Granite production in the Delamerian Orogen, South Australia

Abstract

In the South Australian sector of the Cambro-Ordovician Ross–Delamerian Orogen, granites range in age from Mid-Cambrian to Early Ordovician. Their occurrence is largely confined to deep, Early Cambrian, sediment-filled basins where they are associated with mafic rocks. The syntectonic suites have compositions forming a continuum between I- and S-type granites. After the cessation of convergent deformation at c. 490 Ma an abrupt transition to a bimodal magmatic association of mafic intrusions and felsic granites and volcanic rocks of S- and A-type affinities occurred. As exposed on the south coast of Kangaroo Island, S-type granite originated as in situ partial melts of the Early Cambrian sediments locally intruded by either mafic magmas or I–S granite magmas. These migmatite complexes were mingled with intrusions from the magmas that provided the underlying heat sources. Also on Kangaroo Island, composite S-type rhyodacite–dolerite dykes indicate that crustal melting involved mantle-derived melts. Field observations, major and trace element data and Nd–Sr isotopic data indicate that granite magmas in this fold belt result from mixing of crustal and mantle source components, and from fractional crystallization (AFC-type processes). Whereas the Nd–Sr compositions of granite suites from the Delamerian Orogen form a continuous geochemical trend between the crust and the mantle melts, the A- and I-types cluster towards the mantle endmember and the S-types towards the crustal endmember. This dichotomy reflects three granite magma production situations: (1) lower-crustal mafic magma chambers that are contaminated by, and mingled with, melts of the local metasediments producing I-type magmas; (2) crustal melts formed in the heated zones above upwelling mantle or close to mafic or I-type granite intrusions producing S-type magmas; (3) upper-crustal mafic intrusions where closed-system fractionation dominates to produce A-type granite. The extent of fractionation and crustal assimilation varied progressively through the c. 30 Ma deformation history (514–485 Ma) of this orogen. Importantly in this sector of the Ross–Delamerian Orogen, the crustal endmember is represented only by the Cambrian basin sedimentary fill (Kanmantoo Group) and expressly excludes the older Precambrian crust.