CanadianMineralogist VoL 12, pp. 469-47aQ97a)

S UB.GR EENSCI-IIST METAMORPHIC ASSEM.BLAGES IN N ORTHERN MAINE

DOROTIIY A. RICHTER* eNo DAVID C. ROY**

AssrRAcr vided to us by R. Horodyski; these samples allowed us to incorporate their assemblagein- Prehnite-pumpellyite facies mineral assemblages formation on the map shown in Figure 2. Our have Ordovician Silurian rocks beenrecognized in and determine the extent of the in northern Maine. Systematicvariation of ttre min- purpose was to already eral asse.mblagespermits the mapping of three zones prehnite-pumpellyite facies in a region of increasing grade within the prehnite-pumpellyite extensivelymapped by Roy & Mencher (in press) facies: a prehnite-analcimem\e, a prehnite-pumpel- and shown to contain rocks of suitable composi- lyite zone,and a pumpellyite-epidote-actinolitezone. tion to support mineral assemblagesindicative Local variations in pco, are suggestedto explain the of sub-greenschistmetamorphism. In addition, erratic distribution of chlorite-calcite assemblages. we wished to determine the distributions of sub- Acadian metamorphismdid not exceedthe prehnite- facies vrithin tle prehnite-pumpellyite facies sug- pumpellyite facies. However, the presenceof litlic gested,but not mapped, by Coombs er al. clasts showing prehnite-pumpellyite assemblagesin Silurian greywackessuggests tlre possibility of Taco- nian metamorphiunto the samegtade. GBNnner, Gsorocv

INtnooucrrox The geology of central Aroostook County' Maine (Fig. 1) has been mapped by Boucot ef (1964), (1968), Roy (1970), Boone Several axens of sub-greenschist metamorph- ol. Horodyski (1970), Roy & Mencher (in press). Rocks ism have recently been described in the north- and from Middle Ordovician to ern Appalachians. The first report of the preh- ranging in age Devonian are exposd in the nite-pumpellyite fasies in lower Paleozoic rocks Early Middle Pennington Mountain Anti- of the northern Appalachians region is that of northeast-trending Asbland Synclinorium and numer- Coombs et aL. (197O) in which they describe clinorium and anticlines and synclines such as the the facies in the Big Machias Lake region of ous smaller Anticline and the Chapman Syn- Aroostook County, Maine (Fig. 1). They found Castle Hill The Pennington Mountain Anticlinorium diagnostic metamorphic assemblages in inter- cline. extension of the Munsungun mediate to mafic volcanic rocks and grey- is the northeast of Hall (1970) and the Bronson wackes ranging in age from Middle Ordovician Anticlinorium Hill-Boundary Mountain Anticlinorium. The to Early Devonian. Zen (in press), Mossman & oldest rocks exposedin the core of the Penning- Bachinski (L972), and Papezik (1972) have re- Mountain Anticlinorium are Ordovician, but ported other occurrences of sub-greenschist ton complex synclinal infolds of Siluro-Devonian facies mineraL assemblages in Pennsylvania, rocks are present. Silurian and Devonian rocks New York, New Brunswick, and New- Quebec, predominate in the Ashland Synclinorium which foundland. to the east of the Pennington Mountain This report summarizesthe results of a petro- lies Anticlinorium and to the west of the Aroostook- graphic study of rocks, from a large region to Matapedia Anticlinorium of Pavlides et al. the north and east of the area studied by Coomibs (r964). et al. (1970), which are discussedin more de- tail in Richter & Roy (MS submitted 1974).In- The main rock types in the area are shale cluded in our study were thin sections exam- and slate, lithic greywacke and areniteoconglo- (with ined by Coombs et al. which were kindly pro- merate abundant volcanic clasts), subma- rine mafic and intermediate volcanic rocks (in- cluding abundant pyroclastic rocks), and bedded chert @oy & Mencher, in press).The two forma- * Dept. of Earth and Planetary Sciences, Massa- tions which are of most interest to this study chusetts Institute of Technology, Cambridge, are the Winterville and Frenchville formations, Mass. 02139. the distributions of which are shown in Figure 1. x*Dept. of Geology and Geophysics, Boston Col- The Winterville Formation is Middle to Late lege, Chestnut Hill, Mass. 02167. Ordovician in age. It is exposed in the core of 469 470 THE CANADIAN MINERALOGIST

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DEVONIAN SILURIAT{ ORDOVIGIAN/SILURIAN ORDOVIOIAN @ uoptoron rn. Ilil ,lratlcrr Fn. @l toorloot Rir.r Fn @ xcdororto Lal. Fm. tr lairoslv. Root. Fnaolrlllr Fa. El E ca?r! Iills Fm. El wlnf.rvill. Fm. (lnclud.r liil Dogtcadorff €roop lgd iN Sr.d.o Fn. Pyl€Xt. ArgillitE in Co!tl3Hill Aaticlino) iill uadttteronrtoreo B Sprogurvrtle Fm. (lcdlll.d tto6 Rot ed I.ac!.r,la SILURIAN/OEVONIAN J6l upprr Sllrrlon Un.litt Frtr) lGl rosetta Hlll Fra. Frc. I. Generalized geologic and tectonic map of central Aroostook County, Maine. The inset of Maine shows the location of the study area and the boundary of Aroostook County. the Pennington Mountain daligliasrirrm and in pathic sandstonemember, sandstone-slatemem- the cores of anticlines in the Ashland area and ber, and the quartzose sandstonemember. The ttre Castle ltrill Anticline. The formation is a greywacke and conglomerate members contain series of more than 1500 m of mafic to inter- abundant pebbles of volcanic rocks clearly de- mediate volcanic rocks in'terbeddedwith shale, rived from the Winterville Formation. Meta- bedded chert, and greywacke. The volcanic morphic mineral assemblages characteristic of rocks include some pil,low lavas and shallow in- the prehnite-prrmpellyite facies are found rnost trusive bodies,but are predominantly pyroclastic. commonly in those two members. The Winterville correlates with similar rocks in Two other formations have yielded critical the Munsungen Anticlinorium in the Spider mineral assemblages- the Aroostook River and Lake area of Maine (Hall 1970), and regionally Jemtland formations. The Aroostook River with the Ammonoosuc-Dixville volcanic belt Formation is 6@ m thick and consistsof lithic farther southwestin central New England. greywacke, conglomerate, and slate. The form- The Frenchville Formation is a sequ€nceof ation is pre-Frenchville in age. The volcanic 1100 m of Early Silurian clastic sedimentary detritus which is abundant in the formation was rocks which are largely confined to the AshlanC also derived from the Winterville terrane. Synclinorium. Roy (1973) subdividesthe forma- The Jemtland Formation is a unit of thinly tion into five provisional members: the grey- bedded calcareous shale and calcareous grey- wacke m'ember, conglomerate member, felds- wacke showing turbidite features (Roy 1970). SUB-GREENSCHIST METAMORPHIC ASSEMBLAGES IN NORTIIERN MAINE 47I

The formation overlies the older Silurian unirs sections of the Winterville Formation, 4I of. conformably. A marker horizon of devitrified 101 rhin sections of the Aroostock Riyer and aquagene tuff up to 20 m thick in the middle of Frenchville formations, and 5 of 9 thin sections the formation in the Stockholm area contaisrs of tuff in the Jemtland Formation. pumpellyite in trace quantities. Both the igneous 316 ssdimen'tary rocls re- tain their pre-metamorphic textures in hand spec- MnrarvroRpnrsvr imen. Igneous rocks are uniformly non-foliated throughout the area, regardless of metamorphic Metamorphic mineral assemblages character- zone. Sandstones display a variety of primary 'mpellyite istic of the prehnite-p facies were sedimentary features such as ripple marls, paxal- identified in samples from the Winterville, lel and slsss laminationsoand graded bedding. Aroostook River, Frenchville, and Jemtland In fhin section, the degree of recrystallizatigg forrnations. The mineral assemblagesshow sys- is never so gxeat as to completely obliterate tematic areal variations and we have theref6re primary textures. In the igneous rocks, the ne- defined three metamorphic zones on the basis of tamorphic minerals form veins and amygdule the variations (Fig. 2). The data are based on fillings, or partly or completely replace original exnlination of approximately 25O thin sectionso minerals such as plagioclase and augite. Meta- including 39 which were used in the Coombs morphic minerals also appear as rims and over- et al. (197Q study. The localities bearing critical growths on primary mins1afu and as minute asem,blagesare plotted in Figure 2. Diagnostic euhedral crystals in the matrix of pyroclastic assemblageswere identified in 56 of 138 thin rocks. In the sedimentary rocks, metamoqlhic

Frc. 2. JVIap showing the distribution of metamorphic mineral assemblages and metamorphic zones in central Aroostook County, Maine. Each symbot iepresents ono assemb-iage. Symbols folr more than one sample are combined where the samples are crowded. The dotted lines outline the Winterville and Frenchville formartions. Abhreviations used in the legend are: Act = actinolite; Pr = preh- - - nite; Pu pumpellyite; Ep epidote; ca = calcite; sph - sphene; Ab - albite; Anal = Anal- cime. 472 THE CANADIAN MINERALOGIST

minerals occur as partia:l replacements of lithic prehnite - chlorite - pumpellyite - calcite clasts, overgrowths on monomineralic detrital prehnite - chlorite - pump€llyite grains, and euhedral crystals disseminnledis 1e- pumpellyite - chlorite crystallized matrix. Calcite may form all or part chlorite - calcite of the cement in the sands,tonesand is rarely e seen as a clast. Quartz appears as overgrowths Pumpellyite-epidote-actinolit zone primary quartz on clasts. The highest-grade zone is the pumpellyite- In several thin sections of greywacke it is epidote-actinolite zone. It is well-documented in difficult to decide whether a monomineralic ths Winterville Formation and in Silurian grey- ovoid aggregate of a metamorphic mineral such wackes of the Ashland Synclinorium. The zone as pumpellyite or epidote is detrital, a replace- is defined by the appearance of epidote-pumpel- ment of a clast, or recrystallized from the matrix. lyrte, and actinolite-pumpellyite assemblages. Ilowever, in a few sections delicate aggregates Actinolite, which forms overgrowths on auglte, of epidote and pumpellyite are clearly secondary occurs only in six samples of volcanic rocks of and of metamorphic origin. the Winterville Formation in the southwestern The abundance and grain size of the meta- part of the area and not in greywacke. Epidote, morphic minerals increase with metamorphic on the other hand, is abundant in both grey- grade in the igneous rocks of the Winterville wacke and volcanic rosks of the area. The ob- Formation but not in the sedimentary rocks in served partial assemblagesfor this zor.e arei the Ashland Synclinorium. Metamorphic assemblagesare based on those Prehnils - PumPellYite- chlorite minerals which appear to be stable in a 1 mrn' prehnite - chlorite field of view although the different phases may prehnite - chlorite - clinozoisite not be in actual contact Qan, in press). The prehnite - calcite assemblages discussed here all include al'bite, pumpellyite - chlorite chlorite, and sphene -+ quartz, white mica, and pumpellyite - chlorite - clinozoisite hematite. pumpellyite - calcite epidote - pumpellyite - chlorite Pre hnite-analcime zone epidote - prehnite - chlorite - chlorite The prehnite-analcime zone is the lowest- epidote epidote - calcite grade zone in the area. It is thus far only recog- opidote- prehnite - calcite nized in the northwest part of the area in vol- - - chlorite canic roctrs and lithic greywacke of the Winter- actinolite epidote - pumpellyite - ville Formation. is defined actinolite chlorite The zone bv the co- - existence of analcime with prehni,te in quartz- aotinolite chlorite - prehnite free volcanic rocks. The other assemblagesin the actinolite - prehnite - pumpellyite zone occur most commonly in rocks with minor actinolite amounts of quartz. Partial assemblagesobserved in this zone are: DrscussroN exo Coucr-usroNs - - - analcime prehnite chlorite calcite All the metamorphic mineral assemblages - - of analcime prehnite chlorite central Aroostook County, Maine, are consis- prehnite - - chlorite calcite tent with the prehnite-pumpellyite facies as de- prehnite - chlorite fined by Coombs (1960). The epidote- and acti- prehnite - pumpellyite - - chlorite calcite nolite-bearing rocks of the highest-grade zone pumpellyite - chlorite suggestthat the conditions of metamorphism - ap- chlorite calcite proached those of the greenschistfacies, but the large number of mixed prehnite and pumpellyite P r ehnit e- p ump elly ite zone assemblagesin the same area precludes defining A prehnite-pumpellyitezone seemsto be pres- a change into the greenschistfacies. ent in the central part of the study area (Fig. 2). The zone contains mineral assemblagessimilar Numerous samples in all tlree zones seem to to the prehnite-analcime zone but lacks anal- be of a bulk chemical composition which should cime. The southern limit of this zone is defined support calcium-alumino-silicate minerals diag- greenschist by the first appearanceof epidote and,/or actino- nostic of sub metamorphism. These partial Iite. The observedpartial assemblagesare: rocks instead have the assemblage chlorite-calcite. There does not seem to be a prehnite - chlorite systematic distribution of chlorite-calcite rocks prehnite - chlorite - calcite in the three zones. Coombs et al. (1970) suggest SUB-CREENSCHIST I4ETAMORPHIC ASSEMBLAGES IN NORTHERN MAINE lIJ that chlorite-calcite rocks in the Big Machias might well be explained by pre-Frenchville me- Lake area are a product of local metasomatism. tamorphism of the source rocks. There are also Albee & Zen (L967) show tJrat a relatively high ovoid aggregatesof prmpellyite and epidote in &co, c?rl suppress the format ion of minerals the Aroostock River and Frenchville sandstones characteristic of the prehnite-pumpellyite facies which may be either detrital or authigenic. If in favour of calcite- clay assemblages. the aggregatesare detrital, they can be cited ar further evidence for Taconian metamorphism, Calcite-chlorite is the metamorphic assem- There is no petrographic evidence of retrograde blage found in the intermediate and mafic metamorphism of Ordovician rocks. There is volcanic rocks which lie more than 15 km east also no evidence of high-pressure type meta- of Ashland where all the units are calcareous (for morphism or melange deposits (Roy & Mencher, example, tJre New Sweden and Carys trvlills in press) in the area which would support the formations). The chemical potential of COz suggestionof Bird & Dewey (1970) of an Ordo- was therefore probably higher in the western vician island arc-oceanic trench in northern part of the study area than in the central and Maine. western parts. We also suggest that since p"o, was not uniform regionally, it might also have AcKNoWLEDGMENTS varied locally. Such a local variation might ex- plain why neighbouring mafic rocks of similar We have benefited greatly from discussions major-mineral composition contain either prehn- with E-an Zen, Sharon Bachinski, Gary Boone, ite-pumpellyite or calcite-chlorite assemblages. Priscilla Perkins, James Skehan, S. J. and J. Christopher Hepburn. The loan of 57 thin sec- The determination of the age of the meta- tions by Ely Mencher, and 39 thin sections by morphism is made difficult by the complex Robert Horodyski helped make this study pos- sedimentation and deformation history of this sible. area. Northern Maine was affected by at least three periods of tectonism: the Ordovician Taco- RnrBnBNcrs nian orogeny, the Silurian Salinic disturbance, and the Devonian Acadian orogeny. The Taco- BIRD, J. & DrwEv, J. (1970): Lithosphere plate- nian orogeny produced uplift and gentle folding continental margin tectonics and the evolution in the western part of the area whereas the of the Appalachian orogen. Geol. Soc. Amer. Carys Mills lithofacies in the easternpart of the Bull. 81, 1031-1060. (1970): area remained submarineand sedimentationcon- Boowr, G. M. The Fish River Lake For- mation tinued into the Silurian. and its environments of deposition. The Taconian orogeny Maine Geol. 29. produced Surv. Bull. 27-41. no recognizable cleavage in pre-Silu- Boucor, A. J., Frplo, M. T., Fr-rrcuEn. R., rian rocks. The Salinic disturbance is marked in Fonms, W. H., NAyLoR, R. S. & Pewrors, L. northern Maine by local disconlormities and (1964): Reconnaissance bedrock geology of tho very slightly angular unconformities. The dis- Presque Isle quadrangle, Maine. Maine Geol. turbance does not seem to have produced any Surv. Quad. Map Series 2, l-123. folding or cleavageand was apparently restricted Coor'rss, D. S. (1960): Lower grade mineral fa- to the Presque Isle - Ashland, and Fish River cies in New Zealand. Internat, Geol. Congr. Lake areas et al. L964; Pavlides er a/. XXI, Copenhagen, Rept. Pt. xiii, 339-351. @oucot Nlvron, R. S. L964). T\e Acadian orogeny Honolvsrr, R. I. & was the major tec- (1970): Occurrence of prehnite-pumpellyite fa- tonic event which affected northern Maine. It cies metamorphism in northern Mainz. Amer. produced the major folds and regional cleavage. J. Sci. 268, 142-156. Metamorphic mineral assemblagesdiagnostic Harr., B. A. (1970): Stratigraphy of the southero of sub-greenschist metamorphism have been end of the Munsungun Anticlinorium, Maine. Maine Geol. Surv. Bull. 22. identified in Ordovician and Silurian rocks. Honoovsru, R. J. (1968): Bedrock Geology ol Coombs et al. (L970) report prehnite-pumpellyite Portions of the Fish River Lake, Winterville, pre-Acadian in lower Devonian volcanic rocks Greenlaw, and Mooseleuk Lake Quadrangles, in the Big Machias Lake region. It seemsclearu Aloostook County, Maine. M.S. Thesis, Mass. then, that Acadian metamorphism did not ex- Inst. Technol. ceed the prehnite-pumpellyite facies in the por- MossweN, D. & BlcsrNsrr, D. (1972): Zeolite fa- tion of northern Maine examined in this study. cies metamorphism in the Silurian-Devonian fold New Brunswick. Can. The possibilrty of Taconian belt of northeastern metamorphism J. Earth Sci. 9. 7703-fiA9. cannot be entirely eliminated. The common pre- PApEur., V. S. (1972): Burial metamorphism of sence of pumpellyite, prehnite, and epidote rn late Precambrian sediments near St. Iohn's, volcanic clasts in the Silurian lithic grelwackes Novrfoundland. Can. l. Earth Sci. I, L568-1572. 474 THE CANADIAN MINERALOGIST

PAwIDES, L., MeNcuEn, E., NeyLoR, R. S. & & MrNcurn, E. (in press): Ordovician BoucoT, A. J. (1964): Tectonic features of and Silurian stratigraphy of northeastern Aroo- eastern Aroostook County, MailLe. U.S. Geol. stook County, Maine. Geol. Soc. Amer, Spec. Surv. Prof. Paper 50L4, 28-38. Paper. RoY, D. C. (1970): The Silurian of Northeastern Zell, E-eN (in press): Prehnite and pumpellyite Aroostook County, Mal,ne. Ph.D. Thesis, Mass. bearing mineral assemblages, west side of the InsL Techaol. northern Appalachian fold belt, Pennsylvania (1973): The provenance and tectonic to Newfoundland. .1. Petrology. setting of the Frenchville Formation, north- eastern Maine. Geol. Soc. Amer. Program Manuscript received July 1974, emended Septem- Abstr. 5, 2I4. ber 1974.