Non-buoyancy driven liquid segregation in magma mush systems: implications for the origin of high silica liquids, reversely zoned plutons, and episodic eruption

The origin of high silica granites and rhyolites is a hotly debated topic. These highly evolved compositions require extreme fractionation, traditionally interpreted as minute fractions of residual liquids formed during fractional crystallization or from small degree partial melts of pre-existing rocks. However, because the viscosities of silicic liquids are so high, efficient segregation of these liquids from the crystal matrix in both processes is difficult. Alternative hypotheses for generating granites include gravity-driven compaction of magmatic mushes or Soret diffusion via thermal migration of cations. We conducted a petrologic and geochemical transect across a vertically oriented pluton-wallrock contact in the Cretaceous Peninsular Ranges Batholith in southern California. The pluton interior is tonalite in composition (68-70 % SiO2), but becomes progressively enriched in silica (75 %) within 30 m of the contact with pelitic to quartzitic metamorphic wallrock. This Si-enriched chemical boundary layer also coincides exactly with a depletion in Mg, Ca and Ti towards the wallrock. Mafic enclaves are abundant in the pluton interior and are shown to be thermally metamorphosed fragments of the wallrock, clear evidence that the pluton has assimilated significant amounts of wallrock. Parodoxically, however, enclaves are absent within the 30 m chemical boundary layer. Furthermore, trends in major and trace-elements and oxygen and lead isotopes within the boundary layer show no evidence for mixing with wallrock. The linear compositional arrays, instead, are more easily explained by unmixing of mafic minerals and plagioclase from tonalite. Collectively, these observations can be used to assess various hypotheses for the formation of high Si granites. For example, the horizontal gradient in composition and enclave density cannot be explained by gravitational processes. The identical thickness of each chemical boundary layer and the enclave-depleted halo cannot be explained by Soret-type diffusion or by horizontal migration of crystals and xenoliths into low strain regions in the interior driven by shear-induced pressure gradients, the latter because migration velocities should differ for different grain or clast sizes. We propose, as an alternative hypothesis, that the high Si boundary layer was generated by outward expulsion of Si-rich, eutectic interstitial liquids and accompanying inward compaction of the solid matrix driven by over-pressures within the magma body itself. We speculate that sideways expulsion of silicic liquids at pluton margins can explain the numerous cases of "reversely" zoned plutons. Only after the Si-rich boundary layer reaches a critical thickness, will it have sufficient buoyancy and reduced viscosity (due to its crystal-poor nature) to rise rapidly to the top of the pluton, where it collects as a large silicic magma chamber. During the lifetime of a crystal-mush system, we predict several such pulses of silicic magma generation.