南秦岭柞水地区小磨岭杂岩基性岩类的地球化学特征及其地质意义

Geochemistry and geologic significance of basic rocks in the Xiaomoling complex in the Zhashui area,South Qinling,China.

小磨岭杂岩主要由基性、中性和后期侵入的酸性岩类组成。基性岩类主要由变质玄武岩和辉绿岩组成,二者具有类似的元素地球化学特征,稀土元素配分曲线显示轻稀土元素富集的特征;MORB标准化的微量元素蛛网图显示"大隆起"型的配分形式,具有轻微Nb、Ta亏损及富集大离子亲石元素的特点,La/Nb、Th/Yb及Hf/Ta比值特征揭示岩浆源区受到陆壳混染作用的影响,显示板内伸展背景下幔源岩浆成因的特点。另外,相对高的Zr/Y、Ta/Yb和低Zr/Nb比值亦指示板内岩浆作用的特点。综合地质、地球化学研究结果认为,小磨岭杂岩中的基性岩形成于板内构造环境,在岩浆演化过程中受到了陆壳物质的混染,是南秦岭新元古代末陆内扩张背景下岩浆作用的产物。

The Xiaomoling complex exposed at Mogouxia, Zhashui County, South Qinling, is mainly composed of basic, intermediate and hte-intruded acid rocks. The basic rocks are dominantly made up of metabasalt and metadiabase, which have similar element geochemical characteristics. Their REE distribution patterns show the LREE-enriched feature; the MORB-normalized trace element spidergram shows ＂humped＂ distribution patterns and slight deplefon in Nb and Ta and enrichment in LILE. The high values of La/Nb, Th/Yb and Hf/Ta reveal that the magma source was subjected to contamination with continental crust, suggesting the features of mantle-derived magma in an intraphte extensional setting. In addition, the higher Zr/Y and Ta/Yb ratios and lower Zr/Nb ratios also indicate that the magma formed in an intraphte tectonic setting. According to the integrated geological and geochemical study, it may be concluded that the basic rocks in the Xiaomoling complex formed in an intraphte tectonic setting and were contaminated by continental crust during fractional crystallization （FC）. These all indicate that the basic rocks of the Xiaomoling complex were the product of magmatism occurring in the terminal Neoproterozoic intracontinental extensional setting in the South Qinling.