Neoproterozoic anatexis of 2.9 Ga old granitoids in the Goiás-Crixás archean block, Central Brazil: evidence from new SHRIMP U-Pb data and Sm-Nd isotopes

The first SHRIMP U-Pb ages for granitoid rocks from the southern part of the Goiás-Crixás Archean Block (Caiçara and Uvá complexes) are presented and discussed in combination with Sm-Nd isotopic data in order to elucidate the main aspects of the geological evolution of that part of the Brasília Belt in central Brazil. Zircon grains from a tonalitic gneiss (GOV-4) in the Uvá Complex show that the original tonalite crystallized at 2934 ± 5 Ma. One metamorphic zircon crystal is concordant and indicates an Archean age for the recrystallization episode (2793 ± 3 Ma) and one inherited grain with an age of 3092 ± 9 suggests, together with negative values of µNd(T) (+0.4 and -4.6), that the magma was contaminated with older crust. One leucocratic granite (GOV-1) exposed north of the Goiás greenstone belt crystallized at 626 ± 7 Ma, as indicated by the igneous overgrowths surrounding older inherited cores. The latter indicate a crystallization age of 2893 ± 12 Ma. This rock is interpreted therefore as the product of Neoproterozoic anatexis of ca. 2.89 Ga-old rocks of the Caiçara Complex. This is reinforced by strongly negative µNd(T = 626) values of - 28.0 and -29.0. Its crystallization age is identical to the U-Pb ages of the Itapuranga granite and Uruana quartz syenite, which are exposed to the north of the investigated area and interpreted as syn-tectonic intrusions in relation to the main Brasiliano tectonic event. This represents, therefore, the first evidence of Neoproterozoic magmatism within the Goiás Archean Block and raises the possibility that other leucogranite dykes and stocks identified regionally may also have been formed during the Brasiliano orogeny.No presente estudo são reportadas as primeiras idades U-Pb SHRIMP para rochas granito-gnáissicas dos complexos Uvá e Caiçara, na parte sul dos terrenos arqueanos de Goiás. Combinados com análises isotópicas Sm-Nd, os dados U-Pb permitem aprofundar o conhecimento a respeito da evolução geológica daquela parte da Faixa Brasília. Cristais de zircão do gnaisse tonalítico de Uvá (GOV-4) indicam a idade de 2934 ± 5 Ma para a cristalização do protólito ígneo. Um grão de zircão metamórfico sugere idade de 2793 ± 3 Ma para o metamorfismo que afetou a rocha, enquanto que um grão herdado de zircão com idade de 3092 ± 9, aliado a valores de µNd(T) variando entre +0,4 e -4,6, sugere contaminação com crosta continental mais antiga. Granito leucocrático exposto a norte do greenstone belt de Crixás no Complexo Caiçara apresenta idade de cristalização de 626 ± 7 Ma, dada por sobrecrescimentos ígneos cristalizados em torno de núcleo com idade de 2893 ± 12 Ma. O resultado, combinado com valores fortemente negativos de µNd(T) (entre -28,0 e -29,0), indica que o granito é produto de refusão de rochas arqueanas com ca. 2,89 Ga de idade. A idade do granito é idêntica às idades U-Pb SHRIMP e convencional do álcali-granito Itapuranga e quartzo sienito de Uruana, expostos a norte da área estudada. Trata-se, portanto, do primeiro registro confiável de magmatismo Neoproterozóico no interior dos terrenos arqueanos de Goiás e pode sinalizar que vários dos pequenos corpos e diques de leucogranitos encontrados em meio aos terrenos TTG podem representar magmas gerados durante a orogênese brasiliana

Evolution tectono-métamorphique d'un segment de croûte continentale archéenne. Exemple de l'Amsaga (R.I. Mauritanie), Dorsale Réguibat (Craton Ouest Africain).

Mémoires de Géosciences Rennes, n° 56, 400 p. ISBN: 2-905532-55-6Le but de cette thèse est de déterminer l'évolution tectono-métamorptlique d'un segment de croûte continentale archéenne. L'objet choisi est la région de l'Am saga (R.1. Mauritanie), dans la Dorsale Réguibat (Craton Ouest Africain). L'étude géochronologique (Rb-Sr, Sm-Nd, U-Pb et Pb-Pb) met en évidence au moins quatre épisodes de croissance crustale : vers 3,5 Ga; à 3,0 Ga; vers 2,85 Ga et vers 2,74 Ga. La région est affectée par un événement tectono-métamorphique majeur, synchrone (ou immédiatement postérieur) du dernier épisode de croissance crustale. L'étude pétro-structurale montre que cet épisode est caractérisé par un fort raccourcissement sub-horizontal NW-SE en régime globalement coaxial. La déformation est accommodée par le biais de structures verticales: foliations, plis et décrochements en fin d'évolution et s'accompagne d'un métamorphisme -granulitique HT-BP (800 ± 50°C, 5±1 kb). La migmatisation d'une partie de la région au cours du métamorphisme provoque la diminution de aH20 et permet l'acquisition des paragénèses granulitiques. Le chemin P-T suivi au cours du métamorphisme est un chemin horaire dont l'évolution rétrograde s'effectue dans un premier temps par décompression isotherme. Différents ensembles magmatiques (gabbro et granites), datés à 2,7 Ga se mettent en place, postérieurement au métamorphisme granulitique, pendant la remontée des séries. L'étude géochimique (majeurs, traces et isotopes) de ces massifs indique trois types de source pour les protholithes _ magmatiques: 1/ mantellique (gabbro des Iguilid); 2/ fusion partielle de roches G-rthodérivées (granite de Touijenjert); 3/ fusion partielle de métasédiments (granite d'Ioulguend). La mise en place de ces granites marque donc le début du recyclage crustal dans la région. Le gradient géothermique moyen au cours du métamorphisme granulitique (55°C.km-1), en accord avec le caractère enrichi (LREE) de la source des magmas basiques et les données géochronologiques, permet d'envisager la présence d'un plume mantellique à l'aplomb de la région pendant l'événement tectono-métamorphique principal. L'étude géochronologique révèle que la région est le siège de plusieurs événements thermiques postérieurs au métamorphisme principal : vers 2,3 Ga, vers 1,5 Ga et peut - être vers 2,4-2,5 Ga. Ce réchauffement protérozoïque se fait apparemment sans déformation associée mais provoque : 1/ la rétromorphose des paragénèses granulitiques; 2/ des perturbations géochimiques importantes (éléments majeurs, traces, REE), une partie de celles-ci pouvant également résulter de l'événement granulitique; 3/ des ouvertures successives des différents systèmes isotopiques. Ces phénomènes secondaires sont essentiellement localisés aux * abords des grands accidents mylonitiques qui découpent la région. La synthèse de ces différents résultats et des données existantes sur le reste de la Dorsale Réguibat archéenne et protérozoïque permet de suggèrer un modèle d'évolution géodynamique de cette partie du craton Ouest africain.No abstrac

Kinematics of long-lasting Paleoproterozoic transpression within the Thompson Nickel Belt, Manitoba, Canada

This paper presents a reappraisal of kinematics and tectonic history of the Thompson Nickel Belt, a major Paleoproterozoic deformation zone along the western boundary of the Archean Superior Province in Manitoba. The study, based on the analysis of foliation trajectory maps and associated shear zone arrays, emphasizes that the overall strain pattern results from transpression. Strains are large, marked by strong subvertical stretch and NW-SE subhorizontal shortening, combined with along-strike stretch. From these, we infer distributed crustal shortening, involving steeply plunging flow rather than large horizontal displacements, although kinematic indicators show that top-to-the-west motions were dominant. Along-strike shear indicators are poorly expressed but suggest a component of bulk dextral strike-slip. Synkinematic thermal indicators and new geochronological data indicate that transpression could have been a long-lasting, circa 100 Ma, event in the area

Isotope geochemistry and revised geochronology of the Purrido Ophiolite (Cabo Ortegal Complex, NW Iberian Massif): Devonian magmatism with mixed sources and involved Mesoproterozoic basement

In the Purrido Ophiolite (Cabo Ortegal Complex), new U–Pb zircon dating of the amphibolite G03-8 (by laser ablation inductively coupled plasma mass spectrometry) confirms the existence of a dominant Mesoproterozoic zircon population with a refined age of 1155 14 Ma. However, the U–Pb zircon dating of two more amphibolite samples (by sensitive high-resolution ion microprobe) has provided new ages of 395 3 Ma and 395 2 Ma, respectively, interpreted as the crystallization age. Hf isotope data for zircon show that most of the Devonian zircons crystallized from a juvenile depleted mantle source. The Mesoproterozoic zircons have relatively juvenile Hf isotopic composition reflecting some influence of an older component. A few Devonian zircon crystals show evidence of mixing with an older component represented by the Mesoproterozoic zircons. The whole-rock Sm–Nd isotope data indicate an important heterogeneity in the composition of the Purrido amphibolites, only compatible with the generation of their protoliths from two sources. We interpret these puzzling data as resulting from the mixing of a Devonian mantle-derived magma with a Mesoproterozoic basement. These new data provide new perspectives in the interpretation of the most common ophiolites across the Variscan suture in Europe

Chronology of transpression, magmatism, and sedimentation in the Thompson Nickel Belt(Manitoba, Canada) and timing of Trans-Hudson Orogen - Superior Province collision

International audienceThe Thompson Nickel Belt marks the boundary between the Archean Superior Province and the Trans-Hudson Orogen in Canada. It comprises Archean gneisses, and Paleoproterozoic rocks with metasediments and metavolcanites (Ospwagan Group) and intrusions. The gneisses are frequently migmatitic and host numerous pegmatites. The western belt boundary is a fault contact with the Kisseynew Domain of the Reindeer Zone. In the south, the transition zone between the belt and the Kisseynew Domain comprises granitoids and a detrital sequence (Grass River Group), part of which grades into turbidites in the Kisseynew Domain. The eastern belt boundary is a diffuse zone where the Archean east­west (E­W) structural trend changes into the north-northeast (NNE) trend of the belt. This paper presents U­Pb ages for granitoids and 207Pb/206Pb detrital zircon ages from the Ospwagan and Grass River groups. Ages and a comparison of events in the belt and in the eastern Reindeer Zone have major implications. The change from stable platform deposits to syn-tectonic filling and emplacement of mafic intrusions in the Ospwagan Group are attributed to the convergence between the Reindeer Zone and the Superior Province at 1891­-1885 Ma. At ca. 1850 Ma, continuing convergence led to drowning of marginal basins of the Superior craton and to the development of a transpressive regime in the belt, the onset of which could be as old as ca. 1885 Ma. Metamorphic ages of 1818­-1785 record closure of the Kisseynew basin and crustal thickening. Collision of the new continental block with the Superior Province was accommodated by transpression until 1750-­1720 Ma

Neoproterozoic Anatexis of 2.9 Ga Old Granitoids in the Goiás-Crixás Archean Block, Central Brazil: Evidence From New SHRIMP U-Pb Data and Sm-Nd Isotopes

The first SHRIMP U-Pb ages for granitoid rocks from the southern part of the Goiás-Crixás Archean Block (Caiçara andUvá complexes) are presented and discussed in combination with Sm-Nd isotopic data in order to elucidate the main aspectsof the geological evolution of that part of the Brasília Belt in central Brazil. Zircon grains from a tonalitic gneiss (GOV-4) inthe Uvá Complex show that the original tonalite crystallized at 2934 ± 5 Ma. One metamorphic zircon crystal is concordantand indicates an Archean age for the recrystallization episode (2793 ± 3 Ma) and one inherited grain with an age of 3092 ± 9suggests, together with negative values of eNd(T) (+0.4 and –4.6), that the magma was contaminated with older crust. Oneleucocratic granite (GOV-1) exposed north of the Goiás greenstone belt crystallized at 626 ± 7 Ma, as indicated by theigneous overgrowths surrounding older inherited cores. The latter indicate a crystallization age of 2893 ± 12 Ma. This rockis interpreted therefore as the product of Neoproterozoic anatexis of ca. 2.89 Ga-old rocks of the Caiçara Complex. This isreinforced by strongly negative eNd(T = 626) values of – 28.0 and –29.0. Its crystallization age is identical to the U-Pb agesof the Itapuranga granite and Uruana quartz syenite, which are exposed to the north of the investigated area and interpretedas syn-tectonic intrusions in relation to the main Brasiliano tectonic event. This represents, therefore, the first evidence ofNeoproterozoic magmatism within the Goiás Archean Block and raises the possibility that other leucogranite dykes andstocks identified regionally may also have been formed during the Brasiliano orogeny

Isotope geochemistry and revised geochronology of the Purrido Ophiolite (Cabo Ortegal Complex, NW Iberian Massif): Devonian magmatism with mixed sources and involved Mesoproterozoic basement

<p>In the Purrido Ophiolite (Cabo Ortegal Complex), new U–Pb zircon dating of the amphibolite G03-8 (by laser ablation inductively coupled plasma mass spectrometry) confirms the existence of a dominant Mesoproterozoic zircon population with a refined age of 1155 ± 14 Ma. However, the U–Pb zircon dating of two more amphibolite samples (by sensitive high-resolution ion microprobe) has provided new ages of 395 ± 3 Ma and 395 ± 2 Ma, respectively, interpreted as the crystallization age. Hf isotope data for zircon show that most of the Devonian zircons crystallized from a juvenile depleted mantle source. The Mesoproterozoic zircons have relatively juvenile Hf isotopic composition reflecting some influence of an older component. A few Devonian zircon crystals show evidence of mixing with an older component represented by the Mesoproterozoic zircons. The whole-rock Sm–Nd isotope data indicate an important heterogeneity in the composition of the Purrido amphibolites, only compatible with the generation of their protoliths from two sources. We interpret these puzzling data as resulting from the mixing of a Devonian mantle-derived magma with a Mesoproterozoic basement. These new data provide new perspectives in the interpretation of the most common ophiolites across the Variscan suture in Europe. </p