Temporal, Isotopic and Spatial Relations of Early Paleozoic Gondwana-Margin Arc Magmatism, Central Transantarctic Mountains, Antarctica

The Cambrian-Ordovician Ross Orogeny in Antarctica produced a voluminous magmatic belt composed mainly of post-orogenic granitoids. This magmatic belt has strong calc-alkaline characteristics reflecting a convergent-margin origin associated with subduction of paleo-Pacific oceanic lithosphere beneath cratonic East Antarctica. However, it is unclear how and when magmatism began, and to what degree magmatism was associated with syn-orogenic deformation and intra-arc extension. New U-Pb zircon ages, and whole-rock geochemical and Sr-Nd isotope data for granitoids sampled along a transect across the Ross Orogen in the Nimrod Glacier area of the central Transantarctic Mountains provide constraints on the timing, spatial variation, and origin of the magmatism in this area. This transect is one of the few places where the orogenic arc extends into the East Antarctic cratonic basement, thus helping to constrain both craton and arc evolution. New U-Pb ages show that magmatism was initiated as early as similar to 590 Ma following latest Neoproterozoic rifting, that the magmatic belt is long-lived, lasting over about 100 Myr, and that the locus of magmatism shifted oceanward over time. Early syn-orogenic magmatism was focused within the leading edge of the cratonic basement, perhaps guided by strain partitioning during oblique subduction; younger magmas intruded a forearc sedimentary molasse basin, itself eroded from the earlier established arc system. Broadening of the arc during the later phases of Ross convergence indicates rollback of the subducting plate hinge and thickening of the developing forearc during continuing orogenic contraction. The granitoids mainly have calc-alkaline geochemical characteristics, but they show some similarity to adakitic compositions indicative of melt fractionation from the subducting slab; they do not show the alkaline signatures attributed in other areas to crustal extension. Inherited zircon components are compatible with melting of lower crust similar to that exposed at present in the Nimrod Group; however, the paucity of older cores suggests that melt production involved relatively large degrees of fractional melting at high temperature. Whole-rock Sr and Nd isotopic compositions vary systematically across the belt, as expected in a convergent-margin arc setting, and they show significant cratonic influence, with initial Sr-87/Sr-86 compositions as high as 0 center dot 750 and epsilon Nd values as low as -15 for granitoids intruding Nimrod Group basement. Although there is an isotopic discontinuity associated with the inferred cratonic rifted margin, the later phase of magmatism is characterized by uniformly low Sr-87/Sr-86 and high epsilon Nd, indicating that melt compositions are controlled more by subduction processes than by assimilation of existing crust in the cratonic upper plate.