Systematic petrochemical differences in andesite suites

When the chemical compositions of andesites are computed by a least mean squares technique at 60 % silica for different volcanoes and provinces, systematic differences appear. While island arcs in general have lower K, Rb, Ba, Th, Sr and higher Ca than the continental andesites, individual volcanoes within a province often show up to orders of magnitude differences in these elements. Of the seven main Quaternary volcanic provinces of the western American Cordillera (Alcutian, Cascade, Central Mexican, Central American, Columbian, North Chilean and South Central Andes) the Cascade region has the lowest K₂O and highest Sr (1.25%, 1400 ppm) and the North Chilean region has the highest K₂O (2.8) and low Sr. Ba appears to follow K while K, Rb, Sr appear to vary independently in different regions though K and Sr often vary inversely within a region.