Abstract
Abstract Sedimentary processes constitute essential factors in the evolution of the crust and the atmosphere. Their role during Precambrian time is relatively little known, however, due to the frequent alteration of Precambrian rocks by deformation and metamorphism. The objective of the present paper is to document the occurrence of a metasedimentary sequence with inferred abundant dolostone—limestone—evaporite—shale—arkose, and interbedded basalt/dolerite—rhyolite/granite, metamorphosed to granulite grade in middle Proterozoic time (ca. 1000 Ma ago). The basis for this interpretation is the abundance of sulfur and chlorine-rich scapolites, sulfides, alkalic feldspar, and sometimes gypsum and anhydrite in calc-silicates, interlayered with magnesian marble, quartzo-feldspathic gneisses, pyroxene amphibolites and aluminous ultramafic granulites within the paragneissic section of the Oaxacan Complex. The calc-silicates are interpreted to have been formed in part from evaporitic beds, the marbles from siliceous dolostones, the quartzo-feldspathic gneisses from arkosic red beds and from rhyolite/granite igneous rocks, the amphibolites from chloritic marls and from alkalic basalt/dolerite, and the ultramafics from magnesian clays. This lithologic association is comparable to deposits associated with the development of continental rifts and, therefore, a similar initial depositional environment is considered likely for part of the sequence. The common description of high-grade terranes of early to middle Proterozoic age with similar lithologies in all continents, suggests the prevalence at that time of very extensive regions with arid climates, under tensional tectonic conditions. It also indicates a possibly much greater abundance of evaporites in the Precambrian than is generally believed.
Published Version
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