NEW FINDINGS IN THE MASSABESIC GNEISS COMPLEX, SOUTHEASTERN NEW HAMPSHIRE

Recent large scale bedrock mapping of the Massabesic Gneiss Complex and geochemical analyses of metasedimentary rocks, migmatites and granites suggest that partial melting and mobilization of these melts occurred after the late-Proterozoic. Gradational textural relationships suggest that the metasedimentary rocks were migmatized and melts mobilized along a structural arch forming migmatite textures and copious granites. INAA and XRF whole rock Rare Earth, major and trace element analyses of the three types of rocks do not support the progression of melting of the metasedimentary rocks into migmatites and pooling the mobilized melt to form granites, as hypothesized during the mapping phase of this project. Instead, log-log plots of the incompatible elements Ba and Sr verses the Europium anomaly for the granites support that they are fractionated from a Permian Milford Granite source, suggesting that the granites in and around the Massabesic Gneiss Complex are most likely Permian. Plotting the migmatites and the Devonian Barrington Granite on the granite fractionation plots indicate they have distinct magma sources even though outcrop exposures demonstrate that melts from the migmatites and granites (except the Barrington Granite) co-existed. Partial melt modeling of the rock analyses shows that the metasedimentary rocks are most likely the protoliths of the migmatites and that some of the migmatite textures are restitic. These results are interpreted as follows: The metasedimentary rocks (Silurian?) were partially melted producing several migmatite textures during a single migmatization event. The granites, originally thought to be Devonian and part of the Barrington Granite are most likely Permian and part of the Milford granite, and the Massabesic Gneiss Complex has undergone at least three melt events since the late-Proterozoic, one in which the Barrington Granite was produced, one that produced the migmatites and another producing Permian granites.