略阳白雀寺-石瓮子双峰式侵入岩锆石U-Pb定年、地球化学特征及意义

在碧口地体东段勉略宁地区,发现侵位于碧口群双峰式火山岩中的白雀寺-石瓮子杂岩体具有双峰式侵入岩的特征,其主体由辉长岩与二长花岗岩组成。二者的锆石U-Pb年龄分别为836. 2 Ma与835. 1 Ma,年龄非常接近且略晚于碧口群火山岩(850～840 Ma)。与碧口群双峰式火山岩类似,该双峰式侵入岩中酸性端元所占比例较少,以基性端元为主,且二者岩石地球化学特征存差异并非巨大,据此初步推断二者可能具有相同的岩石圈地幔源区。辉长岩具有的碱性玄武岩与拉斑玄武岩特征,以及二长花岗岩具有的弧向板内过渡特征,并非绝对的陆内(板内)裂谷特征,而为陆间裂谷所特有;其形成于陈家坝岛弧(或弧后)的局限裂解环境,这与碧口群双峰式火山岩、以及在区内苍社-太阳岭一带的岩浆成因碳酸岩在时空上高度耦合,共同代表了新元古代扬子北缘碧口局限洋盆俯冲(会聚)作用过后的弧后(局限)伸展构造体制下幔源岩浆事件。

Zircon U-Pb geochronology and geochemistry of Baiquesi-Shiwengzi bimodal intrusive rocks in Lueyang, and their significance

Intruding into the bimodal volcanic rocks of Bikou Group in Mian-Lue-Ning area, eastern section of Bikou terrane, The Baiquesi-Shiwengzi complex is characterized by bimodal intrusive rocks, the main body of which is composed of gabbro and monzonitic granites. The zircon U-Pb ages of the two groups are 836.2 Ma and 835.1 Ma, respectively, which are very similar and slightly later than those of the Bikou Group volcanic rocks (850-840 Ma). Similar to the bimodal volcanic rocks of Bikou Group, the bimodal intrusive rocks have relatively small proportion of acidic series, mainly basic series, and their petrochemical characteristics are not very different. Therefore, it is preliminarily inferred that they may have the same lithospheric mantle sources. The alkaline basalt and tholeiite characteristics of gabbro and the arc-to-intraplate transitional characteristics of monzonitic granites are not absolutely intra-continental (intra-plate) rift characteristics, but are unique to inter-continental rift; they were formed in the limited cracking environment of Chenjiaba island arc (or back-arc), which is similar to the bimodal volcanic rocks of Bikou group and the magmatic carbonate rocks in Cangshe-Taiyangling area. Highly coupled in time and space, they together represent the Mantle-derived magmatic events under the back-arc (limited) extensional tectonic system after the subduction (convergence) of the Bikou restricted oceanic basin in the northern margin of the Neoproterozoic Yangtze.