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Textural and geochemical evidence for melt extraction from a magma mush in Aztec Wash Pluton (Nevada, USA)

Elizabeth Teeter , Guilherme Gualda, Calvin Miller

Abstract

Magmatic textures and compositions illuminate magma dynamics during the emplacement of a large (2 m diameter) felsic enclave (LFE) within granitic mush in Aztec Wash Pluton (15.7 Ma, NV-USA). The LFE is an ellipsoidal microgranite mass inferred to have been nearly solid when it settled at the top of a crystal-rich zone of the magma body. Backscattered Electron (BSE) imaging, Energy Dispersive Spectroscopy (EDS), and X-ray Fluorescence whole-rock data for samples from the granite surrounding the LFE show key textural and compositional differences between granite immediately beneath the LFE and granite located more distant and to the side ("far-field"). Alkali feldspar crystals beneath the enclave have well-defined euhedral rims, while those in the far-field have irregular overgrowths, suggesting continuing growth into larger melt pools. The samples underneath the enclave (69.6-70.0% SiO2, 1100-1170 ppm Ba, 400-430 ppm Sr) are less felsic than the far-field sample (72.1% SiO2, 750 ppm Ba, 330 ppm Sr), also suggesting less retained melt. We conclude that impingement of the LFE led to enhanced melt extraction. The composition of the far-field sample is typical of Aztec Wash pluton samples interpreted to represent cumulate mush (70.0-72.4% SiO2, 640-1100 ppm Ba, 250-440 ppm Sr; Harper et al. 2004 and our new data). For comparison, "non-cumulate" Aztec Wash samples that may represent input magma are distinctly more felsic (72.3-73.9% SiO2, 570-690 ppm Ba, 170-270 ppm Sr). Our beneath-LFE compositions indicate the greatest melt depletion identified among Aztec Wash granites, substantially more than in the typical cumulates.