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The Neoproterozoic Tsagaan-Olom Group is exposed in the Zavkhan Terrane of southwestern Mongolia and hosts unique geochemical, paleoclimate, and paleontological records that have become central to our understanding of this pivotal interval of Earth history. New sedimentological, stratigraphic, geochronological, and geochemical data provide context for and further develop these records. Detrital zircon provenance indicates that Neoproterozoic strata of the Zavkhan Terrane were derived from basement with age peaks between 1950 to 2100 and 2400 to 2600 Ma. At ϳ800 Ma, the Zavkhan Terrane transformed from an active arc and back-arc complex to a rifted ribbon continent with passive margins on both sides. Deposition was accommodated by extension, which is recorded with syn-sedimentary normal faulting and alluvial fan deposition in the Zavkhan and Khasagt formations. Passive margin sedimentation in the overlying Tsagaan-Olom Group begins with the glacigenic Maikhan-Uul Formation, which consists of two massive diamictite units separated by clast-poor graded beds of the middle member. Detrital zircon at the base of the middle member of the Maikhan-Uul Formation were dated with U-Pb chemical abrasion isotopedilution thermal ionization mass spectrometry and constrained its age to <729.8 ؎ 1.4 Ma. This, along with chemostratigraphy and Re/Os geochronological constraints from the overlying Taishir Formation, supports our correlation of the Maikhan-Uul Formation with the ϳ717 to 660 Ma Sturtian glaciation. The Taishir Formation was deposited on a carbonate ramp in four large-scale sequence tracts that thin to the southwest. The Taishir Formation preserves a large negative ␦ 13 C excursion referred to as the Taishir excursion that covaries in carbonate and organic carbon isotopes in limestone sections. A dolomitization front at the top of the Taishir Formation also results in depleted ␦ 13 C values, however, these are related to local processes and do not represent a global Trezona excursion. Although ␦ 13 C values in the Ol Formation are highly variable along strike, 0.70756 initial strontium isotope values in limestone of the upper Ol Formation are consistent with earliest Ediacaran values. A sandstone-filled karst surface at the top of the Shuurgat Formation that overlies the Ol Formation defines the top of the Tsagaan-Olom Group and is interpreted to mark a major unconformity. Carbon and strontium isotope values in the uppermost Shuurgat Formation are also consistent with early Ediacaran values and suggest that most of the late Ediacaran Period is missing in the Zavkhan Terrane of Mongolia. Carbon isotope profiles from sections preserved as limestone and dolostone display large differences and indicate that isotopic data from

The Zavkhan terrane is a Proterozoic cratonic fragment in southwestern Mongolia that forms the core of the Central Asian orogenic belt. We provide new geologic and U-Pb zircon geochronologic constraints on the Neoproterozoic and early Paleozoic tectonic evolution of the terrane. Orthogneisses dated as ca. 1967 and ca. 839 Ma form the basement and are intruded and overlain by ca. 811-787 Ma arc-volcanic and volcaniclastic rocks that lack a gneissic fabric, suggestive of a mid-Neoproterozoic metamorphic event. Rifting and formation of the Zavkhan ribbon continent occurred from ca. 770-717 Ma and was followed by passive margin sedimentation between 717 and 580 Ma. During the latest Ediacaran to Cambrian, the southern margin of the Zavkhan terrane was reactivated with the obduction of the Lake terrane, slab break-off and reversal, and ca. 509-507 Ma magmatism. Metamorphosed Proterozoic and Cambrian units are cut by undeformed ca. 496 Ma gabbro, providing a tight constraint on the age of Cambrian metamorphism. Late Ordovician to Silurian rifting is marked by bimodal magmatism and deposition in narrow fault-bound basins. Our data indicate that the Zavkhan terrane traveled alone in the Neoproterozoic, collided with the Lake terrane in the late Ediacaran to Cambrian, accreted an unknown crustal block during Cambrian Epoch 2-Epoch 3, and then rifted away in the Ordovician. We suggest the majority of continental growth in Mongolia occurred through the trapping and oroclinal bending of ribbon continents rather than long-lived accretion on the margin of a major craton.