赣南桃山复式岩体的锆石U-Pb年代学及其产铀性探讨

桃山复式花岗岩体位于南岭东段北部,区内产有我国最大的花岗岩型铀矿田。宝华山(蔡江)岩体和黄陂岩体是桃山复式岩体中面积最大的两个。前人测得宝华山岩体成岩时代为与铀矿关系更密切的印支期,但是该岩体仅发现个别铀矿床,是否具有更大的产铀潜力还有待探讨;前人测得黄陂岩体与产铀的打鼓寨岩体年龄一致,均为(154±2)Ma,与打鼓寨岩体侵入到黄陂岩体的野外地质现象存在矛盾,有必要进一步厘定其形成年龄。本文利用高精度的激光剥蚀-多接收器电感耦合等离子体质谱法(LA-MC-ICPMS)重新测定了宝华山岩体和黄陂岩体的锆石U-Pb年龄,证实宝华山岩体形成于印支期(229.98±0.98)Ma,重新厘定黄陂岩体形成于燕山早期(160.9±2.4)Ma,略早于产铀的打鼓寨岩体,更符合打鼓寨岩体侵入于黄陂岩体的地质事实。结合前人的研究,本文对桃山复式岩体的岩浆演化序列、构造背景及产铀性进行了讨论,将桃山岩体岩浆演化过程分为五个主要阶段:由印支期的宝华山岩体、燕山早期的黄陂岩体和打鼓寨岩体,以及燕山晚期的罗布里岩体和菜山岩体等组成,不同阶段的岩体对应不同的花岗岩成因类型;推测印支期和燕山期花岗岩的形成均与太平洋板块俯冲造成的伸展拉伸环境有关;指出岩石成因类型是控制花岗岩产铀/不产铀的重要因素,桃山岩体中的S型花岗岩即打鼓寨岩体的产铀能力最强,今后应对花岗岩成因类型控制产铀性的深层原因进一步研究。

Zircon U-Pb Geochronology and Uranium-Production Capacity of the Taoshan Complex in Southern Jiangxi

The Taoshan granite complex located at the North-East part of the Nanling region, hosts the largest granite-related uranuim ore field in China. Baohuashan granite and Huangpi granite are two major components of the Taoshan granite complex. The formation age of Baohuashan granite was in the Indosinian period, which is more closely associated with uranuium ore, however, only individual uranium ore has been found in Baohuashan granite. Huangpi and Daguzhai granites have the same age of (154±2) Ma, but this is in conflict with the field geological phenomena, i.e. Huangpi granite was intruded by Daguzhai granite. High precision techniques of Laser Ablation-Multicollector Inductively Coupled Plasma-Mass Spectrometry (LA-MC-ICPMS) have been used to re-determine the emplacement age of these two plutons and the method is reported in this paper. The results indicate that Baohuashan granite was formed in the Indosinian era (229.98±0.98) Ma, and the Huangpi granite was formed in the early Yanshanian era (160.9±2.4) Ma. The emplacement age of Huangpi granite is a little earlier than Daguzhai granite, which is more aligned with the geological facts. Combining with previous studies, the magma evolutionary sequence, tectonic environment and uranium-production capacity of Taoshan complex are discussed. The magma evolutionary sequence of Taoshan complex was classified into five main stages, which is composed of Baohuashan granite in Indonisian, Huangpi and Daguzhai granites in early Yanshanian, Luobuli and Caishan granite in late Yanshanian. Granite formed at deferent stage belongs to deferent genesis type. The formation of Indonian and Yanshanian granite in Taoshan complex was speculated to related extension tectonic environment caused by subduction of pacific plate. We pointed out that the genesis type of granite is a primary controlling factor for uranium-production capacity of granites. The Daguzhai granit, which is an S-type granite, has the greatest uranium-production capacity. There is a need for further study to determine the underlying reasons for the genesis type controlling uranium-production capacity of granite.