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 conta ct aureole only centimeters wide in the adjacent host rock. Mean solid bitumen reflectance ranges from about 2.4 to 2.7% and shows little ch ange up to 6m away from the dike contact. Carbon and oxygen isotope va lues also show little change except at the dike contact. The most dete ctable 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 bri ght orange color. By 1 cm away from the contact, however, CL shows pre servation of sharp boundaries in finely zoned burial cements. A region al study of thermal maturation and a geothermometer based on solid bit umen reflectance indicate paleotemperatures near 200 degrees C were wi despread in the Buchan Rift during peak burial near the time of the De vonian Tabberabberan Deformation, The lack of response of the Buchan C aves Limestone to contact metamorphism is attributed to: (1) prior hea ting to near 200 degrees C; and (2) the fact that the dike intruded in to cool, near surface, low-porosity rocks which may have been in the v adose zone.