Abstract
Biotite gneiss xenoliths included in a basanitic flow in the Causses region (France) show several kinds of partial melting. Rhyolitic glasses appear at the expense of the quartz + feldspar assemblage; initially undersaturated latitic glasses at the expense of biotite + feldspar (+ quartz); lastly hyperaluminous products derivating from large phenoblasts of alkali feldspar.
The microprobe study of these glasses, of their contacts with the host lava, as well as that of the neogenic mineral phases, show that contamination occurs through, at least, four possible processes:
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1.
Diffusion at the interface between two liquids of contrasted composition (rhyolitic and basanitic).
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2.
Mechanical mixing of two liquids of less contrasted chemistry (latitic and basanitic).
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3.
Progressive solution of a solid phase (quartz) into the magma.
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4.
Vapour phase transfers.
Qualitative magmatic contamination in alkalies (mainly K) and silica results from all these processes; quantitatively, the most important of them seems to be 4.
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Maury, R.C., Bizouard, H. Melting of acid xenoliths into a basanite: An approach to the possible mechanisms of crustal contamination. Contrib. Mineral. Petrol. 48, 275–286 (1974). https://doi.org/10.1007/BF00951335
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DOI: https://doi.org/10.1007/BF00951335