MIOCENE-PLIOCENE HIGH-Sr INTERMEDIATE AND ACID ROCKS IN THE SIERRA DEL NEVADO, PAYENIA

RUBIANO LORENZONI, GEORGINA; ORIHASHI YUJI; BERTOTTO GUSTAVO W.; JALOWITZKI, TIAGO; SUMINO, HIROCHIKA; PONCE ALEXIS; BERNARDI, MAURO I.; HIRATA, DAIJI

Congreso; XXI Congreso Geológico Argentino; 2022

Late Cenozoic magmatism located in the northeastern area of the Andean Southern Volcanic Zone (34°-38°S) is characterized by voluminous eruptions of intermediate to felsic rocks and by monogenetic volcanism. Sierra del Nevado (35º25?-35º45ºS and 68º10?-68º47?W) represents an example of silicic back-arc volcanism situated more than 200 km from the Andean volcanic arc (e.g., Holmberg 1973, Sepúlveda et al. 2007). Three main Miocene-Pleistocene volcanic units have been described in this area: a) felsic to intermediate rocks of Cortaderas Formation, b) intermediate to ma c rocks of Nevado Formation, and c) basaltic rocks of Chapúa Group. In this work, new petrographic and geochemical data coupled with K-Ar ages of rocks from Cortaderas Formation are presented. These complement previous  eld observations, petrographic descriptions, whole-rock geochemistry, and chronological data from Bermúdez (1989), Sepúlveda et al. (2007), Ramos et al. (2012) and Litvak et al. (2015). Samples from Los Cerritos, Sierra Chorreada, Peceño, Llume, Pelado, Puntudo, Plateado, and Los Cerritos de Afuera eruptive centers were collected. Based on  eld descriptions and petrography classiffications, most samples are dacitic and andesitic lavas and domes of, except for the obsidian breached dome of Cerro Peceño. The dominant texture is porphyritic with the groundmass usually seriate, but also in lesser extent felsitic and pilotaxitic. The  phenocrystals (60-25%) are mostly composed by: plagioclase + hornblende + biotite+ opaques minerals, with rarely quartz ± clinopyroxene ± allanite. Plagioclase is usually the main crystal phase and the larger phenocrysts (4.2-1.53mm) that normally exhibit growing-related and morphological micro-textures (Renjith 2014). The  rst are coarse/ ne-sieve morphology,  ne-scale oscillatory zoning in the rims or in the whole crystals, as well as in resorption surfaces. On the other hand, morphological micro-textures like glomerocrysts, synneusis and broken crystals are also common. As suggested in previous studies (Ramos et al. 2012, Litvak et al. 2015), the Cortaderas Formation is subdivided into two chronological stages: 11.73-7.40 Ma (older volcanic centers: Los Cerritos, Puntudo, Sierra Chorreada) and 4.54-3.55 Ma (younger centers: Llume, Pelado, Plateado without the Peceño whose age is 0.8 Ma). Following TAS diagram (Fig. 1A), they are all subalkaline with the  rst group composed by andesites, except for one sample from Sierra Chorreada, which classi ed as a basaltic andesite. Conversely, the younger centers have a variable composition that includes andesite, dacite and trachytes. The rocks from Cortaderas Formation are plotting within the medium- to high-K series on the K2O-SiO2 diagram (Le Maitre et al. 1989). The majority of these andesites and dacites show SiO2-enrichment trend coupled with insigni cant variation in FeO*/MgO ratios. They belong to calc-alkaline series (Miyashiro 1974),except for four samples from Peceño, Los Cerritos, Sierra Chorreada and Pelado, which plot near the limit between the tholeiitic and calc-alkaline series. The CIPW normative minerals are: albite 31.1-46.6, orthoclase 11.7-23.9, anorthite 6.0-27.4, quartz 8.6-31.4, with ± corindon ± hiperstene ± diopside ± wollastonite ± magnetite ± hematite ± ilmenite ± titanite ± apatite. MORB-normalized trace element patterns of studied samples show enrichment of LILE over HFSE with Ba/Nb ratios of 55.68-238.20. Moreover, these patterns have negative anomalies in Nb-Ta, and slight positive Eu anomalies (1.01-1.29). In a REE chondrite-normalized diagram, all samples have LREE enriched compared to HREE (La/YbN = 11.43-23.47). The Cortaderas rocks are enriched in Sr (>500 ppm). Both Miocene and Pliocene rocks can be divided based on their Sr/Y ratios, where older centers have higher ratios (46.18-121.51) than younger ones (36.16-44.49 with an outlier of 88.03). Seven samples from Cerro Plateado, Los Cerritos, Los Cerritos de Afuera, and Pelado strictly plot on the ?adakite?  eld in the Sr/Y vs. Y and La/Yb vs. Yb diagrams (e.g., Defant andDrummond 1990, Castillo 2012) (Figs. 1B and 1C). They belong to the group of ?High-SiO2 adakites? (Martin et al. 2005), with SiO2= 60.32-70.65 wt.%, Mg#: 33.95-52.21, Sr = 556-992 ppm (with few>1100 ppm), low