Spatiotemporal magmatic focusing in upper-mid crustal plutons of the Sierra Nevada arc

In the upper crustal sections of arcs, spatiotemporal focusing of magmatism involves the inward migration and younging of many volcanic centers, as well as homogenization of erupted magma, over spatial scales of ∼102 to 105 km2 and temporal scales of 105-107 yr. Magma focusing occurs during, and may be a consequence of, magmatic flare-ups in arc systems. The occurrence of magmatic focusing is well documented in volcanic systems, but has been largely inferred for the underlying plutonic footprints. At the plutonic level, focusing predicts similar inward migration and younging of plutonic bodies and the growth of larger, long-lived magma chambers in which magma compositions shift towards “monotonous” biotite-hornblende granodiorite that potentially feed large volcanic eruptions. In addition, focusing predicts increased magma devolatilization, the formation of ore deposits, and increased rates of emplacement-related regional deformation. In the central Sierra Nevada, CA, we find: (1) a pattern of spatial and temporal focusing in a field of Cretaceous plutons between ca. 102 and 85 Ma, with the Tuolumne Intrusive Complex (TIC) involved in and occurring at the center of this focusing; (2) metamorphic pendants in this same area preserve volcanic rocks between 102 and 95 Ma involved in focusing; (3) a number of coeval subvolcanic porphyry intrusions in this area that are potential links between these volcanic and plutonic fields. Plutons show an inward younging, increase in the size of final plutonic complexes, and trends towards hybrid compositions, with restriction in the range of 87Sr/86Sri and εNd isotope ratios. Thus, plutonic focusing contributes to the construction and longevity of large, central magma chambers that increase the potential for large-scale fractionation and thorough mixing, and provide a source for large volcanic eruptions. Although focusing is likely a transcrustal process, this study documents the continuation of magma focusing from surface to upper-mid crustal levels. We speculate that other Late Cretaceous intrusive complexes in the Sierra Nevada, California (e.g. Sonora, John Muir, and Whitney intrusive suites) may all reflect focused systems.