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Proterozoic ophiolites from Yanbian and Shimian (Sichuan Province, China): petrography, geochemistry, petrogenesis, and geotectonic environment

Publication date1992
Abstract

The Middle Proterozoic ophiolites from the Yanbian and Shimian areas (Sichuan Province, China), which occur as allochthonous fragments embedded within the Late Proterozoic orogenic belt on the western rim of the Yangtse Craton, consist of mantle-derived tectonite peridotites, plutonic cumulates, and submarine volcanics. The mantle sequence is made up of serpentinized harzburgites, representing an impoverished residual material after a high degree partial melting of upper mantle (> 30%). The plutonic sequence comprises ultramafic cumulates, Mg-gabbros (troctolites, Ol-anorthosites and Ol-gabbros), gabbronorites (including Ol-free gabbros), ferrogabbros, and albitites. The volcanic sequence consists of massive basalts with "sheet-flow" textures, pillowed basalts with associated hyaloclastites, and small amounts of ferrobasalts and keratophyres. This sequence is injected by sills and two types of dikes (dikes I and dikes II). Pelagic sediments occur as intercalations and cover of the volcanic piles. Two magmatic suites are present. Suite I includes all the plutonic rocks and the great majority of volcanic and hypabyssal rocks. These rocks are characterized by an early crystallization of plagioclase prior to clinopyroxene and comparable to those from N-type Mid-Ocean Ridges. Such a magma would have been the liquid derived by a low degree partial melting (about 15%) from a slightly depleted mantle, and then submitted to a differentiation at low pressure (< 5 kb). In contrast, suite II is only represented by the dikes II, which are characterized by an early appearance of clinopyroxene as phenocrysts, and chemically comparable to plume-type MORB. This magma would have been derived from an enriched mantle also by a low partial melting (about 15%), undergoing later a differentiation at a relatively higher pressure (> 5 kb). None of the two suites can be considered as representing the liquid extracted from the studied peridotites. Using the 1-atm pseudo-quaternary system Ol-Pl-Di-Q and the distribution coefficient between mineral and liquid, the formation ranges of different plutonic rocks and the magmatic correspondence between plutonic and the suite I volcanic rocks are also determined. Comparing with the characteristics of different present-day oceanic environments, it is likely that the Yanbian and Shimian Proterozoic ophiolites represent oceanic remnants which might have been formed at hot-spot- influenced spreading centers of small incipient oceanic basins, such centers being probably located in the vicinity of transform faults. The similarity of these ophiolites with present-day oceanic rocks indicates that the modern plate- tectonic processes may already have been active as early as Middle Proterozoic time.

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Keywords
  • Proterozoic
  • Ophiolites
  • Petrography
  • Geochemistry
  • Petrogenesis
  • Geotectonic environment
  • Sichuan Province, China
Citation (ISO format)
SUN, Chuan Min, VUAGNAT, Marc. Proterozoic ophiolites from Yanbian and Shimian (Sichuan Province, China): petrography, geochemistry, petrogenesis, and geotectonic environment. In: Schweizerische mineralogische und petrographische Mitteilungen, 1992, vol. 72, n° 3, p. 389–413. doi: 10.5169/seals-54921
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ISSN of the journal0036-7699
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