华北克拉通北缘中段多伦地区早二叠世双峰式岩浆作用

Early Permian Bimodal Magmatism in the Duolun Area of the Central Section of the Northern Margin of the North China Craton

摘要

东大山地区双峰式岩浆岩位于内蒙古多伦县东部，处于华北板块北缘. 岩体由石英二长岩和玄武岩组成. 对其开展了全岩地球化学、锆石U-Pb年代学和Hf同位素组成研究. 研究结果表明：石英二长岩和玄武岩锆石LA-ICP-MS U-Pb定年结果分别为283±1.6 Ma、280±2.9 Ma，形成于早二叠世；二者锆石Hf同位素组成整体较相似，两阶段模式年龄（t _DM2）普遍较为古老（1 760~2 354 Ma，仅玄武岩内测得324 Ma、824 Ma两个较新年龄），与华北板块基底接近；岩体SiO₂含量在50.64%~65.87%存在明显的间断，具有双峰式岩浆岩特征，都相对富K、Na、Al，亏Ca、Mg，石英二长岩较玄武岩明显亏损Ti、P，具有明显的互补迹象. 结合前人研究成果，认为东大山地区双峰式岩浆岩可能是板块俯冲、后撤引起的伸展背景下，古亚洲洋俯冲板片脱水引起上地幔部分熔融形成的幔源岩浆向上运移、聚集于华北板块之下引起古老基底部分熔融的产物，研究区早二叠世期间构造环境为活动大陆边缘.

Abstract

This paper presents petrological,Hf isotopes, geochronological and geochemical study on the early Permian Bimodal magmatites in Duolun area, Inner Mongolia, which is tectonically located in the northern margin of North China Craton, comprising quartz monzonite and basalt. U-Pb dating of zircons from the samples by LA-ICP-MS yield weighed mean ²⁰⁶Pb /²³⁸U ages of 283±1.6 Ma and 280±2.9 Ma respectively. They have similar Hf isotopes. Hf isotopes t _DM2 was oldgenerally (from 1 760 to 2 354 Ma, merely two youngs:324 Ma and 824 Ma) and closed to these reported in ancient basement of North China Plate. The quartz monzonite and basalt exhibit SiO₂ geochemical discontinuity (50.64%~65.87%), showing typically bimodal features. They also significantly enrich K, Na, Al and deplete Ca, Mg. Besides, the quartz monzonite show significantly depletions in Ti and P compared with the basalt, showing complementary relations. This paper, combined with previous data, suggest that Bimodal magmatitesare the products of partial melting of the ancient basement of the North China Plate under the background of extension, which was caused by the upward migrating and gathering of the mantle magma formed from partial melting of mantle resulted in dewatering of subducted plate of the ancient Asian Ocean. In the early Permian, Dongdashan area is under the background of active continental margin.

关键词

双峰式岩浆岩 / 华北板块北缘 / 伸展背景 / 活动大陆边缘 / 早二叠世 / 地球化学

Key words

bimodal magmatites / Northern margin of North China Craton / extension setting / active continental margin / the early Permian / geochemistry

基金资助

国家自然科学基金(41872234;41872194)

内蒙古自治区地质勘查基金项目(NMKD2015-35)

中国核工业地质局项目(202101;202203-4;202204-1)