Aziouz Ouabadi

The origin and geodynamic significance of the Alpine cordierite-bearing granitoids of northern Algeria. A combined petrological, mineralogical, geochemical and isotopic (O, H, Sr, Nd) study

In northern Algeria, the coastal area displays a very variegated magmatism of Miocene age, extending E–W over 900 km with a width of less than 50 km. This magmatism includes both plutonic and volcanic massifs containing basic, intermediate to acidic rock types. Two groups may be identified on the basis of the chemistry and mineralogy. The first group has a calc-alkaline metaluminous chemistry and contains mainly “ high-K” types, sometimes “ medium-K” and exceptionally “low-K” types. They are gabbros and basalts, diorites and andesites, tonalites and dacites. Some granitoids (quartz-monzonites, granodiorites and syenogranites) possess affinities with this magmatism (Bejaia–Amizour and Thenia intrusions). Only mafic to intermediate enclaves of magmatic origin are found in granitoids from this group. The second group has strongly peraluminous compositions: they are cordierite-bearing granodiorites, monzogranites and microgranites, as well as dacites and rhyolites (ignimbrites). Two types of enclaves exist in these granitoids: (i) microgranular metaluminous hornblende-bearing enclaves of igneous derivation, (ii) strongly peraluminous enclaves of metapelitic compositions. Textures and mineralogical associations indicate that the enclaves experienced heating at decreasing pressures and also partial melting. Therefore, they are indicative of assimilation, not of restite unmixing. After removing the local effects of hydrothermal alteration, the O, Sr, Nd isotopic compositions of parent magmas were determined: they span a very large range of values, e.g., eNd (T) varies from +12 (Cap Bougaroun gabbros) to −10 (Crd-bearing granitoids). From the covariation of Sr–Nd isotopic compositions and the constraints of O isotopic ratios, the following sources are proposed for the different rock types: Mafic to intermediate rocks contain a subduction-related component but most of them require contamination at crustal levels. The source of metaluminous granitoids is predominantly crustal as constrained by their high O isotopic composition (δ18O ranging from +9.3‰ up to +13‰), but they might contain a minor component derived from the mafic sources. Most of the cordierite-bearing granitoids are thought to be formed through assimilation of pelitic metasediments by a less aluminous parental magma. The general tectonic setting of this Neogene (limited, linear but variegated) magmatism is explanable in terms of the slab detachment models recently advocated by several authors in the western Mediterranean domains. This study argues that cordierite-bearing granitoids may be produced in areas of high heat flow, in crustal domains which did not experience important thickening.

Eocene exhumation of the Tuareg Shield (Sahara Desert, Africa)

The arch-and-basin geometry that characterizes North Africa was achieved at the end of Paleozoic times. It has been subsequently reactivated during the Mesozoic-Cenozoic with, in particular, the development of large topographic anomalies. Among these, the Tuareg Shield forms a topographic high in which the Pan-African basement reaches 2400 m above sea level (Hoggar core). While Cretaceous sedimentary remnants suggest a possible stage of subsidence during the Mesozoic, currently the area forms a swell, emphasized by Cenozoic volcanic episodes since 35 Ma. In this context, we present the first apatite (U-Th)/He thermochronological data acquired across this swell, with mean ages ranging from 78 ± 22 Ma to 13 ± 3 Ma. These results demonstrate the existence of a widespread Eocene exhumation of the shield before volcanic activity began, which reflects large-scale vertical processes. In the northeastern part of the swell, Cretaceous continental sedimentary remnants unconformably lying on the basement close to our samples evidence that they were near the surface at that time. This study shows that basement rocks have undergone subsequent heating at ∼60–80 °C, suggesting a burial of more than 1 km after the Early Cretaceous. This conclusion can be possibly extended over the whole Tuareg Shield.

Stable isotope study of the igneous, metamorphic and mineralized rocks of the Edough complex, Annaba, Northeast Algeria

The petrogenesis of igneous, metamorphic and mineralized rocks in the Edough massif, NE Algeria, indicates an interplay between crustal and magmatic sources, and magmatic and surface fluids, as determined by sulphur and oxygen isotopic analyses. The Tertiary igneous rocks (microgranites and rhyolites) show a tendency towards I-type granitoids with δ34S values of +5.4±2.2‰ (1σ) and δ18O between +5.6‰ and +6.9‰, with the most hydrothermally altered rocks having the lowest δ18O values. This is indicative of seawater being a major component of the hydrothermal fluids. Seawater interaction with the granitoids produced enrichment in 34S of sulphide-sulphur in both the microgranites and associated skarn mineralization. However, in the Beleleita W–Sn–(Au) deposit, magmatic fluids dominated the mineralization, reflected by δ34S values of +1.7‰ and +1.8‰. The basement amphibolites of Kef Lakhal indicate derivation from a basaltic magma with δ18O ranging from +4.9‰ to +8.6‰. Locally altered amphibolite has the lowest δ18O values suggestive of meteoric water interaction, whereas the highest δ18O indicate the incorporation of crustal material during the genesis of the amphibolites. Crustal contamination is also reflected in their δ34S of −18‰ to +2.1‰, with the most contaminated rocks having the lowest δ34S. Host Palaeozoic mica schists with δ18O of +12.7‰ and δ34S of −13.2‰ and associated marbles (δ34S −9.9‰ to −17.4‰) are thought to be the most likely contaminants. At the Ain Barbar Fe–Cu–Pb–Zn deposit, δ34S values range between −9.6‰ and −10.8‰, indicating that the sulphide-sulphur and base-metals were likely leached from the host Cretaceous flysch and/or the underlying mica schists, with local Tertiary magmatic rocks providing the heat for local convection cells. At Boumaiza Fe-deposit, δ34S values of the sulphide-sulphur extend from −1.2‰ to −8.1‰ indicative of magmatic sulphur with the incorporation of substantial amounts of sulphur derived from the host mica schists.

The 600 Ma-Old Pan-African Magmatism in the In Ouzzal Terrane (Tuareg Shield, Algeria): Witness of the Metacratonisation of a Rigid Block

The high-level sub-circular North Tihimatine granitic pluton, intrusive in the In Ouzzal terrane, has been dated at 600 ± 5 Ma (LA-ICP-MS U–Pb zircon) and at 602 ± 4 Ma (SHRIMP U–Pb zircon). At this time, while Tihimatine intruded a brittle In Ouzzal without major metamorphism, large high-K calc-alkaline granitoid batholiths emplaced in the adjacent terranes under ductile conditions and regional amphibolite facies metamorphism. Outside In Ouzzal, high-level plutons emplaced under brittle conditions are known only at c. 580 Ma. The In Ouzzal terrane (500 km × 80 to 5 km), made of c. 2 Ga very high-temperature granulitic lithologies with Archean protoliths, is the sole terrane within the Tuareg Shield to have been largely unaffected by the Pan-African orogeny. The field, petrographic, geochemical and isotopic characteristics of the In Ouzzal granitic plutons studied herein, give keys for the understanding of the atypical behavior of the In Ouzzal terrane. The In Ouzzal Pan-African granitoids present chemical compositions varying from medium-K to high-K calc-alkaline to alkaline compositions. This is recorded by the Sr and Nd radiogenic isotopes (−4 < ɛNd < −30; 0.704 < ISr < 0.713), pointing to a mixing between a heterogeneous and old Rb-depleted source, the Eburnean granulitic In Ouzzal crust, and a Pan-African mantle. The latter is represented by the nearby bimodal Tin Zebane dyke swarm (ɛNd = +6.2, ISr = 0.7028; Hadj Kaddour et al. in Lithos 45:223–243, 1998), emplaced along the mega-shear zone bounding the In Ouzzal terrane to the west. Trace element composition and Sr–Nd isotope modeling indicate that 20–40% of different crustal lithologies outcropping in the In Ouzzal terrane mixed with mantle melts. At least two, most probably three, Eburnean granulitic reservoirs with Archean protoliths are needed to explain the chemical variability of the In Ouzzal plutons. The Pan-African post-collisional period is related to a northward tectonic escape of the Tuareg terranes, including the rigid In Ouzzal terrane, bounded by major shear zones. Blocking of the movement of the In Ouzzal terrane, which occurred 20 Ma earlier (at 600 Ma) on the western side than on the eastern side, induced its fracturing along oblique faults inside the terrane. This process allowed asthenosphere to rise and to locally melt the In Ouzzal crust, giving rise to the studied plutons. This corresponds to a metacratonization process. The In Ouzzal terrane demonstrates that a relatively small rigid block can survive within a major orogen affected by a post-collisional tectonic escape at the cost of a metacratonization, particularly at depth along faults.