Degassing Driving Magma Hybridization in Plutons: Structural Evidence from the Famatinian Arc in the Puna Plateau, Argentina

Abstract

Arc magmas are rich in fluids that exsolve forming a volatile phase at some point during ascent and crystallization. Despite expectations that this phase could have a significant impact on the evolution of magma reservoirs, there is relatively little recorded evidence of this impact or even of their passage, other than the upper reaches of the chamber. We hypothesize that this mismatch could be due to a lack of recognition of possible magmatic structures associated with this process. Here, we use meso- and microstructures found in variably disrupted dioritic sills and dykes, forming microdioritic and microgranitic enclaves of a large range of sizes, to explore features that could record the passage of a volatile phase and to argue that its passage facilitated hybridization between the two magmas. We focus on structures formed in three plutons of the Early Paleozoic Famatinian Arc in Argentina.The diorites in these plutons are commonly heterogeneous at cm- to dm-scale and have open and filled miaroles, indicative of the presence of a volatile phase. These miaroles tend to form swarms in the heterogeneous rock that also has quartz ocelli and feldspars with rapakivi texture – features indicative of magma hybridization. These heterogeneous rocks are also associated with a number of curious magmatic structures that we speculate record degassing processes and facilitate hybridization by intensifying melt flow and reactivity between the magmas. These structures indicate that the volatile phase caused bubbly diapirs, gas-driven filter-pressing, local catastrophic transfer of crystals across magmatic contacts, and it likely facilitated pervasive flow of invading leucogranitic melt through the pore space of a mushy diorite. Despite the speculative nature of these conclusions, we argue that there is a range of magmatic structures that remain unexplained and that require magma dynamics that could be explained by flow related to degassing.