New chemical analyses for major elements in 76 Hawaiian rocks are presented and bring the total number of such modern analyses to about 470. Many determinations of minor elements also are becoming available. Hawaiian petrology is discussed against this total background. The three major rock suites, tholeiitic, alkalic, and nephelinic, are chemically intergradational. The main mass of the volcanoes is tholeiitic, followed by a relatively small volume (generally less than 1 percent) of alkalic lavas; the two types of lavas are interbedded in a thin transitional zone. The nephelinic lavas are separated from the others by a long time interval that is marked by a profound erosional unconformity.Variations within the rock suites are largely the result of crystal differentiation. All three rock suites probably are derived from a single type of parent magma, which varies slightly from one volcanic center to another, of olivine tholeiite composition. Crystallization of this magma in shallow magma chambers leads to eruptible magmas of tholeiitic composition. In the last stages of volcanism, consolidation of the upper part of the magma body leads to crystallization at deeper levels under higher pressure and to production of alkalic magmas. Finally, crystallization at depths of several tens of kilometers produces nephelinic magmas that are erupted after a long period of volcanic quiescence. 477 on July 14, 2015 memoirs.gsapubs.org Downloaded from 478 STUDIES IN VOLCANOLOGY CONTENTS on July 14, 2015 memoirs.gsapubs.org Downloaded from on July 14, 2015 memoirs.gsapubs.org Downloaded from 482 STUDIES IN VOLCANOLOGY trachyte, and picrite-basalt of ankaramite type, as well as rocks transitional from mugearite to trachyte that have been termed benmoreite by Tilley, Yoder, and Schairer (1965). Oceanites belonging to the alkalic suite have not been found in the Hawaiian Islands, though they are fairly common in some other regions, such as Samoa. The nephelinic suite includes nephelinite, melilite nephelinite, basanite, alkalic olivine basalt, and small amounts of ankaratrite and picrite-basalt of mimosite type. The rock names are used herein as defined in previous papers (Macdonald, 1949a, p. 1544; 1960, p. 172-175; Macdonald and Katsura, 1964, p. 84-86). The rocks of the shield-building stage of Hawaiian volcanism are wholly tholeiitic. Most contain considerably more K,0 than the deep ocean basalts described by Engel, Engel, and Havens (1965), though such low-K rocks are by no means unknown among subaerial Hawaiian lavas. They are particularly abundant in Kohala volcano, where all of the 14 analyzed samples of the shield-building lavas (Pololu volcanic series) contain less than 0.3 percent K 2 0, and 11 of them contain less than 0.2 percent (Macdonald and Katsura, 1964, p.
117). It would be of much interest to determine whether low-K rocks increase in abundance at increasing depths below sea level within the volcanoes, that is, whether there is a general increase in the abundance of K (and related elements) throughout the life of t...
