Geology, geochemistry, isotope geochemistry and fluid inclusions of the Early Carboniferous granitic rocks from Bajo de La Leona, Deseado Massif (Santa Cruz, Argentina) and geological relationships with the Triassic-Jurassic magmatism

Abstract

Paleozoic plutonic rocks from the eastern Deseado Massif, southern Patagonia Argentina, generally appear as stocks and sills of variable lithological composition from gabbro-diorite to leucogranite, with pegmatite-aplite differentiations. Their ages vary from Early-Middle Ordovician up to Early Carboniferous.The Early Carboniferous Laguna Turbia Leucogranite (LTL) crops out in the Bajo de La Leona area as a small stock located between the Permian conglomerates of La Golondrina Formation and the Late Triassic-Early Jurassic plutonic rocks of La Leona Formation. It shows a peneplanized landscape due to erosion processes and has microscopic evidence of solid-state deformation features. The geochemical signature is calc-alkaline, typical of magmatic arc environments. The high K contents and FI data are consistent with a crustal provenance.Four kilometers to the SSW of the LTL appears a biotitic granite whose petrographic, geochemical and isotopic characteristics indicate that it belongs to a different magmatic cycle, of possible Late Devonian-Early Carboniferous age. It has been named Laguna Larga Granite (LLG).The Late Triassic-Early Jurassic intrusive rocks (La Leona Formation) occur as several plutons in close spatial association with pre-Permian igneous-metamorphic basement rocks in the eastern part of the Deseado Massif. These granitoid suites crop out in erosional windows through the widespread Middle to Late Jurassic Bahía Laura Complex and younger cover.The main isotopic feature of the granitic rocks of Bajo de La Leona is their lack of equilibrium, which is especially evident in the case of the LLG, given its extraordinarily low values of δ18O.Fluid inclusion data obtained allowed the identification of different types of fluid formation environments: (a) a porphyry type, related to the Early Carboniferous leucogranite suggesting an origin from cooling magmatic solutions; (b) late magmatic/high salinity fluids related to Mesozoic quartz monzonite; (c) solutions of hydrothermal/mesothermal origin that generated the quartz gold veins (LSV and LTV); and (d) solutions associated with an epithermal system superimposed on quartz monzonite of La Leona Formation, which gave rise to Cu mineralizations in quartz-veins with adularia of La Leona and Schultz mines. The FI and stable isotope data suggest that ore fluids related to quartz veins were product of mixing igneous and meteoric solutions.The emplacement of the LTL extends the known Paleozoic magmatic history of the eastern Deseado Massif into the Early Carboniferous. Magmatism of that age is scarcely known in southern Patagonia.