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Late Ordovician 40Ar/39Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex of Inner Mongolia (northern China) and implications for the early Palaeozoic history of continental blocks in Central and East Asia

Abstract : We obtained 453.2 ± 1.8 Ma and 449.4 ± 1.8 Ma (2σ) laser step-heating Ar/Ar plateau ages for phengite from mylonitic meta-chert from the blueschist-facies Ondor Sum subduction-accretion complex in Inner Mongolia. These ages are within error of the inverse isochron ages calculated using the plateau steps and the weighted mean ages of total fusion of single grains. The compositional change from glaucophane in the cores to crossite in the rims of blue amphiboles, as revealed by electron microprobe analysis, points to decompression, probably caused by progressive exhumation of the subducted material. The Late Ordovician ages were not affected by excess or inherited argon because in all likelihood the oceanic sediments were wet on arrival at the trench and free of older detrital mica. The ca. 450 Ma ages are, hence, interpreted as the time of crystallization during mylonitization under high fluid activity at fairly low temperatures. This means that accretion of the meta-chert unit occurred about 200 Ma before final closure of the Palaeo-Asian Ocean, amalgamation of the Siberian, Tarim and North China cratons, and formation of the end-Permian Solonker suture zone that can be followed from Kyrgyzstan in the west to northernmost North Korea in the east. We argue that the early Palaeozoic evolution of the Ondor Sum complex occurred along the Cimmerian margin of East Gondwana, which was composed of micro-continents fringed by subduction-accretion complexes and island arcs. Only the later evolution took place during the building of the Eurasian continent following middle Devonian and younger rifting along the Gondwanan margin and northward drift of the detached North China craton. Such a two-stage scenario probably also applies to the geodynamic evolution of other micro-continents in Asia. This implies that a vast orogenic system, comprising an archipelago of micro-continents, seems to have existed along the Cimmerian margin of East Gondwana in early Palaeozoic time in which the ultrahigh-pressure metamorphism that characterizes the early evolution of many of the Asian micro-continents occurred. < Am. J. Sci., in press >
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https://hal-insu.archives-ouvertes.fr/hal-00120365
Contributor : Nathalie Pothier <>
Submitted on : Thursday, December 14, 2006 - 3:59:05 PM
Last modification on : Friday, April 5, 2019 - 8:08:16 PM

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Koenraad de Jong, Wenjiao Xiao, Brian F. Windley, Hideki Masago, Ching-Hua Lo. Late Ordovician 40Ar/39Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex of Inner Mongolia (northern China) and implications for the early Palaeozoic history of continental blocks in Central and East Asia. 2006, 1p. ⟨hal-00120365⟩

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