Mélanges and ophiolites during the Pan-African orogeny: the case of the Bou-Azzer ophiolite suite (Morocco)

Abstract

Since the discovery of ophiolite sequences, the Bou-Azzer inlier has been considered a key area for understanding the evolution of the northern margin of the West African craton during the Pan-African orogeny. For about 20 years, it had been commonly accepted that the Bou-Azzer inlier represents an accretionary mélange accreted onto the West African craton under blueschist metamorphic conditions, similar to the Franciscan Complex and the Sanbagawa facies series. This would imply that a low geothermal gradient was prevalent during the subduction of the Pan-African oceanic plate, and that the ocean was subducted with a high convergence rate. A reinvestigation of the metamorphic conditions by a thermodynamic approach shows that the ophiolite sequence of Bou-Azzer underwent HT greenschist metamorphic conditions instead of blueschist metamorphic conditions. We propose that the ophiolites of Bou-Azzer are not similar to the Sanbagawa facies series or to the Franciscan Complex, but bear similarities to the Albanian or Cyprus ophiolites, which represent dismembered ophiolite sequences overprinted by greenschist conditions.

The Tachakoucht–Iriri–Tourtit arc complex (Moroccan Anti-Atlas): Neoproterozoic records of polyphased subduction-accretion dynamics during the Pan-African orogeny

“…Stronger arguments are needed as the occurrence of subduction-796 related low-T high-P relics and typical fore-arc assemblages that could give a polarity to the initial 797 subduction zone / locate paleo-subduction channel. On this topic, Hefferan et al(2002) described 798 blueschist-bearing mélange in the northern section of Bou Azzer ophiolite but the estimation of its P-T 799 conditions has been discussed byBousquet et al (2008). 800The 760-730 Ma intra-oceanic arc (IOA) has been intensively deformed and metamorphized through 801 dismembering, partial subduction and a general stacking of arc blocks and SSZ oceanic crust (i.e.…”

The relations between Gondwana and the adjacent peripheral Cadomian domain—constrains on the origin, history, and paleogeography of the peripheral domain

“…Major deformation, metamorphism, and syntectonic granitoid plutonism at ca. 660-630 Ma record the closure of the basin, southward obduction of the Bou Azzer ophiolite and the older subduction-related rocks over the margin of the WAC, and probably also collision with an ancient block coming from the north (Thomas et al, 2004;Soulaimani et al, 2006;Bousquet et al, 2008). Thus subduction over N50 Ma is indicated.…”

The Bou Azzer Co–Ni–Fe–As(±Au ± Ag) District of Central Anti-Atlas (Morocco): A Long-Lived Late Hercynian to Triassic Magmatic-Hydrothermal to Low-Sulphidation Epithermal System

“…Several syn-to post-kinematic, high-K calc-alkaline, subduction-related diorite-tonalite stocks (655-635 ± 5 Ma) intruded the ophiolitic sequence at the end of subduction and the beginning of exhumation-obduction (Inglis et al 2005;El Hadi et al 2010). To the north of the Bou Azzer district, the mafic-ultramafic succession is bounded by a tectonic sliver <300 m wide composed of a blueschist-facies metamorphic assemblage suggesting maximum pressure conditions of 5-6 kbar and temperatures of 400-600°C (Hefferan et al 2002;Bousquet et al 2008). Younger, post-collisional igneous rocks including the Bleida granodiorite dated at 579.4 ± 1.2 and 578.5 ± 1.2 Ma (U-Pb on zircon; Inglis et al 2004) constrain the cessation of principal Pan-African deformation to before ca.…”

“…Mined since antiquity and well-described in the literature (Leblanc 1975, and references herein), the Bou Azzer district has produced more than spatial distribution of main historically mined Co-As-FeNi(±Ag ± Au) orebodies, projected to surface (adapted and modified after Bousquet et al 2008) 55,000 t of cobalt (Leblanc and Billaud 1982) and contains more than one hundred Co-NiFe-arsenide ± Au ± Ag orebodies distributed along Hercynian reactivated major Paleoproterozoic Eburnian (*2.1-2.0 Ga) and Neoproterozoic Pan-African (*560-550 Ma) faults, contributing as much as *2 % of the world's cobalt production (U.S. Geological Survey 2013). In addition to Co, sub-economic gold and silver concentrations are also recovered as by products, with total production of these two metals ranging from 2 to 13 t (Leblanc and Billaud 1982;Buisson and Leblanc 1987).…”