大兴安岭中段二叠系大石寨组细碧岩的岩石学地球化学特征及其成因探讨

The petrological and geochemical characteristics and petrogenesis significance of spilite from lower Permian Dashizai Group in the middle section of Da Hinggan Mountains

大兴安岭中段林西地区二叠纪大石寨组下部的细碧角斑岩建造记录了大石寨裂陷槽早期的发育历史,因而对其成因和构造环境的研究具有一定的意义。本文在详细的野外工作基础上,对二叠系大石组的细碧岩进行了较为系统的岩石学、元素地球化学及同位素地球化学研究。结果表明,细碧岩的岩石类型为玄武岩、玄武安山岩、安山岩、橄榄粗安岩和粗安岩(或歪长粗面岩),属于拉斑玄武岩岩石系列;细碧岩的主量元素质量分数及变化范围类似于N-MORB玄武岩;与正常洋中脊玄武岩相比较,细碧岩的LILE元素明显富集,HFSE元素的质量分数与大洋中脊玄武岩(N-MORB)相接近,Nb、Ta明显亏损;细碧岩的稀土元素分配模式具平坦型;(^(87)Sr/^(86)Sr)t为0.7013～0.7056。细碧岩形成于大石寨裂陷槽早期裂陷阶段弧后盆地局部扩张的构造环境;细碧岩是由拉斑玄武质岩浆在上升过程中经热卤水的严重混染后喷出地表结晶而形成的。

The spilite-keratophyre formation from lower Permian Dashizhai group in the middle section of Da Hinggan Mountains recorded the early development of Dashizai taphrogenic trough, so it is significant to study their tectonic setting and petrogenesis. Based on detailed fieldwork, petrological and geochemical studies on spilites have been carried out. The results show that the spilites, composed of basalt, basaltic andesite, olivine trachy-andesite and trachy-andesite belong to tholeiitic series. The major elements show that they are similar to the N-MORB in EPR. Compared to the N-MORB, the spilites are enriched in LILE, and HFSE are similar to that in N-MORB, Nb and Ta are depleted, and REE patterns are generally flat; the (Sr-87/Sr-86)t varies from 0.7013similar to0.7056. The results suggest that the spilites were developed in the back-arc or within-arc spreading basin during the early development of Dashizai taphrogenic trough, and spilites were crystallized in the surface from tholeiitic magma which are mixed seriously by salt water.