PETROLOGIC EXAMINATION OF A MESOZOIC SUPER-ERUPTION: TUFF OF SKELTON LAKE, MT. MORRISON PENDANT, SIERRA NEVADA, CALIFORNIA

In the early Mesozoic, the subduction of the ancient Farallon plate beneath the North American plate gave rise to the Sierra Nevada batholith, a vast magmatic system that stretches along much of modern-day eastern California. Within this batholith is the tuff of Skelton Lake in the Mount Morrison Pendant, southwest of Mammoth Lakes, California. The tuff is the remnants of a volcanic event that exhumed approximately 160 km³ of tephra. This tuff offers a glimpse into one of these ancient magma chambers to better understand the evolution of Sierran arc magmatism throughout time.

Three transects were sampled throughout the tuff beginning at Duck Lake, through Barney and Red lakes, and ending at Arrowhead Lake; however, these transect samples are homogenous with respect to phenocryst abundance and silica concentration. The tuff is moderately crystal rich, with 15 to 37% feldspar and euhedral to anhedral (broken) quartz phenocrysts. Undulose extinction and subgrain development in quartz phenocrysts record post-emplacement deformation. Bulk-rock analyses indicate the tuff is rhyolitic, with silica ranging from 71 to 75% and an average of 73.2% by weight. Moderate and variable K2O (3.3 – 6%) and low Na2O abundances (<1.5%) are indicative of post-deposition hydrothermal alteration. The tuff samples are light rare earth enriched, with slightly concave-up middle and heavy rare earths and modest Eu anomalies (Eu/Eu* 0.7 – 0.9).

The phenocryst abundances and correlations between silica, alumina, and trace element geochemical data suggest that crystal fractionation of plagioclase + quartz + zircon + sphene within the magma chamber can explain the observed mineralogic and chemical variations. This process may also correlate the tuff of Skelton Lake to modern arc intrusive rocks such as those that may underly the Okataina Volcanic Center in the Taupo Volcanic Zone of New Zealand, where the anatomy of such systems has yet to be determined.