Do coeval mafic and felsic magmas in post-collisional to within-plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review

Abstract

The end of major orogenic episodes is marked by uplift and erosion, transcurrent to extensional tectonic regimes induced partly by gravitational collapse of the thickened crust, partly by delamination of the lithosphere, and emplacement of voluminous igneous formations. This period starts soon after the completion of the continent–continent collisional event. Contrasting igneous suites, regarding their nature, evolution and original sources, are emplaced. Two distinctive and successive igneous associations can be evidenced: (i) The post-collisional association is the more complex. Peraluminous silicic rocks, bearing Al–Fe–Mg silicates, such as garnet, cordierite, and sillimanite, are coeval with metaluminous mafic–felsic igneous suites, ranging from medium-K to high-K calc-alkaline to shoshonitic to ultrapotassic. (ii) The postorogenic association yields less potassic and more sodic compositions. The igneous suites, comprising mafic and felsic rocks, range from alkali-calcic metaluminous to alkaline and peralkaline. They evolve progressively into more markedly alkaline within-plate suites. The post-collisional association identifies two contrasting sources. Peraluminous granitoids and related volcanic rocks contain frequently mafic enclaves corresponding to blobs of undercooled shoshonitic to ultrapotassic basic to intermediate magmas. The peraluminous suite originates by dehydration incongruent melting of muscovite±biotite in the continental crust. Medium-K to high-K calc-alkaline suites originate in an amphibole–spinel peridotite metasomatised lithospheric source. Shoshonitic to ultrapotassic metaluminous suites are issued from partial melting involving phlogopite of a garnet-bearing depleted lithospheric upper mantle, metasomatised by subducted material. The onset of the successive short-lived magmatic episodes is induced by lithosphere stacking and slab breakoff. Mantle-derived magmas emplaced within lower crust provide enough heat to enhance crustal anatexis and, then, they are injected into and mix with the crustal-derived liquids. During postorogenic and within-plate episodes, commingling of mafic and felsic magmas is documented by synplutonic dykes and sills, and by mafic enclaves. Radiogenic isotope systematics suggest that coexisting mafic and felsic magmas come from the same mantle source yielding depleted but LILE-enriched compositions, with subsequent wall-rock crustal contamination. The onset of the magmatic episode is promoted by lithosphere delamination. The role played by continental crust, though still disputed, becomes increasingly minor with time to null. The transition from post-collisional to within-plate geodynamic settings documents the waning role played by crustal anatexis in magma generation, with regard to the increasing role of enriched OIB mantle sources.