Mantle and Crustal Sources in the Genesis of Late-Hercynian Granitoids (NW Portugal): Geochemical and Sr-Nd Isotopic Constraints

Abstract

Large volumes of granitoids were emplaced in the Hercynian Central Iberian Zone during the last ductile deformation phase (D3, 300-320 Ma). The biotite-rich granitoids are the most abundant: (1) syn-D3 granodiorites-monzogranites (313-319 Ma) with calc-alkaline and aluminopotassic affinities; (2) late-D3 granodiorites-monzogranites (306-311 Ma), related to subalkaline and aluminopotassic series. These granitoids are associated with coeval gabbro-norite to granodiorite bodies and/or mafic microgranular enclaves. Both granitoids and basic-intermediate rocks show petrological, geochemical and isotopic evidence of interaction between felsic and mafic magmas. The mantle-derived melts, represented by shoshonitic gabbro-norites, were probably derived from an enriched and isotopically homogeneous source (Srl = 0.7049 to 0.7053, E~~ =-2.1 to-2.5). In some synand late-D3 plutons there are evidences of essentially crustal granites, represented by moderately peraluminous monzogranites of aluminopotassic affinity. They have similar Nd model ages (1.4 Ga) but different isotopic compositions (Srl = 0.7089 to 0.7106, cNd =-5.6 to-6.8), revealing a heterogeneous crust. Potential protoliths are metasedimentary (immature sediments) and/or fclsic meta-igneous lower crust materials. Large amounts of hybrid magmas were generated by the interaction of these coeval mantle-and crust-derived liquids, giving rise to slightly peraluminous monzogranites/granodiorites of calc-alkaline and subalkaline affinities, which display more depleted isotopic compositions than the crustal end-members (Sr, = 0.7064 to 0.7085, E~~ =-4.4 to-6.2). Petrogenetic processes involving mingling and/or mixing and fractional crystallization (at variable degrees) in multiple reservoirs are suggested. A major crustal growth event occurred in late-Hercynian times (-305-320 Ma) related to the input of juvenile mantle magmas and leading to the genesis of composite calc-alkaline and subalkaline plutons, largely represented in the Central Iberian Zone.

Late-Variscan emplacement and genesis of the Vieira do Minho composite pluton, Central Iberian Zone: Constraints from U–Pb zircon geochronology, AMS data and Sr–Nd–O isotope geochemistry

“…Isotopic Sr, Nd signatures from the studied granites are very similar to those found in other syn-to-post-D3 Variscan granites from northern Portugal, and also from other areas of the Central Iberian Zone (e.g. Bea et al, 1999;Dias et al, 2002;Villaseca et al, 1998Villaseca et al, , 2008.…”

“…The presence of more basic rocks (gabbro-norites, quartz-diorites, and vaugnerites) associated with these granites, especially within the VMG south of the studied area, have been reported by Andrade and Noronha (1981), Dias and Leterrier (1994) and Dias et al (2002).…”

“…The late deformation phase, D 3 , Namurian-Westphalian in age, is correlated to the post-thickening and extension which marked the end of the collision process (Dias and Ribeiro, 1995;Lagarde et al, 1992). The events related to this tectonic phase were marked by vertical folds with subhorizontal axes and strike-slip sinistral and dextral, subvertical shear zones which controlled the large volume of orogenic granitic rocks emplaced in the Central Iberian Zone, (Aguado et al, 2005;Almeida et al, 2002;Dias et al, 1998Dias et al, , 2002Ferreira et al, 1987;Martins et al, 2009Martins et al, , 2011Noronha et al, 1979). Based on field and geochronological data, these Variscan granites were divided into two main groups (Dias et al, 1998;Ferreira et al, 1987;Martins et al, 2009Martins et al, , 2011: synorogenic (syn-late and late to post-D 3 ; 320-300 Ma) and late to post-orogenic (post-D 3 ; 299-290 Ma).…”

“…The available isotopic data from the MRG were obtained by Dias et al (2002). This granite presents a ( 87 Sr/ 86 Sr) i between 0.7070 and 0.7075 with initial ε Nd = − 6.0 to − 5.2 (Table 5).…”

“…Although controversial, the type and nature of the processes responsible for the genesis of different granitic magmas can involve partial melting of distinct source rock materials, hybridisation of magmas with contrasting compositions, fractional crystallization, crustal contamination and more complex models, such as assimilation-fractional crystallization process (Bea et al, 1999;Bonin, 2004;Castro et al, 1999;Collins et al, 2000;Dias and Leterrier, 1994;Dias et al, 2002;Janousek et al, 2004;Moreno-Ventas et al, 1995;Patiño-Douce, 1999;Renna et al, 2006;Silva and Neiva, 2004;Slaby and Martin, 2008;Villaseca et al, 1998Villaseca et al, , 2008Villaseca et al, , 2009). Thus, their study offers an important contribute to a better knowledge of the mechanisms of magma genesis and the deep continental crust evolution, giving together insights about the nature of source rocks that are melted.…”