辽宁弓长岭鞍山群时代与物源——来自石榴十字云母片岩碎屑锆石U-Pb-Hf同位素特征与稀土元素组成的约束

鞍山群是华北克拉通东部陆块最重要的含BIF变质表壳岩之一，经历了绿片岩相—角闪岩相变质-变形作用，准确限定其沉积时代与物质来源，对于探究华北克拉通BIF的构造环境具有重要意义。利用LA-MC-ICP-MS（多接收激光剥蚀电感耦合等离子质谱仪）对弓长岭鞍山群石榴十字云母石英片岩进行了碎屑锆石U-Pb-Hf同位素与稀土元素原位分析。根据碎屑锆石阴极发光图像特征与U-Pb年龄结果，获得石榴十字云母石英片岩中最年轻一组碎屑锆石峰值年龄为2528 Ma，限定了其原岩的最大沉积时代，结合区内约2500 Ma齐大山富钾花岗岩侵入鞍山群的野外地质关系，以及鞍山群经历了2510~2470 Ma变质作用，认为弓长岭地区鞍山群沉积时代为2528~2510 Ma，属于新太古代晚期。鞍山群十字石榴云母石英片岩碎屑锆石年龄集中在2931~2454 Ma之间，最主要的峰值年龄为2528 Ma。结合碎屑锆石内部结构特征，认为鞍山群主要物源为新太古代晚期酸性岩浆岩。代表性碎屑锆石Hf两阶段模式年龄为3532~2711 Ma，峰值年龄为2830 Ma，进一步揭示了中太古代晚期是鞍山—本溪地区地壳最重要的生长阶段。综合鞍山—本溪地区已发表的相关地球化学数据，推测弓长岭鞍山群可能沉积于活动大陆边缘的弧后盆地。

Depositional age and provenance of the Anshan Group in the Gongchangling area,Liaoning Province: Constraints from detrital zircon U-Pb-Hf isotopic and rare earth element composition in the garnet-staurolite-mica-quartz schist

The Anshan Group, which has undergone greenschist to amphibolite facies metamorphism and polyphase deformation, is one of the most important banded iron formation(BIF)-bearing meta-supracrustal rocks in the Eastern Block of the North China Craton.Depositional age and provenance of the Anshan Group are vital for understanding the tectonic setting of the BIF in the North China Craton.In this study, LA-MC-ICP-MS(laser ablation inductively coupled plasma mass spectrometry) was employed to analyze detrital zircon U-Pb-Hf isotope and rare earth element compositions of garnet-staurolite-mica-quartz schist from the Anshan Group in the Gongchangling area.Based on the internal structure, and U-Pb ages of detrital zircons, the peak age of 2528 Ma recorded by the youngest group of detrital zircons is chosen as the maximal depositional age of the Anshan Group, coupled with the geological relationship which the Anshan Group was intruded by the ca.2500 Ma Qidashan K-rich granites and 2510~2470 Ma metamorphic age of the Anshan Group, suggest that the Anshan Group was deposited between 2528 Ma and 2510 Ma, late Neoarchean.The 207Pb/206Pb ages of the detrital zircons from the studied sample is between 2931 Ma and 2454 Ma, and detrital zircon age patterns of the studied sample is characterized by the major peak of 2528 Ma.Combined with the internal structure of the detrital zircons, our new U-Pb ages reveal that the main provenance of Anshan Group is the late Neoarchean acid magmatic rocks.The two-stage Hf model ages of the typical detrital zircons are 3532~2711 Ma with a peak age of 2830 Ma, which further reveals that the intensive crustal growth in the Anshan-Benxi area was concentrated in Late Middle Archean.Integrated with previous geochemical data from Anshan Group in the Anshan-Benxi area, it is speculated that the Gongchangling BIF-bearing meta-supracrustal rocks of Anshan Group were probably deposited in a back-arc basin on the ancient continental margin.