Polymetamorphic Al, Mg-rich granulites with orthopyroxene–sillimanite and sapphirine parageneses in Archaean rocks from the Hoggar, Algeria

Kiénast, Jean-Robert; Ouzegane, Khadidja

doi:10.1002/gj.3350220607

Cited by 28 publications

(14 citation statements)

References 50 publications

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“…( , 1991 found this assemblage stable above 1050" C. However, we have seen that because of Fe3+, Sa-Qz from In Ouzzal certainly crystallized between this high-T limit and the invariant point [Qz, Co] which is believed to occur above 900°C (Hensen, 1987). Independent pressure estimates based on garnet-plagioclase-sillimanite-quartz barometry give pressures in the range 8-11 kbar at 900°C (Kienast & Ouzegane, 1987;Ouzegane, 1987), whereas two-pyroxene thermometry yields temperatures as high as 930°C (Ouzegane, 1987). These results are in good agreement with our calculations.…”

Section: P-t Pathmentioning

confidence: 94%

P–T–X relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria

Bertrand

Ouzegane²,

Kiénast

1992

Journal Metamorphic Geology

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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 Fe0-Mg0-A1,03-SiO, system. The sequence of mineral reactions can be subdivided into three distinct stages: (1) a highd 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 A1203 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 f 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 , value between orthopyroxene and garnet (K, = 2.06-1.99), the high alumina content in pyroxene (up to 11.8%) and the high magnesium content in garnet [lo0 Mg/(Mg + Fe) = 60.61. Mineral thermometry based on Fe-Mg exchange between garnet and pyroxene and on Al-solubility in pyroxene gives temperatures close to 970 f 70" C at 10 f 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.

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Section: P-t Pathmentioning

confidence: 94%

P–T–X relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria

Bertrand

Ouzegane²,

Kiénast

1992

Journal Metamorphic Geology

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“…Grew, 1982), Hoggar (e.g. Kienast & Ouzegane, 1987), Italian Alps (Droop & Bucher‐Nurminen, 1984) and Scotland (Baba, 1999). Quantitative pressure and temperature estimates and P‐T trajectories of Peruvian UHT migmatites will be dealt with in a separate paper.…”

Section: Introduction and Geological Settingmentioning

confidence: 99%

Regional‐scale Grenvillian‐age UHT metamorphism in the Mollendo–Camana block (basement of the Peruvian Andes)

Martignole¹,

Martelat²

2002

Journal Metamorphic Geology

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The Mollendo-Camana Block (MCB) is a 50 · 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U-Pb and U-Th-Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K-feldspar (Kfs), orthopyroxene (X Mg Opx ¼ 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (X Mg Grt ¼ 0.60), sapphirine (X Mg Spr ¼ 0.87), cordierite (X Mg Crd ¼ 0.92) and biotite (X Mg Bt ¼ 0.83). The ubiquitous peak mineral assemblage is Opx-Sil-Kfs-Qtz-(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al 2 O 3 ) and the local coexistence of sapphirine-quartz, attest to regional UHT metamorphism (> 900°C) at pressures in excess of 1.0 GPa. Fluid-absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo-Cocachacra areas). In this area, a first stage of decompression from 1.1-1.2 to 0.8-0.9 GPa at temperatures in excess of 950°C, is marked by the breakdown of Sil-Opx to Spr-Opx-Crd assemblages according to several bivariant FMAS reactions. High-T decompression is also shown by Mg-rich garnet being replaced by Crd-Spr-and Crd-Opxbearing symplectites, and reacting with quartz to produce low-Al-Opx-Sil symplectites in quartz-rich migmatites. Neither osumilite nor spinel-quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt-Qtz-(± pl) symplectites in quartz-rich migmatites (melt + Opx + Sil ¼ Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid-absent melting, compatible with a second stage of decompression below 900°C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large-scale penetration of hot asthenospheric mantle at the base of an over-thickened crust.

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“…This evolution appears similar to that established in the neighbouring granulitic terrains of the In Ouzzal massif (Fig. 1) Ouzegane et al, 2003), where complex magmatic and tectonometamorphic histories are recognized between 3.3 and 2.6 Ga before a very-high-temperature metamorphic event at 2.0 Ga (Kienast and Ouzegane, 1987;Bertrand et al, 1992). The aim of the present study was to check the occurrence of a 3.5 Ga early Archean event in the Gour Oumelalen area that could be similar to that described, for example, in the West African Craton of Mauritania by Potrel et al (1996Potrel et al ( , 1998.…”

Section: Introductionmentioning

confidence: 55%

U–Pb zircon (TIMS and SIMS) and Sm–Nd whole-rock geochronology of the Gour Oumelalen granulitic basement, Hoggar massif, Tuareg shield, Algeria

Peucat

Drareni

Latouche

et al. 2003

Journal of African Earth Sciences

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Polymetamorphic Al, Mg-rich granulites with orthopyroxene–sillimanite and sapphirine parageneses in Archaean rocks from the Hoggar, Algeria

Cited by 28 publications

References 50 publications

P–T–X relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria

P–T–X relationships in the Precambrian Al–Mg‐rich granulites from In Ouzzal, Hoggar, Algeria

Regional‐scale Grenvillian‐age UHT metamorphism in the Mollendo–Camana block (basement of the Peruvian Andes)

U–Pb zircon (TIMS and SIMS) and Sm–Nd whole-rock geochronology of the Gour Oumelalen granulitic basement, Hoggar massif, Tuareg shield, Algeria

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