The Precambrian of Hoggar, Tuareg shield: history and perspective

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The Precambrian of Hoggar, Tuareg Shield

11 articlesinfo:eu-repo/semantics/publishe

Dedication to Louis Latouche

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Granulitic metamorphism in the Laouni terrane (Central Hoggar, Tuareg Shield, Algeria)

In the Laouni terrane, which belongs to the polycyclic Central Hoggar domain, various areas contain outcrops of formations showing granulite-facies parageneses. This high-temperature metamorphism was accompanied by migmatization and the emplacement of two types of magmatic suite, one of continental affinity (garnet pyroxenites and granulites with orthoferrossilite-fayalite-quartz), and the other of arc affinity (layered metanorites). Paragenetic, thermobarometric and fluid-inclusion studies of the migmatitic metapelites and metabasites make it possible to reconstruct the P-T-H2O path undergone by these formations. This path is clockwise in the three studied areas, being characterized by a major decompression (Tamanrasset: 10.5 kbar at 825 °C to 6 kbar at 700 °C; Tidjenouine: 7.5 kbar at 875 °C; to 3.5 kbar at 700 °C; Tin Begane: 13.5 kbar at 850 °C; to 5 kbar at 720 °C), followed by amphibolitization that corresponds to a fall of temperature (from 700 to 600 °C) and an increase in water activity (from 0.2-0.4 to almost 1). The main observed features are in favour of petrogenesis and exhumation related to the Eburnean orogeny. However, the lacks of good-quality dating work and a comparison with juvenile Pan-African formations having undergone high-pressure metamorphism, in some cases reaching the eclogite facies, do not rule out the possibility that high-temperature parageneses are locally due to Pan-African events. © 2004 Elsevier Ltd. All rights reserve

The post-collisional Gabbro-anorthosite complex from the Tin Zebane area (western Hoggar, Algeria)

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Ternary feldspar thermometry of Paleoproterozoic granulites from In-Ouzzal terrane (Western Hoggar, southern Algeria)

International audienceThe In Ouzzal terrane in western Hoggar (Southern Algeria) preserves evidence of ultrahigh temperature (UHT) crustal metamorphism. It consists in Archean crustal units, composed of orthogneissic domes and greenstone belts, strongly remobilized during the Paleoproterozoic orogeny which was recognized as an UHT event (peak T > 1000 °C and P ≈ 9–12 kbar). This metamorphism was essentially defined locally in Al-Mg granulites, Al-Fe granulites and quartzites outcropping in the Northern part of the In Ouzzal terrane (IOT).In order to test and verify the regional spread of the UHT metamorphism in this terrane, ternary feldspar thermometry on varied rock types (Metanorite, Granulite Al-Mg and Orthogneiss) and samples that crop out in different zones of the In Ouzzal terrane. These rocks contain either perthitic, antiperthitic or mesoperthitic parageneses. Ternary feldspars used in this study have clearly a metamorphic origin.The obtained results combined with previous works show that this UHT metamorphism (>900 °C) affected the whole In Ouzzal crustal block. This is of major importance as for future discussion on the geodynamic context responsible for this regional UHT metamorphism

The Syenite–Carbonatite Complex of Ihouhaouene (Western Hoggar, Algeria): Interplay Between Alkaline Magma Differentiation and Hybridization of Cumulus Crystal Mushes

International audienceThe 2 Ga-old Ihouhaouene alkaline complex (Western Hoggar, Algeria) is among the oldest known carbonatite occurrences on Earth. The carbonatites are calciocarbonatites hosted by syenites, the predominant rock type in the complex. Both rock types are characterized by medium-grained to pegmatitic textures and contain clinopyroxene, apatite, and wollastonite, associated with K-feldspar in syenites and a groundmass of calcite in carbonatites. The rock suite shows a continuous range of compositions from 57–65 wt.% SiO 2 and 0.1–0.4 wt.% CO 2 in red syenites to 52–58 wt.% SiO 2 and 0.1–6.5 wt.% CO 2 in white syenites, 20–35 wt.% SiO 2 and 11–24 wt.% CO 2 in Si-rich carbonatites (>10% silicate minerals), and <20 wt.% SiO 2 and 24–36 wt.% CO 2 in Si-poor carbonatites (<5% silicate minerals). Calculation of mineral equilibrium melts reveals that apatite and clinopyroxene are in disequilibrium with each other and were most likely crystallized from different parental magmas before being assembled in the studied rocks. They are subtle in the red syenites, whereas the white syenites and the Si-rich carbonatites bear evidence for parental magmas of highly contrasted compositions. Apatite was equilibrated with LREE-enriched (Ce/Lu = 1,690–6,182) carbonate melts, also characterized by elevated Nb/Ta ratio (>50), whereas clinopyroxene was precipitated from silicate liquids characterized by lower LREE/HREE (Ce/Lu = 49–234) and variable Nb/Ta ratios (Nb/Ta = 2–30). The Si-poor carbonatites resemble the Si-rich carbonatites and the white syenites with elevated REE contents in apatite equilibrium melts compared to clinopyroxene. However, apatite equilibrium melt in Si-poor carbonatite shows a majority of subchondritic values (Nb/Ta<10) and clinopyroxene has chondritic-to-superchondritic values (Nb/Ta = 15–50). Although paradoxical at first sight, this Nb-Ta signature may simply reflect the segregation of the carbonatite from highly evolved silicate melts characterized by extremely low Nb/Ta values. Altogether, our results suggest an evolutionary scheme whereby slow cooling of a silico-carbonated mantle melt resulted in the segregation of both cumulus minerals and immiscible silicate and carbonate melt fractions, resulting in the overall differentiation of the complex. This process was however counterbalanced by intermingling of partially crystallized melt fractions, which resulted in the formation of hybrid alkaline cumulates composed of disequilibrium cumulus phases and variable proportions of carbonate or K-feldspar

Ultrabasic and basic rocks from the Tileouine Area (Western Hoggar): from subduction-related metamorphism to post-collisional magmatism

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