皖南南华系上溪群羊栈岭组中段细碎屑岩的地球化学特征及其地质意义

皖南地区前寒武纪地层广泛出露，包括上溪群和溪口群，其中上溪群羊栈岭组中段岩性以泥岩、粉砂岩和细砂岩等细碎屑岩为主，并含有多套砾岩层。通过对绩溪县杨溪镇和东至县葛公镇剖面羊栈岭组中段细碎屑岩的地球化学分析，发现羊栈岭组中段样品具有相对较高的Al₂O₃/TiO₂比值（21.57）、较高的LREE/HREE比值（6.98）和明显的负Eu异常（0.71），在TiO₂和Al₂O₃二元图解、Zr和TiO₂二元图解及La/Th-Hf二元图解中，大多数样品落入长英质物源区中。表明它们的物源以长英质岩石为主。此外，反映源区风化程度的参数CIA、CIW和PIA值均较低，平均值分别为71.6、75.4和71.2，分别低于PAAS的76.4、82.7和79.0，表明源区的风化程度较低。将样品的地球化学特征和皖南新元古代岩体和火山岩进行比较，发现它们具有亲缘性，结合其它岩石学和地层学证据认为这些岩体和火山岩为沉积物提供了重要的物源。在主量元素判别图中，样品主要落入活动大陆边缘区并受大陆岛弧的影响，在微量元素构造背景判别图中，样品均落入大陆岛弧区。此外，样品的各种REE元素特征值属于大陆岛弧来源的沉积物的范围，同时又与活动大陆边缘沉积物相近，指示羊栈岭组中段可能代表了一套活动大陆边缘的弧后盆地沉积。而羊栈岭组中段作为一次完整的海侵-海退序列中的一部分可以代表整个上溪群形成的构造背景。

Geochemical characteristics and its geological significance of fine-grained clastic sedimentary rocks in the middle member of Yangzhanling Formation of Nanhua System in South Anhui

Weak metamorphic rock series of Nanhua System widely distribute in South Anhui. They include Shangxi Group and Xikou Group. Shangxi Group can be divided into Xiangdong Formation and Yangzhanling Formation from bottom to top. The middle member of Yangzhanling Formation is composed predominantly of mudstones,siltstones and sandstones,and minor polymictic conglomerates. In this study,we take 11 samples of fine-grained clastic rocks from Zhentou profile in Jixi County and Gegongzhen profile in Dongzhi County where they are well-exposed to measure their geochemistry characteristics and discuss their provenance and tectonic setting. Relatively high Al₂O₃/TiO₂ ratios(21.57 on average),LREE/HREE ratios(6.98 on average)and obviously negative Eu anomalies(Eu/Eu^* =0.71 on average)of samples from both two profiles,combining with the TiO₂ vs Al₂ O₃, TiO₂ vs Zr,and La/Th-Hf plot,suggest that their provenance are dominantly felsic rocks. Indicators of CIA,CIW and PIA for these samples are small(71.6, 75.4 and 71.2,respectively),implying that their provenance suffered lower weathering. In addition,the geochemistry characteristic of this clastic rocks are similar to the granodiorites and volcanic rocks surrounding themselves and together with the new evidences of petrology and stratigraphy,we believe that these granodiorites and volcanic rocks would be the important source of these sedimentary rocks. The discrimination diagrams of tectonic setting based on major elements and trace elements and characteristic values of rare elements show the feature of both continental island arc and active continental margin,which may indicate that these rocks represent the infill of active continental back arc basin. This conclusion can be used to the Shangxi Group because the middle member of Yangzhanling Formation belongs to the component of transgression-regression sequence of Shangxi Group.