TEXTURAL AND COMPOSITIONAL DISTINCTIONS BETWEEN MID-CRETACEOUS TONALITES OF THE SOUTHEASTERN PETERSBURG QUADRANGLE, ALASKA

This study compares the mineralogy, texture, and geochemistry of the Wrangell Island and Bell Island tonalitic plutons. Both the Wrangell Island and Bell Island plutons are part of the central Admiralty-Revillagigedo magmatic belt in central southeastern Alaska. They are separated by the Fool's Inlet fault, an inferred steeply dipping NNW trending thrust fault. The purpose of this research was to determine if the Wrangell Island and the Bell Island tonalites are the same igneous intrusion and to assess the lithologic discontinuity across the Fool's Inlet fault.

This study examined tonalite samples collected from near and around the Fool's Inlet fault zone, and compared them to existing data from Wrangell Island tonalite and Bell Island tonalite samples. The two plutons are similar in their mineralogy, with the exception of certain accessory minerals. The Wrangell Island body contains ubiquitous apatite and titanite, which are considerably less abundant in the Bell Island pluton. Texturally, the Wrangell Island tonalite is moderately to strongly foliated and shows evidence of solid-state deformation of quartz and plagioclase feldspar. The Bell Island pluton is strongly lineated to weakly foliated; mineral shapes and inter-granular relationships indicate fabric development in the magmatic state. Geochemical differences that distinguish the two plutons include greater abundance of P, Nd, Ce, La, and K in the Wrangell Island pluton and greater Ba and Na in the Bell Island pluton. Both follow a calc-alkaline trend on an AFM diagram, with the Wrangell Island body showing relative enrichment in FeO. These results suggest that the Fool's Inlet fault defines a structural boundary between the Wrangell Island pluton and the Bell Island pluton. The textural and geochemical distinctions between these two Mid-Cretaceous tonalites probably reflect differences in age, cooling history, and magma evolution.