辽东明安硼镁矿床混合花岗岩的锆石U-Pb年龄及对成矿时代的制约

Zircon U-Pb dating of migmatitic granites of Ming'an Mg-borate deposit in Kuandian area, eastern Liaoning Province, and its constrains on mineralization age

辽东硼矿赋存于辽吉裂谷中央凹陷带内的里尔峪组地层中，形成于火山活动与蒸发沉积交替旋回的环境，后期经历了低压角闪岩相至混合岩化变质作用，属沉积-变质型硼矿床，主矿区包括有后仙峪、翁泉沟和砖庙-杨木杆三处。混合岩化作用使裂谷早期喷发的酸性火山岩发生原地重熔，形成大量层状产出的混合花岗岩、变粒岩和浅粒岩。文章利用LA-MC-ICP—MS技术对砖庙一杨木杆矿区明安硼镁矿体下盘，与上部地层发生同构造变形的层状混合花岗岩进行了锆石UPb年龄的测定，尝试解决明安硼矿的成矿时代问题。混合花岗岩的锆石阴极发光图像显示，锆石具有核一边结构，核部的震荡环带为岩浆成因，边部为变质增生成因。锆石核部的U-Pb定年结果表明：207Pb／206Pb加权平均年龄为（2229±3）Ma（MSWD=0．8），Th／U值为0．4479～1．1309。因此，笔者认为（2229±3）Ma年龄代表了层状混合花岗岩原岩——酸性火山岩的喷发时代，也代表了硼镁矿的初始含硼蒸发岩形成时代的下限。

Borate deposits in eastern Liaoning belong to a metamorphic volcano-sedimentary mineral type, and are distributed in the central part of ＂Liao-Ji rift＂. They were formed in an environment of volcanism and evaporation, and commonly experienced amphibolite facies metamorphism and migmatitization. The main ore districts include Houxianyu, Wenquangou and Zhuangmiao-Yangmugan Mg and Mg-Fe borate ore districts. Due to the regional intense migmatitization, the acid volcanic rocks were melted in-situ, and large mounts of layered migmatitic granites, leptites and leptynites were formed. This paper reports an integrated study of U-Pb zircon age （LA-MC-ICP-MS） of the layered migmatitic granites in the Ming＇an borate deposit of the Zhuangmiao-Yangmugan ore district. This study may give a solution of the timing of the volcanic-sedimentary association and borate deposits. The zircon CL images of the migmatitic granites show that they have uniform core-rim microstructures, the cores have preserved the magmatic zone, and the rims display metamorphic structure. The U-Pb analyses of the magmatie core show a weighted mean 207pb/206pb age of （2229 ± 3） Ma （MSWD = 0.8） and Th/U ratios of 0.4479- 1. 1309. It is thus held that the weighted mean age of （2229 ± 3） Ma might represent the lower limit forming age of the boron-bearing evaporites.