Abstract:

        This paper briefly describes and compares  two  international classifications of  the  rock  types of TTG 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 5 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 TTG  rock  types 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 TTG 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 TTG  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 TTG  rock  types;  (3) A unified classification  is needed  for regional  igneous petrological  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 of TTG rock types.

Key words:  TTG rock types, QAP modal classification, An-Ab-Or normative classification, procedure of recognition