Chronology and geochemical characteristics of the highly fractionated alkali feldspar granite from the Maodeng deposit in the southern Great Xing'an Range

Abstract

毛登矿床位于内蒙古锡林浩特境内，是大兴安岭南段典型的钼铋锡铜矿床，产于阿鲁包格山杂岩体晚阶段侵位的碱长花岗岩和花岗斑岩以及下-中二叠统大石寨组火山角砾岩中。本文以碱长花岗岩为主要研究对象，开展了岩石地球化学、Sr-Nd-Hf同位素、锆石U-Pb定年和微量元素地球化学研究，以探讨成岩时代、岩浆结晶的物理化学条件及其对成矿的制约。研究显示，碱长花岗岩锆石结晶年龄为140.5±0.8Ma，侵位于早白垩世。岩体高硅，富碱，贫钙、镁、铝，富集Rb、F、Th、Nd、Sm、Zr和Hf等元素，亏损Eu、Ba、Sr、P、Nb、Ti和轻稀土等元素，轻、重稀土分馏较小，具稀土四分组效应。碱长花岗岩锆石Ti温度为654~817℃，Ce⁴⁺/Ce³⁺值为0.1~19，岩浆氧逸度lgf（O₂）为-25.7~-18.6，表明原生岩浆为高温、低氧逸度的熔体。岩体具较高的ε_Nd（t）值（-0.1~+2.9）和正的ε_Hf（t）值（+4.0~+9.3）以及年轻的二阶段模式年龄（t^Nd_DM2=699~940Ma；t_HfDM^C=600~936Ma），表明岩浆源区来自含大量幔源组分的新生地壳的部分熔融。碱长花岗岩经历了高度分异演化和熔体-流体作用，具高F、低氧逸度特征，是毛登矿区钼、铋、锡、铜金属矿化形成的重要先决条件。;Located in Xinlinhot of Inner Mongolia in the southern segment of Great Xing'an Range, the Maodeng deposit is a typical Mo-Bi-Sn-Cu deposit occurred in the alkali feldspar granite and granite porphyry in the late emplacement of Alubaogeshan complex, as well as the volcanic breccia of Lower-Middle Permian Dashizhai Formation. In this paper, whole-rock geochemistry, Sr-Nd-Hf isotope, zircon U-Pb age and trace element geochemistry of alkali feldspar granite have been studied, aiming to investigate the rock-forming age, the formation conditions, the significance for petrogenesis and metallogenesis. The zircon dating yields a weighted mean U-Pb age of 140±0.8Ma, which indicates the emplacement of the alkali feldspar granite took place during the Early Cretaceous. Geochemically, the alkali feldspar granite is characterized by high concentrations of SiO₂ and (Na₂O+K₂O), with low contents of CaO, MgO and Al₂O₃. The rock is enriched in Rb, F, Th, Nd, Sm, Zr and Hf, while depleted in Eu, Ba, Sr, P, Nb, Ti and LREE. It is especially characterised by a low LREE/HREE ratio, with a huge Eu depletion and a REE tetrad pattern. The zircon of the alkali feldspar granite shows high temperature (654~817℃) and low Ce⁴⁺/Ce³⁺ ratio (0.1~19), oxygen fugacity (lgf(O₂)=-19.6~-14.4) of the magma. The granite has relatively high ε_Nd(t) (-0.1~+2.9), positive ε_Hf(t) (+4.0~+9.3) and young two-stage Nd and Hf model ages (t^Nd_DM2=699~940Ma; t_HfDM^C=600~936Ma), suggesting that the rock is dominantly derived from the partial melting of a juvenile lower crust which was originated from source rocks or magma separated shortly from the upper mantle, followed by strong fractional crystallization during magma ascent. Based on the above reasearch, it is proposed that under the condition of relatively low oxygen fugacity and high temperature, the magma evolves by highly fractional crystallisation and melt-fluid interaction during the late stage. Therefore, it is believed that the alkali feldspar granite plays an important role in Mo-Bi-Sn-Cu mineralization in the Maodeng deposit.