首页|Petrogenesis and tectonic setting of mid-Neoproterozoic low-delta O-18 metamafic rocks from the Leeuwin Complex, southwestern Australia
Petrogenesis and tectonic setting of mid-Neoproterozoic low-delta O-18 metamafic rocks from the Leeuwin Complex, southwestern Australia
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NSTL
Elsevier
Mid-Neoproterozoic low-delta O-18 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 epsilon(Nd)(t) values of 1.4 to + 1.5 and zircon epsilon(Hf)(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 delta O-18 values of 0.89 to 4.10 parts per thousand, which are lower than normal mantle values (5.3 +/- 0.3 parts per thousand). These cores preserve oscillatory zoning or banding in cathodoluminescence images, and individual samples have concordant ages and preserve a narrow range of delta O-18 values, suggesting that the low-delta O-18 signatures are of primary magmatic origin. It is inferred that these low-delta O-18 metamafic rocks were generated by contamination by low-delta O-18 felsic crustal wall rocks and interaction of the magma with surface water at shallow depths in an extensional regime during the mid-Neoproterozoic.
NSTL
Elsevier
