FROM SHALLOW TO DEEP: CRUSTAL MAGMA PROCESSES BASED ON COMPARISON OF THE AUCANQUILCHA VOLCANIC CLUSTER, CENTRAL ANDES, AND THE TUOLUMNE INTRUSIVE SERIES, SIERRA NEVADA

The Aucanquilcha Volcanic Cluster of the central Andes and the Tuolumne Intrusive Series of the Sierra Nevada Batholith have remarkably similar magmatic evolution, bespeaking a commonality of processes in protracted continental arc magmatism. Although emplaced on different continents and in different periods, the two magmatic loci share the following attributes: life span of about 10 m.y.; a footprint of about 1000 km²; and a crudely concentric distribution from less silicic (andesitic/diorite) to more silicic (dacite/granite) compositions inward. Compositions over 70% SiO₂ are absent in the AVC. Both have a dominant volume pulse of emplacement about halfway through the history and crustal magma residence depths of ~600-100 MPa. In both systems, recycling of zircon antecrysts during successive magmatic injections yields modest age dispersion of zircon ages and is compatible with progressive growth of a large, long-lived, crystal mush body. Geothermometry for AVC reveals that the time of most magma flux corresponds with highest temperature and resetting of the zircon antecryst record. The AVC erupted ~ 320 km³ of magma, which when compared to the 5000 to 10,000 km³ of the TIS, based on present exposure, yields minimum intrusive to extrusive ratios of about 15:1 to 30:1, or ~6-3 % eruptible magma.

The two suites have increasing ⁸⁷Sr/⁸⁶Sr_i with decreasing e_Nd, over a similar range of values. This crustal signature increases with time and corresponds with temporal increase in Sr/Y and Dy/Yb. We infer an increasing role of residual garnet with time. This can be achieved through an expanding MASH zone. Downward expansion through addition of magma drives thickening and so expands the role of residual garnet at depth. Upward expansion through crustal heating can contribute mid to upper crustal isotopic crustal components. On the other hand, the garnet signature and the crustal isotopic signature can be directly coupled if upper crustal emplacement of magma in a growing batholith leads to thickening and loading, thus displacing fertile middle crustal lithologies into the garnet residual zone. The striking similarity of the AVC with the TIS indicates that a volcanic arc complex has a thick (>10 km) plutonic root of similar composition and that fosters involvement of shallow crustal lithologies with the deep crustal magma nursery.