TECTONOTHERMAL HISTORY OF THE WESTERN LACHLAN FOLD BELT, AUSTRALIA - INSIGHTS FROM WHITE MICA STUDIES

Detailed b lattice parameter and illite crystallinity (IC) studies of K-white micas in slates from the Stawell and Ballarat-Bendigo Zones (S Z, BBZ) in the western Lachlan Fold Belt of Victoria, Australia, revea l a metamorphic pattern characterized by regional metamorphism associa ted with crustal thickening and younger contact metamorphism accompani ed by deformation. The IC data indicate that rocks regionally metamorp hosed prior to the intrusion of the Early and Late Devonian granitoids , vary in grade from epizonal (greenschist facies) to diagenetic (zeol ite facies) and that most are of epizonal to anchizonal (prehnite-pump ellyite facies) grade. In the BBZ, a decrease in grade from west to ea st occurs. Across fault zones, IC values show little change, indicatin g that limited vertical displacement has occurred. This is in accord w ith the thin skinned deformation model proposed for the western Lachla n Fold Belt. The b lattice parameters (x = 9.022 Angstrom n = 137, sig ma(n) = 0.009) indicate baric conditions intermediate between those of New Hampshire (P = Al2SiO5 triple point) and Otago (intermediate P). Thus, a moderately low geothermal gradient existed 450-430 Ma ago, whe n these rocks were deformed. K-D Fe/Mg (actinolite)/Fe/Mg (chlorite) v alues (0.52-0.70) obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high-P (c. 4 kbar) metamorphism suggested by the b cell parameter values. The metamorphi c conditions indicated by these data are contrary to the low-P/high-T conditions proposed by previous authors, who inferred an intimate asso ciation between deformation, granitoid intrusion and Sold mineralizati on. The b lattice parameter of white micas in slates adjacent to Early Devonian (c. 400 Ma) granitoids with schist bearing aureoles in the n orth-eastern part of the BBZ (x = 9.002 Angstrom; n = 27; sigma(n) = 0 .007), indicate pressures in the order of c. 2.5 kbar which are in acc ord with those obtained from andalusite-cordierite and zoisite-garnet bearing assemblages observed in the higher grade metapelitic and calca reous rocks. This contrasts with the higher pressure (c. 4 kbar) exist ing during regional metamorphism and implies that c. 6.5-8 km of metas edimentary rocks in the BBZ were removed before the emplacement of the Early Devonian granitoids. Metamorphic assemblages in hornfelses asso ciated with Late Devonian granitoids indicate a further 5-6 km of meta sediment were removed in the next 40 Ma prior to their emplacement. Th is study shows the value of white mica studies in elucidating the tect onothermal history of a low-grade metamorphic terrane dominated by met apelitic rocks.