| 北京密云地区BIF锆石SHRIMP U-Pb定年及其对成矿与变质时代的约束 |
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| 引用本文：董津蒙,吕向光,史志强,王鹏,梁景利,张明宇,于文涛,王振,石玉若.2026.北京密云地区BIF锆石SHRIMP U-Pb定年及其对成矿与变质时代的约束[J].地球学报,47(4):853-867. |
| DOI：10.3975/cagsb.2026.042721 |
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| 基金项目:本文由天津华北地质勘查局基础地质调查项目“冀北地区一带沉积变质型铁矿成矿规律研究与找矿预测”(编号: HK2024-B13)、市场委托项目“丰宁建宇铁矿有限公司杨营铁矿深部勘查”和华北地质勘查局五一四地质大队科研项目“冀北杨营地区构造复杂环境找矿技术综合研究”(编号: HKWYS2023-C1)联合资助。 |
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| 中文摘要:密云地区是华北克拉通北缘早前寒武纪变质基底的重要出露区, 广泛发育典型的条带状铁建造(BIF)。为约束该区BIF的形成与变质时代, 本文对沙厂铁矿透辉磁铁角闪石岩(SY01)和冯家峪铁矿磁铁石英岩(MF01)开展了锆石SHRIMP U-Pb定年研究。沙厂透辉磁铁角闪石岩的锆石给出了两组加权平均年龄: (2 531±7) Ma(MSWD=2.6)和(1 798±15) Ma(MSWD=1.2)。其中部分2.53 Ga的锆石具典型的振荡环带, Th/U比值较高(多数>0.4), 为外来锆石, 表明存在新太古代晚期地壳物质; 1.80 Ga锆石为变质重结晶锆石, 记录了古元古代晚期构造热事件。冯家峪磁铁石英岩的锆石同样给出了两组加权平均年龄: (2 456±8) Ma (MSWD=1.6)和(1 777±9) Ma(MSWD=1.3)。~2.45 Ga锆石来自亮白色残留核部, 保留微弱岩浆环带, Th/U比值较高, 解释为岩浆锆石在古元古代晚期(~1.8 Ga)强烈构造热事件改造下发生古铅丢失的结果, 原锆石年龄应为~2.5 Ga或更大。~1.78 Ga锆石来自暗黑色重结晶区域或变质增生边, 呈均一无结构特征, Th/U比值较低, 为典型的变质重结晶锆石。本研究为BIF成矿时代提供了关键年代学约束。透辉磁铁角闪石岩中~2.53 Ga继承性锆石指示了成矿时代的下限; 磁铁石英岩记录的~2.45 Ga锆石为BIF形成时代提供了最大沉积年龄约束。综合华北克拉通BIF成矿高峰期(2.50~2.55 Ga)的区域背景, 推断密云BIF铁矿形成于新太古代晚期 (~2.5 Ga)。BIF形成后经历了~1.8 Ga的重大构造热事件的叠加改造。本研究为深入理解华北克拉通早前寒武纪BIF成矿与地壳演化之间的耦合关系提供了新的年代学依据。 |
| 中文关键词:条带状铁建造(BIF) 锆石SHRIMP U-Pb定年 变质事件 华北克拉通 |
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| Zircon SHRIMP U-Pb Dating of the BIF in Miyun, Beijing and Its Constraints on the Timing of Mineralization and Metamorphism |
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| Abstract:Miyun is a key area of the North China Craton (NCC), where early Precambrian basement and banded iron formations (BIFs) are well developed. To constrain the timing of BIF formation and metamorphism in this area, we conducted zircon SHRIMP U-Pb dating on diopside magnetite hornblendite (sample SY01) from the Shachang iron deposit and magnetite quartzite (sample MF01) from the Fengjiayu iron deposit. Zircons from sample SY01 yield two weighted mean ages of (2 531±7) Ma (MSWD=2.6) and (1 798±15) Ma (MSWD=1.2). Some ca. 2.53 Ga zircons exhibit typical oscillatory zoning with high Th/U ratios (mostly >0.4), interpreted as xenocrystic zircons, suggesting the presence of late Neoarchean crustal material. The ca. 1.80 Ga zircons are recrystallized zircons, recording a late Paleoproterozoic tectonothermal event. Zircons from sample MF01 also yield two weighted mean ages of (2 456±8) Ma (MSWD=1.6) and (1 777±9) Ma (MSWD=1.3). The ca. 2.45 Ga zircons are derived from bright-white residual cores, with weak magmatic zoning and high Th/U ratios; the original magmatic zircon should be ca. 2.5 Ga or older. The age younging is interpreted as ancient lead loss caused by the overprinting of strong late Paleoproterozoic tectonothermal modification. The ca. 1.78 Ga zircons, derived from dark recrystallized domains or metamorphic overgrowths, are homogeneous with low Th/U ratios, typical of metamorphic zircons. This study provides key constraints on the age of BIF mineralization. The ca. 2.53 Ga xenocrystic zircons in the diopside magnetite hornblendite define the oldest age of the BIF, whereas the ca. 2.45 Ga zircons in the magnetite quartzite provide the youngest age. The Miyun BIF is inferred to have been deposited in the late Neoarchean (~2.5 Ga), consistent with the peak BIF mineralization period (2.50–2.55 Ga) in the NCC. Subsequently, the BIF underwent a superimposed modification by a major tectonothermal event during the Late Paleoproterozoic. This study provides new geochronological evidence to better understand the coupled relationship between early Precambrian BIF mineralization and crustal evolution in the NCC. |
| keywords:banded iron formation (BIF) zircon SHRIMP U-Pb dating metamorphic events North China Craton |
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