Abstract

The Saint Malo massif presents a continuous metamorphic series from gneiss to greenschist facies to anatectic granites; in addition, it shows evidence of all stages of progressive melting. A modal and geochemical study shows that incongruent melting of biotite according to the reactiongraphicplays a major role during anatexis. The distribution of trace elements between the liquid and residual solid states is closely bound to the melting of this mineral phase.Analytical results are compared to the results forecast from a theoretical model of sequential melting at equilibrium. With the exception of strontium, there is good agreement between the theory and the analytical data, which confirms the fundamental role played by the biotite.Rb proceeds normally in the liquid during the melting of biotite; Cr, Ni, and V, on the other hand, take refuge in the cordierite, whereas Co, which is not plentiful in the biotite, is not influenced by the destruction of the latter. In contrast, Sr shows a different behavior from that predicted from the mathematical models. This seems to be due to a mass separation of the liquid during the initial stages of the anatectic process. The liquid, which is injected into the adjacent nonmobilized gneiss, is rich in Sr and Co and poor in Rb, Cr, Ni, and V. This results in a relative impoverishment of the residual solid, in Sr and a little in Co (not predicted from the theoretical models). [Translated by the Journal]

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