PARTIAL MELTING ALONG SUBGRAIN BOUNDARIES: A POTENTIAL FACTOR INFLUENCING THE RHEOLOGY OF CRUSTAL ROCKS

Migmatitic melt networks and pathways extend throughout areas of the middle and lower crust, and remnants of these features have been recognized over a range of scales from kilometers to nanometers. Increased melt presence leads to rock weakening, with potentially dramatic effects on the overall strength of the crust. However, rheologic weakening depends not just on the volume of melt, but also on how well it is connected. Here, we present new data from former melt networks along grain and subgrain boundaries, demonstrating that partial melting along subgrain boundaries is pervasive and may play an important but previously unrecognized role in influencing crustal rheology.

We conducted a combined energy dispersive spectroscopy (EDS)-electron backscatter diffraction (EBSD) study of six domains that contain former melt in two thin sections from the Wet Mountains in central Colorado. EDS data was used both to document phases present in thin section, but also to identify the former melt, which is now preserved as plagioclase. We calculated the crystallographic misorientation across these former melt channels using EBSD data to determine whether melting proceeded along grain or subgrain boundaries in quartz. Data from all six domains are remarkably consistent, with approximately 30% of all former melt located along subgrain boundaries within quartz grains. Partial melting along subgrain boundaries has only recently been documented, but these observations indicate it may be a common process. The development of melt networks along subgrain boundaries will promote integration of an interconnected melt network, facilitating rheologic weakening in strained migmatitic rocks.