J.-J. Peucat

Late Archaean (2550-2520 Ma) juvenile magmatism in the Eastern Dharwar craton, southern India : Constraints from geochronology, Nd-Sr isotopes and whole rock geochemistry

The results of field, geochronologic, geochemical and isotopic studies are presented for the granitoids that occur east of the Closepet batholith up to the Kolar schist belt (KSB). Field data, such as common foliation, strong shear deformation occasionally leading to mylonitization, together with petrographic data, including reduction in grain size with corroded borders, show characteristics of the syn-kinematic emplacement of the granitoids. Single zircon evaporation ages define a minimum age of 3127 Ma for the tonalitic–trondhjemitic–granodioritic (TTG) basement and 2552–2534 Ma plateau ages for the emplacement of the granitoids, which slightly predate (20–30 Ma) the emplacement of the 2518 Ma Closepet batholith.Major and trace element data, together with isotopic data, suggest at least four magmatic suites from Closepet batholith to the east, which have independent magmatic evolution histories. The observed data are compatible with magma mixing for the Closepet batholith, melting of TTG and assimilation–fractional crystallization processes for Bangalore granites, either melting of heterogeneous source or different degree of melting of the same source for the granitoids of Hoskote–Kolar and fractional crystallization for the western margin of the KSB. Isotopic (Nd–Sr) and geochemical data (LREE and LIL elements) suggest highly enriched mantle and ancient TTG crust for the Closepet batholith, enriched mantle and TTG crust for the Bangalore granites, c.a. chondritic mantle source for the granitoids of Hoskote–Kolar and the quartz monzonites of the western margin of the KSB and slightly depleted mantle for granodiorites of the eastern margin of the KSB.We interpret all these geochronologic, geochemical and isotopic characteristics of granitoids from the Closepet batholith to the east up to the KSB in terms of a plume model. The centre of the plume would be an enriched ‘hot spot’ in the mantle that lies below the present exposure level of the Closepet batholith. Melting of such an enriched mantle hot spot produces high temperature magmas (Closepet) that penetrate overlying ancient crust, where they strongly interact and induce partial melting of the surrounding crust. These magmas cool very slowly, as the hot spot maintains high temperatures for a long time; thus they appear younger (2518 Ma). On the contrary, to the east the plume induces melting of c.a. chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas, which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older (2552–2534 Ma). This plume model can also account for late Archaean geodynamic evolution, including juvenile magmatism, heat source for reworking, inverse diapirism and granulite metamorphism in the Dharwar craton.

Late Archaean 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 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic phyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr/86Sr (0.7017–0.7035) and ɛNd(−0.9to −4.1). (b) A later SiO2-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (87Sr/86Sr ratios=0.7028–0.7336 and ɛNd values from-2.7 to-8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518–2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the ɛSr vs ɛNd and elemental variation diagrams. The continuous chemical 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 additional 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 associated with granulite facies metamorphism 2.51±0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.

Crustal evolution in the early Archaean of South America: example of the Sete Voltas Massif, Bahia State, Brazil

The Sete Voltas massif (Sao Francisco craton, Brazil) appears as a composite crustal segment built in at least three successive accretional events dated by RbSr, 207Pb206Pb monozircon and U-Pb SHRIMP methods. (a) At ca 3.4 Ga, generation and emplacement of magmatic precursors to the old grey gneisses. These are the oldest rocks so far recognized in South America. Their composition is typical of Archaean TTG (Tonalite, Trondhjemite and Granodiorite) and geochemical modelling indicates that they were produced by partial melting of an Archaean tholeiite, leaving a hornblende garnet residue. (b) Between 3.17 and 3.15 Ga, younger grey gneisses and porphyritic granodiorites intruded the old grey gneisses. Their geochemical composition is clearly different from that of typical TTG and geochemical modelling shows that they were produced by partial melting of an older continental crust. (c) Grey granite dykes emplaced at ca 2.6 Ga, during a late-stage magmatic event in the Sete Voltas massif. All the units belonging to the Sete Voltas massif yield homogeneous TDM ages at 3.66 Ga which are interpreted as reflecting derivation from older protoliths, or rather their contamination by older pre-existing continental crust. It is tentatively proposed that a 3.66-Ga-old continental crust existed prior to the emplacement of the older grey gneisses. Although such crustal material is unknown at the present day in South America, Nutman and Cordani (1993) have nevertheless reported a single zircon core from Sete Voltas giving a 207Pb206Pb age of 3.473 ± 0.008 Ga. The present study, together with some other recent data (Mougeot et al., 1995; Santos Pinto et al., 1995a) indicates that the 3.4-Ga-old South American continental crust, until now considered as of very small geographical extent, in fact made up a true continental block corresponding in size to at least the whole present-day Gaviao block. Major and trace element petrogenetic modelling for the young grey gneisses and the porphyritic granodiorites indicates that the precursor magmas are derived from the melting of pre-existing crustal rocks similar to the old grey gneisses. This conclusion is supported by the intense migmatization of the old grey gneisses which took place before the intrusion of the younger intrusives. The calculated residual mineral assemblage in equilibrium with the magmatic liquid is quartz + hornblende + plagioclase, a condition that is achieved between P = 10 kbar, T ≈ 800°C, <5% weight H2O, or P = 15 kbar, T ≈ 700°C, ∼15% weight H2O (Johnson and Wyllie, 1988). This suggests that the hydrous melting of the old grey gneisses occurred at ca 3.17 Ga at depths of ca 30–45 km, thus giving a minimum estimate of the thickness of this part of the Archaean crust. This estimate, as well as the development of an intense migmatization and the strong foliation in the old grey gneisses, can be interpreted as classical features of collisional thickening. Taken with the existence of horizontal tectonics in Archaean cratons older than 3.0 Ga, such a conclusion indicates that, more or less modern types of plate tectonic mechanisms operated in the early Archaean in the studied terrain.

Evolution of the precambrian In-Ouzzal block (Central Sahara, Algeria)

Abstract New structural and geochronological data are used to constrain the evolution of the Precambrian In-Ouzzal block situated on the eastern side of the West African Craton. The results of this study can be summarized as follows: 1. (i) The block can be subdivided into two series; one is mainly composed of Archaean granitic gneisses (3.2–2.5 Ga old), while the other contains metasedimentary and basic to ultrabasic rocks which can be compared with those described in Archaean supracrustal terrains. 2. (ii) The entire block is affected by very high temperature granulite-facies metamorphism. 3. (iii) The metamorphism is associated with two main deformational phases, the first of which is responsible for a regional foliation foliation refolded during the second phase. 4. (iv) The major tectono-metamorphic event in the area is related to a 2.0 Ga-old (Eburnian) orogeny which ended with carbonatite emplacement. Nearly all evidence of the earliest crustal evolution events (i.e. the initial emplacement and deformation of the granitic and supracrustal materials) has been erased by the Eburnian event.