Granite production in the Delamerian Orogen, South Australia

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 occured. 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 of 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.