T162 Initiation and Evolution of Arc-Forearc Systems in Cascadia and Beyond (Posters)

Tuesday, 12 October 2021
9:00 AM - 1:00 PM
Exhibit Hall A

GEOCHRONOLOGY AND THERMOBAROMETRY OF THE GARNET AMPHIBOLITE AT THE BASE OF THE TUOLUMNE OPHIOLITE, SIERRA NEVADA CENTRAL BELT, CALIFORNIA

Paper number: T162-4
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

  • Jo Black Presenting Author
    California State University, Sacramento, Sacramento, CA
  • David Shimabukuro
    California State University, Sacramento, Sacramento, CA
  • Steven Skinner
    California State University, Sacramento, Sacramento, CA
  • Sierra Rack
    California State University, Sacramento, Sacramento, CA

Abstract

The Central Belt of the Sierra Nevada consists of ophiolitic fragments, some of which are high-pressure and high-temperature amphibolite, along with Triassic to Jurassic arc rocks. One of the more notable amphibolite exposures occurs in the Tuolumne Ophiolite, west of the town of Chinese Camp, where previously-published work has described chert and coarse-grained garnet-rutile-epidote mafic amphibolite lenses with a K-Ar age of 200±10 Ma and Ar-Ar ages of 200-178 Ma on hornblende. The position of the high-grade rocks concentrated at the base of the ultramafic sequence has led to proposals that this is a metamorphic sole. In order to understand the timing, origin, and P-T history of these rocks we implement a combination of outcrop-scale field mapping, geochronology, and quantitative mineral thermobarometry.

Rock samples were collected from the garnet amphibolite and sent for U-Pb zircon geochronology with the LA-ICP-MS at the Arizona LaserChron Center. Thin sections samples were analyzed for mineral chemistry with an electron microprobe and the resulting data used to determine pressure and temperature conditions through garnet-plagioclase-hornblende-quartz barometry and garnet-hornblende thermometry.

Field mapping indicates that the amphibolite occurs as lenses at multiple levels within the structurally lowest part of the ultramafic body. Preliminary thermobarometry indicates metamorphism at a pressure and temperature of 9.0-9.8 kbar at 470oC. U-Pb zircon geochronology on one sample yields ages of ~216 Ma which along with the existing K-Ar hornblende age of 200±10 Ma indicates a short time span between formation of the mafic protolith and high-pressure and high-temperature metamorphism, consistent with formation during initiation of subduction near a mid-ocean ridge.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-370496

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