The gedrite-anthophyllite solvus and the composition limits of orthoamphibote

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 equilibrium 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). Coexisting orthoamphiboles display exsolution parallel to (010); exsolution is much more pronounced in gedrite than in anthophyllite, which suggests that the solvus is steeper on 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 described as gaps in the edenite (NaAl?2 n Si) and tschermakite (AlvrAllvaMgSi) substitutions. The width of the solvus is a function of Fel(Fe+Mg), with Fe-rich samples showing the widest gap. Distribution of elements between 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 coexistence with phases more enriched in certain elements: 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 analyses reveals that the crest of the solvus probably lies at approximately 6fi)+25"C. Introduction The existence of a solvus in the orthoamphiboles between low-Al anthophyllite and hiCh-Al gedrite has been noted by Robinson et al. (1969, 1970, l91la), Ross et al. (1969), Stout (1969, t970, 1971, 1972), Spear (1977), and Sroddard (1979). In the southwestern corner of the Vermont section of the Mt. Cube Quadrangle, New Hampshire and Vermont, the Post Pond Volcanic Member of the Orfordville Formation consists of a variety of rock types that contain coexisting orthoamphiboles. The locality is particularly well suited for a chemographic study of these minerals, because within this one area a highly diverse range of bulk compositions is found, and the entire composition range of orthoamphiboles at this metamorphic grade can be delineated. Moreover, the solvus in the orthoamphiboles from the Mt. Cube Quadrangle shows the widest compositional gap yet reported for these minerals. This paper describes the textures, mineral chemistry, and phase relations of orthoamphiboles and coexisting minerals from the Post Pond Volcanics with the purpose of (l) outlining the compositional field defined by these minerals, including the limits of the miscibility gap between anthophyllite and gedrite; (2) investigating substitution mechanisms in orthoamphiboles; and (3) estimating the temperature of the crest of the anthophyllite-gedrite solvus.' Geologic setting The samples were collected from the Post Pond Member of the Orfordville Formation (Hadley,1942) from the Vermont portion of the southwest corner of the Mt. Cube Quadrangle, New flampshire and Vermont (see location map, Fig. l). This unit strikes to I In tlis paper the names anthophyllite and gedrite are used to describe varieties of orthoamphiboles with Na(A)+Alrv<1.0 and Na(A)+Alrv>1.0, following the nomenclaturc of Leake (1978). 0003-o04x/80/l I 12-l 103$02.00 I 103 I 104 SPEAR GEDRITE-ANTHOPHYLLITE SOLVUS the northeast and dips to the northwest at 39-73o. To the west, and structurally above the Post Pond Volcanics, are the pelitic schists of the Brick Hill Member of the Orfordville Formation (Hadley, 1942). Rumble (1969) reported quartzite boudins from along the contact between the Post Pond Volcanics and Brick Hill Member, which he interpreted as Silurian Clough Formation, and therefore correlated the Post Pond Volcanics with the middle Ordovician Ammonoosuc Volcanics, which occupy the same stratigraphic position to the east (ThomFson et aI., 1968). In the southwest corner of the Mt. Cube Quadrangle, the Post Pond Volcanics are composed of massive to finely bedded mafic to aluminous amphibolites, with minor calc-sitcates, biotite gneiss, orthoamphibole gneiss, and cordierite gneiss. There are mafic dikes and agglomerate textures in the rocks exposed in two quarries near Interstate Highway 91, attesting to the original volcanic nature of at least part of this unit. In the roadcut along U.S. Route 5, EXPLANATION [-G-l uowgrode rock, undifferenlioled l-Dl-l Littteton Formotion lEl ctougn Formolion Orfordville Formolion Block Schist Member Posl Pond Volconics Member higtrly aluminous layers are found, which probably represent volcanic material with an admixture of detritus. To the west, at the upper part of the section' just below the Siluro-Devonian unconformity, there are cordierite and orthoamphibole gneisses high in MgO and low in CaO. The rocks are massive to finely laminated and possibly represent a fossil leached zone or soil horizon at the top ofthe volcanic pile, or a volcanic sequence that has been hydrothermally altered, possibly by seawater. Metamorphic grade is in the staurolite-kyanite zone, as shown by abundant staurolite and kyanite in the Brick Hill Member and staurolite in the Post Pond Volcanics. Metamorphic temperatures were estimated with two geothermometers. Oxygen isoto,pe fractionation between quartz and magnetite (1000 tna : 9.8+0.11) yields a temperature of 530+10"C when the calibration of Downs and Deines (1978) is used and 490+10"C when using the calibration of Bottinga and Javoy (1973). The partitioning of Fe and Mg between garnet and biotite yielded a temper43" 47' 5s,>, Strike ond dip of composifionol loy€ring 9 Strir" ond dip of schistositY 461 Trend ond plunge of minor folds . ?z-37 Somple locol ion 43" Locotron of Mt. Cube Quodrongle Fig. l. Geologic map of the southwestern corner 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). SPEAK GEDRITE-ANT:HOPHYLLITE SOLVUS ature of 535+30"C when compared to the calibration of Ferry and Spear (1978). If one assumes a temperature of 530oC, the presence of kyanite implies a minimgn pressure of 4.0 kbar, from the ALSiO, triple point of Holdaway (1971). The samples were collected from an area approximately 2 km by I km and are believed to have been equibrated under approximately isothermal, isobaric conditions. Textural relations between anthophyllite and gedrite Orthoamphiboles form large bladed crystals typically 0.1-5.0 mm long, but up to several cm in length in rocks of diverse bulk comFosition and mineral assemblages (see Table l). They occur in the foliation plane but also cut across it, indicating that these minerals may have formed late in the crystallization sequence. For comparison, hornblende and cummingtonite from this area almost invariably lie in the foliation. The orthoamphiboles are distinguished from the clinoamphiboles by their parallel extinction. Anthophyllite is colorless and can be distinguished from Ferich gedrite by the moderate pleochroism in gedrite. There is a correlation between the degree of absorption in gedrite and its composition. The strongest pleochroism is observed in A1-, Na-, and Fe-rich gedrites; the weakest in gedrites with high Mg and low Al and Na. In Fe-rich samples the absorption is pale-gray to yellow-gray parallel to a and gray to blue-gray parallel to B and y (absorption is y > B > c). In more Mg-rich samples the pleochroism scheme in gedrite is similar, but much weaker. In the most Mg-rich samples [Fel(Fe+Mg) in anthophyllite =0.301, gedrite and anthophyllite are di-fficult to distinguish on the basis of pleochroism; distinction was made in these samples with the electron microprobe. Anthophyllite and gedrite coexist in a variety of textures. They occur as discrete grains in some sam'ples, and sharp optical contacts can be seen where the grains touch. They also occur as oriented intergrowths that are optically continuous and exhibit several habits including bladed intergrowths (Fig. 2A), patchy intergrowths, and overgrowths (Fig. 2B). In most occurrenoes the overgrowth is anthophyllite on gedrite, but in a few samples gedrite overgrowths appear on anthophyllite. In several samples a lamellar intergrowth of anthophyllite and gedrite is observed (see Figs. 2B, C, D). Lamellae are oriented approximately parallel to (120) and are readily observed in sections cut normal to c. In Figures 28 and C the amphibole in the core of the grain is gedrite, whereas anthophyllite makes up the rim. This is the case in most of the samples observed with this intergrowth texture, but in some samples the textural juxtaposition is reversed. Figure 2D shows the distribution of Al between the anthophyllite rim (ow Al) and gedrite core (high Al). The (120) lamellae are readily observed in this figure because of the sharp contrast in Al content between the lamellae and the anthophyllite rim. Electron microprobe analysis revealed that the lamellae have approximately the same composition as the core of the grain (re., gedrite composition). The origin of this textural intergrowth is not Table l. Representative nineral assemblages of 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