赣南地区早南华世钾质斑脱岩的发现及其大地构造意义

The discovery of Early Nanhuan K-bentonite in southern Jiangxi and its tectonic significance

赣南地区(华夏陆块西缘)早南华世上施组中含多层乳白色、黄绿色凝灰质黏土岩。化学成分分析结果显示富K2O,属钾质斑脱岩;微量元素分析结果显示其源岩为中酸性岩体,源于火山岛弧构造环境。应用LA-ICP-MS对其中的锆石进行U-Pb测年,得到(756.0±7.5)、(774.3±8.5)、(774.1±8.8)Ma 3组谐合年龄。锆石Hf同位素分析显示εHf(t)为-20.1～5.8(正值较少且多出现在774 Ma),其t_(DM2)为130 6～3 030 Ma,表明早期岩浆(774 Ma)具有少量的地幔物质贡献,而晚期岩浆(756 Ma)几乎完全是壳源物质局部熔融的结果。综合构造背景及时限性,认为774～756 Ma间华夏内部还存在洋壳俯冲形成的火山弧岩浆活动,进一步证实扬子与华夏在早南华世还未发生碰撞拼贴。

The Early Nanhua Shangshi Formation in southern Jiangxi area（ western margin of Catheaysia block） contains several beds of milky white and kelly tufaceous clay stones. Geochemical analysis reveals that the clay beds are of typical K-bentonite,rich in K2 O.Geochemical composition of the bentonite by trace elements analysis suggests that their source rock is intermediate-acidic magma,derived from volcanic island arc environment. Zircon U-Pb dating by LA-ICP-MS yields three sets of ages：（ 756. 0±7. 5） Ma,（ 774.3±8.5） Ma and（ 774.1±8.8） Ma. Zircon Hf isotope compositions have εHf（ t） values of-20.1 to 5.8（ positive values are less and almost concentrate at 774 Ma） and two-stage Hf-isotope model ages of 1 306-3 030 Ma. In-situ zircon Lu-Hf isotopic data revealed a contribution of mantle-derived materials to the earlier magmatic rocks（ 774 Ma）,while the latter magmatic rocks（ 756 Ma） seem to be completely resulted from partial melting of crustal rocks. Tectonic setting and time limitation together indicated that there were volcanic arc magma resulted from the subduction of the oceanic crust in 774-756 Ma,which further proves that the Catheaysia block and the Yangtze block did not collide or assemble in the Early Nanhua period.