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The Gedrite-Anthophyllite Solvus and the Composition Limits of Orthoamphibote from the Post Pond Volcanics,Vermont

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The Gedrite-Anthophyllite Solvus and the Composition Limits of Orthoamphibote from the Post Pond Volcanics,Vermont AmericanMineralogist, Volume 65, pages 1103-lI //8, 1980 The gedrite-anthophyllite solvus and the composition limits of orthoamphibote from the Post Pond Volcanics,Vermont FneNr S. Speln Department of Earth and Planetary Sciences Massachusetts Institute of Technolo gy Cambridge, Massachusetts 02 I 39 Abstract Coexisting orthoalnphiboles (high Al-Na gedrites and low Al-Na anthophyllites) from the Post Pond Volcanics, Vermont, display a variety of textures that are interpreted to be equilib- rium growth textures. These textures include: (l) discrete grains; (2) bladed intergrowths; (3) patchy intergrowths; (4) overgrofihs; and (5) lamellar intergrowths with lamellae parallel to (120). Coexistingorthoamphiboles display exsolution parallel to (010); exsolution is much more pronounced in gedrite than in anthophyllite, which suggeststhat the solvus is steeperon the anthophyllite limb. The solvus is defaed by discontinuities in the amount of Si, AlIv, Alvr, and Na(A) in the orthoamphiboles, which can be describedas gapsin the edenite (NaAl?2 n Si) and tscherma- kite (AlvrAllvaMgSi) substitutions.The width of the solvusis a function of Fel(Fe+Mg), with Fe-rich samplesshowing the widest gap. Distribution of elementsbetween anthophyllite and gedrite is systematic,with gedrite enriched in Alrv, AlvI, Fe2*, Na, Ti Ca, and Mn and anthophyllite enriched in Si and Mg. The composition timits of orthoamphiboles at this metamorphic grade are controlled by coexistencewith phasesmore enriched in certain ele- ments: the most Si-rich orthoamphiboles coexist with quartz, the most Al-rich with cordierite or staurolite, the most Fe-rich with garnet, the most Mg-rich with talc, chlorite or cordierite, the most Ca-rich with hornblende, and the most Ti-rich with ilmenite or rutile. Evaluation of published orthoamphibole analysesreveals that the crest of the solvus prob- ably lies at approximately6fi)+25"C. Introduction This paper describesthe textures,mineral chemis- The existence of a solvus in the orthoamphiboles try, and phase relations of orthoamphiboles and co- between low-Al anthophyllite and hiCh-Al gedrite existing minerals from the Post Pond Volcanics with has been noted by Robinson et al. (1969, 1970, the purpose of (l) outlining the compositionalfield l91la), Ross et al. (1969), Stout (1969, t970, 1971, defined by these minerals, including the limits of the 1972),Spear (1977), and Sroddard(1979). miscibility gap between anthophyllite and gedrite; (2) In the southwesterncorner of the Vermont section investigating substitution mechanisms in ortho- (3) of the Mt. Cube Quadrangle, New Hampshire and amphiboles; and estimating the temperature of Vermont, the PostPond Volcanic Member of the Or- the crest of the anthophyllite-gedrite solvus.' fordville Formation consists of a variety of rock types Geologic setting that contain coexistingorthoamphiboles. The locality is particularly well suited for a chemographic study The samples were collected from the Post Pond of these minerals, becausewithin this one area a Member of the Orfordville Formation (Hadley,1942) highly diverse range of bulk compositions is found, from the Vermont portion of the southwestcorner of and the entire composition range of orthoamphiboles the Mt. Cube Quadrangle, New flampshire and Ver- at this metamorphic grade can be delineated. More- mont (seelocation map, Fig. l). This unit strikes to over, the solvus in the orthoamphiboles from the Mt. I In tlis paper the names anthophyllite and gedrite are used to Cube Quadrangle shows the widest compositional describe varieties of orthoamphiboles with Na(A)+Alrv<1.0 and gap yet reported for theseminerals. Na(A)+Alrv>1.0, following the nomenclaturcof Leake (1978). 0003-o04x/80/lI 12-l 103$02.00 I 103 I 104 SPEAR GEDRITE-ANTHOPHYLLITE SOLVUS EXPLANATION [-G-l uowgroderock, undifferenlioled l-Dl-l Littteton Formotion lEl ctougn Formolion OrfordvilleFormolion BlockSchist Member 43" PoslPond Volconics Member 47' 5s,>, Strike ond dip of composifionolloy€ring 9 Strir" ond dip of schistositY 461 Trend ond plunge of minor folds . ?z-37 Somple locolion 43" Locotronof Mt.Cube Quodrongle Fig. l. Geologic map of the southwesterncorner of the Mt. Cube Quadrangle, New Hampshire and Vermon! showing the location of the study area. Geology is after Hadley (1942) arrd Rumble (1969, personal communication). the northeastand dips to the northwestat 39-73o.To higtrly aluminous layers are found, which probably the west, and structurally above the Post Pond Vol- represent volcanic material with an admixture of de- canics, are the pelitic schistsof the Brick Hill Mem- tritus. To the west, at the upper part of the section' ber of the Orfordville Formation (Hadley, 1942). just below the Siluro-Devonian unconformity, there Rumble (1969) reported quartzite boudins from are cordierite and orthoamphibole gneisseshigh in along the contact between the Post Pond Volcanics MgO and low in CaO. The rocks are massive to and Brick Hill Member, which he interpreted as Silu- finely laminated and possibly represent a fossil rian Clough Formation, and therefore correlated the leached zone or soil horizon at the top ofthe volcanic Post Pond Volcanics with the middle Ordovician pile, or a volcanic sequence that has been hydro- Ammonoosuc Volcanics, which occupy the same thermally altered, possibly by seawater. stratigraphic position to the east (ThomFson et aI., Metamorphic grade is in the staurolite-kyanite 1968). zone, as shown by abundant staurolite and kyanite in In the southwest corner of the Mt. Cube Quad- the Brick Hill Member and staurolite in the Post rangle, the Post Pond Volcanics are composed of Pond Volcanics. Metamorphic temperatureswere es- massive to finely bedded mafic to aluminous am- timated with two geothermometers.Oxygen isoto,pe phibolites, with minor calc-sitcates, biotite gneiss, fractionation between quartz and magnetite (1000 orthoamphibole gneiss, and cordierite gneiss. There tna : 9.8+0.11)yields a temperatureof 530+10"C are mafic dikes and agglomeratetextures in the rocks when the calibration of Downs and Deines (1978) is exposedin two quarries near InterstateHighway 91, used and 490+10"C when using the calibration of attesting to the original volcanic nature of at least Bottinga and Javoy (1973). The partitioning of Fe part of this unit. In the roadcut along U.S. Route 5, and Mg betweengarnet and biotite yielded a temper- SPEAK GEDRITE-ANT:HOPHYLLITE SOLVUS ature of 535+30"C when comparedto the calibration Mg-rich samples [Fel(Fe+Mg) in anthophyllite of Ferry and Spear(1978). If one assumesa temper- =0.301, gedrite and anthophyllite are di-fficult to dis- ature of 530oC, the presenceof kyanite implies a tinguish on the basis of pleochroism; distinction was minimgn pressure of 4.0 kbar, from the ALSiO, made in these sampleswith the electron microprobe. triple point of Holdaway (1971).The sampleswere Anthophyllite and gedrite coexist in a variety of collected from an area approximately 2 km by I km textures. They occur as discrete grains in some sam- 'ples, and are believedto have been equibratedunder ap- and sharp optical contacts can be seen where proximately isothermal, isobaric conditions. the grains touch. They also occur as oriented inter- growths that are optically continuous and exhibit Textural relations betweenanthophyllite and gedrite several habits including bladed intergrowths (Fig. Orthoamphibolesform large bladed crystalstypi- 2A), patchy intergrowths, and overgrowths (Fig. 2B). cally 0.1-5.0 mm long, but up to severalcm in length In most occurrenoesthe overgrowth is anthophyllite in rocks of diverse bulk comFosition and mineral as- on gedrite, but in a few samplesgedrite overgrowths semblages(see Table l). They occur in the foliation appear on anthophyllite. plane but also cut acrossit, indicating that thesemin- In several samples a lamellar intergrowth of an- erals may have formed late in the crystallization se- thophyllite and gedrite is observed(see Figs. 2B, C, quence. For comparison, hornblende and cum- D). Lamellae are oriented approximately parallel to mingtonite from this area almost invariably lie in the (120) and are readily observedin sectionscut normal foliation. to c. In Figures 28 and C the amphibole in the core The orthoamphiboles are distinguished from the of the grain is gedrite, whereas anthophyllite makes clinoamphibolesby their parallel extinction. Anthoph- up the rim. This is the case in most of the samples yllite is colorless and can be distinguished from Fe- observed with this intergrowth texture, but in some rich gedrite by the moderate pleochroism in gedrite. samplesthe textural juxtaposition is reversed.Figure There is a correlation between the degree of ab- 2D shows the distribution of Al between the an- sorption in gedriteand its composition.The strongest thophyllite rim (ow Al) and gedrite core (high Al). pleochroism is observed in A1-, Na-, and Fe-rich The (120) lamellae are readily observedin this figure gedrites; the weakest in gedrites with high Mg and becauseof the sharp contrast in Al content between low Al and Na. In Fe-rich samplesthe absorptionis the lamellae and the anthophyllite rim. Electron mi- pale-gray to yellow-gray parallel to a and gray to croprobe analysis revealed that the lamellae have ap- blue-gray parallel to B and y (absorptionis y > B > proximately the same composition as the core of the c). In more Mg-rich samplesthe pleochroism scheme grain (re., gedrite composition). in gedrite is similar, but much weaker. In the most The origin of this textural intergrowth is not Table l. Representativenineral assemblagesof orthoamphibole-bearing rocks from the Post Pond Volcanics Sample /l Ged** Anth Hbld Cum Gar Stau Cord Plag O.Ez Biot Naph Talc Ch Iln Rut 7 3-20D +* + ff ^3, x* x 73-30r + ++ An^- x x J) 77-56c + ++ An^^ x x JJ 77-37N + + h29-40 * xx 68-43211 An^^ x x x JJ 68-432U h15-29 * x x 68-432D + + Atro-30 * xxx x 7 3-29D + ++ At3o x 68-432A + ++ ^27 x * The symbol + indicates that an analysis is presented in Tables 2-5. The symbol x indicates the mlneral is present but no analysls is presented.
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