Original paper

Relationship of amphibole composition to host-rock geochemistry: the A-type gabbro-granite Wentworth pluton, Cobequid shear zone, eastern Canada

Pe-Piper, Georgia

Abstract

The Wentworth pluton is the most alkaline of a series of Late Devonian metaluminous to mildly peralkaline plutons in the Cobequid shear zone. It includes numerous granitoid bodies of slightly differing composition, which may contain sodic, sodic-calcic or calcic amphiboles. This study examines the relative importance of host rock composition including volatiles, fO2, and temperature in controlling the type of amphibole present. Electron microprobe analyses and back-scattered electron images were used to characterize the amphiboles and whole-rock geochemical analyses were made from the host granites. Magmatic sodic amphiboles are arfvedsonite, ferrorichterite and katophorite. Ferrowinchite and riebeckite developed by subsolidus oxidation reactions with loss of fluorine. Sub-solidus changes in Fe-Ti oxides make reliable estimation of fO2 difficult. Regionally, sodic amphiboles are found only in the Wentworth pluton, suggesting the importance of peralkaline geochemistry and perhaps higher temperature as a result of voluminous gabbro in the pluton. However, the presence of sodic or calcic amphibole in granite with >73% SiO2 in the Wentworth pluton shows no systematic difference with either host-rock geochemistry or distance from the gabbroic heat source, except that rocks with sodic amphibole have high Li, suggesting abundance of free water.

Keywords

amphibolegranitenova scotiacobequid shear zonelithiumalkalis