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Hartmann L.A.,Federal University of Rio Grande do Sul | Philipp R.P.,Federal University of Rio Grande do Sul | Santos J.O.S.,Redstone Resources | McNaughton N.J.,Curtin University Australia
Gondwana Research

The time frame of the three main geological events in the Neoproterozoic Cambaí Complex, juvenile São Gabriel belt in the southern Brazilian Shield is established by integrating field mapping, back-scattered electron imaging and sensitive high-resolution ion microprobe (SHRIMP II) U-Pb dating of 96 zircon crystals from nine granitic and metasedimentary rock samples. The three events are: (1) voluminous flat-lying paragneisses (Cambaizinho Complex) and orthogneisses (Vila Nova gneisses) between 735 and 718. Ma, (2) tonalite-trondhjemite association (Lagoa da Meia-Lua Suite) between 710 and 690. Ma, and (3) late granodiorite intrusions (Sanga do Jobim Suite) at 680. Ma. An additional older volcanic event (Campestre Formation) was dated at 753. Ma. These results are most significant for the reconstruction of West Gondwana. © 2010 International Association for Gondwana Research. Source

Teixeira W.,University of Sao Paulo | Geraldes M.C.,State University of Rio de Janeiro | D'Agrella-Filho M.S.,University of Sao Paulo | Santos J.O.S.,Redstone Resources | And 3 more authors.
Journal of South American Earth Sciences

The Figueira Branca Suite (FBS) comprises a layered mafic-ultramafic complex which together with mafic-felsic plugs makes up a string of NW-trending intrusive bodies that are emplaced into the Jauru domain (Rio Negro-Juruena province; 1.80-1.60Ga). This domain comprises Orosirian calc-alkaline rocks and coeval metamorphic volcanic-sedimentary associations, intruded by voluminous granitoid plutons resulted from outboard Cachoeirinha (1587-1522Ma) and Santa Helena (1485-1420Ma) accretionary orogens that eventually created the Rondonian-San Ignacio province along the SW margin of the proto-Amazonian Craton. SHRIMP U-Pb age in zircon for one cumulatic gabbro from the FBS yielded a concordia intercept age of 1425.5±8.0Ma (MSWD=1.11). Another gabbroic plug which crops out to the East gives a similar within error concordia intercept zircon age of 1415.9±6.9Ma (MSWD=0.25), whereas a nearby monzogranite yields a concordia intercept zircon age of 1428.9±2.8Ma (MSWD=1.30). All these results are crystallization ages and constrain an important intraplate magmatic event within the Orosirian continental crust at the time of outboard Santa Helena orogen. On the other hand, igneous titanite from another gabbro located to the West of the FBS yielded a weighted mean 207Pb/ 206Pb crystallization age of 1541±23Ma (MSWD=0.74). Therefore this rock is not genetically associated with the FBS, as previously suggested by the field information. Additional Nd-Sr isotopic analyses of the FBS mafic-ultramafic rocks and coeval gabbro showed comparable e{open} Nd(1.42Ga) values (+3.0 to +4.7) and variable e{open} Sr(1.42Ga) ones (-39.1 to -8.1). These data plot in the depleted field quadrant of the Nd-Sr diagram, indicating a significant influence of the MORB end-member reservoir in the magma genesis. This interpretation is similarly supported by comparison of the Nd evolutionary path of the FBS with those that characterize the isotopic evolution of the Jauru crust and the Cachoeirinha and Santa Helena intrusive plutons. The bulk isotopic signature allows the genetic relationship among the Santa Helena Suite and the FBS and coeval rocks. The anorogenic character of the FBS is supported by the geologic framework of the Jauru domain, given that the emplacement took place under an extensional regime, associated with the recognized regional NW-trending structures. 40Ar- 39Ar analyses were carried out in mafic rocks that are assigned to the FBS. Biotite from a gabbronorite yielded an ideogram age of 1222±5Ma while a nearby troctolite yielded plateau biotite ages of 1275±4Ma and 1268±4Ma. These ages are minimum estimates of the regional cooling of the FBS. One gabbro of the Alto Jauru Group yields a 40Ar- 39Ar plateau age of 1781±15Ma, interpreted as the time of regional cooling that succeeded continental accretion and metamorphism at Orosirian times. Later on heterogenous crustal thickening and uplift took place from West to East, as response from the Cachoeirinha and Santa Helena orogen dynamics, which is marked by the 1539-1510Ma and 1452-1322Ma 40Ar- 39Ar age-patterns, respectively. The available apparent ages suggest that Cachoeirinha crust was subjected to a fast exhumation rate (ca. 50m.y.), as estimated by the zircon-titanite-hornblende-biotite time-path, while the Santa Helena crust seems to display a lower exhumation rate. Particularly, the youngest 40Ar- 39Ar biotite age (1322Ma) available for the Santa Helena crust signals the post-tectonic phase of the Rondonian-San Ignacio province. © 2011 Elsevier Ltd. Source

Zappettini E.O.,Servicio Geologico Minero Argentino | Chernicoff C.J.,Servicio Geologico Minero Argentino | Chernicoff C.J.,CONICET | Santos J.O.S.,University of Western Australia | And 2 more authors.
Revista de la Asociacion Geologica Argentina

The Santa Helena Schists comprise biotite-garnet schists and minor sillimanite-garnet gneissoid lenses, reaching the higher amphibolite to lower granulite facies metamorphism. Suboutcrops of the schists occur at Estancia Santa Helena. U-Pb SHRIMP dating of the Santa Elena Schists detrital zircon yielded mostly Mesoproterozoic and Neoproterozoic ages, with a small number of Paleoproterozoic grains. Almost all the Mesoproterozoic and Neoproterozoic grains are magmatic and crystallized from a juvenile source (positive εHf), with TDM (Hf) ranging 1127 to 1625 Ma, and 948 to 1274 Ma, respectively. The Paleoproterozoic zircons are also magmatic, with a TDM (Hf) of 2310 Ma. The age of the youngest detrital zircon is 556 Ma, which corresponds to the maximum age for the onset of sedimentation, indicating that deposition probably started by Neoproterozoic times. The absolute absence of metamorphic zircons of Pampean age suggests that during the time of deposition of the schists, the metamorphic nucleous of the Pampean orogen was still not formed and exposed. This implies a great contrast with the provenance pattern of the nearby Green Schists, dominated by Cambrian metamorphic zircons, deposited between ca. 500 Ma and ca. 465 Ma, when the Pampean orogen was already exhumed and subject to denudation. It is interpreted that the Santa Elena Schists would pertain to a supracrustal sequence deposited between 556 and ca. 530-520 Ma in the southwestern Gondwana margin, in a foreland basin equivalent to the later stage of the Puncoviscana basin. It is also interpreted that the main source of sediments was derived from the denudation of a Mesoproterozoic magmatic arc, located to the east of the schists. This arc, equivalent to the precollisional magmatism of the Juruena and Sunsás orogens, would have been accreted to the southwestern margin of the Rio de la Plata craton and, at present, would form part of the substratum of its southwestern Paleoproterozoic ́rim ́ and the substratum of the para-authochtonous Pampia terrane. This mechanism of accretion would be similar to that of other Archean-Paleoproterozoic cratonic nuclei, such as the Amazon, Kalahari and Congo. Finally, the other important source of provenance of the Santa Elena Schists indicates the existence of a belt of Neoproterozoic magmatic rocks that would pertain to an arc developed on the eastern margin of Pampia. Source

Chernicoff C.J.,CONICET | Zappettini E.O.,Argentine Geological Mining Survey SEGEMAR | Santos J.O.S.,University of Western Australia | Santos J.O.S.,Redstone Resources | And 2 more authors.
Gondwana Research

The present study led to the identification of a largely hidden southern segment of the Famatinian magmatic arc in the province of La Pampa, south-central Argentina. The arc is represented by scarce outcrops of metaigneous rocks (mostly meta-quartz-diorites and metagabbros) but stands out in the aeromagnetic data as a conspicuous and continuous, NNW-trending magnetic anomaly. The geochemical signature of this magmatic suite points to a pre-collisional continental magmatic arc. Its crystallization age (U-Pb SHRIMP dating on magmatic zircon) ranges from ca. 476 to 466 Ma, being comparable, therefore, to the central and northern segments of the Famatinian arc. All dated zircon of the metagabbro have similar 176Hf/177Hf ratios, negative eHf (from - 5.02 to - 3.62) and average Lu-Hf model ages of ca. 1.7 Ga, indicating a crustal contamination of a mantle-derived mafic magma (of ca. 466 Ma) with much older rocks, probably older than 2 Ga, hence suggesting that part of the underlying basement of the southernmost Pampia terrane is at least this old. U-Pb SHRIMP dating on metamorphic zircon rims/areas yielded ca 454 Ma, that is within the 465-450 Ma age range of Famatinian metamorphism previously recorded in La Pampa. The amphibolite-facies metamorphism that affects the magmatic arc in La Pampa is attributed to the collision of the Cuyania (greater Precordillera) terrane against the western margin of Gondwana. © 2009 International Association for Gondwana Research. Source

Zappettini E.O.,Argentine Geological Mining Survey SEGEMAR | Chernicoff C.J.,Argentine Geological Mining Survey SEGEMAR | Chernicoff C.J.,CONICET | Santos J.O.S.,University of Western Australia | And 4 more authors.
International Journal of Earth Sciences

The studied Carboniferous units comprise metasedimentary (Guaraco Norte Formation), pyroclastic (Arroyo del Torreón Formation), and sedimentary (Huaraco Formation) rocks that crop out in the northwestern Neuquén province, Argentina. They form part of the basement of the Neuquén Basin and are mostly coeval with the Late Paleozoic accretionary prism complex of the Coastal Cordillera, south-central Chile. U-Pb SHRIMP dating of detrital zircon yielded a maximum depositional age of 374 Ma (Upper Devonian) for the Guaraco Norte Formation and 389 Ma for the Arroyo del Torreón Formation. Detrital magmatic zircon from the Guaraco Norte Formation are grouped into two main populations of Devonian and Ordovician (Famatinian) ages. In the Arroyo del Torreón Formation, zircon populations are also of Devonian and Ordovician (Famatinian), as well as of Late Neoproterozoic and Mesoproterozoic ages. In both units, there is a conspicuous population of Devonian magmatic zircon grains (from 406 ± 4 Ma to 369 ± 5 Ma), indicative of active magmatism at that time range. The ε Hf values of this population range between -2. 84 and -0. 7, and the TDM-(Hf) are mostly Mesoproterozoic, suggesting that the primary sources of the Devonian magmatism contained small amounts of Mesoproterozoic recycled crustal components. The chemical composition of the Guaraco Norte Formation corresponds to recycled, mature polycyclic sediment of mature continental provenance, pointing to a passive margin with minor inputs from continental margin magmatic rocks. The chemical signature of the Huaraco Formation indicates that a magmatic arc was the main provenance for sediments of this unit, which is consistent with the occurrence of tuff-mostly in the Arroyo del Torreón Formation and very scarcely in the Huaraco Formation-with a volcanic-arc signature, jointly indicating the occurrence of a Carboniferous active arc magmatism during the deposition of the two units. The Guaraco Norte Formation is interpreted to represent passive margin deposits of mostly Lower Carboniferous age (younger than 374 Ma and older than 326 Ma) that precede the onset of the accretionary prism in Chile and extend into the earliest stage of the accretion, in a retrowedge position. The Arroyo del Torreón and Huaraco formations are considered to be retrowedge basin deposits to the early frontal accretionary prism (Eastern Series) of Chile. The presence of volcanism with arc signature in the units provides evidence of a Mississippian magmatic arc that can be correlated with limited exposures of the same age in the Frontal Cordillera (Argentina). The arc would have migrated to the West (Coastal Batholith) during Pennsylvanian-Permian times (coevally with the later basal accretionary prism/Western Series). The source of a conspicuous population of Devonian detrital zircon interpreted to be of magmatic origin in the studied units is discussed in various possible geotectonic scenarios, the preferred model being a magmatic arc developed in the Chilenia block, related to a west-dipping subduction beneath Chilenia before and shortly after its collision against Cuyania/Gondwana, at around 390 Ma and not linked to the independent, Devonian-Mississippian arc, developed to the south, in Patagonia. © 2012 Springer-Verlag. Source

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