20 Structural Geology and Tectonics

Wednesday, 6 April 2016
8:30 AM - 12:30 PM
Meeting Room 203 C

NEW GARNET PRESSURE-TEMPERATURE PATHS FROM THE CHLORIDE CLIFF AREA, FUNERAL MOUNTAINS, CALIFORNIA, REVEAL A HISTORY OF LATE JURASSIC BURIAL FOLLOWED BY EXHUMATION

Paper number: 20-8
Program: Technical Sessions
Time: 10:45 AM – 11:05 AM

Authors

Abstract

The Funeral Mountains lie within three thrust belts that converge in the area of Death Valley: the Permian Death Valley thrust belt, the Jurassic East Sierran thrust belt, and the Jurassic to early Tertiary Sevier fold-thrust belt. A recently determined garnet age of 158 Ma from the Proterozoic Johnnie Formation in the central Funeral Mountains, Indian Pass area, suggests that burial was associated with the earliest stages of Sevier orogenesis. Pressure-temperature (P-T) conditions and paths in the Indian Pass area indicate a depth of metamorphism of ~20 km, requiring at least 9 km of thrust related burial (Hoisch et al. 2014, Geology). The grade increases to the northwest, from upper greenschist facies at Indian Pass to middle amphibolite facies at Chloride Cliff, across a distance of ~6 km. At Chloride Cliff we are undertaking new studies of P-T paths in the metamorphosed Kingston Peak Formation, in schist containing the primary mineral assemblage quartz + muscovite + biotite + staurolite + plagioclase + garnet. P-T paths were determined using a G-minimization method (Theriak/Domino) overlain by the MATLAB script of Moynihan and Pattison (2013, JMG), which finds the best-fit intersection of the Xsps, Xprp, Xalm and Xgrs isopleths for each point in core-to-rim traverses while adjusting the bulk composition to account for the fractionation of garnet as it grows. Simulations from two samples (4 garnets from one sample and 2 garnets from another) are strongly consistent and show a pressure increase of 0.6 kb following by a decrease of 1.2 kb, all occurring during a temperature increase of 30 °C. The path starts at conditions of ~545 °C and ~5.6 kb (isopleth intersections for the garnet cores). The garnets grew from the breakdown of chlorite in the absence of staurolite. Staurolite grew in conjunction with partial garnet consumption when the P-T path crossed the staurolite-in reaction during decompression. We interpret the initial pressure increase to represent the same burial event recorded in garnets at Indian Pass (Late Jurassic) and hypothesize that the decompression is older than the 67-78 Ma muscovite Ar-Ar ages from the Chloride Cliff area, as the temperatures associated with the decompression segment of the garnet growth history (555-575 °C) are significantly higher than Ar closure in muscovite.

Geological Society of America Abstracts with Programs. Vol. 48, No. 4

doi: 10.1130/abs/2016CD-274485

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