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Abstract: We propose a review of the tectonic, magmatic and hydrothermal evolution of slow spreading ridges, focusing on concepts which we think are most relevant to discuss the rift to drift transition at magma-poor ocean–continent transitions (OCTs). Based on this review, we emphasize the importance of the thermal regime as a key parameter and we propose that the thermal regime of magma-poor OCTs evolves in the following principal phases: the onset of decompression mantle melting, the onset of localized plate divergence, the installation of a ridge-type thermal regime (active heat balance, no inheritance), which we argue is the most practical definition for continental breakup, and, possibly, the onset of ridge-type, focused mantle upwelling. We propose this phased evolution as a framework in which to develop further research, combining our understanding of rifting and seafloor spreading, in order to better quantify the processes that control continental breakup. [Copyright &y& Elsevier]

Abstract: Permo-Triassic intermediate–felsic magmatism is developed along the Truong Son fold belt, located in the eastern margin of the Indochina Block. It comprises a succession of the active continental margin associations: calc-alkaline volcano-plutonic associations (272–248Ma), peraluminous granites (259–245Ma), and subalkaline felsic volcano-plutonic associations (younger than 245Ma). Detailed study of geochemical characteristics such as trace elements (LILE, REE, HFSE) and isotopes (Sr, Nd, Pb) indicates that they are homogeneous and that they are products of the Palaeotethys subduction process in relation to Indochina (IC)/North Vietnam–South China (NV–SC) amalgamation (S.L. Chung et al., Abstr., GEOSEA 98, Malaysia, 1998, pp. 17–19). The Indosinian characteristics are represented by mantle–crust interaction in magma generation, controlled by their emplacement localities in relation to the Kontum Uplift. The spatial and temporal evolution of Permo-Triassic magmatism allows reconstructing the geodynamic history of the Indosinian orogeny. It confirms that this event ended in Early to Middle Triassic (246–240Ma, after C. Lepvrier et al., Tectonophysics 393 (2004) 87–118). [Copyright &y& Elsevier]

Abstract: Palaeozoic formations of the Tassilis Oua-n-Ahaggar (southeastern Hoggar) include magmatic rocks in the Tin Serririne syncline. Slight contact metamorphism of the overlying bed and studies of anisotropy of magnetic susceptibility of these rocks show that the latter correspond to sills and NW–SE or north–south dykes. 40K/40Ar dating of separated feldspars and whole rock for one sample and of whole rock for two other samples give a mean age of (at the 2-σ level), thus corresponding to a Lower Carboniferous (Tournaisian) age. Taking into account both the age of this magmatism and the stratigraphic and structural data for this region suggests that dolerites were emplaced within distensive zones that are related to the reactivation of Panafrican faults. To cite this article: H. Djellit et al., C. R. Geoscience 338 (2006). [Copyright &y& Elsevier]