川西冕宁-德昌REE成矿带成矿年代学研究：热液系统维系时限和构造控矿模型约束

Geochronology of REE deposits in Mianning-Dechang REE metallogenic belt:Constraints on duration of hydrothermal activities and tectonic model for carbonatite-alkalic complexes in southwestern Sichuan

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矿床以及一系列矿点和矿化点。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,岩体主要侵位于元古代结晶基底岩石和古生代—中生代沉积盖层中。40Ar/39Ar快中子活化分析表明,大陆槽白云母、里庄黑云母和木落寨金云母的40Ar/39Ar坪年龄分别为13·2Ma、30·6Ma和35·5Ma,结合前人成岩成矿年龄资料,牦牛坪矿床岩浆-热液活动时限约12Ma,大陆槽矿床约5Ma,而里庄矿床则为2Ma,热液活动持续时间与矿床的REE储量存在正相关关系。研究发现,冕宁-德昌REE矿带从北到南,成岩成矿年龄逐渐变新,时间跨度长达27Ma;碳酸岩-正长岩岩浆初始活动时间与雅砻江左旋走滑断裂开始活动时间相一致,成岩成矿年龄的系统变化,有可能是由于雅砻江左旋走滑断裂右侧板片在相对固定的幔源岩浆"热点"上从南至北滑移造成的(相对于左侧板片)。

The Mianning-Dechang REE metallogenic belt in western Sichuan, is one of the most significant REE belts in China and consists of a superlarge （Maoniuping）, a large （Dalucao）, an intermediate-sized （Muluozhai） and a number of small REE deposits （e. g., Lizhuang） as well as ore spots. REE mineralization is associated with the carbonatite-alkalic complexes, controlled by a series of Cenozoic strike-slip faults in eastern Indo-Asian collision zone. These complexes consist of carbonatite sills or dykes and associated alkalic syenite stocks, which intruded the Proterozoic basement and Devonian-Cretaceous sedimentary sequence. Using the 40Ar-39Ar isotope chronological method, the authors obtained the plateau age of 13.2 Ma for muscovite （Dalucao）, 30.6 Ma for biotite （Lizhuang） and 35.5 Ma for phlogopite （Muluozhai）, respectively. Combined with other rock-forming and metallogenic ages, it is held that the duration periods of rnagmatic-hydrothermal activities are about 12 Ma in Maoniuping, 5 Ma in Dalucao, and 2 Ma in Lizhuang, which are positively related to their REE ore reserves. It is also found that the rock-forming and metallogenic ages decrease systematically from north to south, covering a period of 27 Ma. The beginning of carbonatite-syenite magmatism coincided with the onset of Yalongjiang sinistral strike-slip fault. Age sequence can be plausibly explained by assuming that mantle magmatism was a relatively fixed ＂hot spot＂, and the right plate moved from south to north along Yalongjiang sinistral strike-slip fault relative to the left plate.