Paper No. 5
Presentation Time: 9:10 AM
1.67-1.66 GA PEAK METAMORPHISM AND DEFORMATION IN THE IVANPAH MOUNTAINS, MOJAVE PROVINCE
STRICKLAND, Ariel, Department of Geoscience, University of Wisconsin - Madison, 1215 W Dayton St, Madison, WI 53706, WOODEN, Joseph L., USGS-Stanford Ion Microprobe Laboratory, Stanford University, Stanford, CA 94305, MATTINSON, Christopher G., Geological Sciences, Central Washington University, 400 E University Way, MS 7418, Ellensburg, WA 98926, MILLER, David M., U.S. Geological Survey, 345 Middlefield Road MS 973, Menlo Park, CA 94025, USHIKUBO, Takayuki, Department of Geoscience, University of Wisconsin, 1215 West Dayton Street, Madison, WI 53706, KOZDON, Reinhard, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706 and VALLEY, John W., Department of Geoscience, University of Wisconsin, Madison, WI 53706, strick@geology.wisc.edu
Accessory minerals from Proterozoic migmatites in the Ivanpah Mountains of southeastern California preserve age and d
18O 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 δ
18O analyses of these grains reveal detrital age populations of 2.8-2.4 Ga and 2.0-1.8 Ga with δ
18O values of 6.50 ± 0.57‰ and 7.38 ± 3.22 ‰, respectively, consistent with Archean vs. Proterozoic igneous d
18O worldwide. The metamorphic zircon overgrowths have been dated ca. 1.76, 1.74, 1.70, and 1.66-1.67 Ga and have δ
18O values that are consistently >2‰ higher than detrital cores (~6-11 ‰), suggesting that the rims reflect partial melting of enclosing high d
18O 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.
