THE AMERICAN MINERALOGIST, VOL 47, IVLY AUGUST, 1962 CORDIERITE-GARNET GNEISS AND ASSOCIATED MICRO- CLINE-RICH PEGMATITE AT STURBRIDGE, I,{ASSA- CHUSETTS AND UNION, CONNECTICUTI Fnor B.cnrBn, [/. S. GeologicalSurttey, Washington,D. C. Aesrnacr Gneiss of argillaceous composition at Sturbridge, Massachusetts, and at Union, Connecticut, 10 miles to the south, consists of the assemblagebiotite-cordierite-garnet- magnetite-microcline-quartz-plagioclase-sillimanite. The conclusion is made that this assemblagedoes not violate the phase rule. The cordierite contains 32 mole per cent of Fe- end member, the biotite is aluminous and its ratio MgO: (MgOf I'eO) is 0.54, and the gar- net is alm6e5 pyr26.agro2.espe1.2.Lenses of microcline-quartz pegmatite are intimately as- sociated with the gneissl some are concordant, others cut acrossthe foliation and banding of the gneiss. The pegmatites also contain small amounts of biotite, cordierite, garnet, graphite, plagioclase, and sillimanite; each mineral is similar in optical properties to the corresponding one in the gneiss. It is suggestedthat muscovite was a former constituent of the gneiss at a lower grade of metamorphism, and that it decomposedwith increasing metamorphism, and reacted with quartz to form siliimanite in situ and at lerst part of the microcline of the gneiss and pegmatites These rocks are compared with similar rocks of Fennoscandia and Canada. INtnouucrroN Cordierite-garnet-sillimanitegneisses that contain microcline-quartz pegmatiteare found in Sturbridge,Massachusetts, and Union, Connecti- cut. The locality (Fig. 1) at Sturbridgeis on the south sideof the \{assa- chusettsTurnpike at the overpassof the New Boston Road; this is about 1 mile west of the interchangeof Route 15 with the Turnpike. The other locality is about 1] miieseast of the villageof Union, on the north side of Bigeiow Hollow Road, Connecticut Route 198, about 100 feet west of Bigelow Pond. Thesegneisses are part of the widespreadBrimfield Schist (Emerson,l9l7 , p. 68-72;and Rodgersand others,1959, p. 46 and map). The two localitiesare about 10 milesapart. DrscuprroN ol RocKS The gneissat Sturbridgeis gray, medium- to coarse-grained,and shows a rough compositional banding of mafic and quartzo-feldspathic units from a few millimeters to a few inches in thickness.Irregular folia of intergrown biotite and sillimanite with scattered anhedral garnet and cordieriteare set in a matrix of massiveaggregates of plagioclase,q\artz, microcline,and minor amountsof biotite. Nluscoviteis absent. l,{agnetite, graphite, and pyrite are present in small amounts. Proportions of the minerals vary considerablyfrom one part of the gneissto another. 1 Publication authorized by the Director, U. S. Geological Survey. 907 I'-RED BARKNR T.,ASSACIIUSEITS OONNECTICIII +NoRrIrI ffi Frc. 1. Map showing locations of cordierite-garnet-sillimanite gneiss in Sturbridge, Massachusetts, at A, and in Union, Connecticut, at B. An analysisof typical gneiss,free of pegmatite,from Sturbridge was madeby chemistsof the U. S. GeologicalSurvey usingthe rapid methods of Shapiroand Brannock (1956): Sio:.. 53.1 N.rO. 1.7 Tioz |.2 KzO 3.5 Alzor. 24.1 HzO 7.+ FzO: 1 4 P:Os 0.06 IreO 8.4 MnO 0.10 Mgo 3.7 co: .. o.o5 CaO. 15 Total IOO 2 The compositionof the gneissis comparableto that of a shale,except for its low content of HzO and its high ratio of FeO: FezOr.The low content of H2O probably is due to dehydration during metamorphism.The original sedimentmay have containedenough ferrous clay to accountfor the 8.4 per cent of FeO, but it is more probable that original Fe2O3,in hydrous iron oxides,was partially reducedto FeO during metamorphism'perhaps by carbonaceousmaterial. The gneissat Union is similar to that at Sturbridge, except that the folia of biotite and sillimanite form only 10 to 20 per cent of the rock; cordierite and garnet ar'e much more abundant; and both garnet and cordieriteoccur in the quartzo-feldspathicportions. There is a gradation CORDI DRI T E-GA RN ET G1I/E1.'S lrarn l'rc.2. Mafic band in gneiss from llnion, connecticut. Biotite, bio; cordierite, cdt; garnet, gar; microcline, mic; myrmekite, myr; plagioclase,pl; qtartz, qtzl and sillimanite, sil. from scatteredgrains and clustersof microclineto small, lenticular and blob-likepegmatites that appearto have replacedthe gneiss.An example of the latter is shownin Fig. 4. rn thin section the folia of biotite and sillimanite in the gneissat union wrap around equant, anhedral garnets and irregular grains of cordierite. small amounts of plagioclaseand quartz are interstitial to these folia; microclinerarely occursinside them, but commonly embays their mar- gins.A typical view is given in Fig.2. The quartzo-feldspathicaggregates are granular mosaics of andesine, qtartz, microcline microperthite, and biotite, with occasional cordierite, garnet, and sillimanite. Rims of myrmekite lie betweenplagioclase and microcline;they are similarto ones in migmatite pictured by Schreyer (1958, Taf. 10, Abb.2). The volu- metric ratio of myrmekite to the adjacent microclineis about 1: 10. The amount of microcline varies greatly from place to place, even as seenin thin section.The relative amounts of microcline and plagioclasecom- monly vary inversely;the feldsparin one thin sectionmay be almost al1 plagioclase,whereas that in a section cut from adiacent rock mav be FRED BARKER : i', . r ..,.r, r. ::'::'.... .' ..rrr.InnFt.l,,,.:,,-.' Frc. 3. Small aggregateof microcline in cordierite-rich gneiss,union connecticut' The grain of myrmekite in right center has only a core remaining of the original grain of andesine.Biotite, bio; cordierite, cdt;garnet, gar;microcline, mic;myrmekite, myr;plagio- clase,pl; quartz, qtz, sillimanite, sil; magnetite, mt; and zircon, zr ' Plane light' nearly ali microcline.Some grains of microclineinterfinger into grains of biotite as thin sheetsparallel to the cleavageof the latter. Grains of garnet and cordierite that are embayed by microcline are seenin thin section,as in Fig. 3. Thin shellsof sillimanite are presentbetween much of the microclineand embayedgarnet. DnscnrprroN oF MTNERALS Analysesof biotite,cordierite,and garnetfrom the gneissat Sturbridge are given in Table 1. The formula of the biotite, calculatedon a basisof 24 anions,is: CORDIERITE.GARNET GTZYSS 911 Tegr,r 1. ANu,vsns ol BrorrrB. Connremrr. lm GanNrr rnou Brornr- Conornmm-GenNnr-Me.cNnrrrn-Pllcrocr,asr-Quantz- Srrr,rMANrrE GNrrss, Srunnnrocn, MassecRusners Biotitel Cordierite2 Garnet2 SiOz 37.3 48.2 39.L TiOz t\ 0.00 0.03 AlzOs r7.3 32.9 22.2 FezO: 0.7r 0.00 0.00 l'eO 15.5 7.4 30.8 Mso 10.3 8.78 6.58 CaO 0.42 0.05 1.04 NurO 0.19 n.d. n.d. KzO 9.6 n.d. n.d. HzO3 J.l 1.66 n.d. F 0.54 n.d. n.d. MnO (0.otq)n 0.06 0.56 Sums 99.3 99.05 1 Analysis by B. L. Ingram. 2 Analysis by C. A. Kinser. 3 Determined by Penfield method. a Determination by semiquantitative spectrographic analysis. 6 Oxygen equivalent for fluorine subtracted from total. n.d.: not determined, That of cordierite,on a basisof 18 atoms of oxygen,is: Mgr.soFeo64A14 00Si4 sgOrs' 0.57HzO Schreyerand Yoder (1960,p. 94) have suggestedthat HrO may be pres- ent in molecularform in intersticesin the cordieritestructure, and so the formula of cordierite is given with H2O as water of hydration. The for- mula of the garnet on a basisof 24 atomsof oxygenis: Fea eMgr s:Cae 17Mn6 67Ala 67Si6e7o2a The molar ratios MgO: (MgOf FeO) of theseminerals are: biotite, 0.54; cordierite,0.68;and garnet,0.28.Formula of the garnetin termsof end numbersis: alm6e bpyr26 agro2 espet 2 The 0 and 7 indices of refraction in sodium light of biotite at both Sturbridgeand Union are about 1.640.The biotite is pleochroicfrom pale yellowish brown to red-brown.Indices of the cordieritefrom Sturbridge arei ot:1.541,0:1.547, and 7: 1.553,all *0.002; thoseof Union cordi- erite are similar exceptthat a : 1.542.For garnetat Sturbridge n : t.795, + 0.004,and that from Union is L792. 912 FRED BARKER The distortion index of the cordierite,as definedby Miyashiro (1957, p. 44-45), is 0.25+0.02. Garnet was *-rayed by R. O. Fournierl the aver- agesof four determinations(hhl: 12.2.0, lI'5'2, 12'6'0, and 10'9'3) from Sturbridgegarnet and of five determinaLions(hkl: 12'2'0, ll'5'2, t2' 6' O,12' 6' 2,and 10.9' 3) of Union garnetwere each o: 11.49 + 0.05A. n{ost grainsof microclineshow grid twinning, but somedo not. About half of the grains of this mineral are microperthitic with fine filaments or irregular patchesof sodic plagioclase.The volume of plagioclasein the perthite rangesfrom a trace to about 20 per cent. Microcline from a large crystal in pegmatite at Sturbridge was homogenizedby heating at 900'C. for 60 hours and then r-rayed. The (201) method with KBrOs as an internal standard was used (Orville, 1958, p. 208) and indicated a compositionof 82 weight per cent of KAISiaOe. The plagioclaseis sodic andesine. Gradationally zoned grains are not uncommon;these have rims of calcicandesine or sodiclabradorite. l,{rralronpruc PEGMATTTES Lensesof pegmatitefrom a fraction of an inch to about 10feet in thick- nesslie scatteredin the gneiss.The lensescommonly lie parallel to the foiiation of the gneiss,which locally wraps around them. A few lenses, however,transect the foliation. Blobs or blotchesof pegmatite of very irregular shape,from an inch to about 10 feet in maximum dimension, are alsofound in the gneiss.These latter pegmatitesmostly transect the foliation of the gneisswithout displacingit, and many of them decrease in grain size toward the margins and fade into the quartzo-feldspathic aggregatesof the gneiss.Typical examplesare shown in Fig. 4. These blobs are interpreted to have replacedthe gneiss.Both forms of pegma- tite consistof about 80 to 95 per cent by volume of microcline,with the remainder qvartz, biotite, and accessoryminerals.