Some regional compositional differences within Tertiary—Recent orogenic magmas

Abstract

Regional differences in the compositions of Recent orogenic magmas, mainly from the circum-Pacific areas, are investigated with the aid of a data file of over 7000 analyses, which contains major-oxide data and, where available, trace elements and modal mineralogy. Each analysis is also coded as belonging to one of the following magma series: calc-alkali, low-K, high-K or shoshonitic, as defined by Peccerillo and Taylor. The lavas can be divided chemically into two broad groups. One group, which includes the western U.S.A. (but not the Cascades), S. America and the Mediterranean, can be regarded as “continental”, while the other, which includes the S.W. and N.W. Pacific, and the Aleutians, Alaska and Cascades, can be regarded as “island-arc”. The inclusion of the Cascades in the latter is, however, somewhat anomalous and indicates that the geochemistry of the lavas is not controlled entirely by their tectonic setting. Central America seems to be intermediate to these two groups. Cluster analysis and multiple discriminant analysis have been used to investigate the chemical and mineralogical differences between these groups. The mineralogical characteristics are consistent with the idea that increasingly K-rich magma series in the orogenic regions may tend to be increasingly hydrous. It is concluded that the K-enrichment in the “continental” group is due to crustal interaction.