Hybridization of crustal magmas and direct evidence for two mantle sources during Early-Ordovician magmatism at the NW Gondwana margin

Abstract

In the Malpica-Tui Complex, granitic gneisses, metagranodiorite and eclogite-facies gneisses represent a former subalkaline association of hypersolvus granite, granodiorite and tonalite. Besides, peralkaline gneisses, ferropargasite-bearing gneisses and alaskitic gneisses derive from an alkaline association of subsolvus alkali-feldspar granites and quartz alkali-feldspar syenites. The subalkaline association formed by open-system processes: anatectic granitoids contain comagmatic, subalkaline basic enclaves from a depleted mantle source [Y/Nb = 3–10; ε_Nd(475) = −0.8 to +5.3]. Also, the presence of HFSE-rich granodiorite with ferropargasite-bearing trondhjemite enclaves points to hybridization with a subordinate alkaline source. In the alkaline association, peraluminous alkali-feldspar granites (alaskites) contain the same basic enclaves as the subalkaline association, but also comagmatic, alkaline mafic rocks [Y/Nb = 0.5–1.1; ε_Nd(475) = +1.3 to +3.6]. Peralkaline granite, metaluminous alkali-feldspar granite and quartz alkali-feldspar syenite evolved from alkaline mantle melts through fractional crystallisation, with minor crustal assimilation. The two igneous associations developed in a regional context of continental rifting in Late Cambrian - Early Ordovician. In this setting, heat and mass transfer from rising mantle materials induced crustal anatexis and hybridization of the resulting partial melts with mantle-derived subalkaline magmas, presumably in the Late Cambrian. Subsequently, minor amounts of alkaline melts were also involved in the generation of subalkaline granitoids in the Early Ordovician (481 ± 1 Ma). The increasing relative contribution of alkaline melts progressively changed the composition of the acid magmas, leading to the emplacement of peraluminous alkali-feldspar granites, and finally, of mantle-derived, peralkaline and metaluminous granitoids at 472 ± 1 Ma.