Seeking Evidence for Reactive Assimilation in a Hybrid Pluton

Virginia Museum Of Natural History Foundation, Martinsville VA

Investigators

Abstract

The PI seeks funding to identify source components in hybrid mixed plutonic rocks and to understand the mechanisms by which they are incorporated. He proposes that the fundamental process involves assimilation by disaggregation and reaction of crystalline materials with silicate melts. In particular, wholesale melting of assimilant may not be required. Instead, solids formed during partial melting (e.g. dehydration melting) of the assimilant are incorporated into the magma as the xenolith disaggregates. Reactions during assimilation and crystallization promote both chemical mixing and physical disintegration of xenoliths and are largely responsible for the cryptic character of bulk assimilation. The goal of this project is to identify mesoscale and microscale regions within igneous rocks that reflect varying degrees of crust vs. mantle input. The work will focus on petrographic and mineralogical heterogeneities that may preserve evidence of fine-scale, cryptic mixing even if original textures and mineralogy have been modified by reactive processes during assimilation and crystallization. The work is specifically aimed at the identification and characterization of minerals - either phenocrysts or xenocrysts - that predate the hybridization event as well as reactant minerals that carry a hybrid signature. The PI proposes to investigate these processes in the Martinsville Igneous Complex (MIC) in southern Virginia, a gabbro-diorite complex with a hybrid isotopic signature. The MIC is well-suited for this study because; 1) isotope chemistry is strongly correlated with major and trace element chemistry within the MIC; 2) the system (a basaltic intrusion into pelitic rocks) is geologically well-characterized, highly contrasting, and simple and; 3) minerals and mineral clots have been identified in the pluton that are tentatively interpreted as being derived from the country rocks. Additionally, petrographically distinctive phases, including zoned, dusty plagioclase in the pluton and blue quartz in the country rocks provide obvious analytical targets. Data to be acquired unclude a suite of whole rock analyses, and mineral chemical and zoning data including major and trace elements and oxygen isotopes. Broader impacts: The PI will incorporate information from this study into his ongoing program of K-12 teacher training, which already includes a large component on the origin and composition of granitic magmas. If this proposal is supported, additional support will be sought to create an inquiry-based exhibit on igneous rocks that emphasizes the results of the proposed research. This museum is a focal point for school visitation and alternative learning in an economically depressed region (Enterprise Zone) where approximately 50% of the school-age population is African-American and Hispanic.