新疆尼勒克县圆头山后碰撞花岗斑岩的同位素年代学及地球化学特征

圆头山黑云母花岗斑岩位于尼勒克县城南,岩体普遍发育的铜矿化,以中部碎裂带中矿化最强。岩石具有典型的斑状结构,斑晶由碱性长石和黑云母组成,基质以长石、云母和石英为主,并含有少量的硫化物。圆头山黑云母花岗斑岩的轻重稀土分馏明显,并且明显富集大离子亲石元素(LILE) (除Sr外)和Pb,同时亏损高场强元素(HFSE)和Nb、Ta、P、Ti,显示出弧火山岩的地球化学特征。圆头山黑云母花岗形成年龄为269±3Ma,其源区以具有弧火山岩特征的下地壳为主,同时含有少量的幔源物质,而且在岩体的侵位过程中有高(⁸⁷Sr/⁸⁶Sr)_i (0.706054~0.709228)和高Pb含量(5.05×10^-6~32.5×10^-6)的陆壳物质混染。在下地壳物质熔融过程中,位于下地壳底部的富水和成矿元素的MASH带物质也被圈入,使原始岩浆富水和成矿物质。圆头山黑云母花岗斑形成于碰撞后阶段,岩石的形成主要受岩石圈地幔拆沉作用的控制,而岩石圈地幔的拆沉可能与中亚造山带的垮塌或塔里木地幔柱活动有关。

Zircon U-Pb geochronology and geochemistry of post-collisional granitic porphyry from Yuantoushan, Nileke, Xinjiang

The Yuantoushan biotite granitic porphyry intrusion is located at the southern Nileke. Copper mineralization prevalently occurs within the intrusion,especially in the shattered zone. The granite has typical porphyritic texture, where phenocrysts are composed of alkali feldspar and biotite, and matrix is mainly composed of feldspar, biotite, quartz and minor sulfide. Trace element patterns for the granite are significantly depleted in HSFE, Nb, Ta, P, Ti and enriched in Pb and LILE (except Sr), indicating products of arc magmatism. Zircon U-Pb age for the granite is 269±3Ma. According to its geochemical features, we inferred that the granite were derived from a source mainly is composed of the lower continental crust materials with minor mantle substance and the lower continental crust has similar geochemical features with arc magma. In addition, the Yuantoushan granite has high (⁸⁷Sr/⁸⁶Sr)_i ratios (0.706054 to 0.709228) and Pb contents (5.05×10^-6~32.5×10^-6) indicate the occurrence of contamination of continental crust materials during its emplacement. The involvement of fertilized materials from the melting-assimilation-storage homogenization (MASH) zones, which settled in the bottom of lower continental crust, induced the primary magma of the Yuantoushan intrusion enriched in water and metals. The geochemical data of the Yuantoushan granite indicates it was formed at post-collision setting and controlled by a delamination of mantle lithosphere. The delamination could be induced by the collapse of Central Asian Orogenic Belt or by the activity of Tarim mantle plume.