花岗质复式岩体成因及其与W-Mo成矿的关系——以广西油麻坡岩体为例

Petrogenesis of Granitic Complexes and Implications for the W-Mo Mineralization:A Case Study from the Youmapo Pluton,Guangxi Province

广西陆川米场-博白三滩成矿带是近几年新发现的以W、Mo矿化为主的多金属成矿带。本文对带内与W-Mo矿床有关的油麻坡复式岩体进行了锆石U-Pb年代学、岩石学、地球化学和Sm-Nd同位素研究。油麻坡岩体由花岗闪长岩(主体)和白云母花岗岩(补体)两部分组成。传统观点一般认为补体是主体经历分离结晶后的残余岩浆,而本文LA-ICP-MS锆石U-Pb定年结果表明,主体和补体分别形成于109.0±0.3Ma和100.7±0.5Ma,后者明显晚于前者约10Ma,说明两者不大可能是同一岩浆房演化的产物。来自岩石学、地球化学和同位素方面的证据也不支持两者是分离结晶关系,主要包括:①主体含自形角闪石,常见榍石和磁铁矿等副矿物,表明岩浆富水、高氧逸度的性质。而补体不含角闪石,常见萤石、钛铁矿和硫化物等副矿物,表明岩浆富F和偏还原的性质。这与两者的分离结晶演化关系不吻合;②补体锆石的U含量极高(平均高达10602×10-6),从而出现特有的蜕晶现象、裂纹等放射性损伤结构,明显不同于主体锆石(平均U含量为860×10-6);③补体强烈亏损Sr、Ba、Eu和LREE,富集Na、Rb、Ga以及W、Sn等成矿元素,并具有典型的稀土四分组效应和non-CHARAC特征,微量元素模拟计算表明,补体与主体没有演化关系;④主体和补体的Nd同位素组成差别较大(εNd(t)分别为-5.1^-4.0和-9.6^-8.6)。本文提出高度演化的补体花岗岩是新的幔源岩浆底侵导致下地壳部分熔融而形成,由于其富F,导致岩浆固相线大幅下降(岩浆期延长),所以经历了比主体岩浆更为强烈的熔-流体相互作用,有利于萃取W、Mo等金属元素并使之富集成矿。

The Michang-Bobai belt is a newly recognized W-Mo metallogenic belt in the Guangxi province,SE China.In this paper,we report new U-Pb ages,and petrological,geochemical and Sm-Nd isotopic data for the Youmapo pluton associated with W-Mo mineralization.The Youmapo complex consists of granodiorite （predominant phase） and muscovite granite （supplementary phase）.Traditionally,the supplementary phase was considered to be residual melt through fractionation of the predominant phase.However,our dating result by zircon LA-ICP-MS method show that the supplementary phase formed at 100.7±0.5 Ma,which is significantly younger than the predominant granodiorite （109.0±0.3 Ma）.Other evidence further preclude the genetic relationship of fractional crystallization between the two units：（1） Hornblende,sphene and magnetite are common minerals in the granodiorite,suggesting water-rich and high oxygen fugacity features,which is different from the F-rich and low oxygen fugacity nature for the muscovite-granite as indicated by the presence of fluorite,ilmenite and sulfide.（2） Zircons from the muscovite-granite are characterized by extremely high U contents （10602× 10-6）,rugged surfaces and contain microfractures caused probably by radioactive damage,while zircons from the granodiorite have low U contents （860× 10^-6）.（3） The muscovite-granite is depleted in Sr,Ba,Eu and LREE,and enriched in Na,Rb,Ga,W and Sn,with tetrad REE patterns and non CHARAC behavior.Trace element modeling using Rb Ba and Sr-Ba co-variations suggests that the supplementary phase is unlikely to be the derivative from the predominant phase via fractionation.（4） The predominant and supplementary phases have different Nd isotopic compositions （εNd （t） =-5.1～ 4.0 and-9.6～-8.6,respectively）.We conclude that the muscovite-granite probably formed through a new partial melting event related to basaltic underplating.High F contents reduced the solidus temperature and caused the highly evolved characteristics of the muscovite-granite.Therefore,the supplementary magmas have gone through intense interaction between melts and fluids,and played a significant role in W-Mo mineralization.