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. Norridgewock 9. BryantPond 10. Buckfield 11. Livermore 12. Augusta 13. Norway 14. Poland 15. Lewiston 16. Cardiner

Fig. 1. Isograd map for M, and M, in west-centralMaine. Isogradsshown are M, staurolite (S0, M, sillimanite (Sil), and M, K-feldspar-sillimanite (Kfs-Sil). Whole numbers refer to analyzedspecimens. Bold decimal numbers indicate P determinationsfrom Table l0 in kbar. Pressuresin parenthesesare those from grade 7 (Table 10) that may representdisequilibrium as discussedin text. Acidic plutonic units-pattern: MB : Mooselookmegunticbatholith, PB : Phillips batholith, LG : Livermore Falls group, HG : Hallowell group, RB : Reddington batholith, LB : l,exington batholith, SkB : Skowheganbatholith, SoB : Songobatholith, SeB : Sebagobatholith (Hercynian).Gabbroic plutonic units-stipple. For namesof quadrangles,see diagram and list at right. For complete distribution of mineral assemblages,see Holdaway et al. (1982). Modified from Holdaway et al. (1982), Henry (1974), Dutrow (1985), and Evans and Guidotti (1966). Igneous contacts from Osberget al. (1985). M, metamorphism is not shown and will be discussedin a future contribution (seealso Dickerson and Holdaway, ms.; Holdaway et a1.,1986a). Isograds stop near Skowhegan batholith becauseits metamorphism involves andalusiteand mav be M,. east ofthe Skowheganpluton (Fig. l) is the closestdated Ms pluton (whole-rock Rb-Sr, Dallmeyer et"al., 1982)at 362 Ma, the ageHoldaway et al. (1982)assumed for Mo. We Recent isotopic studies for the Sebagobatholith (Fig. tentatively concludethat Mo did not affect the areaunder l) give Hercynian agesof 325 Ma (Hayward and Gau- study. dette, 1984) and 324 + 3 Ma (Aleinikoff et al., 1985) HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRALMAINE z5 usingRb-Sr whole rock and U-Pb on zircon,respectively. two grains was used. The core and intermediate-zoneanalyses Lux and Guidotti (1985)and Guidotti et al. (1986)have were grouped and averagedon the basis of chemical similarity. garnet rim pointed out that much of the higher-grade area to the Garnet-biotite geothermometry indicated that all (- pm from rim) showedretrograde temperatures, and south of the M. plutons has probably equilibrated to the analyses 5 thesevalues are not given here. Biotite analyseswere taken from thermal events associatedwith the emplacement of this severalgrains within 30 to 100 pm ofthe gamet being analyzed. younger preserving post- Sebagobatholith, thereby only Each biotite was analyzed immediately before or after the co- M. effects. The sillimanite-K-feldspar isograd (Fig. l) existing garnet using the sameanalysis file. shows a general spatial relation to the Sebagocontact. Presenceof graphite and/or nearly pure ilmenite (=4o/ohe- Much of this area is highly migmatitic and pegmatitic, matite solid solution) in every rock indicatesthat Fer* should be and the ageof theseigneous rocks may well correlatewith minor and approximately constantin most of the minerals. Con- that of the Sebagobatholith. Lux and Guidotti (1985)and sistentwith the assumptionof low Fe3*,mineral stoichiometries Guidotti et al. (1986) have suggestedthat the effectsof were determinedby normalizing to the indicated number of ions: 8 oxygens, the Sebagoevent may extend as far as 30 km to the north mtcas-22 oxygens,chlorite- 28 oxygens,feldspars- garnet-8 and ilmenite-2 cations. For staurolite, stoi- and certainly are responsiblefor the sillimanite-K feld- cations, chiometry was determined by normalizing (Si + Al) to 25.53 as spar isograd. One possible shortcoming of this interpre- discussedbelow. "hinge" (Gui- tation is the absenceof an obvious effect Ion-microprobe analyseswere done on two ilmenite speci- dotti, 1970),a retrogradedzone of M, rockswhere lower mens (see Table 8) using an AEI IM-20ion microprobe at the temperaturesfrom the Sebagoevent might have down- University of Chicago. The instrumental configuration was de- graded the rocks far north of the pluton. However, Gui- scribedby Steeleet al. (1981),and the analyticalconfigurations dotti et al. (1986) have noted some downgradingin the were similar to those describedby Jonesand Smith (198a). The Rumford quadrangle.We designatethis Hercynian higher- ion yields were basedon Corning borosilicate glasses,except Li P, higher-T event M, and suggestthat those rocks south (staurolite),Na (plagioclase),Sc (hibonite), and Nb (assumedto from mass of the secondsillimanite isograd(grades 6.5 and 7) suf- be the sameas Zr). Analyseswere made by scanning for 1 s at eachmass. Elements not listed fered its effects.It should be pointed out that M, and M, 100 to mass7, counting in Table 8 were not detected.These semiquantitative analyses were originally mapped as a single metamorphic se- will be affectedby matrix-dependent factors and are estimated quence(e.9., 1982).The principal rea- Holdaway et al., to be between0.5 and 2 times the correct values. sons for separating M, and M, are the age differences In order to save spacebut provide the maximum amount of betweenplutons, the spatial associationwith the plutons, data, the major-element analysesare presentedin Appendix Ta- and important differencesin lt and P as discussedbelow. ble I as stoichiometric analysesonly, plus the oxide total. Anal- The rocks under considerationfor this report have suf- ysesare listed in order ofincreasinggrade designation (see above). fered M. and/or M, depending on their grade and map Within eachgrade, analyses are given in order of increasingtem- location. M, is discussedin more detail by Holdaway et perature using a slightly modified Ganguly-Saxena(1984) bio- geother- al. (1986a) for a four-quadrangle area in the Kingfield tite-garnetgeothermometer (discussed in the sectionon In the text, tablesare provided to illustratethe average region, by Dickerson and Holdaway (ms.) for the Bing- mometry). compositions of minerals with increasinggrade. ham area, north ofthe Kingfield and Anson quadrangles, and by Dutrow and Holdaway (in prep.) for the Far- mlngton area. MrNnru.r, cHEMISTRY AND PETRoLocIC INTERPRETATION ANar-vrrca.l METHoDS Petrologic interpretation of the present study area is Forthe areaof M, andM, in Figure1,40 polishedsections provided in a paperby Holdaway et al. (1982).This sec- of low-variancerocks were prepared for microprobework. Lo- tion adds aspectsto their interpretation that have become by numbersin Figurel. Microprobeanalyses cationsare shown possible as a result of detailed mineral chemistry. weredone on an automatedrcol-rr: microprobeat SMU with To a very good approximation, the metamorphic rocks KRTsELautomation that usesthe conectionprocedure of Bence given (M. or M') andAlbee (1968) modified by Albeeand Ray (1970). Beam cur- studied within a metamorphic event rent was 20 nA, acceleratingpotential was 15 kV, and beam may be regardedas a simple sequenceof mineral assem- diameterwas focusedto -2 prnfor staurolite,garnet, and il- blagesthat have similar bulk compositions,/o, controlled meniteand to l0 pm for phyllosilicatesand plagioclase.Stan- at a reducing value, Xpr6 controlled over a nalrow range, dardsincluded a numberof analyzedsilicates and oxides.For and Z (metamorphic grade) as the principal intensive eachmineral, five or six analyseswere taken from oneportion variable. The thin sectionschosen for study were selected of the thin sectionas nearas possibleto the gametbeing ana- on the basis of minimum variance, and each grade des- lyzed.Analyses were for 30 s or 60000counts. For staurolite, ignation (given above) representsa single assemblage. all majorelements were renormalized to standardsto minimize Thus, changesin mineral assemblagesand zoning are drift, andAl andFe values were multiplied by constantsto bring mainly a responseto changesin Z. In support of this the analysesinto agreementwith previousgood-quality micro- (limited nature probe and wet-chemicalanalyses, a proceduredescribed by simplified view are (l) the thermal time) Holdawayet al. (1986b).Ilmenite (Fe, Ti, Mn) andplagioclase of the metamorphism, (2) the prevalenceof graphite and (Na, Ca)analyses were also renormalized to standardsto mini- nearly pure ilmenite, and (3) the fact that, with a few mizedrift. For garnet,a regularlyspaced traverse across one or noted exceptions, the approach works. The section on 24 HOLDAWAY ET AL.: METAMORPHISM IN WEST.CENTRAL MAINE

TneLe1. Summaryof averagemuscovite stoichiometry by grade, based on 22 oxygens

5l 6.07(3) 6.0s(3) 6.04(3) 6.04(2) 6.05(2) 6.07(1) 6.04 .Al 1.s3(3) 1.s5(3) 1.96(3) 1.96(2) 1.e5(2) 1.97(1) 1.96 'rAl 3.78(8) 3.82(4) 3.80(1) 3.76(4) 3.74(21 3.71(2) 3.69 Ti 0.03(1) 0.03(1) 0.05(1) 0.06(1) 0.08(1) 0.08(2) 0.11 FE 0.13(4) 0.10(1) 0.10(1) 0.11(2) 0.12(1 ) 0.13(1) 0.12 Mg 0.13(8) 0.09(2) 0.10(1) 0.11(2) 0.110) 0.12(1) 0.13 >(vi) 4.07(4) 4.04(1) 4.04(1) 4.04(1) 4.050) 4.04(1) 4.05 K 1.61(2) 1.56(s) 1.se(s) 1.67(8) 1.71(s) 1.83(3) 1.85 Na 0.2s(6) 0.35(e) 0.34(6) 0.26(7) 0.21(5) 0.12(2') 0.09 Ba 0.01(0) 0.01(1) 0.01(0) 0.01(0) 0.01(1) 0.01(0) 0.01 >(xii) 1.s1(3) 1.93(2) 1.s5(2) 1.ss(2) 1.93(3) 1.95(1) 1.9s Ba-ms 0.005 0.005 0.005 0.005 0.005 0.005 0.005 Ti-ms 0.015 0.015 0.025 0.030 0.040 0.040 0.055 phl-ann 0.035 0.020 0.020 0.020 0.025 0.020 0.025 ce 0.010 0.020 0.015 0.020 0.025 prl 0 038 0.023 0.018 0.015 0.033 0.020 0.025 pg 0.145 0j75 0.170 0.130 0.105 0.060 0.045 ms 0.752 0.753 0.747 0.780 o.792 0.830 0.845 Nofe.-Grade3 is representedby two specimensand grade 8 by one. Value in parenthesesis one standard deviation. See text for grade assemblages. Abbreviations (Kretz, 1983): Ba-ms-"Ba- muscovite"; Ti-ms-"Ti-muscovite"; KTi(Fe,Mg)AlSi"O,lOH)";phl-ann-phlogopite-annite; ce-cel- adonite; KAI(Fe,Mg)SinO,o(OH),;prl-pyrophyllite: pg-paragonite; ms-mus@vite.

metamorphic conditions demonstrates that these as- stitution TiR'z*Al ,. These same relations can be seenrn sumptions are reasonable. the present data (Table l) if zones 3 (2 specimens)and 8 (l specimen)are ignored. Muscovite The average end-member muscovite composition of Analysesfor muscovite are provided in Appendix Ta- each zone was calculated from the 22-oxygen stoichi- ble I and summarized by zone in Table 1. All specimens ometry by first dividing by 2 and then using the following contain <0.001 Ca and <0.004 Mn ions. Basedon anal- procedure:(l) "Ba-muscovite"' : Ba1'(2) "Ti-musco- yses of 6 specimens over a range of grade, muscovite vite,"' KTi(Fe,Mg)AlSi.O,o(OH),: Ti; (3) phlogopite- contains between0.02 and 0. l3 wto/oF, correspondingto annire : [>(vi) - 2]; (4) celadonite (ce) : {Fe + Mg - an average 1.2 + lo/o substitution of OH by F. These Ti - 3[>(vi) -2]]; (5) pyrophylliteis the averageof [l - valuesare comparableto thoseof Evans(1969). >(xii)l and (Si - ce + Ba - 3) (thesedifer by between0 In both muscovite and biotite, all substitutions involve and2.5 mol7o);(6) paragonite: Na; (7) muscovite is the either (l) octahedral sites, (2) octahedral and tetrahedral remainder. The dominant substitutions in these musco- sites, or (3) tetrahedral and l2-fold sites. Nothing in the vites are TiR'z*Al-, for "Ti-muscovite," Rr2+Al,tr-, for analysessuggests the need for any coupled substitutions phlogopite-annite, R,*SiAl , for celadonite, trSiIL,Al_, involving a combination of octahedral and l2-fold sites. for pyrophyllite, and NaK-, for paragonite. The muscovite substitutionscan be viewed using the con- It should be noted that the celadonite values near 2 cept of exchangecomponents (Thompson, 1982) with molo/oare partly a function of the other end members muscovite (KAlrAlSi3Or(OH)r) as the additive compo- assumedand the sequencein which they were calculated, nent. and partly a function of the standardsused for analysis. The octahedral site occupancyof all the muscovites is Becauseof the inaccuracy of estimating octahedral ions remarkably constant averaging4.044 + 0.012 calculated above 2, phlogopite-annite and celadonite contents must on a 22-oxygenbasis. This suggeststhat (l) any substi- have large errors relative to the other components.How- tutions involving octahedral vacancies must be nearly ever, low celadonite contents are reasonablefor the pres- constant in amount for all the muscovites and (2) there suresat which these muscovites crystallized. is a limited and constant amount of trioctahedral substi- The effectsof grade on composition of muscovite are tution. Most studies of muscovite have shown excess very subtle and approximately the same as those noted octahedralions (Guidotti, 1984),and in the presentmus- by Guidotti for muscovite in comparable assemblages covite this takes the form of about 2 molo/ophlogopite- (1978;1984,p. 407)for the Rangeleyarea, Maine. Briefly annite. The substitution involving Ti, since it is variable summarized,they are (l) Ti and R2* increasewith grade, in extent, cannot involve a change in vacancy content. (2) "'et decreaseswith grade, and (3) paragonite content Holdaway (1980) showedthat for pelitic muscoviteco- existing with ilmenite and an Al-rich mineral, R2* in- t "Ba-muscovite," "Ba-biotite," "Ti-muscovite," and "Ti-bio- creasesand "iAl decreaseswith increasingTi, with slopes tite" are given in quotation marks to emphasizethat they are of 0.98 and,-2.02, respectively,consistent with the sub- hypothetical end-member componentsand not mineral names. HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRALMAINE 25

Tnele 2. Summary of average biotite stoichiometryby grade, based on 22 oxygens

Grade:34566.578 Si 5.42(0) 5.38(4) 5.38(2) 5.37(1) 5.35(1) 5.34(2) s.36 toAl 2.58(0) 2.62(4) 2.62(21 2 63(1) 2.65(1) 2.66(2) 2.64 'rAl 0.84(4) 0.92(3) 0.s0(2) 0.88(10) 0.78(4) 0.79(3) 0.86 Ti 0.18(0) 0.17(21 0.23(4) 0.25(5) 0.33(4) 0.38(3) 0.33 Fe 2.51(39) 2.52(30) 2.53(15) 2.51(21) 2.s5(71 2.53(6) 2.33 Mg 2.23(421 2.17(311 2.07(10) 2.Os(21) 1.57(121 1.85(10) 2.O5 Mn 0.02(1) 0.01(1) 0.01(0) 0.01(1) 0.02(0) 0.01(1) 0.01 >(vi) 5.78(1) 5.7s(5) 5.73(3) 5.71(s) 5.65(4) 5.s7(4) 5.59 K 't.74(4) 1.68(7) 1.71(21 1.74(5) 1.80(5) 1.90(3) 1.86 Na 0.04(2) o.o7l2) 0.07(21 0.07(1) 0.06(1) 0.03(1) 0.03 >(xii)- 1.78(s) 1.76(7) 1.78121 1.81(5) 1.82(4) 1.94(2) 1.90 Fe/(Fe + Mg) 0.53(9) 0.54(6) 0.55(3) 0.s5(4) 0.57(2) 0.58(2) 0.53 Ba-bt 0.003 0.003 0.003 0.003 0.003 0.003 0.003 Ti-bt 0.090 0.085 0.115 0.125 0.165 0.190 0.165 tlc-mi 0.110 0j20 0.110 0.090 0.070 0.030 0.050 ms 0.015 0.018 0.018 0.019 0.004 0.021 0.035 eas-sid 0.390 0.424 0.414 0.402 0-382 0.353 0.360 won 0.020 0.035 0.035 0.035 0.030 0.015 0.015 phl-ann 0.372 0.315 0.305 0.326 0.346 0.388 0.372 Note: Abbreviations (Kretz, 1983): Ba-bt-"Ba-biotite"; Ti-bt-"Ti-biotite"; KTitr(Fe,Mg)AlSi"O,o(OH).;tlc.mi-talc-minnesotaite; ms-muscovite; eas-sid-eastonite-sidero- phyllite, KAI(Fe,Mg),Al,Si,O1o(OH),;won-wonesite; phl-ann-phlogopite-annite. " Includes0.005 Ba (see text).

first increasesand then decreasesas grade rises. Most of is TiDR'*r, the most important Ti substitution proposed the changesin muscovite composition reflect the chang- by Dymek (1983).(The remaining0.04 units of vacancy ing saturation levels of "Ti-muscovite" and paragonite may be explained by an averagemuscovite content of 2 content. Severalof the most subtle changesseen by Gui- molo/0.)The end member associatedwith Ti substitution dotti, e.g.,decreasing Z(vi) and decreasingSi, are not seen is KTitr(Fe,Mg)AlSi,O,o(OH)r, the sameas the "Ti-mus- in the presentmuscovites. Diferences in averageSi con- covite" end member. However, in biotite the ions could tent between the two suites clearly reflect diferences in be disorderedover both the M(l) and M(2) sites,whereas standardizationfor analysis. in muscovite they are presumably all at M(2) sites. The combination of l2-fold site vacanciesand consis- Biotite tently lower values for (t"At - 2) than for "iAl indicates Microprobe analysesfor biotite are provided in Ap- the presenceof significant amounts of talc-minnesotaite pendix Table I and summarizedby zone in Table 2. All component in the biotite. The averageend-member com- specimenscontain <0.001 Ca ions. Ba in representative position ofeach zone was calculatedfrom the 22-oxygen biotite is about half the amount in coexisting muscovite. stoichiometryassuming 0.005 Ba ions,dividingby 2,and The Fe3*contents were not measured,but must be low using the following procedure:(l) "Ba-biotite" : Ba; (2) and nearly constant, and would produce a small amount "Ti-biorite" : Tl (3) talc-minnesotaite: [1 - >(xii)];(a) - - t/2lviAl- of ferri-eastoniteor oxy-biotite (Dymek, 1983).Analyses muscovite : the averageof [3 >(vi) Ti] and of six biotites over a rangein gradeindicated 0. I 3 to 0.4 I I'Al + 2(xii)l (these differ by < I mol0/o);(5) eastonite- wto/oF (about 4 times that of coexisting muscovite), cor- siderophyllite : ("'At - 2ms); (6) wonesite : Na; (7) responding to an average substitution of 6.9 + 2.4o/oF phlogopite-annite comprises the remainder. The domi- for OH. Valuesof F for biotitesanalyzed by Evans(1969) nant substitutions in these biotites are Ti!R1+, for "Ti- are0.l2 to 0.50 wt0/0.Munoz and Ludington(1974, 1977) biotite," trSiK rAl-r for talc-minnesotaite, Al,trR1J for have shown experimentally that Mg-bearing biotite par- muscovite, AlrRlisi , for eastonite-siderophyllite,and titions F to a much greater extent than coexisting mus- NaK-, for wonesite.This procedureis comparableto that covrte. used by Dymek (1983) exceptthat Dymek's more oxi- In biotite the best selectionfor the additive component dized rocks necessitatedan attempt to calculateFe3* val- is annite. As is the casewith all metamorphic biotite (e.g., ues,whereas the reasonableassumption of zero Fe3*con- Dymek, 1983;Guidotti, 1984),octahedral vacancies are tent for these biotites makes the procedure simpler and common. A plot of >(vi) against Ti (Fig. 2) reveals that perhapsmore accurate. for the present biotites, most of which coexist with il- The effectsof grade on composition of biotite are more menite, the vacanciesare mainly associatedwith Ti con- noticeable than those for muscovite and are essentially tent. The sum of )(vi) + Ti for all the biotites is 5.96 + the same as those observed by Guidotti (1984, p. 446). 0.04. Clearly most or all of the variation in vacancy con- In summary, the changeswith grade are (l) slight de- tent is a linear function of Ti. The relevant substitution creasein Si and corresponding increase in toAl, (2) de- 26 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE

Quabbin Reservoirarea, Massachusetts, also contains 190/o "Ti-biotite" on the average.It appears that despite ab- senceof ilmenite in some rocks, the biotite from Maine is essentiallysaturated with respectto Ti. Also the Quab- bin Reservoir rocks, which probably formed at somewhat higher P, do not appear to have formed at higher Z or they would have had higher Ti in biotite coexisting with ilmenite. The explanation may lie in the fact that high- graderocks in both areasappear to involve vapor-absent partial melting of muscovite-bearing pelites. According to Kerrick (1972), this melting should occur at a nearly constant I over a P range of severalkilobars. The increasein averageFe/(Fe + Mg) followed by a possible decreaseat the highest grades results from the interplay of two factors: (l) The reactionssummarized by Holdaway et al. (1982)for theserocks (Fig. 3) imply an increasein Fe/(Fe + Mg) as Fe-rich staurolite reactsaway and then Fe-rich garnet partially reacts with muscovite, but at the highest grades(7, 8) biotite starts to react with sillimanite to grow more Fe-rich garnet and K-feldspar thus reducing Fe/(Fe + Mg) in biotite (Thompson, 1976). Ti (2) As gradeincreases, biotite and garnet must move clos- er togetherin Fe/(Fe + Mg) ratio as a consequenceof the Fig,2. Plot of )(vi) vs.Ti in ionsper 22-oxygen formula unit Z effect on the garnet-biotite exchangereaction (Ferry for biotites.The average sum of)(vi) + Ti is 5.96+ 0.04.The lineis drawnthrough this intercept with a slopeof - I indicating and Spear,1978). that mostofthe Ti substitutionin biotiteis charge-balancedby vacanciesin theoctahedral layer. Box is estimated( I o)analytical Garnet precrsron. Stoichiometry of the cores and intermediate zones of garnetsis provided in Appendix Table I and summarized creasein -A1, (3) a pronounced increasein Ti and cor- by grade in Table 3. Compositional changesof garnets respondingdecrease in 2(vi), (4) an increasein )(xii), and with grade are as follows: (a) almandine and pyrope con- (5) a slight increasein Fe/(Fe + MC). Thesechanges with tents first increasewhile staurolite is present (grades4, grade primarily reflect a progressiveincrease in "Ti-bio- 5), then decreaseas garnetbegins to react(grades 6,6.5), tite" and progressivedecreases in talc-minnesotaite and and then increase again with the onset of new growth eastonite-siderophylliteas $ade increases. (grades7, 8); (b) spessartinecontent behavesopposite to Several of the highest-graderocks contain little or no almandineand pyrope; (c) grossularcontent remainsmore ilmenite. Biotite in grade 7 of the Maine rocks (Fig. l) or less constant at about 4 molo/o;(d) Fel(Fe + Mg) de- containsan averageof l9 molo/o"Ti-biotite." Biotite ana- creaseswith increasing grade. Zoning in garnet is char- lyzed by Tracy (1978) from ilmenite-bearing rocks of the acterized (a) by a slight outward decreasein Fe/(Fe +

TneLe3. Summaryof averagegarnet stoichiometry by grade

Core alm 0.60(7) 0.71(8) 0.74(4) 0.68(5) 0.70(1) 0.74(4) 0.70 prp 0.06(2) 0.0e(2) 0.10(1) 0.10(2) 0.11(1) 0.12(1) 0.18 sps 0.2s(6) 0.15(7) 0.12(4) 0.17(3) 0.16(2) 0.10(s) 0.06 grs 0.04(1) 0.05(2) 0.04(2) 0.04(21 0.03(1) 0.03(2) 0.06 Fe/(Fe+ Mg) 0.e0(4) 0.89(2) 0.88(1) 0.87(3) 0.86(1) 0.86(1) 0.80 Intermediatezone atm 0.68(12) 0.73(7',) 0.75(4) 0.70(s) 0.71(1) 0.74(4) 0.72 prp 0.08(1) 0.09(2) 0.10(1) 0.10(2) 0.11(1 ) 0.11(1) 0.16 sps 0.1s(12) 0.13(6) 0.12(41 0.16(4) 0.15(2) 0.11(4) 0.06 grs 0.04(1) 0.0s(2) 0.04(2) 0.04(2) 0.03(1) 0.03(2) 0.06 Fe/(Fe+ Mg) 0.89(3) 0.89(2) 0.88(2) 0.88(2) 0.87(1) 0.87(0) 082 r pro. pro. retro. retro. retro. retro. retro. /Vote:Abbreviations (Kretz, 1983): alm-almandine; prp-pyrope; sps-spessartine; grs-gros- sular. . Based on averagegamet-biotite temperatures (Table 8), intermediatezones are designatedpro- gressive or retrogressive. HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 27

u.e9D9#

E J 6 o E (L

Temperature

Fig. 3. Tentative P-7 diagram showing divariant reactions involving garnet in pelitic rocks. Lower curves are Fe end-member boundaries,and upper curves are Mg end-member boundaries.The three AFM model univariant curyes are also shown as narrow divariant fields. Numbers representgrade designationsfor M, and Ms, +Mn indicates increaseor decreasein Mn from core to intermediate zone ofgarnet, and arrow indicates increaseor decreasein 1'from core to intermediate zone. The signs of slopesare consistentwith observationson M. and M, metamorphism. At high or low 4 signsof slopesmay change(e.9., Spear and Selverstone, 1983). Within the wide fields, the divariant reaction is the only reaction involving Fe-Mg components.The tentative conclusion that progradechlorite disappearsearly in the staurolite zone is discussedin the sectionon chlorite mineral chemistry and petrologic interpretation.

Mg) at garnet and staurolite grades (3, 4) and a slight lower averageMn content of grades6.5 and 7 is incon- outward increasein Fe/(Fe + Mg) in all successivegrades sistent with the garnet-muscovitereaction, which should and (b) by spessartinecontent decreasing outward in increaseMn. In the narrow T range of zone 7, a simple grades 3 and 4, remaining nearly constant in grade 5, Z increaseshould leave the averagegarnet more or less decreasingoutward in grades 6 and 6.5, and increasing unchangedwhile muscovite and K-feldspar adjust their outward in grades7 and 8. Na content. The lower Mn content of grades6.5 and 7 is Prograde changes in garnet composition. The steady a problem that can be most easily explained by a higher changein averageFe/Mg ratio of garnet with grade can P of crystallization for theserocks (Fig. 3) than for grades be seenin the cores,the intermediate zones,and even in 3 to 6. The rocks of grades 6.5 and 7 are not only the the peak metamorphic portion of the garnet (the inter- southernmost rocks of the region, but they were meta- mediate zone for grades 3 and 4 and the core for higher morphosed during the younger M, event. A single rock grades).With increasing Z, the garnet moves composi- in zone 8 has still lower Mn in the garnet, but this can tionally closer to the biotite (K" approachesunity) as a be explained in part by the reaction ofbiotite and silli- consequenceofthe Ieffect on the garnet-biotite exchange manite to garnet and K-feldspar in a rock where most of reaction (Ferry and Spear, 1978). the muscovite has reactedaway. The progressivechanges in averageMn content are ex- Garnet zoning. Most garnets are zoned continuously plained in Figure 3. Becausegarnet is the only Mn-rich from core to rim. The compositional behavior of zoned phasein the rocks, Mn content of garnet is an especially garnets(Table 3) can be explained with the aid of Figure good monitor of growth or reaction. Garnet decreasesin 3 and the garnet-biotite temperatures (details in a sub- Mn as it growsat garnetand staurolitegrades (3, 4). When sequent section; see Table 8). Garnet-biotite geother- garnet reaches staurolite-sillimanite grade (5), it has mometry demonstratesthat at garnet and staurolite grade reached a minimum Mn content relative to adjacent (3, 4), garnet zoning is mainly of a prograde nature, re- grades4 and 6. Once staurolite disappears,Mn content flecting a temperatureincrease, whereas at higher grades, increasesin grade 6 as garnet and muscovite react. The the zoning is dominantly retrograde with the intermedi- 28 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE

TeeLe4. Summary of average staurolite and chlorite stoichiometry by grade

Staurolite

Si 7.62(6) 7.64(11) DI 5.19(5) 5.14(2) "Al 0.38(6) 0.36(11) toAl 2.81(s) 2.86(2) 'rAl 17.s3(1) 17.53(4) 'rAl 2.92(61 2.93(3) Ti 0.10(1) 0.11(1) Ti 0.01(0) 0.01(1) Fe 3.31(18) 3.09(37) he 4.46(81) 4.60(62) Mg 0.60(10) 0.s3(e) M9 4.48(83) 4.36(65) Mn 0.06(4) 0.07(21 Mn 0.04(3) 0.02(2) Zn 0.0s(8) 0.18(14) Ca 0 00(0) 0.00(0) Li' t0.311 [0.s1] 4vi) 11.90(0) 11.93(2) H [2.e71 t3.04] Fe/(Fe+ Mg) 0.050(9) 0.s1(7) >(R4+Li +H). 5.86(4) s.s1(17) Fe/(Fe + Mg) 0.8s(2) 0.86(1)

Note: See text for calculationof stoichiometryof staurolite. Error in "'Al is artificiallylow because of this procedure.Brackets indicatethat these entries are based mainly on estimated values. . For analysesfor which Li2Owas not determined,Li is estimatedas 0.3 atoms (see App. Table 1). ate zone of the garnetindicating a lower temperaturethan Staurolite the core. A decreasein Mn content of the garnet from Of the 16 staurolite specimensanalyzed in this study core to intermediate zone indicates that the garnet was (App. Table l, App. l, and Table 4), 4 werc analyzedfor growing (growth zoning), and an increasein Mn content H,O (Holdaway et al., 1986c)and 3 were analyzedfor indicates garnet that the underwent late-stageresorption Li,O (Dutrow et al., 1986). Holdaway et al. (1986b) or diffusionzoning (Tracy, 1982). showed that, for a larger group of complete staurolite zoning grades Growth is indicated in 3 and 4 where analyses,(Si + Al) was nearly constant on a 48-oxygen increasing I coincides with decreasingMn. Garnet grew basis:for 3l staurolites,(Si + Al) :25.43 + 0.13;for 28 flrst in response to reaction of chlorite to garnet in the of these 3l staurolitesnot containing high Fe3*,(Si + garnet zone and then as staurolite-biotite reacted to al- Al): 25.46 I 0.09; and for 6 Maine specimensthat all mandine-muscovite in the staurolite zone. The consistent formed under reducing conditions with graphite, (Si + decreasein Mn in garnets zoned of the staurolite zone is Al) : 25.53 + 0.03. In order to producea stoichiometry supporting evidence garnet- that the staurolite-biotite to that is both accurateand consistentwith those specimens muscovite equilibrium (see has a negative slope also analyzedfor water, we normalized the stoichiometry of Holdaway et al., 1982). probably This reaction was not the 12 specimensnot analyzedfor HrO and LirO to 25.53 affected by traces of chlorite in several staurolite-grade (Si + Al). The total positive chargewas then subtracted rocks, because the chlorite is believed to be retrograde, from 96 to give an approximation of the (H + Li) ions. as discussedbelow. At grade staurolite-sillimanite (5), the Through a propagation of errors it was determined that constancy Mn garnet of indicates that was neither being the approximate (H * Li) content has an error (lo) of creatednor destroyed,but the intermediate zonesofgar- +0.3 ions. Basedon the Li analysesof five specimens generally net equilibrated at lower temperaturesthan the from Maine, the averagenumber of Li ions for the re- (see cores Table 8)., This diffusion zoning must have been maining staurolites can be estimated at 0.3, allowing for produced in a retrograde fashion with biotite and silli- reasonableestimates of H ions that agreewell with those manite tending to react to garnet, but with slight stauro- analyzed for water. production garnet lite consuming (Fig. 3). In grades6 and The overall stoichiometry of the staurolite agreesvery 6.5, decreaseof Mn with decreasingI in the garnetsin- well with that given for the 3l staurolites by Holdaway dicates a retrograde growth zoning (Cygan and Lasaga, et al. (1986b).Mn and Ti are consistentlylow, whereas 1982). Retrograde reaction of biotite and sillimanite to Zn and Li are somewhat variable. This variability be- garnet and muscovite accounts for the Mn decrease.In comesmore apparentin the specimensfrom grade5 where grades 7 and 8, increaseof Mn with decreasing7 (diffu- staurolite was breaking down by reaction with muscovite sion zoning) results from decreasein garnet content as it and quartz to sillimanite, garnet, and biotite (e.g., Gui- reacts with K-feldspar during the early stagesof cooling. dotti, 1974). The most unusual staurolite is specimen 6 from near East Winthrop (Fig. 1). This staurolite forms '?Retrograde zoning in grade 5 is partly an artifact of the fact about loloof the rock but occursin small euhedralcrystals that biotite used for core and intermediate-zoneI calculation is rather than as remnantsin mica pseudomorphs,the more the samefinal biotite. For grades3 to 6, Fe/(Fe + Mg) in biotite common occurrenceof the mineral as it reactsout (Hold- increases0.02 molo/o/K(Tables 2, 8). Lessthan half of the garnet awayet al., 1982;Guidotti, 1968).It may have beensta- re-equilibratedwith biotite during the (average)15 .C retrogres- - I Li ion per 48 (see sion. Thus, biotite changedabout -0.15 mol0/0,indicating that bilized by the oxygen also Dutrow actlualT values for grade 5 cores were about 2 'C lower than et al.,1986). measured(Ferry and Spear, 1978). Basedin part on the structure determination of Smith HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 29

(1968),Holdaway et al. (1986b)suggested that 0.25 Fe3" ions be assignedto staurolite Al octahedral sites, 0.25 =2 (Mn + Fe'z*)be assignedto U sites, and the remaining o 4.71t0./t8 Fe, Mg, Zn,Li, Ti, and vacanciesbe assignedto the four o 6 tetrahedralsites, allowing for the determination of an ap- 6 o6 1 , proximate activity model. Becausethe presentstaurolites grew under more reducing conditions than the average staurolites used for HrO analysesand structure determi- nations, it is reasonableto assumethat, for the present staurolite,about 0.15 rather than 0.25 Fe3t replaceoc- tahedral Al and the remaining Fe is Fe'?*.This value is approximately equal to the lowest Fe3*determinations of 0,6{x,x.o.ffi Juurinen(1956). Values of Fe/(Fe + Mg) for staurolitesrange from 0.81 to 0.89. Staurolites show no zoning and no consistent pattern of compositional changewith grade.If the Li-rich specimen is discarded, the other four staurolites from L 5 grade 5 range from 0.849 to 0.853, averaging0.851 + o 0.002, suggestingthat the staurolite breakdown approxi- 6 o.76h0.69 matesa univariant reaction. As shown in Figure 4, the staurolite-biotite Ko is ap- parently unaffected by grade whereas the average stau- rolite-garnet Ko appearsto increaseslightly from grade 4 to grade 5. Specimen 6 exhibits anomalous Ko values, i.e., the staurolite Fe/(Fe + Mg) is about 7 molo/omore Fe-rich than would be indicated by the biotite and garnet compositions in the specimen. Apparently the effect of high Li on staurolite is to impose a positive deviation from ideality on the Fe-Mg solid solution or increasepos- StauroliteFelMg (1986b) sible Fe-Mg immiscibility. Holdaway et al. sug- Fig. 4. Ko Fe-Mgplots for staurolite-biotiteand staurolite- gestedthat for a given P and T, there are limits on the garnet(using peak metamorphic zone of garnet).Dots represent amount of smaller ions (Zn, Mg, Li, and Ti) that the grade4 and circlesrepresent grade 5. Specimen6, Li-rich stau- tetrahedral Fe sites can hold. An increasein Li must de- rolite,is identified,and plots at garnetFe/Mg : 4.07.Lines give creasethe maximum possibleamount of Mg, other things averageKo for staurolite-biotiteand averageK" by grade(des- being equal. This phenomenon is also shown by increas- ignatedas 4 or 5) for staurolite-gamet,excluding specimen 6. ing Li with increasingFe/Mg in staurolite (Dutrow et al., AverageKo and standarddeviation are given. 1986,Fig. 4). In both the biotite and garnet Ko plots there is a ten- dency for Mg-rich staurolite to exhibit a slightly higher in specimen94 where chlorite discontinuously surrounds KDthan more Fe-rich staurolite.As Mg content increases, and partly replacesstaurolite. Simple stoichiometry in- the Fe/(Fe + Mg) of staurolite is slightly higher than would volving Si, Al, Mg, Fe, and minor Mn characterizesthe be predicted from ideal solid solution, indicating positive chlorites. The numbers of Al and Si ions are very nearly deviation from ideality in staurolite (assuming that the constant for Al-saturated rock compositions and resem- biotite and garnet Fe-Mg solid solutions are closeto ide- ble thosereported by Novak and Holdaway(1981), Gui- al). If some of the Fe is subtracted from staurolite Fe dotti (1974),and Dutrow (1985)for chloritesofgraphite- values to account for the minor U-site and octahedral bearing pelites from Maine. The present chlorites and Al-site occupancy as discussedabove, the absolute Ko those of Novak and Holdaway (198 l), using a different values are changed somewhat, but the relationships re- microprobeand standards,show 0.086 + 0.041 higher main the same.Zn and Li contents of most of the stau- "iAl than toAl and slightly low )(vi). To a lesserdeg,ree, rolites are low enough to have no measurableeffect on the same tendencyis seenin the data of Guidotti (1974). KD. These observations cannot be explained by Fe3* substi- tuting for "iAl or by intercalated biotite, talc, or wonesite Chlorite layers (Veblen, 1983) becauseall theseeffects tend to in- Chlorite is an important mineral in garnet-grade(3) creaseoctahedral R3* or increase Si, which would also rocks, but forms only <2o/oof 8 of 11 analyzedstaurolite- increaseapparent octahedralAl. erade (4) rocks. Only well-crystallized chlorite, not seen Three possibleexplanations for these observationsare replacing biotite, garnet, or staurolite, was chosen for suggested.(l) They may result from a calibration error in analysis(App. Table l; Table 4). One exception to this is microprobe analyses of about lolo relative for SiO, or JU HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE

AlrO.. Although correcting for such an error would bring *Al o2 "iAl and into agreement,it would lower )(vi) still more. (2) The total number of anions per formula unit L O.ft9!o.Oln I may not be stoichiometric. Specifically,the chlorite may o contain a few percent ofintercalated brucite layers (Veb- 1 len, 1983,p. 5'74).To allow for extra brucite layers,the oxygenbasis needs to be increasedslightly from 28, which would also increase>(vi). (3) The chlorite may contain very limited dioctahedral substitution (Eggleston and Bailey, 1967;Tompkins et al., 1984).In the absenceof rnu work it is impossible to distinguish between these last two possibilities. The main compositional variation in chlorite is in Fel Mg ratio. Garnet-chlorite K, varies with grade, whereas biotite-chlorite Ko appears to be constant (Fig. 5). The 0.122!O.Ot1 recently calibrated garnet-chlorite geothermometer > ru (Dickenson and Hewitt, 1986) gives temperatures that o L averageabout 5 "C below those of the Ferry and Spear ( I 978) garnet-biotite geothermometer(Table 5), suggest- 3g ing that garnet-chloriteK" gives a reasonableestimate of prograde Z in these rocks. The AFM univariant assemblagestaurolite-garnet-biotite- chlorite-muscovite-quartzoccurred over a I interval of about 'C. 50 In rockssuch as these,in which P, Xr,o, and/o, were nearlyconstant, one might expectsignificant variations in non- AFM componentsin one or more mineralsto allow for this assemblageto existover such a 1'range.Such variations are not seen,and the variationsthat do occurare random in natureor 1 15 2 easilyexplained by otherreactions (see App. Table1). A number Chlorite FelMg ofargumentsbased on the presentwork and observationselse- wheresuggest that manyof the chloritesin the rocksof grade4 Fig. 5. Ko Fe-Mg plots for chlorite-biotite and chlorite-garnet areretrograde and result from minor reactionofbiotite andoth- (usingpeak metamorphic zone ofgarnet). Circlesrepresent grade er mineralsto chlorite:(1) Biotitecoexisting with stauroliteand 3 and dots representgrade 4. Lines representaverage Ko. Average progradechlorite should have a restrictedrange ofFe/(Fe + Mg) K" and standarddeviation are given. that shouldbe lowerthan that ratio in rockscontaining stauro-

TneLe5. Comparisonof garnet-chloriteand garnet-biotitetemperatures; calculated equilibriumconstants for Reaction1 Temperature("C) Speci- Grade men ct ln ,C 3 2 461 467 470 477 0.366 3 vb 442 443 461 460 0.214 4 52 455 470 480 494 0.503 4 3 4U 499 484 499 0.526 4 61 500 498 495 494 0.433 4 84 484 481 484 481 0.252 4 129 494 497 483 486 0.378 4 114 520 513 507 501 0.469 4 94 537 544 537 544 0.378 4 27 527 534 541 546 0.302 Average 490 + 32 495 + 31 494 + 27 498 + 27 0.382+ 0.104 Note. Sequenceis that of increasingfgiven in AppendixTable 1, based on Table 8. Abbreviations: C : core; | : intermediatezone. . Approximate equilibriumconstant; values of In ,C for Reaction 1 using activity models given in text. .. Temperature based on garnet-biotitecompositions and Dickensonand Hewitt (1986) geother- mometer modifiedby M. P. Dickenson(pers. comm., 1987). t Temperaturebased on garnet-biotitegeothermometry (Table 8). Ferry and Spear (1978) calibra- tion was used because the garnet-chloritegeothermometer was calibrated with Ferry and Spear (1978). HOLDAWAY ET AL.: METAMORPHISM IN WEST.CENTRAL MAINE 3l lite-biotite without chlorite (e.g.,see AFM diagram of Guidotti, with 7 (Fig. 5). The composition of retrogradechlorite is con- 1974). ln grade 4 biotites, Fe/(Fe + Mg) ranges from 0.47 to trolled by the dominant biotite in the rock and the chlorite- 0.69 in chlorite-bearingrocks, and from 0.47 to 0.57 in rocks biotite KD. In M, (Dickerson, 1984),retrograde chlorite is wide- without chlorite. Small differencesin X"ro of the fluid phase spread, and almost all chlorite is clearly retrograde.Here the could account for some variability but are unlikely to produce chlorite-biotiteKo is 0.858 + 0.040 comparedwith 0.869 t the observed degreeof randomness during prograde metamor- 0.022 for the presentrocks. The closecompositional relationship phism. (2) If chlorite and garnetwere an equilibrium pair in the between chlorite and biotite is maintained despite the wide- present rocks, then the Fe end-member equilibrium (Reaction l) spreadretrogression in Mr.

0.250Fe0'Al,o?(Alr43sir57)O10(OH)8+0.0l0KerAI,rrFeo,r(AlonrSiro3)Oro(OH),+0.458SiO, chlorite Muscovite Quartz

: 0. 367FerAl.Si3O,, + 0.0 I 1Ko rrFer rrAlo o7(Alr3 r Si2 6e)O ro(OH)r + HrO (1) Almandine Biotite Fluid

would apply to all rocks of grades3 and 4 that contain chlorite. Based on these observations,we tentatively conclude Approximate mineral equilibrium constants(Kx) were calculat- that much of the chlorite in grade 4 grew from biotite ed to take into account dilution of Fe and K by other compo- and other minerals during cooling or M5 reheatingof the 'C nents, assumingthat Al varies little from the averageamounts rocks at temperatures in the range of 450-500 and (Tables 1-4). Activity models basedon mineral compositions in perhapsat pressureshigher than those of Mr. Equilibrium Reaction I are as follows: retrograde domain assemblagesmust have been garnet- chlorite-staurolite-biotite, garnet-chlorite-staurolite,gar- : Chlorite ao".n, [Fel(Fe + Mg + Mn)]+st net-chlorite-biotite, and staurolite-chlorite-biotite (all as- : Muscovite a,," [K./(K + Na)]0'g?[Fel(Fe+ Mg)lo'' semblagescontaining muscovite * quartz). This conclu- : Almandine a",- (XFJ)' sion suggestsa re-evaluation of the first sillimanite- : Biotite a""", [K./(K * Na)]o'at>< forming reaction proposed by Guidotti (1974) for the lFe/(Fe+ Mg + Mn + 2Ti;1:s:. Rangeley area on the western side of the present study area and by Holdaway et al. (1982) for the present area. The calculated values of Kx are given in Table 5 in order of Instead of staurolite-chlorite-muscovitereacting to silli- increasing7 (basedon Table 8). The relation betweenIC and T manite-biotite in normal M, rocks, we suggestthat stau- for an equilibrium reaction (Kerrick, 1974) is rolite-muscovitereacted to sillimanite-biotite-garnet.The AI: R[ln(X*a1,o)AS;'. (2) chlorite-staurolite tie line in AFM projection must break immediately after staurolite forms, allowing garnet-chlo- Thus as 7 increasesand AZ rises relative to the equilibrium Z rite-staurolite-biotite to exist over a limited I range,con- calculatedfor the end-member reaction (e.g.,Fig. 3), ln K* must sistentwith the high AS involved in chlorite dehydration. increaseifequilibrium prevails. It is clear that prograde 1"is not (4) M, of Maine occupynarrow the controlling factor for -Kx. Variation in Kx can result from The staurolite-grade rocks changesin the HrO activity as shown by Equation 2, but in compositional and pressureranges such that neither alu- reducedgraphitic rocks, such variation is not expectedto exceed minum silicate3 nor chlorite occur as equilibrium pro- | 5o/o(e.g.,see Table I 2), leading to a maximum variation of 0. 16 grademinerals. At lower P during Mr, staurolitewas more in ln K*. With the possibleexception of the first four specimens Fe-rich and occurredwith andalusiteover a rangeofgrade listed in Table 5, Kx for Reaction I does not support chlorite as as indicated by field relations (Dickerson and Holdaway, an equilibrium progrademineral in the staurolite zone.(3) Chlo- ms.).At this lower P the common bulk compositionsin- rite occursas small amounts ofwell-crystallizedgrainsofvariable tersect the AFM fields of staurolite-biotite-garnet and mainly cross-cuttingthe foliation defined by muscovite and size staurolite-biotite-andalusite. some of the biotite. It commonly occurs near porphyroblastsof staurolite or biotite. (4) Aluminum silicate and Mg-bearing bio- tite are a common lower staurolile-zoneassemblage elsewhere, Ilmenite suggestingthat the alternative staurolite-chloriteassemblage be- The only significant components detected in ilmenite comesunstable early in the staurolite zone. Examplesare M, in were Fe, Ti, and Mn (App. Table l; Table 6). Ilmenite where reducing conditions, andalusite-biotite be- Maine under content varies from 89 to 99o/o,pyrophanite (MnTiO) comes stable immediately after staurolite (Dickerson and Hol- from I to 1.lo/o,and hematite from 0 to 4o/o.The only daway, ms.); the Picuris Mountains in New Mexico where an- pattern is a strong tendency for dalusite-biotiteand sillimanite-biotite occur next to normal lower obvious compositional staurolite-graderocks (Holdaway,1978); and the SnakeRange, the pyrophanite content to mirror spessartinecontent of Nevada, where kyanite-biotite occurs above normal staurolite- the garnet. The lower Mn in the ilmenite combined with graderocks in a sequenceofdownward-increasinggrade (Geving, much lower ilmenite content than garnet in the rocks I 987). indicates that only a few percent of the Mn in the rocks One important question remains to be answered:If much of is contained in ilmenite and most remaining Mn occurs the chlorite of grade4 is retrograde,why does it give reasonable in garnet. ?'valueswith the garnet-chloritegeothermometer (Table 5)?The answer apparently lies in the constancy of chlorite-biotite KD 3 But seeNote added in proof. 5Z HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRALMAINE

Tneue6. Summaryof averageilmenite composition by grade TABLE7. Semiquantitativeion-microprobe analyses of two ilmenites from o.c grade5 ilm 0 s4(5) 0.s7(21 0.e6(2) 0.e4(4) 0.95(1) pyr 0.06(5) 0.03(2) 0.04(2) 0.0s(3) 0.04(2) Element Sample14 Sample 6 nem 0.00(0) 0.01(1 0.00(0) 0.01(1) 0.01(1) ) ?Li 5.4 167 eBe /Vote.' Abbreviations (Kretz, 1983): ilm-ilmenite; pyr-pyrophanite 1.0 0.2 (MnTiO.); hem-hematite "E 1.7 0.6 23Na 21 52 ,nMg 281 195 27Al 554 282 Carefulinspection ofpolished sectionsofgrades 6.5 to 28Si 804 466 3eK 165 170 8 shows that most of the opaque grains are graphite. In @Ca 34 several specimens,no ilmenite was detected;and in oth- 6Sc JU 55 ers,only traceamounts were seen(see App. Table l). This 46Ti [31.58%] t31.58%l 100 85 scarcity of ilmenite is not related to Fe/(Fe + Mg) of 52Cr 18 20 biotite, nor is rutile presentin any of the specimens.Pre- sFe 36.23./" 37.83'/" ssMn sumably, low ilmenite content results from increasedTi 3.15% 3.67% 68Sr 1 10 solubility in biotite at high grades.The most hematite- s3Nb 227 61 rich ilmenite (4o/oin specimen 63) coexists with a trace Note.' Analyses were normalized to Ti : 31.58%. of graphite. Although graphite is difrcult to recognizebe- Analyses are upper limits because background mea- causeof fine grain size in some lower-grade specimens, surements were not made. True values are probably presence between 0.5 and 2 times the recordedvalues because its obvious in all high-grade specimenscom- of matrix-dependenteffects. Unless specifiedas per- bined with the low hematite content of all the ilmenite cent, values are given as parts per million(by weight). implies that graphite is presentin all specimens. On the basisof 2 (Fe,Ti,Mn,Mg) cations, five ilmenites contain more than l.0l Ti atoms. The theoretical limit of Ti ions in ilmenite is 1.00 since the larger octahedral mens show no more than trace amounts of Mg, Ti, and sitespresumably exclude Ti. Ti ions rangefrom 1.013to Mn, and 0.19 to 0.35 wto/oFerOr. The averagecontent of 1.028,and oxide totalsrange from 97.36 to 98.54010.The FerO, is 0.27o/ocorresponding to 0.005 Fe atoms per for- remaining ilmenites have oxide totals (correctedfor FerO, mula. This amount of Fe is of no consequencein calcu- on basis of hematite content) ranging from 98.49 to lating reaction boundaries from experimental equilibria. 100.91o/o.These results suggestthat some element may not have been analyzed in the anomalous specimens. CoNnrrroxs oF MToTAMoRPHISM Specimens6 and 14, the two most extreme cases,were Metamorphic conditions were deduced on the basis of examined semiquantitatively with the ion microprobe. (l) estimates of metamorphic temperatures by garnet- The results (Table 7) indicate that no other ion is present biotite geothermometry, (2) metamorphic pressureesti- in significant amounts. The high Li of the sraurolite in mates by comparison of the andalusite-sillimanite phase specimen 6 is reflected by 167 ppmw of Li in the coex- diagram (Holdaway, l97l) with garnet-biotite geother- isting ilmenite. A microprobe wavelength scan of ilmen- mometry (grades 5 and 6 of Mr), and (3) estimates of ite in specimen 6 revealed only Ti, Fe, and Mn. There- pressurefor grades6.5 and 7 of M, basedon calibration fore, high stoichiometric content of Ti is probably an of the muscovite-almandine-biotite-sillimanite (MABS) artifact of the analysis procedure (small grain size, con- geobarometerusing the results of (2) above. Finally, P duction, thicknessofcarbon coat, etc.). and estimatesfor grades 5 and 7 were compared with dehydration equilibria in order to test for accuracy and Other analyzedminerals evaluate fluid-composition models. Plagioclaseis commonly observedin rocks of grades5 and higher (see App. Table l) and is present in small Geothermometry amounts at lower grades.It is Anrn in one specimen of The only reasonably well calibrated geothermometer grade5, An,r-r, in grade6, An,u-r,in grade6.5, and An,r_r, applicable to medium- and high-grade pelitic metamor- in grades7 and 8. The two most calcic compositions,Ano, phic rocks is that based on the garnet-biotite exchange and Anr., suggestpartial removal of albite component reaction. This geothermometeris ideal for the M, and M, throughreaction with muscovite(Tracy, 1978). Maine rocks becausethe biotite/garnet ratio is high and Orthoclasein grades7 and 8 varies from Orro to Orno evidencefor extensiveretrogressive effects is virtually ab- (see App. Table l). These feldspars are comparable in sent. Table 8 provides garnet-biotite temperatures for composition to those of the southwest part of the study garnetcore or intermediate zonesand surrounding biotite area analyzedby Guidotti et al. (1973), e.g., Or* to Orno grains not in physical contact with the garnet, using four in grade 7 schists.Celsian content ofalkali feldsparsav- recent quantitative calibrations: (l) Ferry and Spear eragesabout I molo/0. (1978), (2) Hodges and Spear(1982), (3) Ganguly and Most sillimanite coarse enough for analysis is from Saxena(1984, 1985), and (4) Gangulyand Saxena(1984, grades 7 and 8. Analyses of sillimanite from six speci- 1985)with AlZ,. reducedfrom 3000 + 500 to 2500 cal. HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE JJ

Tnele 8. Garnet-biotitetemperatures ('C)

Ferry-Spear Hodges-Spear Ganguly-Saxena Ganguly-Saxena' No. c Peak f Grade 3 (M3) 2 467 477 481 490 535 531 506 506 506 YO 443 460 462 479 515 525 494 507 507 Avg. 455(18) 469(12) 472(131 48s(8) 525(14) 528(4) 500(8) 507(1) 507(1) Grade 4 (M3) 1A 431 492 465 521 503 522 479 508 508 az 470 494 488 509 513 519 498 508 508 3A 499 499 513 513 523 523 508 508 508 61A 498 494 514 510 534 527 515 510 515 84-- 481 481 493 493 540 540 521 521 521 129* 497 486 518 509 534 515 525 509 525 114 513 501 537 524 531 520 528 517 528 5A 526 531 553 cc/ ccv 560 543 544 544 A6** 502 518 528 545 546 554 534 545 545 94 544 544 564 564 568 568 5J/ 55/ CC/ 27 534 546 544 555 583 589 J/b 5U 584 Avg. 500(32) 508(23) 520(30) 527(24) 539(24) 540(2s) 526(27) 528(26) 531(24) Grade 5 (M3) 137 543 514 557 526 552 s30 545 522 545 53 556 540 570 554 568 ccc 559 546 559 14A 537 507 543 516 576 552 560 537 560 o c/c 584 603 611 600 605 585 591 591 23 598 572 614 585 616 594 605 583 60s Avg. 562(25) 543(34) 577(30) s58(40) s82(261 567(31) 5711241 5s6(30) s72(2slI Grade 6 (M3) Tstf 487 494 506 514 538 542 526 532 532 4A 541 553 574 582 586 586 566 568 568 47 c/J 548 596 568 595 572 578 556 578 19 c/c 551 58s 562 606 579 589 566 589 112 576 530 589 538 614 577 595 557 595 50 595 578 606 586 619 599 607 589 607 63 587 cc/ 606 576 626 600 610 585 610 30 595 559 615 578 639 606 616 584 616 8 631 611 654 635 646 625 628 609 628 140 625 588 637 600 652 615 632 598 632 Avg. 589(27) 564(241 607(25) 581(27) 620(23) 59s(18) 602(22) s7e(18) 603(22) Grade 6.5 (Ms) 77-3 624 598 635 609 634 611 619 597 619 77-2 636 606 M7 617 646 620 630 605 630 90 632 623 640 631 657 645 637 626 637 to 636 605 647 616 660 630 641 613 641 co 636 593 652 609 657 620 641 606 641 ol 656 627 ooo 637 689 661 669 643 669 Avg. 637(11 ) 609(14) 648(11 ) 620(12) 6s7(18) 631(1 e) 640(17) 61s(17) 640(17) Grade7 (M5) 73 630 601 643 614 660 637 a0 616 640 87 665 629 690 654 666 640 657 631 657 143 675 621 686 633 668 631 658 620 658 145 666 ooo 675 675 674 674 663 663 663 96 712 656 718 662 682 u4 675 636 675 Avg. 670(29) 635(2s) 682(271 648(241 670(8) 64s(17) 659(13) 633(19) 659(13)+ Grade8 (Ms) 11 779 716 805 741 726 682 719 675 719++ Avg.$ 25.8 23.8 25.2 24.8 20.7 21.8 21.7 22.O Avg.$$ 22.5 23.2 20.3 20.8 Note.'References:Ferry and Spear (1978); Hodges and Spear (1982); Ganguly and Saxena (1984, 1985).A nominal Pof 3500 bars was assumed. "4" with sample number indicatespresence of M, andalusite.Values in parenthesesare one stanclarddeviation. - Gangulyand Saxena with AtyM"reduced from 3000 + 500 to 2500 cal. Peak f italicizedand given in last column. -'These three specimensare probably Mr. qC, t lncfusionof three specimensstudied by Dutrow(1985) (237, 580'C; 330, 5929;267,613 rc) increasesthis valueto 581(24) presumablya more accurate value because rof specimen137 might be retrograde. ft Specimenhas textural indicationsof retrogression;excluded from all averages and from geobarometry. + This value may be reduced by a maximum of 7'C to compensatefor change in biotite compositionduring early cooling (see t€xt). f+ May be reduced by a maximum of 20 rc (see text). $ Weighted average of standard deviations,grades 4-7- $$ Weighted average of peak temperaturestandard deviations,grades 4-7. 34 HOLDAWAY ET AL.: METAMORPHISM IN WEST.CENTRAL MAINE

Most calibrations after 1978 use Ferry-Spear1(" data and Once K-feldspar had formed in grades7 and 8, garnet empirically correct for nonideality, principally from Ca may have reacted with K-feldspar during retrograde and Mn in garnet.A nominal pressureof 3.5 kbar was metamorphism (Fig. 3) to make biotite more Fe-rich than usedfor the calculations. that at equilibrium with the garnet cores.Thus the garnet The quality of these calibrations as they relate to the core temperaturesin grades7 and 8 may be higher than Maine rocks may be evaluated by comparing the tem- equilibrium values, whereasthe intermediate-zonetem- peratureswith temperature estimatesbased on dehydra- peraturesare more accuratebut are retrogradein nature. tion equilibria (discussedbelow) and by comparing the An estimate of the maximum extent of this efect may be scatterof the data and the overlap betweengrades. Com- made by comparing the averagedifference between core parison of the calibrations centersmainly on grades5 and and intermediate-zone temperatures for grades 6.5 and 'C 7 . Each of thesezones may representa relatively naffow 7. In grade6.5, LT is 25 oC,and in grade7 it is 32 (if Z range (assuminga limited range of XHrq)because each the unzoned specimen 145 is excluded). Thus, the grade involves a univariant reaction in the model KFMASH 7 rocks from the same region as grade 6.5 rocks have a system.For grade 5 (staurolite breakdown), the modified maximum Z effect of about 7 oC becauseof the change Ganguly-Saxenacalibration involves the smallest stan- in biotite composition. For the single grade 8 specimen, dard deviation, and the Hodges-Spearinvolves the larg- the maximum effect is 20 "C. est (see also Dutrow, 1985). For grade 7 (muscovite Peaktemperatures from grade 5 (staurolite breakdown) breakdown),the original Ganguly-Saxenacalibration ap- average572 + 25 "C. However, the temperaturefor spec- pears best, and the Ferry-Spearappears worst. If all the imen 137 (545 'C) may be retrograde.Addition of three standard deviations ofgrades 4-7 are averaged,the orig- specimens from the Farmington quadrangle [Dutrow, inal Ganguly-Saxenais slightly lower than the modified 1985;Table 8, grade5 (Mr) averagelincreases the average Ganguly-Saxena(Table 8). This may be due to the fact to 581 + 24 "C, which may be a better averagefor grade5. that the Ganguly-Saxenacorrections are partly basedon Approximatetemperatures are grade 3, < 510; gfade4, best-fit calculations of data similar to ours. 510*560;grade 5, 560-595;grade 6, 595-625;grade 6'5, A partial test of the accuracy of the geothermometric 625-645; grade7, 645-670;grade 8, >670 "C (Table 8). methods is provided by the two garnet-zonespecimens. The largestdeviation of any specimenfrom these ranges Stability considerations for staurolite-almandine (e.9., is 25 'C which is an approximate indication of (2o) pre- Ganguly, 1969) and fluid-composition considerations(e.g., cision. This is in rough agreementwith analysesof two Holdaway, 1978) suggestthat in graphite-bearingrocks different garnet grains in each of three samples (5, 19, the stauroliteisograd should occur at about 520'C at 3- and 56) that averagea 19'C differenceand with analyses 4-kbar pressure.The garnet-zonespecimens come from of two specimensfrom 4 m apart in the same outcrop field positions very near the staurolite isograd (chloritoid ('77-3,77-2,Table 8) that differ by I I "C. A further test does not occur in M.). The high Mn content (up to 28 of the accuracyof these temperaturesis given in a sub- molo/o)of garnet-zonegarnets causes the various calibra- sequentsection. It is important to note that grades6 and tions to spread out over a wide Z range. The modified 6.5, which have the samemineral assemblages,show al- Ganguly-Saxenacalibration appears to provide the best most no overlap in calculatedtemperatures. In our opin- temperaturesfor this zone, while the original Ganguly- ion, this is becausethey crystallizedduring different events Saxenaprovides slightly higher temperaturesthat overlap at different pressuresas discussedbelow. with several of the staurolite-zone temperatures. It ap- pears that the Ganguly-Saxenacalibration wilh AW*,: Geobarometry 3000 cal slightly overcompensatesfor nonideality of spes- Three geobarometershave possible value in medium- sartine component. to high-grade pelitic metamorphic rocks: the phengite Temperaturesthat estimate peak metamorphic condi- geobarometer(e.g., Powell and Evans, 1983);the garnet- tions are listed in the last column of Table 8. In grades3 aluminum silicate-plagioclase(GASP) geobarometer, and 4, much ofthe zoning is ofa progradenature, where- most recentlycalibrated by Newton and Haselton(1981) as in grades5 through 8, diffusion zoning and retrograde and improved by the garnet mixing models of Ganguly growth zoning are prominent as indicated by intermedi- and Saxena(198a); and the muscovite-almandine-bio- ate-zonetemperatures that are lower than core tempera- tite-sillimanite geobarometer(MABS) approximately cal- tures.4In grades5 through 8, the intermediate-zonetem- ibrated by Spearand Selverstone(1983) and Robinson peraturesare more accurate than the core temperatures (1933). There are presently large uncertaintiesin the becausethe adjacent biotite was equilibrated with inter- phengite geobarometer,especially for low-pressurerocks mediate-zone compositions last. However, temperature where the phengite content of muscovite is small and estimatesrecord an event during the cooling cycleat which highly dependenton the method of calculatingend mem- diffusion betweengarnet and biotite was frozen in. bers and activity models (seemineral-chemistry section). Also the method requires the presenceof primary chlo- 4 was used for Note that although final biotite composition rite, and many of these rocks contain only chlorite inter- and intermediate-zonegarnet-biotite temperatures,the ?" core geobarometer efect on biotite composition is quantitatively unimportant as preted to be retrograde.The GASP suffers shown in footnote 2. from the low grossular contents of garnets and variable HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 35 zoning of plagioclasecompositions in these low-pressure l', aoo: l. Fe was assumedto be disorderedon the M(l) rocks. Although in need offurther calibration and testing, and M(2) octahedral sites of biotite. Activity coeftcients the MABS geobarometermay becomethe best barometer were calculatedfor almandine component following Gan- for pelitic rocks becauseall the components involved in gulyand Saxena(1984) with AW-^:2500 cal.The value the reaction are major componentsin the minerals. of (rFJ),varies from 0.90 to l.l l. Activity coefficientsfor Pressureof zone 5. The first appearanceof sillimanite K in muscovitewere evaluated assuming that pyrophyllite in M, is by reaction of staurolite with quartz in grade 5. component behaved like muscovite component and ,yf" Garnet-biotite temperatures for this zone indicate that was based on paragonitecomponent only. The equation sillimanite beganto form at -560 "C (Table 8). M. is of Chatterjeeand Flux (1986) for muscovite was used. defined as metamorphism in which the aluminum silicate Biotite was assumedto be ideal. This is partly becauseof produced as a staurolite breakdown product was silli- the absenceof data and partly becausethe amounts of manite. The aluminum silicate diagram of Holdaway most of the other componentswere small enoughto have (1971)shows that this must have occurredat a pressure negligible effectson the major components.The possible above -3.0 kbar. BecauseM. immediatelyfollowed Mr, nonideality of Al in biotites tends to cancel out because and locally the transition from M, to M, is nearly contin- the calibration was done on Al-rich biotites. Tetrahedral uous (Novak and Holdaway, l98l), it is reasonableto siteswere assumedto be coupled to octahedralor l2-fold assumethat averageM, pressurewas only slightly above srtes. this value. There is also an andalusite locality near Far- The value of AP corrected for observed mineral com- mington (Dutrow, 1985)that cannotbe explainedas ear- positions is given by lier M., but may well representa slight northward P de- LP: -AV tRZIn Kx, (4) creaseduring Mr. If the averageM, pressurewas 3.5 kbar, as determined by Ferry (1976) and generallyaccepted by whereAIlis given in Table 9. A plot of M againstZwas other workers in the area, garnet-biotite temperatures used to determine the end-member P-T curve for Reac- show that some relict M, andalusites would have oc- tion 3. The resultingequation is curred in the sillimanite P-Z field, without nucleation of P: l5(r * 968)+ AP, (5) sillimanite. For the purposeof calibrating the MABS geo- barometerwe haveassumed an averageP for M, of 3.1 + where P is in bars and ?"is in kelvins. 0.25 kbar. Although the 3.l-kbar P estimateis adopted Calculatedpressures for the rocks from grades5 and 6 as the averagepressure for grades5 and 6 and is reason- (MJ from Dutrow ( I 985) and the presentstudy are shown able for grades 3 and 4 (which show close spatial asso- in Figure 6, whereasall calculatedpressures from the pres- ciation with grades 5 and 6), one cannot assume that ent study are given in Table 10. The approximate con- pressureremained constant in grades6.5 through 8. These stancy of pressurefor grades 5 and 6 indicates that the zones are exposedconsiderably south of most M3 rocks slope of about 15 bars/K is reasonable.Attempts were and sufferedlater M, effects(Fig. l). made to calculatethe slopefrom entropy and volume data MABS geobarometry.For grades 5 and 6, the present (Table 9). For Reaction 3, AS varies between -6.8 and analysesand thoseof Dutrow (1985)in the southern400/o +5.6 J/K dependingon the choice of entropy for alman- of the Farmington quadrangle provide a uniquf oppor- dine and sillimanite. This would allow a maximum slope tunity to calibrate the geobarometerinvolving the assem- of +3.6 bars/K for the reaction, suggestingthat the ther- blagemuscovite-almandine-biotite-sillimanite (Fig. 3) that modynamic data are lessprecise than the presentcalibra- occurs in all rocks from grades 5 to 8. The ideal end- tion. If we assumethat the best values for sillimanite and member reaction is almandine entropy are those of Robie and Hemingway (1984) and Chatillon-Colinetet al. (1983),respectively, *Lo.lfl"'.9*(oH)'+ t'l*l1l1f '' and that the muscovite entropy is accurate,then the annite entropy would need to be about 29 J/K higher than the : **{"1*.o,0(oH),* * (3)value given by Helgesonet al. (1978)in order to be con- 1tlf"lR,t'9;,' sistentwith the l5 bars/K slope. Severalarguments suggestthat much of the scatter in All analyzedM, specimensfrom grades5 and 6 were used Figure 6 (3.09 + 0.38 kbar) results from analytical error, with the exceptionof one retrogradedspecimen each from temperatureerror, and imperfect equilibration rather than the presentanalyses and from the Dutrow (l 985) analyses. real variation in P: (l) For grades 5 and 6, the Dutrow All specimenswere assumed to have crystallized at an (1985)specimens show as much scatterin P as the spec- averageP of 3. I kbar and peak temperatureas defined by imens from this report collectedover a much wider area. the modified Ganguly and Saxena(1984, 1985)garnet- (2) The distribution of P values for M, is neaily random biotite geothermometer(Table 8). on the map as shown in Figure l. Considering these ob- The equilibrium constantlKx was evaluated using the servations and the smaller spread of P values for grade following activity models to adjust actual mineral com- 6.5 (t0.19), we estimatethe (2o)error for the method at positions to ideal end-member compositionst ex^ : 0.5 kbar. (XFI)'(rfI)'; a,, : (XXi)'XH"XX'?X';a^"" : ffi!)3Xft,; a.,,: Specimensfrom grade6.5 (M5)indicate consistentpres- 36 HOLDAWAY ET AL.: METAMORPHISM IN WEST.CENTRAL MAINE

o 5

o5 5

o J Tt. ni t4 .6 QA 4o 6 o 6 t6 o 6 L 6A o .t 6 6o o \6 5A >,3 500 550 600 650 700 Temperature('C) Fig.6. P-T plot for M, metamorphicgrades 4,5, and 6 using presentdata (dots)and those of Dutrow (1985) for southern Farmington quadrangle(circles). "A" indicates M, andalusitepresent. Temperature is that of the modified Ganguly-Saxena(198a) garnet-biotitegeothermometer (Table 8), and pressureis basedon the MABS geobarometer(Table l0). Sillimanite-absentrocks are plotted at 3.1 kbar, the averageP for M, (seetext). Only two specimens(a 6,{ and a 5A containing M, andalusiteand M, sillimanite) plot incorrectly in the andalusitefield. Much ofthe scatterfrom the average3.1 kbar representsanalytical error, temperatureerror, and incomplete equilibration. Temperature error (2o) is +25 'C. suresof3.8l + 0.19kbar (Table10, Figs. l, 7), or about 700 bars above the prevailing P during Mr. This is con- Dehydration equilibria sistent with the recent discovery of kyanite in Mr, 35 km Univariant dehydration reactions in the model southwest of Lewiston (Fig. l) on the south side of the KFMASH systeminvolving breakdown of staurolite and Sebagobatholith (Thomson and Guidotti, 1986;Hussey muscovite were applied to grades 5 and 7, respectively. et al., 1986).For grades7 and 8, from the sameM, area Two alternative approachesare (l) to assume that the asgrade 6.5, pressures from all but the lowest-Ispecimen determined P and Z values are correct and solve for Xtro of grade 7 are higher and rather variable (Table 10, in in the fluid, or (2) to assumevalues for &ro consistent brackets). Possibly, the presenceof K-feldspar, imply- with presenceof graphite in reducing rocks and test for ing partial reaction of biotite and sillimanite to garnet, consistencybetween P, T, and X"ro. ln our opinion, X"ro K-feldspar, and fluid (Fig. 3), disturbs the equilibration in graphitic pelites is better constrainedthan P or T, and ofgarnet and muscovite with biotite and sillimanite. Con- thus we have chosenthe secondapproach. sequently,the geobarometershould be used with caution HrO content of fluid. As shown by Ohmoto and Ker- in K-feldspar-bearing rocks. An alternative explanation t''ck (1977), the mole fraction of HrO in the fluid of is that the M, rocks share a complicated metamorphic graphite-bearingrocks is a slowly varying function of P history, and the grade 6.5 rocks have most completely and T and a rapidly varying function of the relative equilibrated to a single (presumablythe latest) metamor- amounts of CO, and CHo in the fluid (or, alternatively, phic event. of the6r). We have calculatedvalues of 4H2oat the equil-

TaaLe9. AS and AVfor Reactions3. 7. and I

Reaction Irc) P (kbar) aS, (J/K) ay, (J/bar) References,notes' (3) Alm + Mus 610 -6.84 2.082 a,b,d 610 -3.32 a,D 610 2.11 a,c,o 610 5.63 (7) Mus + Qtz 660 3.8 75.77 a,f,i (8) St + Qtz 660 4 125 122.49 3.550 a,b,d,e,f,g,h 660 4.125 109.09 a,c,d,f,g,h 610 3.1 128.64 a,b,d,f,g,h * a : Helgesonet al. (1978) (S, y, and Cp),except as noted. b : Metz et al- (1983) (Alm S); Helgeson et al. (1978)(Alm Cp).c : Chatillon-Colinetet al. (1983)(Alm S); Helgesonet al. (1978)(Alm CJ. d : Robie and Hemingway(1984) (Sil S, Cr. e : Richardson(1966) (St 14.f : Burnham et al. (1969) 1n,O % s). g : Hemingwayand Robie(1984) (St S, C,). h : See appendixfor calculationof chemical and disorder effect on staurolite. Staurolite formula given in Reaction 8. i : Helgesonet al. (1978) data used tor internalconsistencv. HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 31

TABLE10. Pressuredeterminations based on the almandine- muscovite geobarometer

rPT-P-' No. fC) (kba4 No. ('C) (kbar) Grade 5 (M3) Grade 6.5 (Ms) 137 545 3.18 77-3 619 4.03 53 559 3.28 77-2 630 3.96 14A 560 2.24 90 637 3.52 6 591 3.41 76 641 3.77 23 605 3.37 56 641 3.67 Avg. s72(25) 3.12(50) 91 669 3.90 Avg. 640(17) 3.81(19) Grade 6 (M3) 4A 568 2.77 Grade 7 (M5) E 47 578 2.83 73 640 3.80 19 589 2.80 87 657 14.7'tl o 112 595 3.19 143 658 [5.08] 50 607 3.59 145 663 t5.131 63 610 2.60 86 67s [s.93] 30 616 2.60 Avg. 659(13) 14.93(7711 I 628 3.31 140 632 3.33 Grade I (Ms) Avs. 603(22) 3.00(36) 11 699 [6.24] 600 . Peak garnet-biotite f (Table 8). ('C) '. Brackets indicate uncertainvalues (see text). Temperature Fig.1. Comparisonofaverage Zdetermined by garnet-biotite geothermometry$oxes) and average7 determinedfrom dehy- drationequilibria (curves) for Reactions7 and8 in grades7 (M') ibration Z (calculatedfrom garnet-biotiteequilibrium) for and5 (Mr),respectively. Numbers in parenthesesindicate models Reactions 7 and 8 for P-T conditions ofgrades 5 and 7 for reactionofwater with graphitediscussed in thetext. For grade on the basis of two models: (1) The pelitic rocks acted as 5, two possibilitiesare indicated: W" : 0 kJ per staurolitetet- closed systems,and graphite reacted with HrO of dehy- rahedralsite and Wc: 14.5kJ persite as discussed in text.Data dration to produce equimolar amounts of CO, and CHo: from Tables12 and 13. 2C + 2HrO: CO, + CHo. (6) (2) COr, being severaltimes the sizeof CHo,escaped from where Kx is the equilibrium constant based on solid so- the rocks more slowly and/or was added to the pelites lution of muscovite and K-feldspar, AS, is given in Table from the rare metacarbonaterocks of the area. Metacar- 9, and n is the number of moles of water in the reaction bonate rocks equivalent to grades5 and 6 at the eastern (Kerrick, 1974).At 3.8 kbar, the pressureassumed for all edgeof the presentarea contained fluids with Xrro > 0.78 M, in the area (Table l0), the pure phasesof Reaction 7 (Ferry, 1980).We assumea limiting caseof Xco2: l)X.ru are stableat 664"C (Chatterjeeand Johannes,1974).Fi- The majority of graphite-bearing pelites with low /o, nal results of calculations are given in Table 12 and il- should have Xnro values between these extremes. The lustrated in Figure 7. sourcesof data and results of these calculationsare sum- The agreementbetween garnet-biotite and muscovite- marized in Table ll. Other fluid species(Hr, HrS, CO, quartz temperaturesis excellent.The accuracyof calibra- etc.) are assumed to be quantitatively unimportant tion of each geothermometeris estimated to be + 15 oC, (Ohmoto and Kerrick, 1977). Muscovite-quartzreaction. In grade 7, the model uni- variant reaction *1f,l*y.!o"''* TABLE1 1. Molefractions for water in graphite-bearing 319";:YJll;9' oelites . . Q) Model 1 Model 2 *,lf:"o;H:3 rfC) P(kbar) 4"o dr"o 660 3.8 0.903 0.831 was in progressover a relatively narrow ?" range in all 620 3.1 0.902 0.831 the rocks. Using the activity model discussedabove for 580 3.1 0.919 0.860 muscovite, .Xlr" for K-feldspar, and activity coefficients /Vote:Calculated using data of Ohmoto and Kerrick (1977) calculatedfrom Chatterjeeand Flux (1986) for muscovite for thermodynamicdata of Reaction6, Burnhamand Wall (un- and Waldbaum and Thompson (1969) for K-feldspar, the pub. data) for CO, fugacity coefficients,Burnham et al. (1969) for H,O fugacity coetficients,Ryzhenko and Volkov (1971)for Z separationbetween the experimental curve and natural CH4 fugacity coefiicients,and Kerrick and Jacobs (1981) for occuffenceswas calculated.The relationship is H2O-CO,mixing relations.Because of its nonpolar nature and similar fugacity coefficientto COr, CH1 was assumed to mix AI: R?l.ln(Kxa6,o)AS;,, (2) with the same mixing activity coefficientas COr. 38 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE

TnaLe12. Temperatures('C) of grade 7 (MJ based on mus- Staurolite-quartz reaction. In grade 5 all specimens covite-quartz reaction and presence of graphite at aontain an assemblagerelating to the FASH model re- 3.8 kbar actron No. K Io_"** I-Model 1t l-Model 2t H.Fer.Al,rnrSiru.Ou,+ 4.l66SiO, : 7.527AlrSiOj 73 1.143 640 oo/ Staurolite Quartz Sillimanite 6t 1.166 oc/ ooY 661 143 1 108 658 664 o50 + l.433FerAl,Siro,,+ 1.5HrO. (8) 145 1.125 663 ooo 657 Almandine Fluid 86 1.178 o/c 670 662 Avg. 1.14(3) 6s9(13) 667(2) 6s9(3) The staurolite formula used above is an Fe end-member . le includesactivity coefficientslor muscoviteand KJeldspar. formula for graphite-bearing rocks based on the chemical ". Peak garnet-biotite f oable 8). t See text and Table 11 Calculatedusing Equation2, initial fof 664 rc study of Holdaway et al. (1986b). The H content is con- (Chatterieeand Johannes,1974), and Ag:75.77 UK (Table9). sistent with analyzed H values from the present study (Table 4). As shown in Appendix l, an activity model for staurolite can be formulated using the results of Holda- way er al. (1986b). suggestingthat the averagesof the two methods should Fe end-member staurolite breaksdown along a stability curve of 645 and 660 'C at 3.25 kbar and agreewithin about 2l 'C (1zT5t+ Tf;. Model | (X.o,: bracketedby temperatures 675 and 690'C (but very closeto 690 eC)at 5 kbar (Dutrow and X.") yields Z 8 'C higher and model 2 (Xcor: l0X."o) Holdaway, I 983; Dutrow, I 985; Dutrow and Holdaway,in prep.). yields garnet-biotite I equal to the average temperature This stability curve for Reaction 8 results from a careful sru for grade 7. The maximum separationbetween a garnet- study of reaction products and is consistentwith the earlier ex- biotite and the conesponding muscovite dehydration ?" perimentsof Richardson(1968) at moderatepressures. Calcu- based on precision alone (950/oconfidence level) should lated AS values for Reaction 8 (Table 9) were compared with be lessthan2r/TT +T or 27 "C. For model I the max- slope calculationsbased on the experimental results.The exper- imum diference between the methods is 27 "C, and for iments indicate an averageslope between 39 and 117 bars,/K, model2 this maximum is 19 "C (Table l2). This analysis whereas the Metz et al. (1983) almandine entropy indicates a suggeststhat (l) much of the scatterin garnet-biotite tem- slope of 34.5 bars/K, and the Chatillon-Colinetet al. (1983) almandine entropy indicates a slope of 30.7 bars/K (Table 9). peraturesfor grade 7 is due to precision error and does The agreementis good consideringthe magnitude oferrors both not representreal variation in 1"and (2) model 2 may be in AS and in the experimental slope.A value of 128.64J/K was slightly better than model I for muscovite breakdown, chosenfor the approximate midpoint betweenthe experimental although real variation in Xrro is probable. There is noth- and natural Iconditions, consistentwith the Metz et al. (1983) ing in the resultsto suggestlower valuesof XHrothan those almandine entropy. The slope data ar€ not precise enough to of model 2 (0.78). Inspection of the values of K* and 7 selectbetween the Metz et aI. (1983) and Chatillon-Colinet et al. from model 2 (Table 12) shows that the effectsof small (1983) almandine entropies,but they do suggestthat both entro- amounts of octahedral Fe, Mg, and Ti in muscovit€s pies are reasonable. roughly cancelout the effectsof CO, and CHo in the fluid Using Equation 2, the almandine activity model discussed and greaterNa in K-feldspar than in muscovite. previously, the staurolite activity model given in Appendix l, and the fugacity ratios of Table I 1, values of 7 were calculated The excellent agreementbetween the muscovite dehy- and are tabulated in Table I 3 for both models of fluid behavior. dration equilibrium and garnet-biotitetemperatures gives An initial T of 645 oC was chosen for the experimental end- credenceto the averagevalues of garnet-biotite temper- member reaction at 3.1 kbar, consistent with the experiments atures and to the two models for the behavior of HrO in and with the calculatedslope values. Despite the improvement reduced graphitic systems.In addition, the preservation in AS, staurolite stability relations, staurolite formula, and gar- of graphite and reduced nature of the ilmenite indicate net-biotite geothermometry,there remains a 5742'C discrep- that large volumes of pure HrO have not moved through ancy betweenaverage I calculatedfrom garnet-biotite geother- these rocks and that reasonable amounts of CHo re- mometry and that determined from staurolite stability relations mained in the equilibrium fluid. (Fig. 7). Such discrepancieshave also been noted by many other workers (e.g., Pigageand Greenwood, 1982; Mclellan, 1985). -IT".ioot Previously, we believed that thesediscrepancies resulted mainly *orkers(e.g., Speer, 1984, p. 329)have implied that from problems in the crystal chemistry and stability relations of thedata of Velde(1965) require staurolite (Holdaway et al., 1986b). However, it now becomes thatceladonite 'fhecomponent must reducethe thermalstability of muscovite. initial efect of clear that there must be additional causes,the most likely being celadonitecomponent must relate to Reaction7. BecauseAl in nonideality in staurolite tetrahedral sites. muscoviteis diluted much more than Al in sillimaniteand Nonideality in staurolite solid solution is further evidencedby K-feldspar,muscovite stability must first be increased,reach a values of Kx for Reaction 8 Oable l3). Since the staurolites all maximum,and then be decreasedaccording a reactionsuch as crystallized over a limited T range,K* should be roughly con- celadonite + sillimanite: muscovite+ biotite.The reaction ,, (Table l2). Instead (* is high for studiedby Veldeis not directlyapplicable to mostpelitic rocks stant, as it is for muscovite becauseit involvesK-rich bulk compositiqnsproducing K-feld- stauroliteslow in Fe and low for stauroliteshigh in Fe. The 1(x spar in the absenceof aluminumsilicate. Velde's study does values indicate that, for three specimens,solid solution reduces demonstratethat muscovite.celadonitesolid solution cannot be equilibrium 7and, for the other five, solid solution increases7l far from ideal. if water dilution is ignored. The temperaturescalculated for the HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 39 mostFe-rich staurolite (specimen 53) most closely approximate TneLe13. Temperatures('C) of grade5 (M.) the averagegarnet-biotite temperature, whereas those for the t4lc: 15 kJisite most Mg-rich staurolitesare fartherfrom the averagegarnet- We-O biotitetemperature (Table l3), againsuggesting nonideality. Re- No K lu-u I(1) T (2) r(1) r (2) centhigh-P experimental studies ofRice (1985) can only be ex- 137 1 152 545 647 642 619 615 plainedby nonidealsolid solution. 53 0.953 559 636 631 613 608 Ifwe excludespecimen 6, a high-Li staurolite,all the stauro- 14 1.272 560 653 648 609 605 great 237'. 1.095 s80 644 639 618 613 litesof thisstudy contain at least3.01 Fe atoms. The ma- 643 jority o 1.246 591 6s2 [578] [573] of staurolitesare Fe-rich, and thosewith high Mg appear 330'. 0.993 592 639 634 609 604 to be high-P,high-Z staurolites (e.g., Schreyer et al., 1984). 23 1.006 605 639 634 609 604 Holdawayet al. (1986b)provide strong evidence, based on high- 267.' 1 110 613 646 640 612 608 qualitycomplete chemical analyses, that Fe,Mg, Zn, andLi may Avg. 1.10(12) 581(24) 645(6) 639(6) 608(13) 604(13) Aug 643(6) 638(6) 613(4) 608(4) be assignedto the tetrahedralFe sites.Using this assumption, + we postulatea regular-solution(pseudobinary) model for stau- Noter Temoeraturesbased on staurollte-quartzreaction and presence rolite with tetrahedralions groupedin two categories:Fe and of graphiteai 3.1 kbar.Calculated usinE Equation 2 and initialf of 645'C (Dutrowand Holdaway,in prep.).AS. for the calculationswas 128.64J/K vacanciesas one,and Mg, Zn, and Li as the other.The justifi- given ifaote S;. f (1) - 11st model 1; r(2): from model2. values in cationsfor this are(1) vacancycontent oftetrahedral sites may 6rackets correspond to a Li-rich staurolite whose activity coefficient is vary substantiallywith pre$enceor absenceof coexistingR'?* overcorrectedby the pseudobinarymodel discussedin the text. ' : phasesand (2) ionic radii and maximumsolid solutionof Mg, Eouilibriumconstant K* " soecimens 237,330, and 267 are from the Farmingtonquadrangle Zn, andLi are comparable(e.g., Holdaway et al., 1986b).The (Dutrow,1985). See also Table8, grade5 (Ms)average. approachcan only be expectedto work well for an (Fe,Mg)- + Excludingspecimen 6. dominatedstaurolite with minor Li. Zn. andvacancies. A Mar- gulesparameter of 15 kJ per tetrahedralsite produces a solvus that passesthrough 7-X." valuesfor five of the stauroliteanal- present yses,leaves all theremaining analyses outside ofthe solvus,and certain that staurolite behaves nonideally. The hasa crestat 600'C. From this maximumpossible Wo value, model distributes the diluting ions Mg, Zn, and Li over activitycoefrcients were calculated. The activity modelfor the about four sites. No ideal model can be used to account presentsituation becomes rl:/ae x ilxls v ux?;ts. for the -50'C differencebetween garnet-biotite T and T The results (Table 13) show that the use of 15 kJ for basedon Reaction 8, when one considersthat Fe-Mg K" Wo overconectsthe Li-rich specimen,suggesting that the values between staurolite and garnet are close to unity Fe-Li interaction has a IZo lower than 15 kJ. For the other and Zn and Li in staurolite approximately compensate staurolites,a tighter spreadin Tresults. For model l, the for Mn in garnet (note Kx values averaging 1. l0 in Table I averages32 "C above that indicated by garnet-biotite l3). The only way Kx could be reduced slightly would be geothermometry, whereasmodel 2 indicates an average if the diluting ions in staurolite were distributed over less f that is 27 "C above the garnet-biotite average(Fig. 7). than four sites (presumably 2). This is very unlikely be- Specimenswith large diferences in Fe content but similar causeMg-rich staurolitescontain more than 2 (Mg + Zn) Li contents(e.g., 53 and l4) show similar calculatedtem- (Schreyeret al., 1984),and syntheticMg staurolitecon- peratures.The 27-32 "C difference in average I is very tains substantiallymore than 2 Mg (Schreyerand Seifert, reasonablein light ofthe various errors involved in garnet- 1969).(2) The assignmentof Fe, Mg, Zn, and Li to tet- biotite geothermometry, in the oversimplified staurolite rahedral sites is slightly less certain, although Holdaway solution model, in estimating Wo, and in the fluid-com- et al. (1986b)have made a strongcase for it. (3) Assuming position models. nonideality in the tetrahedral Fe sites, approximations The behavior of staurolite may be somewhatanalogous that require further testing and refinement are the pseu- to that of the muscovite-paragonitesystem. A (metasta- dobinary model, the regular-solutionassumption, and the ble) breakdown curve for the Mg staurolite end member exact value of Wo. The value of l/o chosen is the max- in equilibrium with muscovite and quartz probably oc- imum possibleone, and it still doesn'tfully compensate curs at lower temperaturesthan that for the Fe staurolite for the I'diference betweengarnet-biotite geothermome- end member. Even though the Mg end member is unsta- try and staurolite stability. ble at low pressures,the Fe limb of the Fe-Mg solvus still The srrggestionof a Margules parameter of -15 kJ approximately controls the limiting composition of Fe- per site for staurolite may provide an explanation for rich staurolite in the sameway that the muscovite-parag- reversals that occur in Fe-Mg partitioning of staurolite onite solvus controls the limiting composition of mus- vs. other minerals.Grambling (1983)has observeda re- covite even at conditions where paragonite is unstable. versal in staurolite-chloritoid partitioning caused by an Analogous to paragonite, Mg staurolite is stable at high increasein Mg. Chloritoid contains more Fe than stau- pressures(Schreyer and Seifert, 1969).This model may rolite in very Fe-rich compositions and more Mg than be testedby seeking(experimentally and in the field) co- staurolite at more Mg-rich compositions. A comparable existing Fe- and Mg-rich staurolitesat high P and T that reversal may occur with staurolite-almandine at much equilibrated below 600 'C. more Mg-rich compositions than those observed here It is important to distinguish betweenwhat can be said (Schreyeret al., 1984).The estimated Z" should be tested regardingstaurolite nonideality with reasonablecertainty againstthese Ko reversals,which may well provide a very and what remainsas conjecture. (l) We canbe reasonably sensitive indicator. 40 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE

3.5 ,E

o ll J M3 379-394m.y. E? le ta aa C) o-

2.5

2r 500 550 600 650

TemperotureCC)

Fig. 8. Rangeof P-Zconditions for various metamorphic eventsin west-centralMaine. Approximate ageof eachevent is given. Data arc from Dickerson and Holdaway (ms.) and the presentreport. The sillimanite-andalusiteboundary is from Holdaway (197l), the first sillimanite curve is the beginning of Reaction 8, and the averagesillimanite-K-feldspar curve is the averageappearance of this assemblageby Reaction 7.

The results of geothermometry,geobarometry, and de- of metamorphismmoved southwardduring this time from hydration geothermometryare consistentwhen staurolite Caratunk, 30 km north-northeast of Kingfield (Dickerson nonideality is taken into account (Fig. 7). These results and Holdaway,ms.; Holdawayet al., 1986a),to the Nor- are in agreementwith reasonablemodels for fluids (Xrro: way-Lewiston region, a distance of about 125 km in a 0.78-0.91) in reducedgraphite-bearing rocks. In some south-southwest direction. In discussing the causes of previous studies,overestimation of pressureor the stau- metamorphism, temperatureand pressurewill be treated rolite problem has led workers to suggestunrealistically separately.Although a detailed discussionof M, is not a low valuesof Xrro. topic of the present contribution, evidence from other sourceswill be usedhere to tie M, into the generalpicture of metamorphismlor the region. C.l,usBs oF METAMORpHICcoNDITToNS The thermal eventsrecognized as Mr, Mr, and M5 pro- Temperature vide a discontinuous record of pressureincrease during a With the exception of low-grade metamorphism M', time interval between400 and 325 Ma (Fig. 8). The locus all the metamorphic events in the area of Figure I show HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 4l a generalspatial associationwith plutonic rocks (Fig. l; increase:(1) an increasefrom north-northeast to south- Holdaway et al., 1986a).Many of the intrusive rocks of southwestat any given time and (2) an increasewith time the area are sill-like bodies that pass gently beneath the at any given place.Ifone considersonly the time, general surrounding metamorphic rocks, commonly with a spatial distribution, and P-T conditions of the three northern dip (Hodgeet al., 1982;Carnese, 1983). Mod- events,either ofthese two causesofP increasecould have eling studiesof Lux et al. (1986) and DeYoreo (1987) acted alone. Evidence cited below shows that both were demonstratethat conditions suchas those achievedat the necessary,perhaps each being responsiblefor about half culmination of Mr, Mr, and M, require the close prox- of the pressureincrease. imity of magmatic heat. Even though isogradsrepresent- Changein P from north-northeast to south-southwest ing substantiallyincreased Z are commonly up to 10 km at a given time is difrcult to document quairtitatively, and rarely as much as 30 km from the exposedigneous but a number offactors suggestthat this has occurred:(l) rocks (Fig. l; Holdaway et al., I 986a),the overall igneous The northern lobe of the Lexington batholith produced spatial association and the gently dipping character of cordierite-andalusite(Mr", -2.35 ! 0.25 kbar), whereas many of the igneousrocks imply that the batholithic heat the central and southern lobes produced staurolite-an- sourceslay no more than I to 3 km below the Mr, Mr, dalusite(M2, -2.7 + 0.25 kbar;Mr", -2.75 + 0.25 kbar) and M, isograds. This is shown by Lux and Guidotii (Dickersonand Holdaway,ms.t Holdaway et al. 1986a). (1985)and Guidotti et al. (1986)for the relationbetween The Gaudetteand Boone(1985 and pers.comm.) ageof the Sebagobatholith and the M, sillimanite-K-feldspar 400 Ma is based on all three lobes, and none is distin- isograd. In areaswhere isogradslie far from the igneous guishablefrom the others beyond the limits of error. (2) contacts and detailed geophysicalstudies fail to demon- Farther south, M, in the Kingfield and Anson quadran- strate that igneous rocks lie at depth, there is the possi- gles produceslittle or no garnet zone, and andalusite ap- bility that gently dipping plutonic bodies once lay above pears immediately after staurolite (Holdaway et a1., the metamorphic rocks and acted as a heat source for 1986a).To the south near Farmington, the gamet zone metamorphism. widens to about 6 km, and andalusite first appearsin the The conditions of grade7, M, metamorphism (3.8 kbar, Dixfield quadrangle several kilometers west of the first 660 "C) are very close to the invariant point at which the M, staurolite (Dutrow, 1985). These observations are vapor-absentmelting curve for muscovite-quartz-plagio- consistent with a southward P increasein Mr. (3) Dall- claseoccurs in graphitic pelites (Kerrick, 1972; Ohmoto meyer(1979) has shown with incrementalAr-release ages and Kerrick, 1977). In this area (Fig. I in Holdaway et on metamorphic biotites in an areaalong the easternedge al., 1982) K-feldspar-absentrocks (6.5) are regionally in- of Figure 1 (M, and M5) that agesdecrease progressively terspersedwith K-feldspar-bearing rocks (7) that clearly from about 330 Ma in the northeast to 230 Ma in the formed at a higher average ?'than grade 6.5 (Table 8). southwest. This slower cooling toward the southwest is The large number of small granitic bodies and pegmatites consistent with a longer period of erosion to the biotite suggeststhat incipient melting occurred in the rocks im- blocking isotherm or a higher initial Z, hence the possi- mediately below the present exposures,and varying de- bility of a deeperburial or slower uplift to the southwest. greesof upward transmissionof magma and heat resulted. (4) In M, there must have been a P increasesouthwest The incipient melting of theselocal magmas and the Se- acrossthe Sebagobatholith as indicated by P about 3.8 bagogranites at slightly greaterdepth probably had a buF kbar on the north side comparedwith P of 5 kbar or more ering effect on the rocks, preventing the total destruction on the south side where kyanite routinely occurs with of muscovite at the prevailing P, exceptin the singlegrade almandine-rich garnet (Thomson and Guidotti, 1986; 8 specimen (ll, Table 8) in which muscovite probably Husseyet al., 1986; Carmichael,1978). These observa- grew after the thermal culmination for the locality. In tions require additional documentation, but a case for addition to Z differences,minor differencesin Xrro be- south-southwestincrease in P appears to be emerging. tween grades6.5 and 7 are possible. This increase(perhaps equivalent to 2-3 km) probably representsa combination of slight tilting during post- Pressure metamorphic uplift and higher topography to the south- southwestduring metamorphism. In west-central Maine, pressureincreased from north The progressive increase in P with each subsequent to south and with time. The pressureof 2.35 kbar for the event also requires changesin P with time as shown by lowest-P portion M, (Dickerson and Holdaway, ms.), of the following evidence: (l) The transition from cordier- averageP of 3.1 kbar for M., and averageP of 3.8 kbar ite-andalusite(Mr,) to staurolite-andalusite(Mr, Mr.) oc- for M, (Fig. 8) correspondto approximate depths of 8.2, curs abruptly at the junction between the northern and 10.9,and 13.3km, respectively,during the three events. central lobe of the kxington batholith (Dickerson and Mr, M,, and M, occurredat 400,394-379, and 325 Ma, Holdaway,ms.; Holdaway et al., 1986a),suggesting a rapid respectively, and at average distances of about 30, 80, P increaseover a small time interval near 400 Ma. This and 1l0 km south-southwestof Caratunk where the low- transition does not involve a fault, but does involve a est-P M, begins. This increasein P with map position change in igneous lithology. (2) In the Rangeley, Rum- and time may have been produced by two kinds of P ford, Phillips, Dixfield, Farmington, and Augusta quad- 42 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE

rangles (Fig. l), M, (no andalusite) is superimposedon involving a combination of octahedraland l2-fold sites M. (andalusite-bearing),indicating a P increasewith time is required. throughoutthese areas (Holdaway et al., 1982).(3) In the 3. In garnet, the progressive changesin Mn content Bryant Pond, Buckfield, Livermore, and Lewiston quad- with Zmay be usedto confirm previously suggestedphase rangles,there is a much less obvious superimposition of relations betweengarnet, staurolite, biotite, and silliman- M5 (-3.8 kbar) on M3 (-3.1 kbar). In part this is less ite. In the staurolite zone, the occurrenceofa wide range obvious becauseof the absenceof a P-sensitive phase of biotite compositions with chlorite, along with other transition like andalusite-sillimanite. However, the ap- evidence,suggests that chlorite is mainly a retrogrademin- pearanceof retrogressivechlorite in the M, staurolite- eral at this grade.A related observation is that the stau- grade rocks may be due to this efect. rolite-chlorite tie line in AFM projection must break early The progressiveincrease in P with time requires the in the staurolite zone; rocks of appropriate composition addition of perhaps 2-3 km of rock above the present would form aluminum silicate immediately after stau- level of exposureover a 75-m.y. period at a rate more rolite. rapid than erosion. This increase in overburden could 4. Staurolite H values are near 3, basedon 48 oxygens. have occurred over the whole region or could have been Staurolite-garnetand staurolite-biotite KD plots suggest addedin larger amounts in the southern and central parts positive deviation from ideality in Fe-Mg solid solution. ofthe area shown in Figure I than in the northern parts. 5. Ilmenite is very low in hematite content and coexists The rock may well have been added both at the surface with graphite in most pelitic rocks, supporting low fo, and at shallow crustal levels. There are several possible valuesnear QFM. mechanisms whereby P may have increasedwith time: 6. Modified Ganguly-Saxenatemperatures indicate that (1) Marine sedimentary and/or volcanic deposition could the staurolite-sillimanite rocks in M, formed at 581 + 24 have continued throughout much of the time in question. oC and sillimanite-K-feldspar-muscovite rocks in M, This would have resultedfrom continuedsubsidence while formedat 659 + l3'C. plutonic rocks were being emplaced in great volume. 7. The formation of the first sillimanite from staurolite However, the likelihood of surfacesubsidence during such in M,, directly above the andalusite field, suggestsP : a time is not great. (2) Nonmarine deposition could have 3.1 + 0.25 kbar.The muscovite-almandine-biotite-silli- been extensivewith the build-up of a widespreadvolca- manite (MABS) geobarometer has been calibrated and nic highland. The P increase could have been accom- yieldsP : 3.8 kbar for M,. plished by subsidenceat the presentlevel combined with 8. The muscovitebreakdown reaction in M, is consis- intrusion at shallower levels and extrusion at the surface tent with a Icalculated from garnet-biotite,P: 3.8 kbar, (seealso Holdaway et al., 1982). The shallower igneous and a graphite-presentfluid model in which P.o, is be- rocks would have been correlatives of the batholiths ex- tween P."o and l0P.ro. The staurolite breakdown reac- posedat presentlevels. (3) A final possibility is emplace- tion in M. is not consistent with garnet-biotite Z, but ment of crust by overthrusting, presumablyfrom the east. requires ? Wo = 15 kJ per tetrahedral Fe site in order to This model has problems becauseof the generalabsence bring the two setsof data into better agreement.This non- of evidenceof Middle Devonian to Mississippian thrust- rdeality suggestsa staurolite solvus at about 600 "C. ing. We prefer the model of emplacement of magmatic 9. Heat for Mr, Mr, and M, came from shallow sill-like rocks at the surfaceand at levels shallower than present batholiths either below or above the presentschists. Max- exposures.It should also be pointed out that the youngest imum temperaturein M, leveled out at about 660 "C (3.8 rocks in west-central Maine are the Middle Devonian kbar) due to bufering causedby incipient vapor-absent Tomhegan Formation, which is exposed30 km north of muscovite melting, ubiquitous local pegmatitesand gran- the Lexington batholith and contains felsic tuffs, tuffa- ite, and the Sebagobatholith below. ceous sedimentary rocks, and garnet rhyolite (Rankin, 10. Pressureincrease from M, to M. to M, (Fig. 8) 1968).It is tempting to speculatethat theseare the ear- correspondsto about 5 km over a 75-m.y.period. About liest remnants of a more widespreadvolcanic terrane. half of this is related to a south-southwestmovement of the locus of igneousactivity and metamorphism and as- CoNcr,usroNs sociatedP increasewith map position. The remainder is l. In west-centralMaine, M3 Q94-379 Ma) includes a P increasewith time that may well be associatedwith the biotite, garnet, staurolite, staurolite-sillimanite, and emplacement of shallower intrusive rocks and develop- sillimanitezones. The youngerM5 (325 Ma) includessil- ment of an extensive volcanic highland during the De- limanite and sillimanite-K-feldspar within the secondsil- vonian and Mississippian. limanite isograd. 2. In thesegraphite-bearing pelitic rocks, the micas are AcxNowr-BncMENTS well expressedin terms of conventional end members We acknowledgewith thanks the assistanceof Dwight Deuring on the and "Ti-biotite" (KTitr(Fe,Mg)AlSi,O,JOHD or'.Ti- electron microprobe. We thank Jo Ann Weber for writing a computer program muscovite" (KTi(Fe,Mg)AlSi3Or'(OH)r). Celadonirecon- to calculateactivity coefficientsfor almandine component.Mary Lee Eggartat LSU drafted the figures.Myrtle Watson is thanked for her tent of muscovite is very low and has a potentially large cxpert typing of many drafls of the manuscript. M.J.H. acknowledgesthe error. In determining mica end members,no substitution supportof the National ScienceFoundation (grants EAR-8306389 and HOLDAWAY ET AL.: METAMORPHISM IN WEST.CENTRAL MAINE 43

EAR-8606489)and B L D the Inslitutefor the Study of Earth and Man lite + quartz at medium to low pressures.Geological Society ofAmerica at SMU R.W H acknowledgessupport from NASA (grantNA99-51 to Abstractswith Programs,15, 563 R N Clayton). The project has benefitedgreatly from discussionswith Dutrow, B L., Holdaway,M J., and Hinton, R W. (1986)Lithium in stau- C V Guidotli and his colleaguesat University of Maine and with Darrell rolite and its petrologic significance Contributions to Mineralogy and Henry at LSU. The manuscript has been reviewed formally by M. P Petrology,94, 496-506. Dickenson III, Jo Laird, and Eileen Mclellan and informally by C V Dymek, R F (1983)Titanium, aluminum, and interlayercation substi- Guidotti and Jinny Sisson tutions in biotite from high-gradegneisses, West Greenland American Mineralogist,68, 880-899. Eggleston,R.A, and Bailey,S.W. (1967)Structural aspects ofdioctahe- Rnrnnpucns crrpu dral chlorite American Mineralogist, 52, 673-689. (1969) fluorinein micasof pelitic schistsfrom Albee, AL, and Ray, L. (1970) Correctionfactors for electronprobe Evans.B W. Chlorineand isograd, Maine American Mineralogtst, 54, microanalysisof silicates,oxides, carbonates, phosphates, and sulfates the sillimanite-orthoclase AnalyticalChemistry, 42, 1408-1414. r209-t2tI C.v. (1966)The sillimanite-potashfeldspar Aleinikofl,J.J, Moench,R H, and Lyons,J.B. (1985) Carboniferous U-Pb Evans,B.W., and Guidotti, U S A Contributions to Mineralogy and ageof the Sebagobatholith, southwesternMaine. GeologicalSociety of isograd in western Maine, AmericaBulletin, 69, 990-996. Petrology,12,25-62. Ferry, M. (1976)P, T,f"o,, andf"reduring metamorphism of calcareous Bence,A.E , and Albee,A.L (1968)Empirical correction factors for elec- J in the Waterville-Vassalboroarea, south-centralMaine. tron microanalysisof silicatesand oxides Journal of Geology,7 6, 382- sediments Petrology,5'l, l19-143. 403 Contributionsto Mineralogyand -(1980) A casestudy of the amount and distributionof heat and Burnham,C W., Holloway,J.R., and Davis,N F (1969)Thermodynamic fluid during metamorphism Contributions to Mineralogy and Petrol- propertiesofwater to 1,000"C and 10,000bars. Geological Society of America SpecialPaper I 32. ogy,71, 373-385. (1978) ofpartition- Carmichael,D.M (1978) Metamorphicbathozones and bathograds:A Ferry,J.M., and Spear,F.S. Experimentalcalibration and garnel. Contributions 10 Min- measure of the depth of post-metamorphic uplift and erosion on a ing of Fe and Mg between biotite l7 regionalscale American Journal of Science,278, 769-797 eralogyand Petrology,66, I l3-1 (i958) Metamorphicreactions Carnese,MJ (1983)Gravity studiesofintrusive rocks in west-central Fyfe,W.S., Turner, F J., and Verboogen,J Societyof America Memoir 73. Maine M S thesis,University of New Hampshire,Durhirm, New and metamorphic facies Geological relatedparageneses: Hampshire. Ganguly,Jibamitra. (1969) Chloritoid stabilityand American Science, Chatillon-Colinet,C., Kleppa,O.J., Newton, R.C., and Perkins,D, III. Theory, experiments,and applications. Joumal of (1983)Enthalpy of formationof FerAlSi,O,, (almandine)by highrem- 267,910-944. 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(1981)Ion of plutonism in low-pressuremetamorphic belt formation. Nature, 323, microprobe techniquesand analysis of olivines and low-calcium py- 794-797 roxenes.American Mineralogist, 66, 526-546. HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 45

Thompson,A.B. (1976)Mineral reactionsin pelitic rocks:I. Prediction Disorder correction.In order to make the disorder correction, of P-I-X(Fo-Mg) phase relations. American Journal of Science,276, two assumptionswere made: (l) The staurolite is as ordered as 40t-424 possible, i.e., minimum corrections were made; (2) the correc- Thompson, J.B. (1982) Composition space:An algebraicand geometric tions that were made are correctionsthat can be reasonablyas- approach. Mineralogical Society of America Reviews rn Mineralogy, sumed to be constant for Fe "end-member" staurolite and for t0, l-31. the natural staurolitesfrom west-centralMaine. The "end-mem- Thomson, J A., and Guidotti, C.V. (1986) The occurrenceof kyanite in (HrEr)o(FerrEot).- southernMaine and its metamorphic implications. GeologicalSociety ber" staurolite formula may be rewritten as of America Abstractswith Programs, 17, 59. (A1,,rFeo ,rEo rr)r(Feo.rEr rr)14.116(Si7 66A10 3o)EOo*. Following the Tompkins, LA, Bailey, S.W, and Haggerty,S.E. (1984) Kimberlitic maximum-order structuremodel of Holdaway et al. ( 1986b)with chlorites from Sierra Leone, West Africa: Unusual chemistries and P(lB), A(3A), U(1) sites partially filled and P(lA), A1(3B),and structural polytypes.American Mineralogist, 69, 237-249. U(2) sites empty, we have the following disorder terms with Tracy, R.J (1978) High grade metamorphic reactionsand partial melting AS-o: -R[(Xln X) + (l - X)ln(l - X))] per site:(l) -Fe site in pelitic schists,west-cenlral Massachusetts. American Joumal of Sci- disorderis -4R[0.025 ln 0.025+ 0.975ln 0.975]: 0.4676R' ence,278,150-178. (2) P(lB) site disorderinvolves only 3.95 sitesand 2.9 ions be- - ( I 982) Compositional zoning and inclusionsin metamorphic min- H on P(lA) and P(lB) is coupled to the 0.1 erals Mineralogical Society of America Reviews in Mineralogy, 10, cause0.05 each i"Fe. -3.95R[0.734 + 0.266 ln 355-397 vacancyin Disorder is ln 0.734 Veblen, D.R (1983) Microstructures and mixed layering in intergrown 0.2661:2.2880R.(3) Al(3A) sitedisorderincludes U(1) disorder -2R[0.790 wonesite,chlorite, talc, biotite, and kaolinite. American Mineralogist, to which it is coupled.Disorder is ln 0.790 + 0.075 68, 566-580 ln 0.075 + 0.135ln 0.1351: 1.3017R.(4) Si site disorderis Velde, B. (1965) Phengitemicas: Synthesis,stability and natural occur- -8R[0.958 ln 0.958+ 0.042ln 0.042]: 1.4065R.Total disorder rence.American Journal of Science,263, 886-9 I 3. entropyis 5.4638R: 45.42J/K. Waldbaum,D.R., and Thompson,J.B, Jr. (1969)Mixing propeniesof Hemingway and Robie's (1984) Table 6, which already has a sanidine crystalline solutions: IV. Phasediagrams from equations of + 121.0-J/K chemical and disorder correction, may be adapted state.American Mineralogist, 54, 1274-1298. presentFe "end-member" staurolite by subtracting53.59 Zen, E-an. (1981) Metamorphic mineral assemblagesof slightly calcic to the pelitic rocks in and around the Taconic allochthon, southwesternMas- J/K (- 121.0+ 2r.99 + 45.42). sachusettsand adjacent Connecticut and New York. U.S. Geological SurveyProfessional Paper I I 13. Activity model Becausethe above disorder corrections apply to the Fe "end- M,tNuscnryr REcETvEDMlncs 20, 1987 member" staurolite in hydrothermal experimentsand in nature, M,asrrscnrvr AccEprEDSer"rsr4ssr 14. 1987 none ofthem should be taken into accountin the activity model. In determining the model and calculating staurolite activities, the following reasonableassumptions were made (Juurinen, 1956; AppnNorx 1. Srlunor,rrE DATA Holdawayet al., 1986b).(1) Assumethat the H contentof the Entropy staurolitesis 3 and fix thei"Fe site vacancyat 0.1 ions pfu. Thus i"Fe Hemingway and Robie (1984) have provided entropy data for occupancytotals 3.9 (to allow this quantity to vary in these staurolite355-1, originally studied by Zen (1981). specimenswould produce variations resulting largely from an- Chenical correction.Holdaway et al. (1986b) have shown that alytical error). (2) 0.25 (Mn + Fe) pfu is assignedto U(1) and this staurolite has the formula Hrrr(FerruMg,orrMnoorfnoorrl-iorru- 0.15 Fe pfu is assignedto Al(3A). Where Li is not known, an Croo,rCooorTioos.)Al,?7s7si?2130or.6In order to correct for the averagevalue of 0.3 pfu is assumed.Remaining Fe, Mg, Zn, and toFe chemical differencesbetween this staurolite and pure Fe "end- Li are assignedto sites. member" staurolite(HrFeo.Al,rrrSiruuOor) as usedin Reaction 8, appropriate amounts of entropy of the following compounds The activity model below restricts additional solid solution to i'Fe : were added or subtractedfrom the raw data of Hemingway and the and U(l) sites (i end-member values): Robie (1984):MgO, LirO, pyrophyllite, Cr,Or, ZnO, TiO,, MnO, --l};' x t',YB;lx uXY,!' x uXl:'!' CoO, SiOr, AlrOr, and fayalite. The molar volumes of the sub- : tu)(ps x ux|25. t *X2:] x x uXl:lt tracted componentssummed to 3.114 J/bar, and the sum for the ")G:., "X3?,', addedcomponents was 3.055 J/bar. This slight compressionwas The mole fractions are basedon the analytical quantities, per 48 corrected for by subtracting 1.48 J/K from the entropy (25.1 oxygens: J/K for eachJ/bar; Fyfe et al., 1958).The total chemical correc- 0.25- Mn tion was +23.47 - 1.48: +21.99 J/K. 0.25 -u co ir gi*n as 0.02 to account for small amounts of co, v, The value 0.4 corrects for Mn and Fe in the U(1) and Al(3A) and Zr. sites(0.25 and 0.15, respectively). 46 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE

AppENDrxrABLE 1. Stoichiometriccompositions of minerals"

Grad€ 4t5*5 Speclmen 1A 52 3A 61A 84 L29 114 5A 65 94 2t 137 53 l4A 6 23 79 4A si 6.090 6.045 6.059 6.002 6.0r8 6.049 6.ll2 6.063 6.050 6.045 6.080 6.055 6.036 5.083 6.051 5,014 6.015 6.013 6.017 6.034 -. Al 5.535 5.792 5.706 5.874 5.826 5.758 s.675 5,777 5.812 5.734 5.704 5.762 s.81s s.701 5.737 5.800 5.776 5.785 5.786 5.727 I Ti 0.031 0.023 0.043 0.023 0.031 0.027 0.027 0.023 0.027 0.043 0.043 0.035 0.020 0.047 0.047 0,040 0.043 0.0s9 0.062 0.047 E r" 0.154 0.103 0.114 0.078 0,089 o.o9r 0.rzz 0,082 0.078 o.106 o.toz o.rot o.tio o.no 0:098 0:to6 o.tos o.ogl o.l0z 0.114 I Ms 0.r85 0.079 0.122 0.074 0,085 0.095 0.110 0.090 0.070 0.ll8 0.105 0.093 0.059 0.098 0.098 0.083 0.rr7 0.090 0.078 0.134 K -5 1.62i 1.599 1.679 r.444 1.490 1,625 1.525 1.533 1.418 1.670 1.662 1.542 1.524 r.559 1.545 1.594 1.559 1.590 1.540 r.727 ffa 0.252 0.336 0.255 0.488 0.409 0,328 0.264 0.388 0.490 0.248 0.235 0.367 0.406 0.352 0.40{ 0.356 0.247 0.334 0.387 0.225 Ba 0.008 0.004 0,012 0.007 0,019 0.004 0.008 0,008 0.027 0,016 0.012 0.007 0.012 0.008 0.012 0.016 0.016 0.012 0.008 0,015 t(0x) 94.14 93.54 94.71 94.54 94.89 93.50 94.12 94,24 94.03 94.56 94.59 94,75 93.97 94.09 94.43 93.88 94.88 94.23 94.74 94.s6 sl 5.420 5.421 5.386 5.405 5.4r1 5.404 s.400 5.362 5.415 5.375 5.279 s,42s 5.337 s.384 5,394 5.373 5.405 5.353 5.348 5.375 -. Al 3.394 3.452 3.514 3.505 3.530 3.490 3.530 3.548 3.544 3.541 3.559 3.5?3 3.525 3.507 3.514 3.515 3.517 3.531 3.591 3.534 ! Tl 0.186 0,179 0.166 0.174 0.159 0.144 0.154 0.167 0.161 0.183 0.182 0.163 0.203 0.203 0,193 0,209 0.278 0.245 0.1s8 0.264 E Fe 2.236 2,789 2.195 2.439 2.209 2,2s7 2.69t 2.636 2,6u 2.261 2.183 2.507 3.I74 2.490 2.522 2.6U 2.310 2.654 2.848 2.199 ils -; 2.52? 1.938 2.495 2.252 2.473 2.497 1.979 2.r01 2.U7 2.370 2.r01 2.109 1.416 2.140 2,127 t.947 2.r57 1.958 1.8r1 2.318 iln 0.027 0.009 0.013 0.009 0.013 0,018 0.022 0.004 0.004 0.0u 0.005 0.009 0.00't 0.009 0.009 0.018 0.013 0.009 0.009 0.013 K L.752 L.lL2 1.783 1.550 1.637 r.758 1.697 1.628 1.616 1.774 r,560 1.694 1.694 1.721 1.659 t,706 L.1?4 1.7r3 1.698 1.723 lla 0,050 0.027 0.058 0.080 0.1010.$1 0.055 0.054 0.085 0.057 0,059 0,085 0.087 0.088 0.085 0.092 0.031 0.059 0.095 0.057 t(0x) 94.23 93.15 94.12 94.76 95.27 93.11 93.70 93.72 93.88 94.45 93.98 93.85 94.26 93.87 94.49 93.95 93.68 94.30 94,84 95.44 aln 0.554 0.554 0.556 0.698 0.687 0.654 0.683 0.73s 0.810 0.658 0,731 0.732 0.84i 0.781 0.754 0.73r 0.673 0.749 0.763 0.604 n.- prp 0.078 0.051 0.058 0.081 0.107 0.101 0.066 0.081 0.094 0.105 0.078 0.100 0.059 0.109 0.109 0,084 0.113 0.105 0,065 0.103 F fi sps 0.333 0.247 0.286 0.r74 0.157 0.204 0.218 0.128 0.035 0.168 0.124 0.115 0.074 0.075 0.094 0.169 0.144 0,105 0.123 0.211 --,i.E srs 0.035 0.048 0.090 0.048 0.038 0.041 0.033 0.055 0,060 0.059 0.067 0.05? 0.026 0.034 0.034 0.016 0.070 0.041 0,049 0.083 t(0x) 99.78 98.49 99.92100.45 100.42 99.40 98.95100.73 99.40100.39 99.69 99.80100.45 99.73100.74 100.15 99.80100.37 100.18 100.86

-: aln 0,600 0.583 0.671 0.750 0.587 0,665 0.583 0.790 0.817 0.666 0.748 0.732 0.862 0.788 0.759 0.733 0.676 0.760 0.769 0.632 1JI prp 0.087 0.057 0.104 0.095 0.107 0.101 0.066 0.084 0.091 0.109 0.084 0.100 0.063 0.100 0,104 0.077 0.156 0.099 0.067 0.112 F E sps 0.279 0.210 0.150 0.1i7 0,167 0.i90 0.218 0,067 0.032 0,161 0.098 0.116 0.053 0.080 0.092 0.169 0,139 0,110 0.113 0.184 ,iE grs 0.034 0.050 0.075 0.038 0.038 0.044 0.033 0.059 0.060 0.065 0.059 0.052 0.022 0.032 0.035 0.021 0.059 0.03r 0.051 0.072 t(ox) 99.83 99.21100.41 100.71 100.42 99.36 98.95 99.94 99.52100.45 100,52 99.80100.32 100.25 100.5s 100.19 100.79 100,49 100,48 100.91

sl 7.621 7.632 7,638 7.578 7.699 7.592 7.521 7.734 7.660 7.581 7.505 7.703 7.s05 7.617 7.823 7.595 AI 17.90917.898 17,911 17.952 17.$r 17.93817.891 17.795 17.870 17.949 18.024 17.827 18,006 17,913 17.898 17.935 -_n 0.1I0 0.090 0.114 0.103 0.080 0.097 0.093 0.114 0.086 0.104 0.080 0.106 0.105 0.119 0.083 0.115 !Fe 3.036 3.408 3.300 3.126 3,121 3.374 3.383 3.201 3.418 3.422 3,662 3.293 3.441 3.008 2,ss9 3,249 0,610 0.661 0.790 0,623 0.465 0.577 0,561 0.583 0.567 0.590 0.443 0.586 0.609 0,519 0.374 0.550 e;; 0.082 0.038 0.114 0.101 0.121 0.029 0.017 0,076 0.045 0.040 0.0r7 0.045 0.054 0.079 0.101 0.054 Er; 0.069 0.060 0.037 0.039 0.270 0.064 0.060 0.230 0.037 0.062 0.062 0.083 0.050 0.407 0.202 0.r73 6Ll n.d. , n.d. n.d. n.d. n.d. n.d. 0,418 n.d. n.d. n.d. n.d. n,d. 0.259 n.d. 0.993cn.d. H 3.755u3.084 2.677 3.642 3.437 3.342 2.903 2.840 3.272 3.I85 3.216 3.289 2.969 3.291 3,312 3.283 t (0x) 10uJT-10U3r100.10 ry ry ry ee.84 e93f-l0r.o9-g9-tr]0u.54- ry100. 51 sg;?T-es.87 10lr50- sl 5.225 s.157 5.131 5.r3r 5.172 5.156 5.143 5.123 5.105 5.142 Al 5.549 5.808 5.801 s.793 5,714 5.770 5.771 5.855 s.814 5.828 ri 0.012 'i$ 0.013 0.012 0.012 0.012 0.025 0.012 0.005 0.012 0.013 Fe 3.887 5.029 4.282 3.838 3.99s 4.87I 4.7t0 4.731 4.516 5.847 I lls s.065 3.887 4.697 5.r07 s.010 4.030 4,296 4.193 4.484 3.065 6 l,tn 0.061 0.019 0.012 0.042 0.037 0.0s7 0.012 0.006 0,019 0.006 Ca 0.006 0.000 0.006 0.000 0.000 0.000 0.000 0.000 0.000 0.000 l(0x) 85.89 86.28 87.04 86.85 85.91 85.72 87.04 86.29 86.55 86,98

r ilE 0.906 0.973 0.955 0.974 0.948 0.929 P 0.977 0.982 0.963 0.970 0.961 0.994 0.972 0.977 0.941 0.939 0.97r 0.976 0.928 2 pyr 0.094 0.027 0.045 0,019 0.052 0.050 0.013 0.005 0.030 0.022 0.018 0.005 0.0r8 0.022 0.0s9 0.060 0.027 0.024 0.060 5 Sei 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.003 0.000 0.000 0.006 0.000 0.000 0.001 0.000 0.00r 0,00r 0.000 0.005 t hen 0.000 0.000 0.000 0.007 0.000 0.020 0.010 0.010 0,007 0.008 0.0r5 0.000 0.010 0.000 0.000 0.000 0.00r 0.000 0.007 i t(0x) 98.44 98.98 98,65 99.52 98.15 98.68 98.80 99.24 99.64 98.89 99.41 98.92 99,92 98.61 97.92 91,36 98.49 98.54 98.99

.anP 0.388 p P 0.340 oaD 0.6r1 0.558 :or * 0.001 0.002 r(ox) 100.58 100.48 HOLDAWAY ET AL.: METAMORPHISM IN WEST-CENTRAL MAINE 4'l

APPENDfx r ABLE1 - Continued

r r8 Grade * 6.5 I speclmn t9 r40 77-3 77-2 90 76 56 91 73 87 i43 14s 86 11 si 5.060 6.016 6.014 5.027 6.053 5.0s1 6.053 6.037 6.053 6.026 5.037 6.071 6.063 6.035 6.055 5.073 5.072 6.055 5.072 5.038 Af s.704 s.743 5,775 5.770 5,700 5.597 s.603 5.716 5.665 5.715 5.706 5.679 5.705 5.680 5.530 5.652 s.531 5.666 5.653 o Ti 0.048 0.053 0.063 0.048 0.065 0.055 0.075 0.067 0.081 0.081 0.083 0.072 0.064 0.091 0.103 0.074 0.071 0.r08 0.060 0.111 f re 0.110 0.115 0.090 0.106 0.115 0.110 0.154 0.115 0.115 0.tt0 0.114 0.117 0.110 0.134 0.125 0.1310.r17 0,126 0.i34 0.118 : l.fq 0.126 O.lO3 0.098 0.099 0.107 o.ll8 0.150 0,107 0.119 0.114 0.122 0.t25 0.098 0.103 0.125 0.130 0.108 0.r10 0,118 0.125 9 x- r.668 1.646 1.585 1.s85 r,lLi t.i4B t,'tg2 r.7r7 1.702 1.666 1.674 L.749 r.698 1.792 1.816 1.875 1.940 1.816 1.811 1,853 i Na 0.2G5 0.301 0.326 0.328 0.206 0.20s 0.163 0.206 0.2s8 0.251 0.236 0.137 0.236 0.1s0 0.137 0.083 0.126 0.130 0.114 0.090 Ba 0.008 0.016 0.008 0,008 o.ol2 0.015 0.020 0.012 0.015 0.020 0.008 0.004 0.000 0.008 0.008 0.004 0.004 0.012 0.008 0.008 t(0x) 93.95 94.07 94.18 93,71 93.81 94.58 94.32 93.81 93.95 94.79 94.35 94.93 94.21 94.42 95.o4 94.56 94.45 94.38 94.7s 95.26

5t 5.372 5.361 5.397 5.369 5,365 5.361 5.376 5.374 5.356 5,354 5.354 5.343 5.353 5.325 5,351 5.344 5,331 5.349 5,312 5.359 AI 3.527 3.465 3.727 3.489 3.457 3.525 3.396 3.397 3.473 3.44s 3.409 3.4i2 3.386 3.435 3.4r3 3.455 3.446 3.485 3.457 3.505 ETi 0.158 0.243 0.2t7 0.298 0.265 0.257 0.326 0.27! 0.292 0.281 0.337 0.360 0.335 0.399 0.419 0.356 0.346 0.407 0.364 0.331 EFe 2.343 2.533 2.402 2.693 2.725 2.425 2.298 2.579 2.463 2.529 2,531 2.543 2.505 2.658 2.443 2.511 2.549 ?.5U 2.579 2.328 2.357 2.054 1.855 1.830 r.A75 2.064 2.244 2.090 2.0s0 2,087 1.986 1.967 1.959 1.749 1.878 r.897 1.898 1,679 1.899 2.050 oil; 0.009 0.013 0.023 0.004 0,013 0.027 0.013 0.013 0.013 0.013 0.022 0.018 0.020 0.022 0.022 0.009 0.018 0.013 0.009 0.013 K r.711 1.709 1.698 1.712 1.795 1.812 1.843 r.746 1.712 1.738 r.779 1,805 1.834 r.872 r.872 1.911 1,939 1.895 1.891 1.852 NA 0.071 0.072 0.073 0.072 0.054 0.052 0.049 0.086 0.057 0.071 0.067 0.045 0.052 0.036 0.044 0.027 0.035 0.036 0.027 0.031 [(0x) 94.84 94.79 93.93 94,69 94.50 95.49 94.96 94.35 94.65 95.22 94,69 94.85 94,73 94.59 95,45 94.51 94,19 94.70 95.25 94.65

aln 0.663 0.709 0.591 0.764 0.711 0.629 0.641 0.573 0.7r0 0.704 0.581 0.685 0.704 0.595 0.675 0.731 0.751 0.755 0.778 0.702 0.119 0.099 0,096 0.099 0.091 0.101 0.131 0.112 0.121 0.123 0.112 0.112 0.112 0.102 0.108 0.1260.130 0,r14 0.145 0.r80 Eg sps 0.155 0.158 0.181 0.ll5 0.150 0.219 0.172 0.185 0.143 0,147 0.187 0.177 0.146 0,177 0.184 0.082 0.091 0.099 0.052 0.055 38 grs 0.052 0,024 0.032 0.022 0.048 0.051 0.056 0.029 0.02s 0.02s 0.020 0.025 0.038 0.024 0.033 0.051 0.028 0,021 0.0r5 0.053 t (0x) 100.89100.04 r00.54 100.06100.05 101.73 100.94 99.9910l.OO 101.02 99.91 r00.15 100.37 99.51100.32 100.10 99.48100.04 100.34 100.85 alm 0.674 0.739 0.595 0.785 o.7rg 0.542 0.662 0.701 0.125 o.7tg 0.698 0.706 0.719 0.713 0.681 0.734 0.750 0.765 0.783 0.717 d 0.115 0.114 0.114 0.128 0.161 El prp o.rrz o.ogo 0,084 0,096 0.084 0.093 0.r28 0.106 0.115 0.116 0.112 0.107 0.102 0.097 0,101 i6 sps 0.155 0.139 0.201 0.099 0.148 0.218 0.151 0.165 0.133 0.138 0.170 0.162 o.l4l 0.157 0.187 0.089 0.107 0.099 0.073 0,062 3E gts o.oso o:o2G 0:oz0 0.ozo 0.049 0.047 0.0s9 0.029 0.027 o.oz7 0.020 0.025 0.038 0.023 0.032 0.062 0.029 0.021 0.015 0.060 - t(0x) 101.05100.52 100,25 100.45 100.30 101.49 roo.68 100.28 100.80 100.92 99.48100.54 100.38 100.05 r00.63 100.32 99.43100.04 100.57 101.21

0,840 0.899 0,862 0.835 0.883 0,888 Sor 0.093 0.129 0.146 0.101 0.101 :ab 0.152 lan 0.000 0-000 0.000 0.000 0.000 0.000 5.. 0.008 0.008 0.009 0.0I8 0.016 0.011 t (0x) 100.39 99.94 99.58 99,30 99.16 99.77

o lln 0,957 0.969 0.909 0.978 0,895 0.887 0.891 0.968 0.961 0.952 0.933 0.954 P P # pyr 0.041 0.028 0.068 0.017 0.067 0.111 0.086 0.032 0.026 0,027 0.066 0.044 E gei 0.000 0.003 0.001 0.000 0.000 0,000 0.001 0.000 0.003 0.003 0.000 0,000 ! hm 0.002 0.000 0.022 0.005 0,037 0.002 0.022 0,000 0.010 0.018 0.001 0.002 - t(0x) 99.49 99,55 99.92 99.59100.97 99.15100.29 99.45 99.85 100.70 99.25 98,99

, an 0.293 0.155 0.297 0.314 0.389 0.r8i 0.179 0.180 P 0,195 0.265 0.157 0.230 0.529 P 0.115 0.479 3 ab 0.705 0.832 0.701 0.583 0.508 0.817 0.817 0.817 0.798 0.732 0.836 0.751 0.463 0.879 0.515 E or 0.002 0.002 0.002 0.003 0.003 0,002 0.004 0.003 0.007 0.003 0.007 0.009 0,008 0.006 0.006 t(0x) 100.88100.39 100.55100.78 101.09 99.73 99.92r00,47 100.49100.35 r00.53 100.25 100.41 100.r0100.94

astoichimtry is basedon 22 orygensfor nicas, endremberproportions for garnets, feldspars, and ilmnite, 28 orygensfor chlorite, and 25.53 (si+Al) for .staurolite. P lndicates nineral-present but not abundantenough for anatyiis. A after nurber indlcates specirencontains tl2 andalusite. ovaluesunderlined are aDDroxiMtecalculated values for (H+Li)-and[(0x). If Li is assunedto be 0.3 pfu, the reminderls-an approximtelndication of H. cll content of thls staul.blite is about 1.39 pfu and variable. Thenulser usedhere is from a mre accurateAA analysis from an otheruise identical staurolite from the sare outcrop (Dutror et al., 1986).

Note added in proof. We have recently observedthat the four andalusite-bearingrocks of grade4 (App. Table l, Fig. 1) from the Augusta quadrangleare compositionally distinct from all other rocks in grade 4. These andalusite-bearingrocks contain higher Mg/Fe in staurolite, biotite, and garnet and higher spessartine+ grossularin garnet, indicating that they representthe compositional limit of staurolite stability. Becausethese specimensare compositionally unique, the andalusite need not have an M, origin. Other evidence(e'g., local cordierite) suggestsincreasing P during metamorphism. Novak and Holdaway ( 198l) suggestedthat M, and M3 tend to merge in time and metamorphic conditions in this area. The probability that andalusite formed or re-equilibrated during M, in the Augusta area in no way affectsthe conditions of metamorphism deducedfor the region. This matter will be consideredin detail in a future contribution.