2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 227-16
Presentation Time: 9:00 AM-6:30 PM


ARDILL, Katie E. and PATERSON, Scott R., Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy, Zumberge Hall of Science (ZHS), Los Angeles, CA 90089-0740, kardill@usc.edu

It is rare to find coeval volcanic and plutonic rocks or volcanic feeder systems preserved in the calc-alkaline Mesozoic Sierran arc, making studies of the links between magmatic and volcanic systems challenging. Previous workers proposed that a Triassic caldera system was exposed at Tioga Pass. Our new field mapping and LA-ICP-MS U-Pb zircon ages indicate that the Tioga Lake granodiorite and intrusive dacite porphyry exposed at Tioga Pass are not Triassic but ~97-99 Ma. A large displaced and rotated block of westward younging Koip sequence Triassic volcanics and interbedded volcaniclastics, the latter with a ~217 Ma U-Pb detrital zircon minimum peak, is intruded by the dacite porphyry. It is cross cut by dacitic-rhyolitic dykes. Nearby volcanics to the north range from ~95-113 Ma, indicating that these units are within uncertainty coeval to the Tioga Pass magmatism. The dacite porphyry makes up a significant proportion of the sub-volcanic system and discordantly intrudes Upper Paleozoic/Lower Triassic host rocks of the Saddlebag Lake Pendant. It is particularly heterogeneous, consisting of crystal rich flow bands, clastic and tuffaceous bands, as well as mafic-intermediate blocks of host rock material. The biotite-hornblende rich granodiorite has in places, a gradational contact with the dacite porphyry. Whole rock elemental data from Tioga Pass show some differentiation trends between the dacite porphyry and granodiorite. The Cretaceous dacite porphyry (SiO2 range 63-71%) may be an intermediate-felsic shallow component of the magmatic system, relative to the deeper Tioga Lake granodiorite (SiO2 range 56-63%). It is thus permissible that the dacite porphyry and Tioga Lake granodiorite may be co-genetic as well as coeval, representing an upper crustal section of a magmatic feeder system to Cretaceous volcanics. New mapping and geochronology elsewhere in the central Sierra establish that 95-100 Ma plutons and volcanics are widespread around the Tuolumne Intrusive Complex, indicating that this feeder system is a part an extensive Late Cretaceous volcanic-plutonic system.