GEOCHEMICAL ANALYSIS OF MELT GENERATION IN MAINE MIGMATITES

Migmatites are rocks that represent the first occurrence of crustal anatexis in orogenic events and preserve characteristics of both their metamorphic precursors and the products of their melting. The presence of migmatites has important implications for how mountains and the continental crust are created during orogenies. However, many questions concerning these processes are not fully understood, including the actual mechanism for heating such rocks and the nature of the geochemistry of the first stages of anatexis. The geochemistry is particularly problematic because the leucosome that represents the “molten” portion of the migmatite may have been subjected to fractional crystallization and re-crystallization processes. As a result, bulk rock studies of migmatites may not necessarily indicate what is happening over the course of melting from beginning to end. This study aims at addressing this question by conducting geochemical microanalysis of a group of migmatites collected from the Tumbledown and Weld Anatectic Domains, Maine. Bulk rock geochemistry of these samples has been determined by previous authors, but there is a lack of microanalytical data to compare and constrain the results of melting models. Thin sections were made in order to study the petrography of the samples and subsequent electron microprobe analysis (EPMA) was conducted in order to determine the exact chemical composition of various minerals within the samples and examine chemical zonation of these minerals as well. Using this data, specific mechanisms responsible for melt generation and variations over time were determined.