Search

An Early Cambrian pluton, known as the Guasayan pluton, has been identified in the central area of Sierra de Guasayan, northwestern Argentina. A U-Pb zircon Concordia age of 533±4 Ma was obtained by LA-MC-ICP-MS and represents the first report of robustly dated Early Cambrian magmatism for the northwestern Sierras Pampeanas. The pluton was emplaced in low-grade metasedimentary rocks and its magmatic assemblage consists of K-feldspar (phenocrysts)+plagioclase+quartz+biotite, with zircon, apatite, ilmenite, magnetite and monazite as accessory minerals. Geochemically, the granitic rock is a metaluminous subalkaline felsic granodiorite with SiO2=69.24%, Na2O+K2O=7.08%, CaO=2.45%, Na2O/ K2O=0.71 and FeO/MgO=3.58%. Rare earth element patterns show moderate slope (LaN/YbN=8.05) with a slightly negative Eu anomalies (Eu/Eu*=0.76). We report the first in situ Hf isotopes data (εHft=-0.12 to -4.76) from crystallized zircons in the Early Cambrian granites of the Sierras Pampeanas, helping to constrain the magma source and enabling comparison with other Pampean granites. The Guasayan pluton might provide a link between Early Cambrian magmatism of the central Sierras Pampeanas and that of the Eastern Cordillera, contributing to define the western boundary of the Pampean paleo-arc. Un plutón de edad Cámbrica temprana, conocido como plutón Guasayán, ha sido identificado en el área central de la sierra de Guasayán, noroeste de Argentina. Una edad U-Pb en concordia de 533±4 Ma fue obtenida en circones mediante LA-MC-ICP-MS. Esta edad representa el primer reporte de magmatismo Cámbrico temprano para el noroeste de las Sierras Pampeanas. El mismo está emplazado en rocas metasedimentarias de bajo grado y se caracteriza por una asociación magmática de K-feldespato (fenocristales)+plagioclasa+cuarzo+biotita, con circón, apatita, ilmenita, magnetita y monacita como minerales accesorios. Geoquímicamente, la roca granítica se clasifica como una granodiorita félsica metaluminosa subalcalina con contenidos de SiO2=69,24%, Na2O+K2O=7,08%, CaO=2,45% y relaciones de Na2O/ K2O=0,71 y FeO/MgO=3,58%. Los patrones de elementos de tierras rara muestran una pendiente moderada (LaN/YbN=8,05) con una ligera anomalía negativa de Eu (Eu/Eu*=0,76). Nosotros reportamos los primeros datos in situ de isótopos de Hf (εHft=-0,12 a -4,76) para circones cristalizados en granitos del Cámbrico temprano de Sierras Pampeanas, lo que aporta información crítica sobre la fuente de los magmas, permitiendo la comparación con otros granitos pampeanos. El plutón Guasayán podría proveer el enlace entre el magmatismo del Cámbrico temprano del sector central de las Sierras Pampeanas y aquel de la cordillera Oriental, contribuyendio a definir el límite occidental del paleoarco Pampeano.

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.

Two tectono-thermal metamorphic events, M1-D1 (S1, with associated white mica and chlorite: WM1-Chl1) and M2-D2 (S2, with development of WM2-Chl2), are established from polyphase white mica growth for low-grade units from the Ordovician metasedimentary successions of La Cébila Metamorphic Complex in the Famatinian belt (western-central Argentina). The thermobarometric characterization of the M1 main event was carried out by means of clay-mineral analysis and crystallo-chemical parameter measurements. Epizonal (temperatures between 300 and 400°C) and low-pressure conditions are suggested for M1 event, based in Kübler index values ranging from 0.23 to 0.17 A°20, white mica b parameter values between 9.004 and 9.022 Á (mean of 9.014 Á, n=16) and Si contents between 3.13-3.29 a.p.f.u. Temperatures of ~180-270°C are estimated for the M2 event, with Kübler index values ranging from 0.31 to 0.46 A°20. The M1-D1 event of La Cébila could be linked to high-strain heating tectono-metamorphic Ordovician regime recorded in others complexes from Famatinian foreland region of Sierras Pampeanas. Dos eventos tectono-metamórficos fueron establecidos a partir de la blastesis superpuesta de mica blanca en sucesiones metasedimentarias ordovícicas de bajo grado del Complejo Metamórfico La Cébila, cinturón Famati-niano (centro oeste de Argentina): M1-D1 (S1, con blastesis asociada de mica blanca y clorita: WM1-Chl1) y M2-D2 (S2, con desarrollo de WM2-Chl2). La caracterización termobarométrica del evento principal M1 fue llevada a cabo a través del análisis de minerales de arcilla y de la medición de parámetros cristaloquímicos. Las condiciones de epizona con temperaturas entre 300 y 400°C, y baja presión fueron estimadas para el evento M1 sobre la base de valores de índice de Kübler de 0,23 a 0,17 A°20, parámetro b de la mica blanca entre 9,004 y 9,022 Á (valor medio de 9,014 Á, n=16) y contenidos de Si entre 3,13-3,29 a.p.f.u. Se estimaron temperaturas de ~180-270°C para el evento M2, con valores de índice de Kübler entre 0,31 y 0,46 A°20. El evento M1-D1 registrado en La Cébila podría ser vinculado al evento tectono-metamórfico ordovícico con calentamiento bajo un régimen de alta deformación ocurrido en otros complejos de la región del antepaís famatiniano de las Sierras Pampenas.