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

Polymetamorphism in medium- to high-grade pelitic metamorphic rocks, west-central Maine

M. J. HOLDAWAY Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 CHARLES V. GUIDOTTI» Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 JAMES M. NOVAK Air Force Geophysics Laboratory, Hanscom Air Force Base, Massachusetts 01731 WILLIAM E. HENRY 235 G.B., Research Center, Phillips Petroleum Company, Bartlesville, Oklahoma 74004

ABSTRACT throughout the area at a given time, but range in grade from chlorite zone to K- increased about 0.5 kbar or more between feldspar-sillimanite zone (Osberg and oth- A regional petrologic study of pelitic M2 and M3 metamorphism. The more ers, 1968). metamorphic rocks has been undertaken, regional distribution of isograds to the With the exception of a small syenite with emphasis on the area bounded by the south is consistent with higher initial tem- body near Litchfield and two gabbro intru- cities of Augusta, Lewiston, Norway, peratures and more closely spaced mag- sions (Fig. 1), the plutonic rocks are biotite- Rangeley, Kingfield, and Madison. This matic heat sources, but not necessarily with muscovite-garnet quartz monzonites and report involves a regional isograd map, increasing pressure to the south. The pres- biotite-garnet granodiorites of the New interpretation of the nature of the various sure increase between M2 and M3 might Hampshire Magma Series. For the purpose reactions, and discussion of petrologic prob- have been produced by intrusion of magma of this report, the plutons are identified as lems of polymetamorphism, isograd distri- above the present level and/or extrusion of batholiths or groups of smaller plutons bution, and widespread occurrence of mus- rhyolitic volcanics. (Fig. 1) that may be cupolas or roots of covite in the K-feldspar-sillimanite zone. A batholiths at deeper or shallower levels. future contribution will deal more specifi- INTRODUCTION From west to east, the major centers of plu- cally with mineral chemistry, geotherrnome- tonic activity are (1) the Mooselookmegun- try, and geobarometry of much of the area. West-central Maine is unique from a tic batholith (2) the Phillips batholith, (3) After a low-grade metamorphism, Mi, petrologic standpoint because, as noted by the Livermore Falls group with the Skow- the region suffered a higher-grade early numerous previous workers, it is in the hegan batholith at the northeast end, and metamorphism, M2, which in many places transition zone from isolated plutonic rocks (4) the Hallo well group. On the northern produced andalusite and cordierite as well of the New Hampshire Magma Series and edge of the area are the Reddington and as staurolite. This was followed by a some- associated contact metamorphism on the Lexington batholiths, and in the southwest what higher-pressure metamorphism, M3, north to regional metamorphism with asso- part of the area is the composite Sebago involving staurolite-chlorite and sillimanite. ciated plutonic bodies on the south (Doyle, batholith, the largest plutonic complex in In some areas, M3 has produced extensive 1967). The area involved in this report is Maine. retrograde effects. With increasing grade in within a polygon bounded by the cities of Previous work in the area has brought to M3, the major mineral reactions are (1) Augusta, Lewiston, Norway, Rangeley, light a number of significant problems, formation of staurolite from reaction of Kingfield, and Madison, Maine (Fig. 1). some of which need to be considered using garnet, chlorite, and muscovite; (2) forma- Although three or more metamorphic the regional approach involved in this tion of sillimanite from staurolite, chlorite, episodes were involved, the associated plu- study. Moreover, an overview will serve to and muscovite; (3) reaction of staurolite to tonic rocks span a narrow age range better identify remaining questions. The form sillimanite, biotite, and in many rocks, between 395 and 360 m.y. (Guidotti, 1970a; first is the problem of polymetamorphism. garnet; and (4) formation of K-feldspar Dallmeyerand Vanbreeman, 1978; Moench Can the various metamorphic events found from the partial reaction of muscovite, and Zartman, 1976). The earliest metamor- in parts of the area be traced and correlated quartz, and plagioclase. A possible fourth phic event affected Early Devonian rocks, throughout the area? Guidotti (1970b) metamorphism, M4, produced narrow au- and the latest is associated with 360-m.y.- found evidence for an early low-grade reoles with andalusite and staurolite around old plutons. Thus all the metamorphic metamorphism, Mi, followed by at least the three latest plutons. events were associated with the Acadian two major metamorphic events in the Range- Analysis of the isograd pattern demon- orogeny, at least in the broad sense (Naylor, ley and surrounding quadrangles. These strates that the pressure was rather uniform 1971). The metamorphic rocks are domi- are M2, which produced staurolite, andalus- nantly pelitic, with lesser amounts of meta- ite, and cordierite in pelitic rocks; and M3, graywacke and calcareous rocks, and they which produced sillimanite in pelitic rocks •Present address: Department of Geological Sciences, University of Maine, Orono, Maine are mainly Silurian and Early Devonian in at appropriate grades, but no andalusite or 04473. age (Pankiwskyj and others, 1976). They cordierite. In the eastern Augusta Quadran-

Geological Society of America Bulletin, v. 93, p. 572-584, 3 figs., 2 tables, July 1982.

572

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1 2 3 4

5 6 7 6

9 10 II 12

13 14 15 16

1 Rangeley 2 Phillips 3 Kingfield 4 Anson 5 Rumford 6 Dixfield 7 Farmington 8 Norridgewock 9 Bryant Pond 10 Buckfield 11 Livermore Falls 12 Augusta 13 Norway 14 Poland 15 Lewiston 16 Gardiner

MINERAL ASSEMBLAGES 1 Chi 2 Biot-Chl° 3 Alm-Biot-Chl° 4 St-Alm-Biot-Chl0 5 Sill-St-Alm-Biot0 6 Sill-Aim-Biot0 7 Ksp-Sill-Alm-Biot a And c Cord 0 Aim or Chi missing in soma specimens

ISOGRADS S Staurolite Si Sillimanite KS Ksp-Sill 4 M4

Figure 1. Isograd map of west-central Maine. Control for the isograds is based on representative localities shown and on references given in text. Isograd designation is on high-grade side. Mineral assemblages are given in legend; abbreviations: Chi = chlorite, Aim = almandine-bearing garnet, Biot = biotite, St = staurolite, Sill = sillimanite, Ksp = K-feldspar, Cord = cordierite, And = andalusite. Table 1 footnote gives additional phases present. Underlined numbers refer to data of Guidotti and various co-workers at Wisconsin. Other numbers refer to data of Holdaway and Novak included in Table 2. Acidic plutonic units, stipple: MB = Mooselookmeguntic batholith, PB = Phillips batholith, LG = Livermore Falls group, HG = Hallowell group, RB = Reddington batholith, LB = Lexington batholith, SkB = Skowhegan batholith, SeB = Sebago batholith. Random line pattern: late gabbro or syenite. The M4 isograds around the Lexington and Skowhegan batholiths are largely schematic. Northwestern Anson Quadrangle is under study by Holdaway; Pankiwskyj (1979) shows extensive andalusite and staurolite, suggesting a northeastern extension of the M2 isograd.

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gle, 75 km southeast of Rangeley, Osberg The present study is part of a long-term dominant metamorphism of the area is M3, (1968, 1971, and 1974) described only one effort in the region by several workers. The with reaction isograds shown by solid lines. metamorphism and concluded that the work falls broadly into two categories: (1) An M2 staurolite isograd is shown by a sillimanite-bearing rocks of the southern description and understanding of the petrol- dashed line with long dashes, and an M4 part of the area represent the higher ogy and metamorphic history of the area staurolit e isograd is shown by a dashed line temperatures of the metamorphism that and (2) determination of mineral chemistry with short dashes. Justification for the produced extensive andalusite and cordier- and distribution of temperature, confining chronology shown in Figure 1 will be pro- ite to the north. In the western Augusta pressure, and Xh2o in the pelitic rocks dur- vided in the sections that follow. Only the Quadrangle, Novak and Holdaway (1981) ing each metamorphic episode. The present staurolite, sillimanite, and K-feldspar-sil- found evidence that, at medium grades, paper involves the petrology and metamor- limanite isograds are shown. Pluton config- chlorite has overprinted low-pressure min- phic history, including a careful analysis of urations are based on the 1973 Preliminary eral assemblages; at slightly higher grades, pelitic mineral equilibria as a basis for Litholoj;ic Map of Maine compiled by P. H. sillimanite has formed from reaction of future work on mineral compositions, geo- Osberg ( Maine Geological Survey). chlorite and staurolite. The pattern that thermometry, and geobarometry. This re- appears to be emerging for the whole region port combines data collected by Guidotti SUMMARY OF extending from Rangeley to Augusta is that and his co-workers mainly from west of METAMORPHIC ZONES of early metamorphism, M2, which pro- longitude 70° 15', with data collected by duced andalusite-biotite and, depending on Holdaway and his co-workers mainly from The metamorphic isograd map (Fig. 1) bulk Mg/Fe ratio, staurolite or cordierite; east of longitude 70° 15'. Results of the illustrates four major zones, the low-grade and that of a later, higher-pressure meta- work of the two groups coincide and com- zone, the staurolite zone (which locally con- morphism, M3, which produced sillimanite plement each other. tains andalusite and/or cordierite), the sil- and staurolite, but not andalusite or cor- During field seasons in 1977 and 1978, limanite zone (which locally contains anda- dierite. M3 shows a spacial relationship to Holdaway and Novak visited 210 localities. lusite), and the K-feldspar-sillimanite zone. several, but not all, of the plutons in the Several specimens were collected from most In order to understand the mineral reac- region. A late, low-pressure metamorphic localities, and 322 thin sections of pelitic tions and isograd configurations, it is neces- event around the northern plutons (here rocks were examined. Emphasis was placed sary to have a tentative sequence of events. defined as M4) must also be considered. on pelitic rocks with Mg/ Fe ratios near one, The sequence given below is based on pre- A second closely related problem involves in an effort to minimize compositional vious work in specific areas. More-detailed the extensive areas west of Phillips and effects. All rocks reported here contained documentation of the evidence is available Farmington and near Kingfield (Fig. 1), muscovite. Psammitic rocks with abundant from Guidotti (1970a, 1970b, 1974), Henry where staurolite-bearing rocks have been biotite, plagioclase, K-feldspar, or quartz (1974), Novak and Holdaway (1981), Boone retrograded to garnet-, biotite-, or chlorite- were not included, nor were sulfidic gra- (1973), and Guidotti and Cheney (unpub. zone assemblages. In the simplest interpre- phitic rocks. For this reason, when cordier- data). tation, these areas represent staurolite-zone ite is reported here, it occurs in "normal" 1. Low-grade regional metamorphism M2 metamorphism overprinted by low- pelitic rocks rather than Mg-rich sulfidic (Mi) in the chlorite zone was widespread, at grade M3 metamorphism. graphitic rocks (Guidotti, 1975). In collect- least in the western part of the region (Gui- A third problem is the distribution of iso- ing and sectioning pelitic rocks, minimum- dotti, 1970b). Synchronous with this meta- grads apparently related to M3 metamor- variance specimens were emphasized, so morphism were formation of prevalent S2 phism (Fig. 1). In the northern part of the that samples were biased in favor of rocks schistosity (S] = bedding), which was axial region, isograds are closely related to pluton containing several of the phases staurolite, plane to large northeast-trending folds. Mi contacts. Farther south, isograds diverge garnet, sillimanite, andalusite, and cordier- is in places difficult to distinguish from later from contacts and appear to be more ite. Also used were the results of Evans and low-grade metamorphism. regional in nature. Even where the isograds Guidotti (1966), Guidotti (1965, 1970b, and 2. Regional metamorphism (M2) pro- are less clearly related to. pluton contacts, 1974), Henry (1974), and unpublished data duced staurolite, andalusite, and locally, they show sharp re-entrant angles, suggest- of Guidotti. These data were used in a cordierite in rocks of appropriate grade in ing local sources of heat. In some ins tances, fashion consistent with that employed for the Rangeley and Phillips Quadrangles the irregularities result from gently d ipping the Holdaway and Novak specimens. (Guidotti, 1970b) and in the eastern part of isogradic surfaces. Figure 1 shows the distribution of pelitic the Dixfield Quadrangle (Henry, 1974). In A fourth problem involves the occurrence mineral assemblages in the study area. The the area where this metamorphism is recog- of muscovite along with sillimanite and K- isograds shown are reaction-isograds, as nized, it postdates a prominent slip cleav- feldspar throughout the K-feldspar-silli- defined by Winkler (1979). Each isograd age, S3, the last major tectonic event in the manite zone. This could be a retrogressive marks the occurence of a particular reac- area. In these quadrangles, M2 is always feature, an XH2O buffering phenomenon, or tion, given later in this report. The best map affected by later (M3) metamorphism. In could reflect a leveling of the metamorphic control is in the Rangeley, Dixfield, Bryant those portions where later metamorphism temperatures, as suggested by Evans and Pond, Buckfield, and Augusta Quadran- was retrograde, staurolite and andalusite Guidotti (1966) for the Bryant Pond Quad- gles; elsewhere the mapping is of a more are seen as fine, micaceous pseudomorphs, rangle. This problem and the sporadic dis- reconnaissance nature. One important as- whereas rocks that experienced later meta- tribution of sillimanite-K-feldspar will be pect is interpretive—the relative timing of morphism of comparable grade have staur- considered in this report. the three metamorphic episodes shown. The olite pseudomorphs partly replaced by new,

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finer-grained, oriented staurolite (Guidotti, the textural arguments of Novak and Hold- In some specimens, biotite is partly replaced 1970b). In areas where later metamorphism away suggest that andalusite may have by chlorite, suggesting the possibility of two was higher in grade than M2, staurolite has been unstable by no more than 100 to 300 metamorphic episodes, M2 and M3. been destroyed and sillimanite is abundant. bars during M3 metamorphism by the Hal- Low-grade rocks from 10 km north of To date, no isograds have been mapped in lowell group of plutons. Augusta and near Farmington are banded recognized M2 metamorphism. Areas of 4. Contact metamorphism (here desig- gray phyllites and fine-grained schists, typi- early M2 metamorphism appear to bear no nated M4) affected 1-km-wide zones around cally with small garnet megacrysts. In thin relationship to existing plutons. No silli- the Reddington, Lexington, and Skowhe- section, the rocks show well-formed garnets manite-grade rocks have yet been ascribed gan batholiths (Moench, 1971; Boone, 1973; in a matrix of partially oriented phyllosili- to M2. Pankiwskyj, 1979). Relatively fresh assem- cates (B in Table 1). Biotite is the coarsest In the western 70% of the Augusta Quad- blages including staurolite, andalusite, and phyllosilicate, and chlorite is the finest. In rangle, Novak and Holdaway (1981) have cordierite are common. The tentative age specimens from the area north of Augusta, tentatively identified M2 metamorphism assignment, younger than M3, is based on muscovite randomly cuts across biotite and that has produced staurolite, andalusite, the fact that the associated plutons are the chlorite. In several specimens from near and cordierite and that has apparently been youngest in the area, with an age of 360 m.y. Farmington, garnet is partly pseudo- affected by a later event at similar grade. (Naylor, 1973). morphed by chlorite. Here, textural evidence is less distinct than Interpretation of metamorphic history in Low-grade rocks in the area between in the areas mentioned above, but textural west-central Maine suffers from two prob- the above-mentioned dashed isograds are and AFM topologic arguments (discussed lems. (1) M2 metamorphism cannot be rec- coarser and locally lighter in color than in more detail below) still support polyme- ognized with certainty unless it is at least of those to the east, and they show evidence of tamorphism in the area. In the Kingfield staurolite grade, contains andalusite, or the retrogression from the staurolite zone (Gui- and Anson Quadrangles, Pankiwskyj (1979) rocks show effects of later M3 metamor- dotti, 1970b). In hand specimen, the rocks has identified metamorphism that produced phism. (2) M3 metamorphism in the upper are greenish-black to light green with abun- staurolite and andalusite and that has been part of the sillimanite zone or the K- dant pseudomorphs. Many pseudomorphs extensively retrograded. These effects might feldspar-sillimanite zone obliterates effects have staurolite-like or andalusite-like shapes also be due to In both the Augusta and of M2. Description of the individual zones and are composed of fine mica with or Kingfield-Anson areas, the effects tenta- follows. without chlorite. One specimen contains tively ascribed to M2 show no relation to relict staurolite. There are also "islands" of distribution of present plutonic rocks. Low-Grade Zone nearly unaffected staurolite-mica schist in 3. Regional metamorphism (M3) at some- the Rangeley and Phillips Quadrangles, 7 what higher pressure produced either gar- With the exception of the Dixfield Quad- km west-northwest of Phillips and 2 km net-chlorite, staurolite-chlorite, or silliman- rangle, the low-grade zone (represented by southwest of the Reddington batholith on ite, depending on grade, in the area west of rocks containing muscovite and various State Highway 4. These assemblages are longitude 70° 15' (Guidotti, 1970b; Henry, combinations of chlorite, biotite, and gar- included in Table 1 (see D). Both Moench 1974; Fig. 1, this report). This metamor- net; assemblages 1-3 in Fig. 1) was studied (1971) and Guidotti (1970b and unpub. phism is clearly related to the Mooselook- in somewhat less detail than the other data) have shown such "island-like" areas in meguntic and Phillips batholiths. The sharp zones. Because of the limited number of the Rangeley and Phillips Quadrangles, re-entrant angles shown by the isograds localities visited and the complexities of where staurolite-grade rocks have apparent- between these plutons imply that M3 meta- mineral distribution and rock composition, ly survived retrogressive effects. Moench morphism was not everywhere synchronous M3 isograds were not drawn within this notes andalusite in some of the less-affected but instead was related in time to the intru- zone. However, on the basis of rocks stu- areas southeast of Rangeley, especially in sion of the plutonic rocks. died, the low-grade region can be broadly the Smalls Falls Formation. West of the Novak and Holdaway (1981) have tenta- subdivided according to the dashed isograd western dashed isograd, near Rangeley, tively identified the later sillimanite-pro- in the Kingfield and Farmington Quadran- pseudomorphs are absent, indicating lower ducing events in the Augusta Quadrangle as gles (Fig. 1). Most of the rocks east of the grade during the earlier metamorphism. M3 metamorphism. If true, this adds a third line appear to have never reached staurolite- Observed mineral assemblages of the ret- and a fourth lobe of igneous activity asso- zone conditions, whereas those to the west rograded rocks are listed in Table 1 (see C). ciated with M3. Novak and Holdaway pro- are retrograde from the staurolite zone. Establishing the correct grade for such vide evidence to show that the M3 meta- Similarly, in the Rangeley Quadrangle, rocks is difficult. Assuming external control rocks west of the dashed M2 isograd never morphism associated with the Hallowell °F XH2O. the retrograde reaction of stauro- group of plutons is somewhat older than reached staurolite grade, and those to the lite- or andalusite-bearing rocks must con- that associated with the Livermore Falls east and in the low-grade part of the Phillips sume staurolite, andalusite, and biotite as group of plutons. Quadrangle have been retrograded from the the garnet-chlorite tie line is established It should be emphasized that in none of staurolite zone. (Fig. 2), but some biotite should remain the areas presently designated as M3 meta- Low-grade rocks between Kingfield and stable with garnet and chlorite. These rocks, morphism was andalusite stable. Cordierite Farmington (Fig. 1) are phyllites and pri- however, show textural evidence of various was unstable in all rocks except the most marily contain chlorite, biotite, or chlorite- degrees of replacement of both garnet and magnesian sulfidic rocks (Guidotti and oth- biotite (A in Table 1). Some have porphy- biotite. The mineral-zone designations for ers, 1975). In the Augusta area, however, roblasts of interleaved biotite and chlorite. Figure 1 were commonly determined by

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TABLE 1. CHARACTERIZATION OF PELITIC ROCKS FROM VARIOUS studying more than one specimen from a MINERAL ZONES AND AREAS locality. Minerals that were only partly replaced were considered stable, and those Assemblage No. of thin sections largely replaced were considered unstable. The areas west of Farmington and south of A. Chlorite, biotite zones, prograde rocks, between Kingfield and Farmington Rangeley are mainly in the garnet zone, and 1. Chi 6 the remainder of the retrograded region is in 2. Biot-Chl 4 the biotite and chlorite zones. This ap- 3. Biot 4 proach presupposes a lack of complete B. Garnet zone, prograde rocks, north of Augusta, Farmington area 1. Alm-Biot-Chl 11 equilibration during retrogressive metamor- 2. Alm-Biot 1 phism. 3. Biot 2 Guidotti (1970b) interprets the retrograd- C. Chlorite, biotite, garnet zones, retrograde rocks, Farmington, Phillips, and Rangeley Quadrangles (in order of increasing grade) ed rocks of the Rangeley and Phillips Quad- 1. Chi 9 rangles as the result of low-grade M3 2. Biot-Chl 10 superimposed on staurolite-zone M2. The 3. Alm-Chl 7 M3 metamorphism produces a "hinge ef- 4. Alm-Biot-Chl 15 fect," downgrading the M2 rocks away from 5. St-Alm-Biot-Chl 1 the Phillips and Mooselookmeguntic batho- D. Staurolite zone, exclusive of Augusta area 1. St-Alm-Biot-Chl 20 liths and upgrading the M2 rocks nearer the 2. St-Alm-Biot 6 batholiths. It is logical to extend this rea- 3. St-Biot-Chl 3 soning east into the Kingfield and Farming- 4. Alm-Biot 1 ton Quadrangles. However, farther east in 2 5. Biot-Chl the northeastern Farmington and south- 6. Biot 1 E. Staurolite zone, northwest of Augusta eastern Kingfield Quadrangles, the grade of 1. St-Alm-Biot-Chl 26 M2 was lower, and one cannot distinguish 2. St-Alm-Biot 8 between Mi, M2, and M3. The retrogressive 3. St-Biot-Chl 2 effects were probably caused by water from 4. Alm-Biot-Chl 3 the plutons. The incomplete nature of these 5. Alm-Biot 1 6. Biot-Chl 1 effects could have resulted from inadequate 7. Biot 2 time and /or inadequate supply of water, AS A 8. And-St-Alm-Biot-Chl 7 allowing bed-to-bed variation of XH2O 9. And-St-Alm-Biot 6 result of local buffering. 10. And-St-Biot-Chl 5 11. And-Alm-Biot 3 12. And-Biot-Chl 1 Staurolite Zone 13. Cord-And-St-Biot-Chl* 3 14. Cord-And-Biot-Chl 2 15. Cord-St-Biot-Chl* 1 All rocks that contain staurolite-chlorite F. Andalusite zone, contact metamorphism and no sillimanite are designated staurolite 1. St-Biot-Chl 1 zone. Such rocks are subdivided into three . 2. St-Alm-Biot-Chl 1 categories: (1) regional staurolite-zone 3. And-St-Alm-Biot-Chl 1 rocks, exclusive of the Augusta area, (2) 4. And-St-Biot-Chl 3 5. And-Alm-Biot-Chl 1 regional staurolite-zone rocks from the area G. Sillimanite zone northwest of Augusta, and (3) rocks from 1. Sill-And-St-Alm-Biot (Chi 1) 5 the narrow contact zones of the Skowhegan 2. Sill-And-St-Biotj (Chi 2) 3 and Reddington plutons. 3. Sill-And-Alm-Biot (Chi 2) 9 4. Sill-St-Alm-Biot (Chi 5) 13 1. Regional staurolite-zone rocks col- 5. Sill-St-Biot (Chi 1) 3 lected by Holdaway (D in Table 1) exclusive 6. Sill-Alm-Biotf (Chi 17) 51 of the Augusta area contain neither anda- 7. Sill-Biot (Chi 5) 11 lusite nor cordierite. However, the more 8. Alm-Biot (Chi 2) 4 detailed studies of Guidotti and Henry 9. Biot 5 H. K-feldspar-sillimanite zone (all + plagioclase) reveal the presence of both minerals. In the 1. Ksp-Sill-Alm-Biot (Chi 6) 15 area south of Rangeley (Fig. 1), Guidotti 2. Ksp-Sill-Biot (Chi 1) 2 (1974) has found andalusite megacrysts in 3. Sill-Alm-Biot*» (Chi 5) 28 25% of the specimens, and cordierite meg- 4. Sill-Biot (Chi 3) 8 acrysts in a few. East of the central part of 5. Alm-Biot (Chi 1) 1 6. Biot 1 the Phillips pluton, Henry (1974) found andalusite in a few specimens. The andalu- Note: See Figure 1 caption for mineral abbreviations. All assemblages contain quartz, muscovite, sites are highly sieved and locally rimmed and ilmenite; most contain plagioclase; many contain graphite and tourmaline. Only specimens col- by muscovite. Guidotti (1970b, 1974) has lected by Holdaway and Novak are included. Refer to Figure 1 (underlined localities) and referenced presented arguments to show that the publications of Guidotti and his co-workers for mineral assemblages from west of longitude 70° 15'. andalusite and cordierite are metastable •Trace of garnet in three specimens. fCordierite present in one specimen each. (Chi 3) Retrograde chlorite present in three specimens. **Muscovite absent from one specimen. relicts from M2.

In most respects, the staurolite-zone Following the interpretation of Guidotti dimensionally isolated segments. The larg- rocks around the Phillips pluton and north (1974) and Henry (1974), we believe that all est crystals commonly show strain, as of the Livermore Falls group (Fig. 1) fit of these staurolite-zone rocks have most indicated by a lack of perfect optical conti- Guidotti's (1974) description of the Range- recently suffered M3 metamorphism, at nuity. At crystal boundaries, andalusite typ- ley rocks. The rocks are coarser than those approximately the same grade as the earlier ically becomes more skeletal and grades of the garnet zone and contain megacrysts M2 metamorphism. The rocks are a reason- into the matrix. In rocks containing anda- of garnet and staurolite. Garnet is typically able higher-grade continuation of the lusite and chlorite, much of the chlorite is in euhedral crystals with some quartz inclu- garnet-zone M3 rocks, but in this case ret- concentrated near andalusite grain boun- sions, while the staurolite contains more rogression was not always involved, because daries. In three specimens, sieved andalusite poikiloblastic quartz and shows less euhe- M2 metamorphism was also in the staurolite is partly rimmed by staurolite. The reverse dral form. The staurolite grew (or recrystal- zone or lower in grade. If cordierite devel- relationship has not been seen. Coarse poi- lized) after the formation of a slip cleavage oped during M2 in "normal pelites," all kiloblastic cordierite crystals contain abun- (S3 of Guidotti, 1970b), as shown by the evidence for it is now gone except in the dant inclusions of quartz and micas and are helicitic texture (Spry, 1969) in several area south of Rangeley (Guidotti, 1970 and partly rimmed by chlorite. Staurolite occurs specimens. 1974; Moench, 1971) and in the area near as coarse poikiloblastic megacrysts, with Variable amounts of chlorite are present Augusta to be discussed below. The miner- sharp boundaries, whereas garnet mega- in many samples. Its growth or recrystalli- alogy appears to have equilibrated to M3 crysts are still more euhedral and nearly free zation was late, as suggested by random partly as a response to emplacement of the of inclusions. In all cases, staurolite and orientation of plates or close association plutonic rocks. As discussed above, M3 was almandine have fewer inclusions and better with garnet or staurolite. In the majority of presumably not synchronous throughout crystal form than coexisting andalusite or rocks, chlorite does not continuously rim the region. cordierite. In most rocks, chlorite occurs as staurolite or garnet. While chlorite has 2. Rocks of the staurolite zone from randomly oriented laths. Micas define a partly replaced some portions of staurolite north and west of Augusta (Novak and weak foliation, with biotite coarser than grains, the textures suggest that at the cul- Holdaway, 1981) contain andalusite and/or muscovite and less oriented. mination of metamorphism, garnet, stauro- cordierite in many normal pelitic specimens In a north-south zone about 3 km east of lite, biotite, and chlorite were stable to- (E in Table 1; Fig. 1). Andalusite occurs the sillimanite isograd in the western part of gether in most rocks. Garnet was probably as large, irregular poikiloblastic crystals the Augusta Quadrangle, chlorite exten- stabilized in chlorite-bearing rocks by Mn with abundant inclusions of muscovite, bio- sively rims and replaces staurolite in many and Ca in solid solution, as suggested by tite, and quartz. Commonly these inclusions specimens, but in most such rocks, garnet is Thompson (1957). separate each crystal into numerous two- not rimmed (Novak and Holdaway, 1981). This suggests that metamorphism from the AI2O3 Livermore Falls group of plutons was late. In most of the rocks outside the north-south zone, chlorite appears to have developed in M, ALMANDINE equilibrium with associated staurolite, garnet, and biotite. Textures related to the Liv- ZONE ermore Falls lobe of M3 metamorphism are Aim ? not considered in the discussion that follows. The spacial distribution (Fig. 1) and complexity of mineral assemblages (Table 1) pose problems in the Augusta area. For the following discussion, let us assume the XH2O IN the fluid phase of the pelitic rocks

AI2O3 was maintained within a relatively narrow range. We then must consider whether that assumption is reasonable and what bearing M3 STAUROLITE M3 SILLIMANITE it might have on the possibility of polymetamorphism in the Augusta area (exclusive ZONE ZONE of the effects of the Livermore Falls group of plutons). In the Rangeley area, staurolite-bearing rocks normally show a rapid transition from rocks rich in staurolite to rocks richer in aluminum silicate, with increasing grade (Guidotti, 1974). In the context of the assemblages listed in Table 1 (see E), a rea- Figure 2. Idealized AFM projections for M2 and for garnet (almandine), staurolite, and sonable sequence of mineral zones in the sillimanite zones of M3. Only those portions of the diagrams which relate to observed Augusta area would include (1) staurolite- assemblages in pelitic rocks are shown. Each diagram is drawn for a limited range of chlorite-biotite, (2) staurolite-andalusite-bio-

temperature, pressure, and XH2O- tite, (3) staurolite-sillimanite-biotite, and (4)

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almandine-sillimanite-biotite. Although sil- producing staurolite and chlorite. The im- allowed internal buffering with respect to

limanite forms and staurolite disappears plication is that M3 was rather pervasive XH2O> leaving andalusite almost unaffected along distinct isograds (Fig. 1; Novak and and destroyed andalusite and cordierite in in some rocks. Because of the near-coinci- Holdaway, 1981; see below also), the forma- many of the staurolite-zone rocks. The vir- dence in time for the two events, the stauro- tion of andalusite cannot be mapped, tual absence of pseudomorphs poses a lite isograds coincide. After a short period because its occurrence is too irregular. problem. of cooling, the Livermore Falls phase of M3 Occurrences of andalusite-biotite aire ran- We now provide a revision and fine tun- metamorphism affected the rocks in the far domly interspersed with those of stauro- ing of the polymetamorphism concept as western Augusta Quadrangle. This model lite ± chlorite, indicating crossed tie lines on applied to the Augusta area. Most of the differs from that evoked by Guidotti (1970b

an AFM projection (assuming fixed XH2O)- minerals in the staurolite zone, excluding and 1974) in the Rangeley area in that he The variance of the assemblages is a use- chlorite, owe their origin to M2 metamor- proposed a distinct cooling between M2 and ful indicator of possible disequilibrium. If phism, at pressures such that andalusite and M3. This difference may explain the abun- we assume that the phases garnet, plagio- cordierite were stable. If plutonic activity dant development of pseudomorphs in the clase, muscovite, ilmenite, and quartz may existed, plutons were at depth and provided Rangeley area and their relative scarcity in be ignored because they each may reflect a no external fluid at the present level. The the Augusta area. non-AFM component or projection from random distribution of andalusite relative 3. On the basis of work by Moench quartz and muscovite, and consider only the to staurolite was related to variations in (1971), Boone (1973), and Pankiwskyj AFM phases andalusite, cordierite, stauro- fluid and bulk-rock composition and to the (1979), as well as our own limited sampling, lite, chlorite, and biotite, the staurolite zone existence of subhorizontal isotherms. With- a narrow staurolite-andalusite zone appears (exclusive of the Augusta and Rangeley out cooling, M2 metamorphism merged into to surround the Reddington, Lexington, areas) contains few univariant and no invar- M3 metamorphism with the emplacement of and Skowhegan batholiths. A very narrow iant specimens (D in Table 1), while the the Hallowell group of plutons and modest andalusite zone also surrounds the small staurolite zone of the Augusta area contains increase in pressure. As temperature rose pluton 8 km northeast of Farmington 15 univariant specimens (see 8, 10, 14, 15 near the plutons, the isotherms steepened, (Boone, 1955). Close to the contacts, silli- under E in Table 1) and three invariant and sillimanite began to form as water manite also occurs at least near the Red- specimens (13 under E in Table 1). If the moved through the rocks and produced a dington and Lexington batholiths (isograd assemblages represent equilibrium, assem- more constant value of XH2O (Ferry, not shown in Fig. 1). In the few specimens blages 8, 10, and 13 under E should line up 1980b). The effect of M3 on the staurolite- examined (F in Table 1), staurolite is minor, along an isograd indicating reaction of zone rocks was an H20-induced retrogres- and andalusite is locally rimmed or partly staurolite, chlorite, and muscovite to anda- sion of some rocks, as manifest by partial replaced by muscovite. Such narrow con- lusite and biotite, with only staurolite- destruction of cordierite and andalusite and tact zones imply that the country rocks were chlorite-biotite on the low-grade side and formation of additional staurolite or staurolite- not hot at the time and place of intrusion of andalusite-biotite-staurolite on the high- chlorite. Away from the plutons, these these plutons. This contact metamorphism grade side (Fig. 2). Inspection of Figure 1 slight retrograde effects could well have is given the tentative designation M4, based shows this is not the case. Similar arguments may be made concerning other reactions between phases in assemblages 13, 14, TABLE 2. TENTATIVE SEQUENCE OF ACADIAN METAMORPHIC and 15 under E. One is forced to conclude AND PLUTONIC EVENTS that either several assemblages in the Au- gusta area are disequilibrium assemblages Metamorphism Type Approximate Synchronous Age (m.y.) or that the Augusta rocks are unique in pressure batholith or group some way that produces additional degrees of freedom (for example, local variations in Reddingtontf M Contact 3.0 kbar Lexingtonft 360 ± 12* XH2O)- 4 HartlandTf** 360 ±1 If Two solutions to the problem have been Skowheganft proposed: (1) In reviews of an earlier ver- Period of cooling and pressure decrease sion of this report, John Ferry (also Ferry, L.ivermore Falls group 1980a) and P. H. Osberg argued strongly Sebagotf for a single metamorphic event and bed-to- M3 Regional-contact 3.8 kbar Phillips Mooselookmeguntic 379 ± 6§ bed variation of XH2O to explain the irregu- Hallowell group 387 ± 4j lar distribution of mineral assemblages. The 394 ± 8f explanation does not account for the tex- Period of cooling (except near Hallowell group) and pressure increase tures discussed above nor for the distinct sillimanite isograd in the area. (2) Novak M2 Regional 3.3 kbar No known plutons >394 ±8 and Holdaway (1981) have argued for two metamorphic events over much of the area. *Naylor (1973). The first (M2) produced andalusite, garnet, •fDallmeyer and Vanbreeman (1978). staurolite, and some cordierite, amd the § Moench and Zartman (1976). "The Hartland pluton is 15 km northeast of the Skowhegan, outside the map area. second (M3, related to the Hallowell. group) tfUnknown age relations to other plutons in category. destroyed andalusite and cordierite while

on its purely contact origin and on the rela- replaced by coarse-grained muscovite, as ite, and of the localities containing pelitic tively younger age of the Lexington batho- described by Guidotti (1968). This musco- rocks, over one-third have no specimens lith than those of the batholiths associated vite is a prograde feature as contrasted with with K-feldspar-sillimanite. In most local- with M3 (Table 2). fine-grained muscovite (and chlorite), which ities where two to four specimens were To summarize, most of the regional replaces staurolite in the low-grade zone collected from a several-hundred-metre in- metamorphic rocks of the staurolite zone during retrograde M3 metamorphism. In a terval, only one contained K-feldspar- have suffered an early M2 metamorphism few rocks, these coarse muscovite pseudo- sillimanite. Similar observations were made followed by later M3 metamorphism. Anda- morphs persist to the K-feldspar-sillimanite by Evans and Guidotti (1966) in the Bryant lusite and cordierite were best developed isograd. Almandine remains euhedral while Pond Quadrangle. In Figure 1, wherever during M2 in the east-central and western staurolite exists in the rocks but becomes one or more specimens from a locality con- parts of the region (Fig. 1). These minerals irregular in shape, scarce, and in places tain K-feldspar-sillimanite, that locality is were not stable in most "normal pelites" skeletal toward the upper part of the silli- labeled 7. In the area northwest of Lewis- during M3. The more complete preservation manite zone. Biotite crystals in the micace- ton, exposures are scarce, and most are of andalusite and cordierite during M3 in ous layers tend to grow coarser and more psammites or pegmatites. Of the few pelitic the Augusta area implies that pressure porphyroblastic with increasing grade. Chlo- rocks collected here, most contain no might have been somewhat lower here than rite occurs in 34% of sillimanite-zone spec- K-feldspar-sillimanite; more detailed sam- throughout the rest of the area during this imens and in 29% of specimens found in the pling would probably reveal additional first phase of M3. Contact metamorphism K-feldspar-sillimanite zone, as opposed to K-feldspar-sillimanite localities. (M4) produced andalusite and staurolite 70% of the staurolite-zone rocks (Table 1). The sporadic distribution of the highest around the northern and northeastern plu- Moreover, almost all of the chlorite in the grade specimens and the universal occur- tons, presumably after M3. rocks of sillimanite grade or higher is typi- rence of muscovite could be explained by cally anomalous blue, minor in amount, variable prograde effects or variable retro- Sillimanite Zone and clearly late and retrogressive after bio- grade effects. The prograde origin of most tite (post M3), compared with the anoma- muscovite is supported by the fact that it is lous brown apparently prograde (M3) chlo- The sillimanite zone is defined to include coarse and in some specimens still appears rite in most staurolite-zone rocks. the area between the first appearance of sil- as pseudomorphs after staurolite. More- limanite and the first appearance of K- Textural evidence concerning the first over, muscovite is rarely seen replacing feldspar with sillimanite (Fig. 1). Of the sillimanite-forming reaction is equivocal. other minerals. Prograde origin is also sup- sillimanite-zone mineral assemblages, those However, the disappearance of primary ported by the systematic chemical data on containing andalusite (see 1, 2, and 3 under chlorite at the sillimanite isograd and the muscovite given by Evans and Guidotti G in Table 1) have been found only in the field relations over the whole region support (1966), Cheney and Guidotti (1979), and areas west of Augusta, east of the middle of the reaction of staurolite, chlorite, and mus- Guidotti and others (1973). Sillimanite and the Phillips pluton (Henry, 1974), and covite to sillimanite and biotite. Thus, K-feldspar are unaltered in the majority of southwest of Rangeley (Guidotti, 1974). throughout the region, the sillimanite iso- rocks which contain them. Most significant Assemblages involving staurolite and/or grad has the same significance as that in is the fact that swirls of unaltered fibrolite andalusite are restricted to a zone within 2 the Rangely and Dixfield Quadrangles, occur in much of the coarse muscovite. If to 3 km of the isograd (LSZ of Guidotti, described in detail by Guidotti (1974) and the muscovite had formed retrogressively, it 1974). The remaining assemblages (G in by Henry (1974). Andalusite, where present, might have enclosed coarse sillimanite, but Table 1) are most common throughout the must also begin reacting to sillimanite at it is unlikely to have grown around fibrolite remainder of the zone (USZ of Guidotti, this stage. without altering it. Finally, there is no correlation between the minor retrogressive 1970a). The predominant metamorphism of the chlorite and the absence of K-feldspar- Texturally, the rocks are medium-grained sillimanite zone is M3, and it thoroughly sillimanite (H in Table 2). schists with garnet megacrysts. Sillimanite reconstitutes rocks which first experienced occurs as fibrolitic knots and coarse crystals M2. The only well-established remnants of The pattern described above extends that intergrown with the micas. Andalusite oc- M2 in the sillimanite zone are relict andalu- described by Guidotti and his co-workers in curs as granular masses in which the gran- site crystals seen in the Augusta, Rangeley, the southwest part of the area shown in ules become sparse and finally disappear and Dixfield Quadrangles (Fig. 1). Figure 1. We conclude that the observed at higher grades. With increasing grade, distribution of mineral assemblages is a andalusite decreases as sillimanite increases, K-Feldspar-Sillimanite Zone prograde effect of M3 resulting from a level- and the last andalusite disappears at about ing of horizontal temperature gradients at

the same grade as the last staurolite. Silli- The K-feldspar-sillimanite zone is de- variable XH2O- manite crystals may occur in andalusite, but fined to include the area from where K- most sillimanite is in other parts of the rock. feldspar first occurs with sillimanite, south REACTIONS AND EQUILIBRIA Throughout the whole region shown in Fig- to the Sebago batholithic complex. Many of ure 1, there is an inverse relationship the rocks in the zone contain coarse musco- A schematic sequence of AFM diagrams between sillimanite and staurolite, with vite visible in hand specimen, and almost all for pelitic rocks from the two metamorphic staurolite abundance decreasing rapidly as contain muscovite and quartz as seen in thin episodes is given in Figure 2. The occur- sillimanite becomes common with increas- section. Of pelitic specimens collected by rence of coarse staurolite with cordierite ing grade. Staurolite crystals become skele- Holdaway (H in Table 2), over one-half and muscovite in the lower-grade part of the tal and in some rocks are rimmed and have muscovite without K-feldspar-silliman- staurolite zone at West Sidney (Osberg,

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1971) implies the possible stability of this that follow, we have omitted quartz for a small Ti content, which must slowly assemblage during M2 near the beginning of simplicity and ignored the reaction coeffi- increase with grade (Guidotti and others, the staurolite zone, under certain conditions cients. The coefficients are left out because 1977). However, this Ti will not affect the (see also Holdaway and Lee, 1977). The they depend strongly on the formulae variance of the reactions. Sodic plagioclase alternative assemblage andalusite-biotite is chosen for the minerals. (Holdaway, 1980, has a similar tendency to buffer muscovite much more common and was probably sta- has made a case for writing such equations compositions with limited Na content. Pla- ble through most of the M2 staurolite zone. entirely with solid solution-free end mem- gioclase itself has a variable composition, Some occurrences of staurolite-cordierite- bers, and then comparing them with exper- however, and so the effect of Na could still muscovite (13 and 15 under E in Table 2), in imental or theoretical reaction curves, using result in an increased degree of freedom. which cordierite is coarse and staurolite activity models.) The relative partitioning of Isochemical metamorphism is assumed; is finer in grain size, could result from Fe and Mg between minerals as given by where large-scale movement of components the superposition of M3 conditions on Thompson (1976) is useful in deducing has occurred, some reactions must become an M2 assemblage, with the partial con- compositional changes for continuous reac- equilibrium relations. version of unstable cordierite to staurolite- tions, with Fe/Mg decreasing in the se- biotite-chlorite. quence almandine > staurolite > biotite > Staurolite-Forming Reaction Because all of the rock compositions lie chlorite. All of the reactions discussed below the garnet-chlorite join on an AFM below are model reactions in the simple sys- The first appearance of staurolite occurs diagram, no rocks contained chloritoid at tem K20-Fe0-Mg0-Al203-Si02-H20. The when the garnet-chlorite tie line is replaced low grade. The occurrence of garnet with effects of other components may be consi- by a staurolite-biotite tie line (Fig. 2). The chlorite-biotite-staurolite in the staurolite dered as dilutants to the existing phases, discontinuous reaction in the AFM model zone and garnet with sillimanite-biotite- which will add to the variance of each reac- system is staurolite in the lower sillimanite: zone tion. The universal presence of nearly pure probably resulted from Mn and Ca in the ilmenite buffers biotite and muscovite with Aim + Chi + Mus = St + Biot + W. (1) garnet stabilizing it beyond its normal stability range. The discussion that follows concerns only the equilibrium reactions that occurred during Mj. Andalusite and cordierite are excluded from further considera- tion. Figure 3, based on analysis of AFM reactions by Thompson (1976), schematically illustrates Fe end-member reactions (light lines) and Fe-Mg discontinuous reactions (heavy lines) among the phases muscovite, sillimanite, K-feldspar, biotite, staurolite, garnet, and chlorite. Invariant points A and B are metastable relative to assemblages with chloritoid and andalusite (A) or chloritoid only (B). All invariant points are Fe end-member invariant points, and each generates a discontinuous solid-solution reaction as Mg increases in the Fe-Mg phases. Arrows show the directions Fe invariant points would move if Mn and Ca were added, mainly diluting the garnet. Dashed lines show the positions of Fe-Mg reactions resulting from addition of a fixed, small amount of Mn and Ca. The sequence of reactions and equilibria taking place in west-central Maine may be deduced from Figure 3 and from AFM topologies, which in turn are based on thin- section petrography and the map distribution of assemblages. Thorough understanding of reactions allows prediction of how minerals will change composition or be- TEMPERATURE > come zoned with increasing temperature. Figure 3. Schematic pressure-temperature curves for Fe end-member model reactions Chemical observations, which will be part (light lines) and Fe-Mg solid-solution model reactions (heavy lines). Invariant points A and of subsequent reports on the area, m ay then B are metastable as are portions of curves radiating from them (see text). Arrows indicate be used to confirm the predictions of pro- the directions invariant points would move with addition of Mn + Ca, and dashed curves grade changes or demonstrate that retro- represent Fe-Mg solid solution curves for a fixed small amount of Mn + Ca. Additional grade effects have occurred. In the reactions abbreviations: Mus = muscovite, W = water.

Once staurolite forms, the complete reac- by plagioclase, and plagioclase composition The complete reaction is divariant over a tion assemblage persists in many rocks until changes little (Guidotti, 1974). Thus the Zn range of conditions due primarily to varia- sillimanite first appears. This divariance in and XH2O effects could each increase the ble XH2O and Na in muscovite and K- nature is explained by increasing Mn + Ca limited variance of equation 3. Sillimanite feldspar. [A balanced reaction involving all in garnet, increasing Na in muscovite, or appears by reaction 5 in chlorite-absent the phases and components would include increasing XH20- The most important single rocks. plagioclase as a reactant, as shown by Evans compositional effect is probably from Mn, Once sillimanite is present, garnet-bear- and Guidotti (1966). However, reaction 7 because plagioclase tends to buffer the Na ing rocks exhibit the continuous equilibrium adequately models the reaction for calcula- content of muscovite and the Ca content of tion of intensive parameters as long as activ- garnet so that they vary smoothly with Aim + Mus = Biot + Sill. (4) ity corrections are made.] The universal grade. presence of muscovite indicates that equili- In rocks that contain no chlorite, increas- However, this cannot become a reaction brium temperatures were never reached for = ing grade (temperature) is reflected in the until staurolite is gone. pure muscovite and Xh2o 1- continuous reaction The sequence of reactions given in Figure Disappearance of Staurolite 3 and discussed above may be used to St + Biot = Aim + Mus + W. (2) understand garnet or staurolite zoning pat- Once sillimanite has formed, staurolite terns. The problem is complicated by the The stability curve for this reaction has a begins to diminish in amount, even in the fact that zoning may be prograde or retro- nearly flat PT slope (Tracy and others, absence of chlorite. In rocks that contain no grade, a mineral (for example, chlorite) may 1976; Novak and Holdaway, 1981), so that garnet, the reaction may be modeled by the be late in origin, or the zoning may partly increasing temperature causes none of the continuous reaction reflect an earlier stage of metamorphism phases to react, grow, or change composi- when another mineral (for example, stauro- tion. This is consistent with the euhedral St + Mus = Biot + Sill + W. (5) lite) was also present. Clearly, a complete character of both garnet and staurolite in understanding of the predicted prograde this zone. This reaction marks the first appearance of changes in composition is needed to begin In chlorite-bearing garnet-staurolite sillimanite in chlorite-absent rocks. As reac- solving such problems. schists, equation 2 is an equilibrium, but the tion progresses, Mg/Fe decreases for all main reaction taking place as grade in- phases, staurolite increases in Zn, and XH2O REGIONAL SIGNIFICANCE creases is by equation 1. In the presence of may increase. OF ISOGRAD PATTERN stable chlorite, garnet decreases in amount In garnet-bearing rocks, staurolite break- and increases in Mn + Ca, and all phases down is by the AFM discontinuous reaction A long-term goal of work by various increase in Mg/ Fe. In order for equation 2 researchers in Maine is to establish as well to remain at equilibrium, increase in Mn St + Mus = Aim + Sill + Biot + W. (6) as possible the temperatures, pressures, and + Ca in garnet is compensated for by the mole fractions of components in fluids in increase in Mg/ Fe (Fig. 3). Thus, prograde The reaction in nature is divariant and, as pelitic rocks during each of the metamor- zoning behavior of garnet should differ reaction proceeds, staurolite increases in phic episodes such as M2, M3, and M4, de- depending on whether or not chlorite was a Zn, all phases decrease in Mg/Fe (because scribed in this report. This will not be stable phase. Moreover, garnet zoning pro- equation S must also maintain equilibrium), possible for some time due to the fact that vides a test for main-stage versus retrograde and garnet decreases in Mn + Ca as it grows some of the calculations depend on experi- chlorite. For the staurolite zone, the (Fig. 3; Novak and Holdaway, 1981). mental work still under way, such as that at AFM diagram should show little or With further increase in grade after the SMU hydrothermal laboratory, and on no counterclockwise rotation of the al- staurolite is gone, almandine and muscovite further field studies with associated micro- mandine-staurolite-biotite triangle and larger now react continuously to form sillimanite probe analyses. Nonetheless, at present, counterclockwise rotation of the staurolite- and biotite, according to equation 4. During qualitative statements may be made con- biotite-chlorite triangle with increasing reaction, garnet increases in Mn + Ca as it cerning the significance of the metamor- temperature. reacts, and Mg/Fe of the phases continues phism. to decrease. Sillimanite-Forming Reaction Polymetamorphism The staurolite-chlorite tie line breaks Appearance of K-Feldspar with Sillimanite when sillimanite first forms in west-central The areal limit of the M2 staurolite- Maine (Fig. 2). The discontinuous reaction Appearance of K-feldspar in pelitic rocks bearing rocks (Fig. 1, dashed line) is the is may be modeled by the reaction only approximation of an isograd for the earlier metamorphism that can be recog- St + Chi + Mus = Sill + Biot + W. (3) Mus = Sill + Ksp + W. (7) nized with any certainty. There appears to be no sillimanite isograd related to M2 The studies of Guidotti (1974), Henry In garnet-bearing rocks, the reaction signals metamorphism, although sillimanite did (1974), and Novak and Holdaway (1981) the termination of the almandine-muscovite form in the Augusta area during the transi- show that the reaction is nearly univariant reaction and the beginning of reaction of tion between M2 and M3. The random pat- and takes place over a narrow field interval. biotite-sillimanite to almandine-K-feldspar tern of development of andalusite versus Factors that might allow increased variance (Fig. 3) as temperature rises. Because mus- staurolite seen in the Augusta Quadrangle are variable Zn in staurolite, Na in musco- covite is never totally destroyed, conditions and apparently occurring in the Kingfield, vite, and XH2O- Na in muscovite is buffered are always at equilibrium for equation 7. Phillips, and Rangeley Quadrangles is con-

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sistent with very gently dipping isothermal more regional nature for M3 could have ments suggesting that M3 may have been at surfaces, bulk compositional control of been caused in part by a tendency for pluton a pressure greater than that of M2 (we sug- these phases, and local buffering of XH2O- contacts to dip less steeply in the south. gest 0.5 kbar greater). More intrusions and Heat sources for this metamorphism were Kane and Bromery (1968) have shown that contact metamorphism M4 followed at concentrated in the western and southern many of the plutons are sheet-like, and about 360 m.y. Limited evidence suggests parts of the area. However, the lack of any Moench and Zartman (1976) have corre- that this intrusion and the associated con- clear relationship to plutonic bodies sug- lated sheet-like plutons with areas of high- tact metamorphism occurred as a discrete gests that if increased grade was related to grade metamorphism. The local geothermal event: the ages of the Lexington and Hart- plutonism, it was at considerably greater gradient at the time of inception of M3 must land pluton (15 km northeast of the Skow- depths than the present level of exposure. also have been a factor. The southern part hegan batholith) are at the lower limit of On the basis of the widespread occurrence of the region may have been intruded by the available ages for the area, the meta- of andalusite, staurolite, and muscovite, larger amounts of magma not only at the morphism is entirely contact in nature, and pressure of 3.3 ± 0.4 kbar and temperature present level of exposure but also at shal- over distances of 7 km or less, medium- of 525 ± 30 °C are estimated (Richardson, lower and deeper levels, such that convec- grade metamorphism changes from stauro- 1968; Holdaway, 1971) for the staurolite tive transfer of heat and water was more li te-only (M3) to andalusite-dominant (M4) zone of M2. important during metamorphism. This south of the Reddington batholith and east The metamorphic isograds for M3 are would have resulted in a distribution of of Farmington. Pressure of M4 was pre- clearly related to the distribution of granite isograds less controlled by individual plu- sumably comparable to or slightly lower plutons, excluding the Reddington, Lexing- tons. Furthermore, if M2 and M3 were not than that of M2. ton, and Skowhegan batholiths. In the widely separated in time as proposed above In west-central Maine, one of several fac- northern part of the area, the M3 staurolite for the southeast part of the area, the areas tors may have been responsible for a pres- and sillimanite isograds follow nearly paral- most strongly affected by M2 may not have sure increase over a relatively short period lel within 1 to 2 km of the existing plutonic cooled to ambient temperatures before of time: (1) deposition of an additional 2 km contacts but gradually diverge to as much as inception of M3. of sediment during the period involved; (2) 7 km from the contacts at the southern lim- Finally, Dallmeyer and Vanbreeman overthrusting, such that 2 km of nappe its of the same plutons or plutonic groups. (1978) have dated plutonic (Table 2) and structure was added to the overburden; (3) In the southwestern part of the region, the metamorphic rocks near the eastern edge of emplacement of large volumes of magma K-feldspar-sillimanite isograd roughly par- the study area. Their work suggests progres- into the overlying rocks to produce the allels the large Sebago batholithic complex sively younger cooling ages of the meta- major uplift of the area into a mountain at distances between 5 and 30 km from the morphic rocks to the south. This probably system; or (4) extrusion of vast amounts of contact. However, the position of this iso- reflects hotter initial temperatures and, rhyolitic volcanics on the surface to produce grad is also affected by the Livermore Falls thus, longer cooling times to the south. the necessary overburden increase. The first group of plutons. As Guidotti (1974) has Their results do not support or conflict with two factors can probably be eliminated pointed out, individual isograds are trun- the polymetamorphic model proposed here, because such a sedimentary sequence would cated by the plutons at acute angles. Near because their metamorphic rock ages pre- require sedimentation at a time of uplift, the plutons, isothermal surfaces steepened sumably date the last cooling of the area. and most or all of the tectonic activity considerably due to heat conduction and However, their results appear to be compat- appears to have preceded M2. The last two convection of water and heat (Ferry, 1980b). ible with the increasingly more regional factors, possibly in combination, appear to This effect was apparently most pronounced aspect to the metamorphism in a southward be viable explanations for a pressure in- near the sillimanite isograd, where XH2O direction. crease on the order of 0.5 kbar. Possible was externally controlled, leading to a well- pressure decrease before contact metamor- defined isograd. At low grades, retrogres- Variation of Pressure with Time phism M4 would have resulted from a 20 sive effects of M3 were variable and prob- m.y. period of erosion during a time of ably related to flatter isothermal surfaces reduced igneous activity, followed by more Variation of pressure throughout the and more internal buffering of XH2O- At intrusion. region under study appears to be more highest grades, isothermal surfaces again related to time than to map position. All of became nearly horizontal near the Sebago the metamorphic events are Acadian in age; Relation between Metamorphic batholithic complex as igneous tempera- they postdate the youngest exposed Lower and Plutonic Events tures were approached. M3 becomes in- Devonian rocks (Pankiwskyj and others, creasingly more regional in nature to the 1976) and are older than or synchronous The sequence of events given in Table 2 south, but there is presently no compelling with the youngest plutonic rocks at 360 m.y. must presently be considered as a working evidence for or against an increase in pres- (Table 2). M2 slightly predates the intrusion model. The broad aspects of the plutonic sure to the south during this event. Pressure of the Hallowell group as discussed pre- sequence are based on relative ages and the of metamorphism during M3 was on the viously and is thus older than 394 m.y. M3 nature of the associated metamorphism. order of 3.8 kbar, such that neither andalus- was synchronous with intrusion of several The sequence of plutonic events during M3 ite nor kyanite formed (Novak and Holda- plutonic groups dated from 394 to 379 m.y. metamorphism is based on how well M2 way, 1981; Guidotti, 1974). The change to a (Table 2). Guidotti (1970b) presented argu- andalusite and cordierite are preserved in

M3 "normal pelitic" rocks. This is predi- location and partly related to time. The map of Maine: Maine Geological Survey, cated on the assumption that pressure metamorphic map of New England (Thomp- scale 1:500,000. son and Norton, 1968) shows that the area Evans, B. W„ and Guidotti, C. V., 1966, The increased slightly during M3. Minor differ- sillimanite-potash feldspar isograd in west- ences in pressure with map location might of this report marks the main southward ern Maine, U.S.A.: Contributions to Miner- have also played a role. Clearly, the se- transition from contact to regional meta- alogy and Petrology, v. 12, p. 25-62. quence of plutonic events within M3 is not morphism. Hence, some of the ideas ex- Ferry, J. M., 1980a, A comparative study of well known. Lack of information on relative pressed here might be applied elsewhere in geothermometers and geobarometers in peli- New England. We also emphasize that tic schists from south-central Maine: Ameri- age within a metamorphic episode is desig- can Mineralogist, v. 65, p. 720-732. nated by a footnote in Table 2. paleopressures relating to the triple point 1980b, A case study of the amount and dis- The high degree of preservation of M2 isobar (Thompson and Norton, 1968) or tribution of heat and fluid during metamor- andalusite and the apparent coincidence of bathograds (Carmichael, 1978) should also phism: Contributions to Mineralogy and M2 and M3 staurolite isograds northwest of take into account variation of pressure with Petrology, v. 71, p. 373-385. time and the possibility that lateral differ- Guidotti, C. V., 1965, Geology of the Bryant Augusta are consistent with a tendency for Pond Quadrangle, Maine: Maine Geological M2 and M3 to converge in temperature, ences in pressure may in some cases result Survey, Quadrangle Mapping Series No. 3. pressure, and time in this area. The close from different time of metamorphism. 1968, Prograde muscovite pseudomorphs relationship between plutonism and meta- after staurolite in the Rangeley-Oquossoc morphism for M3 and M4 is obvious. Each areas, Maine: American Mineralogist, v. 53, ACKNOWLEDGMENTS p. 1368-1376. batholith or plutonic group may represent a 1970a, The mineralogy and petrology of the separate pulse of metamorphism. The sharp Research support for this project was transition from lower to upper sillimanite re-entrant angles between plutonic groups provided by National Science Foundation zone in the Oquossoc area, Maine: Journal provide strong support for this suggestion grants EAR7612463 and EAR8025891 to of Petrology, v. 11, p. 277-336. (Fig. 1). However, the distribution of iso- 1970b, Metamorphic petrology, mineralogy Holdaway and EAR7704521 and EAR and polymetamorphism in a portion of N. W. grads does not allow one to conclude that 7902597 to Guidotti. The Maine Geological Maine: Guidebook for field trips in the Range- the plutons were the only heat source for the Survey provided support and field assist- ley Lakes-Dead River Basin region, west- metamorphism. More probably, the plu- ance. John Ferry, Philip Osberg, and Jef- ern Maine: New England Intercollegiate Geological Conference, 62nd Annual Meet- tonic intrusions during M3 modified the frey Warner provided constructive reviews. gross distribution of isograds in a terrane ing Field Trip B-2, p. 1-23. 1973, Compositional variation of muscovite that was transitional between contact and REFERENCES CITED as a function of metamorphic grade and regional metamorphism. The leveling of the assemblage in metapelites from N.W. Maine: highest-grade temperatures south westward, Contributions to Mineralogy and Petrology, Al-Mishwt, A. T., 1972, Contact metamorphism v. 42, p. 33-42. toward the Sebago batholith, a muscovite- and polymetamorphism in northwestern 1974, Transition from staurolite to silliman- bearing pluton, does support a close cause Oquossoc Quadrangle, Maine [M.A. thesis]: ite zone, Rangeley Quadrangle, Maine: Geo- Madison, Wisconsin, University of Wis- and effect relation between metamorphism logical Society of America Bulletin, v. 85, p. consin. and plutonism. 475-490. Bickel, C. E., 1974, Metamorphism in the Belfast area, Maine: New England Intercollegiate Guidotti, C. V., Cheney, J. T„ and Conatore, P. We hope that future work on igneous D., 1975, Coexisting cordierite + biotite + rocks of the area will concentrate on subdi- Geological Conference, 66th Annual Meet- ing Field Trip A-l, p. 1-22. chlorite from the Rumford Quadrangle, vision of the New Hampshire Magma Series Boone, G. M., 1955, Petrology of the Farming- Maine: Geology, v. 3, p. 147-148. into subseries, and on dating of emplace- ton, Maine area, in Trefethen, J. M., Report Guidotti, C. V., Cheney, J. T., and Guggenheim, ment times in the hope of documenting and of the state geologist 1953-1954: Maine S., 1977, Distribution of titanium between Development Commission, p. 46-62. coexisting muscovite and biotite in pelitic improving on the proposed sequence and its schists from northwestern Maine: American 1973, Metamorphic stratigraphy, petrology relation to metamorphic events. Mineralogist, v. 62, p. 438-448. and structural geology of the Little Bigelow Mountain map area, western Maine: Maine Guidotti, C. V., Herd, H. H„ and Tuttle, C. L„ CONCLUSIONS Geological Survey Bulletin 24. 1973, Composition and structural state of K- Carmichael, D. M., 1978, Metamorphic batho- feldspars from K-feldspar + sillimanite grade rocks in northwestern Maine: American The metamorphic events we have sum- zones and bathograds: A measure of the depth of post-metamorphic uplift and ero- Mineralogist, v. 58, p. 705-716. marized occur over a distance of 100 km, sion on the regional scale: American Journal Henry, W. E., 1974, Metamorphism of the pelitic from Oquossoc to Augusta. The work of of Science, v. 278, p. 769-797. schists in the Dixfield Quadrangle, NW Guidotti (1970b) and Al-Mishwt (1972) Cheney, J. T., and Guidotti, C. V., 1979, Maine [Ph.D. dissert.]: Madison, Wisconsin, Muscovite-plagioclase equilibria in silliman- University of Wisconsin. shows that similar polymetamorphic rela- Holdaway, M. J., 1971, Stability of andalusite tionships may extend west into northern ite + quartz bearing metapelites, Puzzle Mountain area, northwest Maine: American and the aluminum silicate phase diagram: New Hampshire. Such effects probably also Journal of Science, v. 279, p. 411-434. American Journal of Science, v. 271, extend eastward toward the coast of Maine Dallmeyer, R. D., and Vanbreeman, O., 1978, p. 97-131. (Bickel, 1974; Guidotti, unpub. data). «Ar/^Ar and Rb-Sr ages in west-central 1980, Chemical-formulae and activity models for biotite, muscovite and chlorite appli- If the apparent trends of plutonic and Maine; their bearing on the chronology of tectono-thermal events: Geological Society cable to pelitic metamorphic rocks: Amer- metamorphic ages shown in Table 2 are of America Abstracts with Programs, v. 10, ican Mineralogist, v. 65, p. 711-719. valid, the distribution of contact versus p. 38. Holdaway, M. J., and Lee, S. M„ 1977, Fe-Mg regional metamorphism is partly related to Doyle, R. G. (ed.), 1967, Preliminary geologic cordierite stability in high-grade pelitic rocks based on experimental, theoretical, and nat-

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