FLUID INCLUSION AND VITRINITE REFLECTANCE GEOTHERMOMETRY COMPARED TO HEAT-FLOW MODELS OF MAXIMUM PALEOTEMPERATURE NEXT TO DIKES, WESTERN ONSHORE GIPPSLAND BASIN, AUSTRALIA

Nine basalt dikes, ranging from 6 cm to 40 m thick, intruding the Uppe r Jurassic-Lower Cretaceous Strzelecki Group, western onshore Gippslan d Basin, were used to study maximum temperatures (T-max) reached next to dikes. T-max was estimated from fluid inclusion and vitrinite-refle ctance geothermometry and compared to temperatures calculated using he at-flow models of contact metamorphism. Thermal history reconstruction suggests that at the time of dike intrusion the host rock was at a te mperature of 100-135 degrees C. Fracture-bound fluid inclusions in the host rocks next to thin dikes (< 3.4 m thick) suggest T-max systemati cally increases towards the dike margin to at least 500 degrees C. The estimated T-max next to the thickest dike (thickness (D) = 40 m) sugg ests an extended zone of elevated Rv-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 the ir thickness. In contrast, the pattern seen next to the thin dikes is a relatively narrow zone of elevated Rv-r. Heat-flow modeling, along w ith whole rack elemental and isotopic data, suggests that the extended zone of elevated Rv-r is caused by a convection cell with local recha rge of the hydrothermal fluids. The narrow zone of elevated Rv-r found next to thin dikes is attributed to the rise of the less dense, heate d fluids at the dike contact causing a flow of cooler groundwater towa rds the dike and thereby limiting its heating effects. The lack of ext ended heating effects suggests that next to thin dikes an incipient co nvection system may form in which the heated fluid starts to travel up ward along the dike but cooling occurs before a complete convection ce ll can form. Close to the dike contact at X/D < 0.3, Rv-r often decrea ses even though fluid inclusion evidence indicates that T-max is still increasing. Further, fluid inclusion evidence indicates that the evol ution of water vapor or supercritical fluids in the rock pores corresp onds to the zone where Rv-r begins to decrease. The generation of the water vapor or supercritical fluids near the dike contact seems to cha nge vitrinite evolution reactions. These metamorphic conditions, close r to the dike than X/D = 0.3 make vitrinite-reflectance unreliable as a geothermometer. The form of the Rv-r profile, as it indicates T-max, can be interpreted using temperature profiles estimated from various heat-now models to infer whether the dike cooled by conduction, incipi ent convection, or a convection cell. A contact aureole that consists of decreasing Rv-r or T-max extending out to X/D greater than or equal to 2 and that has a T-contact much greater than (T-magma+T-host)/2 ap pears to be a signature of simple conductive cooling. Incipient convec tion is indicated by a Rv-r profile that decreases to background level s at X/D < 1. A convection cell is indicated by a wave-like form of th e Rv-r profile and consistently high Rv-r that may not decrease to bac kground levels until beyond distances of X/D > 1.5. (C) 1998 Elsevier Science B.V. All rights reserved.