Proterozoic Evolution of the Mojave Crustal Province as Preserved in the Ivanpah Mountains, Southeastern California
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In the western U.S., numerous exposures of Precambrian crustal rocks were exhumed from mid-crustal depths during Cenozoic tectonic events and provide a view into the deep crust. This natural laboratory provides an excellent opportunity to study orogenic and continent-forming processes at deeper levels. The Ivanpah Mountains of southeastern California contain exposures of Paleoproterozoic migmatites and banded gneisses that belong to the Mojave crustal province. New, detailed U–Pb geochronology of monazite and zircon from ortho- and paragneisses reveal the timing of the formation of these rocks and multiple periods of metamorphism and magmatism. Paragneisses have detrital zircon populations of dominantly 1.80–2.15 Ga (dominant) and 2.4–2.8 Ga, although the youngest detrital zircon grain is 1.754 ± 24 Ga. Values of δ18O(zircon) of the two detrital age populations are 7.0 ± 2.2‰ and 6.3 ± 1.4‰ (2SD), respectively, consistent with original, igneous values. Based on the 1.76 Ga age of metamorphic zircon rims, the metasediments were intruded by a calc-alkaline magmatic suite including gabbro, tonalite, and porphyritic granite at 1.76 Ga, although the contacts are obscured by pervasive metamorphism and deformation.
U–Pb ages of metamorphic overgrowths on detrital zircons from the paragneisses indicate multiple events at ∼1.76, 1.74, 1.70 and 1.67 Ga which documents a periodicity of tectonism through time. The oxygen isotope ratios of the metamorphic zircon overgrowths from the paragneisses remained relatively constant through time within each sample, consistent with growth during in situ partial melting. However, oxygen isotope ratios of zircons from 1.74 Ga leucocratic material are lower in δ18O, and therefore could not have been generated by partial melting of the adjacent paragneiss. Monazite from both igneous and metasedimentary rocks preserves two periods of growth at 1.75 and 1.67 Ga. In situ, U–Pb dating of monazite reveals that 1.67 Ga monazite inclusions are found in major rock-forming minerals, and therefore we interpret this as the timing of fabric formation and migmitization, which is younger than previously believed and is interpreted to be the dominant metamorphic pulse in the protracted middle crustal tectonism. Thermobarometry indicates peak metamorphic conditions of ∼3.5 kb and ∼740 °C, which is consistent with partial melting and the metamorphic mineral assemblage in these rocks (sillimanite + K-feldspar, muscovite absent). This paper shows (1) multiple pulses identified within 100 Ma orogeny, (2) zircon components and their overgrowths demonstrate mixing of Archean, 1.8 and younger reservoirs, (3) low pressure granulite facies metamorphism occurred at 1.67 Ga and may have been related to continental extension of a tectonically over-thickened crust.
Strickland, A., et al. (2013). Proterozoic evolution of the Mojave crustal province as preserved in the Ivanpah Mountains, southeastern California. Precambrian Research, 244, 222-241. DOI: 10.1016/j.precamres.2012.09.006
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