T266 Accessory Minerals as Monitors of Magmatic Processes: New Ideas, Applications, and Pitfalls (Posters)

Wednesday, 3 November 2010
8:00 AM - 6:00 PM
Hall D

NEW U-PB SHRIMP-RG ZIRCON AGES OF MINERAL KING ROOF PENDANT RHYOLITE UNITS REVISE THE VOLCANIC HISTORY OF THE SIERRA NEVADA, CALIFORNIA

Paper number: T266-8
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

New U/Pb SHRIMP-RG ages of meta-rhyolites from the Mineral King roof pendant document an extended and more complex early volcanic history in the southern Sierra Nevada, CA. The meta-rhyolites of the Mineral King roof pendant in the southern Sierra Nevada of California are thought to represent a series of submarine calderas that periodically erupted during the beginning of Sierran subduction-induced magmatism. Of the five rhyolite units (R0–R4), previous multigrain zircon U/Pb TIMS analyses (Busby-Spera, 1983) yielded only two concordant ages. Units R3 and R4 yielded eruption ages of 217 Ma and 190 Ma, respectively. Two meta-rhyolite units (R and R4) were dated using in situ SHRIMP-RG zircon U/Pb analyses. These new ages provide a mean age of ~136 Ma for unit R0, with a range of 120 to 140 Ma, and further refine the previously determined age of R4 to a mean age ~196 Ma. For most analyzed zircon, the core and rim ages of the respective zircon populations were within error. However, one zircon from R0 was found to have a core of ~1.6 Ga, suggesting inheritance of zircon from a much older source previously unrecognized in the Mineral King pendant. The R4 zircon are very uniform in their trace element composition while the R0 zircon exhibit a wider range of compositions, suggesting changing magmatic sources over the development of the early Sierran arc.

These new ages modify the previous interpretation of the volcanic history of the Mineral King roof pendant. Previous studies suggested that the eastern portion of the roof pendant becomes younger to the east. However, the R0 rhyolite is located to the west of both R3 and R4, and this, combined with its ~136 Ma age, suggests the distinct units within the roof pendant have a more complex spatial relationship than previously thought. Additionally, the age of R0 expands the duration of rhyolitic volcanism recorded in the Mineral King roof pendant from ~30 Ma to ~80 Ma. Few volcanic or plutonic rocks have been identified within the southern Sierra Nevada that have ages in the range of the 120-140 Ma R0 meta-rhyolite. This suggests that the R0 rhyolite may be one of the few records of magmatism during what is currently thought to be a period of magmatic quiescence within the Sierran arc (ca. 145-120 Ma).