CONSTRAINING THE HIGH-TEMPERATURE POLYMETAMORPHIC HISTORY OF THE MARTINSVILLE CONTACT AUREOLE
Petrographic analysis reveals several distinct garnet inclusion suites and textures that likely record dehydration melting reactions. Several garnet crystals contain inherited poikioblastic cores, inclusion-free mantles, and inclusion-rich rims, with WDS X-ray mapping revealing a sharp increase in grossular content associated with these inclusion-rich rims. Thermodynamic modeling constrains peak P-T conditions upon contact heating to approximately 850˚C and 5 kbar, while also providing information about the early cooling history. Models reincorporate melt lost during contact heating to match the average composition of pelitic schists outside the contact aureole. This allows constraints to be placed on the prograde heating path and supports field and petrographic evidence for rapid heating and extensive anatexis due to the contact heating. Furthermore, thermodynamic constraints suggest that calcic garnet rims grew during cooling and crystallization of an internally derived melt that was hybridized by addition of a gabbroic component from the adjacent intrusion.
In a regional tectonic setting, these findings have significant implications for the thermal structure of the crust during the waning stages of the Taconic orogeny. The MIC is one of numerous mafic intrusives that occur along strike up to and beyond the Cortlandt Complex in New York. Together, these intrusives represent significant advection of heat and mass into the crust towards the end of an orogenic cycle.