Because of extensive ice cover and sparse remote-sensing data, the geology of the Precambrian East Antarctic Shield (EAS) remains largely unexplored with information limited to coastal outcrops from the African, Indian and Australian sectors. The East Antarctic lithosphere is globally important: as one of the largest coherent Precambrian shields, including rocks as old as ~3.8 Ga, it played an important role in global crustal growth; it is a key piece in assembly of the Rodinia and Gondwana supercontinents; it is the substrate to Earth?s major ice cap, including numerous sub-glacial lakes, and influences its thermal state and mechanical stability; and its geotectonic association with formerly adjacent continental blocks in South Africa, India and Australia suggest that it might harbor important mineral resources. This project will increase understanding of the age and composition of the western EAS lithosphere underlying and adjacent to the Transantarctic Mountains (TAM) using U-Pb ages, and Hf- and O-isotope analysis of zircon in early Paleozoic granitoids and Pleistocene glacial tills. TAM granites of the early Paleozoic Ross Orogen represent an areally extensive continental-margin arc suite that can provide direct information about the EAS crust from which it melted and/or through which it passed. Large rock clasts of igneous and metamorphic lithologies entrained in glacial tills at the head of major outlet glaciers traversing the TAM provide eroded samples of the proximal EAS basement. Zircons in these materials will provide data about age and inheritance (U-Pb), crustal vs. mantle origin (O isotopes), and crustal sources and evolution (Hf isotopes). Integrated along a significant part of the TAM, these data will help define broader crustal provinces that can be correlated with geophysical data and used to test models of crustal assembly.
Broader impacts: This project will provide a research opportunity for undergraduate and graduate students. Undergraduates will be involved as Research Assistants in sample preparation, imaging, and analytical procedures, and conducting their own independent research. The two main elements of this project will form the basis of MS thesis projects for two graduate students at UMD. Through this project they will gain a good understanding of petrology, isotope geochemistry, and analytical methods. The broader scientific impacts of this work are that it will help develop a better understanding of the origin and evolution of East Antarctic lithosphere underlying and adjacent to the TAM, which will be of value to the broader earth science and glaciological community. Furthermore, knowledge of East Antarctic geology is of continuing interest to the general public because of strong curiosity about past supercontinents, what?s under the ice, and the impact of global warming on ice-sheet stability.
Our main objective was to sample material from rock outcrop and glacial deposits that can help build a better picture of the continent hidden beneath the polar ice cap of Antarctica. In this project, our team sampled granitic rocks exposed in the Transantarctic Mountains and glacial boulders stranded in the ice next to the mountains that have been scraped off of the East Antarctic continent. Granites are produced by the melting of existing crust. Granites made in this way give us geochemical tracers of the rocks that were melted, and that we cannot see at the surface. The East Antarctic ice sheet is a huge lens of ice with an average thickness of over 8,000 feet. As the ice in this lens spreads laterally, it erodes rock of the continent below and carries this debris to the edge of Antarctica. Where the ice flows up against the Transantarctic Mountains, it carries these rocks along with it, and the ice ablates by wind erosion, leaving behind a lag of rock clasts. Glacial boulders transported from the interior of the continent by ice flow thus represent pieces of the hidden Antarctic crust that were eroded by glaciers flowing from the spreading ice sheet. Rock samples we collect from glacial moraines can give us geochemical clues about what is otherwise hidden by the ice cap. We completed a field season in Antarctica in 2010-11 with a 5-person field party. Ten sampling sites along the Transantarctic Mountains from the Convoy Range to Hatcher Bluffs were visited by helicopter or fixed-wing aircraft (see figure), where rock samples were collected. All samples were returned to the University of Minnesota-Duluth, where they were prepared for laboratory study. Laboratory work includes examination of polished thin sections by optical microscope and scanning electron microscope to determine textures, mineral assemblages, and mineral compositions. Samples of igneous and metamorphic rock clasts were crushed in order to isolate the mineral zircon; zircon from these samples was analyzed by U-Pb, O and Hf isotopic analysis in order to determine their ages and isotopic character. Monazite was identified in selected samples for U-Pb age dating in polished thin section. A suite of Ross Orogen granitoids was also prepared for zircon separation and for whole-rock geochemical analysis to determine the origin of the granitic melts. Monazite U-Pb ages were combined with geothermobarometry to interpret P-T-t histories for the metamorphic rocks. Briefly, we found the following: Igneous glacial clasts. New U-Pb zircon ages from glacial igneous clasts collected from drainages crossing the Transantarctic Mountains demonstrate that the crust in central East Antarctica was formed by a series of 2.00-1.90, 1.88-1.85, 1.75-1.73, 1.58-1.55, 1.48-1.43, and 1.20-1.10 billion year-old (Ga) magmatic events. The dominant igneous populations are 1.85, 1.45 and 1.18 Ga, with some showing metamorphic overprinting at 1.15-1.18 Ga. Together, these disparate and unique lines of evidence indicate the presence in cratonic East Antarctica of a large, composite Proterozoic igneous province that reflects crustal growth across central East Gondwana, and they provide direct geologic support for Rodinia and Nuna supercontinent reconstructions. The persistence of ~1.1 Ga Grenville-age igneous and metamorphic signatures in the interior may reflect the latest stages of Rodinia assembly, and may provide the best sampling to date of crust underlying the Gamburtsev Subglacial Mountains. Igneous isotopic compositions. In addition to U-Pb ages, we determined zircon Hf and O isotopic compositions of the igneous clasts in order to evaluate crustal history. Among the granitoid age populations, the age and isotopic data provide the first glimpse of crustal growth in central East Antarctica and suggest a varied history of relatively juvenile Proterozoic magmatism. Metamorphic glacial clasts. In addition to the igneous clasts, petrologic, geochemical, and isotopic analysis of metamorphic clasts from glacial moraines improve our understanding of the geologic history of the ice-covered shield. Seven metamorphic clasts from three different sample sites are semi-pelitic leucogneisses with a moderate- to high-P assemblages. In-situ SHRIMP U-Pb monazite ages record previously unknown Paleoproterozic tectonometamorphic events in central East Antarctica, and a younger Mesoproterozoic metamorphism corresponds to recently identified magmatic events. Ages from younger samples (~545-595 Ma) provide evidence that Ross metamorphism was initiated significantly earlier than previously thought. Ross Orogen granitoids. Granitoids from the Transantarctic Mountains serve as proxies of crustal age and evolution. New U-Pb age data, coupled with whole-rock geochemical compositions and zircon O- and Hf-isotope analyses, from a geographically diverse suite of granite samples provide a wealth of new geochronologic, tracer and inheritance information. SHRIMP zircon U-Pb ages from these samples range from 476-549 Ma. Geochemically the Ross magmatic suite shows major and trace element characteristics of Cordilleran-type, calc-alkaline volcanic-arc magmas. SHRIMP zircon δ18O values range from mantle to crustally-derived magmatic values.There is a strong inverse correlation between εHf and δ18O indicates that more crustal-like O-isotope compositions correlate with less radiogenic Hf.
