华北克拉通五台山-2520Ma辉长岩侵入体的成因及其地质意义

Petrogenesis and geological implications of the ca.2520Ma gabbroic intrusions in Wutai Mountain of the North China Craton

本次研究从华北克拉通五台地区基底中识别出一套-25亿年的基性侵入体(主要为岩株)。其中,龙王堂岩株出露面积约5km2,侵入新太古界五台群台怀亚群,岩石类型主要为粗粒辉长岩。一条北东向基性岩墙侵入该岩株,岩墙边部发育明显的冷凝边,岩性为中粒辉长岩。LA-ICP-MS锆石U-Pb同位素年代学分析显示龙王堂粗粒辉长岩岩株形成时代为新太古代晚期(-2520Ma),中粒辉长岩岩墙形成时代为晚三叠世(-230Ma)。锆石原位Lu-Hf同位素分析显示龙王堂岩株具有正的εHf(t)值(4.75-7.95,峰值为6.3),其亏损地幔Hf模式年龄与岩浆年龄接近(tDM=2.64-2.53Ga),指示龙王堂粗粒辉长岩的原始岩浆来自于亏损地幔端元。全岩主微量元素地球化学分析显示龙王堂粗粒辉长岩表现出与洋岛玄武岩(OIB)相似的地球化学特征,并经历了橄榄石和单斜辉石等的结晶分异。结合前人研究成果,本文认为龙王堂新太古代晚期辉长岩岩株与区域上绿岩带(五台群)及同时期花岗侵入体形成时代接近,可能共同指示早期地幔直接分异形成陆壳物质的过程。这可能与俯冲构造体制不同,而与地幔柱作用产生的岩浆特征更为相似。

A suit of 2.5 Ga mafic intrusive bodies(mainly stocks) are newly identified in the basement in Wutai area of the North China Craton in this study. Among them, the Longwangtang mafic stock intruded in the Neoarchean Taihuai Subgroup of the Wutai Group and outcropped within an area of 5 km~2. The main rock type of this intrusion is coarse-grained gabbro. One NE-striking mafic dyke of medium-grained gabbro intruded in this mafic stock with clear chilled margins. The LA-ICPMS zircon U-Pb isotopic geochronology analysis shows that the Longwangtang coarse-grained gabbro stock was emplaced at the late Neoarchean(-2520 Ma), while the medium-grained gabbro dyke emplaced at the Late Triassic(-230 Ma). The in-situ zircon Lu-Hf isotopic analysis shows that the Longwangtang coarse-grained gabbro stock has positive εHf(t) values(4.75-7.95) with a peak at 6.3. The similarity between the depleted mantle model Hf ages(2.53-2.64 Ga) and the magmatic age(-2.52 Ga) indicates that the parental magma is from a depleted mantle source. The whole-rock geochemical analysis shows that Longwangtang coarse-grained gabbro stock has similar geochemical characteristics with the oceanic island basalt(OIB), and its magma experienced a fractional crystallization dominated by olivine and clinopyroxene. Combined with previous studies, we proposed that the Neoarchean Longwangtang gabbro stock, the greenstone belt(Wutai Group) and the simultaneous granites in the Wutai area were formed almost at the same period during the fractional process of the continental crust from an early mantle, i.e., the formation of the magmas is different from those in a subduction belt but similar to those in a mantle plume.