Chemical and isotopic evidence from the laterally asymmetric Bega Batholith for protracted Devonian fluid-induced infracrustal partial melting, eastern Lachlan Fold Belt, southeastern Australia

Abstract

The Devonian Bega Batholith in far southeastern Australia is composed of meridional strips of granites (supersuites) 10–20 km wide and up to 300 km long, each with distinctive chemical and isotopic compositions. The granites are predominantly granodiorites, with tonalites, some monzogranite and rare gabbro. West to east there are systematic changes in their chemical compositions, including increases in Na, P, Sr and Al, and decreases in Ca, Fe, Y and Rb, but relatively constant K, Mg and Ti. Similarly, there is also a systematic west to east trend towards more primitive isotopic compositions, with falling whole rock initial ⁸⁷Sr/⁸⁶Sr (Sr_i), and rising whole rock ε_Nd and zircon ε_Hf values. The bulk of the granites were emplaced by a belt of magmatic activity up to 50 km wide that moved stepwise west to east over a period of about 25 Ma, followed about 15 Ma later by a separate episode of A-type magmatism. At any given time, particularly when the western part of the batholith was forming, granites belonging to as many as four different supersuites were being intruded simultaneously as coherent strips at widely separated locations. The granites within individual supersuites were not emplaced simultaneously, but over significant periods of time, in some cases ≥10 Ma. The main determinant of the chemical and isotopic composition of a pluton was its location, not its emplacement age, i.e. ‘where’ was more important than ‘when’. There are no systematic changes in granite age or composition along the length of the batholith. The western supersuites have relatively similar evolved isotopic compositions, consistent with a sediment contribution to the magmas. This accords with the relatively common inherited zircon cores in those granites, the age pattern of which matches that in the detrital zircon from the early Palaeozoic flysch into which the granites were emplaced. The eastern supersuites contain much less inherited zircon (although with the same age pattern) and show distinct trends in isotopic composition both between and within supersuites, becoming progressively more primitive eastwards and with decreasing age. In the abundance of elements such as Ca, Na, P, Sr, V and Sc, and in isotopic signatures (Sr_i, ε_Nd and ε_Hf), there is a sharp step in composition between the supersuites in the west and east. In contrast, throughout the batholith the isotopic compositions of feldspar Pb and whole rock and zircon O (δ¹⁸O) are relatively uniform. These features are difficult to reconcile with infracrustal mixing between different mantle-derived magmas and significant amounts of a high level crustal component such as the Palaeozoic flysch. Those magmas would have to have different compositions in different linear strips at any given time, and to maintain those compositions as the belt of magmatism stepped eastwards. More likely the lower crust, from which the granite magmas were derived, consisted of meridional belts or strips of different chemical and isotopic compositions. As that crust was partially melted under the influence of fluids and/or melts released either by underplating or dehydration reactions within an underlying subduction-modified mantle wedge, the chemical and isotopic characteristics of the crust were reflected in the resultant granite magmas—the granites are images of their source.

The isotopically more primitive eastern granites of the small, isolated Moruya supersuite and adjacent Cobargo supersuite provide examples of the likely role of plate-boundary subduction processes. The internal trends in their isotopic compositions are best explained by fluids and partial melts from subducted altered oceanic crust (SAOC) together with a limited contribution from the Palaeozoic flysch. In particular, the high Sr contents and island-arc like range of Sr_i and δ¹⁸O above MORB values provide a distinctive signature of low degree partial melting of SAOC. A small component of subducted flysch (<5 %) is necessary to account for the shifts in ε_Nd and ε_Hf towards more continental crustal values, as well as the presence of small but persistent numbers of inherited cores in zircon. While there is no evidence in the western supersuites for direct slab-derived melts, the limited range of isotopic compositions in mobile elements such as Pb and O indicates that fluids played a major role in the production of the granitic magmas by partial melting of igneous lower crust.