Timing of initiation of extension in the Tianshan, based on structural, geochemical and geochronological analyses of bimodal volcanism and olistostrome in the Bogda Shan (NW China)

摘要: This paper describes an olistostrome formation and accompanied bimodal volcanic rocks occurring in the Baiyanggou area, south of Bogda Shan. The main lithotectonic units consist olistostrome, turbidite. is tectonically underlain by Upper Carboniferous limestone sandstone along a NEE-trending detachment fault. Paleo-growth fault locally observed. unit includes plenty blocks limestone, sandstone, rhyolite volcaniclastic rocks, matrix graywacke. Limestone are dated as Pennsylvanian-Bashkirian age coral brachiopod fossils that extensively recognized strata. consists pillowed massive basalt rhyolite, latter occur 8- to 10-meter-thick layer above unit. turbidite mainly composed chert, siliceous mudstone within which Bouma sequence can be recognized. Meter-wide gabbro diabase dykes intrude these three units. Geochemically, rhyolites characterized high ACNK value >1.1, depletion Ba, Nb Sm, enrichment Rb, Th Zr. Basaltic rich K2O, they show LREE-enriched pattern Zr, Ti, Ce Hf, similar continental rift-type tholeiite series. A porphyrite intruding was at 288 ± 3 Ma sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb method, 297 ± 2 Ma laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS) method. accompanying series linked extensional setting developed Several lines evidence, e.g. occurrence large-scale strike-slip shear zones, large number mantle-derived magmatic available geochronological data, demonstrate significant geodynamic change from convergence extension Chinese Tianshan belt, even whole Central Asian Orogenic Belt. belt initiated ca. 300 Ma, i.e. around Carboniferous–Permian boundary times, peak period intra-plate magmatism occurred interval 300–250 Ma.