Formation age of the Neoarchaean Jining Group(banded iron formation) in the western Shandong Province: Constraints from SHRIMP zircon U-Pb dating
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摘要: 济宁群以隐伏的地质体分布于华北克拉通鲁西地区,以往被划归古元古代。近年来在济宁群的钻探和地球物理研究中,发现存在较大规模条带状硅铁建造,其形成时代引起地学界极大关注。本文对济宁群变质碎屑沉积岩和长英质火山岩进行了锆石SHRIMP 年龄测定。含砾绿泥绢云千枚岩的碎屑锆石年龄主要集中在~2.7Ga,可靠的最年轻碎屑锆石年龄为2.61±0.01Ga。变质长英质火山岩岩浆锆石年龄为2.56±0.02Ga。研究表明,济宁群形成于新太古代晚期,而不是以往认为的古元古代。华北克拉通条带状铁建造主要形成于新太古代晚期。济宁群在岩石组合、变质作用方面与鞍山、冀东以及五台地区同时代含硅铁建造的表壳岩系有差别,但是与本溪南芬铁矿的岩石组合有相似之处。这些可能为我国的隐伏太古宙BIF铁矿的研究与勘探提供了重要依据。Abstract: The Jining Group, located in the Jining area, western Shandong Province, is covered by young sediments and was once considered to be formed during the Early Palaeoproterozoic. Much attention was paid on the supracrustal rocks because of the discovery of a huge banded iron formation (BIF) through drilling and geophysical study in recent years. There were arguments about the formation age and origin of the BIF. This paper reports SHRIMP U-Pb zircon ages for the metamorphic rocks of the Jining Group. A pebble-bearing chlorite sericite phyllite sample has detrital zircons mainly of ~2.7Ga, with a grain with a believable age of 2.61±0.01Ga. An upper intercept age of 2.56±0.02Ga was obtained for magmatic zircons from meta-felsic volcanic rock. These indicate that the Jining Group was formed at the end of Neoarchaean, not Palaeoproterozoic, as thought before. In the North China Craton, BIFs were mainly formed during the Late Neoarchaean. The Jining Group is different in rock association and metamorphism from the BIF-bearing supracrustal rcoks in the Anshan, eastern Hebei and Wutai areas, but similar to ones in the Nanfen-Benxi area. This is important for the research and exploration of Archaean BIF-type iron deposits in China.
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Key words:
- Jining Group /
- Neoarchaean /
- Felsic volcanic rock /
- BIF /
- SHRIMP dating /
- Western Shandong Province /
- North China Craton
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