Fluid inclusion and vitrinite-reflectance geothermometry compared to heat-flow models of maximum paleotemperature next to dikes, western onshore Gippsland Basin, Australia

Abstract

Nine basalt dikes, ranging from 6 cm to 40 m thick, intruding the Upper Jurassic–Lower Cretaceous Strzelecki Group, western onshore Gippsland Basin, were used to study maximum temperatures ( T max) reached next to dikes. T max was estimated from fluid inclusion and vitrinite-reflectance geothermometry and compared to temperatures calculated using heat-flow models of contact metamorphism. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a temperature of 100–135°C. Fracture-bound fluid inclusions in the host rocks next to thin dikes (<3.4 m thick) suggest T max systematically increases towards the dike margin to at least 500°C. The estimated T max next to the thickest dike (thickness ( D)=40 m) suggests an extended zone of elevated R v-r to at least a distance from the dike contact ( X) of 60 m or at X/ D>1.5, using a normalized distance ratio used for comparing measurements between dikes regardless of their thickness. In contrast, the pattern seen next to the thin dikes is a relatively narrow zone of elevated R v-r. Heat-flow modeling, along with whole rock elemental and isotopic data, suggests that the extended zone of elevated R v-r is caused by a convection cell with local recharge of the hydrothermal fluids. The narrow zone of elevated R v-r found next to thin dikes is attributed to the rise of the less dense, heated fluids at the dike contact causing a flow of cooler groundwater towards the dike and thereby limiting its heating effects. The lack of extended heating effects suggests that next to thin dikes an incipient convection system may form in which the heated fluid starts to travel upward along the dike but cooling occurs before a complete convection cell can form. Close to the dike contact at X/ D<0.3, R v-r often decreases even though fluid inclusion evidence indicates that T max is still increasing. Further, fluid inclusion evidence indicates that the evolution of water vapor or supercritical fluids in the rock pores corresponds to the zone where R v-r begins to decrease. The generation of the water vapor or supercritical fluids near the dike contact seems to change vitrinite evolution reactions. These metamorphic conditions, closer to the dike than X/ D=0.3 make vitrinite-reflectance unreliable as a geothermometer. The form of the R v-r profile, as it indicates T max, can be interpreted using temperature profiles estimated from various heat-flow models to infer whether the dike cooled by conduction, incipient convection, or a convection cell. A contact aureole that consists of decreasing R v-r or T max extending out to X/ D≥2 and that has a T contact≫( T magma+ T host)/2 appears to be a signature of simple conductive cooling. Incipient convection is indicated by a R v-r profile that decreases to background levels at X/ D<1. A convection cell is indicated by a wave-like form of the R v-r profile and consistently high R v-r that may not decrease to background levels until beyond distances of X/ D>1.5.