VOLCANIC PETROLOGY INFORMS PLUTONOLOGY OF THE AUCANQUILCHA VOLCANIC CLUSTER, ANDES, AND COMPARISON TO THE TUOLUMNE INTRUSIVE SERIES

Volcanic rocks give intriguing clues to their plutonic history stored in the assemblage and composition of their constituent phases. We here consider how volcanic rocks erupted at the Aucanquilcha volcanic cluster (AVC) in the central Andes both reveal and obscure their plutonic record. Aucanquilcha volcano lies at the center of a ~250 km² cluster of more than 20 volcanoes that record eleven million years of magmatic history in the central Andes, 21°S, Chile. The footprint, age span, compositional range, temporal magma emplacement history and depth of magma residence mimic those of the Tuolumne Intrusive Series (TIS).

Magmas erupted in the AVC are almost exclusively lavas. These range from 56 to 68 wt. % SiO₂, and ⁸⁷Sr/⁸⁶Sr_i of 0.7052-0.7068 with both generally increasing toward the center of the cluster, analogous to the normally zoned TIS which has main range of 56-72 wt. SiO₂ and ⁸⁷Sr/⁸⁶Sr_i of 0.7057-0.7069. A sharp increase in eruption rate in the AVC after about 5 m.y. coincides with a change from sampling igneous processes occurring between 0.8-0.1 GPa and 1100 - 650 °C to sampling a restricted range of 0.250 ± 0.1 GPa and 900 ± 50 °C, based on amphibole, pyroxene, Fe-Ti oxide and zircon thermobarometry. The staging depth of the dominant erupted volume is consistent with depth of emplacement of the TIS 0.35 to 0.05 GPa, with most at 0.2 GPa (~6 km) and with pre-eruptive depth for many ignimbrites.

A deeper, more cryptic and more voluminous intrusive history underlies the AVC. Persistent input of magma at depth is required to drive the thermal waxing of the system and to support crustal assimilation mandated by radiogenic isotope data. We can infer that the depth from which magmas are delivered to the pre-eruptive staging region has deepened in time, because Dy/Yb ranges to higher values in time, consistent with an increasing role for residual garnet. Petrologic models require a vast, largely mafic intrusive volume, although no mafic magmas were erupted. As much a 5 km were added to the crust, as both fractionated residua and as crystallized magma that delivers heat to support crustal assimilation.