西昆仑阿克萨依铁矿二长花岗岩岩石成因及动力学背景:年代学、地球化学及Sr-Nd-Pb-Hf同位素约束

对甜水海地块东北部阿克萨依矽卡岩型铁矿区内与矿化有关的二长花岗岩开展LA-ICP-MS锆石U-Pb年代学、全岩主微量元素、Sr-Nd-Pb同位素及锆石Lu-Hf同位素研究,旨在厘定花岗岩的形成时代、岩石成因及成矿动力学背景。LA-ICP-MS锆石U-Pb定年结果表明,二长花岗岩侵位年龄为12.37±0.18 Ma(中新世高拉瓦尔期)。地球化学特征表明二长花岗岩属准铝质?弱过铝质高钾钙碱性?钾玄岩系列,具有高Sr(432~1174μg/g),低Y(3.08~18.11μg/g)和Yb(0.35~1.39μg/g)以及高Sr/Y值(64.8~203)的埃达克岩特征。在Harker图解上SiO2与其他主量元素呈负相关关系,La/Sm-La以及La/Yb-La图解显示具有分离结晶趋势,表明岩浆演化以分离结晶为主。二长花岗岩SiO2与Y、Yb呈负相关关系而与Sr/Y值呈正相关关系,并且矿区内存在同源同期闪长玢岩,因此认为阿克萨依二长花岗岩的埃达克岩性质是玄武质岩浆的分离结晶作用引起的。全岩Sr-Nd-Pb同位素分析表明,阿克萨依二长花岗岩与青藏高原北部地球化学省具有相似的同位素组成特征,岩浆起源于受俯冲板片交代的富集地幔,而变化范围较大的εHf(t)值以及较低的Mg#暗示岩石侵位过程中受到地壳混染。结合前人研究,我们认为阿克萨依二长花岗岩及相关的矽卡岩铁矿形成于中新世西昆仑受巨型地幔低速体上涌影响的伸展构造背景下。

Petrogenesis and Geodynamic Setting of Akesayi Cenozoic Monzo-granite in West Kunlun: Constraints from Geochronology,Geochemistry and Sr-Nd-Pb-Hf Isotopes

This study focus on the monzogranite intrusion related to the Akesayi skarn Fe deposit which is located in the eastern part of the Tianshuihai terrane,West Kunlun.In this paper,we report new results of laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)U-Pb zircon dating,chemical and Sr-Nd-Pb isotopic compositions of the whole-rock samples,and zircon Lu-Hf isotopic compositions,and discuss the petrogenesis and geodynamic settings of the intrusions in Akesayi.LA-ICP-MS zircon U-Pb dating of the monzogranite yields a concordant age of 12.37±0.18 Ma,which means that the intrusion was formed in the Miocene.The monzogranite belongs to high-K calc-alkaline and shoshonite series,and is characterized by moderate fractionated REE patterns with weak negative Eu anomalies.The monzogranite has geochemical signatures resembling that of the adakites,such as high Sr(432–1174μg/g)and Sr/Y ratio(64.8–203),with low Y(3.08–18.11μg/g)and Yb(0.35–1.39μg/g)contents.The Harker,La/Sm-La and La/Yb-La diagrams show that the magmatic evolution is dominated by fractional crystallization.Combined with the negative correlation between SiO2 and Y,Yb,positive correlation between SiO2 and Sr/Y,it can be inferred that the adakitic monzogranite may be derived from fractional crystallization of mantle-derived basaltic magmas.The Sr-Nd-Pb isotopic compositions of the rocks show affinity of the Northern Tibetan Plateau Geochemical Province(NTPGP),which indicate that the magma may be originated from the enriched mantle source that had been metasomatismed by the subduction fluid.Contamination of the continental crust may account for the highly variedεHf(t)values and relatively low Mg#of the monzogranite.Integrating with previous geochronological and geological results,we propose that the monzogranite and related Fe mineralization in the Akesayi deposit were likely formed at a regional extension background induced by the huge mantle upwelling.