Mafic rocks of the Ordovician Famatinian magmatic arc (NW Argentina): New insights into the mantle contribution.

ALASINO, P.H.; CASQUET , C.; PANKHURST, R.J.; RAPELA, C.W.; DAHLQUIST, J.A.; GALINDO, C.; LARROVERE, M.; RECIO, C.; PATERSON, S.; COLOMBO, F.; BALDO, E.

GEOLOGICAL SOCIETY OF AMERICA BULLETIN

GEOLOGICAL SOC AMER, INC

Lugar: Boulder; Año: 2016 vol. 128 p. 1105 - 1120

0016-7606

The petrogenesis of mafic magmatism in the Famatinian arc has been studied in thewestern Sierra Famatina (NW Argentina), a middle crustal section of the EarlyOrdovician in the proto-Andean margin of the Gondwana. Mafic rock types consist ofamphibolite, gabbro and metagabbro, and pod- and dyke-like bodies of gabbro-dioritecomposition. Field relations together with geochemical and isotopic data for maficrocks of the western Sierra de Famatina (at 29° S) define two contrasting groups,which are correlated with similar distinctions noted in other parts of the orogen.Amphibolite, metagabbro and gabbro bodies mostly form the oldest intrusive rocks (>480 Ma), with the tonalite and post-tonalite mafic dykes being slightly younger. The first group, consisting of the pre- to syn-tonalite mafic magmas, was tholeiitic to calcalkaline and isotopically evolved (excluding amphibolite). The second group, calcalkaline, of syn-tonalite mafic intrusive rocks, often having subduction geochemical signatures, was isotopically more juvenile. Whole-rock chemical composition and isotopic analyses are compatible with a progressive mixing of different isotopic reservoirs. Pyroxenites (±garnet) are likely the dominant source of the older gabbroic magmatism whereas peridotite dominates in the second one, implying that the mafic magma experienced a progressive shift towards more juvenile compositions though time (over 20 m.y.). Pyroxenite-derived melts could be generated by lithospheric foundering followed by upwelling of primitive melts by adiabatic decompression of mantle wedge peridotite.