The two-component model for the genesis of granitic rocks in southeastern Australia — Nature of the metasedimentary-derived and basaltic end members

Abstract

Granitic magmas in three areas of southeastern Australia are the product of melting of quartzofeldspathic metasedimentary rock (high-silica end member) combined with varying degrees of incorporation of mafic (dominantly basaltic) magma (low-silica end member). The link between granitic magma and local metasedimentary rock is established by identifying shared geochemical signatures in Sr, Na 2O and K 2O contents. High-silica end members have compositions formed in a genetic spectrum from total source mobilisation to partial melt segregation from metasedimentary rock. The position of a granitic end member in this spectrum is monitored by titanium contents varying from normal metasedimentary to the strongly depleted induced by partial melting. The titanium signature of partial melting is established in a type area with unequivocal links between migmatite mesosomes and leucosomes and the high-silica end members of granitic variation systems that extend to lower silica. The low-silica end members approximate high-alumina basalts on the basis of observed gabbros and projection of granitic variation lines to equivalent gabbroic compositions. High-alumina basalt is postulated to be a fundamental ingredient in the petrogenesis of orogenic granitic rocks.