Distribution and significance of cordierite in paragneisses of the Hudson Highlands,southeastern New York |
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| Authors: | R D Dallmeyer R T Dodd |
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| Institution: | (1) Department of Earth and Space Sciences, State University of New York at Stony Brook, 11790 Stony Brook, New York, USA;(2) Present address: Max-Planck-Institut for Kernphysik, D-6900 Heidelberg, German Federal Republic |
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| Abstract: | Cordierite occurs locally and sporadically in biotite-quartz-two feldspar paragneisses of the Precambrian Highlands complex in southeastern New York. Cordieritic and associated non-cordieritic gneisses were compared to determine the significance of cordierite for the metamorphic history of the complex.Microprobe analyses of the ferromagnesian phases show the following ranges in Fe/Mg (mol.): cordierite 0.19–0.43; biotite 0.33–0.73; garnet 1.98–3.56. Feldspar compositional ranges are: plagioclase An25–53; K-feldspar in microperthite Or62–87Ab12–37 An0–1. Garnet and plagioclase associated with cordierite are depleted in Ca relative to those in cordierite-free assemblages.Textural evidence, phase rule considerations and consistent distribution coefficients for FeO and MgO in coexisting garnet, cordierite and biotite from each locality examined suggest that all phases formed in at least local equilibrium during the hornblende-granulite subfacies metamorphism. The assemblages studied limit the conditions of metamorphism to between 700 and 750° C and 3.0 to 5.5. Kb. total pressure, with P
T greater than
. Differences in mineral compositions and partitioning coefficients among the sampled areas suggest slight local differences within these ranges.Mineral and modal analyses of cordierite-bearing and cordierite-free gneisses show the latter to be enriched in calcium and potassium and depleted in alumina relative to the former. We conclude that the rarity of cordierite in the Highlands paragneisses reflects a scarcity of parent rocks of suitable composition rather than unfavorable physical conditions. |
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