TEMPORAL, SPATIAL AND GEOCHEMICAL MAGMATIC FOCUSING OF PLUTONIC AND VOLCANIC FIELDS IN THE CENTRAL SIERRA NEVADA, CALIFORNIA

The temporal, spatial and geochemical focusing of magmatism during continental arc flare-ups can result in volumetrically large volcanic eruptions, increased tectonic activity and ore genesis, as has been established at volcanic clusters in the San Juan volcanic region (USA) and Aucanquilcha (Chilean Andes, South America). However, these trends have not been explored in the plutonic footprints beneath such volcanic fields. We present mapping, geochronology and whole-rock geochemistry from a belt of Cretaceous plutons, sub-volcanic porphyritic feeder zones and erupted volcanic products exposed in the central Sierra Nevada that offer a view of magmatic focusing through the upper crustal magma plumbing system.

During the Late Cretaceous flare-up of this arc (ca. 130 Ma - 85 Ma), plutons, porphyry feeder zones and volcanic deposits show temporal and geochemical focusing in one area of the central Sierra Nevada from at least 115 Ma to 85 Ma. The Tuolumne Intrusive Complex was involved in, and formed the last recorded locus of magmatism. The recognition of localized porphyritic feeder zones of the same age and isotopic character as plutons and volcanics in the focusing region, are exciting field laboratories, where the plutonic and volcanic fields are linked and provide the opportunity to study the vertical collection and transport of magmas during focusing in an arc flare-up.

The spatial scale of focusing is comparable to the San Juans and larger than the Aucanquilcha volcanic cluster; the temporal duration of focusing is 2-3 times longer in the Sierras than documented in the above volcanic examples. In all three areas plutonic/volcanic ratios are estimated to be ~30/1. The geochemistry of plutons, porphyries and volcanic rocks over this 30 Myr period in the central Sierra Nevada evolves with spatiotemporal trends in the focusing region. Changing Sr/Y ratios indicate a period of crustal thickening from ~30 to 60 km during the magmatic focusing, in agreement with isostatic mass balance modeling, while εNd and Sr_i isotopes in both plutonic and volcanic units trend towards more evolved compositions. We suggest that this focusing and growth of large intrusive complexes in the central Sierra reflects thermal maturation of the system whereas the evolving chemistries reflect both crustal thickening and internal magma dynamics.