Perspectives on Magma Mush Deformation: Textural and Geochemical Constraints on Compaction and Melt Extraction in a Granitic Magma Body
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2025-07-09
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Magmatic textures and compositions illuminate magma dynamics, such as compaction and melt extraction. Preserved textures in plutons (crystallized magma bodies) or magma mush fragments (crystal-rich, melt bearing clasts potentially genetically related to the erupted magmas) can provide important perspective on mush processes in both inactive and active magmatic systems respectively. The emplacement of a large (2 m diameter) felsic enclave (LFE) within granitic mush in Aztec Wash pluton (15.7 Ma, NV-USA) is preserved within a granitic body. 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), Electron Backscatter Diffraction (EBSD), 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. We calculate that emplacement of the LFE in a magma mush enhanced melt extraction, segregating 10-20% melt from the crystal-rich framework.
Magma mush fragments – crystal-rich, glass bearing clasts potentially genetically related to the erupted magmas – can provide important perspective on mush processes in active magmatic systems. Granitoid clasts from the Rotoiti Ignimbrite, Taupō Volcanic Zone (TVZ), include magma mush fragments that can provide insights on magmatic systems of one of the most active areas of rhyolitic volcanism in the world. This study focuses on a large (35 cm) clast found within a lag-breccia of the 64 ka Rotoiti Ignimbrite, one of the largest eruptions from the Ōkataina Volcanic Center (TVZ). This fragment is layered and can be divided into 5 zones based on compositions and textures. Backscattered Electron (BSE), Cathodoluminescence (CL), and Energy Dispersive Spectrometry (EDS) techniques highlight the textural and mineralogical differences between the zones. In this study, we focus on a fine-grained, biotite granite zone with microcrystalline patches and sparse granophyric texture. Quartz grains range from euhedral to subhedral, with complex zoning patterns that are visible in both BSE and CL. Some feldspar grains are zoned, others are not. The microcrystalline patches make up less than 10% of the sample and they are surrounded by subhedral grains (both quartz and feldspar). The interstitial texture and distribution of the microcrystalline patches indicate that this fragment is representative of a magma mush with <10% trapped melt. The average compositions of this microcrystalline material range from 72-77% SiO2 and 4-6% K2O and yield rhyolite-MELTS model pressures of ~100 MPa. This melt composition does not correspond with Rotoiti eruptive products; this mush fragment may illuminate non-eruptive sequences of the TVZ. Tying textures and compositions in both active and inactive magmatic systems can provide key perspective on the generation of eruptible magma bodies.
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Granite, magma rheology, magma mush, plutonic lithic