阿尔金山东段黑沟脑侵入体地球化学特征及锆石U-Pb年龄

Geochemical characteristics and zircon U-Pb age of the Heigounao intrusive in the eastern part of the Altun Mountains

阿尔金山东段黑沟脑广泛发育中酸性侵入体,黑沟脑地区主要由花岗斑岩、正长花岗岩构成。岩石地球化学显示,该岩体具有较高的SiO2含量,全碱Na_(2)O+K_(2)O含量平均为9.65%,富铝、富钾的特征,里特曼指数(δ)介于1.73~7.63,均值为4.11,铝质指数(A/CNK)为0.88~1.03,平均为0.97,为碱性系列准铝质岩石;具有较高的分异程度;稀土元素总量SREE介于211.54×10^(-6)~569.58×10^(-6),均值为350.72×10^(-6),其稀土配分模式右倾,整体表现出轻重稀土分馏明显,(La/Yb)N=8.15~21.80,δEu为0.31~0.60,均值为0.45,表现出强烈的Eu负异常,富集高场强元素Zr和Hf,Nb、Ta表现出亏损,亏损Ba、Sr等大离子亲石元素(LILE)。通过对岩石成因、源岩及构造环境分析认为,本区侵入体可能以壳源变质杂砂岩的部分熔融物质为主,并有幔源物质参与,为经历了结晶分异作用形成的Ⅰ型花岗岩类;LA‑ICP‑MS锆石U‑Pb定年确定其形成时代为490.4±2.9 Ma,表明寒武纪在阿尔金东段南阿尔金洋已开始向南俯冲且与北祁连花岗质岩浆事件有较好的对应性。

There are large-scale intermediate-acid intrusive rocks distributed of the Heigounao in the eastern part of the Altun Mountains.The typical plutons include granite porphyry and syenogranite.The rock geochemistry shows that the rock mass has high SiO2 content,the average content total alkaloids Na_(2)O+K_(2)O is 9.65%,rich in aluminum,sodium and potassium.The Rittman index(δ)is between 1.73~7.63,the average value is 4.11,and the aluminum index(A/CNK)is 0.88~1.03,0.97 on average,is alkaline series quasi aluminum rock,with high degree of differentiation.The total amount of rare earth elements(ΣREE)is 211.54×10^(-6)~569.58×10^(-6),with an average value of 350.72×10^(-6),and its rare earth distribution pattern is right inclined,showing significant light and heavy rare earth fractionation as a whole.(La/Yb)N=8.15~21.80,δEu is 0.31~0.60,with an average value of 0.45,showing a strong negative anomaly of Eu,which gathers high field strength elements and has a large loss of ion lithophile elements,enrichment of HFSE(Zr and Hf)and depletion of LILE(Ba and Sr).Based on the analysis of petrogenesis,source rock and tectonic environment,it is suggested that the intrusions in this area may be formed in the subduction volcanic arc environment and later syn-collisional tectonic environment,and the basic rocks of shell source amphibolite facies are mainly partially melted,with the partial melting of the crust derived metasandstone as the main part,and theⅠ-type granites formed through crystallization differentiation.LA-ICP-MS zircon U-Pb isotopic dating has acquired age of 490.4±2.9 Ma.It shows that the South Altun Ocean in the east part of Altun has begun to subduct to the south and has a good correspondence with the granitic magmatic event in the North Qilian.