RUI: Fabric in Granite: the Vinalhaven and Cadillac Mountain Intrusive Complexes, Maine

Mount Holyoke College, South Hadley MA

Investigators

Abstract

Granitoid plutons constitute much of the earth's crust, and most show magmatic foliation and lineation (i.e. fabric). Although structural geologists have made great strides in quantifying magmatic fabric and interpreting its significance in the context of pluton emplacement, problems remain in using fabric to understand tectonic and magmatic processes, identifying fabric overprinting or modification, and integrating different scales of observation. Does fabric form during magma chamber processes such as deposition of crystal mush on the magma chamber floor, during tectonic processes such as deformation in an active shear zone, or as a composite record of multiple events? Although many processes probably contribute to fabric formation in granites, and the mechanisms by which fabric forms in one pluton may be different from those in another, few widely accepted criteria exist for testing the different models of fabric formation against one another. This project integrates structural and petrological methods in order to test hypotheses about fabric formation in and emplacement of two shallow Silurian plutons in coastal Maine: the Cadillac Mountain and Vinalhaven Intrusive Complexes. Both plutons show quantifiable variations in magmatic fabric, and neither shows evidence for solid-state deformation. In addition to developing new methods for quantifying magmatic fabric in granite, this proposal takes a hypothesis-testing approach to interpreting the origins of fabric. It also supports research by six undergraduate women in a field in which they are under-represented. The research goals are three-fold: Goal 1: Test models for fabric development and modification by quantifying outcrop- scale gradients in granite fabric, texture, and composition. Working on mutually orthogonal faces of large, oriented hand samples of granite, the PI's will use a state-of-the-art fabric analysis technique to quantify the strength and symmetry of fabric defined by each mineral population. They will also use a petrological technique that has never been integrated into a structural study: qualitative assessment of the relative volumes of cumulus crystals and melt present during fabric development. The focus is on gradients in three settings: (a) granite that is associated with a preserved magma chamber floor because it is interlayered with mafic sheets, (b) typical homogeneous granite, and (c) granite associated with mafic schlieren. Goal 2: Conduct pluton-scale studies of magmatic fabric in order to further constrain our interpretations of the origin and the significance of magmatic fabric in both plutons. Using the anisotropy of magnetic susceptibility (AMS), the PI's will map the orientations of magmatic foliation and lineation with respect to both the wall-rock contact and major boundaries between granite facies. The AMS study will be accompanied by detailed work on magnetic mineralogy and the relation of the AMS ellipsoid to rock fabric. Goal 3: Test the hypothesis that the magma chamber floor sagged during pluton emplacement. Previous work, based on mapping of polarity criteria and paleo-horizontal indicators preserved in mafic and felsic layers, has generated a testable model for the emplacement of these plutons by the downward sagging of the pluton floor. With alternating field demagnetization measurements on all cores collected for Goal #2, the PI's will use a paleomagnetic study to attempt to quantify the relative rotation of different parts of the pluton.