Abstract

Siliciclastic rocks in the Transantarctic Mountains record the tectonic transformation from a Neoproterozoic rift-margin setting to a passive-margin and ultimately to an active early Paleozoic orogenic setting along the paleo–Pacific margin of East Antarctica. New U-Pb detrital-zircon ages constrain both the depositional age and sedimentary provenance of these strata. In the central Trans-antarctic Mountains, mature quartz arenites of the late Neoproterozoic Beardmore Group contain Archean and Proterozoic zircons, reflecting distal input from the adjacent East Antarctic shield, Mesoproterozoic igneous provinces, and Grenville-age parts of East Gondwana. Similarly, basal sandstones of the Lower Cambrian Shackleton Limestone (lower Byrd Group) contain zircons reflecting a dominantly cratonic shield source; the autochthonous Shackleton was deposited during early Ross orogenesis, yet its basal sandstone indicates that the inner shelf was locally quiescent. Detrital zircons from the Koettlitz Group in southern Victoria Land show a similar age signature and constrain its depositional age to be ≤ 670 Ma. Significant populations (up to 22%) of ca. 1.4 Ga zircons in these Neoproterozoic and Lower Cambrian sandstone deposits suggest a unique source of Mesoproterozoic igneous material in the East Antarctic craton; comparison with the trans-Laurentian igneous province of this age suggests paleogeographic linkage between East Antarctica and Laurentia prior to ca. 1.0 Ga. In strong contrast, detrital zircons from upper Byrd Group sandstones are dominated by young components derived from proximal igneous and metamorphic rocks of the emerging Ross orogen. Zircon ages restrict deposition of this syn- to late-orogenic succession to ≤ 520 Ma (Early Cambrian or younger). Sandstone samples in the Pensacola Mountains are dominated by Grenville and Pan-African zircon ages, suggesting a source in western Dronning Maud Land equivalents of the East African orogen. When integrated with stratigraphic relationships, the detrital-zircon age patterns can be explained by a tectonic model involving Neoproterozoic rifting and development of a passive-margin platform, followed by a rapid transition in the late Early Cambrian (Botomian) to an active continental-margin arc and forearc setting. Large volumes of molassic sediment were shed to forearc marginal basins between Middle Cambrian and Ordovician time, primarily by erosion of volcanic rocks in the early Ross magmatic arc. The forearc deposits were themselves intruded by late-orogenic plutons as the locus of magmatism shifted trenchward during trench retreat. Profound syntectonic denudation, followed by Devonian peneplanation, removed the entire volcanic carapace and exposed the plutonic roots of the arc.

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