Southeastern Section - 66th Annual Meeting - 2017

Paper No. 10-6
Presentation Time: 2:40 PM


BROADWELL, Kirkland, CADDICK, Mark and CATON, Summer, Dept. of Geosciences, Virginia Tech, Blacksburg, VA 24061,

Granulite facies metamorphic rocks can provide a sensitive but often enigmatic record of heat and mass transfer in Earth’s crust. Contact aureoles provide an ideal natural laboratory in which to study these processes and their effects on the crust. At the Martinsville Igneous Complex (MIC), contact heating of pelitic schists adjacent to a mafic intrusion produced rapid heating to high-temperature conditions, accompanied by significant partial melting, at relatively shallow crustal depths in and around Martinsville, VA. Here, we combine petrographic analysis with thermodynamic modeling and accessory phase geothermometry to reconstruct the polymetamorphic history of this terrane, providing new insights into the thermo-tectonic evolution of the eastern margin of Laurentia during the waning stages of the Taconic orogeny.

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.