SHRIMP U-Pb Geochronology of Transantarctic Mountains Basement

Southern Methodist University, Dallas TX

Investigators

Abstract

Goodge OPP 9912081 This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, provides funds for a geochronological investigation relating to the tectonic development of the East Antarctic continental shield. The East Antarctic shield represents one of Earth's oldest and largest cratonic provinces, with a long-lived Archean to Proterozoic history. It is the central piece in the eastern Gondwanaland mosaic, and it likely played an important role in the Neoproterozoic amalgamation of the Rodinia supercontinent. Because much of the shield is covered by the polar ice cap, our knowledge of early Archean to Neoproterozoic geologic events comes mostly from coastal outcrops in areas such as Enderby Land and Queen Maud Land. However, crustal thicknesses and geophysical patterns show that the Transantarctic Mountains orogenic belt forms the Pacific edge of the East Antarctic shield. Underlying the central Transantarctic Mountains are crystalline basement rocks of the shield itself (Nimrod Group), juxtaposed against Neoproterozoic and lower Paleozoic marginal-basin sedimentary assemblages (Beardmore and Byrd groups). Together, these rock units record Archean crustal growth, Proterozoic crustal modifications, sediment deposition across a Neoproterozoic rift margin, and early Paleozoic plate-margin convergence. The central Transantarctic Mountains is therefore a critical area along the Pacific margin of East Antarctica to study both the Precambrian crystalline shield and its deformed supracrustal cover. Recent U-Pb age dating of the Nimrod and Beardmore groups by use of the Super High Resolution Ion MicroProbe (SHRIMP) has greatly expanded our understanding of East Antarctic shield evolution and the transition from rift- to active-margin tectonics during the late Neoproterozoic and early Paleozoic. From a relatively small number of samples analyzed to date, SHRIMP analysis has demonstrated a multi-stage Archean and Proterozoic history of the Nimrod Group, and placed new constraints on the depositional age and sedimentary provenance of the Beardmore Group. This new project will significantly expand the geochronological framework of basement and supracrustal assemblages in the Transantarctic Mountains, with goals as follows: 1. Detailed study of metamorphic and igneous rocks from the Nimrod Group, in order to fully characterize the Archean and Proterozoic history of Precambrian igneous, metamorphic and detrital zircon components, as well as the Ross-age pre-, syn- and post-tectonic magmatic sequence. 2. Expanded study of siliciclastic rocks from the Beardmore Group and related lower Paleozoic sequences, primarily by age analysis of detrital zircons in order to constrain depositional ages and sedimentary provenance. With existing sample material and some provided by other researchers from other parts of the Transantarctic Mountains, it will be possible to address regional provenance variations among these transitional Neoproterozoic-lower Paleozoic sequences. Diagenetic xenotime will also be analyzed in order to attempt direct dating of the depositional age of the different rock units. To complete this study, SHRIMP U-Pb zircon and xenotime analyses will be completed in collaboration with researchers at the Australian National University (ANU) in Canberra. Analyses will be completed in the SHRIMP facility during a 6-month period from July to December, 2000. ANU will provide sample processing at no charge and instrument access at collaborative rates. Therefore, the full costs of this work are to be shared between NSF (SHRIMP machine time and incidental support costs), Southern Methodist University (salary and basic office support), and ANU (reduced in-house instrument rates, no charge for sample processing, and sabbatical support with office space, library access, and computer support).