22 Undergraduate and Graduate Geoscience Student Lightning Talk Showcase (Posters)

Thursday, 3 April 2025
8:00 AM - 5:30 PM
Sierra Ballroom

ARE SHEAR SENSE INDICATORS PRESERVED IN THE METAMORPHIC SOLE OF THE TUOLUMNE OPHIOLITE IN THE SIERRA NEVADA?

Paper number: 22-15
Program: Theme Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

Tuolumne Ophiolite of the Sierra Nevada Central Belt consists of an ultramafic unit, overlain by Jasper Point and Peñon Blanco Volcanics. The Jasper Point and Peñon Blanco basaltic and andesitic rocks are interpreted to be forearc and nascent arc volcanics, and are cross cut by gabbros that have reported U-Pb ages of 196 and 200 Ma. The ultramafic unit is underlain by a thin strip of high temperature/high-pressure metaclastic, metabasaltic, and metachert that have been interpreted as a metamorphic sole. Metabasalt from the sole is garnet amphibolite that records an early eclogite-facies metamorphic event, indicated by omphacite and rutile in inclusions in garnet. This high-pressure event is then overprinted by a high-temperature event, evidenced by partial melt textures and temperatures of up to 800 °C determined by thermometry on amphibole and pyroxene inclusions in garnet. Two high-temperature metamorphic events are recorded by U-Pb geochronology of zircons. The first at 216-220 Ma is preserved in the metabasalt, while another at 185 Ma is preserved in the metaclastic rock. While the high-pressure minerals indicate the rocks are subducted, it is not clear what the direction of subduction is, with both west-dipping and east-dipping subduction proposed in the literature.

In this presentation we present field and petrographic observations from the metaclastic and metachert rocks that make up the sole. Field observations show that both types of rocks are strongly foliated in the field; however, these preliminary observations don't yield the oriented mineral grains making it difficult to establish the sense-of-shear plane. Additional updated observations will be presented with the poster.

Geological Society of America Abstracts with Programs. Vol. 57, No. 4

doi: 10.1130/abs/2025CD-410194

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