim .787 .054 .108 .052 in the sequence of samples NS118 (R2) - NS18 (R4) - NS87 (R5) 6A-438 6 Rim .844 .015 .092 .048 (squares) represents the progressive exposure of Mn- and Ca-rich garnet 7 .862 .020 .063 .056 8 .775 .085 .045 .094 cores while the prograde garnet zoning remained intact. The relict garnet 9 Core .703 .166 .036 .095 fragment in sample NS85 (R5, hexagon) is from a position near the rim of NS7I 10 Rim .829 .050 .048 .073 the original crystal (Fig. 5B), and it is also intermediate between typical 11 .809 .059 .052 .081 12 .787 .083 .044 .086 core and rim compositions. Garnets from iron-rich assemblages also ap- 13 Core .773 .098 .039 .091 pear to retain prograde zoning patterns. Analyses from the partially altered 6A-273 14 Rim .881 .030 .059 .031 garnet in sample NS71 (R5) lie largely within the field of 6A-438 garnet 15 .872 .037 .064 .026 16 Core .822 .053 .070 .055 analyses. Note that in NS71, analysis 13 is from near the now-vanished

NS118 17 Rim .853 .012 .116 .020 garnet core and is therefore higher in Mn and Ca than is analysis 10, taken from the rim (Fig. 5E). Therefore, garnet did not re-equilibrate during NS18 18 Rim .812 .046 .090 .051 rétrogradation, and garnet breakdown was a disequilibrium process. NS87 19 Rim .775 .084 .072 .070 Biotite occurs as anhedral to subhedral, red-brown to orange-brown Note: rim analyses for samples NS85, NS118, NSI8, and NS87 are averages; all others are individual analyses. grains. In zones R2 to R6, biotite is associated with chlorite and fine- •Aim. = Fe/(Fe + Mn + Mg + Ca), and so on. grained muscovite, most of which cut biotite at random angles to the cleavage and have sharp contacts with biotite. Minor amounts of chlorite occur as thin layers parallel to the cleavage. In some samples, lenses of Representative garnet compositions are given in Table 4, and the retrograde K-feldspar occur parallel to the biotite cleavage. There is no locations of these analyses within individual garnets are shown in Figure 5. textural evidence for the retrograde production of biotite. Biotite is homo- Samples UOl A, NS118, and 6A-273, which have virtually unretrograded geneous in composition in single thin sections, and biotite compositions are garnets (zone R2, Fig. 6), have zoning trends that are typical of garnets typical of prograde biotite in pelitic schists at sillimanite-staurolite grade from low- to medium-grade prograde metamorphic terranes (Hollister, (for example, Guidotti, 1984; Robinson and others, 1982b; Guidotti and 1966; Anderson and Glenn, 1973; Tracy and others, 1976; Tracy, 1982); others, 1977). The analyzed biotites (representative analyses in Table 5) absolute Mn and Ca contents and the Fe/M+2, Mn/M+2, and Ca/M+2 have 0.094-0.116 Ti per formula unit, A-site occupancies of 0.77-0.85, molar ratios are high in the garnet cores and decrease smoothly toward the \\VIcontents of 0.394-0.474, and Al7^contents of 1.30-1.34. There is no rims. The slight hook-shaped trend toward higher Fe contents for garnet in systematic change in these parameters with rétrogradation. sample UOl A is probably due to high-temperature retrograde ion ex- Prograde muscovite occurs as parallel plates, forming the predomi- change reactions, which are unrelated to the retrograde metamorphism nant foliation. Retrograde muscovite occurs as small plates and irregular discussed herein. Evidence for retrograde changes in garnet rim composi- shapes, commonly intergrown with chlorite as pseudomorphs after silli- tions has been found, but rétrogradation apparently occurred at high manite, staurolite, garnet, and biotite. In zones R1 and R2, muscovite is temperatures (>600 °C, for example, Tracy and others, 1976; Hodges and homogeneous in single thin sections (representative analyses in Table 5). Royden, 1984). Certain chemical parameters remain relatively constant with retrograda-

Gornet

Muscovite, minor biotite

Staurolite Figure 5. Schematic sketches of garnets, Quartz, minor mica showing the locations of microprobe analysis points for the individual garnet compositions ¿rjirj Chlorite, minor muscovite given in Table 4. Each scale bar is 1 mm in length. 6A-273

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/98/6/621/3434577/i0016-7606-98-6-621.pdf by guest on 01 October 2021 Figure 6. Paitial ternary plots of garnet analyses in the systems Fe-Mn-Mg (upper diagrams) and Fe-Ca-Mg (lower diagrams). Diagrams A and D show analy ses for samples 6A-438 and UOl A. Diagrams B and E show analyses for samples NS71 and 6A-273. Diagrams C and F show data for average garnet rim compositions only. Stippled fields are from diagrams A and D. Open circles are individual analyses from Table 4; squares and hexagon, average rim analyses from Table 4; and solid dots, analyses from Hollocher (1981a).

TABLE 5. REPRESENTATIVE SAMPLE AVERAGE ANALYSES OF BIOTITE, MUSCOVITE, AND CHLORITE

Mineral Biotite Muscovite Chlorite

Zone R2 R5 R2 R5 R2 R5 Sample UOl A NS87 UOl A NS87 UOIA NS87

Si02 35.57 35.16 46.16 45.50 24.58 24.07 Ti02 1.72 2.02 0.40 0.39 0.10 0.08

AI2O3 20.03 19.09 37.45 35.74 24.10 22.90 Cr203 0.02 0.05 0.02 0.04 0.01 0.05 FeO 19.86 23.48 0.83 1.12 25.44 30.65 MnO 0.01 0.07 0 0.01 0 0.22 MgO 10.33 7.42 0.40 0.48 15.40 10.57 ZnO 0.05 0.11 0 0.04 0 0.03 CaO 0 0 0 0 0 0.01

Na20 0.38 0.05 2.50 1.51 0.02 0 k2o 8.35 8.19 7.59 9.35 0 0.01

Total 96.32 95.64 95.35 94.18 89.65 88.59

Cation proportions:

A-site Na 0.055 0.007 0.318 0.197 Alkalis Ca 0 0.001 K 0.798 0.802 0.635 0.801 Na 0.003 0 Sum 0.853 0.809 0.953 0.998 K 0 0.001 Sum 0.003 0.002

Si sites Si 2.663 2.700 3.026 3.052 AI 1.337 1.300 0.974 0.948 Si sites Si 2.526 2.585 Sum 4.000 4.000 4.000 4.000 Al 1.474 1.415 Sum 4.000 4.000

VI sites AI 0.432 0.429 1.923 1.880 Cr 0.002 0.002 0.002 0.002 VI sites AI 1.447 1.491 Ti 0.097 0.116 0.020 0.020 Cr 0.001 0.004 Zn 0.003 0.006 0 0.002 Ti 0.008 0.007 Mg 1.154 0.850 0.040 0.048 Zn 0 0.003 Fe 1.244 1.508 0.046 0.063 Mg 2.360 1.683 Mn 0 0.004 0 0.001 Fe 2.188 2.756 Sum 2.932 2.915 2.031 2.016 Mn 0 0.020 Sum 6.004 5.964 Total 7.785 7.724 6.984 7.014 Total 10.007 9.966

Fe/(Fe + Mg) 0.519 0.640 0.481 0.621 Mn/(Mn • Fe * Mg) 0 0.002 0 0.005 K/(K + Na) 0.936 0.991 0.666 0.803

Note: mica formulae calculated to 11 oxygens; chlorite formulae calculated to 14 oxygens.

tion, including A-site occupancies of -0.968, octahedral site occupancies atase is composed almost entirely of Ti02 (Table 3), but the low analysis of -0.027, and A\IV contents of -0.962 per formula unit. Ti contents total is not understood. The analyses were carefully done with adequate range from 0.011-0.022 between samples but do not vary in any syste- standards, and an X-ray scan revealed that no elements with atomic matic way. These Ti contents are somewhat lower than those typical for numbers higher than Na were present in concentrations greater than 1%, muscovite from prograde metamorphic rocks of sillimanite-staurolite the approximate limit of sensitivity for the scan. The low total may be grade (Cheney and Guidotti, 1979; Guidotti, 1973, 1978, 1984). related to very abundant submicroscopic fluid inclusions (unlikely, as the Chlorite is visible in hand specimen as plates and sprays in all retro- anatase is transparent) or to OH" in the anatase structure (I know of no grade zones, although it is rare in zone R1 (sillimanite present). Chlorite is other data supporting this). The abundant, minute crystalline inclusions in homogeneous in composition in single thin sections (representative anal- anatase probably include sphene, as indicated by the presence of as much /(/ w yses in Table 5). Chlorite contains -1.46 AI and Al , 0.005-0.009 Ti as 0.25% CaO and 0.52% Si02 in the anatase analyses. per formula unit, and no significant quantity of alkalis. Sphene, occurring as subhedral to anhedral crystals, has been posi- Homogeneous ilmenite, occurring as large tabular crystals, is the only tively identified in only three samples (including NS90). Some sphene has oxide mineral of prograde origin in schists of the study area. Thin platelets oscillatory zoning with respect to refractive index, A1 and Ti contents, and of ilmenite that occur around retrograded biotite grains are probably of the oxide weight total (a measure of OH" content; see Table 3 for an retrograde origin. Ilmenite compositions (representative analyses in Table average analysis). The systematics of A1 substitution indicate a roughly 3) are typical of graphitic schists, containing 0-0.001 Mg per formula unit constant 3% replacement of Si by Al, probably by the exchange reaction and low (0.7-4.4 mole %) Fe2C>3 component. Si02 ^ AIO(OH), and a variable 5%-16% replacement of Ti by Al, Chloritoid occurs only in biotite-free parts of outcrop NS71 (R5) as probably by the exchange reaction Ti02 ^ AIO(OH). The second, more subhedral plates as much as l.S mm across. Chloritoid is chemically significant substitution has been found in other low-grade sphene (Coombs homogeneous and has a composition typical for chloritoid from pelitic and others, 1976; Boles and Coombs, 1977; Ribbe, 1980). schists (analysis in Table 3). Pyrite occurs as rare, small discrete cubes and clumps of cubes as Sodic feldspar, largely albite, occurs as translucent grains with much as 2 mm across. The occurrence of pyrite is in contrast to the almost abundant quartz inclusions or as smaller inclusion-free crystals. The two exclusive occurrence of pyrrhotite as the prograde sulfide in medium- and types are never found in the same sample. Sodic feldspars range in compo- high-grade schists in central Massachusetts. In prograde assemblages in sition from An 1 to An 13, with constant Or 0.3 and Cn 0. Optically schists, pyrite occurs only in unusual, highly sulfidic rocks with very Mg- distinguishable zoning of as much as 6 An% is common (measured by rich silicates (Robinson, 1977; Robinson and others, 1982b). Pyrite is electron probe), the calcic cores always grading smoothly to more sodic common, however, in low-grade metamorphic rocks (McNamara, 1965; rims. There is no textural or other evidence that plagioclase was involved Condie, 1967; Ramsay, 1973; Coombs and others, 1976) and is therefore in retrograde metamorphic reactions. not unexpected in the retrograded rocks in New Salem. K-feldspar (optically low sanidine) is exclusively of retrograde origin in the schists studied. It occurs in zones R2-R4 as small, thin lenses within CATION EXCHANGE EQUILIBRIA biotite, parallel to the cleavage. In zones R5 and R6, K-feldspar lenses increase in size and in some cases have coalesced into polycrystalline The Fe/(Fe+Mg) ratios of coexisting biotite and chlorite tend to masses as much as 1 mm across. In zones R5 and R6, K-feldspar is seen in increase systematically from zone R2 to R6 because of Fe released from contact with quartz, muscovite, chlorite, and albite, in addition to biotite. the breakdown of staurolite, garnet, ilmenite, and biotite (Fig. 7). The The K-feldspar composition is Or 99.1, Ab 0.5, Cn 0.4, and An 0, with no Fe/(Fe+Mg) ratios increase from 0.524 for biotite and 0.481 for chlorite detectable variation. in sample UOl A (R2) to 0.640 for biotite and 0.621 for chlorite in sample Anatase (colorless to blue) occurs only as pseudomorphs after ilmenite. Anatase nucleated on ilmenite, and growth proceeded along sharp, highly irregular surfaces that were not crystallographically controlled. An-

T ' 1 ' T NS7I A

' I I I I Figure 8. Sample average compositions for biotite and muscovite, 0.7 0.6 0.5 0.4 with octahedral site occupancy plotted versus K/(K+Na). Sample Fe /(Fe + Mg) NS71 (R5) apparently never contained any biotite. Sample NS90 (R6) Figure 7. Sample average compositions for biotite and chlorite, no longer contains biotite but does contain substantial amounts of with Al/(A1+Si) plotted versus Fe/(Fe+Mg). K-feldspar.

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NS87 (R5). The molar distribution coefficient KFe/M% for biotite/chlorite rétrogradation, from -0.08 per formula unit in staurolite-bearing samples remains approximately constant near 1.1. Sample NS90 (R6), in which to -0.15 in the more severely retrograded samples. Individual analyses biotite has been destroyed, contains a more Fe-rich chlorite than do any (dots) include three data pairs (circles) that represent core and rim analyses biotite-bearing samples [Fe/(Fe+Mg) = 0.637], as expected. The of zoned muscovite crystals. K/(K+Na) ratios clearly increased in parallel chloritoid-bearing sample NS71 (R5) has an even more Fe-rich chlorite with M+2 in muscovite during retrograde metamorphism (Fig. 9B). Fig- [Fe/(Fe+Mg) = 0.701], also as expected for this assemblage. ures 9C and 9D show that M+2 also increased with decreasing A1vl. The +2 In Figure 8, sample average K/(K+Na) ratios of biotite and muscovite three core-rim analysis pairs in Figure 9D also show the increase in M VI are plotted versus octahedral site occupancy. The K/(K+Na) ratios of both with decreasing A1 with rétrogradation. The mechanism for substituting +2 biotite and muscovite tend to increase from zone R2 to R6. In addition, M into muscovite was estimated by linear regression using 98 muscovite the molar distribution coefficient KK/Na for biotite/muscovite increases analyses and is apparently the Tschermak exchange reaction, 2A1 ^ +2 from ~7 in sample UOIA (R2) to ~56 in sample NS85 (R5), indicating Si+M . at least partial re-equilibration of biotite and muscovite at low tempera- Mn released during garnet breakdown in zones R3 through R5 was tures. The continuous increase in K/(K+Na) ratios with rétrogradation taken up by chlorite and relict biotite and ilmenite. Although the amount occurred because the biotite component released during biotite breakdown of Mn in these three minerals is small, the increase in Mn content with had a K/(K+Na) ratio less than that of relict biotite but greater than that of rétrogradation is consistent. From least to most retrograded samples, Mn coexisting muscovite. As a result, the biotite was driven nearly to the K contents ranged from 0-0.004 per formula unit in chlorite, 0-0.003 in end-member comjjosition, and muscovite tended to approach the original biotite, and 0.007-0.047 in ilmenite. These values are taken from linear bulk mica K/K+Na) ratio. Retrograde production of K-feldspar prevented regression lines through sample average data, which are necessarily some- muscovite from becoming as K rich as in K-feldspar-free rocks. The effect what scattered. of removal of K by K-feldspar is evident in sample NS90 (R6), in which Chlorite Al/(A1+Si) ratios, which are related to M+2, Si, Al/(/, and all biotite has broken down, leaving 2% modal K-feldspar and a muscovite A1Vl contents, vary systematically with the host assemblage. Chlorite from somewhat more sodic than in sample NS85 (R5), which is less retrograded all samples except NS71 (R5) and NS90 (R6) have Al/(A1+Si) ratios in and has biotite bu : not K-feldspar. the narrow range 0.529-0.542. Chlorite in sample NS71 occurs in an Figure 9 A shows that Mn+Mg+Fe in muscovite also increased with aluminous chloritoid-garnet assemblage and has a high Al/(Al+Si) ratio

0.6

0.7 - + K (K + Na) Figure 9. Muscovite analyses 0.8 • • / • with K/(K+Na) plotted versus Fe NS90 R6 +Mg+Mn (upper diagrams) and NS85 R5 with A\VI plotted versus Fe+Mg+ 0.9 Mn (lower diagrams). In diagrams + Zone R2 o Analyses I —10 A and C, individual analyses are given, showing the trends of • Zones R3 to R6 A Sample averages increasing Fe+Mg+Mn and de- 1.0 creasing \\VI and K/(K+Na) with ' 0 0.1 retrograde metamorphism. In diagrams B and D,, these trends are c. D. shown by sample average analyses (triangles) and fa>yindividua l muscovite analyses (circles, see Table AI3ZI D, GSA Data Repository). Arrows connect core (arrow tail) and rim (arrow head) compositions of analyzed zoned muscovite crystals. 1.9 -

NS87

1.8 0 0.1 0.2 0.1 0.2 M n + Fe + M g

Figure 10. Ternary projections from muscovite in the AKFM system. (A) Probable prograde phase relations in metamorphic rocks from the sillimanite-muscovite-staurolite zone (zone II of Tracy and others, 1976) that surrounds the retrograded area. (B) Postulated retrograde phase relations in retrograde zone R6. (C) Sample average mineral compositions as projected from their respective average muscovite compositions. The fields are patterned as follows. Light stipple, UOIA (R2) and NS87 (R5); unpatterned, NS71 (R5) and NS85 (R5); heavy stipple, 6A-273 (R2). Sample NS90 is represented only by a chlorite-K-feldspar tie line in this projection. Samples NS118 (R2), 6A-473 (R4), and NS18 (R4) were omitted for clarity.

of 0.555. Chlorite in sample NS90 occurs in a K-feldspar-bearing, garnet- relict biotite became progressively more Fe rich while more or less consist- free assemblage and has a low Al/(A1+Si) ratio of 0.514. These variations ent tie lines were maintained. In rocks with bulk compositions less alumi- in chlorite composition are as expected for the host assemblages. nous than the chlorite composition field, biotite was replaced by Figure 10A shows the prograde phase relations in muscovite projec- chlorite-K-feldspar tie lines. tion for the sillimanite-staurolite-grade rocks in central Massachusetts (Robinson, 1963; Hall, 1970; Tracy and others, 1976; Hollocher, 1981a). RETROGRADE REACTIONS Cordierite may occur in very Mg-rich rocks, but chlorite apparently does not occur in muscovite-bearing prograde assemblages at this grade. The sequence of retrograde assemblages and isograds from zones R1 Figure 10B shows the partly conjectural phase relations in zone R6 of to R6 is superficially similar to Barrovian-type sequences of prograde the retrograde zone. Biotite has certainly broken down in all rocks that assemblages in pelitic schists (for example, Thompson and others, 1977; have Fe/(Fe+Mg) ratios less than or equal to that of sample NS90 (R6), Zen, 1960; Albee, 1965, 1968; Evans and Guidotti, 1966; Turner, 1968; having been replaced by a set of chlorite-K-feldspar tie lines. Biotite may Seifert, 1970; Thompson, 1976; Tracy and others, 1976). The differences still occur in unusually Fe-rich compositions. Because no garnet was found between the retrograde and typical prograde reactions are probably due to in zone R6, garnet was probably not stable in the AKFM system, even disequilibrium between prograde minerals (Fig. 10A) and the stable retro- with the modest amounts of Mn present in the natural system. It is as- grade assemblages (Fig. 10B). sumed that a chlorite-chloritoid-aluminosilicate limiting assemblage occurs The retrograde reactions were determined on the basis of field evi- in Al-rich rocks that are more Mg rich than is sample NS71 (R5). dence, textural relationships in thin section, and chemical compositions of Figure 10C shows the actual mineral compositions in muscovite coexisting minerals. It was assumed that retrograde metamorphism was projection. Chlorite probably first became stable in Mg-rich rocks in zones isochemical, except for gain of H2O. That this is a good assumption is R1 and R2, although no extremely Mg-rich chlorite was actually observed suggested by the fact that the balanced reactions are petrographically owing to the available bulk compositions. With increasing rétrogradation, reasonable and by the fact that prograde rocks with typical assemblages the stable chlorite compositions became more Fe rich, intersecting the bulk and modes can be modeled to react to form retrograde rocks with typical composition of sample UOIA (R2). Because of Fe released from the retrograde assemblages and modes (example given below). breakdown of garnet, staurolite, ilmenite, and biotite, the chlorite and Reactions were balanced using the chemical components Si, Al+Cr,

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+2 +3 Ti, Ca, Fe +Fe +Mn+Mg+Zn, Na+K, and H20. Reactions were solved R6 have little or no biotite remaining, yet there is no evidence that silli- by using linear algebraic techniques (for example, Spear and others, 1982), manite, staurolite, or garnet ever existed in those rocks. Obviously, an using the average mineral compositions given in the text and appendix additional reaction must have consumed biotite in the absence of prograde

tables unless otherwise noted. Anatase was taken as pure Ti02. Abbrevia- garnet, staurolite, or sillimanite. A retrograde reaction that consumes bio- tions in the reactions are as follows: ANA, anatase; BIO, biotite; CEL, tite in the absence of these three minerals is similar to reaction 10.

celadonite; CHL, chlorite; CTD, chloritoid; GAR, garnet; ILM, ilmenite; (12) 133 BIO+42 MUS+144 QTZ+100 H20 - 151 KSP+69 KSP, K-feldspar; MUS, muscovite; QTZ, quartz; SIL, sillimanite; SPH, CHL+14 ILM,

sphene; and ST A, staurolite. Several reactions given below are not bal- which results in the BI0+H20 — CHL+KSP three-phase field sweeping anced. In these reactions, the participation of quartz, muscovite, ilmenite, toward higher Fe compositions with retrograde metamorphism. and other components is implied. Each prograde reaction is labeled as The presence of partially altered biotite in zones R5 and R6, in the continuous or discot.tinuous with respect to the muscovite projection. absence of K-feldspar or pseudomorphs after sillimanite, staurolite, or Sillimanite first appears in pelitic schists by several different reactions, garnet, precludes all of the retrograde biotite-consuming reactions 3, 6, 8, an depending on P, T, iih2o> d bulk composition. The following prograde and 12. Another retrograde biotite-consuming reaction may be modeled sillimanite-producing reactions are typical of pelitic rocks that are moder- after the reverse of reaction 11 using two muscovite compositions calcu- ately low in An and have intermediate Fe/(Fe+Mg) compositions. lated from the average analysis: one recalculated to contain 7% celadonite

(1) STA ^ SIL+BI0+H20, continuous, and component (about the composition of prograde muscovite) and one to (2) STA+CHL ^ SIL+BI0+H20, discontinuous. contain 15% (the maximum found on retrograde muscovite rims). Average Reaction 1 is probably responsible for the prograde sillimanite-in isograd and recalculated compositions are given in the appendix tables. The pro- (Fig. 3 A) in the New Salem area. The reverse of reactions 1 and 2 requires posed reaction is

the production of retrograde staurolite, which is not seen. The suggested (13) 90 BIO+846 MUS (7% CEL)+133 QTZ+100 H20 - 922 MUS retrograde reaction is (15% CEL)+29 CHL+8 ILM.

(3) 58 SIL+55 BIO+5 QTZ+100 H20 - 46 MUS+27 CHL+5 ILM. The progress of this reaction may have been limited by the rate of diffusion Chlorite probably first appears in muscovite projection at the Mg sideline of reaction components into and out of muscovite at very low retrograde via a reaction similar to reaction 3 but involving rutile instead of ilmenite. temperatures, resulting in the observed zoned muscovite crystals with Chlorite appears in progressively more Fe-rich rocks as the SIL+BIO+ celadonite-rich (and K-rich) rims.

H20 — CHL three-phase field sweeps toward higher Fe compositions. Chloritoid occurs in only one outcrop (NS71, R5) and two related Staurolite commonly first appears in pelitic schists by the reactions assemblages: quartz-muscovite-chlorite-chloritoid-ilmenite-anatase-

(4) GAR+CHL ^ STA+BI0+H20, discontinuous, or sphene(?)±garnet. Using garnet analysis 11 (Table 4), two chloritoid-

(5) CHL ^ STA+BI0+H20, continuous. producing reactions may be written. The first represents the reverse of a The reverse of reaction 4 would require garnet production in zone R2, typical prograde reaction. which is not seen. The reverse of reaction 5, however, is similar to the (14) CTD+CHL ^ GAR+H20, continuous, apparent retrograde reaction which may be balanced for the retrograde hydration reaction

(6) 14 STA+49 BIO+19 QTZ+100 H20 - 42 MUS+30 CHL+ (15) 37 GAR+9 ILM+100 H20 - 0.4 CTD+ 246 CHL+9 SPH 5 ILM. +38 QTZ.

As the STA+BI0+H;;0 — CHL three-phase field sweeps toward higher Fe Unfortunately, this reaction produces much chlorite but little chloritoid. compositions with retrograde metamorphism, a second generation of Such a reaction may be responsible for the chlorite-rich rims on garnets in chlorite is produced, in addition to that produced by the sillimanite-out sample NS71 (R5, Fig. 4E). A second reaction is necessary to produce reaction. This reaction is quartz consuming and was probably responsible chloritoid in garnet-free assemblages, and one may be written that pro- for the lack of quarto in muscovite-chlorite pseudomorphs after staurolite duces celadonitic muscovite and chloritoid at the expense of prograde which were derived, from prograde staurolite with abundant quartz celadonite-poor muscovite.

inclusions. (16) 1,257 MUS (7% CEL)+196 ILM+200 QTZ+100 H20 - 1,256 In general, garnet appears during prograde metamorphism by the MUS (15% CEL)+50 CTD+191 ANA. reaction The production of anatase by this reaction is consistent with the abundant

(7) CHL ^ BI0+GAR+H20, continuous. anatase in sample NS71 (R5). The reverse of this reaction is similar to the likely retrograde garnet break- The anatase-producing reaction 16 applies only to the unique down reaction. The grossular component of garnet requires a Ca sink, chloritoid-bearing sample NS71. Another anatase-producing reaction which apparently wa:> sphene. must therefore exist because anatase replaces at least some ilmenite in all

(8) 37 GAR+1 BIO+2 ILM+100 H20 - 1 MUS+25 CHL+2 retrograde zones and assemblages. The proposed general anatase- SPH+45 QTZ, for Ca-poor garnet rims, and producing reaction is

(9) 37 GAR+0 BIO+IO ILM+100 H20 - 0 MUS+25 CHL+11 (17) 456 MUS (7% CEL)+146 ILM+100 QTZ+100 HzO - 456 SPH+39 QTZ, for Ca-rich garnet cores. MUS (15% CEL)+25 CHL+142 ANA. These reactions yield a third generation of chlorite, which occurs princi- This reaction is similar to reaction 16, except that chlorite replaces pally in the spherical pseudomorphs after garnet. Anatase may have been chloritoid. another Ti source foi sphene production, but replacement of ilmenite by As stated above, the typical prograde sulfide in medium-grade schists anatase in the reactions would not qualitatively change the reactions. is pyrrhotite, and pyrite is restricted to rocks that contain extremely Mg- Biotite may appear in schists during prograde metamorphism by rich silicates. The following highly schematic reactions summarize the reactions such as pyrite-producing retrograde reaction sequence proposed in Hollocher

(10) CHL+KSP ^ BI0+MUS+H20, continuous, or (1981a). (11) celadonitic MUS+CHL ^ less celadonitic MUS+BI0+H20, (18) FeS i _x+H20+C02 - BIO+C+FeS2, and discontinuous. (19) FeS i_x+H20+C02 - CHL+C+FeS2. Retrograde reactions 3, 6, and 8 all consume biotite. Some rocks in zone The net result of these and related reactions is that pyrrhotite is converted

to FeS2 at constant sulfur, and the resulting Fe+2 is utilized to produce model rock into the retrograde assemblages of zones R2, R3-R5, and R6. retrograde minerals such as chlorite. The resulting retrograde assemblage and mode calculated in this way for zone R6 is very similar to those in sample NS90 (R6, Table 2), supporting CONDITIONS OF RETROGRADE METAMORPHISM the idea that retrograde metamorphism was isochemical except for H20. The results of these calculations are given in Table 6. 2 A variety of methods were used to estimate pressures and tempera- The entire area required -0.26 km of H20 (at STP) to drive the tures of retrograde metamorphism (see Hollocher, 1981a, for a complete observed retrograde hydration reactions and resulted in a volume increase discussion). The following parameters yielded the closest constraints for of-0.23 km3 for each kilometre of vertical extent of the retrograde region. retrograde P and T. The ductile style of post-dome-stage (post-late Acadian) deformation in A minimum retrograde pressure of 1.5 kbar corresponds to the depth the retrograde area indicates that the increased volume was accommo- of erosion since the formation of the nearby Mesozoic Connecticut Valley dated by folding. basin (structural reconstruction of Robinson, 1979). In addition, all data for chlorite-K-feldspar assemblages used in this study occur below 4 kbar. WATER SOURCE This suggests that chlorite-K-feldspar tie lines are not stable at higher pressures with respect to celadonitic muscovite, at least in reduced rocks The Prescott intrusive complex was emplaced during formation of that have intermediate Fe/(Fe+Mg) ratios. The complete reaction of K- the Acadian gneiss domes of the Bronson Hill anticlinorium and was feldspar to mica in hydrothermal experiments conducted at 10 kbar in the probably cold and solid at the time of retrograde metamorphism. The sanidine-Mg-chlorite-H20 system (Fawcett, 1963) supports this idea, as intrusion has little evidence of rétrogradation and is 3 km south of the most does work by Velde (1965) and Guidotti and Sassi (1976). severely retrograded rocks. The Prescott intrusion therefore was probably Coexisting K-feldspar and chlorite in zeolite and lower greenschist not the water source. facies metamorphic rocks and in hydrothermally altered rocks is rather Spear and Selverstone (1983) have suggested that decreasing pressure

common (Coombs, 1954; Chayes, 1955; Zen, 1960; Fawcett, 1963; Harri- during erosion and uplift after prograde metamorphism releases H20 dis- son and Campbell, 1963; Velde, 1965; McNamara, 1965; Albee, 1968; solved in various minerals. This H2O may then migrate from a large Coombs and others, 1970; Mather, 1970; Boles and Coombs, 1977; Ferry, volume of rock to accumulate locally (pond), causing extensive local 1978). Data for chlorite-K-feldspar assemblages used in this study con- retrograde effects. Spear and Selverstone have calculated that a 26,000- strain retrograde temperatures to between -200 and -310 °C. km3 volume of rock could have served as a water source for each kilome- Anatase is metastable with respect to rutile under all reasonable tre of vertical extent of the New Salem retrograde zone.

conditions in the crust (Rao, 1961; McNamara, 1965; Schuling and Vink, Regardless of the H20 source, accumulation of H2O in the New 1967; Jamieson and Olinger, 1969; Rumble, 1976). Reaction rate data for Salem area may have been related to a pressure shadow effect around the

the conversion of anatase to rutile in H20 solutions suggest that anatase north end of the Prescott complex (originally suggested by H. W. Jaffe to may persist for millions of years only below -300 °C, consistent with the Peter Robinson, 1966). chlorite-K-feldspar assemblage (Rao, 1961; McNamara, 1965). The relatively coarse grain size of retrograde sheet silicates (typically SUMMARY AND CONCLUSIONS 0.5-5 mm) suggests that retrograde temperatures and pressures were near the maximum limit of 300 °C and the upper parts of the range of 1.5-4 The retrograde metamorphic rocks occupy a 26-km2 area straddling kbar discussed above. A best estimate for the T and P of retrograde the axial surface of the Prescott syncline. Within the retrograde zone, metamorphism is therefore -280 ± 100 °C and -3.5 ± 2 kbar. there are three concentric retrograde isograds representing the completion of sillimanite-out, staurolite-out, and garnet-out retrograde reactions. On QUANTITY OF WATER REQUIRED the basis of these isograds and the degree of replacement of garnet by FOR RETROGRADE METAMORPHISM chlorite, the area has been subdivided into six retrograde metamorphic zones, designated R1 to R6, from least to most retrograded. Calculations were made to estimate the amount of water necessary to The prograde limiting assemblage in schists of quartz-albite- produce the degree of rétrogradation observed in the New Salem area. The muscovite-biotite-garnet-staurolite-ilmenite-pyrrhotite-sillimanite-graphite mapped sillimanite-out and staurolite-out isograds (Figs. 3B and 3C) were was retrograded to the common zone R6 assemblage of quartz-albite- extrapolated south to the Prescott complex to enclose a 26-km2 area. muscovite-chlorite-anatase-sphene-K-feldspar-pyrite-graphite. Another Sample UOIA (R2, Table 2; chlorite excluded) was chosen as a model prograde assemblage, probably quartz-muscovite-ilmenite-graphiteigar- prograde rock. Reactions 6,8,12,13, and 14 were applied to convert this net, was partially retrograded to the assemblage quartz-muscovite-chlorite- chloritoid-sphene-anatase. TABLE 6. MODEL VOLUME CHANGES AND WATER QUALITY The zoning in garnets is clearly relict from prograde metamorphism, FOR RETROGRADE METAMORPHISM with no observed low-temperature retrograde change in rim compositions. No zoning is evident in staurolite. In contrast, the systematic compositional Metamorphic zone Surface Water Total Total area volume volume water changes of the sheet silicates and ilmenite demonstrate that these minerals (km2) percent increase volume km3 (km3) (km3) maintained at least partial equilibrium during retrograde metamorphism. There is no evidence for the retrograde production of staurolite, R6 0.7 3.0% 0.021 0.023 garnet, or biotite, suggesting that many retrograde reactions were not the R3-R5 12.3 1.2% 0.142 0.165 same as the reverse of superficially similar prograde reactions. This was R2 13.0 0.5% 0.065 0.072 probably due to disequilibrium between the prograde and retrograde

Total per kilometre of assemblages. vertical extent 26.0 km2 (0.9%)* 0.23 km3" 0.26 km3" Retrograde metamorphism required -0.26 km3 of water (at STP) 3 'Area-weighted sums. and —0.23 km of volume increase for each kilometre of vertical extent of the retrograde zone. The conditions of retrograde metamorphism are con-

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strained to -280 - 100 °C and -3.5 ± 2 kbar, compared to -600 °C and Hollister, L. S., 1966, Garnet zoning: An interpretation based on the Rayleigh fractionation model: Sdence, v. 154, p. 1647-1651. -6 kbar estimated for prograde metamorphism. Hollocher, Kurt, 1980, Retrograde metamorphism of the Lower Devonian Littleton Formation, New Salem area, Bronson Hilt anticlinorium, Massachusetts: Geological Scdety of America Abstracts with Programs, v. 12, p. 42. 1981a, Retrograde metamorphism of the Lower Devonian Littleton Formation in the New Salem area, west- central Massachusetts [M.S. thesis]: Amherst, Massachusetts, University of Massachusetts, Department of Geology ACKNOWLEDGMENTS and Geography, Contribution 37, 268 p. 1981b, Systematic sheet silicate composition changes in retrograded Littleton Formation schists, west-central Massachusetts: Geological Sodety of America Abstracts with Programs, v. 13, p. 475.

Most of this paper is from my Master of Science thesis (Hollocher, Jamieson, J. C., and Olinger, Bart, 1969, Pressure-temperature studies of anatase, brookite, rutile, and Ti02—II: A discussion: American Mineralogist, v. 54, p. 1477-1481. 1981a). I thank my committee members, Leo M. Hall, J. C. Cheney, D. U. Kato, T. T., 1985, Pre-Andean orogenesis in the Coast Ranges of central Chile: Geological Sodety of America Bulletin, Wise, and particularly Peter Robinson, for their help in the original proj- v. 96, p. 918-924. Mather, J. D., 1970, The biotite isograd and the lower greenschist fades in the Dalradian rocks of Scotland: Journal of ect. I thank Stephen Haggerty and David Leonard for making available Petrology, v. 11, p. 253-275. McNamara, M. J., 1965, The lower greenschist fades in the Scottish Highlands: Geologiska Fóreningens i Stockholm the electron microprobe at the Department of Geology and Geography, Fárhandlingar, v. 87, p. 347-389. University of Massachusetts, Amherst. I thank David D. Ashenden, geolo- Michener, Stuart, 1983, Bedrock geology of the Pelham-Shutesbury syndine, Pelham dome, west-central Massachusetts [M.S. thesis]: Amherst, Massachusetts, University of Massachusetts, Department of Geology and Geography, gist for the Boston Metropolitan District Commission, for his assistance Contribution 43,101 p., 5 plates. Ramsay, C. R., 1973, The origin of biotite in Archaean metasediments near Yellowknife, N.W.T., Canada: Contributions and for giving access to the MDC-6A drill core. I also thank Lincoln to Mineralogy and Petrology, v. 42, p. 43-54. Hollister and John M. Ferry for their helpful reviews of this manuscript. Rao, C.N.R., 1961, Kinetics and thermodynamics of the crystal structure transformation of spectroscopically pure anatase to rutile: Canadian Journal of Chemistry, v. 36, p. 498-500. Ribbe, P. H., 1980, Titanite, in Ribbe, P. H., ed., Orthosilicates: Mineralogical Sodety of America Reviews of Mineralogy, v. 5, p. 137-154. 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