湖北宜昌埃迪卡拉系陡山沱组底部盖帽碳酸盐岩中具极负碳同位素值方解石的U-Pb年龄

U-PB AGE OF HIGHLY^(13)C-DEPLETED CALCITE FROM THE BASAL EDIACARAN CAP CARBONATE IN YICHANG,HUBEI PROVINCE

作为新元古代全球冰期结束的标志,盖帽碳酸盐岩在全球广泛分布。盖帽碳酸盐岩特殊的沉积构造和地球化学特征使其长时间以来成为学界研究新元古代冰期事件的重点关注对象。湖北宜昌三斗坪地区埃迪卡拉系陡山沱组底部盖帽碳酸盐岩中具极负碳同位素值(δ^(13)C最低值小于-40‰,VPDB)方解石的形成与甲烷氧化有关,被认为是甲烷水合物释放导致冰期结束和盖帽碳酸盐岩沉积这一假说的关键证据。但也有学者认为这些具极负碳同位素值方解石是在盖帽碳酸盐岩沉积之后形成的,因此与冰期结束和盖帽碳酸盐岩沉积无关。为探讨盖帽碳酸盐岩中具极负碳同位素值方解石的形成时间,本文利用近年来发展迅速的碳酸盐矿物激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)U-Pb定年技术,分别对湖北宜昌九龙湾剖面陡山沱组底部盖帽碳酸盐岩裂隙和孔洞中的暗色方解石、白云石胶结物、白云石围岩进行了年龄测定。测试结果表明,暗色方解石的形成时间为593.7±8.7 Ma(2σ,n=111,MSWD=3.6),比盖帽白云岩沉积时间晚约40Myr,而白云石胶结物和围岩的分析测试获得了与方解石近似的U-Pb年龄,表明后期富甲烷流体的活动也对裂隙早期胶结物和周围的白云岩造成了影响。

Globally-distributed cap carbonate,marking the termination of the late Cryogenian Marinoan glaciation,has been focused on the study of the severe Neoproterozoic paleoclimatic events due to its distinctive sedimentary structures and geochemical signatures.The origin of the calcite with extremely negativeδ^(13)C values(the nadir is less than-40‰,VPDB),which occurred within cap carbonate in the Ediacaran Doushantuo Formation in Yichang,Hubei Province,was thought to be related with methane oxidization,thus providing critical evidence of methane hydrate destabilization that led to the termination of global glaciation and cap carbonate deposition,while others argued that the highly 13C-depleted calcite was precipitated after the deposition of cap carbonate.Here we report laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)U-Pb ages of black calcite and dolomite cement in fractures and cavities within cap carbonate,as well as adjacent dolomite matrix at the Jiulongwan Section in Yichang of western Hubei Province.Black calcite yields a U-Pb age of 593.7±8.7 Ma(2σ,n=111,MSWD=3.6),indicating its precipitation later than cap carbonate(~635 Ma)by about 40 Myr.Similar U-Pb ages obtained for dolomite cement and matrix suggest that their U-Pb systems may have been influenced by methane-rich fluids from which the black calcite was precipitated.