西天山乌孙山花岗岩和闪长岩年代学、地球化学及岩石成因

Geochronology Geochemistry and Petrogenesis of the Granite and Diorite in Wusun Mountain Western Tianshan

笔者选取位于伊犁盆地内乌孙山北缘察布查尔林场花岗岩为研究对象,其地球化学、地质年代学和岩石成因研究对于探讨西天山南缘壳幔岩浆作用具有较重要的指示意义。岩石地球化学特征显示二长花岗岩属高钾钙碱性准铝质-弱过铝质岩石系列;轻稀土富集,弱的Eu负异常(δEu=0.74~0.84),富集大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素特征,Zr/Hf=42~44,部分样品中含有少量的刚玉标准矿物,显示出壳源花岗岩的特征。闪长岩Al_(2)O_(3)、FeOT、CaO含量明显高于二长花岗岩,Na_(2)O和K_(2)O含量低于二长花岗岩,属于高钾准铝质岩石系列;轻稀土富集,显示出Eu的轻微正异常(δEu=0.90~1.24),富集大离子亲石元素,亏损Nb、Ta等高场强元素,此外,闪长岩还显示出具有高Sr(Sr>400×10^(-6))低Y(12.83×10^(-6))及Yb(1.34×10^(-6))和高Mg^(#)特征,其源岩应为俯冲板片上覆地幔部分熔融产物。二者均显示出岛弧岩浆的特征。二长花岗岩锆石U-Pb年龄为(361.7±1.8)Ma;形成于晚泥盆世末期,结合前人的研究资料认为在~360 Ma由于南天山洋板片的回撤或俯冲流体参与,上覆地幔发生部分熔融产生了基性岩浆,岩浆上涌过程中提供大量的热导致地壳发生部分熔融形成了酸性岩浆,两种岩浆发生不均匀混合作用。上涌的岩浆引起地幔对流,导致伊犁地块内部出现一定的伸展作用(弧后伸展作用)。

The study object is located in the Chabuchaer Forest Farm on the northern margin of Wushun Moun-tain in Yili Basin,the studying of geochemistry,geochronology and petrogenesis have an important indicative significance for discussing crust-mantle magmatism in the southern margin of the West Tianshan.The geochem-ical characteristics show that the monzogranites is a high-potassium-calcium-alkaline and quasi-aluminous-weak peraluminous rocks,the monzogranites is enriched with LREE、weak negative Eu anomaly(δEu=0.74~0.84)、rich in LILEs and deficient in HFSEs(Nb、Ta、Ti、P),its Zr/Hf radios are 42~44,some samples contain a T small amount of corundum mineral,and it show the characteristics of crust-derived granite.The Al_(2)O_(3),FeO and CaO of the diorite is obviously higher than that in granite,but the Na_(2)O and K_(2)O is lower than the granite,it be-longs to high-potassium-calcium-alkaline quasi-aluminous rock;the diorite is enriched with LREE、weak posi-tive anomaly of Eu(δEu=0.90~1.24)、rich in LILEs and deficient in HFSEs(Nb、Ta),in addition,the diorite has high Sr(Sr>400×10^(-6)),low Y(12.83×10^(-6))and Yb(1.34×10^(-6))and high Mg#,therefore,the source rock is the partial melting product of the mantle overlying the subduction slab,which enrichment hornblende.The mon-zogranite and diorite show the characteristics of island arc magma.The zircon U-Pb dating results show that the age of monzogranite is 361.7±1.8 Ma,and belong to the late Devonian.Combined with previous research data,we believed that in~360 Ma,due to the rolling-back or subduction of the southern Tianshan ocean plate,the overlying mantle partially melted and produced the basic magma.During the uppouring process of the magma,a large amount of heat were provided,which led to partial melting of the crust and the formation of acidic magma,and the two kinds of magma had uneven mixing.At the same time,mantle convection caused by upwelling mag-ma leads to a certain extension(back-arc extension)in the Yili block.