Textural and Geochemical Evidence for Melt Extraction from a Magma Mush in Aztec Wash Pluton (Nevada, USA)

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ID: 332517
2026
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Abstract
Abstract Melt extraction is a critical process for the formation of eruptible magmas. Magmatic textures and compositions illuminate magma dynamics during the emplacement of a large (2 m diameter) felsic enclave (LFE) within granitic mush in the 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 imaging, energy-dispersive spectroscopy, 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”). We constrain melt extraction during impingement of the LFE through 1) thin section measurement of areas interpreted to have accumulated crystals and trapped melt, and 2) mass balance calculations based on whole rock analyses. For approach 1): 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. We use these textures to constrain how much melt was present before and after emplacement of the LFE by calculating area percentages based on the phases present. We use alkali feldspar rim, interstitial quartz, and Na-rich plagioclase rim area measurements to estimate a minimum of 31-35% and maximum of 57-60% melt under the LFE and a minimum of 39% and maximum of 63% melt in the far field. The percent difference between retained melt fractions in the far field vs under the enclave indicate 5-20% extracted melt after impingement of the LFE. For approach 2): 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. 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). Simple mass balance calculations using average cumulate granite compositions, under-LFE granite compositions, and high-silica border facies (interpreted to approximate extracted melt composition) produce melt extraction estimates of 15-20%. We conclude that impingement of the LFE led to enhanced melt extraction. We estimate that the Aztec Wash pluton experienced 35% melt depletion from the initial magma to create the cumulate granite we observe, and 45% melt loss to create the enhanced cumulate granite below the LFE. Our beneath-LFE compositions indicate the greatest melt depletion identified among Aztec Wash granites, substantially more than in the typical cumulates.
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Authors Elizabeth Teeter, Guilherme A.R. Gualda, Calvin F. Miller
Journal journal of petrology
Year 2026
DOI
10.1093/petrology/egag089
URL
https://doi.org/10.1093/petrology/egag089
Keywords Keywords not found

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