Amphibole megacrysts of the Cerro Jeu-Jepén Pluton: some constraints in the framework of the Fuegian Potassic Magmatism (Argentina).

RIDOLFI, F.; RENZULLI, A.; CERREDO, M. E.; OBERTI, R.; BOIOCCHI, M; BELLATRECCIA, F; DELLA VENTURA G; MENICHETTI, M.; TASSONE A.

BOLLETTINO DI GEOFISICA TEORICA ED APPLICATA

ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA

Lugar: Trieste; Año: 2010 vol. 51 p. 80 - 83

0006-6729

The Cretaceous plutons of Argentine Tierra del Fuego Island are characterized by mildly alkaline (shoshonitic) to high-K affinity (hereafter Fuegian Potassic Magmatism; González Guillot et al. 2009), with igneous intrusives ranging  from ultramafic rocks to monzogabbro and monzonite/syenite (Cerredo et al. 2000, 2005, 2007; Peroni et al. 2009). By contrast the coeval intrusions of the South Patagonian Batholith (Nelson et al. 1988; Hervé et al. 2007) and the Beagle Channel Plutonic Group of the southern archipelago (Hervé et al., 1984) are characterized by a dominant subalkaline gabbro to granite trend. Among the intrusive bodies belonging to the Fuegian Potassic Magmatism, the Cerro Jeu-Jepén (CJJ) pluton is a small (< 10 km2), Late Cretaceous (93±4 Ma, Acevedo et al. 2000) intrusive body located at the SE tip of the Lago Fagnano which is stretched out along the inferred strike of the Magallanes-Fagnano Fault (MFF) system (Cerredo et al. 2005). In order to unravel the physical-chemical conditions of crystallization of the CJJ amphibole megacryst and the whole evolution of the monzonite body, a comprehensive study based on EMP, single-crystal XRD, SREF, FTIR and thermobarometry, was carried out. A compositional and thermobarometric picture of the genesis and evolution of CJJ magmas was obtained with the application of several phase equilibria methods. This study emphasizes the mantle P-T condition of crystallization of the CJJ magnesio-hastingsite megacrysts, most likely after small degree of partial melting of the peridotite source. In addition, the strong-zoning of monzonite plagioclase and clinopyroxene phenocrysts, and the disequilibrium corona textures around amphibole megacrysts in the studied CJJ monzonite suggest that the crystallization of the shoshonitic magma (which finally gave rise to the pluton at shallow crustal levels) followed a decreasing P-T path just above the upper limit of amphibole stability, probably promoted by transtensive movements along the MFF system.