Tectonic overview of the West Gondwana margin

The oceanic southern margin of Gondwana, from southern South America through South Africa, West Antarctica, New Zealand (in its pre break-up position), and Victoria Land to Eastern Australia is one of the longest and longest-lived active continental margins known. It was the site of the 18,000 km Terra Australis orogen, which was initiated in Neoproterozoic times with the break-up of Rodinia, and evolved into the Mesozoic Australides. The Gondwana margin was completed, in Late Cambrian times, by closure of the Adamastor Ocean (between Brazilian and southwest African components) and the Mozambique Ocean (between East and West Gondwana), forming the Brasiliano-Pan-African mobile belts. During the Early Palaeozoic much of the southern margin was dominated by successive episodes of subduction-accretion. Eastern Australia, Northern Victoria Land and the Transantarctic Mountains were affected by one of the first of these events – the Late Cambrian Ross/Delamerian orogeny, remnants of which may be found in the Antarctic Peninsula – but also contain two accreted terranes of unknown age and origin. Similar events are recognized at the South American end of the margin, where the Cambrian Pampean orogeny occurred with dextral strike-slip along the western edge of the Río de la Plata craton, followed by an Ordovician active margin (Famatinian) associated with the collision of the Precordillera terrane. However, the central part of the margin (the Sierra de la Ventana of eastern Argentina, the Cape Fold Belt of South Africa and the Ellsworth Mountains of West Antarctica) seem to represent a passive margin during the Early Palaeozoic, with the accumulation of predominantly reworked continental sedimentary deposits (Du Toit's ‘Samfrau Geosyncline’). In many of the outer areas, accretion and intense granitic/rhyolitic magmatism continued during the Late Palaeozoic, with collision of several small continental terranes, many of which are nevertheless of Gondwana origin: e.g., southern Patagonia and (possibly) ‘Chilenia’ in the South American–South African sectors, and the Western Province and Median Batholith terranes of New Zealand. The rhyolitic Permo–Triassic LIP of southern South America represents a Permo-Triassic switch to extensional tectonics, which continued into the Early Jurassic, and was followed by the establishment of the Andean subduction margin. Elsewhere at this time the margin largely became passive, with terrane accretion continuing in New Zealand. In the Mesozoic, the Terra Australis Orogen evolved into the accretionary Australides, with episodic orogenesis in the New Zealand, West Antarctic and South American sectors in Late Triassic–Early Jurassic and mid-Cretaceous times, even as Gondwana was breaking up

Sr, C and O isotope composition of marbles from the Sierra de de Ancasti, Eastern Sierras Pampeanas, Argentina: age and constraints for the Neoproterozoic–Lower Paleozoic evolution of the proto-Gondwana margin

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti Formation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician), metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the western side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran depositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and –tentativelythe Corumbá Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean, whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina) and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Rio de la Plata craton, which at the time occupied a position farther to the north.Peer reviewe

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

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

Granate con alto contenido de tierras raras pesadas (HREE) y elevada relación Sm/Nd, en pegmatitas de la Sierra de Valle Fértil (Sierras Pampeanas, Argentina).

2 p.El trabajo se realizó en el marco de lso proyectos PB97-1246 (MEC) y BTE2001-1486 (MCYT) y PICT98-4189 (Argentina

Pacific subduction coeval with the Karoo mantle plume: the Early Jurasssic Subcordilleran belt of northwestern Patagonia

The Early Mesozoic magmatism of southwestern Gondwana is reviewed in the light of new U-Pb SHRIMP zircon ages (181 ± 2 Ma, 181 ± 3 Ma, 185 ± 2 Ma, and 182 ± 2 Ma) that establish an Early Jurassic age for the granites of the Subcordilleran plutonic belt in northwestern Argentine Patagonia. New geochemical and isotopic data confirm that this belt represents an early subduction-related magmatic arc along the proto-Pacific margin of Gondwana. Thus, subduction was synchronous with the initial phase of Chon Aike rhyolite volcanism ascribed to the thermal effects of the Karoo mantle plume and heralding rifting of this part of the supercontinent. Overall, there is clear evidence that successive episodes of calc-alkaline arc magmatism from Late Triassic times until establishment of the Andean Patagonian batholith in the Late Jurassic involved westerly migration and clockwise rotation of the arc. This indicates a changing geodynamic regime during Gondwana break-up and suggests differential rollback of the subducted slab, with accretion of new crustal material and/or asymmetrical ‘scissor-like’ opening of back-arc basins. This almost certainly entailed dextral displacement of continental domains in Patagonia.Centro de Investigaciones Geológica

The continental assembly of SW Gondwana (Ediacaran to Cambrian): a synthesis

SW Gondwana resulted from complex interplay between continental amalgamation and dispersal between ~ 650 and 490 Ma. The main cratons involved were Laurentia, Amazonia– MARA (Proterozoic Maz–Arequipa–Rio Apa, Casquet et al., 2012), Kalahari, Rio de la Plata (RPC), Congo and East Antarctica (Mawson block). Several collisional orogenic belts resulted, notably the East Africa–Antarctica, Brasiliano–Panafrican, Pampean–Saldania, and Ross– Delamerian orogens. East-Antarctica broke away from the western margin of Laurentia in Rodinia. After a long drift and counter-clockwise rotation (Dalziel, 2013) it collided with Congo and Kalahari to produce the southern part of the left-lateral transpressional East Africa–Antarctica orogen between 580 and 550 Ma, completing the amalgamation of East Gondwana. The Trans-Antarctic margin became an active one in the Ediacaran and subduction of the Pacific Ocean lithosphere occurred throughout the Paleozoic, forming a tract of the Terra Australis orogen. NW–SE directed compression in Late Cryogenian and Early Ediacaran times promoted closure of the Adamastor Ocean, resulting in the left-lateral transpressional Brasiliano–Pan African orogeny between 650 and 570 Ma. The Pampean orogenic belt to the west of the RPC resulted from right-lateral collision between Laurentia and its eastern extension MARA on the one hand and Kalahari–RPC on the other. Ocean opening started at ~ 630 Ma and subduction and further collision took place between 540 and 520 Ma, coeval with the northward drift of Laurentia (~ 540 Ma) away from MARA and the consequent formation of the proto-Andean margin of Gondwana. The margins of the intervening Puncoviscana ocean were covered by Laurentia-derived siliciclastic sediments and carbonates on the MARA side between 630 and ~ 540 Ma (Rapela et al, 2014; this symposium), and by the marine siliciclastic Puncoviscana Formation on the other. The latter formation, deposited between a 570 and ~530 Ma, received input from large alluvial fans descending from juvenile Mesoproterozoic and Neproterozoic sources (new Hf isotope evidence) largely located in the southern East Africa–Antarctica orogen. The Pampean orogen extended into the Saldania–Gariep orogen of southern South Africa (545–520 Ma) and was apparently discordant to the earlier Brasiliano–Pan African orogen. In late-Early to late Cambrian times the Pampean–Saldania realm evolved into a passive margin with siliciclastic platform sedimentation. The Pampean-Saldania realm was separated from the active Trans-Antarctic margin of East Antarctica by an inferred transform fault in Ediacaran to Cambrian times. Regional NW–SW shortening in the Ediacaran became N–S directed in the Cambrian, suggesting a major plate reorganization at this time.Peer reviewe

Precise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central Antarctica

The 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

Cordieritite and Leucogranite Formation during Emplacement of Highly Peraluminous Magma: the El Pilón Granite Complex (Sierras Pampeanas, Argentina)

Cordieritites and highly peraluminous granites within the El Pilon granite complex, Sierras Pampeanas, Argentina, were emplaced during a medium-P, high-T metamorphic event during the initial decompression of a Cambrian orogen along the southwestern margin of Gondwana. Very fresh orbicular and massive cordieritite bodies with up to 90% cordieritite are genetically associated with a cordierite monzogranite pluton and a larger body of porphyritic granodiorite. The petrogenesis of this association has been studied using petrographical, mineralogical, thermobarometric, geochemical, geochronological and isotope methods. The granitic magmas were formed by anatexis of mid-crustal metamorphic rocks formed earlier in the Pampean orogeny. The cordieritites appear at the top of feeder conduits that connected the source region located at ∼6 kbar with the pluton emplacement level at 3·7 ± 0·3 kbar. A fall in the liquidus temperature of the melt during emplacement was produced by a marked increase in fluid activity owing to rapid decompression and assimilation of surrounding hydrous metapelitic schists, followed by isobaric crystallization. High-Mg cordierite crystallized early on biotite–sillimanite restitic mineral assemblages of the assimilated schists or at the wall of the feeder conduits. Strong convection in the small magma chamber caused flow segregation of cumulate cordierite and restite, developing leucogranites and highly evolved pegmatoids that are in isotopic equilibrium. Rapid ascent of highly peraluminous magmas might explain why emplacement of these granites was simultaneous with the metamorphic peak registered in neighbouring high-grade migmatite and granulite terranes.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

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