西北天山莱历斯高尔岩体年代学、地球化学及其成因

Geochronology,geochemistry and petrogenesis of the Lailisigaoer intrusion in northwestern Tien Shan,NW China

西北天山莱历斯高尔岩体主要由花岗闪长斑岩和二长花岗斑岩组成,二者具有相似的地球化学特征,可能为同源岩浆分异演化的产物。锆石SHRIMP U-Pb年龄显示岩体形成于晚泥盆世(374±4 Ma)。莱历斯高尔岩体具有较高的SiO_2(67.75%~74.71%)含量和K_2O/Na_2O(1.24~2.20)比值,属于高钾钙碱性系列。稀土元素配分曲线为轻稀土富集型(La/Yb)_N(5.25~10.57),具有中等—弱的Eu负异常δEu(0.55~0.92)。微量元素特征显示富集大离子亲石元素(LILE),而亏损高场强元素(HFSE),具有较低的Sr(122×10^(-6)~356×10^(-6))含量、较高的Y(17.19×10^(-6)~21.82×10^(-6))和Yb(1.78×10^(-6)~2.57×10^(-6))含量。岩体的Sr-Nd同位素特征为Is,=0.707 867~0.709 654,ε_(Nd)(t)=-2.79^-1.46。铅同位素特征为^(206)Pb/^(204)b=18.423~19.915,^(207)Pb/^(204)Pb=15.576~15.685,^(208)Pb/^(204)Pb=38.344~39.305。元素和同位素地球化学特征表明,莱历斯高尔岩体岩浆源区主要为中元古代下地壳的部分熔融,并有部分地幔物质的加入。岩体形成于晚泥盆世准噶尔洋向伊犁—中天山微板块陡角度俯冲的大陆弧构造环境。

The Lailisigaoer intrusion, located in northwestern Tien Shan, mainly consists of granodiorite porphyry and monzogranite porphyry which share the similar geochemistry characteristics and may be derived from the same source. The zircon Sensitive High-Resolution Ion Microprobe （SHRIMP）U-Pb analyses of the granodiorite porphyry yielded a weighted mean age of 374 ＋ 4 Ma, indicating that the intrusion emplaced during the Late Devonian. The Lailisigaoer intrusion has high SiO2 contents （67.75 % - 74.71% ） and K20/Na20 ratios （ 1.24 -2.20 ）and all the rocks show a high-K ealc-alkaline trend. Chondrite-normalized REE patterns are moderately fraetionated （ La/Yb ） N ： （ 5.25 - 10.57 ）. They display moderate to weak negative Eu anomalies 8Eu ： （ 0.55 - 0.92 ） , with enrichment of LILE and depletion of HFSE. The rocks show low Sr（ 122×10-6-356×10-6） contents and high Y（ 17.19×10-6 - 21.82×10-6 ） and Yb（ 1.78×10-6 - 2.57×10-6） contents. Initial Nd-Sr isotopic compositions for the rocks are /St=0. 707 867 ～0. 709 654 and 8Nd（t）= --2. 79 ～-1. 46. The Pb isotopic ratios for the intrusion are 206Pb/204pb = 18.423 ～ 19.915,207Pb/204Pb = 15. 576 - 15.685 and 208Pb/204Pb = 38. 344 ～ 39.305. All the geochemical data, in conjunction with the geodynamie evidence, suggest that the Lailisigaoer intrusion was most likely generated by partial melting of the Mesoproterozoic crust and the involvement of some mantle components. We propose that the Lailisigaoer intrusion was emplaeed in a continental are setting with southward steep subduction of the Junggar ocean to the Yili-central Tien Shan microplate during the Late Devonian.