一种新的岩浆岩分类定量方法

A new quantitative classification method for magmatic rocks

目前,岩浆岩的超基性、基性、中性、酸性的分类均根据SiO_(2)的含量,但这种单一含量判别标准并不严谨,含量经常与分类不完全对应。使用TraceElem 1.0软件对各种岩浆岩进行逐步判别分析、聚类分析和对应分析研究,据此认为超基性岩由MgO和FeO组成的因子确定,基性岩则由P_(2)O_(5)、Fe_(2)O_(3)、TiO_(2)、CaO、MnO组成的因子确定,中性岩由Na_(2)O和Al_(2)O_(3)组成的因子确定,酸性岩由K2O和SiO_(2)组成的因子确定。岩浆岩的分类宜以SiO_(2)为主、以MgO和CaO为辅,尤其要引入MgO。在SiO_(2)含量相同的情况下,MgO、CaO含量越高,基性程度越强。基于这一分析结果,提出一种新的酸度指数(ADI)计算方法,即ADI=w(SiO_(2))-0.75w(MgO)-0.23w(CaO),ADI<38.0%为超基性岩,在38.0%~52.0%之间为基性岩,在52.0%~62.5%之间为中性岩,>62.5%为酸性岩。

The current classification of ultrabasic rocks,basic rocks,neutral rocks,and acidic magmatic rocks is based on SiO_(2) content,but this simple content discrimination criterion is not strict,and the content of SiO_(2) is often not exactly corresponding to the classification.By using a stepwise discriminant analysis,cluster analysis and corresponding analysis study for various magmatic rocks with TraceElem 1.0,it is believed that the characteristics of ultrabasic rocks are determined by factors composed of MgO and FeO,those of basic rocks by P_(2)O_(5),Fe_(2)O_(3),TiO_(2),CaO and MnO,those of neutral rocks by Na_(2)O and Al_(2)O_(3),and those of acidic rocks by K 2O and SiO_(2).The classification of magmatic rocks should be dominated by SiO_(2),supplemented by MgO and CaO,especially MgO.With the same SiO_(2) content,the higher the content of MgO and CaO,the stronger the degree of basicity.Based on this analysis result,a new method for the calculation of acid degree index(ADI)was proposed,namely,ADI=w(SiO_(2))-0.75 w(MgO)-0.23 w(CaO).Magmatic rocks with ADI value less than 38.0%are ultrabasic,38.0%-52.0%are basic,52.0%-62.5%are neutral,and over 62.5%are acidic.