ISSN:
1525-1314
Source:
Blackwell Publishing Journal Backfiles 1879-2005
Topics:
Geosciences
Notes:
Low-pressure crystal-liquid equilibria in pelitic compositions are important in the formation of low-pressure, high-temperature migmatites and in the crystallization of peraluminous leucogranites and S-type granites and their volcanic equivalents. This paper provides data from vapour-present melting of cordierite-bearing pelitic assemblages and augments published data from vapour-present and vapour-absent melting of peraluminous compositions, much of which is at higher pressures. Starting material for the experiments was a pelitic rock from Morton Pass, Wyoming, with the major assemblage quartz-K feldspar-biotite-cordierite, approximately in the system KFMASH. A greater range in starting materials was obtained by addition of quartz and sillimanite to aliquots of this rock. Sixty-one experiments were carried out in cold-seal apparatus at pressures of 1–3.5 kbar (particularly 2 kbar) and temperatures from 700 to 840 °C, with and without the addition of water. In the vapour-present liquidus relations at 2 kbar near the beginning of melting, the sequence of reactions with increasing temperature is: Qtz + Kfs + Crd + Sil + Spl + V = L; Qtz + Kfs + Crd + Spl + Ilm + V = Bt + L; and Qtz + Bt + V = Crd + Opx + Ilm + L. Vapour-absent melting starts at about 800 °C with a reaction of the form Qtz + Bt = Kfs + Crd + Opx + Ilm + L. Between approximately 1–3 kbar the congruent melting reaction is biotite-absent, and biotite is produced by incongruent melting, in contrast to higher-pressure equilibria. Low pressure melts from pelitic compositions are dominated by Qtz-Kfs-Crd. Glasses at 820–840 °C have calculated modes of approximately Qtz42Kfs46Crd12. Granites or granitic leucosomes with more than 10–15% cordierite should be suspected of containing residual cordierite. The low-pressure glasses are quite similar to the higher-pressure glasses from the literature. However, XMg increases from about 0.1–0.3 with increasing pressure from 1 to 10 kbar, and the low-temperature low-pressure glasses are the most Fe-rich of all the experimental glasses from pelitic compositions.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1111/j.1525-1314.2004.00497.x
