青海泽多桌肉地区花岗闪长岩与暗色包体地球化学特征及成因

Geochemistry and genesis of granodiorite and the dark inclusion in Zeduozhuorou area,Qinghai

青海泽多桌肉地区花岗闪长岩中发育大量暗色闪长质包体。通过对花岗闪长岩(寄主岩石)与暗色包体的地球化学研究发现,花岗闪长岩(寄主岩石)的SiO_2含量为w(SiO_2)=61.61%~68.80%,A/CNK=0.91~0.99,属于偏铝质钙碱性花岗岩特征;暗色包体中发育针状磷灰石,暗色包体的SiO_2含量为w(SiO_2)=52.66%~66.99%,碱度率AR=2.08~2.26(大于1),里特曼指数σ=1.89~8.67,固结指数SI=16~24.74,属过铝质钙碱性花岗岩。稀土元素分析显示,花岗闪长岩稀土总量w(ΣREE)=117.44×10^(-6)~265.68×10^(-6);暗色包体稀土总量w(ΣREE)=184.05×10^(-6)~342.11×10^(-6),总量高于寄主岩石;暗色包体和寄主花岗岩有着相似的稀土元素配分模式,显示轻稀土富集,重稀土亏损,轻重稀土元素分馏明显的特征。微量元素分析显示,暗色包体微量元素的分布型式与寄主岩微量元素特征基本一致,大离子亲石元素K、Rb、Ba相对富集,高场强元素Nb、Ta、Zr、Hf相对亏损,反映了岩浆混合作用的特征。在花岗闪长岩中获得LA-ICP-MS锆石U-Pb年龄为202.49 Ma±0.63 Ma,属晚三叠世。表明泽多桌肉花岗闪长岩属于晚三叠世以来的造山带伸展崩塌造成岩石圈较快速减薄以及幔源岩浆底侵作用的发生,并导致地壳岩石接近熔融温度产生壳源岩浆而成岩。

A lot of dark diorite inclusions are hosted by granodiorite in Zeduozhuorou area.w(SiO_2)=61.61%~68.80%,A/CNK=0.91~0.99 of the granodiorite is characteristic of metaluminous calcalkaline granite;w(SiO_2)=52.66%~66.99%,AR=2.08~2.26,Rittmann coefficientδEu=1.89~8.67,SI=16~24.74 of the dark inclusions paraluminous-calcalkaline granite.REE analysis show that w(ΣREE)=184.05×10^(-6)~342.11×10^(-6) of the dark inclusions is higher than w(ΣREE)=117.44×10^(-6)~265.68×10^(-6) of the host granodiorite.REE pattern of the dark inclusions is the same as the host rock with LREE enrichment,HREE depletion and obvious LREE HREE fractionation.Trace element analysis show the same distribution pattern in the host rock as in the dark inclusions with relative enrichment of LTLE K,Rb,Ba and relative depletion of HFSE like Nb,Ta,Zr,Hf which all reflect magma mingling characteristics.Their LA-ICP-MS U-Pb dating are all 202.49 Ma±0.63 Ma belonging to Late Triassic Epoch.The result shows that the Zeduozuorou granodiorite was formed by magma of crustal rock during the orogenic belt extension and related collapse-caused fast lithosphere thinning and the magma diapir which led to the temperature to melt the crustal rocks.