GEOLOGY, August 2011 735 INTRODUCTION Cryogenian glaciations (750–635 Ma) were more severe than any subsequent events and may have recorded the most extreme climate changes in Earth history (e.g., Hoffman and Li, 2009). Anomalously low δ 18 O values of -11‰ to -4‰ reported for Neoproterozoic igneous zircons in ultrahigh-pressure (UHP) eclogite facies metamorphic rocks in the Dabie-Sulu orogenic belt of central-east China (Zheng et al., 2008, and references therein) were often inter- preted as representing primary oxygen isotope compositions of Neoproterozoic rift magma- tism of the northern margin of the South China Block, and genetically linked to Neoproterozoic glaciations (e.g., Zheng et al., 1998, 2004, 2007, 2008; Rumble et al., 2002). A glaciation origin for low-δ 18 O magmas requires magmatic rocks to acquire their oxy- gen isotope signatures through hydrothermally driven meteoric water under conditions of a cold climate, such as an ice age, or through the involvement of water from melting of glacial ice below the base of a continental ice cap (e.g., Rumble et al., 2002; Zheng et al., 2008). In other words, the glaciation origin model predicts that Neoproterozoic magmas should display sharp oxygen isotope shifts corresponding to the onset of ice ages. However, in-situ zircon oxygen and U-Pb analyses of metamorphic rocks in the Dabie Shan orogenic belt show that such low- δ 18 O magmatic zircons started to appear from ca. 810 Ma (Chen et al., 2003), significantly ear- lier than the Chang’an glaciation (no older than 725 Ma; Zhang et al., 2008; Hoffman and Li, 2009), making such a link tenuous. In addition, except for the study of Chen et al. (2003), all the zircon oxygen isotope data from the Dabie Shan orogenic belt were acquired through bulk analysis (e.g., Zheng et al., 1998, 2004, 2007, 2008; Rumble et al., 2002), which tends to pro- duce mixed results between zircon cores and rims, and thus does not discriminate between magmatic and UHP metamorphic signatures. Whole-rock primary oxygen isotope composi- tions of Precambrian igneous rocks can be dif- ficult to determine because they are often reset by post-magmatic alteration. Even in relatively retentive minerals, such as quartz, postmagmatic alteration and recrystallization can obscure mag- matic signatures (e.g., Valley and Graham, 1996). Also, most low-δ 18 O magmas were probably eroded away by younger geological processes due to their shallow genesis (e.g., Bindeman, 2008). Although eroded igneous rocks are lost from the rock record, their depleted 18 O signa- tures could be preserved in sedimentary basins by refractory minerals, such as zircon. Due to its extremely slow rate of oxygen diffusion and high closure temperature, zircon is insensitive to hydrothermal alteration and fractional crystal- lization during cooling, and can therefore retain its primary oxygen isotope composition from the time of crystallization (e.g., Valley, 2003). The Nanhua Rift is the largest failed continen- tal rift basin in the South China Block and has some of the best-preserved sedimentary and vol- canic rocks related to the breakup of the supercon- tinent Rodinia and the Neoproterozoic Snowball Earth (e.g., Wang and Li, 2003; Li et al., 2008) (Fig. 1A). Four fine-grained sedimentary samples were collected from Cryogenian sedimentary sequences in the Nanhua Rift basin (Figs. 1A and 1B) for zircon age and oxygen isotope determina- tions, including one tuff sample (08SC31, grain size is slightly smaller than that of typical silt- stone) and three siltstone samples (08SC07 and 08SC11are tuffaceous siltstones and 08SC74 is siltstone). We analyzed these samples by micro- analytical techniques using a Cameca IMS-1280 secondary ion mass spectrometer (SIMS), which permits rapid, high-precision in-situ analysis of oxygen isotopes in zircon (e.g., Kita et al., 2009; Li et al., 2010a). High analytical precision and spatial resolution made it possible to identify pre- viously undetectable variations in oxygen isotope composition, thus providing new insights into the causes for the large-scale 18 O-depletion event in the Neoproterozoic. RESULTS Zircon oxygen isotope compositions are pre- sented in Figures DR2 and DR3 and in the GSA Data Repository 1 . The data have been divided Geology, August 2011; v. 39; no. 8; p. 735–738; doi:10.1130/G31991.1; 3 figures; Data Repository item 2011223. © 2011 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. *E-mail: X.Wang3@curtin.edu.au. Nonglacial origin for low-δ 18 O Neoproterozoic magmas in the South China Block: Evidence from new in-situ oxygen isotope analyses using SIMS Xuan-Ce Wang 1, 2, 3 *, Zheng-Xiang Li 2 , Xian-Hua Li 1 , Qiu-Li Li 1 , Guo-Qiang Tang 1 , Qi-Rui Zhang 1 , and Yu Liu 1 1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China 2 Institute for Geoscience Research, Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia 3 School of Earth and Environment, the University of Western Australia, Crawley, WA 6009, Australia ABSTRACT Low-δ 18 O signatures in supracrustal rocks have been used as geochemical proxies for cold paleoclimates, e.g., glaciations. Unusual low-δ 18 O values found in Neoproterozoic igneous rocks in parts of the South China Block have thus been genetically linked to Neoprotero- zoic glaciation events. However, we report here new oxygen isotope compositions from Neo- proterozoic magmatic zircons in central southern China using in-situ techniques that argue against such an interpretation. Our results show that (1) low-δ 18 O magmatic zircons started to appear in the South China Block from ca. 870 Ma, coinciding with the tectonic switching from Sibao orogenesis to postorogenic extension, which occurred more than 150 m.y. prior to the first glaciation event. The most abundant low-δ 18 O magmatic zircons have ages of 800– 700 Ma. (2) The 830–700 Ma magmatic zircons are characterized by their bimodal nature of oxygen isotope compositions, i.e., mantle-like δ 18 O values (+4.4‰ to +5.8‰) and high-δ 18 O values (+9.3‰ to +10.8‰). (3) A sharp temporal change in maximum zircon δ 18 O values in the South China Block coincided with the onset of continental rifting and the possible arrival of a plume head. (4) No negative δ 18 O zircons have been identified in this study, contrary to previous studies. These features strongly argue against a glaciation origin for low to negative δ 18 O values in Neoproterozoic magmatic zircons from southern China. We propose that two stages of high-temperature water-magma interaction during plume-driven magmatism and continental rifting best explain the low-δ 18 O magmas. The most important implication of this study is that formation of such low-δ 18 O magmatic zircons was not necessarily related to gla- cial events and should not be used as a geochemical proxy for a cold paleoclimate. 1 GSA Data Repository item 2011223, secondary ion mass spectrometer zircon oxygen and U-Pb ana- lytical methods and data, is available online at www .geosociety.org/pubs/ft2011.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.