Geological Society of America

T169 Proterozoic Crustal Evolution of Southern Laurentia: Tectonic Settings, Petrology, Geochemistry, and Geochronology I

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Room 302

1.67-1.66 GA PEAK METAMORPHISM AND DEFORMATION IN THE IVANPAH MOUNTAINS, MOJAVE PROVINCE

Paper number: T169-5
Program: Technical Sessions
Time: 9:10 AM – 9:25 AM

Authors

Ariel Strickland Presenting Author
University of Wisconsin - Madison, Madison, WI
Joseph L. Wooden
Stanford University, Stanford, CA
Chris Mattinson
Central Washington University Dept. of Geological Sciences, Ellensburg, WA
David M. Miller
U.S. Geological Survey, Menlo Park, CA
Takayuki Ushikubo
University of Wisconsin, Madison, WI
Reinhard Kozdon
University of Wisconsin-Madison, Madison, WI
John W. Valley
University of Wisconsin, Madison, WI

Abstract

Accessory minerals from Proterozoic migmatites in the Ivanpah Mountains of southeastern California preserve age and d¹⁸O evidence for multiple tectonic events. Exposures of N-S striking, steeply west-dipping, garnet + biotite + sillimanite + k-feldspar + plagioclase + cordierite + quartz-bearing metasediments contain abundant detrital zircons that are mantled by metamorphic overgrowths. Combined SHRIMP U-Pb and IMS-1280 δ¹⁸O analyses of these grains reveal detrital age populations of 2.8-2.4 Ga and 2.0-1.8 Ga with δ¹⁸O values of 6.50 ± 0.57‰ and 7.38 ± 3.22 ‰, respectively, consistent with Archean vs. Proterozoic igneous d¹⁸O worldwide. The metamorphic zircon overgrowths have been dated ca. 1.76, 1.74, 1.70, and 1.66-1.67 Ga and have δ¹⁸O values that are consistently >2‰ higher than detrital cores (~6-11 ‰), suggesting that the rims reflect partial melting of enclosing high d¹⁸O metasediment. Co-existing monazites record two periods of metamorphic growth at 1.74 and 1.66-1.67 Ga, and older monazites contain muscovite inclusions, which are absent from the host rock.

The multiple periods of metamorphism preserved by the zircon and monazite correlate with previously reported events in the Mojave province, but the tectonic significance of these ages is not well-constrained in the Ivanpah Mountains. In the field and in thin section, the migmatitic layering is penetrative and metamorphic minerals are coarse-grained and appear strain free. Garnets are ubiquitous in the metasedimentary rocks, and frequently have poorly-developed strain shadows of biotite and inclusions of aligned biotite, suggesting that the garnets are late syn-deformational. In situ U-Pb analyses show that monazite inclusions in garnet, biotite and cordierite are as young as 1.66 Ga, and trace element concentrations in metamorphic zircon show a strong HREE depletion after ~1.70 Ga, consistent with late garnet growth. By linking the ages and chemistry of the accessory minerals to the garnet, we can conclude that much of the garnet and the high-grade migmatitic fabric developed ca. 1.66 Ga, and that pre-existing fabrics are no longer preserved. Thus, peak metamorphism and deformation occurred in the Ivanpah Mountains at 1.67-1.66 Ga, at a time when the Mojave Province is thought to be far inboard of the active continental margin.