A hybrid origin for Lachlan S-type granites: the Murrumbidgee Batholith example

The Murrumbidgee Batholith is typical of other composite S-type granite batholiths in the Lachlan Fold Belt (LFB) of southeastern Australia, consisting of discrete peraluminous, cogenetic granite suites. However, it contains a unique suite (Murrumbucka) at its southern extremity that has chemical and petrological features transitional with metaluminous I-type granites. Previous detailed structural and metamorphic studies have shown that the batholith is tilted northward, exposing subvolcanic plutons in the north and root zones in the south, originally located at depths of similar to10 km. A transition from mafic, foliated, sheet-like granites in the south to felsic, generally less foliated, homogeneous granites in the north is consistent with magma ascent via subvertical, structurally controlled sheets to emplacement in an overlying magma chamber. In the inferred root zones, the Murrumbucka Suite hosts migmatitic metasedimentary and gabbroic rocks, both of which have transitional contacts and show evidence for mafic-felsic interaction with the Murrumbucka Suite. The migmatites extend southward to become part of the high-T, low-P Cooma Metamorphic Complex, which contains a core of heterogeneous remobilised diatexitic granite (Cooma Suite), lenses of which also occur throughout the southern (deeper) parts of the Murrumbidgee Batholith. The composition of the most mafic rocks from the batholith lies on a chemical tie-line between Cooma Suite granites and the gabbros, for almost all elements. This chemical coincidence is interpreted to reflect derivation of parental Murrumbidgee S-type granite magmas by bulk mixing between a felsic (crustal) and matic (mantle-derived) component, consistent with Sr and Nd isotopic results and field observations. Based on the tie-lines and simple numerical modelling, the Murrumbucka suite is estimated to be a 50:50 mix of mantle- and crustal-derived magmas, whereas the more peraluminous and widespread Clear Range Suite, which is very typical of Lachlan S-type granites, is modelled as a 40:60 mix. Chemical variation trends diverge from the mixing lines and suggest that much of the fractionation process operated after mixing. Mixing occurred within sheets during the ascent of the magmas, whereas fractionation was the dominant process generating chemical diversity during emplacement in the magma chamber. Generation of the S-type granites probably occurred in an extensional tectonic environment. (C) 2004 Published by Elsevier B.V.