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P–T–X relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria
Author(s) -
BERTRAND P.,
OUZEGANE KH.,
KIENAST J. R.
Publication year - 1992
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1992.tb00069.x
Subject(s) - pyroxene , granulite , geology , mineral , geochemistry , cordierite , mineralogy , craton , metamorphic rock , biotite , mafic , precambrian , analytical chemistry (journal) , olivine , materials science , chemistry , quartz , metallurgy , paleontology , ceramic , facies , structural basin , tectonics , chromatography
The Al–Mg‐rich granulites from the In Ouzzal craton, Algeria, show a great diversity of mineral reactions which correspond to continuous equilibria as predicted by phase relationships in the FeO–MgO–Al 2 O 3 –SiO 2 system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a high‐ P stage characterized by the growth of coarse mineral assemblages involving sapphirine and the disappearance of early corundum and spinel‐bearing assemblages; (2) a high‐ T stage characterized by the development of Sa–Qz‐bearing assemblages; and (3) a later stage, in which garnet‐bearing assemblages are replaced by more or less fine symplectites involving cordierite. During the course of early mineral reactions, the distribution coefficient, K d , between the various ferromagnesian phases decreased significantly whereas Al 2 O 3 in pyroxene increased concomitantly. These observations, when combined with topological constraints, clearly indicate that the high‐ P stage 1 was accompanied by a significant rise in temperature (estimated at 150 ± 50° C) under near isobaric conditions, in agreement with the reaction textures. By stage 2, pressure and temperature were extreme as evidenced by the low K d value between orthopyroxene and garnet ( K d = 2.06–1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [100 Mg/(Mg + Fe) = 60.6]. Mineral thermometry based on Fe–Mg exchange between garnet and pyroxene and on Al‐solubility in pyroxene gives temperatures close to 970 ± 70° C at 10 ± 1.5 kbar. These results are in agreement with the development of Sa–Qz assemblages on a local scale. Late mineral reactions have been produced during a decompression stage from about 9 to 6 kbar. Except for local re‐equilibration of Mg and Fe at grain boundaries, there is no evidence for further reactions below 700° C. We interpreted the whole set of mineral reactions as due to changes in pressure and temperature during a tectonic episode located at c. 2 Ga. Because of the lack of evidence for further uplift after the thermal relaxation which occurred at c. 6 kbar, it is possible however that the exhumation of this granulitic terrane occurred in a later tectonic event unrelated to its formation.