The petrology and geochemistry of the Acheron intrusion

Abstract

Subalkaline transitional gabbros have intruded the Cretaceous - Paleocene sediments of the Acheron Outlier. The gabbroic magma has differentiated in situ to form a laccolith and sill-like apophysis, as well as having a strong thermal effect on the sediments and especially the coals of the Broken River Formation. This research concentrates on the geochemical characteristics and differentiation processes. Additional research on the thermal alteration of the coals at Acheron has provided data for heating models of the sediments. Thermal alteration of the sediments is extensive with the thermal aureole of the intrusion affecting the entire thickness of Tertiary sediments exposed at Acheron, increasing coal ranks from regional lignite A to anthracite. Vitrinite reflectance data indicate circulating hydrothermal fluids had a strong control on the degree of heating sediments underwent. It is suggested that the Acheron magma was modified by flowage differentiation within the conduit prior to emplacement. This created an olivine phenocryst-rich magma and an olivine phenocryst-poor melt. The latter was emplaced into the Tertiary sediments as a thin proto-laccolith which inflated to form the bulk of the Acheron Intrusion prior to the emplacement of the denser and more viscous olivine-rich magma which formed a basal layer within the intrusion and also breached the laccolith margin to form the apophysis. Crystallisation proceeded from the walls, roof and floor by sidewall crystallisation. This produced the five lithotypes recognised within contact gabbro, olivine mela-gabbro, gabbro, monzogabbro and differentiates. Geochemical analyses of these rocks show differentiation trends consistent with crystal fractionation of a relatively basic melt. Geochemical trends are complicated by the extensive deuteric alteration of the intrusion. Alteration of feldspar by late stagedeuteric fluids has produced secondary analcime and other zeolites, as well as quartz, giving the false impression that the intrusion was originally undersaturated.