Petrogenesis and tectonic setting of mid-Neoproterozoic low-δ18O metamafic rocks from the Leeuwin Complex, southwestern Australia

Abstract

Mid-Neoproterozoic low-δ18O metamafic rocks from the Leeuwin Complex, southwestern Australia, are reported for the first time. Sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating of these upper amphibolite- to granulite-facies mafic rocks yields igneous protolith ages of 674–660 Ma. The metamafic rocks are generally classified as subalkaline tholeiitic rocks with an ocean island basalt (OIB) affinity. They have low Mg# values (22–50) and Cr (0.19–105 ppm) and Ni (0.62–115 ppm) contents, with whole-rock εNd(t) values of − 1.4 to + 1.5 and zircon εHf(t) values of − 0.3 to + 3.5. Using these data in combination with the incompatible trace element characteristics, it is inferred that the protoliths of the rocks were derived from low-degree partial melting of relatively depleted asthenospheric mantle in a continental rift environment, and the magmas underwent some crustal contamination and fractional crystallization of mafic minerals. Zircon cores from the metamafic rocks yield δ18O values of 0.89 to 4.10‰, which are lower than normal mantle values (5.3 ± 0.3‰). These cores preserve oscillatory zoning or banding in cathodoluminescence images, and individual samples have concordant ages and preserve a narrow range of δ18O values, suggesting that the low-δ18O signatures are of primary magmatic origin. It is inferred that these low-δ18O metamafic rocks were generated by contamination by low-δ18O felsic crustal wall rocks and interaction of the magma with surface water at shallow depths in an extensional regime during the mid-Neoproterozoic.