The southern São Francisco Paleocontinent (SFP) comprises Archean nuclei and Paleoproterozoic complexes encompassing magmatic arcs juxtaposed during a Rhyacian to Orosirian orogenic event. The Juiz de Fora Complex (JFC) represents an imbricated thrust system that comprises orthogranulites with a wide compositional range formed in an intra-oceanic setting during the Siderian to the Orosirian and later accreted to the southeastern margin of the SFP. Here we report new petrological, geochemical, whole-rock Nd and Sr data, as well as zircon U–Pb ages from felsic and mafic orthogranulites from the JFC. The new data is combined with a regional compilation that enables an evaluation of the interaction between magmatism and orogenetic episodes in the context of the consolidation of São Francisco Paleocontinent during the Rhyacian–Orosirian. Pre collisional Island Arc tholeiites (IAT), Tonalites-Trondhjemites-Granodiorites (TTGs) and sanukitoid magmatism occurred from 2200 Ma to 2085 Ma. This was followed by post-collisional magmatism, which is represented by hybrid granitoids coeval with the emplacement of E-MORB basic rocks. Crustal signatures for the Rhyacian to Orosirian evolution are highlighted by the dominance of negative εNd(t) associated with Meso- to Neoarchean Nd TDM model ages as well as inherited zircon grains from the hybrid granitoids. The JFC is extensively highlighted in the literature as a primitive intra-oceanic arc, but here we propose the reworking or recycling of ancient crustal segments within the mature arc stage of the JFC, suggesting a Mesoarchean crustal source involved in the JFC evolution. 相似文献
Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings.
Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite, whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 °C.
Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni. These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth. 相似文献
The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than 2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean. 相似文献