Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin

International audienceFrom the Neoproterozoic to the early Paleozoic, the northern Gondwana margin was sequentially shaped by the Cadomian accretionary and the Variscan collisional orogens which offers the opportunity to investigate the relative extent of crust production/reworking in both geodynamic settings. In the eastern part of the Variscan French Massif Central (FMC), the Velay Orthogneiss Formation (VOF) represents a consistent lithological unit of the pre-Variscan basement and comprises augen gneisses and leucogneisses. Such rocks constitute a unique record of the pre-Variscan magmatic history and bear critical information on the crustal evolution of the northern Gondwana margin.Here, we present whole–rock major and trace element compositions indicating that: (i) the VOF shows a remarkable geochemical homogeneity; (ii) the protolith of the augen gneisses corresponds to strongly peraluminous, “S-type” porphyritic granites originating from partial melting of an Ediacaran sedimentary sequence; (iii) the leucogneisses are former leucogranites generated by fractionation of the magma at the origin of the porphyritic granites; and (iv) the whole suite emplaced at shallow crustal levels (< 7 km). U–Pb LA–(MC–)ICP–MS analyses on zircon yielded similar emplacement ages of c. 542 Ma and a narrow range of εHf(t) clustering around 0 for the protoliths of both augen and leucogneisses. This homogeneous Hf isotope signature, notably uncommon for S-type granites, would originate from a sequential process of: (i) inherited zircon dissolution during melting and ascent in the crust due to Zr-undersaturated conditions, (ii) isotopic homogenization of the melt by advection and elemental/isotopic diffusion, followed by (iii) early saturation upon emplacement owing to rapid cooling at shallow crustal levels.We propose that partial melting of Ediacaran sediments occurred during inversion of a Cadomian back-arc basin and was promoted by the high thermal gradient typical of thinned crust domains. Therefore, the VOF and other Cadomian S-type granitoids from the northern Gondwana margin are indicative of substantial crust reworking away from any proper continental collision zone

Exhumation of a migmatitic unit through self-enhanced magmatic weakening enabled by tectonic contact metamorphism (Gruf complex, Central European Alps)

The Central Alpine lower crustal migmatitic Gruf complex was exhumed in contact to the greenschist-grade Chiavenna ophiolite and gneissic Tambo nappe leading to a lateral gradient of similar to 70 degrees C/km within the ophiolite. The 14 km long, E-W striking subvertical contact now bridges metamorphic conditions of similar to 730 degrees C, 6.6 kbar in the migmatitic gneisses and similar to 500 degrees C, 4.2 kbar in the serpentinites and Tambo schists 2-4 km north of the contact. An obvious fault, mylonite or highly sheared rock that could accommodate the similar to 8.5 km vertical displacement is not present. Instead, more than half of the movement was accommodated in a 0.2-1.2 km thick orthogneiss of the Gruf complex that was heterogeneously molten. Discrete bands with high melt fractions (45-65%) now contain variably stretched enclaves of the adjacent MOR-derived amphibolite. In turn, the adjacent amphibolites exhibit tonalitic in-situ leucosomes and dikes i.e., were partially molten. The H2O necessary for fluid-assisted melting of the orthogneiss and amphibolites was likely derived from the tectonic contact metamorphism of the Chiavenna serpentinites, at the contact now in enstatite + olivine-grade. U-Pb dating of zircons shows that partial melting and diking occurred at 29.0-31.5 Ma, concomitant with the calc-alkaline Bergell batholith that intruded the Gruf. The major driving forces of exhumation were hence the strong regional North-South shortening in the Alpine collisional belt and the buoyancy provided by the Bergell magma. The fluids available through tectonic contact metamorphism led to self-enhanced magmatic weakening and concentration of movement in an orthogneiss, where melt-rich bands provided a low friction environment. Continuous heating of the originally greenschist Chiavenna ophiolite and Tambo gneisses + schists by the migmatitic Gruf complex during differential uplift explains the skewed temperature profile, with intensive contact heating in the ophiolite but little cooling in the portion of the now-exposed Gruf complex.ISSN:0010-7999ISSN:1432-096

Cadomian S-type granites as basement rocks of the Variscan belt (Massif Central, France): Implications for the crustal evolution of the north Gondwana margin

International audienceFrom the Neoproterozoic to the early Paleozoic, the northern Gondwana margin was sequentially shaped by the Cadomian accretionary and the Variscan collisional orogens which offers the opportunity to investigate the relative extent of crust production/reworking in both geodynamic settings. In the eastern part of the Variscan French Massif Central (FMC), the Velay Orthogneiss Formation (VOF) represents a consistent lithological unit of the pre-Variscan basement and comprises augen gneisses and leucogneisses. Such rocks constitute a unique record of the pre-Variscan magmatic history and bear critical information on the crustal evolution of the northern Gondwana margin.Here, we present whole–rock major and trace element compositions indicating that: (i) the VOF shows a remarkable geochemical homogeneity; (ii) the protolith of the augen gneisses corresponds to strongly peraluminous, “S-type” porphyritic granites originating from partial melting of an Ediacaran sedimentary sequence; (iii) the leucogneisses are former leucogranites generated by fractionation of the magma at the origin of the porphyritic granites; and (iv) the whole suite emplaced at shallow crustal levels (< 7 km). U–Pb LA–(MC–)ICP–MS analyses on zircon yielded similar emplacement ages of c. 542 Ma and a narrow range of εHf(t) clustering around 0 for the protoliths of both augen and leucogneisses. This homogeneous Hf isotope signature, notably uncommon for S-type granites, would originate from a sequential process of: (i) inherited zircon dissolution during melting and ascent in the crust due to Zr-undersaturated conditions, (ii) isotopic homogenization of the melt by advection and elemental/isotopic diffusion, followed by (iii) early saturation upon emplacement owing to rapid cooling at shallow crustal levels.We propose that partial melting of Ediacaran sediments occurred during inversion of a Cadomian back-arc basin and was promoted by the high thermal gradient typical of thinned crust domains. Therefore, the VOF and other Cadomian S-type granitoids from the northern Gondwana margin are indicative of substantial crust reworking away from any proper continental collision zone