鲁西杨庄条带状铁建造特征及锆石年代学研究

Characteristics of the banded iron formation(BIF)and its zircon chronology in Yangzhuang,western Shandong

泰山群主要分布于鲁西地区中部,是鲁西花岗-绿岩带的一个重要组成部分。近年来,在沂水县杨庄发现了一定规模的沉积变质铁矿,铁矿层位赋存于柳杭岩组上段的斜长角闪岩段内。本文对杨庄铁矿BIF及侵入地层的岩浆岩进行锆石年代学测定,测定的含磁铁矿斜长角闪岩中锆石U-Pb年龄数据主要集中在2.6Ga附近,确定斜长角闪岩的形成年龄为2615±61Ma;铁矿顶板黑云母石英片岩的形成年龄小于2527±66Ma,该岩段又被晚期混合花岗岩穿插,其锆石年龄测定结果为2469±34Ma,所以推测黑云母石英片岩的形成年龄在2.5Ga附近。据此我们认为鲁西地区的"柳杭岩组"似可近一步解体为新太古的斜长角闪岩段(含磁铁矿建造)和古元古的表壳岩段(以片岩系为主)。混合花岗岩的形成年龄属于古元古代早期,似乎可以填补全球地质演化的静寂期(2.3～2.5Ga)。以泰山群为代表的变质岩系在地下较深部位出现或者被中晚元古代盖层覆盖,为探讨华北克拉通早期演化和开展华北克拉通BIF型铁矿研究具有重要意义。

The Taishan Group, mainly distributed in the western Shandong Province, is an important part of western Shansong granite-greenstone belt. Some sedimentary metamorphic iron ores have been found in Yangzhuang, Yishui County, the iron bodies are resident in the Liuhang Formation. This paper studied characteristics of BIF in the Yishui area, determined ages of this BIF by LA-ICP-MS U-Pb method from BIF strata and magmatic intrusion of the upper member of the Taishan Group. Results show that zircon ages of in amphibolite are concentrated in about 2.6Ga, its formation age is considered to be 2615±61Ma;an upper intercept zircon age of 2527±66Ma is obtained from biotite quartz schist, zircon dating of migmatitic granite yields an age of 2469±34Ma. These indicate that the upper member of the Liuhang Formation, Taishan Group, formed from earlier than 2.6Ga to Early Paleoproterozoic. Accordingly, we suggest that the ＂Liuhang Group＂ in western Shandong, could be further separated into two parts, i.e., the lower part of amphibolites （with magnetite formation） formed at c.a. 2.6Ga in Late Archean, and the upper part of surficial rock sequences （mainly with schist）, formed at c.a. 2.5Ga in Early Paleoproterozoic. The formation of migmatitic granite of Early Paleoproterozoic could fill the inactive period of global geological evolution gap. The metamorphic sequences represented by the Taishan Group are distributed deep or covered by Mid-Late Proterozoic rocks, are important for research of early evolution of North China Craton and iron deposits （BIF） in China.