INTRODUCTION The interval leading up to the emergence of animal life in the late Neoproterozoic was a time of extreme climatic oscillation from glacial to in- terglacial conditions. Associated carbonates re- veal large swings in δ 13 C values from highly pos- itive values in the interglacials, falling to negative values during and/or following the glacial inter- vals, and these can be used to characterize partic- ular time intervals, along with 87 Sr/ 86 Sr ratios and biostratigraphic data (Fig. 1). Although instabil- ity in the carbon cycle is clearly implicated, there is much controversy about the models. Some rec- ognize four Cryogenian glaciations, none of which were global (Kaufman et al., 1997), while others accept only two global glaciations (Kennedy et al., 1998). Senarios also differ widely, as between the snowball Earth and high obliquity models (e.g., Hoffman et al., 1998; Williams et al., 1998). Distinction between these scenarios is hindered by a paucity of a reliable chronometric constraints. We show that the Huqf Supergroup of Oman contains volcanic horizons with the potential to calibrate key Neoproterozoic events using U-Pb zircon geochronology. Geology; February 2000; v. 28; no. 2; p. 175–178; 3 figures. 175 New U-Pb zircon dates for the Neoproterozoic Ghubrah glaciation and for the top of the Huqf Supergroup, Oman Martin Brasier Gretta McCarron Robert Tucker Department of Earth and Planetary Sciences, Washington University, Campus Box 1169, One Brookings Drive, St. Louis, Missouri 63130, USA Jonathan Leather Philip Allen Graham Shields Centre de Geochimie de la Surface, CNRS-UMR 7517, Strasbourg, France Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Department of Geology, Trinity College, Dublin, Ireland ABSTRACT The Huqf Supergroup, Oman, contains volcanic horizons with the potential to calibrate Neoproterozoic events and chemostratigraphy using U-Pb zircon geochronology. A tuffaceous bed near the base, within the lower (Ghubrah) diamictite and beneath a lower cap carbonate, provides the first U-Pb zircon date obtained from within a Neoproterozoic glacial deposit. This date of 723 +16/–10 Ma suggests a Sturtian age. Diamictites about 1 km higher in the section are overlain by the Hadash cap dolomite, which compares with cap dolomites above Marinoan glacial units elsewhere. A U-Pb zircon age of 544.5 ± 3.3 Ma from ignimbrites in the Fara For- mation, near the top of the supergroup, is just beneath that of the Precambrian-Cambrian boundary (ca. 543 Ma), consistent with the presence of the Neoproterozoic skeletal-fossil Cloudina in correlative subsurface rocks. This revised chronology confirms the existence of four negative δ 13 C excursions between ca. 723 and 543 Ma, the lower two of which are clearly associated with glaciations. Keywords: U-Pb, geochronology, Proterozoic, Oman, glaciation, carbon isotopes, strontium isotopes. Figure 1. Alternative correlations for Huqf Supergroup of Oman, compared against suggested chemostratigraphic framework and absolute time scale.Triangles show inferred positions of Neoproterozoic glaciations. C and Sr isotope curves are mainly after sources in Kaufman et al. (1997) and Pelechaty (1998). Sr isotope data from Oman: black dots, Burns et al. (1994); white dots, Shields (this paper). N1 to N4 and P1 to P3, major negative and positive C isotopic features. A represents low position for Precambrian-Cambrian boundary (e.g., Kimura et al., 1997). B represents high position for boundary (this paper). Stratigraphic column X assumes that Ghubrah diamictite is of younger, Marinoan age, with zircon age of 723 Ma due to reworked zircons. Stratigraphic column Y assumes that Ghubrah diamictite is of Sturtian age (see text). Stratigraphic column Z gives lithostratigraphic succession for Huqf Supergroup in Oman Mountains, showing levels of marker cap carbon- ates and radiometric dates, including new U-Pb zircon ages (this paper).TNP, terminal Neoproterozoic; PDB, Peedee belemnite.