Geochemistry of the (meta-)mafic rocks from the Gonzalito mining district, northern Patagonia

MARTÍNEZ DOPICO, CARMEN I.; PUGLIESE, FRANCO; LÓPEZ DE LUCHI, MÓNICA G.; CUTTS, KATHRYN A.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2020 vol. 104

0895-9811

AbstractIn spite of hosting one of the most important Pb-Ag-Zn mineralizations in Patagonia, the metamorphic history of Mina Gonzalito Complex (MGC; east of the North Patagonian Massif) is still unclear. The complex consists of schists, para- and ortho-derived gneisses, ranging from greenschist to amphibolite facies, and metamafic rocks. Mica-rich leucogranites and pegmatites were intruded synkinematically. Field, petrological and thermochronological evidence indicates that the MGC experienced an early prograde path and a metamorphic peak during the Early Ordovician (ca. 472 Ma), magmatism and localized post-peak deformation and reequilibrium at lower pressure, followed by uplift during the Late Permian. The MGC is intruded by the calc-alkaline Santa Rosa Diorite (SiO2 = 58.7-60.4 wt%; LaN/YbN = 7.2-10.5) and trachyte dike swarms in the Late Permian- Early Triassic. The mafic intrusions of the MGC form small massive, schistose and banded bodies interlayered within the gneisses and granites and record recrystallization of hornblende + plagioclase + quartz + titanite ± clinopyroxene ± biotite ± ilmenite. The metamafic rocks are mostly tholeiitic gabbros having SiO2 (45.4-52.1 wt.%), TiO2 (0.62-2.88 wt.%), flat REE patterns (LaN/YbN =0.48-2.76), although some pyroxene-banded varieties show higher ratios. Initial P?T modelling in the NCKFMASHTO system for the metamafic rocks defined P-T conditions between 550?730 °C and 1-4 kbar. Our data suggests that the protolith of the metamafic rocks was emplaced at? a shallow environment, associated with underplating of mantle-derived magmas slightly modified by crustal contamination. The intrusion of mantle-derived magmas may have been related either to magmatic arc or continental rift environments. The model involving an Ordovician intracontinental back-arc basin is favored herein because this model can reasonably explain many other geological features of Early Paleozoic basement rocks from the northern Patagonia