Mantle and Crustal Sources in the Genesis of Late-Hercynian Granitoids (NW Portugal): Geochemical and Sr-Nd Isotopic Constraints |
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| Institution: | 1. Dipartimento di Scienze della Terra e del Mare, Università degli Studi di Palermo, Via Archirafi 20-22, 90123 Palermo, Italy;2. UMR CNRS-UPMC-IRD 7159, LOCEAN, Université Pierre et Marie Curie, 4 Place Jussieu, 75252 ParisCedex 05, France;3. CR2P, UMR 7207, Sorbonne Universités -MNHN, CNRS, UPMC-Paris 6, Muséum national d''histoire naturelle, 57 rue Cuvier, CP 48, F-75005 Paris, France |
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| 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 (Sri = 0.7049 to 0.7053, eNd = -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 (Sri = 0.7089 to 0.7106, eNd = -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 (Sri = 0.7064 to 0.7085, eNd = -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. |
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