Different types of Variscan mafic granulites characterize the lower crust of the Serre Massif (Calabria, Southern Italy). They consist of: layered metagabbros (Pl+Opx+Amph±Cpx±Bt±Qtz±Grt), metric bodies of meta-monzogabbros (Pl+K-feld+Opx+Cpx+Bt±Qtz±Grt) and lenses or layers of metabasites (Pl+Opx+Cpx±Bt±Amph±Qtz±Grt) interleaved the overlying migmatitic metapelites and felsic granulites. The Variscan metamorphic events developed in high grade metamorphic conditions in which partial melting and fluid interaction were effective (Fornelli et al., 2011, 2014). However, the whole chemical composition of mafic granulites seems unmodified because migrations of melts at large scale have not been documented. Therefore, the whole rock compositions of granulite mega-samples can give suggestions about the magmatic history of their protoliths. Geochemical data indicate a mantle origin for the Neoproterozoic magmatic protoliths (~570 Ma, Fornelli et al, 2011). Metagabbros and metabasites show sub-alkaline features (Na2O+K2O=1-5%) with K2O<1%, whereas meta-monzogabbros display alkaline character (Na2O+K2O=5-7%) with K2O around 3% testified by Bt and K-feld. In metagabbros, the 87Sr/86Sr(570 Ma) ratio ranges from 0.703 to 0.708 with low values of 87Rb/86Sr (from 0.001 to 0.024). Metabasites have higher 87Sr/86Sr(570 Ma) values (0.707-0.710) with 87Rb/86Sr ratio from 0.025 to 0.237 Meta-monzogabbros show 87Sr/86Sr(570 Ma) ratio around 0.705 with high 87Rb/86Sr (0.489-0.544). The εNd(570 Ma) varies from 4.902 to -0.843 in metagabbros, from 0.741 to -2.388 in metabasites, whereas in meta-monzogabbros assume the values varying from -1.078 to -1.253. The meta-monzogabbros show a strong mantle affinity but are enriched in incompatible elements, their origin seems linked to small degree of enriched mantle melting. The metagabbros and metabasites show crust-mantle interaction for their initial 87Sr/86Sr and εNd(570 Ma) values. We hypothesize that subsequent and incremental partial melting events (after the production of gabbroic magma) produced mantle melts with tholeiitic and calc-alkaline affinities migrating in the overlying metasedimentary crust; the stay in hot lower metasedimentary crust induces a certain grade of crust contamination charged by crustal fluids or melts, evidenced by the increase of initial 87Sr/86Sr ratio and by decrease of εNd(570 Ma) values at least in protoliths of metabasites. On the other hand, the placement of metabasites within metasedimentary crust constrains this suggestion.

Metagabbros, meta-monzogabbros and metabasites from Serre Massif (Calabria-Italy): evidence of a Neoproterozoic magmatism at crust-mantle transition

Fornelli A.;Micheletti F.
2018

Abstract

Different types of Variscan mafic granulites characterize the lower crust of the Serre Massif (Calabria, Southern Italy). They consist of: layered metagabbros (Pl+Opx+Amph±Cpx±Bt±Qtz±Grt), metric bodies of meta-monzogabbros (Pl+K-feld+Opx+Cpx+Bt±Qtz±Grt) and lenses or layers of metabasites (Pl+Opx+Cpx±Bt±Amph±Qtz±Grt) interleaved the overlying migmatitic metapelites and felsic granulites. The Variscan metamorphic events developed in high grade metamorphic conditions in which partial melting and fluid interaction were effective (Fornelli et al., 2011, 2014). However, the whole chemical composition of mafic granulites seems unmodified because migrations of melts at large scale have not been documented. Therefore, the whole rock compositions of granulite mega-samples can give suggestions about the magmatic history of their protoliths. Geochemical data indicate a mantle origin for the Neoproterozoic magmatic protoliths (~570 Ma, Fornelli et al, 2011). Metagabbros and metabasites show sub-alkaline features (Na2O+K2O=1-5%) with K2O<1%, whereas meta-monzogabbros display alkaline character (Na2O+K2O=5-7%) with K2O around 3% testified by Bt and K-feld. In metagabbros, the 87Sr/86Sr(570 Ma) ratio ranges from 0.703 to 0.708 with low values of 87Rb/86Sr (from 0.001 to 0.024). Metabasites have higher 87Sr/86Sr(570 Ma) values (0.707-0.710) with 87Rb/86Sr ratio from 0.025 to 0.237 Meta-monzogabbros show 87Sr/86Sr(570 Ma) ratio around 0.705 with high 87Rb/86Sr (0.489-0.544). The εNd(570 Ma) varies from 4.902 to -0.843 in metagabbros, from 0.741 to -2.388 in metabasites, whereas in meta-monzogabbros assume the values varying from -1.078 to -1.253. The meta-monzogabbros show a strong mantle affinity but are enriched in incompatible elements, their origin seems linked to small degree of enriched mantle melting. The metagabbros and metabasites show crust-mantle interaction for their initial 87Sr/86Sr and εNd(570 Ma) values. We hypothesize that subsequent and incremental partial melting events (after the production of gabbroic magma) produced mantle melts with tholeiitic and calc-alkaline affinities migrating in the overlying metasedimentary crust; the stay in hot lower metasedimentary crust induces a certain grade of crust contamination charged by crustal fluids or melts, evidenced by the increase of initial 87Sr/86Sr ratio and by decrease of εNd(570 Ma) values at least in protoliths of metabasites. On the other hand, the placement of metabasites within metasedimentary crust constrains this suggestion.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11586/227882
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