Silica standardization: a discriminant technique applied to a volcanic arc system

Abstract

A classic tholeiitic, calc-alkaline, and shoshonitic island-arc sequence has developed in western Luzon, the Philippines, as a result of subduction defined by an eastward-dipping Benioff zone. A simple technique of standardizing the data allows chemical concentrations to be recast to “andesite equivalent” compositions. Based on the excellent linear correlations that exist between SiO 2 and other elements, individual analyses were standardized to 55% SiO 2 by projecting parallel to least-squares regression lines. Raw and SiO 2-standardized data were then treated by an univariate one-factor analysis of variance and a multivariate discriminant function analysis. Results of the analysis of variance indicate that at the 99% confidence level; 20 of the 27 chemical variates discriminate between at least two of the three geographically distinct magma series; SiO 2-standardized data generally discriminate better than do raw data; and K 2O, Ba, Rb, Sr, and Zr provide the best discrimination. Better classification of samples into the appropriate series is obtained from results of the discriminant function analysis using the SiO 2-standardized data rather than results using the raw data. With this standardizing technique it was possible to detect offsets of SiO 2-standardized K 2O isopleths along the Manila Bay fracture zone that correspond to offsets in contours of depths for earthquake foci.