Chemical fingerprinting of multiple large-scale magmatic events in the Mesoproterozoic Bangemall Supergroup, Western Australia

Three major igneous events, dated at ∼1465, ∼1070 and ∼500 Ma, are represented in the Proterozoic of central Western Australia, yet their extent is poorly understood. The compositions of dated mafic rocks from the western Bangemall Supergroup of Western Australia have been used to establish a chemical fingerprint for the ∼1465 Ma and ∼1070 Ma intrusive events, and to assign sills of an unknown age to one of the two events. A similar approach has been used to identify the extent of ∼500 Ma magmatism. Low-pressure fractionation or accumulation has exerted a strong influence on the chemistry of rocks from all magmatic events, but distinctive trace-element ratios (e.g. Th/Nb, Nb/Zr) and rare-earth element (REE) chemistry (e.g. Eu/Eu*, (Gd/Yb)CN) can discriminate different events. These ratios remain constant regardless of the degree of fractionation or accumulation, and reflect the chemistry of the respective mantle sources. Based on chemistry, ∼500 Ma igneous rocks are not found in the Bangemall Supergroup. Neodymium model ages for ∼1465 Ma sills overlap with crystallisation ages of subduction-related felsic intrusive rocks from the adjacent Gascoyne Complex, and this, combined with trace-element and REE chemistry, suggests that the mantle source for these sills underwent ∼5% crustal contamination approximately 450 Ma prior to melting in a subduction zone environment. Unrealistically large amounts of contaminant are required to explain the chemistry of most ∼1070 Ma sills, and their chemistry is better explained by melting of a heterogeneous mantle source, consistent with the overlap of Nd model ages and crystallisation ages. However, the relatively low εNd(T) and high 87Sr/86Sr(T), elevated light REE, Rb and Zr concentrations, and high Th/Nb of some ∼1070 Ma eastern Bangemall Supergroup sills are indicative of crustal contamination. These sills are relatively depleted in chalcophile elements and platinum-group elements, consistent with coeval precipitation of sulfides and crustal contamination. The overlap in the maximum depositional age of host-rocks with the crystallisation age of some sills, the confining of most ∼1465 Ma sills to older parts of the stratigraphy, and field evidence showing that some sills belonging to both the ∼1465 and ∼1070 Ma events were intruded into wet sediments, indicate a close temporal relationship between sedimentation and sill intrusion.