中天山卡瓦布拉克杂岩带中闪长岩的锆石U-Pb年龄及Hf同位素特征

卡瓦布拉克杂岩带出露于中天山地块东段卡瓦布拉克—阿克塔格地区,沿卡瓦布拉克断裂呈东西向展布。笔者选择构成该杂岩带的中—基性岩石主体闪长岩,开展了LA-ICP-MS锆石U-Pb年代学和LA-MC-ICP-MS锆石Hf同位素研究。结果表明:闪长岩中锆石呈自形—半自形,发育典型的岩浆锆石振荡生长环带,Th/U值较高(均大于0.40),且Th、U含量呈现较好的正相关关系,为典型的岩浆成因锆石;这些锆石的206Pb/238 U年龄加权平均值为(375±1)Ma,MSWD=0.081,属晚泥盆世,可代表其结晶年龄;锆石具有较均一的Hf同位素组成,初始比值为0.282 655~0.282 747、εHf(t)值为4.0~7.2,其对应的亏损地幔模式年龄为714~842Ma。结合区域地质资料认为,卡瓦布拉克杂岩带中的闪长岩由亏损岩石圈地幔发生部分熔融而形成。

Zircon U Pb Isotopic Chronology and Hf Isotopes of Diorites from Kawabulak Complex,Central Tianshan

Kawabulak complex is distributed within Kawabulak‐Aktagh area in the east section of Central Tianshan block ,and extended nearly latitudinally along the Kawabulak fault .We present zircon LA ICP MS U Pb chronology and LA MC ICP MS Hf isotopes analyses of diorites that comprise the main parts of the intermediate and basic rocks of the complex .The zircons of diorites are mainly euhedral and hypidiomorphic texture with clear rings and high Th /U ratios ( > 0 .40) .The Th and U contents show a well positive correlation . This indicates that the zircons are typically from magma .The results show that the diorites intruded during the Late Devonian period with the 206 Pb/238 U weighted average age of 375 ± 1 Ma (MSWD = 0 .081 ) . Zircon Hf analyses show a relatively homogeneous isotopic composition ; the initial Hf values fall in between 0 .282 655 0 .282 747 and positive εHf (t) values of + 4 .0 + 7 .2 with a depleted mantle mode age (from 714 to 842 Ma ) . Combining with previous studies ,we suggest that the diorites from Kawabulak complex were formed through a partial melting of depleted lithospheric mantle .