南秦岭竹山地区硅质岩的地球化学特征及其形成环境

Geochemical Characteristics and Forming Environment of Siliceous Rocksin Zhushan Area,Southern Qinling Mountains

竹山断裂带属南秦岭造山带,研究区所采集之硅质岩分布于断裂带南缘、基性火山岩之上,与中性火山岩互层产出,硅质岩中可见薄层片理化火山岩,内部可见灰岩薄层。通过对硅质岩系统地球化学研究,探讨硅质岩形成的原因和构造背景。研究表明,①硅质岩的SiO 2含量96.88%~97.85%,Al/(Al+Fe+Mn)值为0.19~0.30,显示热液成因特征;②Al 2 O 3/(Al 2 O 3+Fe 2 O 3)范围大致在0.26~0.39,显示倾向洋中脊沉积环境;③硅质岩大部分微量元素是小于地壳元素丰度的,但是V、Ba、As、Sb、U明显较高,符合热水成因硅质岩特征;④稀土元素北美页岩标准化配分模式图显示,富集重稀土的左倾、Eu正异常、Ce负异常的特征,具有明显热水沉积的成因特征,(La/Lu)N、(La/Yb)N及(La/Ce)N比值分别为0.10~0.15、0.11~0.14、1.23~1.66,显示为洋中脊或洋盆环境。结合前人研究成果,认为研究区硅质岩形成于扬子板块北缘深水盆地,为热水沉积的产物。

Zhushan fault zone belongs to South Qinling orogenic belt.The siliceous rocks collected in this paper are distributed in the southern margin of the fault zone,above the basic volcanic rocks,and interbedded with the neutral volcanic rocks.Thin layers of schistosized volcanic rocks and thin layers of limestone can be seen in the siliceous rocks.Based on the systematic geochemical study of siliceous rocks,this paper discusses the causes and tectonic setting of siliceous rocks.The results show that:①the SiO 2 content of siliceous rocks is 96.88%~97.85%,and the Al/(Al+Fe+Mn)value is 0.19~0.30,indicating the hydrothermal genetic characteristics;②the range of Al 2O 3/(Al 2O 3+Fe 2O 3)is about 0.26~0.39,indicating the tendency to the mid ocean ridge sedimentary environment;③most of the trace elements in siliceous rocks are less than those in the crust,but V,Ba,As,Sb and U are obviously higher.④the normalized REE distribution pattern of North American shale shows that the REE enrichment is characterized by left dip,positive Eu anomaly and negative CE anomaly,which has obvious genetic characteristics of hydrothermal deposition.The ratios of(La/Lu)N,(La/Yb)N and(La/Ce)N are 0.10~0.15,0.11~0.14 and 1.23~1.66,respectively,indicating a mid ocean ridge or ocean basin environment.Combined with the previous research results,it is considered that the siliceous rocks in the study area were formed in the deep-water basin on the northern margin of the Yangtze plate and were the products of hot water deposition.