识别富斜长石花岗岩类的岩石化学图解:评价与辨析  

An evaluation on the petrochemical diagrams for discriminating plagioclase-rich granitoids

作　　者：张金朋 汪洋[1] ZHANG Jinpeng;WANG Yang(School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing,100083)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083

出　　处：《地质论评》2024年第5期1963-1980,共18页Geological Review

基　　金：中国地质调查局地质调查项目(编号:D1912)的成果。

摘　　要：富斜长石花岗岩类指其实际矿物含量中斜长石在长石中的比例超过65%的花岗质侵入岩,包括英云闪长岩(tonalite)、奥长花岗岩(trondhjemite)和花岗闪长岩(granodiorite)。前寒武纪大陆地壳所包含的英云闪长岩、奥长花岗岩和花岗闪长岩组合被称为TTG岩套,现今弧岩浆组合也包括相当数量的英云闪长岩或奥长花岗岩以及花岗闪长岩。有效判别富斜长石花岗岩类对了解大陆地壳的组成以及地壳形成的过程和机制具有重要意义。本文基于全球典型英云闪长岩(含奥长花岗岩)、花岗闪长岩及其相关岩性的687组数据,以实际矿物v(Q)—v(A)—v(P)分类图为基准,对n(An)—n(Ab)—n(Or)、n(Q′)—n(ANOR)和w(SiO_(2)′)—w(CaO)/[w(K_(2)O)+w(CaO)]3种分类图解识别富斜长石花岗岩类的可靠性进行检验。结果表明三种判别图解对富斜长石长英质侵入岩的判别正确率在70%左右。对实际矿物含量v(Q)—v(A)—v(P)图解投影在英云闪长岩(含奥长花岗岩)的样品,w(SiO_(2)′)—w(CaO)/[w(K_(2)O)+w(CaO)]图解的判别准确率达90%,n(Q′)—n(ANOR)图解和n(An)—n(Ab)—n(Or)图解约为75%。对实际矿物含量QAP图解投影在花岗闪长岩的样品,3种图解的判别准确率均在70%左右。w(SiO_(2)′)—w(CaO)/[w(K_(2)O)+w(CaO)]和n(Q′)—n(ANOR)图解是识别英云闪长岩(含奥长花岗岩)有效岩石化学方法。实例研究表明,福建长乐—南澳构造带早白垩世和台湾大南澳晚白垩世侵入岩组合均不属于经典定义的TTG岩套。The plagioclase-rich granitoids include tonalite,trondhjemite,and granodiorite.They are the important component of the Precambrian continental crust.The Phanerozoic arc magma assemblages are also composed of a considerable portion of tonalite/trondhjemite and granodiorite.In this work,three petrochemical diagrams,i.e.,An—Ab—Or,Q′—ANOR,and SiO_(2)—CaO/(K_(2)O+CaO)plot,are tested for their ability to identifying the tonalite,trondhjemite,and granodiorite rocks,using the QAP classification results discriminated by the actual mineral modes as the benchmark.The test dataset is composed of 687 pairs of whole-rock chemical composition and mineral mode from a compilation of typical tonalites(including trondhjemites)and granodiorites as well as the related lithologies.The results indicate that three diagrams perform a 70%precision for distinguishing the plagioclase-rich plutons.For tonalites(including trondhjemites)samples defined by QAP mode classification,the discrimination accuracy is about 90%for SiO_(2)—CaO/(K_(2)O+CaO)diagram,and it is about 75%for Q′—ANOR and An—Ab—Or diagrams.Meanwhile,the discrimination accuracy around 70%are found for the three diagrams to identifying the granodiorite samples defined by QAP mode classification.Accordingly,the accuracy of the commonly used An—Ab—Or triangular is not superior to the other two diagrams for discriminating the plagioclase-rich granitoids.Three case studies suggest that the SiO_(2)′—CaO/(K_(2)O+CaO)and Q′—ANOR diagrams are very effective to identify the tonalites(including trondhjemite)rocks.The Early Cretaceous granitoids in Changle—Nan’ao Tectonic Belt of southeast China are composed of tonalite+granodiorite+granite(s.s.),which is comparative to the typical arc magma assemblage.These granitoids cannot be regarded as the TTG suite as proposed by some researchers.

关 键 词：富斜长石花岗岩类 英云闪长岩 奥长花岗岩 花岗闪长岩 TTG岩套 QAP图解 Q′—ANOR图解 SiO_(2)′—CaO/[K_(2)O+CaO]图解 An—Ab—Or图解 

分 类 号：P588.121[天文地球—岩石学] P584[天文地球—地质学]