Articles | Volume 8, issue 2
Research article
05 Apr 2017
Research article |  | 05 Apr 2017

First magmatism in the New England Batholith, Australia: forearc and arc–back-arc components in the Bakers Creek Suite gabbros

Seann J. McKibbin, Bill Landenberger, and C. Mark Fanning

Abstract. The New England Orogen, eastern Australia, was established as an outboard extension of the Lachlan Orogen through the migration of magmatism into forearc basin and accretionary prism sediments. Widespread S-type granitic rocks of the Hillgrove and Bundarra supersuites represent the first pulse of magmatism, followed by I- and A-types typical of circum-Pacific extensional accretionary orogens. Associated with the former are a number of small tholeiite–gabbroic to intermediate bodies of the Bakers Creek Suite, which sample the heat source for production of granitic magmas and are potential tectonic markers indicating why magmatism moved into the forearc and accretionary complexes rather than rifting the old Lachlan Orogen arc. The Bakers Creek Suite gabbros capture an early ( ∼  305 Ma) forearc basalt-like component with low Th ∕ Nb and with high Y ∕ Zr and Ba ∕ La, recording melting in the mantle wedge with little involvement of a slab flux and indicating forearc rifting. Subsequently, arc–back-arc like gabbroic magmas (305–304 Ma) were emplaced, followed by compositionally diverse magmatism leading up to the main S-type granitic intrusion ( ∼  290 Ma). This trend in magmatic evolution implicates forearc and other mantle wedge melts in the heating and melting of fertile accretion complex sediments and relatively long ( ∼  10 Myr) timescales for such melting.

Short summary
The youngest, easternmost part of continental Australia formed in an ancient subduction zone by rifting of the crust. This drove melting of the underlying mantle, producing basaltic magmas that intruded the crust. We determined the composition and ages of these intrusives, finding that they predate other types of magmatic intrusion in the area. This indicates progression in magma types from basaltic (mantle origin), through mixed, to granitic (crust recycling) during the growth of this region.