Ñadó volcanic complex, north of Acambay graben: evidence of Oligocene magmatism and its relation with the central sector of the Trans-Mexican Volcanic Belt
The Ñado Volcanic Complex is located in the central sector of the Transmexican Volcanic Belt (TMVB), to the north of the active Acambay graben, in the states of Querétaro and México. This complex is formed by 11 stratigraphic units, which were subdivided into three sequences (1. pre-Ñadó, 2. Ñadó, and 3. post-Ñadó). The oldest unit of the pre-Ñadó sequence (La Florida Rhyolite) was dated at 30.81 ± 0.17 Ma and 31.71 ± 0.27 Ma by the U-Pb method in zircon, and can thus be related to the Oligocene felsic magmatism of the Sierra Madre Occidental (SMO). All other sequences can be temporarily correlated with previously reported episodes of late Miocene volcanism in the TMVB. The Ñadó composite volcano was built up during late Miocene by multiple effusive events of dacitic composition, including the emplacement of several lava flows, a spine (Peña Ñadó), and a dome (Cerro Pelón). This acid volcanism was contemporaneous with mafic volcanism, represented by scoria cones and basaltic lava flows. Finally, the post-Ñadó stage is represented by a series of andesites, ignimbrites from Amealco and Huichapan Calderas, mafic volcanism, lahars and fall deposits from Temascalcingo volcano. The geochemistry of the rocks shows some typical characteristics of subduction zones, such as enrichment in large ion lithophile elements (LILE) compared to high field strength elements (HFSE), negative anomalies of Nb, Ta, P, and Ti, as well as positive anomalies of Ba, K, Pb y Sr. The rare earth elements (REE) show an enrichment of light REE with respect to heavy REE, except for the Oligocene La Florida rhyolites, which show REE patterns with a typical “seagull” shape, which suggests a different origin compared with the younger samples. Geochemical characteristics suggest that the main petrological mechanism during the Ñadó evolution was the AFC.