Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur-Hassan area

A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55-2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic-trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550-2530+/-5Ma, as shown by both U-Pb and Pb-207/Pb-206 single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510+/-10 Ma. These data provide precise time constraints and Sr-Nd isotopes confirm the existence of late tonalitic-granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur-Hassan area) are in agreement with some previous Sr ages and range between 3200+/-20 and 3328+/-10 Ma. These gneisses were derived from a 3.3-3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55-2.52-Ga 'juvenile' tonalitic-trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb-Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east-west shearing which displaced the 2.5-Ga juvenile terranes toward the west

Late Archean crust-​mantle interactions: geochemistry of LREE-​enriched mantle derived magmas. Example of the Closepet batholith, southern India

The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Gyr ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3-​3.0 Gyr)​. In the field, it appears as a composite batholith made up of at least two groups of intrusions. An early SiO2-​poor group (clinopyroxene quartz-​monzonite and porphyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr​/86Sr ratio (Sri, 0.7017-​0.7035) and εNd (-​0.9 to -​4.1)​. A later SiO2-​rich group (equigranular gray and pink granites) is located along the interface between the SiO2-​poor group and the Peninsular Gneisses. They progressively grade into migmatized Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (Sri = 0.7028-​0.7336 and εNd values from -​2.7 to -​8.3, at 2.52 Gyr)​. Field and geochronol. evidence shows that the two groups are broadly contemporaneous (2.518-​2.513 Gyr) and mech. mixed. This observation is supported by the chem. data that display well defined mixing trends in the εSr vs εNd and elemental variation diagrams. The continuous chem. variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-​members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses)​. It is proposed that the intrusion of mantle-​derived magmas into mid-​crustal levels occurred along a transcurrent shear zone; these magmas supplied addnl. heat and fluids that initiated anatexis of the surrounding crust. During this event, large-​scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-​derived magmatism is clearly assocd. with granulite-​facies metamorphism 2.51 ± 0.01 Gyr ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity

Archean granite-greenstone tectonics at Kolar (South India): Interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion

[1] The structural study of the Kolar greenstone belt and surrounding granite-gneiss terrains combined with U-Pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern Dharwar craton developed during a major magmatic accretion event between 2550 and 2530 Ma. The granite-greenstone pattern resulted from the interference of gravity-driven sagging of the greenstones (i.e., diapirism), E-W bulk inhomogeneous shortening combined with horizontal N-S stretching, and syntectonic juvenile pluton emplacement. Bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive, N-S trending vertical foliation, shallow stretching lineation, and conjugate strike-slip shear zone pattern within and outside the greenstone belt, resulting in regional horizontal pure shear deformation. The plutons around the greenstone belt record regional contraction by developing one set of strike-slip C-S fabrics of the shear zone pattern. The development of the granite-greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion, melting, and granulite metamorphism in the lower crust. The Kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain, due to mechanical homogeneity and low viscosity provided by large-scale melting during the accretion event. This example further suggests specific boundary conditions to the craton that allowed E-W inhomogeneous shortening to be accommodated by N-S stretching and spreading of the crust without significant tectonic thickening. Such tectonoplutonic pattern is specific to the Archean and may develop as a consequence of mantle plume activity in intracontinental settings

Geochemical and isotopic evidence for the petrogenesis and emplacement tectonics of the Serra dos Orgaos batholith in the Ribeira belt, Rio de Janeiro, Brazil

International audienceThe Serra dos Orgaos batholith in the State of Rio de Janeiro (Brazil) is a NE-SW-trending elongated body that occupies ca. 5000 km(2) in plan view. It is a foliated intrusion, especially at its borders and is crosscut by syn-magmatic shear zones, with foliations that are moderately-to steeply-dipping to the northwest and moderately-to shallow-dipping in the center and to the southeast, in a configuration of a large laccolith. It was emplaced between 560 and 570 Ma, during an extensional episode that was part of a series of events that comprise the Brasiliano Orogeny in SE Brazil, and which includes deformation, metamorphism and granite intrusion during the interval between 630 and 480 Ma. The two main rock types in the batholith are biotite-hornblende monzogranite, and biotite leucogranite, with subordinate tonalite, granodiorite, diorite, quartz diorite (enclaves), aplite and pegmatite. Harker-type diagrams help show two rock groups with similar trends of evolution: a dioritic and a granitic. The first one is tholeiitic, whereas the second is calc-alkaline, with medium-to high-K calc-alkaline affinity and metaluminous to slightly peraluminous character. In both groups strong decrease in Al2O3, MgO, FeOT and CaO relative to silica contents are observed, which is compatible with trends of fractional crystallization involving clinopyroxene and/or hornblende, plagioclase, opaque minerals, apatite, microcline and biotite. The Sr and Nd isotopic data suggest recycling of a Paleoproterozoic crust as an important petrological process to generate the batholith rocks. Geothermometry (amphibole composition) and geobarometry (saturation in zircon and apatite) indicate that most of the batholith solidified at mid to lower crustal levels at about 750 degrees C and between 5 and 5.5 kbar. We consider that Serra dos Orgaos crustal protoliths underwent melting caused by the interaction with hotter mafic magma at the base of the crust. These two magmas, with distinct initial compositions and rheology, probably underwent mixing and mingling. This process continued during the rise of the magma through the crust, which was accompanied by magmatic differentiation. The main feature that characterizes the post-collisional Serra dos Orgaos granite magmatism is the connection with high angle ductile shear zones of continental scale and presence to a greater or lesser extent of mafic magmas

Late-Hercynian Intrusion-related gold deposits: an integrated model on the Tighza polymetallic district, central Morocco

International audienceLate-Hercynian Intrusion-related gold deposits: an integrated model on the Tighza polymetallic district, central Morocco, African Earth Sciences (2015), doi: http://dx.Gold have been recently recognized in the Tighza (formerly Jebel Aouam) district, in the Hercynian belt of central Morocco. This district has long been known for its W mineralization, as well as major Pb-Ag-Zn, and minor Sb-Ba deposits, all geographically associated with late-Hercynian calc-alkaline magmatism. Gold mineralization in the district is mainly hosted by thick W-Au quartz veins located around the “Mine granite” small granitic plug. Within the veins, gold grade is highest (up to 70 g/t) close to the granite but rapidly decreases going outward from the granite, defining a perigranitic zoning. Anomalous gold grades have also been measured in hydrothermal skarn layers close to two other granitic plugs (Kaolin granite and Mispickel granite), associated with disseminated As-Fe sulfides. The paragenetic sequence for the W-Au quartz veins shows three stages: 1) an early oxidized stage with wolframite-scheelite associated with early quartz (Q1), 2) an intermediate Bi-As-Te-Mo-Au sulfide stage with loellingite, bismuth minerals and native gold with a later quartz (Q2), restricted to a narrow distance from the granite, and 3) a late lower temperature As-Cu-Zn–(Pb) stage with abundant massive pyrrhotite, arsenopyrite and sphalerite, locally forming independent veins (“pyrrhotite vein”). Both Q1 hyaline and Q2 saccharoidal gold-bearing quartz display aqua-carbonic fluids with minor H2S and Cu and an homogeneous composition (81 mole% H2O, 18 mole% CO2 and about 1 mole% NaCl). The trapping pressure is estimated to 1.5 to 2 kbar with temperature ranging from 300° to 350°C. Q1 inclusions have exploded indicating an uplift of the Tighza block, that lead to saccharoidal Q2 quartz deposition with multiphase NaCl-saturated fluid inclusions. 40Ar/39Ar dating demonstrates that the “Mine granite”, tungsten skarnoid, scheelite-molybdenite veins, and very likely gold-bearing veins are coeval, emplaced at 286 ± 1 Ma. Multiple and widespread metal sources are indicated by radiogenic isotope studies. Nd and Sr isotope compositions of scheelite and granites suggest the participation of a juvenile component while lead isotopes demonstrate a major participation of the basement.Both gold mineralization and zoning suggest that the system developed at the end of the magmatic activity, accompanying a major transition in magmatic fluid composition. The morphology of the gold-bearing mineralization is dependent of the permeability and the reactivity of host-rocks: focus circulation of fluids through pre-existing tectonic corridors, reactivated by late-Hercynian intrusions favor the formation of large W-type gold veins, while infiltration of fluid within reactive stratigraphic layers gives rise to skarn mineralization. A 40Ar/39Ar date (W1 north vein: 291.8 ± 0.3 Ma) indicates that hydrothermal circulation predates gold and tungsten deposition in open fractures as well as Mine granite emplacement.The W-Au mineralization preceded the onset of a large convective hydrothermal cell around the intrusion that led to the formation of the Pb-Ag-Zn mined veins. The Tighza polymetallic district displays numerous similarities with the R-IRG model that was defined in the American Cordillera, such as thermal and zonation patterns, carbonic hydrothermal fluids and chronology of intrusion and related deposits, but also provides new insight to the R-IRG model such as wide Au-quartz veins instead of sheeted Au-veins, oxidation state of the magma, and Sr-Nd isotopic data. These results establish a major magmatic contribution and discard a direct genetic relationship between gold mineralization and major neighboring Pb-Ag-Zn veins. A large number of classic Pb-Zn district of the Western Hercynides belong to the same clan

Histoire Géologique du massif Armoricain : Actualité de la recherche

National audienceUne part essentielle de l'histoire géologique de la France (et même d'Europe occidentale, avec des roches ayant environ 2000 Ma) est déchiffrable dans le Massif armoricain. Si celui-ci est réputé pour ses excellentes qualités d'affleurement sur le littoral (Armor, ou pays de la mer), certains objets ou structures ne peuvent être observés que dans le bocage (Argoat, ou pays des arbres), où leur lisibilité est souvent problématique. En Armor comme en Argoat, de nombreux sites constituent un réel patrimoine géologique (l'intérêt de certaines localités sera mis en exergue dans le texte), dont la valeur ne peut être jaugée qu'au regard de son intérêt scientifique. Ainsi ce travail - une mise en perspective de nos connaissances scientifiques sur l'évolution géologique du Massif armoricain - est-il basé sur plusieurs synthèses antérieures (par ex. Le Corre et al., 1991 ; Ballèvre et al., 2009), qu'il complète en intégrant les nouvelles données disponibles. Plus qu'un exposé complet des faits, nous visons à clarifier certains débats, et montrer en quoi les recherches en cours changent notre image globale du Massif armoricain. Nous restreindrons notre analyse à la période qui couvre la fin du Protérozoïque (Ediacarien : 635-540 Ma), le Paléozoïque (540-250 Ma) et le début du Mésozoïque (Trias : 250-200 Ma)

Crustal evolution between 2.0 and 3.5 Ga in the southern Gaviao block (Umburanas-Brumado-Aracatu region), Sao Francisco Craton, Brazil: A 3.5-3.8 Ga photo-crust in the Gaviao block?

The main evolution of the Gavião block in the Umburanas-Brumado-Aracatu region, in the state of Bahia, is defined by several sets of tonalitic-trondhjemitic and granodioritic gneisses emplaced during the Paleoarchean. The juvenile Bernada gneisses are e

The mafic-silicic layered intrusions of Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands), Armorican Massif: Gabbro-diorite layering and mafic cumulate-pegmatoid association

International audienceThe Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands) Intrusive Complexes (hereafter referred to as SJIC and NGIC, respectively) are examples of mafic-silicic layered intrusions in the Armorican Massif. Both are characterized by the occurrence of (1) a basal/peripheral gabbroic unit interlayered with sheets (generally dioritic in composition, occasionally gabbroic) and crossed by leucocratic diapirs and pipes (from monzodioritic to Q-monzonitic in composition), (2) peripheral pegmatoids associated with mafic cumulates and (3) coeval granitoids. Beside these main similarities, some contrasted features lead us to propose two distinct models of formation. The Variscan SJIC includes tholeiitic mafic rocks (monzogabbro) that locally mingle and mix with leucocratic components (monzonite or Q-monzonite). The Cadomian NGIC is calc-alkaline. The SJIC sheet-bearing gabbro is homogeneous from a petrologic point of view, whereas the NGIC exhibits gabbroic macrorhythmic sequences with mineral layering. The Sr-Nd isotopic compositions of the SJIC gabbros are significantly different from those of the associated dioritic layers. This is not the case in the NGIC where the magmas could be cogenetic. We argue that the SJIC gabbro was a liquid that crystallized in situ without significant crystal settling. By contrast, the rhythmic sequences of the NGIC are consistent with crystal accumulation. Subsequently, both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition. We interpret the sheets in the SJIC as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below, at levels of neutral buoyancy. Injections and convection in the central part of the reservoir possibly resulted in spectacular mixing/mingling structures. In the NGIC, the emplacement of the dioritic sheets was rather controlled by pre-existing rhythmic cumulative structures. In both intrusions, late differentiated diapirs were extracted from the dioritic sheets. Associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase. This extraction might have been enhanced by the disruption of the peripheral cumulate stack, perhaps following pressur