The Jiao-Liao-Ji belt in the Eastern Block of the North China Craton is a Paleoproterozoic mobile/orogenic belt.Whether the belt extends across the Tan-Lu fault to the Xuzhou-Bengbu area is still in a hot debate and needs further research.Precambrian metamorphic basements in the Bengbu area include the Early Neoarchean Huoqiu Group,the Paleoproterozoic Wuhe and Fengyang groups,and a small quantity of Paleoproterozoic (gneissic) granites.The low-greenschist-facies metamorphosed Fengyang Group is subdivide into the Baiyunshan,Qingshishan and Songji formations from the bottom to up.The protoliths of the Baiyunshan Formation are sandstones,and the Qingshishan Formation is composed of metamorphosed sandstones,siltstones and Mg-rich carbonates.The Songji Formation consists of metamorphosed siltstones,mudstones and marlaceous rocks.In this study,we analyzed two quartzite and one feldspar quartzite samples from the Baiyunshan Formation for detrital zircon U-Pb age and Lu-Hf isotope.Detrital zircon age patterns from the three samples are similar and are characterized by the major range of 2.57 ～ 2.46Ga with the peak of ～ 2.52Ga.εHf(t) values of these zircons range from-7.39 to + 7.37 and 90％ of them are positive,which possibly sourced from the Late Neoarchean granitic rocks in the Bengbu area.Another important age range is between 2.98 and 2.66Ga and the peaks are at ～ 2.94Ga and ～ 2.69Ga.εHf (t) values of these zircons range from-11.8 to + 7.24 and more than 90％ of them are positive,which likely sourced from the Huoqiu Group and the Early Neoarchean granitic rocks.Additionally,a dozen of Paleoarchean detrital zircons with ages of 3.61 ～ 3.34Ga and negative εHf (t) values are also found.These Paleoarchean detrital zircons have similar εHf (t) values (-6.34 to-1.05) and two-stage Hf model ages (3.75 ～4.04Ga) with the similar-aged zircons found in the southern margin of the North China Craton (including the Dengfeng,Jiaozuo,Xinyang and Jiaodong areas),which imply the existence of a Paleoarchean,even Eoarchean,continental crust in the southern North China Craton.The five youngest detrital zircons place constraint on the maximum depositional age of the Baiyunshan Formation of the Fengyang Group at ～2.37Ga.On the other way,the muscovite K-Ar age of 1878Ma from the group and the 1.88 ～ 1.80Ga metamorphic age in the Bengbu area constrain its minimum depositional age.In consideration of the meta-sedimentary rocks with similar detrital zircon age patterns from the Laoling Group and the Dashiqiao Formation of the North Liaohe Group in the Jiao-Liao-Ji belt,we suggest that marine-facies carbonates and the Neoarchean continental crust-derived clastic rocks deposited in a nascent rift at 2.3 ～ 2.1Ga.%华北克拉通东部陆块内的胶-辽-吉带是一条古元古代构造活动带,其是否向南西穿越郯庐断裂带延伸至徐州-蚌埠一带还存在着争议.蚌埠地区早前寒武纪变质基底主要包括晚太古代早期的霍邱群、古元古代五河群、凤阳群以及少量的古元古代花岗(片麻)岩.其中低绿片岩相变质的凤阳群自下而上被分为白云山组、青石山组和宋集组,原始沉积物分别为石英砂岩-砂、粉砂泥质及富镁碳酸盐岩交替-粉砂泥质、泥质和少量泥灰质.本次研究对采自凤阳群底部白云山组的2个石英岩和1个长石石英岩样品的碎屑锆石U-Pb年龄及Lu-Hf同位素研究表明,3个样品的碎屑锆石年龄图谱十分相似,最重要的年龄区间为2.57～2.46Ga,峰值年龄约为2.52Ga,εHf(t)值在-7.39到+7.37之间,其中有大约90％具有正值,来源于蚌埠地区的晚太古代末期的花岗质岩石.另一个重要的年龄区间是2.98～2.66Ga,峰值为2.94Ga和2.69Ga,εHf(t)值在-11.8到+7.24之间,其中超过90％为正值,来源于蚌埠地区的霍邱群以及晚太古代早期花岗质岩体.此外,还有少量具有负εHf(t)值的3.61～3.34Ga的古太古代碎屑锆石被发现.这些古太古代碎屑锆石的εHf(t)值(-6.34～-1.05)以及两阶段模式年龄(3.75～4.07Ga)与华北克拉通南缘邻近地区(登封、焦作、信阳以及胶东)发现的古太古代锆石特征相似,说明华北克拉通南缘可能存在过古太古代,甚至冥古宙古老陆壳.5颗最年轻的锆石可以将凤阳群白云山组的最大沉积年龄限定在2.37Ga左右,而前人报到的白云母K-Ar年龄1878Ma与蚌埠地区1.88 ～ 1.80Ga的主期变质时代为其形成时代的上限做出了限制.结合胶-辽-吉带老岭群和北辽河群大石桥组中具有相似碎屑锆石年龄图谱的变质沉积岩,我们认为在2.3 ～2.1Ga裂谷发育早期活动带内沉积了海相碳酸盐岩及新太古代陆壳来源的碎屑岩.
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