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White micas in slates from an imbricated sequence of deformed turbidites, and clast-in-matrixmelanges that form the Narooma accretionary complex at Batemans Bay, record apparent 40 Ar/ 39 Ar whole rock ages in the range 450 to 500 Ma, with intermediate-temperature average ages of 471 + 13 Ma and 476 + 11 Ma (2 standard deviations). By contrast, two K-Ar whole rock ages are 430.9 + 8.6 Ma and 449.2 + 9.0 Ma. Fractions (52, 3-5 and 10-20 mm) from the same samples show a progressive increase in age with grainsize (52 mm ¼ 413.1 + 8.3-404.7 + 8.1 Ma; 3-5 mm ¼ 417.1 + 8.3 Ma; 10-20 mm ¼ 434 + 8.6-437.5 + 8.8 Ma 2 standard deviations). The older whole rock 40 Ar/ 39 Ar ages are attributed to recoil loss of 39 Ar. The cleavage forming event is believed to have occurred at *460-440 Ma (Upper Ordovician to Lower Silurian). The finest fractions possibly record a younger thermal event at approximately 413 Ma. Kü bler Index values obtained from the Wagonga and Adaminaby Groups indicate lower epizonal (greenschist facies) metamorphic conditions. b 0 lattice parameter values (x ¼ 9.033; n ¼ 21) are characteristic of the intermediate pressure facies series.Assuming a temperature of approximately 3008C and P ¼ 4 kb suggests a depth of burial of approximately 15 km was reached by these rocks and that a geothermal gradient of 208C/km operated during subduction.

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A B S T R A C T. A study of low-grade metamorphism in lateSilurian to early Carboniferous rocks in the North Hill End Synclinorium and adjacent anticlinoria has been made by the determination of illite crystallinity and bo values of K-white mica in eighty slates and phyllites. Illite crystallinity values vary from 0.40 A~ on the Molong Anticlinorium to 0.12 A~ within the axis of the synclinorium, suggesting anchizonal to epizonal metamorphic conditions. This is in agreement with previous observations on Ca Al-hydrosilicate assemblages which indicated a change from prehnite pumpellyite facies in the anticlinoria adjacent to the synclinorium to middle greenschist facies in the axis. Local variations in crystallinity are attributed to variation in a~ in fluids migrating along cleavage zones.The mean bo value obtained from the pelites is 9.017 A (an = 0.008; n = 80) which is in close agreement with that obtained from part of the adjacent Capertee Anticlinorium (ff = 9.019 A; an = 0.007; n = 52). However, 't' tests indicate that two bo populations are present in the synclinorium (Y = 9.010 and 9.022A), with the lower values concentrated in the southern portion of this structure. The two populations are considered to be the result of slightly different metamorphic conditions prevailing during the deformation of the rocks in the synclinorium. A higher geothermal gradient affecting rocks giving the lower bo values is attributed to the presence of granitoids at shallower depths than elsewhere in the synclinorium. rz ~ y w o R D S : illite, mica, metamorphic rocks, North Hill End Synclinorium, New South Wales, Australia.

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