Cryptic climatic signatures and tectonic controls on Cryogenian diamictites in the NW Tarim Craton, China

摘要

Unravelling the origins of Cryogenian diamictites is vital in determining their significance in a snowball Earth context and their relationship to environmental processes. Here, we present new sedimentological and geochemical data for the Cryogenian sedimentary rocks from the Tarim Craton, NW China. The diamictite member of the East Qiaoenbrak Formation contains (1) granitic pebbles with high sphericity, (2) well-developed stratification and (3) bed-scale grain-size changes suggesting cyclicity; also, (4) ice-rafted debris is conspicuously absent. Rather than deposits of direct glacial origin, the succession is instead interpreted as a subaqueous fan complex. The major and trace element ratios and REE patterns are consistent throughout 22 sampled intervals in the Qiaoenbrak Group, suggesting a uniformly felsic source area, probably from an island arc. Chemical index of alteration (CIA) analysis indicates low CIA values (49.5-54.3, average 51.6) throughout the Qiaoenbrak Group. We suggest that both the Qiaoenbrak Group and Yuermeinak Formation were deposited during the Marinoan glaciation, in a long-lasting cold and dry interval. It is likely that the Qiaoenbrak Group was deposited in a faultcontrolled basin adjacent to an active continental margin, although the material may ultimately have derived from a glacial source (e.g. outwash).