4-Dimensional Insights into Silicic Magma Reservoir Assembly from Late Miocene Southern Andean Plutons

Abstract:

Linking the development of magmatic flow fabrics to the T-X-t history of intraplutonic domains using modern structural and petrochronologic methods offers a frontier along which to explore for eruptability in plutonic rocks and better understand how shallow magma systems are assembled. The ~6.2 Ma Risco Bayo and Huemul plutons in the Chilean Andes (~36°S) exhibit a similar compositional spectrum and footprint to the active Laguna del Maule rhyolitic volcanic field nearby. The plutons comprise distinct lithological domains-each on the order of a few km³: gabbro to granite in Risco Bayo and granodiorite to leucogranite in Huemul. Whole rock variations and ⁸⁷Sr/⁸⁶Sr ratios highlight the importance of AFC processes during pluton assembly. Mixing and mingling of magma batches is observed geochemically and in the field as abundant mafic enclaves. U-Pb CA-ID-TIMS zircon ages of 6.36 to 6.18 Ma in Risco Bayo granodiorite and Huemul miarolitic leucogranite overlap with their ⁴⁰Ar/³⁹Ar biotite ages of ~6.24 Ma, suggesting coeval zircon saturation, emplacement, and rapid cooling of two compositionally and structurally distinct domains. A granodiorite domain within Huemul has a more protracted crystallization history and zircons with lower REE than in the adjacent miarolitic leucogranite, suggesting pluton construction by pulses on 10 to 100 kyr timescales. Al-in-hornblende barometry constrains emplacement to 3-4 km depth. Observations from Laguna del Maule (extrusion/intrusion rates, spatio-temporal pattern of mafic/rhyolitic volcanism, etc.) provide volcanic parallels with which to help interpret pluton assembly. AMS fabrics suggest possible upward migration of magma associated with decompression. Late Miocene silicic ignimbrites nearby are also being investigated to determine if they represent erupted products from the Risco Bayo-Huemul plutonic system.