中山大学学报自然科学版 ›› 2019, Vol. 58 ›› Issue (5): 59-72.doi: 10.13471/j.cnki.acta.snus.2019.05.009

• 论文 • 上一篇    下一篇

南秦岭勉略构造带三岔子硅质岩地球化学特征及其地质意义

陈铄,周永章,张彦龙,周维丽   

  1. 中山大学地球科学与工程学院∥广东省地质过程与矿产资源探查重点实验室∥中山大学地球环境与地球资源研究中心,广东 广州 510275
  • 收稿日期:2018-03-23 出版日期:2019-09-25 发布日期:2019-09-25
  • 通讯作者: 周永章(1963年生),男;研究方向:矿床地质与地球化学;E-mail:zhouyz@mail.sysu.edu.cn

Geochemistry characteristics and their geological significances of cherts in the tectonic belt from Mianlue,southern Qinling orogenic belt

CHEN Shuo,ZHOU Yongzhang,ZHANG Yanlong,ZHOU Weili   

  1. School of Earth Science and Geological Engineering∥Guangdong Provincial Key Lab of Geological Processes and Mineral Resource Survey∥Research Center for Earth Environment & Resources,Sun Yat-sen University, Guangzhou 510275, China
  • Received:2018-03-23 Online:2019-09-25 Published:2019-09-25

摘要:

勉略带内广泛发育古生代硅质岩,其对南秦岭造山带古生代沉积环境和构造背景具有重要的指示意义。重点分析了勉略带三岔子地区泥盆系硅质岩地球化学特征,结果显示,三岔子硅质岩SiO2含量较高,为94.04%~98.08%,平均9692%,质地较纯,Al2O3和TiO2含量明显偏低;微量元素中Ba、U含量较高、Zr含量较低,U/Th值和Ba/Sr值较高;稀土元素显示样品的ΣREE值显著偏低,为761〖HT5,3〗~51.04 μg/g,平均22.51 μg/g,经PAAS标准化后的稀土配分曲线表现出富集HREE的左倾特征,Eu正异常和Ce负异常,以及Al/(A1+Fe+Mn)值、Fe/Ti值和(Fe+Mn)/Ti值等指标均指示硅质岩具有典型的热水沉积成因。硅质岩Al/(Al+Fe)值、Al2O3/(Al2O3+Fe2O3+MnO)值、Al2O3/(Al2O3+Fe2O3)值、δCe值以及(La/Ce)N值等地球化学特征指示其主要形成于大陆边缘环境。(La/Lu)N值、(La/Yb)N值及δEu值等指标反映硅质岩形成受到深大断裂相关的岩浆活动的影响。三岔子硅质岩Th/U值、U/Th值、δU值等特征指示其形成于缺氧的沉积环境。综合上述研究,该硅质岩发育地区在古生代处于伸展裂陷的构造背景之下,形成了裂陷或断陷盆地以及区域性同生断裂为主的一系列伸展构造。裂谷内岩浆活动在硅质岩的形成过程中为热水活动提供了热源,区域性同生断裂为热水流体的上升提供了通道;对流热水对周边物质的的淋滤和溶解导致了SiO2的富集;三者的共同作用最终导致一系列不同时代的热水沉积硅质岩在勉略带中形成。

关键词: 勉略构造带, 硅质岩, 地球化学, 沉积环境, 构造背景, 秦岭造山带

Abstract:

Paleozoic cherts outcrops were widely developed in the Mianlue tectonic belt, which could reveal the sedimentary environment and tectonic background of southern Qinling orogenic belt. In Sanchazi area, the cherts were found in the Sanhekou Group of the Middle Devonian. Geochemical analyses showed that Sanchazi cherts were high in SiO2 (94.04%-98.08%, with a mean value of 96.92%) and low in Al2O3 and TiO2; and were relatively high in Ba and U, but was relatively low in Zr for trace elements. The U/Th and Ba/Sr ratios were relatively high too. ΣREE content was significantly low (7.61-51.04 μg/g, with an average valve of 2251 μg/g). PAASnormalized REE distribution pattern inclined to the left, rich in HREE and with positive Eu anomaly and negative Ce anomaly. The Al/(A1+Fe+Mn), Fe/Ti and (Fe+Mn)/Ti values indicated the origin of hydrothermal sedimentation of the cherts. Al/(Al+Fe), Al2O3/(Al2O3+Fe2O3+MnO), Al2O3/(Al2O3+Fe2O3), δCe and (La/Ce)N values indicated that the cherts were mainly formed in the continental margin. The values of (La/Lu)N, (La/Yb)N and δEu could be related to the hydrothermal activities controlled by the deep fractures. Th/U, U/Th, δU values indicated the cherts were formed in the anoxic depositional environment. The above-mentioned geochemical evidences inferred that Sanchazi cherts were formed in a sedimentary environment of rifted basin at continental margin in the north of South China Block during the Paleozoic. During the rifting period, while a series of extensional structures, such as rift or fault basin and syngenetic faults, were formed, accompanied magma activities could provide heat source for the thermal water, and the regional syngenetic faults provided the channel for upwelling of the thermal water. Hence, the convective hot water became rich in SiO2 through the leaching and dissolving of chemical composition from surrounding strata, and contributed to the forming of hydrothermal sediment cherts of different geologic ages in the Mianlue tectonic belt.

Key words: Mianlue tectonic zone, cherts, geochemistry, sedimentary environment, tectonic background, Qinling orogenic belt

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