CRYSTALLINITY AND B(0) SPACING OF K-WHITE MICAS IN A PALEOZOIC ACCRETIONARY COMPLEX, EASTERN AUSTRALIA - METAMORPHISM, PALEOGEOTHERMS, AND STRUCTURAL STYLE OF AN UNDERPLATED SEQUENCE

White micas in slates and meta-sandstones from deformed and imbricated chert-basalt-turbidite sequences of the Narooma Accretionary Complex of the eastern Lachlan Fold Belt, Australia, show b(0) cell values ind icative of intermediate P metamorphic facies conditions ((x) over bar = 9.025; n = 73). Illite crystallinity (IC) values indicate middle to upper anchizonal metamorphic conditions and are constant throughout th e area. They are in general agreement with the inferred structural pos ition representing the mid-upper levels of a subduction complex. IC va lues show little change throughout the area, indicating limited vertic al displacement on later inferred tectonic breaks. Differences in buri al pressure estimates and inferred geothermal gradients occur between the chert-basalt-turbidite sequence (similar to 10 km and 26 degrees C km-l) and the structurally lower chaotic block in-matrix melange (sim ilar to 7 km and 36 degrees C km(-1)). Metamorphism of chaotic block-i n-matrix melange is inferred to have occurred at shallow levels closer to the toe of the accretionary complex, in a higher geotherm prior to underplating beneath the chert-basalt-turbidite sequence. Mica crysta llization ages from 445-450 Ma indicate subduction activity in the Lat e Ordovician very close in time to the age of sedimentation.