Abstract

TORO-TORO, Luz Mary; CARDONA-RIOS, John Jairo; MORENO-SANCHEZ, Mario  and  GOMEZ-CRUZ, Arley de Jesús. Petrography and geochemistry of the pyroclastic and effusives rocks of the Bocas Formation (Upper Triassic - Lower Jurassic) and effusives of the Nogontova Formation (Santander Massif, Colombia). Bol. geol. [online]. 2021, vol.43, n.1, pp.53-75.  Epub Jan 01, 2021. ISSN 0120-0283.  https://doi.org/10.18273/revbol.v43n1-2021003.

The sequence of volcanic and volcano-sedimentary rocks reported in the upper segment of the Bocas Formation, underlying red sediments of the Jordán Formation, is constituted by mafic lava spills and felsic pyroclastic interstratified with mudstones, sandstones and tuffaceous sandstones. This work presents new data field, petrographic and geochemical of one sequence mafic volcanic and felsic volcanoclastic. Near to township of Las Bocas, the basic rocks have low TiO. (1.04 - 1.65% weight) and P.O. (0.3 - 0.7% weight) and high content of Al.O. (15.79 - 15.92% weight), within the tolerance series; these rocks are characterized by a slight enrichment of LREE, positive anomalies in Th, U and Zr and negative anomalies in Nb, Ta and Ti with MORB environment trends. The felsic rocks that correspond to pyroclastic rocks of rhyolitic composition are enriched in alkalis and LREE with negative anomalies Ba, Sr, Nb, Ta and Ti and moderate enrichment in LREE with a characteristic intraplate pattern, on the other hand, the geochemistry of the basalts of the Nogontova Formation indicates a MORB-type environment, these data and those obtained for the rocks of the Bocas Formation suggest a rifting (back arc) environment associated with subduction.

Keywords : Volcanic rocks; Bocas Formation; Nogontova Formation; Tuffs of rhyolitic composition; Intraplate.

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