锆石微量元素地球化学对硅质火山岩浆系统的制约  

Zircon trace element geochemistry constrains on the silicic volcanic system

作　　者：贺振宇[1] 颜丽丽[1] HE Zhen-yu;YAN Li-li(Institute of Geology,Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《岩石矿物学杂志》2021年第5期939-951,共13页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金(42002070,42172070,41772060)。

摘　　要：大型硅质火山作用(喷发体积约102~10^(4)km^(3))的岩浆系统是地壳尺度的,经历了复杂的起源、运移、存储、补给和喷发等过程。揭示岩浆从起源到喷发过程中的结晶分异、堆晶、晶体-熔体分离、地壳混染、岩浆补给、晶粥活化等岩浆作用的细节是认识硅质火山岩浆系统演化的关键。锆石中Th、U、Ti、Hf和REE等微量元素的含量和系统变化反映了锆石结晶熔体的成分、温度、氧逸度和水含量等以及共生的矿物相特征,对示踪火山岩浆系统的演化过程具有重要研究意义。随着岩浆温度降低过程中结晶分异作用的进行,锆石微量元素呈现出Hf含量升高、Ti含量降低以及Th/U、Eu/Eu^(*)和Zr/Hf等比值降低的趋势,这些元素含量和比值可以作为岩浆分异演化程度的指标。成矿斑岩中的锆石一般具有高的Ce^(4+)/Ce^(3+)和Eu/Eu^(*)值,反映了岩浆具有高的氧逸度和水含量。火山岩锆石可能经历多阶段结晶过程,因而形成复杂的核-边结构特征,核部具有熔蚀现象,边部CL较亮并具有低的Hf、U和高的Ti含量以及Eu/Eu^(*)值等,反映了岩浆补给作用和晶粥活化过程。由于锆石颗粒比较微小,在晶体-熔体分离过程可能随提取的熔体进入喷发岩浆房,从而可以连续记录岩浆成分的变化,或者残留在晶粥中记录晶体-熔体的分离。锆石微量元素结合高精度年代学分析,可以精细制约火山岩浆系统的多阶段演化过程及其时间尺度。在锆石微量元素数据的解释和筛选过程中,需注意扇形分区、锆石褪晶化和其他矿物包裹体对分析结果的影响,并同时开展岩相学研究,结合锆石产状和共生矿物组合特征,为制约火山岩浆系统的演化过程提供可靠信息。The magmatic system of large silicic volcanic eruptions( with ejected volumes of about 102~ 10^(4) km^(3))extends through the crust,comprising complex generation,transport,storage,recharge and eruption processes.Critical aspect for understanding the evolution of silicic volcanic system is to reveal the magmatic processes from melt generation to eruption,such as crystal fractionation,crystal accumulation,crystal-melt segregation,crustal assimilation,magma recharge and mush rejuvenation. Zircon incorporates a variety of trace elements,such as Th,U,Ti,Hf and rare earth elements,and their abundances and variations are particularly sensitive to the composition,temperature,oxidation state,water content of the magma and the co-crystallized phases. Therefore,zircon has the outstanding capacity to record the evolution of silicic magmatic system. In general,as the falling temperature of the melt,the Hf concentration increases and the Ti concentration and Th/U,Eu/Eu^(*)and Zr/Hf ratios typically decreases,which are effective indicators of fractionated magmas. Zircon from porphyry intrusions associated with mineral deposits tends to have high Ce^(4+)/Ce^(3+) and Eu/Eu^(*)ratios,indicating strong oxidized conditions and high water concentration. Zircon can show characteristics of multistage crystallization with a core-rim structure,including distinctly resorbed core and CL-bright rim. Compared to the zircon core,CL-bright rim commonly shows lower Hf and U and higher Ti and Eu/Eu^(*) ratios,indicating magma recharge event and the rejuvenation of crystal mush.Due to the mobility of smaller zircon relative to the larger crystals,zircon may continuously be mobilized in extracting melts recording a continuous compositional range of magma evolution,but may also remain in the crystal mushes indicating crystal-melt segregation events. Zircon trace element compositions integrated by high-precision zircon U-Pb geochronology can track the evolution of silicic volcanic system as a function of time. Metamictization,sector zoning and exoti

关 键 词：硅质岩浆系统 结晶分异 晶体-熔体分离 岩浆补给 锆石微量元素 

分 类 号：P578.941[天文地球—矿物学] P588.11[天文地球—地质学]