A strontium isotopic traverse across the granitic rocks of southeastern Australia: Petrogenetic and tectonic implications

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Abstract
Three regional granite associations are resolved by a Sr isotopic traverse across the orogenic structures of southeastern Australia. The associations identified by geochronology and the distribution of maximum initial 87Sr/86Sr ratios are (1) western — ∼500 Ma granites of southeastern South Australia with initial ratios up to ∼0.718; (2) central — 400 and 370 Ma granites of central Victoria with maximum initial ratios of ∼0.712; and (3) eastern — 415 Ma granites of the batholiths which parallel the eastern seaboard and have highest initial ratios of ∼0.720. Strontium isotopic compositions preclude granite magma formation by simple melting of a concealed Proterozoic basement comparable with that exposed in the interior of the continent; the latter would be far too radiogenic at the time of magmatism (87Sr/86Sr> 0.73). However, the maximum initial ratios of the western and eastern granite associations correspond to mean values for their respective enclosing Palaeozoic (meta)sedimentary terrains. This link extends to pseudoisochron diagrams of initial 87Sr/86Sr vs 87Rb/86Sr ratios on which the three associations have distinct trends. The granites are construed to define arrays between the local (meta)sedimentary basement points and a mantle‐like isotopic composition. Granite genesis is described by mixing between a melt derived from the local basement and basaltic material. Crustal sections in the western and eastern associations comprise equivalents of surficial Palaeozoic sequences down to the zone of magma generation. The central association in Victoria is interpreted to contain a concealed ‘isotopically juvenile’ granite source terrain underlying the Ordovician flysch and separated from it by a regional thrust fault. Boundaries between the three regional granite associations are considered the fundamental tectonic discontinuities in southeastern Australia.