Abstract:

Field investigations, petrology, geochemistry, zircon U-Pb geochronology and Hf isotope analysis were performed to investigate the petrogenesis of Ningjiawan pluton, in an attempt to shed light on its geodynamic significance. LA-ICP-MS zircon U-Pb dating from two samples yielded ages of 2364±6 Ma (MSWD=0.13) and 2360±23 Ma (MSWD=4.0) respectively, indicating that the magma emplaced and crystalized in Paleoproterozoic. The pluton contains high concentrations of alkaline, K and Si, had elevated FeO^T/MgO ratios and high field strength element (HFSE) contents, was enriched in Rb, Ba, Th and U elements, was depleted in Ca, Mg, P and Ti, and possessed a “seagull-type” chondrite-normalied REE pattern with significantly negative Eu anomalies (δEu=0.13−0.36) and enrichment in the LREE relative to the HREE, exhibiting the traits of the highly fractionated Itype granite. Relatively high whole-rock Y/Nb values (1.2−2.8), together with positive ε_Hf(t) values (+1.6−+6.4), and t_DM1(Hf) and t_DM2(Hf) ages of 2449−2629 Ma and 2474−2711 Ma respectively on the zircon grains, signified that they were products derived from magma mixing between crust and depleted mantle sources. Moreover, distinctive negative Nb, P and Ti anomalies and positive Ce, Nd and Zr anomalies, characteristics of continental margin arc, combining with the regional geological background, indicate that Ningjiawan pluton is very likely to form in island arc setting.

Key words: highly fractionated I-type granite, zircon U-Pb dating, island arc setting, Ningjiawan pluton, North China Craton

摘要：

对呈小规模岩株状产出的宁家湾岩体进行野外地质、岩石学、岩石地球化学、锆石U-Pb同位素年代学和Hf同位素组成研究, 目的是确定其岩石成因及地球动力学意义。两件样品的LA-ICP-MS锆石U-Pb定年结果分别为2364±6 Ma (MSWD=0.13)和2360±23 Ma (MSWD =4.0), 属古元古代岩浆活动的产物。研究结果表明, 宁家湾岩体富碱、高K和Si, 具有高FeO^T/MgO 比值和高的高场强元素(HFSE)含量, 富集Rb, Ba, Th和U等元素, 具有低的Ca和Mg含量, 明显亏损P和Ti, 具弱轻重稀土分异和强Eu负异常(δEu=0.13~0.36)的海鸥型稀土分配模式, 显示高分异I型花岗岩的特征。较高的Y/Nb值(1.2~2.8)、锆石的ε_Hf(t)均为正值(+1.6~+6.4)以及单阶段模式年龄t_DM1(Hf)和二阶段模式年龄t_DM2(Hf)分别为2449~2629 Ma和2474~2711 Ma的特点, 表明源区可能为壳幔混合。明显的Nb, P 和 Ti负异常及Ce, Nd和Zr正异常, 反映出大陆边缘弧岩浆岩的特征。结合区域地质背景, 认为宁家湾岩体很可能形成于岛弧构造环境。

关键词: 高分异I型花岗岩, 锆石定年, 岛弧构造环境, 宁家湾岩体, 华北克拉通