Proterozoic crustal evolution of central East Antarctica: Age and isotopic evidence from glacial igneous clasts, and links with Australia and Laurentia

Rock clasts entrained in glacial deposits sourced from the continental interior of Antarctica provide an innovative means to determine the age and composition of ice-covered crust. Zircon U-Pb ages from a suite of granitoid clasts collected in glacial catchments draining central East Antarctica through the Transantarctic Mountains show that crust in this region was formed by a series of magmatic events at ∼2.01, 1.88–1.85, ∼1.79, ∼1.57, 1.50–1.41, and 1.20–1.06 Ga. The dominant granitoid populations are ca. 1.85, 1.45 and 1.20–1.06 Ga. None of these igneous ages are known from limited outcrop in the region. In addition to defining a previously unrecognized geologic history, zircon O and Hf isotopic compositions from this suite have: (1) mantle-like δ18O signatures (4.0–4.5‰) and near-chondritic Hf-isotope compositions (εHf ∼ +1.5) for granitoids of ∼2.0 Ga age; (2) mostly crustal δ18O (6.0–8.5‰) and variable Hf-isotope compositions (εHf = −6 to +5) in rocks with ages of ∼1.88–1.85, ∼1.79 and ∼1.57 Ga, in which the ∼1.88–1.79 Ga granitoids require involvement of older crust; (3) mostly juvenile isotopic signatures with low, mantle-like δ18O (∼4–5‰) and radiogenic Hf-isotope signatures (εHf = +6 to +10) in rocks of 1.50–1.41 Ga age, with some showing crustal sources or evidence of alteration; and (4) mixed crustal and mantle δ18O signatures (6.0–7.5‰) and radiogenic Hf isotopes (εHf = +3 to +4) in rocks of ∼1.2 Ga age. Together, these age and isotopic data indicate the presence in cratonic East Antarctica of a large, composite igneous province that formed through a punctuated sequence of relatively juvenile Proterozoic magmatic events. Further, they provide direct support for geological correlation of crust in East Antarctica with both the Gawler Craton of present-day Australia and Proterozoic provinces in western Laurentia. Prominent clast ages of ∼2.0, 1.85, 1.57 and 1.45 Ga, together with sediment source linkages, provide evidence for the temporal and spatial association of these cratonic elements in the Columbia supercontinent. Abundant ∼1.2–1.1 Ga igneous and metamorphic clasts may sample crust underlying the Gamburtsev Subglacial Mountains, indicating the presence of a Mesoproterozoic orogenic belt in the interior of East Antarctica that formed during final assembly of Rodinia.Field and analytical portions of this project were supported by the National Science Foundation (award 0944645)

Las rocas huésped del magmatismo devónico en el macizo norpatagónico y Chaitenia

Trabajos anteriores han demostrado que el magmatismo devónico en los Andes meridionales se produjo en dos cinturones contemporáneos: uno emplazado en la corteza continental del Macizo Norpatagónico y el otro , hacia el oeste, en un arco de islas oceánico, Chaitenia, que más tarde se acrecionó a Gondwana. Las rocas hospedantes de las rocas plutónicas consisten en complejos metasedimentarios que aparecen esporádicamente en los Andes a ambos lados de la frontera entre Argentina y Chile, y adicionalmente de metabasaltos de almohadilla en Chaitenia. Las determinaciones de la edad de U-Pb de circones detríticos en 13 muestras de estas rocas metasedimentarias indican edades deposicionales máximas posibles de ca. 370 a 900 Ma, y se argumenta que la sedimentación es principalmente del Devónico similar a las pizarras fosilíferas de Buill. Procedencia del Ordovícico, del Cámbrico-tardío a Neoproterozoico y “Grenville” se ve en todas las rocas, a excepción de los afloramientos más occidentales donde predominan los zircones detríticos del Devónico. Además de una diferencia en los granos de zircón precámbricos, 76% versus 25% respectivamente, no hay variación sistemática en la procedencia del antepaís patagónico a Chaitenia, por lo que el arco de islas debe haber sido proximal al continente: su corteza más profunda no está expuesta pero se conocen varios afloramientos de rocas ultramáficas. Los bordes metamórficos desarrollados durante el Devónico en circones de las rocas del Macizo Norpatagónico no tienen su equivalente en los circones de las rocas metamórficas de bajo grado del sector chileno. Estas rocas metasedimentarias paleozoicas también fueron intruidas por granitoides del Pennsylvaniano y Jurásico.Previous work has shown that Devonian magmatism in the southern Andes occurred in two contemporaneous belts: one emplaced in the continental crust of the North Patagonian Massif and the other in an oceanic island arc terrane to the west, Chaitenia, which was later accreted to Patagonia. The country rocks of the plutonic rocks consist of metasedimentary complexes which crop out sporadically in the Andes on both sides of the Argentina-Chile border, and additionally of pillow metabasalts for Chaitenia. Detrital zircon SHRIMP U-Pb age determinations in 13 samples of these rocks indicate maximum possible depositional ages from ca. 370 to 900 Ma, and the case is argued for mostly Devonian sedimentation as for the fossiliferous Buill slates. Ordovician, Cambrian-late Neoproterozoic and “Grenville-age” provenance is seen throughout, except for the most westerly outcrops where Devonian detrital zircons predominate. Besides a difference in the Precambrian zircon grains, 76% versus 25% respectively, there is no systematic variation in provenance from the Patagonian foreland to Chaitenia, so that the island arc terrane must have been proximal to the continent: its deeper crust is not exposed but several outcrops of ultramafic rocks are known. Zircons with devonian metamorphic rims in rocks from the North Patagonian Massif have no counterpart in the low metamorphic grade Chilean rocks. These Paleozoic metasedimentary rocks were also intruded by Pennsylvanian and Jurassic granitoids.Centro de Investigaciones Geológica

Burdigalian deposits of the Santa Cruz Formation in the Sierra Baguales, Austral (Magallanes) Basin: Age, depositional environment and vertebrate fossils

Indexación: Web of Science; Scielo.ABSTRACT. A succession of marine and continental strata on the southern flank of Cerro Cono in the Sierra Baguales, northeast of Torres del Paine, can be correlated with stratigraphic units exposed along the southern border of the Lago Argentino region in Santa Cruz Province, Argentina. These include the Estancia 25 de Mayo Formation and the basal part of the Santa Cruz Formation. The lithological correlation is also confirmed by detrital zircon ages (maximum age of 18.23±0.26 Ma) and a rich assemblage of terrestrial vertebrate fossils, biostratigraphically equivalent to a postColhuehuapian, pre-Santacrucian South American Land Mammal Age (SALMA) fauna, suggesting a range of 19 to 17.8 Ma. Similar ages have been obtained from the basal part of the Santa Cruz Formation at Estancia Quién Sabe in southwestern Argentina, supporting the assumption of a regional continuity between these deposits. A measured lithostratigraphic column is presented and the depositional environment is interpreted as a coastal plain with small, meandering rivers and ephemeral floodplain lakes. The sedimentation coincides with intensified uplift of the Patagonian Andes during the ‘Quechua Phase’ of Andean tectonism, which is reflected by a change in paleocurrent directions from northwest to east-northeast. Keywords: Burdigalian, Santa Cruz Formation, Santacrucian SALMA, ‘Notohippidian’ fauna, Meandering rivers.RESUMEN. Una sucesión de estratos marinos y continentales en el flanco meridional del cerro Cono, en la sierra Baguales, al noreste de Torres del Paine, se correlaciona con estratos al sur de la región de lago Argentino en la Provincia de Santa Cruz, República Argentina. Estas unidades incluyen la Formación Estancia 25 de Mayo y la parte basal de la Formación Santa Cruz. La correlación litológica es, además, confirmada por datación de circones detríticos (edad máxima de 18,23±0,26 Ma) y un variado ensamble de vertebrados fósiles terrestres de edad post-Colhuehuapense a pre-Santacrucense en la escala de Edades Mamífero Sudamericanas (EMAS), con un rango temporal de entre 19 a 17,8 Ma. Edades similares han sido reportadas para la parte basal de la Formación Santa Cruz, en estancia Quién Sabe, en el suroeste de Argentina, ratificando la continuidad regional entre estos depósitos. Se presenta una columna estratigráfica y se interpreta el ambiente de depositación como una llanura costera con pequeños ríos sinuosos y lagos efímeros. La edad de sedimentación coincide con el solevantamiento de los Andes Patagónicos durante la 'Fase Quechua', lo que se ve reflejado por un cambio en la dirección de las paleocorrientes desde el noroeste hacia el este-noreste.http://ref.scielo.org/csxwd

Isotopic evidence for the diversity of late Quaternary loess in Nebraska: Glaciogenic and nonglaciogenic sources

Pb isotope compositions of detrital K-feldspars and U-Pb ages of detrital zircons are used as indicators for determining the sources of Peoria Loess deposited during the last glacial period (late Wisconsin, ca. 25–14 ka) in Nebraska and western Iowa. Our new data indicate that only loess adjacent to the Platte River has Pb isotopic characteristics suggesting derivation from this river. Most Peoria Loess in central Nebraska (up to 20 m thick) is non-glaciogenic, on the basis of Pb isotope ratios in K-feldspars and the presence of 34-Ma detrital zircons. These isotopic characteristics suggest derivation primarily from the Oligocene White River Group in southern South Dakota, western Nebraska, southeastern Wyoming, and northeastern Colorado. The occurrence of 10–25 Ma detrital zircons suggests additional minor contributions of silt from the Oligocene-Miocene Arikaree Group and Miocene Ogallala Group

Isotopic evidence for the diversity of late Quaternary loess in Nebraska: Glaciogenic and nonglaciogenic sources

Pb isotope compositions of detrital K-feldspars and U-Pb ages of detrital zircons are used as indicators for determining the sources of Peoria Loess deposited during the last glacial period (late Wisconsin, ca. 25–14 ka) in Nebraska and western Iowa. Our new data indicate that only loess adjacent to the Platte River has Pb isotopic characteristics suggesting derivation from this river. Most Peoria Loess in central Nebraska (up to 20 m thick) is non-glaciogenic, on the basis of Pb isotope ratios in K-feldspars and the presence of 34-Ma detrital zircons. These isotopic characteristics suggest derivation primarily from the Oligocene White River Group in southern South Dakota, western Nebraska, southeastern Wyoming, and northeastern Colorado. The occurrence of 10–25 Ma detrital zircons suggests additional minor contributions of silt from the Oligocene-Miocene Arikaree Group and Miocene Ogallala Group

Provenance Response to Rifting and Separation at the Jan Mayen Microcontinent Margin

Funding This research received no external funding. Acknowledgments We are grateful to John Still (University of Aberdeen) and Iain Macdonald (Cardiff University) for their assistance with acquisition of mineral chemical data, and to Mick Pointon (CASP) for running the “R” code to assign apatite provenance. This research was carried out as part of CASP’s Greenland-Norway Project. Our sponsors’ financial support is gratefully acknowledged. We are grateful for the reviewers comments, which significantly improved the manuscript.Peer reviewedPublisher PD

Hf- and O-isotope data from detrital and granitoid zircons reveal characteristics of the Permian–Triassic magmatic belt along the Antarctic sector of Gondwana

Permian–Triassic strata in the Transantarctic Mountains and West Antarctica carry a significant detrital component derived from a contemporaneous magmatic belt along the Gondwana margin. Hf- and O-isotope characteristics were determined for near-contemporaneous (as shown by U-Pb zircon geochronology) detrital igneous zircons in Upper Permian and Triassic sandstones. Zircons from six granitoids in the contemporaneous magmatic belt were also analyzed for Hf and O isotopes in order to gain insight into the potential detrital zircon sources. Although the ages of these granitoids only loosely correspond with the depositional ages of the sandstones, the initial εHf and δ18O isotope compositions for these igneous zircon grains, in general, overlap those recorded for the detrital igneous zircon grains. Results demonstrate a range of εHf and δ18O values. Features of particular interest are the very low δ18O values in two of the granitoids, and similar low values also recorded in the detrital igneous zircons in two sandstones. The distribution of Permian–Triassic granitoids must be much greater than is apparent from the existing outcrops in the extensively ice-covered region. The Permian and one of the Triassic granitoids have Hf-isotope characteristics similar to the Cretaceous granites and Devonian–Carboniferous plutons of West Antarctica, whereas the other Triassic granite differs from both. Importantly, the zircon isotopic data from the Permian–Triassic rocks suggest that an Hf-defined Upper Mesoproterozoic lithosphere underlies much of the magmatic belt

A-type magmatism in the sierras of Maz and Espinal: A new record of Rodinia break-up in the Western Sierras Pampeanas of Argentina

Two orthogneisses have been recognized in the sierras of Espinal and Maz (Western Sierras Pampeanas, NW Argentina) that were emplaced within a Grenvillian metasedimentary sequence. Microcline, plagioclase and quartz are the main rock-forming minerals, with accessory zircon, apatite-(CaF), magnetite, biotite (Fe/(Fe + Mg) = 0.88-0.91), ferropargasite (Fetotal/(Fetotal + Mg) = 0.88-0.89), titanite (with up to 1.61 wt% Y2O3) and an REE-rich epidote. REE-poor epidote and zoned garnet (Ca and Fe3+-rich) are metamorphic minerals, while muscovite, carbonates and chlorite are secondary phases. Texture is mylonitic. Two representative samples are classified as granite (from Sierra de Espinal) and granodiorite/tonalite (from Sierra de Maz) on the grounds of immobile trace elements. Some trace element contents are rather high (Zr: 603 and 891 ppm, Y: 44 and 76 ppm, 10,000 × Ga/Al: 2.39-3.89) and indicate an affiliation with A-type granites (more specifically, the A2 group). Both samples plot in the field of within-plate granites according to their Y and Nb contents. Concordant crystallization ages (zircon U-Pb SHRIMP) are 842 ± 5 and 846 ± 6 Ma, respectively. 87Sr/86Sri (845) ratios are 0.70681 and 0.70666; εNdi (845) values are -1.5 and +0.3 and depleted-mantle Nd model ages (2TDM*) are 1.59 and 1.45 Ga, respectively. These values indicate the involvement of an isotopically evolved source. 2TDM* values are compatible with the presence of inherited zircon crystals of up to 1480 Ma in one of the rocks, thus implying that magmas incorporated material from Mesoproterozoic continental source. This is also indicated by the relatively high contents of Y, Ga, Nb and Ce compared to magmas derived from sources similar to those of oceanic-island basalts. These orthogneisses represent a period of extension at ca. 845 Ma affecting the Western Sierras Pampeanas continental crust that was already consolidated after the Grenvillian orogeny (1.2-1.0 Ga). They are thus a record of the early stages of Rodinia break-up. Metamorphic conditions during the subsequent Famatinian orogenic cycle (ca. 420 Ma, SHRIMP U-Pb on zircon) attained 7.7 ± 1.2 kbar and 664 ± 70 °C.Centro de Investigaciones Geológica

U-Pb SHRIMP zircon dating of Grenvillian metamorphism in Western Sierras Pampeanas (Argentina) : correlation with the Arequipa-Antofalla craton and constraints on the extent of the Precordillera Terrane

The Sierras Pampeanas of Argentina, the largest outcrop of pre-Andean crystalline basement in southern South America, resulted from plate interactions along the proto-Andean margin of Gondwana, from as early as Mesoproterozoic to Late Paleozoic times (e.g., Ramos, 2004, and references therein). Two discrete Paleozoic orogenic belts have been recognized: the Early Cambrian Pampean belt in the eastern sierras, and the Ordovician Famatinian belt, which partially overprints it to the west (e.g., Rapela et al., 1998). In the Western Sierras Pampeanas, Mesoproterozoic igneous rocks (ca. 1.0–1.2 Ga) have been recognized in the Sierra de Pie de Palo (Fig. 1) (McDonough et al., 1993 M.R. McDonough, V.A. Ramos, C.E. Isachsen, S.A. Bowring and G.I. Vujovich, Edades preliminares de circones del basamento de la Sierra de Pie de Palo, Sierras Pampeanas occidentales de San Juán: sus implicancias para el supercontinente proterozoico de Rodinia, 12° Cong. Geol. Argentino, Actas vol. 3 (1993), pp. 340–342.McDonough et al., 1993, Pankhurst and Rapela, 1998 and Vujovich et al., 2004) that are time-coincident with the Grenvillian orogeny of eastern and northeastern North America (e.g., Rivers, 1997 and Corrievau and van Breemen, 2000). These Grenvillian-age rocks have been considered to be the easternmost exposure of basement to the Precordillera Terrane, a supposed Laurentian continental block accreted to Gondwana during the Famatinian orogeny (Thomas and Astini, 2003, and references therein). However, the boundaries of this Grenvillian belt are still poorly defined, and its alleged allochthoneity has been challenged (Galindo et al., 2004). Moreover, most of the Grenvillian ages so far determined relate to igneous protoliths, and there is no conclusive evidence for a Grenvillian orogenic belt, other than inferred from petrographic evidence alone (Casquet et al., 2001). We provide here the first evidence, based on U–Pb SHRIMP zircon dating at Sierra de Maz, for a Grenville-age granulite facies metamorphism, leading to the conclusion that a continuous mobile belt existed throughout the proto-Andean margin of Gondwana in Grenvillian times