Nb-Ta fractionation in peraluminous granites: A marker of the magmatic-hydrothermal transition: REPLY

International audienceWe thank A. Stepanov and co-authors (Stepanov et al., 2016) forgiving us the opportunity to clarify some important points made in ouroriginal manuscript (Ballouard et al., 2016) and to discuss the issuesraised in their Comment. In Ballouard et al. (2016), we propose that thedecrease of the Nb/Ta ratios to <~5 in peraluminous granites “is theconsequence of both fractional crystallization and sub-solidus hydrothermalalteration,” an interpretation challenged by Stepanov et al. (2016)who argue that low Nb/Ta ratios in peraluminous granites are betterexplained by magmatic fractionation and that the role of magmatichydrothermalprocesses is not significant

Evaluation of the chemical reactivity of the fluid phase through hard-solft acid-base concepts in magmatic intrusions with applications to ore generation.

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Granitic batholiths : from pervasive and continuous melting in the lower crust to discontinuous and spaced plutonism in the upper crust.

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TOWARD A NEW PARADIGM FOR GRANITE GENERATION

Ideas about granite generation have evolved considerably during the last two decades. The present paper lists what ideas were accepted and later modified, concerning the processes acting during the four stages of granite generation: melting, melt segregation and ascent, and emplacement. The active role of the mantle constitutes a fifth stage. Fluid assisted melting, deduced from metamorphic observations, was used to explain granite and granulite formation. Water seepage into meta-sedimentary rocks can produce granitic melt by depleting melting temperature. CO2 released by the mantle helps transforming rocks into granulites. However, dehydration melting is now considered as being the origin of most granitic melts, as confirmed by experimental melting. First hydrous minerals involved are muscovite, then biotite at higher temperature. At even deeper conditions, hornblende dehydration melting leads to calc-alkaline magmas. Melt segregation was first attributed to compaction and gravity forces due to density contrast between melt and its matrix. This was found insufficient for magma segregation in the continental crust because they were transposed from mantle conditions (decompression melting) to crustal conditions (dehydration melting). Rheology of two-phase materials documents that melt segregation is discontinuous in time, occurring in successive bursts. Analogue and numerical models confirm the discontinuous melt segregation. Compaction and shear localisation interact non-linearly, so that melt segregates into tiny conduits. Melt segregation occurs at low degree of melting. Global diapiric ascent and fractional crystallisation in large convective batholiths also shown to be inadequate and at least partly erroneous. Diapiric ascent cannot overcome the crustal brittle-ductile transition. Fracture-induced ascent faces the neutral buoyancy level at which the ascent should stop, but it doesn't. Non-random orientation of magma feeders within the ambient stress field indicates that deformation controls magma ascent. Detailed gravity and structural analyses indicate that granite plutons are built from several magma injections, each of small size and with evolving chemical composition. Detailed mapping of the contact between successive magma batches documents either continuous feeding, leading to normal petrographic zoning, or by periods separated in time, commonly leading to reverse zoning. The local deformation field controls magma emplacement and imposes the shape of plutons. A typical source for granite magmas involves three components, from the mantle, lower and intermediate crusts. The role of the mantle in driving and controlling essential crustal processes appears necessary in providing stress and heat, as well as specific episodes of time for granite generation. These mechanisms constitute a new paradigm for granite generation

The role of discontinuous magma inputs in felsic magma and ore generation.

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Textures and melt-crystal-gas interactions in granites

Felsic intrusions present ubiquitous structures. They result from the differential interactions between the magma components (crystal, melt, gas phase) while it flows or when the flow is perturbed by a new magma injection. The most obvious structure consists in fabrics caused by the interactions of rotating grains in a flowing viscous melt. New magma inputs through dikes affect the buk massif flow, considered as global within each mineral facies. A review of the deformation and flow types developing in a magma chamber identifis the patterns that could be expected. It determines their controlling parameters and summarizes the tools for their quantification. Similarly, a brief review of the rheology of a complex multi-phase magma identifies and suggests interactions between the different components. The specific responses each component presents lead to instability development. In particular, the change in vorticity orientation, associated with the switch between monoclinic to triclinic flow is a cause of many instabilities. Those are preferentially local. Illustrations include fabric development, shear zones and flow banding. They depend of the underlying rheology of interacting magmas. Dikes, enclaves, schlieren and ladder dikes result from the interactions between the magma components and changing boundary conditions. Orbicules, pegmatites, unidirectional solidification textures and miarolitic cavities result from the interaction of the melt with a gaseous phase. The illustrations examine what is relevant to the bulk flow, local structures or boundary conditions. In each case a field observation illustrates the instability. The discussion reformulates instability observations, suggesting new trails for ther description and interpretation in terms of local departure to a bulk flow. A brief look at larger structures and at their evolution tries to relate these instabilities on a broader scale. The helical structures of the Říčany pluton, Czech Republic and by the multiple granitic intrusions of Dolbel, Niger illustrate such events

Rheology of a two-phase material with applications to partially molten rocks, plastic deformation and saturated soils

in press into Flow processes in faults and shear zones Alsop, I. & Holdsworth, R.E. (Eds) Geological Society of London Special PublicationA global model is presented to account for the specific rheology of a two-phase material. Examples of observations are taken from a crystallising magma. Applications are ported to a partially molten rock, plastic deformation and soil liquefaction. The general behaviour of the viscosity is drawn as a function of the strain rate and the amount of solid phase. It constitutes a 3D diagram developing a cubic surface. The cubic equation is justified by thermodynamic considerations. It basically results from the mixing of a Newtonian (n=1) and a power law (n=3) type of deformation. The diagram shows two types of rheological response. At high strain rate values, the viscosity contrast between the two phases is the lowest. A bulk en masse behaviour results, as observed during tectonic activity. It manifests by a homogeneous transport of magma during emplacement and fabric development. An equivalent medium, with averaged viscosity is a good proxy. Conversely, at low strain rate values, the viscosity contrast between the two phases is the highest. The two end members behave according to their respective rheology. In between, a transitional state develops, in which instability occurs depending on the strain rate and stress conditions. It manifests in the 3D diagram by a cusp shape. Rheology presents continuous jumps between the liquid-like and the solid-like rheology. They result in strain localisation or phase segregation. The later preferentially develops during magma crystallisation. Deformation under a constant amount of each phase is also possible, resulting in pressure dissolution-like processes. A major cause for instability results from a bifurcation in the solution plane of the equation of viscous motion. It is comparable with strain softening. A similar situation should develop when mixing Newtonian and power law rheology as during diffusion and dislocation creep, or water saturated sediment deformation. Owing to the continual jumps between the two types of rheology, hysteresis, or memory effect may develop. Rapid cyclic deformation may drive strain to extreme straining. The effect of simple shear seems much more effective than pure shear (compaction) to segregate the weak phase out of its strong matrix. The development of instabilities and continuous jumps from one rheology to the other lead to discontinuous motion of the weak phase. In a molten region, it corresponds to discontinuous burst of magma that are extracted

Addressing ore formation and exploration

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