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)

Abstract

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

High-alumina basalts from the Bogda Mountains suggest an arc setting for Chinese Northern Tianshan during the Late Carboniferous

“…Two different main models have been proposed for the evolution of the B-H belt: 1) a Carboniferous island arc that was subsequently superimposed by a Permian post-collisional orogenic belt (Chen et al, 2011;Laurent-Charvet et al, 2003;Ma et al, 1997;Shu et al, 2011;W.-J. Xiao et al, 2004;Yuan et al, 2010); 2) a Carboniferous-Permian continental rift associated with a mantle plume (Gu et al, 2000(Gu et al, , 2001Xia et al, 2008Xia et al, , 2012.…”

“…Accordingly, most part of the Carboniferous and Permian saw an intraplate setting, and the volcanic rocks erupted during this period may reflect responses to continental rifting (Che et al, 1996;Gu et al, 2000Gu et al, , 2001 or represent a large igneous province associated with a mantle plume (Xia et al, 2004(Xia et al, , 2008(Xia et al, , 2012. Others believed that it closed during the Late Carboniferous (Gao et al, 1998;Shu et al, 2011;Windley et al, 1990). This derives from the idea of a Carboniferous island arc setting to Permian post-collisional orogenic setting (Chen et al, 2011;Laurent-Charvet et al, 2003;Ma et al, 1997;Shu et al, 2011;W.-J.…”

“…Others believed that it closed during the Late Carboniferous (Gao et al, 1998;Shu et al, 2011;Windley et al, 1990). This derives from the idea of a Carboniferous island arc setting to Permian post-collisional orogenic setting (Chen et al, 2011;Laurent-Charvet et al, 2003;Ma et al, 1997;Shu et al, 2011;W.-J. Xiao et al, 2004;Yuan et al, 2010;Zhu et al, 2009).…”

Reconstructing the Late Palaeozoic – Mesozoic topographic evolution of the Chinese Tian Shan: available data and remaining uncertainties

“…2 and 3a). The Permian magmas were then emplaced in a post-collisional setting and several authors suggested that slab break-off and lithospheric delamination may explain their mantle signature (Han et al, 1999(Han et al, , 2009Chen and Jahn, 2004;Zhao et al, 2008;Shu et al, 2010). By Late Carboniferous -Early Permian, the Junggar Basin, disconnected from the Palaeo-Tethys ocean (Sha et al, 2011), initiated as a half-graben structure in a post-collisional extensional setting (Qiu et al, 2005(Qiu et al, , 2008Yang et al, 2012).…”

“…suggested that the continental breakup regime that allowed the emplacement of the Late Permian -Early Triassic volcanic trapps in Siberia (Courtillot et al, 1999) could have generated a widespread extensive event within Central Asia, reaching the Tian Shan area. South of the Bogda Shan and between the Tarim and South Tian Shan, extension allowed the formation of relatively small-scale but deep basins that connected with the close-by Palaeo-Tethys Ocean to the south (in the Tarim) and filled with Permian marine sediments Shu et al, 2010) (Fig. 3b).…”