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... However, scarce amphibolites are distributed in a N-S belt through Sierra del Morro, San Felipe and Villa de Praga. Some tungsten deposits are genetically related to them (de Brodtkorb et al. 2005). ...
The Sierras Pampeanas of Central Argentina are an example of a continuous and fast overlap of episodes of high- to medium- and low-grade metamorphism, deformation, anatexis, magmatism and mineralization along the belts that bounded the margins of the South American cratons during the Neoproterozoic and early Paleozoic. A compilation and critical revision of the massive isotopic and geochemical data for the basement rocks of the Eastern Sierras Pampeanas is presented. The Eastern Sierras Pampeanas are defined by three main events: the Ediacaran to early Cambrian (580–530 Ma) Pampean, the late Cambrian–Ordovician (500–460 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. The mean average crustal residence age (Sm–Nd TDM) varies between 1.8 and 1.7 Ga with εNd(540) (−6 to −8). Pampean and Famatinian granitoids exhibit a similar TDM interval except for the Ordovician TTG suites of the Sierras de Córdoba (TDM 1.3–1.0 Ga). Achalian magmatism exhibits more radiogenic εNd(540) values (0.5 to −4) and TDM ages younger than 1.3 Ga. Two types of Pampean-related mafic rocks are recognized: one with a depleted mantle signature and another younger group with an enriched mantle signature, which is associated with the peak of metamorphism. Ordovician mafic-ultramafic rocks result from mixing/assimilation of depleted mantle and crustal magmas. Detrital zircon data for the metaclastic sequences indicates mainly Grenvillian and Brasiliano sources. The difference between the measured crystallization age for detrital zircon grains and the depositional age of the succession indicates that most of the Pampean basins are collisional, i.e. foreland basins except for Sierra Norte metaclastic host rocks that correspond to a convergent setting signature. The results for the post-Pampean Famatinian basins indicate mostly collisional convergent settings for the Ambato and La Cébila (type locality) metamorphic complexes and the Olta (northern sector) and Achavil formations. The Negro Peinado Formation is the only post-Pampean basin that corresponds to a collisional setting. Data from Green Quarry, Nogolí Metamorphic Complex, and Olta Formation in the central part of the Sierra de Chepes, Suri Formation and La Cébila Metamorphic Complex at Quebrada La Rioja yielded convergent margin settings. These exhibit the highest correlation among the post-Pampean basins showing a relatively large number of Middle Cambrian age detrital zircons apart from having Ordovician magmatic zircons in the detrital record. Sources were apparently more restricted than in the rest of the Famatinian post-Pampean basins. The two samples of post-Pampean basins that exhibit Río de la Plata age peaks (2.2–2.0 Ga; Paleoproterozoic) belong to the collisional convergent group. Therefore the exhumed Pampean rocks probably formed a drainage divide that blocked westward transport of the Río de la Plata-derived sediments. Intense erosion owing to an unstable tectonic scenario would have led to the progressive appearance of the Río de la Plata signature.
... However, scarce amphibolites are distributed in a N–S belt through Sierra del Morro, San Felipe and Villa de Praga. Some tungsten deposits are genetically related to them (de Brodtkorb et al. 2005). ...
... Older T DM ages and lower eNd (540) values suggest that the T DM values of these samples reflect enriched signatures and/or a longer crustal residence time (Arndt and Goldstein 1987). In the northern part of the Sierra Chica (de Brodtkorb et al. 2005Table 2), rocks considered as amphibolites as well as the gabbros of the Sierra de Comechingones (Fig. 6a) have eNd (540) values significantly below that of the contemporary depleted mantle and were probably derived from the sub-continental lithospheric mantle. Tibaldi et al. (2008) proposed that the metasomatic enrichment required for OIB-like magmatism was caused by low-degree partial melting of progressively younger and warmer subducted oceanic lithosphere at the Pampean convergent margin and that subsequent melting of the overlying enriched mantle wedge produced OIB magmas. ...
The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. Geochronological and Sm–Nd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm–Nd results for the metamorphic basement suggest that the T
DM age interval of 1.8–1.7 Ga, which is associated with the less radiogenic values of εNd(540) (−6 to −8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger T
DM age and a more positive εNd(540) value. Pampean pre-540 Ma granitoids form two clusters, one with T
DM ages between 2.0 and 1.75 Ga and another between 1.6 and 1.5 Ga. Pampean post-540 Ma granitoids exhibit more homogenous T
DM ages ranging from 2.0 to 1.75 Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. T
DM values for the Ordovician Famatinian granitoids define a main interval of 1.8–1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger T
DM ages ranging from 1.3 to 1.0 Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic εNd(540) values that range between 0.5 and −4 and T
DM ages younger than 1.3 Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism.
... However, scarce amphibolites are distributed in a N-S belt through Sierra del Morro, San Felipe and Villa de Praga. Some tungsten deposits are genetically related to them (de Brodtkorb et al. 2005). ...
... Our own database for the mafic rocks (Table 2) was combined with data from Pankhurst et al. (1998), Rapela et al. (1998), Sato et al. (2001), de Brodtkorb et al. (2005 and Escayola et al. (2007). Sm-Nd results for the maficultramafic rocks are heterogeneous, which is evident from the wide range in Sm-Nd isotopic compositions (Table 2), with eNd (540) values ranging from -6.37 to ?7 and 147 Sm/ 144 Nd ratios ranging from 0.09 to 0.3. ...
... Older T DM ages and lower eNd (540) values suggest that the T DM values of these samples reflect enriched signatures and/or a longer crustal residence time (Arndt and Goldstein 1987). In the northern part of the Sierra Chica (de Brodtkorb et al. 2005 and Table 2), rocks considered as amphibolites as well as the gabbros of the Sierra de Comechingones (Fig. 6a) have eNd (540) values significantly below that of the contemporary depleted mantle and were probably derived from the sub-continental lithospheric mantle. Tibaldi et al. (2008) proposed that the metasomatic enrichment required for OIB-like magmatism was caused by low-degree partial melting of progressively younger and warmer subducted oceanic lithosphere at the Pampean convergent margin and that subsequent melting of the overlying enriched mantle wedge produced OIB magmas. ...
... Metamorphosed dacitic and rhyodacitic tuffs included in the San Luis Formation, the host for some of the tungsten ores, have a zircon U–Pb age of 529 ± 12 Ma, the presumed crystallization age of the tuffs (Söllner et al., 2000 ). Amphibolites interlayered with higher grade metamorphic units of the most eastern Conlara Metamorphic Complex yield a K–Ar reference isochron age of 660 Ma (Brodtkorb and Ostera, 2006). In the Pampa del Tamboreo study area, the dominant lithologies are phyllites, muscovite-biotite schists, quartzites and silicic metavolcanics of the San Luis Formation (Fig. 1). ...
Tourmalinites that are distally associated with tungsten deposits of the Pampa del Tamboreo area, San Luis, Argentina, contain tourmalines retaining evidence for its origin and evolution. Tourmaline grains uncommonly contain small grains of detrital tourmaline. Analysis of a single detrital tourmaline grain reveals that it is a Ca-rich "oxy-dravite". Proximal to the detrital cores there are inner domains of asymmetric tourmaline overgrowths that developed during low grade metamorphism. Volumetrically dominant tourmaline overgrowths in the outer domain are concentrically zoned aluminous dravite and "oxy-dravite" with Al/(Al + Fe + Mg) = 0.71-0.74 and Mg/(Mg + Fe) = 0.64-0.71. Variability of Al is primarily controlled by the deprotonation substitution R + OH- = Al + O2- (where R = Fe + Mg), and is a function of the activity of H2O. A likely evolutionary scenario is one in which volcanogenic material is altered by hydrothermal fluids in the sea floor resulting in an aluminous and magnesian residuum. With further hydrothermal circulation and incipient metamorphism, boron-rich fluids are expelled from metasedimentary and metavolcanic basement rocks and develop Mg-rich tourmalinites in the aluminous, magnesian host rocks. The tourmalinization process occurs over a range of metamorphic conditions and with fluids of variable activity of H2O. (C) 2009 Elsevier Ltd. All rights reserved.
El cuerpo máfico-ultramáfico Las Águilas aflorante en las sierras de San Luis es portador de una mineralización de sulfuros que consiste esencialmente de pirrotina, pentlandita y calcopirita con participación subordinada de minerales del grupo del platino (MGP). Los resultados dados a conocer aquí fueron obtenidos a partir del estudio de muestras provenientes de los testigos de las perforaciones realizadas por la Dirección General de Fabricaciones Militares en la década del 70-80. Dicha mineralización evidencia texturas de tipo intersticial o "net texture"-textura nodular, diseminada y relleno masivo de venillas, según la relación existente entre los sulfuros y la roca hospedante, la cual se presenta principalmente en los niveles de rocas ultramáficas con minerales del grupo del espinelo. La mineralización se habría originado entre los 600-700° C a partir de la solución sólida de monosulfuro, reconociéndose una componente de mineralización magmática primaria (Po1+Ccp1 +Pn±MGP±Cub) afectada por una importante removilización (Po2+Ccp2+Pn±MGP±Py±Mck). Aunque no se determinaron diferencias químicas entre los sulfuros identificados en ambos tipos de mineralización, las diferenciación de ambos tipos es indicada por evidencias de tipo textural, el predominio de MGP asociados a los metales base removilizados y el hecho que los MGP se presentan en los bordes de los sulfuros, asociados a minerales del grupo de la serpentina y en los bordes de alteración de los espinelos.
