Wallrock Melting and Reaction Effects along the Higganum Diabase Dike in Connecticut: Contamination of a Continental Flood Basalt Feeder

Abstract

The question of whether continental flood basalts are contaminated with crustal material during ascent through the lithosphere is addressed through a study of the feeder dike to the first of the Mesozoic flood basalts in the Hartford basin of Connecticut. Faulting associated with basin formation exposed the dike, not only where it connects with the flood basalt, but also at levels that may have been as deep as 10 km at the time of crystallization. Wallrocks at all levels of exposure and along the entire 250-km length of this 50–60 m wide dike show evidence of partial melting and contamination of the diabase magma. Melting of the wallrocks took place on grain boundaries between quartz and feldspar, especially where these boundaries were fluxed with water released from the breakdown of biotite. The melts were simple mixtures of quartz and the particular feldspar involved rather than equilibrium minimum compositions, probably because heating rates were faster than melting rates. The melts crystallized to form granophyre. Feldspars in direct contact with the diabase partly melted, with plagioclase developing the fingerprint texture and orthoclase being converted to an exceptionally fine-grained mixture of plagioclase and quartz. Melts generated in the wallrocks were forced into the margins of the dike by the volume expansion caused by melting. The earliest melts to enter the dike are preserved as felsic wisps in the chilled margins, but at distances of more than a few centimeters from the edge of the dike they appear to have been assimilated by the diabase magma. Melts that entered the dike after the margins had solidified were intruded as crosscutting granophyre veins. Chemical profiles across the dike reveal that the contaminants were not restricted to narrow marginal zones but entered the main body of the dike. Thus, by the time this magma had risen to the surface and erupted as the Talcott basalt it had assimilated a significant amount of crustal material, which accounts for the change from olivine normative compositions in the deepest level exposures of the dike to quartz normative ones in the basalt. Mass balance calculations indicate that the Talcott basalt may have had ˜6% granophyre added to it during the magma's ascent through the last 10 km of the crust. The amount assimilated in traversing the entire continental crust would therefore be greater than this.