Precise U-Pb zircon ages and geochemistry of Jurassic granit

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TitlePrecise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central AntarcticaAbstractThe Ellsworth-Whitmore Mountain terrane of central Antarctica was part of the early Paleozoic amalgamation of Gondwana, including a 13,000 m section of Cambrian–Permian sediments in the Ellsworth Mountains deposited on Grenville-age crust. The Jurassic breakup of Gondwana involved a regional, bimodal magmatic event during which the Ellsworth-Whitmore terrane was intruded by intraplate granites before translation of the terrane to its present location in central Antarctica. Five widely separated granitic plutons in the Ellsworth-Whitmore terrane were analyzed for their whole-rock geochemistry (X-ray fluorescence), Sr, Nd, and Pb isotopic compositions, and U-Pb zircon ages to investigate the origins of the terrane magmas and their relationships to mafic magmatism of the 183 Ma Karoo-Ferrar large igneous province (LIP). We report high-precision (±0.1 m.y.) isotope dilution–thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from granitic rocks from the Whitmore Mountains (208.0 Ma), Nash Hills (177.4–177.3 Ma), Linck Nunatak (175.3 Ma), Pagano Nunatak (174.8 Ma), and the Pirrit Hills (174.3–173.9 Ma), and U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from the Whitmore Mountains (200 ± 5 Ma), Linck Nunatak (180 ± 4 Ma), Pagano Nunatak (174 ± 4 Ma), and the Pirrit Hills (168 ± 4 Ma). We then compared these results with existing K-Ar ages and Nd model ages, and used initial Sr, Nd, and Pb isotope ratios, combined with xenocrystic zircon U-Pb inheritance, to infer characteristics of the source(s) of the parent magmas. We conclude that the Jurassic plutons were not derived exclusively from crustal melts, but rather they are hybridized magmas composed of convecting mantle, subcontinental lithospheric mantle, and lower continental crustal contributions. The mantle contributions to the granites share isotopic similarities to the sources of other Jurassic LIP mafic magmas, including radiogenic 87Sr/86Sr (0.706–0.708), unradiogenic 143Nd/144Nd (εNd < –5), and Pb isotopes consistent with a low-µ source (where μ = 238U/204Pb). Isotopes and zircon xenocrysts point toward a crustal end member of predominantly Proterozoic provenance (0.5–1.0 Ga; Grenville crust), extending the trends illustrated by Ferrar mafic intrusive rocks, but contrasting with the inferred Archean crustal and/or lithospheric mantle contributions to some basalts of the Karoo sector of the LIP. The Ellsworth-Whitmore terrane granites are the result of mafic rocks underplating the hydrous crust, causing crustal melting, hybridization, and fractionation to produce granitic magmas that were eventually emplaced as post-Ferrar, within-plate melts at higher crustal levels as the Ellsworth-Whitmore terrane rifted off Gondwana (47°S) before migrating to its current position (82°S) in central Antarctica. Copyright 2016 Geological Society of America.AcknowledgementsThis project is a continuation of the early exploration and sample collection of J. Campbell Craddock (1959--1960, 1962--1963, 1964--1965 field seasons; deceased 2006), Ed Thiel (1959--1960, 1960--1961 field seasons; deceased 1961 in an Antarctic plane crash), and Gerald Webers (1964--1965 field season; deceased 2008) in central Antarctica. Details of the early exploration of central Antarctica can be found in the preface of Geological Society of America Memoir 170 (Webers et al., 1992). The original Mylar (2 — 3 ft) geologic maps in Appendices 2--5 (see footnote 1) have been archived at the Byrd Polar Institute, Ohio State University. John P. Craddock acquired these samples and materials in 2002. Anne Grunow contributed paleomagnetic cores (separated for zircons) from Pagano Nunatak. Mark Fanning is thanked for his direction of the sensitive highresolution ion microprobe (SHRIMP) analyses. John Splettstoesser (deceased 2016), Staci Loewy, and an anonymous reviewer greatly improved the clarity of the manuscript.

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1st AuthorCraddock, J.AuthorCraddock, J.Schmitz, M.Crowley, J.Larocque, J.Pankhurst, R.Juda, N.Konstantinou, A.Storey, B.Year2017JournalBulletin of the Geological Society of AmericaVolume129Number1/02/2017Pages118-136DOI10.1130/B31485.1URLhttps://www.scopus.com/inward/recor.....22557b3670c357e74367869beKeywordsGeochemistryGeochronologyGraniteIgneous rocksIsotopesLandformsLeadMass spectrometryMetalsRocksSilicate mineralsStrontiumZircon, Isotopic compositionLarge igneous provincesLithospheric mantleSensitive high-resolution ion microprobeSub-continental lithospheric mantlesThermal ionization mass spectrometryWhole-rock geochemistriesX ray fluorescence, Tectonics

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TypeArticleCitationCraddock, J., Schmitz, M., Crowley, J., Larocque, J., Pankhurst, R., Juda, N., Konstantinou, A. and Storey, B. (2017). Precise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central Antarctica. Bulletin of the Geological Society of America, 129(1-2): 118-136 IdentifierCraddock2017Antarctica NZ supported?YesNZARI?No