Abstract
Post-crystallization evolution of the metatroctolites of the Lago Norquinco zone, Alumine Igneous-Metamorphic Complex, is analyzed. These mafic rocks show coronas around olivine, composed of Opx, Cpx, Spl and Amp (Hbl1 and Hbl2). The temporal evolution of these rocks was evaluated via petrographic analysis and the construction of a suitable pseudosection based on the whole-rock chemical composition. This analysis was complemented with classic geothermobarometry, from which four stages on the P-T path were established: A. Starting point lies in the stability field of the Pl-Ol-Opx-Cpx-H2O, with P-T of 936 oC and 2.7 kbar in granulitic facies conditions; B. another point in the trajectory is represented by the presence of amphibole with P-T conditions of 780 oC and 5.69 kbar for the stability of Hbl1-Pl; C. the third point is characterized by the appearance of Hbl2 and Spl and the disappearance of Ol, leading to the association Pl-Opx-Cpx-Hbl2-Spl-H2O. Equilibrium temperature and pressure for this stage are 780 oC and 5.69 Kbar; and D. the last point implies the disappearance of Cpx and the retrogression of the phases to Chl with a concomitant consumption of excess water. The equilibrium temperature (694.2 oC) for the stable association Pl-Opx-Hbl2-Spl-Chl, whereas the narrow stability pressure range (4.5-5.2 Kbar) is constrained by the absence of olivine and garnet. The defined portion of the P-T path indicates cooling from granulite to amphibolite facies with a concomitant pressure increment, defining a counter-clockwise P-T path. U-Pb zircon ages determined for basement xenoliths in metatroctolites and gabbroic inclusions in granodiorites, allows constraining the emplacement of Norquinco lake metatroctolites to Upper Paleozoic times.
Highlights
In the southern region of the Aluminé IgneousMetamorphic Complex (AIMC) (Urraza et al, 2008a and b, 2009, 2011; Urraza, 2014), mafic bodies of variable dimensions are emplaced in a metapelitic sequence composed by schists, gneisses and amphibolites
U-Pb zircon ages determined for basement xenoliths in metatroctolites and gabbroic inclusions in granodiorites, allows constraining the emplacement of Ñorquinco lake metatroctolites to Upper Paleozoic times
The Ñorquinco metatroctolites constitute the northernmost outcrops of metamafic rocks, and are characterized by the presence of coronas developed around relict-magmatic olivine crystals and different replacement textures involving amphiboles
Summary
In the southern region of the Aluminé IgneousMetamorphic Complex (AIMC) (Urraza et al, 2008a and b, 2009, 2011; Urraza, 2014), mafic bodies of variable dimensions are emplaced in a metapelitic sequence composed by schists, gneisses and amphibolites This sequence of pre-Andean rocks constitutes an accretionary prism related to the subduction of the oceanic plate, bellow the southwestern Gondwana margin during the Paleozoic (Urraza, 2014). Kelyphytic corona textures and different types of symplectites are common in gabbros, in metatroctolites regarding to reactions between olivine and plagioclase (Claeson, 1998; Lang et al, 2004) These microstructures always present great interest as they provide invaluable information in the reconstruction of the metamorphic evolution and P-T conditions of the rocks (Cruciani et al, 2008). This study was focused on metatroctolites that exhibit coronas between olivine and plagioclase, and involve the analysis of mineral assemblages, textures, microstructures, whole-rock/mineral geochemistry, geochronology and metamorphic reactions, in order to define the P-T trajectory of the metatroctolites of Ñorquinco lake
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