The minimal response to contact metamorphism by the Devonian Buchan Caves Limestone, Buchan Rift, Victoria, Australia

A 2.2 m thick, Late Eocene (?) dike that intruded the Devonian Buchan Caves Limestone, near Murrindal, Victoria, has produced a narrow contact aureole only centimeters wide in the adjacent host rock. Mean solid bitumen reflectance ranges from about 2.4 to 2.7% and shows little change up to 6 m away from the dike contact. Carbon and oxygen isotope values also show little change except at the dike contact. The most detectable response of the rock to contact metamorphism is the proportion of solid bitumen grains with finely granular mosaic structure increase towards the dike. Also under cathodoluminescence (CL) excitation, the host rock is initially observed to be recrystallized to a uniform bright orange color. By 1 cm away from the contact, however, CL shows preservation of sharp boundaries in finely zoned burial cements. A regional study of thermal maturation and a geothermometer based on solid bitumen reflectance indicate paleotemperatures near 200°C were widespread in the Buchan Rift during peak burial near the time of the Devonian Tabberabberan Deformation. The lack of response of the Buchan Caves Limestone to contact metamorphism is attributed to: (1) prior heating to near 200°C; and (2) the fact that the dike intruded into cool, near surface, low-porosity rocks which may have been in the vadose zone.