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1 Department of Geology, University of California, Davis, California 95616, USA
A global survey of sutures predating formation of the supercontinent Rodinia, i.e., older than ca. 0.9–1.0 Ga, has yielded evidence for at least 35 described and possible ophiolite complexes. Results indicate that ophiolitic complexes older than 1.0 Ga mostly lack mantle tectonites below the magmatic rocks. Geochemically, these pre-Rodinian ophiolites display a mix of supra-subduction zone (SSZ), mid-oceanic-ridge, and oceanic- island compositions, with higher proportions of komatiite in complexes older than 1.6 Ga. Magmatic ages of identified and inferred complexes tend to cluster at times of 1.0–1.5 Ga, 1.8–2.3 Ga, ca. 2.5–2.7 Ga, and ca. 3.4 Ga. The data mostly support the hypothesis that magmatic oceanic crust was thicker prior to 1.0 Ga and thinned abruptly at 1.0 Ga. Mesozoic oceanic plateaus, e.g., the Caribbean, may provide an analogue to pre–1 Ga oceanic crust. A modified ophiolite columnar section to accommodate pre–1 Ga complexes includes a "1972 Penrose Conference–type" section in which the igneous rocks are overlain by a thick accumulation of interlayered sedimentary and volcanic rocks, as well as high-level intrusive complexes and plutons. Thinning of oceanic crust, the end of the anorthosite-anorogenic granite "event’ at 1000 Ma, and discontinuous hotspot activity through time suggest discontinuous magmatic evolutionary processes in Earth history, similar to other planets. Differences in thickness and "subductability" of oceanic crust may provide clues to the Archean-Proterozoic tectonic transition. The stratigraphic and sea-level effects of the inferred 1 Ga thinning of oceanic crust include widespread continental emergence, development of seasons, and increased atmospheric oxygen leading to the Phanerozoic.
Key Words: Archean • oceanic crust • ophiolite • Precambrian • Proterozoic • tectonics
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