大龙山岩体冷却史及其成矿关系的同位素研究

根据全岩Rb-Sr、锆石U-Pb和角闪石、黑云母、钾长石K-Ar同位素年龄综合测定结果,再造了安庐石英正长岩带中大龙山岩体的冷却史。矿物对氧同位素地质测温结果证实,扩散作用是控制同位素体系封闭的主导因素。假定岩体冷却与地温梯度(100℃/Ma)同步降低,以二维热模式为参照,可以推算出大龙山岩体的原始侵位深度约为8km,成岩温度为800±50℃。早阶段石英正长岩体在136Ma侵位结晶后开始快速的冷却上升,冷却速率为27.4℃/Ma,上升速率为0.27mm/a;经过约18Ma后,岩体上升至地下约3km深处,温度为300±50℃,转为缓慢冷却上升,冷却速率为6.3℃/Ma,上升速率为0.06mm/a.晚阶段碱长花岗岩体于117Ma侵位结晶,嗣后开始快速的冷却上升,冷却速率为58.6℃/Ma,上升速率为0.59mm/a;经过约8Ma后,岩体转为缓慢冷却上升,冷却速率为7.2℃/Ma,上升速率为0.07mm/a.结合对国内外其它深成岩体冷却历史的研究,可见这类岩体的侵位上升一般经历了两个阶段:(1)早期高温岩体快速上升至定位,冷却速率显着大于区域地温梯度降低幅度;(2)晚期低温岩体与区域地质体一起缓慢隆起上升,冷却速率与区域地温梯度降低幅度一致。对形成于大龙山岩体接触带的热液铀矿床进行了沥青铀矿U-Pb同位素年龄测定,得到的矿化时间与黑云母K-Ar体系的封闭时间相近。气液包裹体测温结果指示,矿化温度与黑云母的Ar封闭温度相一致;脉石矿物氧同位素组成研究得到,成矿流体为岩浆期后热液。因此,该热液铀矿床的形成与岩浆结晶分异及嗣后的岩体缓慢冷却密切相关。

AN ISOTOPE STUDY ON THE COOLING HISTORY OF THEDALONGSHAN GRANITIC MASSIF AND ITS BEARINGON MINERALIZING PROCESS

Whole-rock Rb-Sr, zircon U-Pb as well as hornblende, biotite and K-feldspar K-Ar isotopic ages are used to reconstruct the. themal history of the Dalongshan granite massif on the An-Lu quartz syenite belt in Anhui. Oxygen isotope geothermometry of mineral-pairs demonstrates that diffusion is a predominant factor to control the closure of isotopic systems. According to the theory of cooling ages, it is reckoned that the Dalongshan massif would have at first experienced a rapidly cooling process since its crystallization and then become slow. The early phase of quartz syenite in the musif was emplaced in 1 36Ma at 800±50℃ and become cold rapidly to sometime in 118Ma at 300± 50℃, with a cooling rate of 27.4℃/Ma. Afterwards it would have experienced a slowly cooling process up to 94 Ma at 1 50±30℃, with a cooling rate of 6.3℃/Ma. The late phase of alkaline granite would have experienced a rapidly cooling from 1 1 7 Ma to 1 09Ma with a cooling rate of 58.6℃/Ma. Then it becomes slow with a cooling rate of 7.2℃/Ma, which is similar to that for the early phase of quartz syenite. Assuming that the cooling of the massif is determined by local geothermal gradients, an emplacement depth of about 8km is inferred if the local geothermal gradient would be 100℃/km. In this respect the Dalongshan massif was uplifted rapidly in 0.27 to 0.59mm/a at the first stage but slowly in 0.06 to 0.07mm/a at the second stage. A certain hydrothermal uranium deposit was found to be located in the contact zone of the Dalongshan massif with wall-rock sandstone. The time and temperature of the uranium mineralization are identical to the closure of argon diffusion in biotite, suggesting a genetic correlation between the massif cooling history and the uranium mineralizing process.