American Mineralogist, Volume 73, pages20-47, 1988 Devonian and Carboniferousmetamorphism in west-centralMaine: The muscovite-almandinegeobarometer and the staurolite problem revisited M. J. Hololwlv Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275, U.S.A. B. L. Durnow Department of Geology, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A. R. W. HrNroN* The EnricoFermi Institute, The Universityof Chicago,Chicago, Illinois 60637,U.S.A. ABSTRAcT Important thermal metamorphic eventsin west-centralMaine occurred at 400 Ma (Mr), 394-379Ma(Mr), and325 Ma(Mr). EachiscloselyassociatedwithemplacementofS-type granites,such that the isogradpatterns produced in the surrounding pelitic schistsgenerally follow plutonic outlines. From north to south, gradeof metamorphism varies from chlorite to sillimanite-K-feldspar-muscovite. Mineral-chemistry studies on M, and M, indicate the following: (l) Much staurolite- grade chlorite is retrograde as demonstrated by the lack of a consistent relation between biotite composition and presenceor absenceofchlorite, given the restricted range ofX"ro allowed in these reduced graphitic rocks. Consequently,the first sillimanite-forming re- action for many pelitic schists in Maine does not involve chlorite. (2) Garnet zoning patterns are prograde in rocks of the staurolite zone and retrograde in rocks of higher grade. (3) Presenceof graphite and nearly pure ilmenite suggestslow Fe3* in micas and allows for end-membercalculations that include a "Ti biotite" (KTiD(Fe,Mg)AlSi3Oro(OH)r) and a "Ti muscovite" (KTi(Fe,Mg)AlSi3O,o(OH)r).(4) Staurolire contains about 3 H (48- oxygen basis) and shows subtle indications of nonideality in Fe-Mg Ko relationships. Garnet-biotite geothermometryindicates the following averagetemperatures for the first occurrenceof minerals:staurolite-510'C, sillimanite-580 "C, sillimanite-K-feldspar- 660 "C. The muscovite-almandine-biotite-sillimanite (MABS) geobarometeris calibrated on the basisof an averageM. pressureof 3.1 + 0.25 kbar such that staurolite * muscovite break down directly to sillimanite + almandine + biotite immediately above the silli- manite-andalusitephase boundary. On the basis of this geobarometer,the M, rocks inside the secondsillimanite isograd crystallized at 3.8 kbar. The muscovite-quartz dehydration boundary shows excellent agreementwith garnet- biotite temperatures and MABS pressure for the second sillimanite isograd in M, if reasonablemodels for behavior of fluids in reduced graphitic pelites are assumed.The staurolite-quartz dehydration boundary predicts temperaturesabout 60 oC above the gar- net-biotite temperaturesfor the first sillimanite isograd in Mr. However, this discrepancy can be partially eliminated by assuminga regular pseudobinarystaurolite solution model wilh WG: I 5 kJ per tetrahedral Fe site. Nonideality in staurolite solid solution may also explain staurolite Ko relationships that involve reversalsin Ko values. The progressivesouthward increasein pressurefrom early i|l42(2.35kbar) to M3 (3.1 kbar) to M5 (3.8 kbar) over a75-m.y. period requires a net increaseof about 5 km of rock during a time when much of New Englandwas sufferingerosion. This P increaseis believed to have been the combination of two efects: (l) as the locus of metamorphic heat moved southward, the depth of metamorphism increasedby 2 to 3 km over a distanceof 125 km laterally, implying postmetamorphic tilting of the area and/or differencesin original to- pography; and (2) at any given place, P increasedwith time becauseof development of a widespreadDevonian volcanic highland and correlative emplacementof granites at shal- low levels. The extensiveterrane of Mr, which varies from K-feldspar-bearingto K-feldspar-absent rocks, marks incipient granite melting immediately below the present rocks at 3.8 kbar, 660 'C; local ?" variations probably reflect local differencesin degreeof mobilization of magma and fluids. This presumably occurred above the gently north-dipping contact of the Sebagobatholith exposedsouth of the M, rocks. * Presentaddress: Grant Institute of Geology, Edinburgh, Scotland EH93JW. 0003-004x/88/0l 02-0020$02.00 20 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 21 INrnooucuoN M2 West-central Maine provides a unique opportunity to Following M,, a widespread thermal event produced unravel the complexity of Acadian and Hercynian meta- staurolite, staurolite-andalusite,and andalusite in rocks morphic events,as it lies in the transitional zone between of appropriate composition and grade (terms as in Gui- slightly metamorphosedrocks to the north and high-grade dotti, 1970). This event is easily recognizedin much of metamorphism to the south. Within this zone, the dis- the area between Farmington, Kingfield, and Rangeley tinctive overprints of three thermal metamorphic events (Fig. l, Holdaway et al., 1982) where retrograde meta- have been preserved. Previously, the metamorphic pe- morphism has downgraded staurolite- and andalusite- trology, isograds, mineral reactions, and metamorphic bearing assemblagesand in the processproduced distinc- history of this region, bounded by the cities of Augusta, tive pseudomorphs.These authors estimated the timing kwiston, Norway, Rangeley,Kingfield, and Madison (Fig. of this event as >394 + 8 Ma. Recently, Gaudette and l), havebeen summarized by Holdawayet al. (1982)and Boone (1985) have obtained a whole-rock Rb-Sr age of Guidotti et al. (1983). This area is one of dominantly 400 Ma on the Lexington batholith (Fig. l). Dickerson pelitic metamorphic rocks and S-type granites,mainly of and Holdaway (ms.) have correlatedintrusion of this plu- the New Hampshire Magma Series,and it representsthe ton with Mr. M, andalusite occurs as a relict mineral in transition from chlorite-zone rocks to the prevailing up- severalareas most recently affectedby M, (Holdaway et per amphibolite-grade metamorphism that is dominant al., 1982).A regional study of M, will be the topic of a in southern Maine, southern New Hampshire, and cen- future communication. tral Massachusetts.Lithologic units and their ages are summarizedby Holdaway et al. (1982) and references M3 therein. The most prevalent metamorphism in the area is re- The presentreport provides data on mineral chemistry gional-contact metamorphism spatially associatedwith and metamorphic conditions for the dominant M. and the Mooselookmeguntic,Songo, Phillips, Livermore Falls, M, metamorphism of the region covered by Holdaway et and Hallowell batholiths or groups of plutons (Fig. l). al. (1982). Pressureestimates for M, are deducedfrom Whole-rock Rb-Sr agesrange from 394 to 379 Ma (Dall- biotite-garnetgeothermometry combined with aluminum meyer and Vanbreeman, 1978; Moench and Zartman, silicate stability relations. In addition, the potential for a 1976).This metamorphismproduced the gradesequence muscovite-almandine-biotite-sillimanite (MABS) geoba- biotite - almandine - staurolite - sillimanite, all pres- rometer is examined and applied to estimatethe pressure ent with muscovite. Isogradsbroadly follow plutonic out- of Mr. Garnet-biotite geothermometry, combined with crops. No major retrogressiveeffects have been noted. experimental stability data on muscovite-quartz and Grade designationsand their mineral assemblages(ab- staurolite-quartz,allows for a test of models for the be- breviations after Kretz, 1983) for M, and M, are defined havior of C-H-O fluids in pelitic rocks. These data are by number as follows: then integrated with the present understanding of the metamorphic history of Maine to provide insight into the l cht (M3) conditions and causesof metamorphism in Maine. 2 Bt + Chl (M3) These metamorphic rocks are ideal for evaluating and 3 Grt + Bt + Chl (M3) formulating pelitic geothermometersand geobarometers 4 St + Grt + Bt (+ Chl) (M3) becausethe lack of synchronousdeformation allows for 5 Sil + st + Grr + Bt (M3) the assumption of nearly constant pressureduring each 6 Sil + Grt + Bt (M3) thermal event, and ubiquitous presenceof graphite allows 6.5 Sil + Grt + Bt (within grade 7) (M') for the assumptions of low /o, and low and/or approxi- 7 Kfs + Sil + Grt + Br (M5) mately constant Fe3*content in Fe-bearingphases. 8 Kfs + Sil + Grt + Bt (trace Ms) (M,) + + BnrBr suvrnrARy oF METAMOR.HT. E'ENTS All assemblagescontain Ms + Qtz * Gr Pl Tur. Hordawaverar. (1e82) summarized rhesequence of ;"fill",'JffiiiJff:1":J#:11!ffiTiJi?:"tuf:": metamorphic eventsof the region consideredhere' o b::t n""j-o*r"ts M. isograds for tire staurolite zone (grade summary of these events, as they are now interpreted, ii'rift--"rite zone (grades5 and 6), and K-feldspar-sil- follows' hmanite zone (gradei 6.5 and 7 in Mr). Apart from the M, local appearanceof M, andalusite, textures and compo- The first event, slightly older than 400 Ma, produce4 sitions indicate that in grades4 to 8, all progrademinerals widespread chlorite-zone metamorphism synchronous equilibrated with M3 or M, conditions' with S, schistosity (Guidotti, 1970). Much of the area betweenMadison, Farmington, and Kingfield (Fig. l) has M4 suffered only this event, and temperature has not been The recentdating of the Lexington batholith at 400 Ma significantly higher in this area during the subsequent (Gaudette and Boone, 1985) puts the significanceof Mo events. for this areain doubt. The Hartland pluton, 15 km north- 22 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE I 2 3 4 5 6 7 8 9 l0 ll l2 t3 t4 l5 l6 1. Rangeley 2. Phillips 3. Kingfield 4. Anson 5. Rumford 6. Dixfield 7. Farmington 8.