东秦岭俯冲消减古洋壳的地球化学鉴定

A GEOCHEMICAL DETERMINATION OF THE SUBDUCTION OCEAN CRUST IN EAST QINLING MOUNTAINS

高长林 秦德余 吉让寿 殷勇

Gao Changlin Qin Deyu Ji Rangshou Yin Yong

地质矿产部石油地质中心实验室 无锡

Central Laboratory of Petroleum Geology, MGMR

本文据笔者所提出的鉴定俯冲消减古洋壳的地质地球化学方法,证实了东秦岭商南松树沟地区存在有俯冲消减的古洋壳的残余。蛇绿岩套和对火山岩带的存在是鉴定俯冲消减古洋壳的主要可靠证据。松树沟蛇绿岩套中,超基性岩为纯橄榄岩和斜辉辉橄岩,具亏损地幔岩的稀土元素地球化学特征,其中的(变)基性岩为大洋拉斑玄武岩,SiO2含量值小于52%,TiO2含量值大于0.80%,TiO2/SiO2值高,Q值高,K值低,∑REE低,∑Ce/∑Y值低,不富集轻稀土,与大洋玄武岩相类似。而与蛇绿岩套紧邻的丹凤群火山岩为钙碱性岛弧型岩石,其TiO2/SiO2值低,Q值低,K值高,∑REE高,∑Ce/∑Y值高,富轻稀土,与岛弧型火山岩相似,据上述资料及常量元素对应分析结果,表明丹凤群火山岩形成于岛弧环境,松树沟地区的(变)火山岩形成于大洋环境。

The relic of subduction palaeooceanic crust is recognized in Song Shugou area of Shangnan in the eastern Qinling with the geochemical methods proposed by the authors. The existence of ophiolite suite and volcanic rock zone is the reliable evidence for the determination of subduction palaeooceanic crust. Ultrabasic rocks, which are pure olivinfels and orthorhomic pyroxene in Song Shugou ophiolite suite, are geochemically characterized by rare earth elements of loss mantle rocks. The basic (metamorphic) lava mixed in the ophiolite suite is practically a oceanic tholeiites and they have some characteristics such as less SiO2 (52%), high TiO2 (0.80%), high ratios of TiO2 /SiO2, high Q value, low K value, low REE value, low ratios of ΣCe/ΣY and poor in light rare earth elements, these and tholeiites are resemble to oceanic basalts. The volcanic rocks of Danfeng group adjacent to the ophiolite suite are calc-alkali island arc-typed rocks with low ratio of TiO2/SiO2, low Q value, and high values of K, ΣREE, and ΣCe/ΣY, enriched with light rare earthelements, which are similar to the volcanic rocks of island arc type. Based on the data mentioned above and the results of correlation analysis of common elements, the authors suggested that the volcanic rocks of Danfeng group were formed in island arc environment, while the volcanic rocks in Song Shugou area were formed in oceanic environment.

https://doi.org/10.11781/sysydz199002135