18 Syncrystallization Evolution of Granitic Magma in Orogenic Belts

Sunday, 20 March 2011
1:30 PM - 3:00 PM
Lawrence Welk

SCHLIEREN STRUCTURES IN THE MOUNT WALDO GRANITIC PLUTON, MAINE; IMPLICATIONS FOR MAGMA CHAMBER PROCESSES

Paper number: 18-5
Program: Theme Sessions
Time: 2:30 PM – 2:45 PM

Authors

Abstract

The dynamic processes that occur within magma chambers such as magma ascent, convection, flow, and crystallization are not well understood. This is the result of emplacement at depth and the general lack of accessibility to magma chambers. Fortunately, a vast amount of knowledge that may provide insight into magma chamber dynamics may be obtained by evaluating the magmatic structures persevered within granitic plutons. Schlieren are layers or streak-like concentrations composed of medium- to coarse-grained mafic minerals such as biotite and hornblende and often contain anomalously high concentrations of accessory minerals, for example magnetite, apatite, and others. These structures can be highly variable in shape, ranging from planar layers and trough-like features to ellipsoids and cylindrical structures. Various hypotheses have been proposed to describe the formation of these magmatic structures: gravitational settling, magmatic flow, convection currents, magmatic stopping, and disaggregation of enclaves and/or xenoliths. Schlieren preserve evidence of the physical processes and crystallization events occurring within the magma chamber and can provide significant insight to these dynamics systems.

Current analysis of schlieren in the Mount Waldo Granite, Maine is underway in hopes to determine the mode of formation of these structures. Field evidence suggests the possible formation mechanism to be magmatic flow or convection currents. Through the evaluation of crystallographic preferred orientations of schlieren-forming minerals by way of microscopy and electron backscatter diffraction (EBSD), the above hypotheses will be tested in attempt to determine the resulting mechanism. This information will not only provide insight to the development of the Mount Waldo pluton, but will also contribute to a more extensive understanding of the rheology, dynamics, and physical processes that transpire within magmatic complexes.