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CM12 469.Pdf 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 ffi Liaitdeall6!PP.d tcrroa. _- - - Conlocr ---l- Artl.lldo.rla or[ -+- st.ch.o.iua ertr -t- antict'n. --t-- st.cl'!. 46'a!'N 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
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