新疆古伦沟地区古仍格萨拉东岩体的地球化学及锆石U-Pb年龄

摘要

The East Gurenggesala granitic intrusion in Gulungou area on the northern margin of Middle Tianshan Mountains consist of granodiorite-porphyry with porphyry copper mineralization. The intrusion is characterized by enrichment of alkali, with Na2O/K2O ratio changing from 1.95 to 19.00, and depletion of Fe and Mg, accompanied by sub-alkaline (mainly tholeiitic and calc-alkaline series) and weakly peraluminous features (A/CNK=0.98~1.11). REE concentrations are low (∑REE=61.28×10-6~99.50×10-6) and show obvious differentiation between LREE and HREE (LaN/YbN=7.82~22.80), with weak Eu negative anomalies (δEu=0.72~0.97). In addition, the rock mass is relatively rich in such elements as Rb, Ba, Th, U and K, and poor in Nb, Ta, P, Ti etc., suggesting characteristics of volcanic-arc granite (VAG). Zircon LA-ICP-MS U-Pb dating results show that the crystallization of the intrusion took place from (488.9±1.7) Ma to (470.5±3.1) Ma, i.e., in Early Ordovician. Sr-Nd isotopic compositions of East Gurenggesala granitic intrusion are fairly uniform: (87Sr/86Sr)i=0.70677~0.70685,εSr(t)=40.10~41.21, (143Nd/144Nd)i=0.51190~0.51191, εNd(t)=-2.62 ~ -2.30, tDM=1.31~1.38 Ga, implying that magma originated from partial melting of Meso-Proterozoic mantle-derived basic lower crust. Based on both previous and present research results, the authors have reached the conslusion that East Gurenggesala granitic intrusion was formed in the epicontinental arc relevant to the subduction of the paleo-Junggar ocean towards Yili-Central Tianshan plate in Early Ordovician together with porphyry copper mineralization. In general, the emplacement of East Gurenggesala granitic intrusion marked the epoch when the northern margin of Middle Tianshan entered into the stage of active epicontinental arc in connection with subduction in Early Ordovician.%古伦沟地区古仍格萨拉东花岗闪长斑岩体位于中天山构造带北缘。地球化学和锆石 U-P 年龄测定结果显示：岩石高碱富 Na、贫 Fe 和 Mg、弱过铝质(ACNK=0.98~1.11),稀土总量较低(∑REE=61.28×10-6~99.50×10-6)且分异明显(LaN/YbN=7.82~22.80)、铕弱负异常(δEu=0.72~0.97),相对富集Rb、Ba、Th、U、K 等,亏损 Nb、Ta、P、Ti 等,具火山弧花岗岩的特征。岩体具有较均一的 Sr、Nd 同位素组成：(87Sr/86Sr)i=0.70677~0.70685,εSr(t)=40.10~41.21,(143Nd/144Nd)i=0.51190~0.51191,εNd(t)=-2.62~-2.30, tDM＝1.31~1.38 Ga。其锆石U-Pb年龄为(488.9±1.7)~(470.5±3.1) Ma。表明古仍格萨拉东岩体形成于早奥陶世陆缘弧环境,可能是先存的中元古代幔源基性下地壳部分熔融的产物,其形成与古准噶尔洋向伊犁-中天山板块下的俯冲作用有关,标志着中天山北缘于早奥陶世时期已进入与俯冲消减有关的活动陆缘演化阶段。此次岩浆活动导致区内发生斑岩型铜矿化。