湘东北新元古代冷家溪群沉积岩的地球化学特征和碎屑锆石U-Pb年代学

Geochemistry and Detrital Zircon U-Pb Ages of Sedimentary Rocks from Neoproterozoic Lengjiaxi Group in NE Hunan Province

江南造山带已被普遍接受为是由扬子陆块与华夏陆块在新元古代碰撞拼合而形成,正确理解江南造山带形成机制是探索华南前寒武纪地质演化的前提,也是深入剖析华南显生宙构造演化的重要因素,但是人们对其拼贴时限及构造属性仍存在争议.沉积岩的矿物组成及地球化学特征可以对其沉积物源及形成的构造背景起到很好的指示作用.然而,前人多聚焦于岩浆岩的地球化学-年代学研究或地层中碎屑锆石的年代学研究,江南造山带新元古代沉积物的物源示踪研究相对薄弱.针对江南造山带中段平江地区冷家溪群中段小木坪组及黄浒洞组沉积岩开展了详细的岩石学、地球化学和锆石U-Pb年代学研究,结果表明,这些样品具有较高的SiO2(58.82%~70.62%,平均为64.68%)和Al2O3(13.35%~20.99%,平均为16.78%),高的Al2O3/(CaO+Na2O)(3.8~12.4,平均6.6)、K2O/Na2O(0.95~3.20,平均1.8)、La/Sc(2.0~2.7)、Th/Sc(0.84~0.86)比值,与此同时,具有较低的CaO、MgO和Na2O含量(分别为0.19%~2.85%、1.43%~2.13%、和1.44%~2.27%)和Co/Th(0.83~5.30)比值.同时,这些沉积岩富集轻稀土元素(LREE),具有明显的铕负异常(Eu/Eu*=0.62~0.69),与澳大利亚后太古代页岩(PAAS)的稀土元素配分模式相似.其中两件定年样品的碎屑锆石最小年龄峰值分别为856 Ma和860 Ma.综合前人研究成果,江南造山带冷家溪群沉积岩来自于中酸性火成物源区,并经历了快速剥蚀和较弱的风化过程,在860~830 Ma时处于活动大陆边缘的构造环境中.

It is generally accepted that the Jiangnan Orogenic Belt was formed by the amalgamation of the Yangtze and Cathaysian blocks during the Neoproterozoic period. However, the time of the amalgamation and its tectonic properties are still controversial.Comprehensive understanding of the formation mechanism of the Jiangnan Orogenic Belt is a prerequisite for exploring the Precambrian geological evolution, and also an important factor for in-depth analysis of the Phanerozoic geological nature in South China. The mineral composition and geochemical characteristics of sedimentary rocks can be used as a good indicator for the source of sediments and the tectonic setting of their formation. However, previous studies have been focused on the geochemistrygeochronology of magmatic rocks or the geochronology of clastic zircons in the strata, and the tracing of sediments is seldom reported. According to our integrated study on petrology, geochemistry, and zircon geochronology on the Xiaomuping Formation and Huanghudong Formation of Lengjiaxi Group from Pingjiang area, it is shown that(1) whole rock samples have high SiO2(58.82%-70.62%) and Al2 O3(13.35%-20.99%) contents, and high A/CN(3.8-12.4), K2 O/Na2 O(0.95-3.20), La/Sc(2.0-2.7) and Th/Sc(0.84-0.86) ratios, but low CaO, MgO and Na2 O contents(0.19%-2.85%, 1.43%-2.13% and 1.44%-2.27%, respectively) and Co/Th(0.83-5.30) ratio;(2) these samples are enriched in LREE and have significant negative Eu anomaly, which is similar to those of the PAAS.(3) The maximum age peak of detrital zircon collection is 856 Ma and 861 Ma;Hence, we propose that the sedimentary rocks of the Lengjiaxi Group from the Jiangnan Orogen are derived from intermediatefelsic magmatic sources in an active continental margin setting at 860-830 Ma, and then experienced rapid erosion and relatively weak weathering processes.