华北克拉通北缘白云鄂博群和腮林忽洞群底部碎屑锆石U-Pb定年、Hf同位素分析及其地质意义

SHRIMP U-Pb dating and LA-ICPMS Hf isotope analysis of detrital zircons from medium-to coarse-grained sandstones of the Bayan Obo Group and Sailinhudong Group and its geological significances

白云鄂博群位于华北克拉通北缘,由于赋存超大型REE-Nb-Fe矿而受到广泛关注。白云鄂博群形成时代有中元古代、古生代等不同认识。腮林忽洞群位于白云鄂博群的南部,与白云鄂博群的关系也有不同认识。本文报道了白云鄂博群和腮林忽洞群底部中-粗粒砂岩的碎屑锆石SHRIMP U-Pb年龄和LA-ICPMS Hf同位素组成。两个岩群碎屑沉积岩的碎屑锆石在形态和内部结构上类似,按结构特征可分为继承或捕获锆石、岩浆锆石、变质锆石和重结晶锆石等不同类型。年龄都主要分布在1.8～2.1Ga之间和2.4～2.7Ga之间,尽管年龄峰值存在一定区别。它们的Hf同位素组成也类似,εHf（t）和tDM2（Hf）变化范围分别主要在约-6.0～6.0之间和2550～2950Ma之间。结合前人研究,可得出如下结论：白云鄂博群和腮林忽洞群的物源区类似,主要由新太古代晚期和古元古代晚期岩浆岩组成,为新太古代早期陆壳物质再循环产物;部分岩石遭受新太古代晚期和古元古代晚期高级变质作用改造;碎屑物质都来自华北克拉通北缘早前寒武纪变质基底,与其形成时代相同的认识不矛盾。

The Bayan Obo Group,located in the northern margin of the North China Craton,is well-known due to the occurrence of a super-large REE-Nb-Fe deposit. It is debatable that the Bayan Obo Group formed in Mesoproterozoic or Paleozoic. The Sailinhudong Group occurs in the south of the Bayan Obo Group,with different opinions on both being same or not in formation age. This paper reports SHRIMP U-Pb ages and LA-ICPMS Hf isotopes of detrital zircons from medium-to coarse-grained sandstones of the Bayan Obo Group and Sailinhudong Group in the northern margin of the North China Craton. Detrital zircons from the lower parts of the two groups are similar in shape and external and internal textures,and can be divided into four types,namely trapped or inherited,magmatic,metamorphic and incompletely recrystallized zircons. The different types of detrital zircons from the two groups have similar age distributions,mainly ranging from 1. 8Ga to 2. 1Ga and 2. 4Ga to 2. 7Ga,respectively,but with slightly different age peaks. The detrital zircons are also similar in Hf isotopic composition,εHf（ t） and tDM2（ Hf） mainly range from- 6.0 to 6.0 and 2550 Ma to2950Ma,respectively. Combined with previous work,the main conclusions can be drawn as follows： 1） The source region is predominated by Late Neoarchean and Late Paleoproterozoic rocks,partly with the high-grade metamorphic overprinting of the Late Neoarchean and Late Paleoproterozoic; 2） the rocks of different ages in the source region are mainly formed by the crustal recycling of Early Neoarchean material; 3） the Bayan Obo Group and Sailinhudong Group were formed in similar depositing environment and the detrital sediments were derived from Early Precambrian basement in the northern margin of the North China Craton,which is consistent with the conclusion that they were formed at the same time.