Volume 38: January - December 1997

Issue 6: June 1997

Abstract

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

FS. Spear and JC. Markussen
Department of Earth and Environmental Science, Rensselaer Polytechnic Institute, Troy, NY 12180, USA and Corresponding author
ABSTRACT
Detailed petrologic analysis of ten meta-anorthosites and related rocks from the Adirondack highlands, New York, has been used to constrain the P-T history of the region. Metamorphic orthopyroxene and clinopyroxene occur in aggregates of 1 mm diameter grains that are zoned concentrically with decreasing Al and Fe/(Fe+Mg) and increasing Ca towards the rim; moreover, zoning is not related to the presence of garnet. Zoning is interpreted to have been produced during growth by the reaction: igneous pyroxene + plagioclase = metamorphic Opx + metamorphic Cpx along a cooling path with a slope of 6 bars/^oC starting at 850^oC and 6.5-8 kbar. Garnet occurs as isolated clusters of 5-25 grains of 1 mm diameter crystals and as necklaces of 1 mm diameter crystals surrounding metamorphic pyroxenes or oxides. Plagioclase +- quartz typically separates garnet from pyroxene, but locally garnet embays metamorphic pyroxene, suggesting that garnet grew at the expense of pyroxene. Garnet clusters have slightly lower Fe/(Fe+Mg) than garnet in necklaces [Fe/(Fe+Mg)=0.73 and 0.75-0.78, respectively] and slightly higher grossular (Xgrs+0.20 vs 0.19). Neither is zoned in Mn and zoning towards increasing Fe/(Fe+Mg) is restricted to contacts with biotite and locally clinopyroxene. Biotite adjacent to garnet is zoned towards decreasing (Fe/(Fe+Mg). Production of garnet in necklaces is interpreted to be the result of the continuous Fe-Mg reaction Opx+Pl+Grt+Cpx+Qtz. P-T-X-M modeling of this reaction reveals a P-T path with a slope of 6 bars/^oC. No evidence for early, premetamorphic peak garnet is observed, which rules out any possibility of early higher-pressure metamorphism. The P-T path is therefore inferred to be counterclockwise with loading to 6.5-8 kbar at a maximum temperature of 850^oC occurring during or immediately following an influx of heat, presumably from early orogenic plutons.

Keywords: Adirondacks; granulites; P-T paths; garnet zoning; Gibbs method

Pages: 757 - 783

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