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-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U-Pb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-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.
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
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 migrnatitization.The main ore districts include Houxianyu,Wenquangou and Zhuangmiao-Yangmugan Mg and Mg-Fe borate ore districts.Due to the regional intense migrnatitization,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 migrnatitic 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 migrnatitic 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 magrnatic 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.
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