Chemical U -Th -Pb monazite ages from metasedimentary and meta-igneous units of the Willyama Supergroup have confirmed initial SHRIMP U -Pb metamorphic zircon ages constraining the onset of the earliest tectonometamorphic event (the Olarian Orogeny) at ca 1610 Ma in the southern Curnamona Province. An additional episode of high-grade metamorphism and heterogeneously distributed retrograde metamorphism and monazite recrystallisation occurred between ca 1570 and 1550 Ma. On the basis of monazite chemical U -Th -Pb ages from across the southern Curnamona Province, tectonometamorphic models based on ca 1690 Ma low-P, high-T metamorphism in the southern Curnamona Province are not supported. Furthermore, tectonic reconstructions that rely on the correlation of ca 1690 Ma deformation and metamorphism in the Broken Hill region with a similar aged event in the Mojave tarrane of southwestern Laurentia (AUSWUS) are not supported.
Mg-rich and Fe-rich metatholeiites intruded the Willyama Supergroup of the southern Australian Curnamona Province in the Late Palaeoproterozoic at ca 1700 Ma and 1685 Ma, respectively. Intrusion of the Fe-rich metatholeiites occurred during a period of punctuated extension in the Willyama basin. Major-element concentrations are variable (SiO 2 45.4 -56.5 wt%; Fe 2 O 3 * 8.5 -20.7; TiO 2 0.46 -2.52 wt%; Mg# 70.5 -29.1) and, in conjunction with trace-element data, support near-closed-system fractionation of a mantle-derived melt with little or no replenishment. Fractionation produced progressively Fe-rich derivative melts. Crystallising phases were dominated by clinopyroxene and olivine, whereas Fe -(Ti) oxide crystallisation was hindered. Primitive mantle-normalised immobile trace elements are characterised by variable Th, Nb, Sr, P and Ti anomalies. Chondrite-normalised rare-earth element patterns for the most primitive, Mg-rich samples from the western Broken Hill Domain have La N /Sm N 5 1, whereas the most evolved Fe-rich samples from the Olary Domain have ratios of La N /Sm N 4 1. Initial eNd values range between -2.2 and þ 2.7 for the majority of the samples, with the isotopic compositions showing no correlation with differentiation or assimilation. The combined geochemical and isotopic data suggest that the southern Curnamona Province metatholeiites were extracted from a depleted mantle in the western Broken Hill Domain, and a variably enriched, heterogeneous subcontinental lithospheric mantle in the Olary Domain. Magmatism most likely occurred in a backarc basin or intracontinental setting. It is speculated that the geochemically enriched mantle component was derived from subduction-related processes, probably related to pre-Willyama basin accretionary processes along the southern and eastern margins of the North Australian Craton.
Monazite electron microprobe U–Th–Pb and garnet Sm–Nd isotopic data from metapelitic assemblages in the Willyama Supergroup in the southern Curnamona Province, south‐central Australia, indicate that the terrain underwent regional greenschist to amphibolite‐grade metamorphism during the c. 500 Ma Delamerian Orogeny. The Delamerian‐aged mineral assemblages include prograde garnet–staurolite and kyanite‐bearing associations that overprint andalusite‐ and sillimanite‐bearing assemblages that are interpreted to have developed during the c. 1600 Ma Olarian Orogeny. Importantly, the development of secondary kyanite‐bearing assemblages in the southern Curnamona Province has been used previously to suggest that the Olarian Orogeny followed an anticlockwise P–T evolution. If such assemblages are the product of c. 500 Ma metamorphism, then the anticlockwise P–T path is an apparent path, due to the overprint of a distinct metamorphic cycle c. 1100 Ma later. Making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny. This study highlights the utility of in situ geochronology, linking age data to petrologically important phases and assemblages.
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