This geochemical classification of granitic rocks is based upon three INTRODUCTION variables. These are FeO/(FeO + MgO) = Fe-number [or Although granitoids are the most abundant rock types FeO/(FeO + MgO) = Fe∗], the modified alkali–lime index in the continental crust, no single classification scheme (MALI) (Na2O + K2O – CaO) and the aluminum saturation has achieved widespread use. Part of the problem in index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number granite classification is that the same mineral assemblage, (or Fe∗) distinguishes ferroan granitoids, which manifest strong iron quartz and feldspars with a variety of ferromagnesian enrichment, from magnesian granitoids, which do not. The ferroan minerals, can be achieved by a number of processes. and magnesian granitoids can further be classified into alkalic, Granitoids can form from differentiation of any hyalkali–calcic, calc-alkalic, and calcic on the basis of the MALI persthene-normative melt and from partial melting of and subdivided on the basis of the ASI into peraluminous, metamany rock types. Furthermore, granitic melts may be luminous or peralkaline. Because alkalic rocks are not likely to be derived solely from crustal components, may form from peraluminous and calcic and calc-alkalic rocks are not likely to be evolved mantle-derived melts, or may be a mixture peralkaline, this classification leads to 16 possible groups of granitic of crustal and mantle-derived melts. Because of this rocks. In this classification most Cordilleran granitoids are magnesian complexity, petrologists have relied upon geochemical and calc-alkalic or calcic; both metaluminous and peraluminous classifications to distinguish between various types of types are present. A-type granitoids are ferroan alkali–calcic, although granitoids. Approximately 20 different schemes have evolved over the past 30 years [see Barbarin (1990, 1999) some are ferroan alkalic. Most are metaluminous although some are for a summary thereof]. Most of these schemes are either peraluminous. Caledonian post-orogenic granites are predominantly genetic or tectonic in nature. This paper is an attempt magnesian alkali–calcic. Those with <70 wt % SiO2 are domto present a non-genetic, non-tectonic geochemical clasinantly metaluminous, whereas more silica-rich varieties are comsification scheme that incorporates the best qualities of monly peraluminous. Peraluminous leucogranites may be either the previous schemes, and to explain the petrologic magnesian or ferroan and have a MALI that ranges from calcic to processes that makes this scheme work. alkalic.

/pdf/a-geochemical-classification-for-granitic-rocks-j2go5p9d9i.pdf

A geochemical classification for granitic rocks

A review of the relationships between granitoid types, their origins and their geodynamic environments

A-type granites in northeastern China: age and geochemical constraints on their petrogenesis

Post-collisional strongly peraluminous granites

A-type granites and related rocks: Evolution of a concept, problems and prospects

https://www.africamuseum.be/publication_docs/Lithos1998-NYTS.pdf

Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Several late-collision and intraplate features are not entirely integrated in the classical plate tectonic model. The Pan-African orogeny (730–550 Ma) in Saharan Africa provides some insight into the contrasting behaviour of cratons and mobile belts. Simple geophysical considerations and geological observations indicate that rigidity and persistence of cratons are linked to the presence of a thick mechanical boundary layer, the upper brittle part of the continental lithospheric mantle, well attached to an ancient weakly radioactive crust. The surrounding Pan-African mobile belts, characterized by a much thinner mechanical boundary layer and more radioactive crust, were the locus of A-type granitoids, volcanism, tectonic reactivation and basin development during the Phanerozoic. During oceanic closures leading to the assembly of Gondwana, lithosphere behaviour was controlled by its mechanical boundary layer, the crust being much less rigid. We suggest that the 5000 km wide Pan-African domain of Saharan Africa, a collage of juvenile and old reactivated basement terranes, has suffered regional continental lithospheric mantle delamination during the early stages of this orogeny, as has been postulated for the more recent Himalayan orogeny in Tibet. Delamination of the continental lithospheric mantle and juxtaposition of crust against hot asthenosphere can explain many features of the late Pan-African (around 600 Ma): reactivation of old terrains, abundant late-tectonic high-K calc-alkaline granitoids, high temperature-low pressure metamorphism, important displacements along mega-shear zones and mantle-derived post-tectonic granitoids linked to a rapid change in mantle source. Recycling into the asthenosphere of large amounts of continental lithospheric mantle delaminated during the Pan-African, can provide one of the reservoirs needed to explain the isotopic compositions of ocean island basalts. Lastly, the lithospheric control over the location of the alkaline rocks enjoins us to consider the thermal boundary layer (the lower ductile part of the continental lithospheric mantle) as a major mixing source zone for these rocks.

http://www.mantleplumes.org/WebDocuments/Black1993.pdf

Cratons, mobile belts, alkaline rocks and continental lithospheric mantle : the Pan-African testimony

Concepts developed in the recently published model of the Air region (eastern Tuareg shield; Niger, Africa) as a college of three displaced terranes integrated in a single geodynamic model lead us to propose a terrane map of the Tuareg shield (500000 km 2 ). The 23 terranes recognized have their own lithological, metamorphic, magmatic, and tectonic characteristics and are separated by subvertical strike-slip megashear zones that can be traced for hundreds of kilometres, or by major thrust fronts. Some of these boundaries have ophiolitic assemblages or molassic deposits. The Tuareg shield was shaped and partly accreted during the Pan-African orogeny (750-550 Ma), but not as a homogeneous body.

/pdf/pan-african-displaced-terranes-in-the-tuareg-shield-central-b0vl7fe9sx.pdf

Pan-African displaced terranes in the Tuareg shield (central Sahara)

Summary 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.

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

https://www.africamuseum.be/publication_docs/Liegeois1994-PR-Air-terrane-assembly.pdf

Russell Black

Papers

Contrasting origin of post-collisional high-K calc-alkaline and shoshonitic versus alkaline and peralkaline granitoids. The use of sliding normalization

Cratons, mobile belts, alkaline rocks and continental lithospheric mantle : the Pan-African testimony

Pan-African displaced terranes in the Tuareg shield (central Sahara)

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

Early and late Pan-African orogenies in the Aïr assembly of terranes (Tuareg shield, Niger)