结构定量分析揭示熊耳群大斑玄武安山岩岩浆房存续时间与岩浆喷发速率  

Quantitative textural analysis reveals magma chamber residence time and magma eruption rate of porphyritic basaltic andesite in the Xiong'er Group

作　　者：瞿川豪 马建锋 赵太平[1] QU ChuanHao;MA JianFeng;ZHAO TaiPing(Key Laboratory of Mineralogy and Metallogeny,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院广州地球化学研究所矿物学与成矿学重点实验室,广州510640 [2]中国科学院大学,北京100049

出　　处：《岩石学报》2024年第8期2374-2388,共15页Acta Petrologica Sinica

基　　金：国家重点研发计划项目(2020YFA0714803);国家自然科学基金项目(92062103、42230807)联合资助。

摘　　要：熊耳群火山岩是华北克拉通形成后首期大规模岩浆事件的产物,它代表了华北克拉通裂谷系发育和演化的开始,其形成时代与构造背景备受关注。前人研究集中于年代学与地球化学分析,但受限于年代学测试精度,对熊耳群岩浆活动的持续时间尚无法精确限定;同时,熊耳群岩浆岩的地球化学数据并不能很好地解释其成因:地幔柱模型往往伴随着短期内巨量岩浆喷发的特征,通过传统地球化学方法很难识别,所以制约熊耳群形成的构造背景尚需更多的证据来支撑。岩石结构定量分析方法为解析岩浆活动的过程提供了全新的视角,该方法依靠分析岩石中晶体的分布特征,还原晶体结晶过程,从而揭示岩浆房存储状态,并估计岩浆存留时间。许山组玄武质安山岩具有典型的斑状结构,且代表熊耳群熔岩最早期的岩浆活动,记录了早期的岩浆房热状态。本文针对其中的斜长石斑晶开展结构定量分析,探讨熊耳群火山岩早期岩浆房的存储状态与存留时间。数据结果表明,许山组玄武质安山岩深部岩浆房长期处于低过冷度、半开放状态,存续时间为2400~24000yr。结合斜长石结晶实验和许山组斑晶晶体形态研究,估算斑晶斜长石生长速率为10^(-11)~10^(-10)mm/s;结合最大特征长度、剖面特征长度差值与熔岩厚度,计算得到许山组体积喷发速率0.28~13km^(3)/yr,二维喷发速率0.04~1.85m/yr,与大火成岩省的活动特征相似。综上所述,熊耳群许山组形成于一次强烈的热事件中,具有较大的岩浆通量与极高的喷发速率,符合地幔柱背景下的大火成岩省岩浆活动特征。The Xiong'er volcanic rocks,being the first major igneous flare-up following the formation of the North China Craton(NCC),mark the onset of rifting within the craton.As such,understanding their age and tectonic setting is crucial.Although geochronology has provided valuable data,the precise duration of the Xiong'er lavas remains uncertain due to analytical challenges.Additionally,the lavas'geochemical composition is highly debated,obscuring their origin and tectonic setting.A large amount of magma eruption in a short time is one of the characteristics of mantle plume,but it is difficult to prove it only by geochemical methods.Quantitative textural analysis offers a promising approach to decipher magmatic processes.By quantifying crystal distributions and reconstructing crystallization histories,this method can illuminate the condition of magma chambers and estimate the timescale of magmatic events.The Xushan Formation,consisting of porphyritic basaltic andesite,lies at the base of the Xiong'er lava and records early magmatic activity.The phenocrystic plagioclase in that basaltic andesite is suitable for quantitative textural analysis.Our results reveal that the deep magma chamber of the Xushan Formation experienced a low degree of undercooling for a long time,suggesting a long-term thermal stability.Spatial texture parameters further indicate the ongoing replenishment of hot magma within an open system.Characteristics of crystal morphology allow us to estimate the plagioclase growth rate of 10^(-11)~10^(-10) mm/s,which speculating to a residence time of 2400~24,000 years.Based on the thickness of the basaltic andesite layers and the residence time,the eruption rate is estimated to 0.28~13km^(3)/yr.Our findings underscore the rapid and voluminous nature of the early Xiong'er magmatism,implying a genetic link to a mantle plume.The high eruption rate is consistent with models invoking a mantle plume as the driving force behind the NCC's rifting and subsequent volcanism.

关 键 词：熊耳群 结构定量分析 晶体粒度分布 存留时间 喷发速率 

分 类 号：P581[天文地球—岩石学] P588.144[天文地球—地质学]