International audienceIn their late stages of evolution, peraluminous granitic melts exsolve large amounts of fluidswhich can modify the chemical composition of granitic whole-rock samples. The niobium/tantalum (Nb/Ta) ratio is expected to decrease during the magmatic differentiation of graniticmelts, but the behavior of both elements at the magmatic-hydrothermal transition remainsunclear. Using a compilation of whole-rock geochemical data available in the literature, wedemonstrate that fractional crystallization alone is not sufficient to explain the distribution ofNb-Ta in most peraluminous granites. However, we notice that most of the granitic samplesdisplaying evidence of interactions with fluids have Nb/Ta < 5. We propose that the decreaseof the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization andsub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ~5 fingerprints themagmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ~5appears to be a good marker to discriminate mineralized from barren peraluminous granites

Permian large-scale transcurrent tectonics and massive magmatism are prominent features of the Tianshan belt and neighboring regions of the Central Asian Orogenic Belt. Structural, geochronological and geochemical analyses of Carboniferous and Permian intrusive rocks associated with the Kangguer-Huangshan Shear Zone (eastern Chinese North Tianshan) provide constraints on their tectonic setting and the tectonic evolution of the Tianshan belt as well. Carboniferous granitic rocks were emplaced at 338 ؎ 4 Ma and 347 ؎ 2 Ma, respectively, and show geochemical features typical of the calc-alkaline series. These arc-type granites do not display ductile deformation, probably because they were completely cooled at the time of shearing tectonics, and are only offset by brittle strike-slip faults. In contrast, Permian granitoids display pervasive ductile tectonic features diagnostic of synkinematic emplacement. Four gabbro and diorite samples from the East Huangshan intrusive complex yielded zircon U-Pb ages of 267 to 275 Ma, and a granitic dike is dated at 290 ؎ 1 Ma. The granitic dike is cut by en-echelon right-lateral strike-slip faults, and the mafic intrusive complex displays a sigmoidal shape with mylonitic foliation localized at its margins. Other specific pluton shapes (such as tongue and tadpole-like) and synmagmatic deformation can be observed in intrusions of the same age, showing similar fabrics and kinematics consistent with that of the Kangguer-Huangshan Shear Zone. Numerous mafic to felsic dikes occur within and off the shear zone with a dominant SE-NW orientation and minor varieties in N-S or NNE-SSW directions.One gabbro dike that intrudes the early Carboniferous granite of the East Kanggurtag area yielded a magmatic age of 274 ؎ 4 Ma, and contains older zircons (ϳ340 Ma, ϳ390 Ma, ϳ450 Ma, and 1.3-2.2 Ga) probably inherited from intruded rocks. The Permian intrusive rocks have variable chemical compositions suggesting derivation of these rocks from depleted and undepleted (or enriched) mantle sources with involvement of subductionrelated components. We conclude from our integrated analysis of the geological, structural, geochemical and geochronological data that the Permian magmatic rocks were formed in a post-collisional/post-orogenic setting from multiple sources, and were emplaced under the control of large-scale dextral transcurrent tectonics.