扬子北缘奥陶纪—志留纪之交泥岩地球化学特征及其源区、构造背景

Geochemical characteristics of mudstones and its provenance and tectonic setting during the Ordovician-Silurian period in northern margin of Yangtze Block

近年来,扬子地块北缘奥陶纪—志留纪之交沉积构造背景一直存在不同认识。本文对渝东北城口厚坪五峰组—龙马溪组砂、泥岩进行系统采样和主、微量元素测试分析,从地球化学特征角度,来探讨该时期砂、泥岩物源源区及其构造环境。地球化学特征显示,五峰组—龙马溪组砂、泥岩主量元素SiO_(2)含量为39.57%~87.16%,多数高于北美页岩,Al_(2)O_(3)含量为3.41%~14.98%,均低于北美页岩,MgO、CaO、MnO_(2)和P_(2)O_(5)含量明显低于北美页岩,相比于四川盆地川南长宁—双河五峰组—龙马溪组泥岩,渝东北城口具有高SiO_(2)和低MgO、CaO的特点,表明城口五峰组—龙马溪组泥岩碎屑组分主要以粉砂质和硅质等陆缘碎屑为主;微量元素Sc、Sr、Co、Zr、Hf较北美页岩相对亏损,V、U、Sb、Ba相对富集,Cr、Ni、Nb、Ta含量相近。稀土元素北美页岩标准化均表现为略微向右倾斜的配分模式,多数具有δCe负异常,观音桥段和龙马溪组δEu正异常指示了该时期存在海底热液活动。砂、泥岩主、微量元素的物源和构造背景图解表明沉积物源区源岩具有多样的物质组分,以长英质岩为主,同时混入了花岗质和安山质岩石组分;源岩形成时的构造背景主要为被动陆缘环境,少数为活动陆缘和大陆岛弧环境,这一结果与扬子地块北缘新元古代—早古生代处于被动大陆边缘的区域地质背景一致。

Objective:In recent years,It is all along argued that whatever sedimentary and tectonic setting in northern margin of Yangtze Block is during the Ordovician-Silurian Period.Method:Systematically sampling sandstones and mudstones of Wufeng Formation and Longmaxi Formation and analyzing their major and minor elements,their provenance and tectonic setting are discussed from the perspective of their geochemical features in this paper.Conclusions:Major element Si O_(2)contents of sandstones and mudstones range from 39.57%to 87.16%,and mostly are higher than North America Average Shale’Content(NASC),while Al_(2)O_(3)contents change from 3.41%to 14.98%and are all lower than those of NASC.NASC normalized major elements spider diagram show that these contents such as MgO,CaO,MnO_(2)and P_(2)O_(5)are clearly lower than NASCs’.Compared to major elements of contemporary mudstones from Changning-Shuanghe region located in southern margin of Sichuan basin,these sandstones and mudstones in northern margin of Yangtze block are charactered by higher Si O_(2)and lower MgO、CaO contents,which indicates that material components of the latter are mainly dominated in terrigenous clast.Relative to minor elements of NASC,some elements such as Sc,Sr,Co,Zr,Hf are relative depleted,some V,U,Sb,Ba are relative enriched,the other Cr、Ni、Nb、Ta have approximate contents.The ratios of LREE/HREE and (La/Yb)Nshow that sandstones and mudstones of Wufeng Formation and Longmaxi Formation might have a feature with LREE depletion and HREE enrichment.NASC-normalized trace element pattern of sandstones and mudstones in this study are entirely featured by slight right-tilted ones with bulkδCe negative abnormalies,δEu positive abnormalies of Kuanyinchiao Member and Longmaxi Formation indicate some hydrothermal activities of oceanic floor during that time.Provenance and tectonic setting diagram of these sandstones and mudstones display their source rock of provenance have multiple kinds of material components,which principally inherited from felsic rocks and mixed with granitic and andesitic rocks.Tectonic setting during source rocks formation is mostly located in passive continental margin and partly in active continental margin and continental island arc,which is coincident with regional geological background of passive continental margin in northern margin of Yangtze block during the late proterzoic-early palezoic period.