Lithostratigraphy, petrography and geochemistry of the Pachicutza unit in the Cóndor cordillera

Abstract

To South of the Ecuadorian Sub-Andean Zone, in the center-north of the Cóndor cordillera, multiple intrusions of various ages were emplaced in the host-rock (Piuntza Unit), that involves volcanic, volcanoclastic and sedimentary sequences of Triassic and Lower Jurassic. The volcanic series within host-rock have been poorly documented, however, basaltic series have been discriminated and grouped in the Pachicutza Unit, nevertheless, stratigraphic relationships and lithostratigraphic, petrographic, and geochemical detail are unknown. Based on lithological discrimination, documentation of erosion surfaces, a description of stratigraphic architecture, compilation of structural data, petrographic and geochemical analysis (major elements, trace elements and rare earth elements), This research reports the characterization lithostratigraphic, petrological and geochemical of the Pachicutza Unit in the Nangaritza river basin, in the vicinity of the Pachicutza river. Thereby, at the base of the Pachicutza Unit, fining-upwards sequence of clast supported, conglomerates with basalts, sandstones, and limestone clasts and massive lithic-sandstones are observed, whereas, the middle part of the Unit is characterized by interbedding mudstone and fine-tuff. These characteristics indicate depositional environment linked to alluvial fans dominated by debris flows, proximity to a basaltic volcanic source. Whilst, the top is dominated by clast and supported matrix, volcanic breccias, and thick beds of subalkaline, basaltic andesites and basalts, of tholeiitic and transitional affinity, MORB-type and Within-Plate lavas environment, high concentrations Cs, Th and Pb and low concentrations Rb, Nb, Ba, K and Sr suggest to an attenuated continental crust, these characteristics are common in back-arc environment with extensional tectonics.