东昆仑造山带低山头二长花岗岩形成时代及岩石地球化学特征

Formation Age and Geochemical Characteristics of Dishantou Monzonitic Granite in Estern Kunlun Orogenic Belt

对东昆仑造山带中段低山头一带二长花岗岩体开展锆石U-Pb测年和岩石地球化学研究,获得二长花岗岩锆石U-Pb定年为(225.52±0.76)Ma,属晚三叠世印支期。岩石地球化学特征显示:Al_2O_3含量为14.73%～16.42%,里特曼组合指数σ43值为1.18～1.46,属钙碱性系列,K_2O/Na_2O<1,具富钠贫钾的特点,A/NCK值主要为0.94～0.99,属偏铝质岩浆岩;大离子亲石元素(LILE)富集Rb、K、Sr、Ba,高场强元素(HFSE)富集Th、U、La、Ce,而Nb、Ti明显亏损;其轻重稀土比值LREE/HREE为6.06～16.98,δCe为0.95～0.99,显示为负异常,(La/Yb)_N为7.46～28.35,轻稀土显示富集;δEu在0.51～0.92,总体属Eu亏损型;Eu负异常特征显示岩浆分异明显。岩体属I型花岗岩,结合构造环境判别图解,综合判断低山头一带二长花岗岩属东昆仑中段晚三叠世火山弧型花岗岩。

Ziron U Pb dating and petrological geochemistry of Ditoushan monzonitic granite in the middle section of Estern Kunlun Orogenic Belt have carried out in this paper. Ziron U Pb dating of monzonitic granite yields an age of （225.52＋0.76） Ma, showing that the Ditoushan monzonitic granite was generated in late Triassic Indo Chinese epoch. Petrological geochemistry research show that A12 03 content vary from 14. 73% to 16. 42% while Rittmann index change from 1.18 to 1.46, suggesting that these rocks belong to calc alkalic series. Their K2 O/Na2 O ra tio less than 1, showing the features of enriched sodium and depleted potassium. Their A/NCK range from 0.94 to 0.99, belonging to aluminous magmatic rock, with richment of large ionic lithophile element （LILE, such as Rb, K, Sr and Ba） and high strength field elements （HFSE, including Th, U, La and Ce）, but obviously poor of Nb and Ti. Their LREE/HREE ratios vary from 6.06 to 16.98, while ~Ce values range from 0.95 to 0.99, showing negative anomaly. Their （La/Yb）N ratios change from 7.46 to 28.35, showing rich LREE; Their ~Eu values vary from 0.51to 0.92, displaying negative Eu anomaly and showing distinct magmatic differentiations. These rocks belong to I type granite. Combined with geotectonic discrimination diagram, it＇s comprehensively judged that the Ditoushan monzonitic granite belongs to late Triassic volcanic arc granitoids formed in the middle Estern Kunlun Orogenic Belt.