Diffusion-controlled biotite breakdown reaction textures at the solid/liquid transition in the continental crust |
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Authors: | Pierre Barbey |
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Institution: | (1) CRPG-CNRS, Nancy Université, B.P. 20, 54501 Vandoeuvre-lès-Nancy Cedex, France |
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Abstract: | Two-phase quartz intergrowths with garnet, cordierite and tourmaline occur commonly in prograde high-temperature migmatites,
granulites, as well as in the last crystallization stages of biotite granites. Structural, microtextural and mineralogical
data show that they result from the breakdown of biotite in the presence of a melt phase associated with incongruent dissolution
of feldspars into the melt and silica release (giving quartz in silica saturated rocks). Biotite breakdown and growth of Al-rich
ferromagnesian minerals, occurring at the solid–liquid transition in the crust (early melting or final crystallization), is
kinetically controlled by Fe and Mg mass transport, the network-forming cations Si and Al being locally compensated for by
feldspar dissolution/crystallization. This process leads to significant changes with respect to equilibrium dehydration-melting
reactions wherein quartz is a reactant and K-feldspar a reaction product. Therefore, quartz inclusions commonly occurring
in garnets from granulite-facies metapelites and metagraywackes are not simply grains passively included during garnet growth.
They may also correspond to newly crystallized phases. Resorption of feldspar may lead to more alkaline melt and to crystalline
residue richer in Al than expected under equilibrium conditions. Hence, excess alumina in granulite-facies rocks is not necessarily
related to initial alumina-rich whole-rock compositions (as currently considered), but may be due, at least partly, to kinetics
of melting. |
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Keywords: | Chemical diffusion Intergrowths Garnet Cordierite Tourmaline |
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