Petrology of phlogopite‐sapphirine‐bearing Al‐Mg granulites from Ihouhaouene, In Ouzzal, Hoggar, Algeria: an example of phlogopite stability at high temperature

Abstract

A B S TR A CT Al-Mg granulites, with cordierite, garnet, sapphirine, orthopyroxene, sillimanite, spinel, phlogopite, K-feldspar, plagioclase and variable quartz from Ihouhaouene (In Ouzzal, Algeria), display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite, and of garnet. The succession of parageneses suggests that the P-T -t evolution corresponds to decompression with cooling from peak conditions of about 950°C and 10 kbar. This decompression path is obtained from the paragenetic analysis in the FMAS system. However, according to current KFMASH grids, this P-T -t evolution should take place outside the stability field of phlogopite+quartz; yet this assemblage is probably stable during most of the P-T evolution, notably during peak metamorphism. This discrepancy is interpreted as the effect of the high content of F in phlogopite which should shift its stability limit towards higher temperature. The consequences of this shift on the phase relationships in the KFeMASH system are investigated and it is concluded that a topological inversion could exist in the F-bearing system.

A review of Archaean and Paleoproterozoic evolution of the In Ouzzal granulitic terrane (Western Hoggar, Algeria)

“…In addition, the Al-Mg granulites of Ihouhaouene show reactions involving phlogopite at very-high-temperature (900-1000°C) without partial melting in a liquid-absent KFMASH system (Mouri et al, 1993;Mouri, 1995). One of the hypotheses proposed by Mouri et al (1996) to explain the stability of the phlogopite + quartz assemblage at very high temperature--without melting--is the presence of fluorine, which stabilizes biotite under extreme granulite-facies conditions as shown by the experimental work of Carrington and Harley (1995). Indeed, biotites in the Al-Mg granulites of In Ouzzal have high fluorine contents attaining 2-4%.…”

“…All primary and intermediate associations developed during the prograde stage are destabilized during a metamorphic evolution characterized by a significant drop in pressure down to 5 kbar (Ouzegane, 1987;Bertrand et al, 1992;Mouri et al, 1996;Ouzegane and Boumaza, 1996;Adjerid, 2002;Ouzegane et al, 2003). A P-X diagram was constructed in the FMAS system using the program Thermocalc (Holland and Powell, 1998).…”

“…This approach makes it possible to determine the variations of pressure, temperature and X H 2 O . The estimation of the conditions of pressure and temperature has been carried out using Al-Mg granulites (Ouzegane, 1987;Kienast and Ouzegane, 1987;Bertrand et al, 1992;Ait Djafer, 1996;Boumaza, 1996;Mouri et al, 1996;Ouzegane and Boumaza, 1996;Adjerid, 2002), Al-Fe granulites (Ait Djafer, 1996;Djemai, 1996;, corundum-magnetite quartzites , calc-silicate granulites (Benyahia, 1996) and garnet pyroxenites (Ouzegane, 1987;Kienast and Ouzegane, 1987;Djemai, 1996). The most complete sequence of reactions could be identified in the aluminomagnesian granulites, where a prograde metamorphism is recorded under granulite-facies conditions at extreme temperatures (Ouzegane, 1987;Kienast and Ouzegane, 1987;Bertrand et al, 1992;Ouzegane and Boumaza, 1996;Ouzegane et al, 2003).…”

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

“…Natural silicates and synthetic oxides were used as reference for all elements. For details, see Mouri et al (1996).…”

Palaeoproterozoic UHT metamorphism in the Lewisian complex and North Atlantic region

“…Recently, several new terranes, althought narrow belts, suggesting UHT condition were reported from other regions in the world (e.g.. In Ouzzal, Hogger: Bertrand et al, 1992;Ouzegane and Boumaza, 1996;Mouri et al, 1996; Perumalmalai in Eastern Ghats: Raith et al, 1997; Mollendo Camana block: Martignole and Martelat, 2003; Schrimacher Hills, E. Antarctica: Baba et al, 2003). In many cases, UHT assemblages are well preserved in rocks with having high Mg and Al compositions, such as sapphirine bearing rock.…”