赤峰东部宝音图群斜长角闪岩锆石U-Pb年龄及地质意义

Zircon U-Pb Ages and Geological Significance of the Plagioclase Amphibolite in the Baoyintu Group Eastern of Chifeng

宝音图群是华北地台北缘最古老的结晶基底之一。赤峰东部宝音图群的斜长角闪岩中锆石晶型、背散射图像及低Th/U比值、高Hf、低Nb、Ta、低Nb/Ta比值等特征说明其为变质锆石。采用LA-ICP-MS锆石U-Pb测年技术,对变质锆石进行了同位素年代学研究,获得了宝音图群的变质年龄为1860.1±7.9 Ma,此年龄限定了宝音图群的形成时代应早于1860 Ma,说明宝音图群应该形成于此年龄之前的古元古代。这一变质年龄所代表的变质事件与华北地台古元古代变质事件相一致,说明宝音图群为华北地台的一部分。以斜长角闪岩主量、微量及稀土元素为依据,进行原岩恢复及构造环境判别,结果显示斜长角闪岩原岩是形成于活动陆缘岛弧环境的钙碱性玄武岩。

The Baoyintu Group is the oldest crystal basement at the north eage of north China platform. The char-acteristics of crystal type, BSE images and low Th/U ratio, high Hf,low Nb, Ta, low Nb/Ta ratio in the zircons of plagioclase amphibolite in Baoyintu Group east of Chifeng show that are the metamorphic zircons. Analysis of LA-ICP-MS zircon U-Pb ages was conducted for the metamorphic zircons. The results show a metamorphic zir-con age of 1860.5 ± 8.4 Ma for the plagioclase amphibolite which constrain the age of Baoyintu Group is earlier than 1860 Ma. Consistent with the metamorphic event of North China Block in the early Proterozoic, that Baoyin-tu Group is a part of the North China Block. According to the characteristics of the major elements and the trace elements, studies of protolith reconstruction map and differentiation of structure environment are proceeded. The result suggests that the protolith of plagioclase amphibolite in Baoyintu Group is calc-alkali basalt which formed in the island-arc environment at active continental margin.