皖南泾县榔桥岩体锆石U-Pb定年、Hf同位素和地球化学特征及其找矿指示意义

Zircon Dating,Hf Isotope Composition,Geochemistry and Their Prospecting Signification of the Langqiao Intrusion in Jingxian County,South Anhui Province

安徽泾县榔桥花岗闪长岩体为分布于皖南台褶带北缘壳幔同熔型复式岩体。该岩体主侵入期两个花岗闪长岩样品和侵入后期脉岩一个花岗斑岩样品的锆石 U-Pb 年龄分别为135．6±1．8Ma，137．6±2．0Ma，136．4±1．9Ma，显示岩体形成于早白垩世。锆石 Hf同位素分析表明榔桥岩体可能由中元古代地壳物质部分熔融形成，同时混染有少量古元古代地壳物质。根据全岩Zr含量计算出榔桥岩体花岗闪长岩“锆石饱和温度”为749～781℃，脉岩花岗斑岩“锆石饱和温度”为722～745℃，指示岩浆在成岩过程中快速冷却；元素地球化学研究显示，本区岩石在成岩过程中发生斜长石、钾长石以及副矿物磷灰石、钛铁矿的分离结晶作用。锆石Ce（Ⅳ）/Ce（Ⅲ）比值计算表明榔桥岩体成岩过程中的高氧逸度有利于该区域发生Cu-Au矿化，而斜长石的分离结晶使得EuN/EuN＊比值降低，为0．2～0．4。榔桥岩体具有富集大离子亲石元素以及Pb，亏损高场强元素（Nb、Ta、Zr，Ti），以及轻重稀土发生明显的分异，轻稀土相对重稀土明显富集的特征，与岛弧岩浆岩类似；大地构造背景判别表明该岩体的形成与古太平洋板块对欧亚大陆的俯冲碰撞作用密切相关。

The Langqiao granodiorite intrusion is a complex pluton with melted crust-mantle materials,located in the northern margin of the fold belt in Jingxian county of South Anhui province.The U-Pb dating results for zircons from two granodiorite samples formed in main forming stage of the Langqiao intrusion and one porphyry granite samples from the vein rock formed at the later forming stage yielded weight average ages of 135. 6±1. 8Ma,137. 6 ±2. 0Ma,136. 4 ±1. 9Ma,respectively,indicating the intrusion formed in the Early Cretaceous.The zircon Hf isotopic analysis suggested that this intrusion was formed by partial melting of Mesoproterozoic crustal materials,contaminated by a small amount of Paleo-proterozoic crutal materials.The “zircon saturation temperature”calculated by whole-rock Zr contents show the magmatic crystallization temperature of Langqiao granodiorite intrusion and porphyry granite from vein rock is 749-781℃,722-745℃,respectively,indicating rapid cooling history of magma;the element geochemical studies show that the intrusion formed accompanied with fractional crystallization of plagioclase, K-feldspar and the accessory minerals,such as apatite,ilmenite.Zircon Ce（Ⅳ）/Ce（Ⅲ）ratios suggest the high oxygen fugacity in the magmatic process is conducive the Cu-Au mineralization,while the fractional crystallization of plagioclase reduced EuN/EuN＊ to 0. 2-0. 4 .The Langqiao intrusion is enriched in large-ion lithophile elements,Pb,depleted in high field strength elements （Nb,Ta,Zr,Ti）,also has obvious differentiation between LREE and HREE,with more enriched LREE than HREE,similar to the arc magmatic rock;tectonic setting discrimination diagrams suggested the formation of Langqiao intrusion was closely related to the subduction and collision of the Paleo-pacific plate towards the Euroasian continent.