Alkaline magmatism subsequent to collision in the Pan-African belt of the Adrar des Iforas (Mali)

Abstract

SUM MARY: The Pan-African Trans-Saharan belt in the Iforas displays a rapid switch from subduction and collision-related calc-alkaline to typical A-type magmatism, which is accompanied by transcurrent movements along major shear zones inducing weak distension. Detailed Rb-Sr geochronology and geochemical data point to different mantle sources for orogenic (lithospheric depleted mantle + oceanic crust) and within-plate magmatism (more primitive asthenospheric mantle). Both groups suffer lower-crustal contamination. A model is proposed whereby asthenospheric mantle originally underlying the subducted plate has risen to shallow depth beneath the continental lithosphere after the rupture of the cold plunging plate. This source, which is often proposed for alkaline rocks, explains the great similarity of oversaturated alkaline ring-complexes whatever their environment. The peculiarities of the alkaline province, for example the lack of Sn mineralization when compared with the Niger-Nigerian province, may be related to the nature and composition of the basement.

The 1160 Ma Hidderskog meta-charnockite: implications of this A-type pluton for the Sveconorwegian belt in Vest Agder (SW Norway)

“…On the other hand, this diagram shows that the alkaline character of Hidderskog cannot be demonstrated on the basis of the Na,O + K,O content alone: it has been shown (e.g. Liegeois and Black, 1987) that, even for alkaline rocks, the alkali content can strongly decrease during the late stages of differentiation.…”

“…We need either an appropriate crustal source (Rogers and Greenberg, 1990;Creaser et al, 1991) or a mantle-derived (OIB type) magma slightly contaminated by a crustal material of talc-alkaline nature. This latter case is common in a post-collisional tectonic environment where chemical relics related to a former subduction process can be present in A-type granites (Liegeois and Black, 1987) while the former is postulated on a Pb isotope basis for the syn-tectonic charnockitic augen gneiss of Ubergsmoen ( 1120 Ma old) in the Bamble sector (Andersen et al, 1994). This will be constrained only when the early Sveconorwegian phase in RogalandVest Agder (c. 1250 Ma, Demaiffe and Michot, 1985) becomes better understood.…”

Genesis of Ferropotassic A-Type Granitoids of Mühlig-Hofmannfjella, Central Dronning Maud Land, East Antarctica

“…d Agpaitic index (molar (Na/ K) / Al versus SiO2. The limit at 0.87 separates subalkaline metaluminous granitoids from alkaline (Liégeois and Black 1987). e Ferriferous character in M =Mg/Mg+ Fe versus B = Fe + Mg + Ti in cationic classification diagram after Debon and Le Fort (1988).…”

Origins of Early Mesozoic granitoids and their enclaves from West Kunlun, NW China: implications for evolving magmatism related to closure of the Paleo-Tethys ocean

“…Houseman et al 1981;Nelson 1992;Turner 1996) or break-off of a subducted slab (e.g. Liégeois and Black 1987;Davies and von Blankenburg 1995;Bonin 2004;Whalen et al 2006). However, lithospheric delamination processes do not explain the southward migration of the magmatic belt and the linear zone of magmatism in the West Kunlun.…”