关于火山熔岩分类命名的建议

岩浆岩的分类、命名是研究岩浆岩岩石学的基础,愈来愈为岩石学者所重视。在24届国际地质会议上确定了深成岩的分类、命名方案,并已得到广泛应用。但由于火山岩的复杂性,在分类、命名方面,至今尚无统一的口径。因此,火山岩分类方案多如雨后春笋,岩石名称杂乱,对岩石学的发展颇为不利,故迫切需要建立一个国际统一的分类、命名方案。

SUGGESTIONS ON THE CLASSIFICATION AND NOMENCLATURE OF VOLCANIC LAVAS

According to the current level of petrologic knowledge,we consider that some principles must be followed in making a classification and nomenclature of volcanic rocks:1.The classification and nomenclature of volcanic rocks should conform to the same principle as for equivalent plutonic rocks;2.A pure chemical classification is inconvenient for application.Thus it must be combined with the classification in accordance with mineral compositions;3.It must make allowance for the petrographic terms used popularly.In the light of the principles mentioned above,the recommendations and suggestions on the classification and nomenclature of volcanic rocks by Streckeisen (1979) seem to be desirable.A.Streckeisen applies the same double-triangle QAPF to the main volcanic rocks as that used in the classification and nomenclature of the plutonic rocks,which is based on mineral compositions (the model or the normative mineral compositions in Vol.% calculated from bulk chemical analyses of rocks).The other parameters may be used for further classification and nomenclature in detail.We have basically adopted his scheme,but some modifications have been made also. Based on our experience,we know that there exist certain difficulties in classification and nomenclature of volcanic rocks by mineral compositions.It is difficult to directly determine the mode of volcanic rocks on the one hand;there are a lot of questions in calculating mineral components from bulk petrochemical compositions on the other hand,such as:1.The calculated results of the normative mineral components vary with the various methods;2.The calculated results of complex rock-forming minerals are different due to the various combination of the normative mineral components;3.The theoretical bases of these calculating methods are not able to fit entirely with the natural law of consolidation and crystallization of magma so that there is some discrepancy between calculations and the reality.Hence,at the present time the most important thing is to select a more rational,realistic and convenient scheme for calculating mineral compositions by the chemical analyses of rocks.We think that the calculating procedure of the C.I.P.W.norm system is rather simple,but there are many deficiencies,of which the important one is non criterion that can be in accordance with the natural laws to distribute Or,Ab,An molecules among alkaline feldspar (A) and plagioclase (P) after the normative mineral components have been gained.However,the values of A and P take a crucial effect on the classification and nomenclature by means of the scheme of QAPF double-triangle.A.Rittmann’s calculating method (1973) is a better one,by which more approximate mineral compositions of the rocks can be obtained,but it is overelaborated and: inconvenient in practice.In order to improve the calculating method,the A.Rittmann’s principles of distributing Or,,Ab,An to A (alkaline feldspar) and P (plagioclase) and calculating: their relative quantities are introduced into the C.I.P.W.method leaving the other factors out of consideration.This is to say,it can be regarded as a prolongation of C.I.P.W.method.Through the calculation and analysis of some examples we come to a conclusion that the improved C.I.P.W.method for ealculating mineral components may be used-for classifying volcanic rocks.