扬子地块北缘镇巴穿心店地区南华系碎屑锆石U-Pb年龄及其物源示踪

Detrital zircon U-Pb age and provenance tracing of Nanhua System in Zhenba-Chuanxindian area,northern margin of Yangtze Block

扬子地块北缘镇巴穿心店地区发育有完整的南华系—震旦系沉积地层,记录了研究区及邻区南华纪沉积特征,对于研究扬子地块北缘新元古代晚期构造演化历史和Rodinia超大陆演化过程具有重要的地质意义。研究区南华系由下至上可划分为4套岩石地层单位,分别为长安组、古城组、大塘坡组和南沱组。南华系砂岩地球化学特征表明,研究区南华系化学风化作用较弱,经历了较低程度的沉积物再旋回作用。砂岩的CIA、CIW、PIA、Rb/Sr等古气候指标显示,研究区南华系具有由寒冷干燥—有波动的寒冷干燥—趋于稳定的温暖湿润—寒冷干燥的古气候演化趋势。在Dickinson三角图解中清晰地反映了南华系各组砂岩具有再旋回造山带和切割岩浆弧物源的特征,同时砂岩岩石地球化学特征也指示南华系的物源区主体应由中酸性岩浆岩组成。南华系长安组、古城组和南沱组3件碎屑锆石U-Pb年龄样品的年龄分布特征类似,主要年龄区间为新元古代晚期(约780~约641 Ma),并显示出约750~约740 Ma的显著峰值。结合区域地质资料,研究区南华系沉积时代应为约720~约635 Ma,物源应主要来自位于现今研究区西侧的扬子地块北缘汉南—米仓山微地块的新元古代岩浆岩,其沉积事件对应于新元古代中晚期扬子地块北缘伸展-裂解阶段,为Rodinia超大陆裂解过程的沉积响应。

An intact Nanhua-Sinian sedimentary cover has been formed in the Zhenba-Chuanxindian area on the northern margin of the Yangtze block.The sedimentary characteristics during the Nanhua period in the research area and its neighboring areas were recorded,which is of important geological significance for studying the Late Neoproterozoic structural evolution and evolution of the supercontinent Rodinia on the northern margin of the Yangtze Block.The Nanhua system in the research area can be divided into four lithostratigraphical units from bottom to top,namely Chang’an Formation,Gucheng Formation,Datangpo Formation and Nantuo Formation.The geochemical characteristics of sandstones from the Nanhua System suggest weak chemical weathering and low levels of sediment recycling of the Nanhua System in the research area.The paleoclimatic indicators of sandstones such as CIA,CIW,PIA and Rb/Sr indicate a paleoclimatic trend of the Nanhua system in the research area from cold-dry to fluctuating cold-dry to stable warm-humid and then to cold-dry climate.Dickinson’s triangular diagram clearly reflects that the sandstones from each formation in the Nanhua System have the characteristics of recycled Orogen and cut magmatic arc provenances.In addition,the geochemical characteristics of sandstone rocks also indicate that the provenance of the Nanhua System should be mainly composed of intermediate-acidic magmatic rocks.Three of the sampled units in the Nanhua system,Chang’an Formation,Gucheng Formation and Nantuo Formation,have similar distribution characteristics of detrital zircon U-Pb age,mainly in the Late Neoproterozoic(ca.780-ca.641 Ma),presenting a significant peak at ca.750-ca.740 Ma.Combined with regional geological data,the sedimentary age of the Nanhua System in the research area should be ca.720-ca.635 Ma.The provenance should mainly come from the Neoproterozoic magmatic rocks of the Hannan-Micangshan microcontinent located on the northern margin of the Yangtze Block in the west of the current research area.The sedimentary event corresponds to the extensional-rifting phase during the Mid-late Neoproterozoic on the northern margin of the Yangtze Block,which is a sedimentary response to the fragmentation of the supercontinent Rodinia.