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 calc-alkaline magmatic belt composed mainly of post-orogenic granitoids that reflect 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 isotopic data for granitoids in the Nimrod Glacier area constrain the timing, spatial variation, and origin of magmatism across this part of the Ross Orogen. It is one of the few places where the orogenic arc penetrates East Antarctic cratonic basement, thus helping to constrain both craton and arc evolution. New age data indicate that magmatism was initiated as early as 585 Ma following latest Neoproterozoic rifting, that the magmatic belt lasted over 100 m.y., and that the locus of magmatism shifted oceanward over time. Early syn-orogenic magmatism was focused within the leading edge of cratonic basement, perhaps guided by strain partitioning during oblique subduction. Younger magmas intruded a forearc sedimentary molasse basin, itself eroded from the already-established arc system. Broadening of the arc during late-stage convergence indicates rollback of the subducting plate hinge and thickening of the forearc during orogenic contraction. The granitoids are mainly calc-alkaline, but they have some adakitic characteristics indicative of melt fractionation from a subducting slab. Inherited zircon components indicate melting of lower crust like that presently exposed, but the small proportion of older cores suggests relatively large degrees of melting at high temperature. Whole-rock Sr- and Nd-isotopic compositions vary systematically as expected in a convergent-margin arc, and they show significant cratonic influence, with initial ⁸⁷Sr/⁸⁶Sr values as high as 0.750 and εNd as low as -15 for granitoids intruding cratonic basement. An isotopic discontinuity marks the inferred craton rift margin, yet the latter phase of magmatism is characterized by uniformly low ⁸⁷Sr/⁸⁶Sr and high εNd, indicating that melt compositions are controlled more by subduction process than by assimilation of existing crust in the cratonic upper plate.