PRESERVATION OF REGIONAL METAMORPHIC MINERAL COMPOSITIONS THROUGH A LATER HIGH-T CONTACT METAMORPHISM, WESTCHESTER COUNTY NY

Reconstruction of regional metamorphic conditions in metapelites overprinted by a later high-T contact event is typically difficult due to compositional modification of key thermobarometric minerals in the later thermal event. In Westchester County, NY, pelitic schists in aureoles of small sill-like mafic plutons record a 440 Ma high-T (775°C) contact overprint on ca. 465 Ma upper amphibolite-facies Taconic regional metamorphic assemblages. A sample from the innermost aureole of the Croton Falls pluton, a small (<10 km²) gabbroic pluton has a matrix assemblage reflecting contact conditions of qz–plg–bt–grt–sil–ilm–gr–Zn-rich spl, without kfs or ms and strongly zoned plg with Ca-enriched rims. P-T estimates (conventional and Thermocalc) using grt and plg rims yields 775°C at 0.8 GPa. Grt porphyroblasts contain numerous bt, plg, sil, and qz inclusions, both in Ca-enriched rim zones and interiors. Bt inclusions are systematically more Mg-rich in grt interiors than in rim zones and plg inclusions in interiors are more Na-rich; bt and unzoned plg in rim zones are identical to plg rims and bt in the matrix. Calculated P-T conditions for grt interior assemblages using Grt-Bt and GASP yields values of 0.55 GPa at 625°C, identical to values for regional metamorphic rocks outside the aureole at sillimanite-staurolite grade. We interpret these observations as preservation of plg and bt inclusions armored within unmodified grt interiors whereas grt, plg and bt in the matrix and in modified grt rims have re-equilibrated to reflect the significantly higher P-T of the contact event. Apparent depletion in K (and formation of Zn-rich spinel from st breakdown) is likely due to dehydration melting of both ms and bt, and melt extraction during the contact heating. The higher P of the contact event than of the regional metamorphism is difficult to explain but likely reflects shallow crustal thickening in the interval between 465 and 440 Ma (Taconic allochthon emplacement?).