Textural and Mineralogical Record of Low-pressure Melt Extraction and Silicic Cumulate Formation in the Late Miocene Risco Bayo–Huemul Plutonic Complex, Southern Andes

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One model for rhyolite generation in the upper crust is via extraction of interstitial melt from crystal-rich magma reservoirs. Although silicic magma reservoirs may grow incrementally over c. 104–105 yr timescales, they can be remobilized prior to eruption much more rapidly (c. 101–103 yr). This process implies the formation of cumulate residues with a composition complementary to the extracted melt, but the predicted cumulates have so far eluded widespread identification. The 7·2–6·2 Ma Risco Bayo–Huemul plutonic complex comprises ∼150 km3 that is subdivided into at least seven gabbroic to granitic domains emplaced at <7 km depth. Distinct Ti, Zr, Nb, and rare earth element variations in amphibole document pulsed emplacement of the gabbroic to granodioritic Risco Bayo pluton ∼800 kyr prior to the adjacent quartz monzonite to high-silica granite of the Huemul pluton. The quartz monzonite is inferred to be a silicic cumulate on the basis of whole-rock mass balance and enrichments in Ba and Zr concentrations. Here, we combine fine-scale textural analysis using energy-dispersive X-ray spectrometry (EDS) phase mapping with in situ mineral compositions to explore the silicic cumulate hypothesis. Quartz monzonite textures are porphyritic and comprise ∼58–64 modal % of partially interlocking, 2–5 mm long euhedral plagioclase, together with euhedral biotite, orthoclase, and amphibole. The finer-grained interstitial matrix is composed of anhedral orthoclase, plagioclase, and quartz. Calculations using the compositions of the interstitial phases suggest that the matrix represents a highly evolved melt similar in major and trace element chemistry to coeval, high-silica granite inferred to be extracted and frozen rhyolite. The high-silica granites are equigranular and contain dense concentrations of miarolitic cavities implying, together with Al-in-Hbl barometry for the quartz monzonite, emplacement and volatile saturation within the upper crust at the granite minimum. Plagioclase, orthoclase, and biotite in high-silica granite are depleted in Ba, Sr, and Eu, similar to their bulk-rock compositions, and support an origin as highly fractionated products of melt extraction. Miarolitic cavities within the high-silica granite overlying the granite domain suggest that volatiles played an important role in the upward percolation of melt. Our observations indicating transport of volatiles and melt through an underlying crystal mush, including accumulation of vapor bubbles at the roof of the magma reservoir, are consistent with recent numerical simulations of multi-phase fluid dynamics within compositionally zoned silicic reservoirs that feed large explosive eruptions.


Journal of Petrology
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