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岩石学报 2011
U-Pb ages of zircons from ophiolite and related rocks in the Kumishi region at the southern margin of Middle Tianshan, Xinjiang: Evidence of Early Paleozoic oceanic basin
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Abstract:
The Yushugou and Tonghuashan ophiolitic mélange, which occurs in the Kumishi region at the southern margin of the Middle Tianshan, Xinjiang, consists of mantle peridotite, cumulate pyroxenite, gabbro and anorthosite, diabase dikes and mafic lavas overlain by chert. Geochemical data indicate that the ophiolitic units originated in both MORB and SSZ tectonic settings. U-Pb isotope ages of zircons from the ophiolitic and related rocks record at least four tectonic events related to oceanic spreading and basin closure: (1) Ocean basin formation in the Ordovician-Silurian, based on the ages of oceanic crust (439.3±1.8Ma for plagiogranite and 435.1±2.8Ma for anorthosite; (2) Island arc magmatism in the Silurian, based on the ages of island arc volcanic rocks and granites (422.1±2.6Ma for dacite and 423.1±1.8Ma for granodiorite); (3) Shearing in the Devonian (400Ma for mylonite), which was probably related to oblique subduction and collision between plates that formed large-scale, strike-slip faults; (4) Extensional magmatism of the orogen in the Late Carboniferous (294.8±1.2Ma for quartz syenite porphyry that formed perpendicular to the direction of subduction and collision). In addition, some granulites in the Yushugou area are regraded as a separated block from the ophiolite, because most of zircons from granulites have old cores with ages of 500~1800 Ma and appear to have had a complex protolith and a very different metamorphic history from the ophiolite. The granulites appear to have had a complex protolith and a very different metamorphic history than the ophiolite .
