Muscovite breakdown above the sillimanite isograd, Cortlandt Complex, NY

Textural and mineral chemical data reveal that the metamorphic aureole of the Cortlandt Complex (southeast NY) has experienced T/P conditions exceeding those of the second sillimanite (sill) isograd. The intrusion crosscuts Taconian structures and metamorphic isograds. The aureole is superimposed upon regional metamorphic assemblages and consists of a well-exposed 100m wide band of sill-bearing schists. Though the country rock contains abundant muscovite (mus), it is absent in rocks within 25m of the contact. K-bearing phases (except biotite) are absent within 10m of the contact. K-spar appears beyond 10m and mus is present as rare small laths beyond 25m. Beyond 30m, mus dominates samples as large poikiloblasts, formed during the contact event, that grow across foliation and enclose all other mineral phases. Near the contact sill occurs both as mats of fibrolite oriented parallel to foliation, and as randomly oriented prismatic crystals. Garnet-biotite pairs yield temperature estimates of 650-725 C. Pressures of 6-7 kb are obtained using QGAP. The assemblage gnt + sill + bio + ksp + corundum + sta (qtz absent) occurs 10m from the contact. Quartz was exhausted in this reaction and/or by mus + qtz = ksp + sill + H20. Corundum most likely formed by themore » breakdown of mus after the depletion of qtz. These observations suggest that muscovite was present in the country rocks prior to intrusion. Mus breakdown is responsible for the formation of a second generation of sill, and in the absence of quartz, for the production of corundum. Bulk chemical analyses of samples may determine whether migration of K-rich fluids from muscovite breakdown explains the distribution of K-bearing phases.« less