PETROLOGY OF THE TERTIARY BERNAL DE HORCASITAS VOLCANIC NECK, TAMPICO PLAIN, NE MEXICO

In the Tampico Plain (Northeastern Mexico) several isolated and eroded volcanic structures of Tertiary age can be recognized. They are composed mainly by primitive basanites and alkali olivine basalts, as well as by highly evolved peralkaline phonolites. These rocks carried mantle xenoliths during their ascent.

Bernal de Horcasitas (BH, Tamaulipas) is the most imposing structure of them, due its morphology that highly contrasts with the surrounding areas. It is located ca. 15 km west from the small town of Gonzalez, and it can be observed in clear days at distances of more than 50 km. BH top reaches a height of 860 m, ca. 750 m above the plain. Its basis is almost circular, with a diameter of 6 km. Its central part has a ca. 600 m diameter, and shows nearly vertical walls with columnar cooling joints.

BH lithology cannot be directly compared with those of the other volcanic necks. It has been described as an ultrabasic olivine phonolite, composed by anorthoclase, orthoclase, nepheline, analcime, Ti rich diopside, olivine, and Ti-biotite. The rock is fine grained, lightly porphyritic with some elongate aggregates of light minerals. Strong petrographic variations along the structure were not observed. Ni and Cr content is low (90-137, and 28-222 ppm), while the concentration of incompatible trace elements is high (Zr up to 600 ppm, Sr up to 1281 ppm, and La up to 95 ppm) for its SiO₂ content (40 %wt). Our data indicate a Mantle origin by probably deep metasomatism, with a subsequent minor olivine fractionation before the emplacement 28 Ma ago under intraplate conditions.