TTG岩类的识别:讨论与建议

摘要

本文简要叙述了国际上通用的两种TTG岩类分类命名方案及实际应用时需要注意的事项:(1) QAP实际矿物定量分类,应用时必须使用精确的矿物定量数据,而不是用目估或半定量方法估算的矿物含量；(2) O' Connor( 1965 )An-Ab-Or标准矿物分类,应用于石英标准分子(Q-norm)＞10％的长英质岩石.在此基础上,对两种分类命名方案进行了比较,认为无论是实际矿物定量分类,还是标准矿物分类,两种分类方案(或者说,双重分类法)对TTG岩类的分类命名总体上是相符合的,但二者在使用时却存在差异:采用实际矿物定量分类的QAP图解,不能直接区分英云闪长岩和奥长花岗岩,还必须进一步根据暗色矿物(M)含量大于(＞)或小于(＜)10％区分是英云闪长岩,还是归属于浅色英云闪长岩的奥长花岗岩.而采用标准矿物分类An-Ab-Or图解,可直接区分英云闪长岩和奥长花岗岩,在应用上更为方便和快捷.在回顾和讨论了国内某些有关TTG岩类分类命名方案的基础上,通过与国际分类命名方案对比,结合工作实例提出如下建议:(1)在识别TTG岩类时,应按照数据特点采用不同的分类命名方法.当具有精确的实际矿物定量统计时,采用国际地科联提出的QAP分类,并应严格按照操作步骤,对区5的英云闪长岩基本名称再进一步区分为英云闪长岩(狭义)和奥长花岗岩；当不具有精确的实际矿物定量统计,但有主元素化学分析数据时,则采用O'Connor( 1965) An-Ab-Or标准矿物分类命名.如果两种数据均有,则可以同时使用,以便互相校对,获得更正确的TTTG岩类名称.(2)鉴于O'Connor( 1965) An-Ab-Or标准矿物分类命名方案在太古宙灰色片麻岩区和花岗-绿岩带广泛使用,对于显生宙岛弧和活动大陆边缘弧花岗岩类的研究亦可使用,建议对我国造山带中花岗岩类更多地使用O'Connor( 1965) An-Ab-Or标准矿物分类方案,可能对TTG岩类的识别会起到积极作用.(3)构筑统一平台,对于区域火成岩对比和综合研究非常重要,由于主元素化学分析数据比较容易得到,采用O' Conner( 1965) An-Ab-Or标准矿物分类命名方案比较现实,可操作性强.(4)O' Connor( 1965) An-Ab-Or标准矿物分类实质上是一种化学分类,它可实现在统一平台上进行侵入岩与对应的火山岩的对比研究；同时,这种化学分类有利于火成岩与实验熔体之间的对比,这对讨论火成岩的起源、形成的温压条件等具有重要意义.最后,文章提出了TTG岩类识别的两个步骤:首先,应用侵入岩TAS图解判别花岗岩类；然后,应用O'Conner( 1965) An-Ab-Or标准矿物分类命名识别TTG岩类.%This paper briefly describes and compares two international classifications of the rock types of TTC series; the Le Maitre's QAP modal classification based on accurate mineral modes, and the O'Connor's An-Ab-Or normative classification when the Q-norm is > 10%. Both modal and normative classifications are generally consistent with each other. The field 3 of the QAP is called tonalite as the root name. It includes trondhjemite and plagiogranite (adopted by part of the USSR geologists). The latter may be called as light-coloured (M^ 10) tonalite. However, the tonalite and trondhjemite can be directly determined by using the An-Ab-Or normative classification. Based on the comparison between the Le Maitre' s QAP modal classification and some Chinese modified QAP modal classifications, and paying attention to some examples for recognizing the TTC rock types I using O'Connor's An-Ab-Or normative classification by the authors, several suggestions are proposed here as follows: (1) The Le Maitre' s QAP modal classification may be used for recognition of the TTC rock types, when the accurate mineral modes are available, and then the procedure is necessary to be determined whether these rock types are tonalite or trondhjemite using the M indexes. When the chemical data are available but the accurate modes are lacking, the O'Connor's normative classification may be used. When both the accurate mineral modes and the chemical data are available, both classifications may be used for a better recognition of the TTC rock types; (2) In terms of widespread utilization of the O'Connor's classification for the Archean grey gneiss and granite-greenstone belt, and for some Phanerozoic magmatic arcs, a suggestion of using the O'Connor's classification for the orogenic belts in China may be helpful for recognizing the TTC rock types; (3) A unified classification is needed for regional igneous petrologioal investigations, particularly in China, because of large available chemical data rather than accurate mineral modes, then the O'Connor's normative classification may be suggested; (4) The O'Connor's classification is actually a chemical classification, which is good for a comparison between the intrusive and volcanic rocks, as well as between the igneous rocks and the experimental melts. Finally, a two-step recognition procedure is suggested: Firstly, use the TAS classification by Middlemost to discriminate the felsic rocks; and second, use the O'Connor's An-Ab-Or normative classification for recognition ofTTG rock types.