粤北长江铀矿田花岗岩独居石U-Pb同位素定年及其地质意义

长江铀矿田位于诸广山复式岩体中南部,是典型的花岗岩型铀矿田.前人采用锆石U-Pb定年方法对赋矿花岗岩进行了年代学研究,但由于全岩和锆石铀含量较高,锆石往往发生了蜕晶化,可能导致锆石U-Pb定年数据散乱,影响锆石U-Pb年龄的可靠性.独居石是花岗岩中广泛存在的含铀副矿物,铀和钍含量均较高,可达10000×10-6,普通铅...

U-Pb Isotopic Dating of Monazite in Granite from the Changjiang Uranium Orefield, Northern Guangdong Province and Its Geological Significance

BACKGROUND: The Changjiang uranium ore field in the southern margin of the Zhuguangshan complex pluton is a typical granite-type uranium ore field in China. Zircon U-Pb dating has been used to date ore-bearing granites. However, due to the high content of uranium in the whole rock and zircon mineral, some zircons tend to be decrystallized, which may lead to scattered zircon U-Pb data and may affect the reliability of zircon U-Pb dating. Monazite is a widespread accessory mineral in granite, and it is an ideal mineral for U-Pb dating due to its high U and Th contents (up to 10000×10^-6), and low common Pb content of about 100×10^-6-1000×10^-6.OBJECTIVES: In order to constrain the age of the ore-bearing granite and discuss the tectonic setting.METHODS: LA-ICP-MS U-Pb dating method was used to constrain the formation age of monazite from the Youdong and Changjiang plutons. The monazite standard materials USGS44069 and Trebilcock were selected as external standard samples and monitoring standard samples, respectively.RESULTS: The ²⁰⁶Pb/²³⁸U age of monazite in the Youdong pluton is 228.0±1.5Ma (MSWD=0.82, n=23), and that of monazite in the Changjiang pluton is 156.8±1.7Ma (MSWD=0.76, n=14). These ages are consistent with the SHRIMP zircon ages of 232±4Ma and 160±2Ma, respectively, which further confirms that the Youdong pluton is the product of Late Indosinian magmatism, and the Yangtze River pluton is the product of early Yanshanian magmatism.CONCLUSIONS: This study demonstrates that the U-Pb age of monazite in high-uranium granites can effectively constrain their formation age.