Petrology and petrogenesis of the Bokan Granite Complex, southeastern Alaska

Abstract

The Bokan Granite Complex is a peralka-line ring-dike complex emplaced into marine shales, volcanic flows and tuffs, and plutonic rocks. Mineralogically and chemically, the Complex is composed of I-type granites. Aegirine- and riebeckite-bearing granite aplites, porphyries, and pegmatites comprise twelve distinct intrusive episodes. The aegirine-bearing rocks occur in an outer annular zone formed during early crystallization. Subsequent rocks are riebeckite-bearing, due to devolatilization of the magma chamber during a collapse-ring-dike emplacement event. Early crystallization of alkali feldspar occurred during magma ascension from a lower crustal-upper mantle source. At shallow depths, subsolvus crystallization allowed microcline and albite to dominate. Local bodies of aegirine syenite were formed during the early collapse-ring-dike emplacement, in response to the magma devolatilization. The riebeckite granites reflect lower Po2 and possibly declining peralkalinity. The granitic rocks at Bokan all exhibit Na2O contents greater than K2O. Lithophile elements are concentrated in all the rocks, especially in zones where hydrothermal albite and chlorite formed. Rb/Sr ratios increase in progressively younger rocks in the Complex. Agpaitic ratios vary from 0.92 to 2.08 for the granitic rocks.