RUI: Mechanisms of Hybridization in Silicic Magma Chambers Affected by Mafic Injections and Their Imprint on Granite: The Vinalhaven Magmatic Complex, coastal Maine

Franklin And Marshall College, Lancaster PA

Investigators

Abstract

Wiebe/Hawkins EAR-0003260/0003555 The goal of this project is to investigate how the injection of mafic magma into a shallow silicic magma chamber affects the chemical and isotopic composition of rocks preserved in granite plutons. The Vinalhaven pluton is an ideal setting for this study because mafic input to a silicic magma chamber is well preserved in feeders and as layered deposits of mafic and hybrid intermediate to silicic rocks. These rocks provide a spatial and temporal framework that will permit us to evaluate the petrographic, geochemical, and isotopic signature of hybridization both proximal and distal to the point of entry of the individual mafic replenishments, as well as in the overlying "homogeneous" granite. Our work will focus on intragrain trace element and isotopic variations through microanalysis of selected minerals, using laser ablation ICPMS and ID-TIMS. We will also employ high-precision U-Pb geochronology to constrain the active lifetime of the Vinalhaven magma chamber and attempt to link the preserved magmatic record to the preserved eruptive record of genetically related volcanic rocks. In the first year, this project will incorporate a Keck Geology Consortium Project with nine undergraduates who will undertake 4 weeks of field work. This research bears directly on the origin and emplacement of granite. We believe that studies of mineral zoning are likely to show that adjacent crystals have very different histories of crystallization. This information, combined with the common isotopic variability of a granite and field evidence for multiple replenishments of diverse magmas, strongly suggests that the bulk chemical and isotopic compositions of granite are neither the records of a single magma source nor equivalent to volcanic liquids erupted from the chamber.