四川牦牛坪稀土矿床碳酸岩Sm-Nd等时线年龄及其地质意义

摘要

川西冕宁-德昌REE成矿带是中国最重要的REE成矿带之一,包括牦牛坪超大型REE矿床、大陆槽大型REE矿床、木落寨中型REE矿床和里庄小型REE矿床等.REE成矿作用与碳酸岩-碱性杂岩体有关,受印度-亚洲大陆碰撞带的一系列新生代走滑断裂系统控制.碳酸岩-碱性岩杂岩体主要侵位于元古代结晶基底岩石和古生代-中生代沉积盖层.碳酸岩主要为方解石碳酸岩,碱性正长岩以英碱正长岩为主,两者微量元素分布模式及Sr-Nd同位素组成特征相一致,表明两者为岩浆不混溶产物,因此两者的成岩时代应该基本相近.然而,前人研究成果表明,牦牛坪碳酸岩中钠铁闪石K-Ar年龄为31.7 Ma,正长岩全岩K-Ar年龄为40.8 Ma,两者相差10 Ma.此外,研究表明,大陆槽、木落寨和里庄REE矿床碳酸岩-正长岩杂岩体成岩年龄与其相应的成矿年龄基本一致,而牦牛坪REE矿床两者相差甚远.本文利用碳酸岩中方解石进行了Sm-Nd等时线年龄测定,结合前人资料,重新厘定了牦牛坪REE矿床碳酸岩的成岩年龄和矿床的成矿年龄,分别为29.9 Ma和26～27 Ma,两者在误差范围内相一致.%The Mianning-Dechang REE belt in western Sichuan, SW China is one of the most significant REE belt in China, including one giant (Maoniuping), one large (Dalucao), one intermediate (Muluozhai), one small (Lizhuang) and so on. The REE mineralization is associated with the carbonatite-alkalic complexes, controlled by a series of Cenozoic strike-slip faults in the eastern Indo-Asian collision zone. These complexes consist of carbonatitic sill or dyke and associated alkalic syenite stocks, which intruded Proterozoic basement and Devonian-Cretaceous sedimentary sequence. Main phase of host carbonatites are calcite carbonatites and associated alkalic syenites are predominated by nordmarkites. They show similar abundance patterns of trace elements normalized by primitive mantle and Sr-Nd isotopic signatures, suggesting an immiscibility origin, which implys their diagenetic ages should be consistent. However, the formers' results indicate that diage-netic ages of carbonatite and syenite from Maoniuping are 31. 7 Ma (K-Ar method for arfvedsonite) and 40. 8 Ma (K-Ar method for whole rock) , respectively, with the difference between 10 Ma. In addition, the formers' results indicate that diagenetic ages of carbonatite-alkalic complex from Dalucao, Muluozhai and Lizhuang are consistent with their corresponding metallogenic ages, while the ages of carbonatite and syenite from Maoniuping are markedly different from its metallogenic ages. In this work, we present Sm-Nd isochron dating for the carbonatite from Maoniuping. Together with other data, we confirm the diagenetic age of carbonatite and metallogenic age from Maoniuping REE deposit are 29. 9 Ma and 26 — 27 Ma, respectively, within analytical uncertainty.