DISCOVERY OF MARINOAN-LIKE CAP CARBONATE IN THE AKSU-WUSHI AREA, NW CHINA: IMPLICATIONS FOR TARIM’S RECONSTRUCTION IN THE NEOPROTEROZOIC

Field investigations in the Aksu-Wushi area of the northwestern margin of Tarim Craton (NW China) have led to a new discovery of a distinctive Marinoan-like pink cap carbonate (2m thick or less) directly and conformably overlying sporadically exposed diamictite (also 2m thick or less). These units lie atop an angular unconformity that we correlate to an unconformity between two earlier-described glaciogenic diamictite-bearing successions in the Central Wushi Mountains. There, the diamictites have been considered to represent the Sturtian (Dongqiaoenbrak Fm) and Marinoan (Yuermeinak Fm) ice ages. Our discovery of the Marinoan-like cap carbonate above the unconformity strengthens these correlations, as well as correlations with strata in the adjacent Quruqtagh region (NE Tarim): the Dongqiaoenbrak Fm would correlate with the Sturtian-age Bayisi-Altungol Fm, while the Yuermeinak Fm would correlate to the Marinoan-ageTereeken Fm with the similar pinkish cap carbonates atop both.

We summarize the following events across northern Tarim: eruption of 740 -725 Ma bimodal volcanics in Quruqtagh, Aksu blueschist metamorphism and exhumation after 730 Ma, intrusion of NW mafic/alkalic dykes (Korla) at 640 Ma, tholeiitic volcanismin Sugetbrak Fm at 615 Ma, and passive-margin development into Cambrian time. We propose a tentative paleogeographic model that places Tarim adjacent to South China in a combined “missing link” position at the center of Rodinia, between Australia and Laurentia. Because the Aksu dyke pole is similar to that of the Sugetbrak Fm, and should be younger than the ca. 730Ma Aksu blueschists, we suggest that the previous zircon age for the Aksu dykes may have measured only xenocrysts, and that the Aksu dykes more readily correlate with the Korla dykes at ca. 640 Ma. Our hypothetical model, albeit highly speculative, implies two episodes of Tarim’s movement, recording rifting of Rodinia and subsequent evolution toward Gondwana: (1) after 750 – 725 Ma rifting, Tarim rotated about 180° relative to Australia, and recollided with NE Australia to produce the blueschist event coincident with the early part of Sturtian ice age; (2) subsequent rifting at 615 Ma separated Tarim finally from its Australian heritage, starting a tectonic odyssey that would culminate in late Paleozoic incorporation into eastern Asia.