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

Mantle and crustal sources in the genesis of late-hercynian granitoids (NW Portugal) : geochemical and Sr-Nd isotopic constraints

Large volumes of granitoids were emplaced in the Hercynian Central Iberian Zone during the last ductile deformation phase (D3, 300-320 Ma). The biotite-rich granitoids are the most abundant: (1) syn-D3 granodiorites-monzogranites (313-319 Ma) with calc-alkaline and aluminopotassic affinities; (2) late-D3 granodiorites-monzogranites (306-311 Ma), related to subalkaline and aluminopotassic series. These granitoids are associated with coeval gabbro-norite to granodiorite bodies and/or mafic microgranular enclaves. Both granitoids and basic-intermediate rocks show petrological, geochemical and isotopic evidence of interaction between felsic and mafic magmas. The mantle-derived melts, represented by shoshonitic gabbro-norites, were probably derived from an enriched and isotopically homogeneous source (Srl = 0.7049 to 0.7053, eNd= -2.1 to -2.5). In some syn- and late-D3 plutons there are evidences of essentially crustal granites, represented by moderately peraluminous monzogranites of aluminopotassic affinity. They have similar Nd model ages (1.4 Ga) but different isotopic compositions (Srl = 0.7089 to 0.7106, eNd= -5.6 to -6.8), revealing a heterogeneous crust. Potential protoliths are metasedimentary (immature sediments) and/or fclsic meta-igneous lower crust materials. Large amounts of hybrid magmas were generated by the interaction of these coeval mantle- and crust-derived liquids, giving rise to slightly peraluminous monzogranites/granodiorites of calc-alkaline and subalkaline affinities, which display more depleted isotopic compositions than the crustal end-members (Sr, = 0.7064 to 0.7085, eNd = -4.4 to -6.2). Petrogenetic processes involving mingling and/or mixing and fractional crystallization (at variable degrees) in multiple reservoirs are suggested. A major crustal growth event occurred in late-Hercynian times (305-320 Ma) related to the input of juvenile mantle magmas and leading to the genesis of composite calc-alkaline and subalkaline plutons, largely represented in the Central Iberian Zone.Financial support was provided by FCT (project PRAXIS 2/2.1/391/94), France-Portugal Scientific Cooperation Programs and by the University of Minho

Hercynian late-post-tectonic granitic rocks from the Fornos de Algodres area (Northern Central Portugal)

The Fornos de Algodres Complex (FAC) comprises several intrusions of late-post-tectonic Hercynian granitic rocks ranging in composition from hornblende granodiorites and quartz monzodiorites, through coarse porphyritic biotite granites and two-mica granites (coarse-, medium- and fine-grained), to muscovite-rich leucogranites. Field and regional constraints show that the emplacement of this large, composite, batholithic complex post-dates the main Variscan regional deformation phases (D1 + D2 + D3) and associated metamorphic events. Field, petrographic and geochemical data suggest a strong genetic relationship between most of the members of the FAC. However, their Rb-Sr and Sm-Nd isotopic signatures appear to rule out any genetic process involving a single homogeneous source and/or closed-system fractional crystallization of the same parental magma. A model involving hybridization of mantle-derived basaltic liquids with crustal anatectic melts followed by further contamination and fractional crystallization is proposed to explain the isotopic and geochemical variation trends defined by the FAC granitic rocks

High precision zircon U-Pb age of a tonalite from the Archean granite-greenstone terrain, Qingyuan, NE China.

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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 ± 5 Ma, as shown by both U–Pb and 207Pb/206Pb 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

Zircon typology combined with Sm-Nd whole-rock isotope analysis to study Brioverian sediments from the Armorican Massif

The identification of source materials contributing to mature terrigenous clastic deposits is made difficult due to a lack of useful discriminant criteria. Sm-Nd isotope analysis can provide some indication of the presence of mantle-derived constituents in elastic rocks. However, since this method is based on whole-rock samples, it is only possible to obtain the averaged composition of the different source materials involved. Moreover, Sm-Nd isotope systematics can be strongly influenced by the presence of heavy minerals rich in rare earth elements and/or displaying isotopic ratios very different to the sedimentary host. In this manner, 0.1% of zircon or 0.01% of monazite from an extraneous source is sufficient to modify significatively the Sm-Nd signature of the whole-rock. The typological study of zircon populations is an extremely valuable tool, especially since this mineral is highly resistant and exhibits a morphology controlled by the physical and chemical conditions under which it crystallized. Zircons that have been reworked in a sedimentary deposit display typologies that make it possible to identify the different igneous rock-types present in the source region. Moreover the typological study can be associated with a single-grain Pb-Pb dating. This twofold approach was applied to Brioverian sedimentary rocks (Neoproterozoic III to Early Palaeozoic in age) from the Central Brittany Domain (Armorican Massif, NW France). It is proposed that these deposits inherited a major component from juvenile crustal materials (eNd(s4e) = - 1.4 to -6.31, reflecting the presence of igneous precursors with a dominantly mantle-derived origin (zircon sub-types S19, S20, S24, S25 etc.), to which was added another component of crustal anatectic affinity (zircon sub-types S2, S6, S7 etc.). Two igneous source regions are identified on the basis of the present study: the anatectic granitic massifs of the Mancellian Batholith (540 Ma), along with a coeval ignimbritic suite of end-Proterozoic/Eocambrian age, and the syn- and post-tectonic dioritic massifs (590 to 530 Ma). Apart from these main sources of elastic supply, it is very likely that sporadic contributions arose from basic to intermediate volcanic activity of mantle origin linked to the Cadomian orogeny and from the Icartian basement