Squashing Crystals with the Big Felsic Enclave: Magma Dynamics in the Aztec Wash Pluton

Abstract

Analyzing magmatic fabrics in the Aztec Wash Pluton (~15.5 Ma) illuminates magma dynamics during the emplacement of a big felsic enclave (BFE) in a granitic pluton. Evidence suggests that the BFE settled at the boundary between crystal-rich and crystal-poor zones of the magma body at the time of impingement. Textural and compositional analysis using a Scanning Electron Microscope (SEM) allow visualization of a deforming crystal mush. Four thin sections sampled from the granite surrounding the BFE (three located underneath, one in the far field) show a change in alkali feldspar foliation around the enclave. Using the SEM, imaging using Backscatter Electron (BSE) imaging, Energy Dispersive Spectrometry (EDS), and Electron Backscatter Diffraction (EBSD) show the textural and compositional differences between the samples located underneath the BFE and the sample from the far field granite, specifically the existence of an oriented fabric directly underneath the enclave. BSE images show multiple instances of broken feldspar grains in the samples located underneath the enclave. BSE and EDS display large euhedral alkali feldspar crystals located underneath the enclave, while the far field sample has alkali feldspar grains with irregular outermost rims. EBSD data indicate that there is an oriented foliation of the alkali feldspar underneath the BFE that does not exist in the far field. This evidence suggests that a crystal mush was deformed during the emplacement of the BFE, causing the alkali feldspars to reorient and causing some feldspars to break. This deformation indicates that compaction occurred underneath the enclave and may have been paired with melt extraction.