Kaigas

Abstract: The snowball Earth hypothesis predicts globally synchronous glaciations that persisted on a multimillion year time scale. Geochronological tests of this hypothesis have been limited by a dearth of reliable age constraints bracketing these events on multiple cratons. Here we present four new Re-Os geochronology age constraints on Sturtian (717–660 Ma) and Marinoan (635 Ma termination) glacial deposits from three different paleocontinents. A 752.7 ± 5.5 Ma age from the base of the Callison Lake Formation in Yukon, Canada, confirms nonglacial sedimentation on the western margin of Laurentia between ca. 753 and 717 Ma. Coupled with a new 727.3 ± 4.9 Ma age directly below the glacigenic deposits of the Grand Conglomerate on the Congo craton (Africa), these data refute the notion of a global ca. 740 Ma Kaigas glaciation. A 659.0 ± 4.5 Ma age directly above the Maikhan-Uul diamictite in Mongolia confirms previous constraints on a long duration for the 717–660 Ma Sturtian glacial epoch and a relatively short nonglacial interlude. In addition, we provide the first direct radiometric age constraint for the termination of the Marinoan glaciation in Laurentia with an age of 632.3 ± 5.9 Ma from the basal Sheepbed Formation of northwest Canada, which is identical, within uncertainty, to U-Pb zircon ages from China, Australia, and Namibia. Together, these data unite Re-Os and U-Pb geochronological constraints and provide a refined temporal framework for Cryogenian Earth history.

Abstract: Identification of Neoproterozoic low δ 18 O igneous rocks is a key to demonstrate that high-T water-rock interaction and low- 18 O magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the Earth's interior to exterior at the onset of the snowball Earth event. Low δ 18 O values of -10.9 to +4.4 permil characteristic of meteoric origin were observed, on an areal extent over 30,000 km2, for mid-Neoproterozoic metaigneous zircons from the ultrahigh-pressure eclogite-facies metamorphic belt of Triassic ages along the Dabie-Sulu orogenic belt, China. Although the zircon O isotope system could be reset to acquire its low δ 18 O values by exchange with matrix minerals during "wet" eclogite-facies metamorphism, the abnormal 18 O depletion is confirmed by occurrence of Neoproterozoic low δ 18 O zircons in low-grade metaigneous rocks in the same belt. Cathodoluminscence imaging of zircons from low-grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin. Hornblende from two granites of low δ 18 O zircons gave Ar-Ar plateau ages of 747 ± 6 and 776 ± 12 Ma, ruling out that Phanerozoic metamorphism effected mineral O isotope values. Two groups of U-Pb ages were measured at 756 ± 5 Ma and 782 ± 5 Ma, respectively, for the low δ 18 O zircons. This indicates that low δ 18 O zircons crystallized during two episodes of low- 18 O magmatism in response to tectonic evolution from supercontinental rifting at ∼780 Ma to breakup at ∼750 Ma. The second episode of low- 18 O magmatism is contemporaneous with the continental Kaigas iceage. A plate-rift model is advanced to account for the tectonic connection between the low- 18 O magmatism, the continental glaciation and the supercontinental rifting during the Cryogenian period. High-T meteoric water-rock interaction occurred prior to low- 18 O magmatism in rift tectonic zones, and caldera collapse was responsible for melting of hydrothermally altered low δ 18 O rocks during rifting of the South China Block from the Rodinia supercontinent.