华北克拉通南缘栾川群大红口组形成时代及其对新元古代构造演化的制约

华北克拉通南缘豫西地区保存有较为完整的变质结晶基底和中-新元古代沉积盖层,记录了重要的前寒武纪构造演化信息。近年来的年龄研究结果表明原认为是中-新元古代的汝阳群-洛峪群可能形成于中元古代早期(1. 75～1. 60Ga),而沿着华北克拉通南缘与秦岭造山带的拼合带(洛南-栾川断裂带)分布的新元古代盖层(主要为栾川群)的形成时代尚不明确。华北南缘新元古代栾川群主要由大理岩、片岩、千枚岩和碱性火山岩组成,其上部大红口组火山岩以碱性粗面质岩石为主,高硅富钾,与侵入到栾川群中下部的辉长岩构成典型的双峰式岩石组合。栾川群大红口组三个粗面岩样品的锆石U-Pb年龄分别为840±4Ma、845±5Ma和846±6Ma,结合已有的栾川群下伏地层最年轻的碎屑锆石年龄结果(～1000Ma),限定栾川群归属于新元古代早期(1000～840Ma)。大红口组岩浆岩的岩石组合和地球化学特征表明其形成于板内裂谷环境,根据区域构造资料以及前人的研究成果,栾川群火山岩与北秦岭同时期的岩浆活动共同指示了华北南缘与北秦岭在～845Ma均处于板内拉张阶段。

New geochronological constraints on the Dahongkou Formation of the Luanchuan Group and its implications on the Neoproterozoic tectonic evolution of the southern margin of the North China Craton

The Archean-Paleoproterozoic metamorphic basement and Meso-Neoproterozoic sedimentary cover sequences are well preserved at the southern margin of the North China Craton (NCC) and record important Precambrian tectonic evolution information. Recently, new dating results indicate that the Ruyang and Luoyu groups, previously were considered to be Meso-Neoproterozoic sedimentary sequences, should be formed in the Early Mesoproterozoic (1.75~1.60Ga). The age of the Neoproterozoic Luanchuan Group, developing along the Luonan-Luanchuan Fault between the southern margin of the NCC and the Qinling Orogenic Belt, is still undefined. The results could contribute to the Precambrian crustal evolution and build the stratigraphic framework of the southern margin of the NCC. The Luanchuan Group mainly consists of marble, schist, phyllite and alkaline volcanic rocks, and the alkaline volcanic rocks of Dahongkou Formation are dominated by trachyte. The alkaline volcanic rocks are rich in SiO₂ and K₂O, and constitute a typical bimodal rock assemblage with gabbro intrusions. New LA-ICP-MS dating of zircons from three trachyte samples yielded U-Pb ages of 840±4Ma, 845±5Ma and 846±6Ma, respectively, indicating that the Luanchuan Group was formed in the Early Neoproterozoic (1000~840Ma) combined with the available youngest age (~1000Ma) of detrital zircons from the underlying strata. The petrological and geochemical characteristics of the alkaline volcanic rocks and the gabbros indicate an intraplate rift setting. According to regional tectonic data and available research achievements, these alkaline volcanic rocks in the Luanchuan Group and those of contemporaneous magmatic activities in the North Qinling collectively suggest intraplate extension at ca. 845Ma.