白云鄂博矿田碳酸岩墙年代学再研究

Geochronology of the Carbonatite Dykes in the Bayan Obo Orefield: Revisit

白云鄂博地区发育侵入于白云鄂博群的碳酸岩墙。碳酸岩墙(00BYC19)中锆石的SHRIMPU-Pb下交点年龄为1984±180Ma,其ID-TIMS上交点年龄为2085±330Ma。碳酸岩墙(02BY80)中锆石SHRIMP的上交点年龄为2035±51Ma;02BY81中锆石的ID-TIMS上交点年龄为1934±64Ma。主矿北碳酸岩墙(00BYC21)的全岩Pb-Pb等时线年龄为1236±300Ma。本文及前人对白云鄂博地区的碳酸岩墙的定年结果表明该地区碳酸岩墙至少有两个阶段的侵入事件,第一阶段在2.0Ga左右,第二阶段在1.2Ga左右。前者是白云鄂博地区以至阴山地块北缘的一次主要的构造—岩浆事件,标志白云鄂博裂谷和白云鄂博群地层的形成。第二阶段的碳酸岩墙,代表白云鄂博裂谷在中元古代的活化,且与成矿有密切关系。

Carbonatite dykes intruded into the Bayan Obo Group in the Bayan Obo deposit. SHRIMP zircon dating gives a lower intercept age of 1984±180 Ma and ID-TIMS dating the upper intercept age of 2085±330 Ma for carbonatite dyke 00BYC19. SHRIMP result shows an upper intercept age of 2035±51 Ma for the carboantite dyke 02BY80, and ID-TIMS gives an upper intercept age of 1934±64 Ma for the carboantite dyke 02BY81. The whole rock Pb-Pb isochron age of a carbonatite dyke （00BYC21）in the north of the Main Ore body is 1236±300 Ma. Dates of carbonatite dykes reported in this paper together with the previous work suggest that there are two episodes of carbonatite magmatism, the first is at about 2.0 Ga, and second 1.2 Ga. The first episode of carbonatite magmatism manifested the main tectonic event in the Bayan Obo area within the northern margin of the Yinshan Block and the formation of the Bayan Obo rift. The second episode indicates the mobilization of the Bayan Obo rift and is correlated to the formation of the deposit.