TRACKING THE EVOLUTION OF MID CENOZOIC SILICIC MAGMA SYSTEMS IN THE SOUTHERN CHOCOLATE MOUNTAINS REGION, CALIFORNIA USING ZIRCON GEOCHEMISTRY AND QUARTZ AND ZIRCON GEOTHERMOMETRY

The Chocolate Mountains region of southeastern California experienced mid-Cenozoic crustal extension slightly before, during, and after the main pulse of magmatism. Regional extension combined with mid-late Cenozoic faulting to locally uplift plutonic rocks interpreted to represent the plumbing system(s) for volcanic units, allowing an examination of both the extrusive and intrusive products of magmatism.

Zircon U-Pb ages of six magmatic units are late Oligocene to early Miocene and correlate better with stratigraphic relationships than previously reported ages. There are four silicic volcanic units – Quechan volcanic rocks, tuff of Felipe Pass, ignimbrite of Ferguson Wash, and tuff of Black Hills – and two plutonic units – the granites of Mount Barrow and Peter Kane Mountain. Comparing the contemporaneous volcanic and plutonic systems, zircons from the volcanic units commonly record plutonic temperatures, interpreted to be solidus or near solidus temperature. Remobilization may have been a common process leading to eruption of the ignimbrites.

Quartz and zircon thermometers suggest the ignimbrite of Ferguson Wash and tuff of Black Hills magmatic systems evolved differently. Quartz yields temperatures of 700°C to ~750°C in both units with no core-rim trends. Cores of zircons from the ignimbrite of Ferguson Wash yield temperatures between 750°C and 890°C. Zircon rim temperatures are between 875°C and 950°C. Tuff of Black Hills zircon cores generally record temperatures of ~850°C and zircon rim temperatures are ~700°C. Rims from tuff of Black Hills zircon record the same temperature range as zircons from coeval granites.

The tuffs of Black Hills and Ferguson Wash record different thermal histories than young systems such as the Bishop Tuff, in which quartz records reheating prior to eruption that is not recorded in zircons. The temperature increase from core to rim in zircons from the ignimbrite of Ferguson Wash also indicates reheating, yet zircon grew later and at higher temperatures than quartz. The zircon temperatures from tuff of Black Hills reveal a system that was growing quartz and zircon at the same low, nearly solidus temperatures. Processes leading to eruption of this tuff are not readily apparent in the thermal history of zircon and quartz.