Geochemistry of the intrusive Miocene arc magmas from the Malargüe area, southern Mendoza, Argentina: an overview and implications on crustal contamination

OSTERA, H., HALLER, M., LÓPEZ DE LUCHI, M.G.,. CAGNONI, M., RAMOS, A., LINARES, E.,

Barcelona, España

Simposio; . 6th International Symposium on Andean Geodynamics; 2005

Institut de recherche pour le developpement, Universitat de Barcelona

>The Southern Paramillos Altos Intrusive Belt (PAIB) (35º 20´-35º-40´) is located in the thick skinned folded-thrust belt of Malargüe, southwestern Mendoza, Argentina.This belt is characterized by episodic arc magmatism that cover the time span from 20-5 Ma. The Early Miocene magmas have more radiogenic Sr isotopic ratios, flatter REE patterns with less LREE enrichment, and lower Th, Ta, alkali and alkaline earth contents. Such observations rule out a pre-Miocene ?enriched? mantle component in the source. Relatively high Nd ratios and less radiogenic 206Pb/204Pb point to depleted source. The geochemistry of the Early Miocene magmatism suggests local process at the magmatic chamber. Crustal thickening is consistent with the steeper REE patterns for PAIC-SAS that may indicate more amphibole-rich residual mineral assemblages equilibrating with magmas in a thicker crust. At equivalent SiO2content, samples of the PAIC-SAS show lower Nd ratios and 87Sr/86Sr ratios relative to early Miocene magmas which could fit with lower crustal contamination within the thickened crust, as well as with any process of contamination of the mantle wedge. Slightly higher Zr/Nb for this middle Miocene rocks point to reduced sedimentary input or in other words, to a more depleted mantle like source. Geochemical and isotopic signature of the rhyolitic dykes of PCB would imply crustal involvement. The low Zr/Nb point to a sedimentary or upper crustal reservoir