赞皇杂岩翁城地区长城系常州沟组砂岩碎屑锆石U-Pb-Hf同位素研究

常州沟组是华北克拉通太古宙-古元古代变质基底上最早的沉积盖层之一,其碎屑锆石U-Pb年代学与Hf同位素特征分析对探讨长城系形成时代、源区特征以及基底演化均具有重要的研究意义。本文对华北克拉通中部赞皇杂岩瓮城地区长城系常州沟组底部砂岩样品进行了LA-ICP-MS碎屑锆石 U-Pb定年和LA-MC-ICP-MS碎屑锆石Lu-Hf同位素分析。砂岩样品中碎屑锆石主要年龄峰值约为2500 Ma,推断其碎屑物质主要来自于华北克拉通新太古代晚期变质基底,而最年轻的碎屑锆石年龄为1822 Ma,结合赞皇杂岩变质基底普遍经历了1850~1800 Ma变质作用以及区域上上覆大红峪组火山岩中1635 Ma结晶锆石年龄,限定瓮城地区长城系常州沟组沉积时代为1 800~1 635 Ma。砂岩样品中碎屑锆石的εHf(t)值变化于-6.8~+5.2之间,相应的两阶段模式年龄峰值约为2820 Ma,结合赞皇杂岩已发表的2900~2700 Ma岩浆锆石和碎屑锆石Hf同位素结果,进一步表明2900~2700 Ma为赞皇杂岩地壳生长最主要的时期。综合太行山中南部地区已发表的长城系常州沟组的沉积学与新的年代学数据,推测赞皇杂岩瓮城地区长城系常州沟组沉积于陆内裂谷盆地。

Detrital zircon U-Pb-Hf isotopic studies on sandstones in the Changzhougou Formation of the Changcheng System in Wengcheng area, Zanhuang Complex

The Changzhougou Formation is one of the oldest sedimentary cover on the Archean-Paleoproterozoic metamorphic basement of the North China Craton. The analysis for the U-Pb geochronology and Hf isotopes of the detrital zircons from the Changzhougou Formation is of great significance for exploring the formation age and source characteristics of the Changcheng System as well as the basement evolution. In this study, LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) and LA-MC-ICP-MS (laser ablation multiple inductively coupled plasma mass spectrometry) were employed to analyze detrital zircon U-Pb ages and rare earth element composi-tions and Lu-Hf isotopic compositions of samples from the Changzhougou Formation of the Changcheng System in the Wengcheng area, Zanhuang Complex, respectively. The main peak ages of detrital zircons in two sandstone samples is 2 500 Ma, and the detrital material of the quartz sandstones is mainly derived from the late Neoarchean metamorphic basement in the North China Craton, and the ²⁰⁷Pb/²⁰⁶Pb age of youngest detrital zircon is 1 822 Ma, combining with the 1850 ~ 1800 Ma metamorphic ages which was recorded by the Zanhuang metamorphic basement and the 1635 Ma age of magmatic zircon from the volcanic rocks in the upper Dahongyu Formation, further define that the initial depositional timing of Changzhougou Formation of Changcheng System in the Zanhuang Complex is later than 1800 ~ 1635 Ma. The εHf(t) values of the detrital zircon from one quartz sandstone sample vary from -6.8 to +5.2, and the corresponding two-stage model peak age is 2 820 Ma. Combined with the published Hf isotope results of 2900 ~ 2700 Ma magmatic zircons and detrital zircons from the Zanhuang Complex, it is further indicated that 2900 ~ 2700 Ma is the most important period of crustal growth in the Zanhuang Complex. Integrating with the published sedimentary and new geochronological data of the Changzhougou Formation of the central and southern parts of the Taihangshan area, it is speculated that the Changzhougou Formation in Wengcheng area was deposited in the continental rift basin.