Major and Trace Element Abundances in Volcanic Rocks of Orogenic Areas
Major and trace element abundances in volcanic rocks of island arcs vary regularly in both the lateral and in the stratigraphic sequence. The earliest manifestations of island arc evolution are tholeiitic rocks occurring on the oceanic side of recent island arcs. These tholeiites are characterized by chemical features such as low K 2 O content and relatively high iron enrichment, that is, high FeO/MgO in medium SiO 2 (59 percent) rocks. The tholeiites are followed stratigraphically or laterally by calc-alkaline rocks and finally by shoshonites or alkaline rocks, showing progressive increase in K 2 O content, increase in K 2 O/Na 2 O ratio, and decreasing iron enrichment. Trace elements of the potassium type are the most sensitive indicators of compositional variation showing differences between tholeiites and shoshonites by two orders of magnitude. Trace elements and major elements suggest a continuous gradational sequence from tholeiites through calc-alkaline rocks to shoshonites. Rare-earth elements show two diverse abundance patterns: (1) a primitive chondritic-like pattern is characteristic for island arc tholeiitic association; and (2) a fractionated light rare-earth elements enriched pattern is typical for calc-alkaline and shoshonitic and alkaline associations.