Mafic rocks spatially associated with Devonian felsic intrusions of the southern Lachlan Fold Belt: a possible mantle contribution to crustal evolution processes

Devonian basaltic to andesitic dykes and compositionally similar plutons of the southern Lachlan Fold Belt are often temporally and spatially closely associated with large granitic complexes, Mafic intrusions play a major rol e in the transfer of heat into the continental crust, providing a thermal ' engine' which leads to crustal melting, and geochemical/isotopic evidence i ndicates that they contribute chemical constituents to the products of this melting. Studied mafic-intermediate dykes in the southern Lachlan Fold Bel t have tholeiitic to alkaline affinities and include groups with both high and low Ti and K. Several dyke generations may be associated with a single felsic complex. Primitive mantle-normalised trace-element abundance pattern s with negative Nb and Ti anomalies for basaltic/andesitic and gabbroic/dio ritic rocks as young as Early Devonian most resemble those of modern island arcs and suggest an influence of subduction on mantle magma sources. Howev er, some Middle and Late Devonian mafic rocks are enriched in light rare-ea rth elements and other incompatible elements. lack significant Nb anomalies , and confirm the change to continental-rift extensional settings clearly i ndicated by Lachlan Fold Belt geology.