Mesoproterozoic Fraser Complex: Geochemical evidence for multiple subduction-related sources of lower crustal rocks in the Albany-Fraser Orogen, Western Australia

The 1300 Ma Fraser Complex in the Albany-Fraser Orogen of Western Australia is a thrust stack of mainly gabbroic rocks metamorphosed to granulite facies. This package of fault-bounded units was elevated from a deep crustal level onto the margin of the Yilgarn Craton during continental collision between the Mawson and Yilgarn Cratons. Incompatible trace-element distributions demand at least three mantle sources. Primitive-mantle-normalised incompatible-element distributions show strong negative Ta-Nb anomalies, typical of subduction-derived magmas. Three lines of evidence indicate that the mafic magmas did not acquire these anomalies by assimilation of crustal rocks: (i) major-element compositions do not allow appreciable contamination with felsic material; (ii) Ni contents of many mafic rocks are too high for a significant contribution from a felsic assimilant; and (iii) Sr and Nd isotopic data support a largely juvenile source for the magmas that produced the Fraser Complex. Hence, the Ta-Nb anomalies are interpreted to reflect subduction-related magmatic sources. On multielement diagrams, depletions in Sr, Eu, P, and Ti can be explained by fractional crystallisation, whereas Th and Rb depletions in many of the Fraser Complex rocks probably reflect losses during granulite-facies metamorphism. These results suggest that the lower crust in this region at 1300 Ma was dominantly of arc origin, and there is no evidence to support mantle plume components. The Fraser Complex is interpreted as remnants of oceanic arcs that were swept together and tectonically interleaved with the margin of the Mawson Craton just before, or during, collision with the Yilgarn Craton at 1300 Ma.