Chapter 33 Neoproterozoic glaciogenic diamictites of the Tarim Block, NW China
Neoproterozoic glaciogenic outcrops are mostly limited to the southwestern and northwestern margins of the Tarim basin and the Tianshan Mountains. Only two glaciogenic diamictites are recorded in the Neoproterozoic successions from the Tielikeli and Aksu-Wusi areas of the Tarim basin. The lower diamictite from the Polong Formation in the Tielikeli area is c. 800 m thick. The upper diamictite in the area is very thin (Yutang diamictite, 10 m; Yulmeinak diamictite, 51 m). In contrast to the Tarim basin, the Neoproterozoic successions along the northern margin of the Tarim Block in the Tianshan Mountains record three or four glaciogenic diamictites, which are well developed and crop out in the Qurugtagh area. Although the glaciogenic nature of the oldest Beiyixi diamictite remains questionable, all other diamictites (Altungol, Tereeken and Hankalchough) exhibit distinct glaciogenic sedimentary features. SHRIMP zircon U–Pb dating demonstrate that the Bayisi diamictite was deposited between 740±7 Ma and 725±10 Ma, the Altungol and Tereeken diamictites between 725±10 Ma and 615±6 Ma, respectively, and the Hankalchough diamictite between 615±6 Ma and c. 542 Ma. Carbonates atop the Altungol, Tereeken and Hankalchough diamictites show distinct C-isotopic values that are typical for those recorded in ‘cap carbonates’ on other continents. The striking similarity between the cap carbonate of the Tereeken and Marinoan-age Nantuo diamictites further support a 635 Ma ‘Marinoan’ age for the Tereeken glaciation. Unlike the 582 Ma Gaskiers glaciations of Newfoundland, the Hankalchough diamictite is possibly <551 Ma, as suggested by C-isotope chemostratigraphy and biostratigraphy, indicating that post-Marinoan glaciations on different continents may be diachronous.