Petrographic, chemical and geochronological characteristics of the Onzaga Metarhyolite and its correlation with Ordovician magmatic events in the northern Andes, Colombia

Abstract

The macroscopic, microscopic, and chemical characteristics as well as crystallization age of the Onzaga Metarhyolite unit are described. The spatiotemporal relationships among the magmatic events that occurred during the Ordovician in the Santander, La Floresta and Quetame massifs and in the Mérida Mountain range are analyzed. Additionally, a correlation is made with plutonic bodies that have crystallization ages within this same period of time.
 The Onzaga Metarhyolite outcrops in the Santander massif in the surroundings of the municipality of Onzaga-Santander, with an approximate area of 155 km2; it is composed of metavolcanic rocks that are chemically classified as rhyolites, which were affected by metamorphism in green schist facies to low amphibolite. The metarhyolites are composed of quartz porphyroclasts, plagioclase and alkali feldspar, and some retain the textures and relict forms of the phenocrysts, which are found within a microcrystalline matrix, with local development of quartz mosaics and oriented sheets of white mica and biotite.
 The rocks of the Onzaga Metarhyolite have high contents of SiO2 (76.9% to 79.3%), K2O (4.03% to 5.66%) and Al2O3 (11.5% to 14.3%) and low contents of Fe2O3, TiO 2, CaO, MnO and MgO. The rocks have alkali values (Na2O + K2O) between 5.3% and 7.9% and K2O/Na2O> 2. They are classified within the calc-alkaline rocks high in K, peraluminous, and related to magmas that generate type S granites. In the multielement diagrams, Nb has a weak negative anomaly, as does Zr, while the negative anomalies of Ti and Sr are pronounced. Higher values of lithophilic elements of high ionic radius (LILE) Cs, Ba and Th indicate a continental crust affinity. The Eu/Eu * values are low, between 0.12 and 0.74. The normalized values of (La/Yb) N vary between 4 and 24.4, (La/Sm) N varies between 2.2 and 2.5, and (Eu/Yb) N varies between 0.2 and 2.1.
 Two samples of Onzaga Metarhyolite were analyzed by the LA-ICP–MS U-Pb method in zircon. The results analysis indicates a crystallization age that varies between 450.9 ± 2.5 Ma and 449.9 ± 5.9 Ma, with a second population between 475.9 ± 5.4 Ma and 469.8 ± 4.0; these ages are interpreted as magmatic zircons inherited from a lower to middle Ordovician igneous event. The zircons have xenocrysts and inherited nuclei of 555 ± 11 Ma and 565.7 ± 9 Ma and populations of 617 ± 16 Ma (n = 3) and 644.5 ± 6.5 (n = 3), respectively, and three inheritances between 1071 ± 48 Ma and 1171 ± 37 Ma.