ASSIMILATION IN A MID-CRUSTAL GABBRO PLUTON: MARTINSVILLE IGNEOUS COMPLEX, VIRGINIA

The 430 Ma Rich Acres Formation of the Martinsville (Virginia) igneous complex consists of mafic rocks ranging from biotite-pyroxene gabbro and diorite to troctolite. ⁸⁷Sr/⁸⁶Sr in the gabbros increases systematically with P, Si and Ti concentration. At the current level of exposure, the gabbros are intrusive into pelitic gneisses. At the contact, the gneisses are medium-pressure (5-6 kb) garnet-cordierite-Kspar granulites that have undergone extensive partial melting manifested by the development of diatexite and overprinting and destruction of metamorphic foliations. There is physical evidence of significant assimilation of country rock in gabbros in the contact zone including xenoliths that have undergone varying degrees of reaction with the host gabbro. Desilicated xenoliths typically consist of plagioclase or microcline, biotite, opaque minerals, and, locally hogbomite or spinel. This mineralogy is reminiscent of strongly desilicated emery deposits associated with the gabbro-pelite contact elsewhere in the region. More common are biotite-rich clots and segregations that contain concentrations of various accessory phases. Some clots are apatite-rich, but others contain mostly sphene and zircon. Cathodoluminescence textures of zircon in the gabbro indicates that it is, at least in part, inherited. The apatite has a more problematic origin vis a vis the country rock. The apatite has consistent and distinctive CL, texture, and composition and has not been identified in rocks assimilated at the current level of exposure. The current working hypothesis is that at least two stages of assimilation are required to explain observations in the gabbros. First came the assimilation at depth of a radiogenic, Ti and P rich source (possibly Grenville basement). Then, at or near the current level of exposure, zircon and (probably) plagioclase and quartz were incorporated into the hybrid magma as solid phases during bulk assimilation of partially molten pelitic gneiss.