青海省扎日根结扎群火山岩形成时代及构造背景分析

青海扎日根结扎群火山岩中基性岩以贫硅、钾,高钛、钙,中性岩类以低硅、中钾、钛、钙,酸性岩类以高硅、钾,中钛,低钙为特征。根据Fe＊/MgO-TiO2图解上显示出本区火山岩绝大多数火山岩落在岛弧区。微量元素中Th/Nb=0.9〉0.11,Nb/Zr〉0.04显示出其构造背景为陆-陆碰撞形成的岛弧区。扎日根结扎群火山岩时代Rb-Sr同位素等时线给出的年龄为231±28Ma和225±8 Ma,属晚三叠世。另外Sr同位素的初始比值ISr=0.70522±0.00023,小于0.719,表明岩浆（原始）来源于上地幔,并且在上升的过程中受到地壳的混染。

Formation age and analysis on tectonic setting of volcanic rocks in Jiezha group,Zharigen area,Qinghai province

The volcanic rocks of Jiezha group in Zharigen area,Qinghai Province,are characterized by lower silicon and alkalis but high titanium and calcium for intermediate-basic rocks,low silicon and middle content of alkalis,titanium and calcium for intermediate rocks and high silicon and alkalis,middle titanium and low calcium for acid rocks.The TFeO/MgO—TiO2,diagram shows that the volcanic rocks are mainly plotted in the island arc area.Trace elements characteristics（Th/Nb=0.90.11,Nb/Zr0.04） illustrate that the volcanic rocks may be formed in a tectonic setting of island arc as a result of continent-continent collision.The apparent ages of volcanic rocks by Rb-Sr isotope isochronism are 231±28 Ma and 225±8 Ma,which means that the volcanic rocks belongs to the late Triassic.The initial ratio of ISr（0.70522±0.00023） is less than 0.719,which implies that the primary magma was originated from the upper-mantle and contaminated by the crust during its uplifting.