Crustal anatexis and melt extraction during deformation in the restitic xenoliths at El Joyazo (SE Spain)

The dacite of El Joyazo contains abundant metapelitic xenoliths. These can be divided into two main types: garnet-biotite-sillimanite and spinel-cordierite xenoliths. In the xenoliths the widespread occurrence of rhyolitic glass as interstitial films, foliation-parallel layers and primary melt inclusions in all mineral phases indicates that these assemblages developed in the presence of a melt phase, i.e. during anatexis. The composition of the interstitial glass is comparable to that of the melt inclusions, suggesting that melt was locally produced. Phase equilibria indicate that anatexis occurred at P-T conditions of 5–7 kbar and 850±50°C.Several microstructural lines of evidence show that melt extraction was assisted by deformation during foliation development, and that on the scale of the xenoliths (up to 50 cm) melt escaped mainly by flow along foliation planes. The development of a syn-anatectic foliation also suggests that metapelitic rocks were involved in high-grade metamorphism and partial melting prior to fragmentation and dispersion in the host dacite.Mass balance calculations, based on the chemical composition of interstitial glass and melt inclusions in minerals, the bulk xenoliths and representative samples of potential pelitic sources support a model wherein the xenoliths represent restites after the extraction of 30 to 55 wt.% melt from graphitic metapelite protoliths similar to the rocks constituting the surrounding Alpujarride metamorphic complex.

A Further Note on Chlorophaeite and Palagonite

I have read with very great interest Dr. Peacock's paper on the distinction between palagonite and chlorophaeite in the GeologicalMagazine, for April, 1930. On p. 170 thereof, Dr. Peacock gives a brief summary of my conclusions upon the nomenclature of chlorophaeite and palagonite as given in my paper on the same subject. Briefly, my conclusions are that as far as can be judged from existing analyses, the hydrous glassy material forming a portion of the rocks originally called palagonite has a composition that brings it within the range of composition exhibited by various specimens of chlorophaeite, and that, therefore, the term palagonite should be reserved for the impure rock, and the term chlorophaeite used for this hydrous glassy material. It was also recommended by me that the process by which in the case of the basaltic tuffs the primary glass becomes hydrated should be termed palagonitization and that the same term should be applied to the process by which the interstitial glass of basaltic lava flows is altered with production of chlorophaeite. The fact that olivine, if present, a portion of the augite, and occasionally some of the felspar, may also suffer similar alteration or be replaced, is not regarded by me as pointing to an essential difference in the two processes.