大兴安岭中生代双峰式火山岩的地球化学特征

Geochemistry of Mesozoic volcanicr ocks in Da Hinggan Ling:The bimodal volcanic rocks

大兴安岭中生代火山岩的主要活动时期为晚侏罗世至早白垩世,由碱性系列玄武岩和亚碱性系列玄武岩以及伴生的中酸性火山岩组成。它们与邻区俄罗斯和蒙古同时期火山岩构成面形展布的巨型火山岩带。碱性系列玄武岩高度富集不相容元素,其富集程度类似于板内碱性系列玄武岩,但明显亏损Nb和Ta等高场强元素这一点又类似于活动大陆边缘火山弧或岛弧钙碱性玄武岩。亚碱性系列玄武岩适度富集不相容元素而强烈亏损高场强元素的特征类似于活动大陆边缘火山弧或岛弧火山岩,其中低钾玄武岩类似于拉斑玄武岩。大兴安岭酸性火山岩根据地球化学特征可划分为高Sr流纹岩类和低Sr流纹岩类。前者富集Ti、Ba、Sr、Co和Ni而贫Rb、Zr和Th等强不相容元素,类似于大陆溢流玄武岩省分异作用形成的流纹岩;后者明显富集不相容元素Rb、Zr和Th而亏损Ti、Ba、Sr和Co,它们与碱性系列玄武岩之间形成类似于大陆裂谷环境的双峰式火山岩组合。亚碱性系列玄武岩与高Sr流纹岩的成因关系类似于大陆伸张环境的双峰式火山岩,但前者形成从基性到酸性的连续演化系列,并没有形成Daly成分间断。这表明大兴安岭火山岩在源区及其原始岩浆的性质上明显区别于世界大陆溢流玄武岩省。也就是说,大陆溢流玄武岩省的双峰模式起源于干岩浆体系。

The Mesozoic volcanic rocks in Da Hin ggan Ling,mainly formed from late Ju rassic to early Cretaceous,consist of alkaline series basalts,subalkaline series(high-K and low-K)basalts and associated intermediate -acid volcanics.Combining with the volcanic rocks of the same age in Russia and Mongolia,they constitute a mega-sc ale area-volcanic province.The alkali ne series basalts,similar to the int raplate alkaline basalts,are highl y enriched in incompatible elements.However,th ey are depleted in Nb and Ta,which is n evertheless similar to the calc-alk aline basalts from active continental margin and island arc.The sub-alkaline series basalts show intensive deple tion in HFSE and mildly enrichment in incomp atible elements,a characteristic o f basalts from active continental ma rgin and island arc.Amongst them the low - K basalts are similar to island arc th oleiite.Mesozoic rhyolites can be geochemically grouped into high-Sr and low-Sr subgoups.The high-Sr rhyolites,enriched in Ti,Ba,Sr,Co,Ni and depleted in highly incompatible element,such as Rb,Zr and Th,are sim ilar to the those in the continental flood basalt province formed by fractional crystallization.The low-Sr rhyolites are obviously enriched in Rb,Zr and Th and depleted in Ti,Ba,Sr and Co.They form,together with alkaline series basalts,typical bimodal pattern s similar to those in the continental r ift.The genetic association between sub-alkaline basalts and high-Sr r hyolites are comparable to the bimodal volcanic r ocks of continental extension setting.However,they constitute a successive geochemical variation without Daly gap,which suggests that the volcani c rocks in Da Hinggan Ling are much different from those in the continen tal flood basalt provinces in the world.It is therefore suggested that th e bimodality of continental flood basalts province is originated from dry magmatic systems where intermediate -composition magmas are more viscous than erupting basalts and contain le ss volatile than erupting rhyolites.In Da Hinggan Ling,the water -rich primary magma contributes to the oversatur ation of fractionated magma with volatile components,which result in the expl osive eruption of intermediate magm a.