大兴安岭中南段中生代中基性火山岩岩石学地球化学研究

Petrological and Geochemical Studies on the Intermediate-Basic Volcanic Rocks from the Middle-South Part of the Da Hinggan Mountains

大兴安岭中南段中生代火山岩的成因和形成时的构造背景涉及南蒙古 -兴蒙造山带的后期构造演化及区内众多的铜、银多金属矿床的形成 ,因而对其成因的研究具有重要意义。在野外工作基础上 ,对大兴安岭中南段中生代中基性火山岩进行了较为系统的岩石学和元素地球化学研究。结果表明 ,火山岩的岩石类型为玄武粗安岩、粗面安山岩、英安岩及粗面岩 ,属钙碱性岩石系列 ;玄武粗安岩、粗面安山岩、英安岩及粗面岩的常量元素以富集SiO2 ,Na2 O ,K2 O和亏损Fe2 O3,FeO ,MgO ,CaO等为特征 ,微量元素以富集亲石元素和亏损铁族元素及Nb ,Ta为特征 ;火山岩的稀土元素分配模式为轻稀土富集型 ;(87Sr/86 Sr) i 为 0 .70 4 5～ 0 .70 77,且主要集中在 0 .70 4 5～ 0 .70 5 5的区间内 ,εNd(t)多数为正值。火山岩是地幔物质部分熔融的产物 ,形成于张性构造环境 ,它们是天山 -兴安造山带后造山期软流圈或地幔柱上涌所引起的岩石圈伸展作用、内蒙古 -吉黑印支造山带造山期后的伸展作用和蒙古 -鄂霍茨克残余洋“剪刀式”闭合所形成的拉张作用等综合效应的响应事件。

The petrogenesis and tectonic background of Mesozoic volcanic rocks from the middle-south part of the Da Hinggan Mountains are closely related to the late stage evolution of the Southern Mongolia-Da Hinggan Mountains orogenic belt and to the genesis of copper-polymetallic deposits. Based on detailed fieldwork,petrological and geochemical studies on volcanic rocks have been carried on. The results show that the volcanic rocks,including basaltic trachyte,trachyandesite,trachyte and dacite,belong to calc-alkaline series. In major elements they are enriched in SiO_2,Na_2O and K_2O,and depleted in Fe_2O_3,FeO,MgO and CaO;and in trace elements they are enriched in lithophile elements,and depleted in siderophile elements and Nb,Ta,Ti etc. In REE patterns they are generally LREE enriched, ( 87 Sr/ 86 Sr)_t ratios are 0.7045-0.7077,mostly 0.7045-0.7055,and most of ε_ Nd ( t ) values are positive. The results suggest that partial melting of upper mantle in different degrees was the major mechanism for generation of the volcanic rocks in the middle-south part of the Da Hinggan Mountains,and the volcanic rocks were developed in the tensional tectonic setting. The Mesozoic volcanic activity was a response to the extension caused by upwelling of mantle plume or asthenosphere during the post-orogenic period of the Tianshan-Da Hinggan Mountains belt,to the post-orogenic extension background of the Inner Mongolia-Jihei orogenic belt,and to compression background caused by closure of Mongolia-Okhotsk residual ocean in a scissors form.