TEXTURAL AND MINERAL COMPOSITIONAL EVIDENCE OF SILICIC CUMULATE FORMATION VIA RHYOLITE MELT SEGREGATION

Schaen et al. (2017; Geology) used bulk rock compositions and zircon petrochronology to hypothesize that the unmixing of a crystal mush produced complementary silicic cumulates and high-silica granites within the ~7-6 Ma Risco Bayo-Huemul plutonic complex (Chile). Here, we use textural characterization by Energy Dispersive Spectroscopy (EDS) phase mapping as well as plagioclase, amphibole, and orthoclase compositions to further evaluate whether Huemul quartz monzonites, enriched in Zr and Ba, are indeed the silicic cumulate crystal concentrate created during melt extraction. Quartz monzonites that display cumulate bulk rock geochemistry have porphyritic textures including ~44 modal %, partially interlocking, 2-5 mm long euhedral plagioclase crystals which commonly appear in synneusis clusters along with fine-grained interstitial orthoclase, plagioclase, quartz, ± biotite. Equilibrium melt calculations using the compositions of the interstitial mineral phases suggest that this matrix represents trapped melt comparable in Ba and Zr concentrations to the adjacent high-silica granite inferred to be extracted rhyolite. These high-silica granites are equigranular and contain abundant miarolitic cavities implying, together with Al-in-Hbl barometry in the coeval quartz monzonite, emplacement and solidification at less than 5 km depth. Plagioclase and orthoclase from the high-silica granites are depleted in Ba, Sr, and Eu, similar to their bulk rock compositions which are thought to be the highly fractionated product of melt extraction. Subparallel amphibole trace element trends (e.g., Ce, Zr, ΣREE) further distinguish the incremental nature the adjacent Risco Bayo pluton as a potential mafic-intermediate source of heat which may have driven mush zone processes in Huemul.