俯冲带镁铁质堆晶岩的母岩浆恢复:方法与实例  

Estimation of parent magma compositions of mafic cumulate at subduction zones:Methods and examples.

作　　者：郭锋[1,2] GUO Feng(State Key Laboratory of Isotope Geochemistry,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;Excellent Center of Deep Earth Sciences,Chinese Academy of Sciences,Guangzhou 510640,China)

机构地区：[1]中国科学院广州地球化学研究所,同位素国家重点实验室,广州510640 [2]中国科学院深地科学卓越创新中心,广州510640

出　　处：《岩石学报》2022年第12期3647-3658,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42073032、41525006、42021002)资助.

摘　　要：古老俯冲带/造山带结构的恢复是重建古板块构造的基石。然而,古俯冲带/造山带由于受到造山作用和随后强烈的构造改造、隆升与剥蚀作用,绝大多数弧岩浆岩尤其是玄武岩较为缺乏,通常仅保留了代表弧下地壳的超镁铁质-镁铁质堆晶岩。从严格意义上来说,镁铁质堆晶岩是造岩矿物、副矿物与粒间熔体的集合体,其全岩成分不能代表弧岩浆本身,因此运用其地球化学来讨论俯冲带演化存在诸多不确定性和多解性。在前人提出的平衡配分方法基础上,作者近年来建立了基于单斜辉石和角闪石的俯冲带镁铁质堆晶岩的母岩浆恢复方法,并根据样品在原始地幔标准化微量元素蛛网图中的异常变化情况,通过增减相关的副矿物成分予以校正。该方法广泛应用于中国东北、华南、中亚造山带和云开地区等古俯冲带/造山带镁铁质堆晶岩的母岩浆成分恢复。通过对比计算结果与全岩的实测成分,发现两种方法获得的Th/La、Ba/La和Ba/Nb比值差别不大,反映这些微量元素比值可以用于示踪古俯冲带地幔楔的改造过程。然而计算获得的Th/Yb比值则明显比镁铁质堆晶岩的全岩实测值偏高,因此该比值不适合用来示踪俯冲带中沉积物的贡献。The architecture recovery of paleo-subduction zone/orogenic belt is fundamental to reconstruct the paleo-plate tectonics of the Earth.Yet it remains difficult to rebuild the paleo-subduction zone/orogenic belt because most arc magmatic rocks,especially the basaltic lavas,are lacking,in response to orogeny and subsequent crustal deformation,uplifting and unroofing,with only residual ultramafic-mafic cumulates that may represent the lower arc crust.Strictly,mafic cumulate is an aggregate of rock-forming and accessory minerals and trapped melts,and their whole-rock compositions can no longer represent the parental arc basalts.This means that it is problematic to use the mafic cumulate geochemistry to track the tectonic evolution of subduction zones.Based on the previous equilibrium distribution method,the author have recently developed two methods for parent magma recalculation of mafic cumulates respectively approaching by clinopyroxene and hornblende.During the calculation,the author further consider the role of accessory minerals such as apatite,ilmenite,titanite and zircon to eliminate the compositional anomaly in primitive mantle-normalized incompatible element(e.g.,P and Ti)spidergrams,except for usually positive Sr-Eu anomalies indicative of plagioclase accumulation.These methods have been widely applied in the compositional estimation of parent magmas for the mafic cumulates in paleo-subduction/orogenic belts across the eastern China,including NE China,South China,the Central Asian Orogenic Belt,and the Yunkai Region.By comparing the calculated results with the measured whole-rock compositions,the trace elemental ratios like Th/La,Ba/La and Ba/Nb remains nearly constant during the backstripping procedure,indicating that these ratios are useful tools to track the enrichment processes operated beneath the mantle wedge of the paleo-subduction zones.In contrast,the calculated Th/Yb ratio is obviously higher than the measured value,so the measured whole-rock Th/Yb ratio cannot be employed to evaluate the role of

关 键 词：地球化学组成 平衡配分法 母岩浆 镁铁质堆晶岩 俯冲带 

分 类 号：P588.112[天文地球—岩石学] P588.125[天文地球—地质学]