Mineral zoning, P-T-X-M phase relations, and metamorphic evolution of some Adirondack granulites, New York

Detailed petrologic analysis of ten meta-anorthosites and related INTRODUCTION rocks from the Adirondack highlands, New York, has been used to The Adirondack highlands of New York are one of constrain the P–T history of the region. Metamorphic orthopyroxene the best studied granulite facies terranes of the world and clinopyroxene occur in aggregates of ~1 mm diameter grains (e.g. Bohlen et al., 1985; Valley et al., 1990, and that are zoned concentrically with decreasing Al and Fe/(Fe+Mg) references therein). Based on this considerable volume and increasing Ca towards the rim; moreover, zoning is not related of work, the peak metamorphic conditions in the to the presence of garnet. Zoning is interpreted to have been produced Adirondack highlands have been fairly well constrained during growth by the reaction: igneous pyroxene+plagioclase= at pressures of ~7–8 kbar (crustal depths of 25–30 metamorphic Opx+metamorphic Cpx along a cooling path with a km) and temperatures that range from >775°C in slope of 6 bars/°C starting at ~850°C and 6·5–8 kbar. Garnet the central highlands to T<675°C in the southern occurs as isolated clusters of 5–25 grains of ~1 mm diameter Adirondacks (e.g. Bohlen et al., 1985). P–T paths have crystals and as necklaces of ~1 mm diameter crystals surrounding been shown to follow nearly isobaric cooling trajectories metamorphic pyroxenes or oxides. Plagioclase±quartz typically (e.g. Bohlen, 1987). Because of this wealth of inseparates garnet from pyroxene, but locally garnet embays metaformation, some fundamental questions about the origin morphic pyroxene, suggesting that garnet grew at the expense of of the Adirondack granulite facies terrane have emerged pyroxene. Garnet clusters have slightly lower Fe/(Fe+Mg) than (e.g. Bohlen et al., 1985): How did the crust achieve garnet in necklaces [Fe/(Fe+Mg)=0·73 and 0·75–0·78, rethe relatively high temperature conditions necessary to spectively] and slightly higher grossular (Xgrs=0·20 vs 0·19). produce granulite facies rocks at depths of 25–30 km? Neither is zoned in Mn and zoning towards increasing Fe/ And what is the relative importance of magmatic (Fe+Mg) is restricted to contacts with biotite and locally clinoheating vs continental burial in this process? pyroxene. Biotite adjacent to garnet is zoned towards decreasing This study will present new data on mineral comFe/(Fe+Mg). Production of garnet in necklaces is interpreted position and element zoning in garnet, pyroxene, to be the result of the continuous Fe–Mg reaction Opx+Pl= biotite, amphibole and plagioclase from Adirondack Grt+Cpx+Qtz. P–T–X–M modeling of this reaction reveals a meta-anorthosite and related rocks with the goal of P–T path with a slope of 6 bars/°C. No evidence for early, prefurther constraining the metamorphic evolution of this metamorphic peak garnet is observed, which rules out any possibility terrane. Mineral compositions will be used to construct of early higher-pressure metamorphism. The P–T path is therefore phase diagrams in an attempt to evaluate approach to inferred to be counterclockwise with loading to 6·5–8 kbar at a chemical equilibrium. Theoretical analysis will be used maximum temperature of ~850°C occurring during or immediately to construct P–T–X–M phase diagrams depicting how following an influx of heat, presumably from early orogenic plutons. mineral composition and modes change with P and T. This theoretical analysis will then be applied to the mineral zoning to constrain the P–T path, and the results will be discussed in the context of a tectonic