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 it in plutonic rocks, other than the late metasomatic alteration in the upper levels of plutons. We hypothesize that this could be due to a lack of recognition of magmatic structures associated with the passage of this fluid phase through crystallizing plutons. Here, we use meso- and microstructures found in variably disrupted dioritic sills and dikes, forming microdioritic and microgranitic enclaves of a large range of sizes in a granitic host, 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 centimeter- to decimeter-scale and have open and filled miaroles, indicative of the presence of a volatile phase. These miaroles tend to form swarms in heterogeneous rocks that also have 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 an 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.