G. Dias

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 (Sr i = 0.7049 to 0.7053, e Nd = -2.1 to -2.5). In some syn- and 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 (Sr i = 0.7089 to 0.7106, e Nd = -5.6 to -6.8), revealing a heterogeneous crust. Potential protoliths are metasedimentary (immature sediments) and/or felsic 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 i = 0.7064 to 0.7085, e Nd = -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.