SHRIMP zircon U–Pb age constraints on Neoproterozoic Quruqtagh diamictites in NW China

Abstract

The Neoproterozoic Quruqtagh Group in the Tarim Block, NW China, contains multiple diamictites in the Bayisi, Altungol, Tereeken, and Hankalchough formations. These diamictites may represent three or possibly four discrete glaciations, although evidence for a glacial origin of the Bayisi and Altungol diamictite is ambiguous. To constrain their age and duration, we dated three volcanic beds (V1, V2, and V3) in the Quruqtagh Group using the SHRIMP (sensitive high-resolution ion microprobe) zircon U-Pb method. Volcanic bed V1 near the base of the Bayisi diamictite yields a 740 ± 7 Ma age, volcanic bed V2 near the top of the Bayisi Formation gives a 725 ± 10 Ma age, and volcanic bed V3 between the Tereeken and Hankalchough diamictites yields a 615 ± 6 Ma age. V1 and V2 have overlapping ages, and together these dates suggest that the Bayisi diamictite was deposited at around 730 Ma. The Tereeken and Altungol diamictites were deposited between 725 ± 10 Ma and 615 ± 6 Ma, and the Hankalchough diamictite between 615 ± 6 Ma and ~542 Ma (i.e., the Neoproterozoic-Cambrian transition). These dates and previously published chemostratigraphic data are consistent with (but doe not require) the correlation of the Tereeken and Hankalchough diamictites with the 635 Ma Nantuo and 582 Ma Gaskiers glaciations, respectively. However, the new dates are inconsistent with a single and globally synchronous Sturtian glaciation that occurred in the pre-Nantuo Neoproterozoic Era. Instead, currently available data necessitate that either multiple glaciations occurred, or a globally diachronous glacial event developed during a protracted period between ~750 Ma and ~650 Ma.

Geochemistry of Altungol cap dolostones from the Tarim Basin, NW China

“…The Altungol Formation unconformably overlies the Bayisi Formation and is conformably overlain by Tereeken Formation. The Zhaobishan Formation, which is reported in the other sections (such as Xishankou section and Yukengou section) (Shen et al 2008;Xu et al 2009), is missing in the Yaerdany area. In the YI section, the Altungol Formation consists of a ∼45-m-thick tillite, a ∼5-m cap dolostone, and a ∼20-m shale.…”

“…A volcanic unit in the topmost Bayisi Formation gave a SHRIMP zircon U-Pb age of 725 ± 10 Ma (Xu et al 2009). He et al (2014) reported U-Pb zircon ages of 655.9 ± 4.4 and 654 ± 10 Ma from the upper Altungol Formation in Quruqtagh.…”

“…A complete Neoproterozoic succession is distributed in the study area, containing four sets of glaciation sedimentary strata, referred to as Bayisi, Altungol, Tereeken, and Hankalchough Formations (Xu et al 2009;Gao et al 2010). They are correlated to Kiagas, Sturtian, Marinoan, and Gaskier Formations (Macdonald et al 2010a, b;Gao and Chen 2003;Hoffman and Li 2009), respectively, based on the U-Pb zircon age of volcanic units in these formations.…”

“…This study presents stable isotope, major, and trace metal concentrations of a Sturtian-aged cap dolostone overlying the Altungol glacial strata (Gao and Chen 2003;Xu et al 2009;Hoffman and Li 2009;Gao et al 2010). The data of δ 13 C carb chemostratigraphy reported by Kou et al (2008) suggested that Altungol Formation may be similar to the most intensively studied Sturtian cap carbonate in the Rasthof Formation, Namibia (Hurtgen et al 2002;Macdonald et al 2010a, b;Hoffman 2011), and other post-Sturtian cap dolostones (Giddings and Wallace 2009).…”

Petrology and zircon U-Pb dating combined with Hf isotope study of granitic rocks from the Kuluketage Block (Tarim Craton, NW China)

“…Middle Neoproterozoic to Phanerozoic rocks consist of mafic dyke swarms, bimodal volcanics as well as fine sandstones, siltstones, shales, dark limestones and chert nodule-bearing dolomites. The mafic dykes with bimodal volcanics were formed at 820-744 Ma and 650-630 Ma (Zhang et al 2007a;Zhu et al 2008;Xu et al 2009). Late Neoproterozoic glacial deposits are also well exposed (Xu et al 2009).…”