Implication of the chemical index of alteration as a paleoclimatic perturbation indicator: an example from the lower Neoproterozoic strata of Aksu, Xinjiang, NW China

Abstract

The Neoproterozoic successions in the Aksu region, NW China, which lies unconformably on the Precambrian Aksu Group basement, comprises the Qiaoenbrak, Yuermeinak, Sugetbrak, and Chigebrak formations (from bottom to top). The two lowermost units include two distinct glacial diamictites, which indicate distinct episodes of glaciations. We report the major and trace element (including rare earth element) data for the Qiaoenbrak, Yuermeinak, and Sugetbrak formations to identify the paleoclimatic perturbations. The chemical index of alteration (CIA) values show variations from Qiaoenbrak to Yuermeinak, then Sugetbrak formations. The diamictites have relatively lower chemical index of alteration values (45.23–59.64) than inter-, post- and non-glacial sediments (48.28–66.96). This result supported the condition that the diamictites underwent relatively weak chemical weathering from a dry-cold sedimentary environment, which is associated with the sedimentary facies description. The lower Neoproterozoic successions recoded at least two glaciations, one is Qiaoenbrak glaciation and the other is Yuermeinak glaciation.