FROM CRYSTAL TO CRUSTAL-SCALE: ORIGINS OF "NORMAL" AND RECYCLED RHYOLITES, MICRO-ISOTOPIC EVIDENCE AND NUMERICAL MODELS

The advent of chemical and isotopic microanalytical methods permits high-resolution (μm-scale) study of minerals, determining mineral-diffusive timescales, giving rise to rapidly “accelerating” petrogenetic models. In particular, rapid pace of discovery of oxygen isotope diversity in zircons and other minerals in large volume ignimbrites worldwide suggests that this phenomenon characterize many silicic units studied so far by in situ methods. The rejuvenation of interest to 1.5 century-long recognition of mineral zoning in igneous rocks pose questions about its meaning with respect to large-scale, magma-body-wide processes. We here present evidence and advocate for a silicic magma body as dispersed agglomeration of batches with distinct zircons; sometimes batches merge into larger-volume magma bodies, mixing crystals together and erupting quickly. Assembly process results in changing temperature/supersaturation conditions. Concave-up crystal size distributions of zircons and quartz in studied voluminous ignimbrites can be explained by just two episodes of reprecipitation. This may suggests that it takes one to three batch merging episodes before assembly of an eruptable large volume magma body. We here present result of finite element modeling of these processes. We use visco-plastic rheology for surrounding rocks to explain the formation of magma batches generated side by side and merging them together. We observe: 1) Fast convective melting with low heat dissipation. 2) Efficient mixing on large (kilometers) horizontal scales, due to the vigorous flow field induced by compositional convective melting of silicic predecessors by superheated rhyolites. Chaotic, meter-scale vortexes of altering directions cause disintegration of the liquid parcels to diminishing size. The marker method allows us to track particle mixing. 3) Mechanical interaction of the closely-spaced sills in hot visco-plastic upper crust with low yield stress (<100 MPas) leads to mechanical failure (both brittle and plastic) of separating screens leading to the coalescence of sills into a single body. We suggest that analogous processes of rapid two-stage segregation may characterize granitic batholith, and large supervolcanic magma bodies formed by remelting.