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Polymetamorphism in Medium- to High-Grade Pelitic Metamorphic Rocks, West-Central Maine

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Polymetamorphism in Medium- to High-Grade Pelitic Metamorphic Rocks, West-Central Maine Polymetamorphism in medium- to high-grade pelitic metamorphic rocks, west-central Maine M. J. HOLDAWAY Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 CHARLES V. GUIDOTTI» Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 JAMES M. NOVAK Air Force Geophysics Laboratory, Hanscom Air Force Base, Massachusetts 01731 WILLIAM E. HENRY 235 G.B., Research Center, Phillips Petroleum Company, Bartlesville, Oklahoma 74004 ABSTRACT throughout the area at a given time, but range in grade from chlorite zone to K- increased about 0.5 kbar or more between feldspar-sillimanite zone (Osberg and oth- A regional petrologic study of pelitic M2 and M3 metamorphism. The more ers, 1968). metamorphic rocks has been undertaken, regional distribution of isograds to the With the exception of a small syenite with emphasis on the area bounded by the south is consistent with higher initial tem- body near Litchfield and two gabbro intru- cities of Augusta, Lewiston, Norway, peratures and more closely spaced mag- sions (Fig. 1), the plutonic rocks are biotite- Rangeley, Kingfield, and Madison. This matic heat sources, but not necessarily with muscovite-garnet quartz monzonites and report involves a regional isograd map, increasing pressure to the south. The pres- biotite-garnet granodiorites of the New interpretation of the nature of the various sure increase between M2 and M3 might Hampshire Magma Series. For the purpose reactions, and discussion of petrologic prob- have been produced by intrusion of magma of this report, the plutons are identified as lems of polymetamorphism, isograd distri- above the present level and/or extrusion of batholiths or groups of smaller plutons bution, and widespread occurrence of mus- rhyolitic volcanics. (Fig. 1) that may be cupolas or roots of covite in the K-feldspar-sillimanite zone. A batholiths at deeper or shallower levels. future contribution will deal more specifi- INTRODUCTION From west to east, the major centers of plu- cally with mineral chemistry, geotherrnome- tonic activity are (1) the Mooselookmegun- try, and geobarometry of much of the area. West-central Maine is unique from a tic batholith (2) the Phillips batholith, (3) After a low-grade metamorphism, Mi, petrologic standpoint because, as noted by the Livermore Falls group with the Skow- the region suffered a higher-grade early numerous previous workers, it is in the hegan batholith at the northeast end, and metamorphism, M2, which in many places transition zone from isolated plutonic rocks (4) the Hallo well group. On the northern produced andalusite and cordierite as well of the New Hampshire Magma Series and edge of the area are the Reddington and as staurolite. This was followed by a some- associated contact metamorphism on the Lexington batholiths, and in the southwest what higher-pressure metamorphism, M3, north to regional metamorphism with asso- part of the area is the composite Sebago involving staurolite-chlorite and sillimanite. ciated plutonic bodies on the south (Doyle, batholith, the largest plutonic complex in In some areas, M3 has produced extensive 1967). The area involved in this report is Maine. retrograde effects. With increasing grade in within a polygon bounded by the cities of Previous work in the area has brought to M3, the major mineral reactions are (1) Augusta, Lewiston, Norway, Rangeley, light a number of significant problems, formation of staurolite from reaction of Kingfield, and Madison, Maine (Fig. 1). some of which need to be considered using garnet, chlorite, and muscovite; (2) forma- Although three or more metamorphic the regional approach involved in this tion of sillimanite from staurolite, chlorite, episodes were involved, the associated plu- study. Moreover, an overview will serve to and muscovite; (3) reaction of staurolite to tonic rocks span a narrow age range better identify remaining questions. The form sillimanite, biotite, and in many rocks, between 395 and 360 m.y. (Guidotti, 1970a; first is the problem of polymetamorphism. garnet; and (4) formation of K-feldspar Dallmeyerand Vanbreeman, 1978; Moench Can the various metamorphic events found from the partial reaction of muscovite, and Zartman, 1976). The earliest metamor- in parts of the area be traced and correlated quartz, and plagioclase. A possible fourth phic event affected Early Devonian rocks, throughout the area? Guidotti (1970b) metamorphism, M4, produced narrow au- and the latest is associated with 360-m.y.- found evidence for an early low-grade reoles with andalusite and staurolite around old plutons. Thus all the metamorphic metamorphism, Mi, followed by at least the three latest plutons. events were associated with the Acadian two major metamorphic events in the Range- Analysis of the isograd pattern demon- orogeny, at least in the broad sense (Naylor, ley and surrounding quadrangles. These strates that the pressure was rather uniform 1971). The metamorphic rocks are domi- are M2, which produced staurolite, andalus- nantly pelitic, with lesser amounts of meta- ite, and cordierite in pelitic rocks; and M3, graywacke and calcareous rocks, and they which produced sillimanite in pelitic rocks •Present address: Department of Geological Sciences, University of Maine, Orono, Maine are mainly Silurian and Early Devonian in at appropriate grades, but no andalusite or 04473. age (Pankiwskyj and others, 1976). They cordierite. In the eastern Augusta Quadran- Geological Society of America Bulletin, v. 93, p. 572-584, 3 figs., 2 tables, July 1982. 572 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/7/572/3444551/i0016-7606-93-7-572.pdf by guest on 27 September 2021 15' QUADRANGLES 1 2 3 4 5 6 7 6 9 10 II 12 13 14 15 16 1 Rangeley 2 Phillips 3 Kingfield 4 Anson 5 Rumford 6 Dixfield 7 Farmington 8 Norridgewock 9 Bryant Pond 10 Buckfield 11 Livermore Falls 12 Augusta 13 Norway 14 Poland 15 Lewiston 16 Gardiner MINERAL ASSEMBLAGES 1 Chi 2 Biot-Chl° 3 Alm-Biot-Chl° 4 St-Alm-Biot-Chl0 5 Sill-St-Alm-Biot0 6 Sill-Aim-Biot0 7 Ksp-Sill-Alm-Biot a And c Cord 0 Aim or Chi missing in soma specimens ISOGRADS S Staurolite Si Sillimanite KS Ksp-Sill 4 M4 Figure 1. Isograd map of west-central Maine. Control for the isograds is based on representative localities shown and on references given in text. Isograd designation is on high-grade side. Mineral assemblages are given in legend; abbreviations: Chi = chlorite, Aim = almandine-bearing garnet, Biot = biotite, St = staurolite, Sill = sillimanite, Ksp = K-feldspar, Cord = cordierite, And = andalusite. Table 1 footnote gives additional phases present. Underlined numbers refer to data of Guidotti and various co-workers at Wisconsin. Other numbers refer to data of Holdaway and Novak included in Table 2. Acidic plutonic units, stipple: MB = Mooselookmeguntic batholith, PB = Phillips batholith, LG = Livermore Falls group, HG = Hallowell group, RB = Reddington batholith, LB = Lexington batholith, SkB = Skowhegan batholith, SeB = Sebago batholith. Random line pattern: late gabbro or syenite. The M4 isograds around the Lexington and Skowhegan batholiths are largely schematic. Northwestern Anson Quadrangle is under study by Holdaway; Pankiwskyj (1979) shows extensive andalusite and staurolite, suggesting a northeastern extension of the M2 isograd. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/7/572/3444551/i0016-7606-93-7-572.pdf by guest on 27 September 2021 574 HOLDAWAY AND OTHERS gle, 75 km southeast of Rangeley, Osberg The present study is part of a long-term dominant metamorphism of the area is M3, (1968, 1971, and 1974) described only one effort in the region by several workers. The with reaction isograds shown by solid lines. metamorphism and concluded that the work falls broadly into two categories: (1) An M2 staurolite isograd is shown by a sillimanite-bearing rocks of the southern description and understanding of the petrol- dashed line with long dashes, and an M4 part of the area represent the higher ogy and metamorphic history of the area staurolit e isograd is shown by a dashed line temperatures of the metamorphism that and (2) determination of mineral chemistry with short dashes. Justification for the produced extensive andalusite and cordier- and distribution of temperature, confining chronology shown in Figure 1 will be pro- ite to the north. In the western Augusta pressure, and Xh2o in the pelitic rocks dur- vided in the sections that follow. Only the Quadrangle, Novak and Holdaway (1981) ing each metamorphic episode. The present staurolite, sillimanite, and K-feldspar-sil- found evidence that, at medium grades, paper involves the petrology and metamor- limanite isograds are shown. Pluton config- chlorite has overprinted low-pressure min- phic history, including a careful analysis of urations are based on the 1973 Preliminary eral assemblages; at slightly higher grades, pelitic mineral equilibria as a basis for Litholoj;ic Map of Maine compiled by P. H. sillimanite has formed from reaction of future work on mineral compositions, geo- Osberg ( Maine Geological Survey). chlorite and staurolite. The pattern that thermometry, and geobarometry. This re- appears to be emerging for the whole region port combines data collected by Guidotti SUMMARY OF extending from Rangeley to Augusta is that and his co-workers mainly from west of METAMORPHIC ZONES of early metamorphism, M2, which pro- longitude 70° 15', with data collected by duced andalusite-biotite and, depending on Holdaway and his co-workers mainly from The metamorphic isograd map (Fig. 1) bulk Mg/Fe ratio, staurolite or cordierite; east of longitude 70° 15'. Results of the illustrates four major zones, the low-grade and that of a later, higher-pressure meta- work of the two groups coincide and com- zone, the staurolite zone (which locally con- morphism, M3, which produced sillimanite plement each other. tains andalusite and/or cordierite), the sil- and staurolite, but not andalusite or cor- During field seasons in 1977 and 1978, limanite zone (which locally contains anda- dierite.
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