REFINING THE CHRONOLOGY OF INTRACALDERA MAGMATISM FOLLOWING THE FORMATION OF YELLOWSTONE CALDERA

Voluminous rhyolite lavas composing the Upper Basin Member (UBM) of the Plateau Rhyolite provide insights into the timing of magma rejuvenation and the proportion of residual magma within Yellowstone’s subvolcanic reservoir following the climatic caldera-forming eruption of the Lava Creek Tuff (LCT). The Biscuit Basin rhyolite was originally identified as a single lava flow that erupted ca. 520 ka [1]. However, oxygen isotopes, ⁴⁰Ar/³⁹Ar ages, and bulk rock geochemistry [3-5] reveals that the Biscuit Basin rhyolite includes at least five sub-divisions (North, South, East, Middle Biscuit Basin and a new unit introduced here) that likely represent distinct flow units. High-spatial resolution dating of unpolished rims on indium-mounted zircon from the East and Middle Biscuit Basin flows yield ²³⁰Th-corrected ²³⁸U-²⁰⁶Pb crystallization ages of ca. 625 ka, similar to the age of zircon rims from the LCT. Interiors of cross-sectioned zircons from East Biscuit Basin flow yield ages that are ~50 kyr older than their rims. Rims on zircon crystals from a newly identified sub-division of Biscuit Basin yields ages of ca. 590 ka. These results identify the oldest UBM units. The ca. 675-625 ka ages for zircons from the East Biscuit Basin flow suggest it may represent rejuvenation of residual LCT magma “mush”. Unpolished rims on zircon from the Scaup Lake and South Biscuit Basin flows yield U-Pb ages of ca. 250 ka that are indistinguishable from the ⁴⁰Ar/³⁹Ar eruption age [6], with the cores of Scaup Lake zircons yielding ages up to 90 kyr older. Clinopyroxene phenocrysts in the Scaup Lake, South Biscuit Basin and North Biscuit Basin flows all exhibit cores with exsolution lamellae and identical Fe-Mg and trace element zoning, suggesting some common lineage for the younger UBM magmas. The ages for younger UBM rhyolites suggest a similar episode of rejuvenation and magma genesis from ca. 340-250 ka, which resulted in smaller volume eruptions.

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