新疆西天山科克赛岩体年代学、地球化学及地质意义

Geochronology and geochemistry of the Kekesai intrusion in western Tianshan,NW China and its geological implications

新疆西天山科克赛岩体位于赛里木地块北部,岩石类型为二长花岗斑岩,属高钾钙碱性系列。元素地球化学组成表明,该二长花岗斑岩主量元素SiO2、Al2O3和MgO含量分别为70.56%～71.46%、15.27%～16.01%和0.77%～1.02%;轻稀土元素富集,(La/Yb)N为14.99～23.69,重稀土元素亏损,δEu值显示弱负异常或正异常(0.88～1.21);微量元素Sr、Y和Yb含量分别为363×10-6～577×10-6、7.40×10-6～8.50×10-6和0.60×10-6～0.72×10-6。上述特征显示二长花岗斑岩具有埃达克岩的地球化学特征。同位素地球化学特征表明该二长花岗斑岩具有较低的锶初始值(ISr=0.705078～0.705572)、正的εNd(t)值(2.82～3.56)和相对年轻的两阶段Nd模式年龄(774～835Ma)。锆石LA-MC-ICP-MSU-Pb年龄为301.9±1.8Ma,表明岩体形成于晚石炭世末期。元素地球化学和同位素地球化学特征显示科克赛岩体为新元古代增生的地壳物质部分熔融的产物,形成于晚石炭世后碰撞初期阶段。

Kekesai intrusion,located in the north of the Sayram block of western Tianshan,Xinjiang,is composed of monzogranite porphyry of high-K calc-alkaline series.This paper presents the new major and trace element geochemistry and Sr-Nd isotopic compositions of the monzogranite porphyry.The contents of SiO2（70.56%~71.46%）,Al2O3（15.27%~16.01%）,MgO（0.77%~1.02%）,Sr（363×10-6~577×10-6）,Y（7.40×10-6~8.50×10-6）,Yb（0.60×10-6~0.72×10-6） together with the strongly fractionated REE patterns（（La/Yb）N：14.99~23.69） and weak negative-,or positive-Eu anomalies（δEu：0.88~1.21） indicate that the monzogranite porphyry shows characteristics of adakitic rock.The rocks have low ISr（0.705078~0.705572）,positive εNd（t） values（2.82~3.56） and young two-stage Nd-model age（774~835Ma）.The LA-MC-ICP-MS zircon U-Pb age of the monzogranite porphyry indicates that the intrusion was emplaced at the end of the Late Carboniferous（301.9±1.8Ma）.Element and isotope data demonstrate that the Kekesai intrusion was produced by the partial melting of juvenile Neoproterozoic lower crust and associated with initial stage of post-collision process.