Identifying Trapped Melt in Low Pressure Silicic Cumulates Using Textural Mapping and Mineralogy

Abstract:

The mush model posits that rhyolite is generated within upper crustal reservoirs via distillation and extraction of melt from magma that remains in a largely crystalline state for 10⁴-10⁵ yr. This model requires the formation of complementary silicic cumulates in the middle to upper crust, yet these residues have been notoriously difficult to identify in the plutonic rock record. One of the challenges surrounding identification of silicic cumulates is the inherent variability of granitoid textures which are less readily recognized as cumulate than their mafic counterparts. Here, Energy Dispersive Spectroscopy phase mapping reveals textures of quartz monzonites within the ~7-6 Ma Risco Bayo-Huemul plutonic complex (Chile) which display silicic cumulate geochemistry. Textures are porphyritic and comprise ~44 modal % of partially interlocking, 2-5 mm long euhedral plagioclase crystals which are commonly in synneusis clusters, together with euhedral biotite and orthoclase, 1-3 mm long. The fine-grained interstitial matrix is composed of orthoclase, plagioclase, and quartz. Equilibrium melt calculations using the compositions of the interstitial phases suggest that this matrix may represent trapped melt comparable in Ba concentration to the adjacent, coeval, high-silica granite inferred to be extracted and frozen rhyolite. These high-silica granites are equigranular and contain miarolitic cavities implying, together with Al-in-Hbl barometry in the quartz monzonite, emplacement and solidification at ~5 km depth. Plagioclase, orthoclase, and biotite from the high-silica granites are depleted in Ba, Sr, and Eu, similar to their bulk rock compositions and supporting an origin as highly fractionated products of melt extraction. Distinct Ti, Zr, Nb, and REE variations in amphibole document incremental emplacement of the adjacent gabbroic to granodioritic Risco Bayo pluton. The latter is a heat source which for hundreds of kyr may have incubated a shallow crystal mush that occasionally reached conditions ideal for rhyolite melt extraction.