Hercynian UHT metamorphism of crustal xenoliths from Bou Ibalghatene and Tafraoute, Middle Atlas, Morocco

Abstract

The Bou Ibalghatene and Tafraoute volcanoes in the Middle Atlas of Morocco erupted during the Pliocene to Quaternary as alkali basalts (Moukadiri and Bouloton 1998, Moukadiri and Pin 1998), enclosing numerous xenoliths. Both mantle and crustal xenoliths are found, but mantle xenoliths are more abundant.Crustal xenoliths from both volcanoes are the subject of this thesis. There are four types of crustal xenoliths, three of which occur in Bou Ibalghatene and two in Tafraoute: - orthogranulite without garnet (Bou Ibalghatene and Tafraoute) - orthogranulite with garnet (Bou Ibalghatene) - orthogranulite with garnet and orthopyroxene (Bou Ibalghatene) - paragranulite with garnet and sillimanite (Tafraoute) The main focus lies on crustal xenoliths which contain garnet. The xenoliths were studied using optical microscopy, microprobe analyses, SEM analyses, element mapping, monazite dating and Raman spectroscopy. Using various thermobarometric techniques (garnetorthopyroxene thermobarometry, two-feldspar thermometry, and TWQ and Perplex modelling) a PT-path for the xenoliths has been constrained. The PT-path is approximately the same for the xenoliths from the two volcanoes. It consists of 4 stages, of which the first three are representative for the lower crust below the Middle Atlas, whereas the last stage represents the magma upwelling and finally the eruption. The peak metamorphic conditions experienced by the xenoliths, at least 900-950 degrees at 10 kb, were of the Ultra-High- Temperature (UHT) type, and are dated at ~300 Ma, i.e. Hercynian, using monazite dating. This is a new location of UHT type metamorphism in the world. There could be no larger contrast with the low-grade metamorphism that dominates the surface geology. After the peak metamorphism there was a period of cooling, from 900-600 degrees. Based on complete homogenisation of the garnet grains and Fe-Mg diffusion, the cooling was a slow process of at least 8 Ma, but probably in the order of 200 Ma. A garnet decompressional breakdown reaction was obsereved in all xenoliths containing garnet. Garnet breaks down to form a symplectite of orthopyroxene, plagioclase and spinel. The orthopyroxene found in the symplectite has extremely high values of Al content (up to 17.3 wt%). This garnet breakdown reaction took place at very high temperatures during the transport of the xenoliths to the surface, probably within a few days.