A bimodal source for the generation of tonalitic to granitic magmas in a non-subduction-related magmatic belt: An example from the Sierra Chica of Córdoba, Argentina

BOFFADOSSI, MARÍA ALEJANDRA; DÍAZ-ALVARADO, JUAN; PINOTTI, LUCIO PEDRO; JUNG, STEFAN; D'ERAMO, FERNANDO JAVIER; PEDRERA, ANTONIO; MILLÁN-MARTÍNEZ, MARÍA; DEMARTIS, MANUEL; AZOR, ANTONIO; MURATORI, MARÍA EUGENIA

LITHOS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2023

0024-4937

A notable consensusprevails about the hybrid nature of intermediate calc-alkaline magmatism.Accordingly, petrogenetic models envisage the addition of young mantle materialand cortical recycling in those geological settings where intermediate magmasare dominant. Most exhaustively studied examples are: 1) magmatic arcsassociated with subduction zone settings where oceanic crust and sediments areintroduced into the mantle wedge, and 2) deep crustal sections are pervaded byhot, H2O-richmantle-derived liquids. Nevertheless, back-arc or foreland terranes of severalhundred kilometers in width, frequently characterized by high heat flux,crustal melting, recycling and melt fractionation, are crucial areas toevaluate the generation of intermediate magmatism. The Sierra Chica of Córdoba(Argentina) provides an excellent example to study these processes. Thehighgrade crustal segment shows metamorphosed pelitic and basaltic protolithsthat were part of an old sedimentary succession. We evaluate the evolution ofthis bimodal source during two consecutive early Paleozoic orogenic periodswhen it was established as the source area of intermediate to silicic magmatismfar from the contemporary magmatic arcs.Field relations andnew geochemical and geochronological data revealed the occurrence of twomagmatic events during Cambrian and early Ordovician periods. Cambrianmigmatization and partial melting (~528 to 505 Ma) is represented byirregularshaped monzogranites and tonalites, while early Ordovician magmatism(~480 Ma) is characterized by dyke-shaped pegmatites andtonalite-trondhjemites. The preservation of two contrasting igneous lineagessuggests a genetic linkage between metapelitic and amphibolitic migmatites andbetween granitic and tonalitic-trondhjemitic partial melts. Coeval with thismagmatism, Famatinian tonalitic-trondhjemitic to monzogranitic magmas intrudedupper crustal levels in Sierra Chica and other parts of Sierras of Córdoba. Geochemicalsignatures point to a bimodal (mafic/felsic) source for this intermediate to silica-richmagmatism. The evolution of partial melts studied in the Sierra Chica isdepicted on the basis of geochemical projections with fractionation andhybridization models, in order to establish the primary liquids of thisFamatinian magmatic belt. Trends defined by Famatinian tonalites, trondhjemitesand granodiorites suggest that primary Ca-rich melts (tonalites andtrondhjemites of this study) evolved through fractionation of plagioclase (30- 50vol.%) compatible with the preservation of inferred co-magmatic Pl-cumulateswithin these plutonic bodies. Peraluminous granites and monzogranites can beexplained by a simple restite unmixing process involving alkali-rich liquidsgenerated in the metapelitic part of the source. The addition (hybridization)of up to 30 vol.% of granitic melts to the tonalite-trondhjemite end-member mayaccount for the composition of other Famatinian granodioritic to monzograniticintrusive rocks from the plutonic belt.The crustal segmentsof the Sierra Chica have undergone metamorphism and partial melting inout-of-arc settings: the Cambrian fore-arc and the early Ordovician back-arc. Thevoluminous magmatic belt rooted in the paired metapelitic-amphibolic sourcearea is involved in the origin of intermediate to silicic Famatinian magmatismwithout significant mass transfer from the mantle and crustal growth, insteadit is dominated by crustal melting, recycling and fractionation.