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

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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 comple- tion 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 composi- tions 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. 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 SYSTEMATIC RETROGRADE METAMORPHISM OF SCHIST, MASSACHUSETTS 623 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 pro- duce 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 assem- blages 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
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