AGE AND PETROGENESIS OF THE RINCÓN MURILLO GABBRO OF THE SIERRA DE SAN CARLOS ALKALINE COMPLEX, NE MEXICO

The Sierra de San Carlos (SC) is located in the state of Tamaulipas and is the northernmost expression of the Sierras Tamaulipecas, which is a broad anticlinorium orientated NNW-SSE. It is constituted by Mesozoic limestone and marls, slightly folded an uplifted by the Laramide Orogeny during the Late Cretaceous to the Early Cenozoic (93-43 Ma).

Later in the Eocene, magmatic bodies intruded in the Sierra de San Carlos, building plutonic ultrabasic-acid plutons grouped in four major units: (a) San José Monzodiorites, (b) Rincón Murillo Gabbros, (c) Ell Bagre Alkaline Syenites, and (d) El Rosario Nepheline Syenites. After the plutonic emplacement, the magmatism continued with the formation of basic-intermediate subvolcanic and volcanic rocks in the periphery of the SC. In this work, we present a petrological approach of the Rincón Murillo Gabbro based on whole-rock, isotopic and geochronological data.

The Rincón Murillo Gabbro has a nepheline-normative alkaline character, enriched in LILE (Rb, Sr), HSFE (Nb, Y) and LREE. In this work we present the first two U-Pb LA-ICP-MS geochronological analysis performed on titanite crystals yielding Oligocene (Rupelian) crystallization ages of 31.38 ± 1.80 and 31.04 ± 1.72 Ma. This unit presents ⁸⁷Sr/⁸⁶Sr ratios between 0.70341 - 0.70417 and ¹⁴³Nd/¹⁴⁴Nd ratios between 0.51276 - 0.51284, which correspond to a mantle origin with a source close to the HIMU-type, without evidence of crustal contamination. The PM-normalized spider diagrams patterns, the tectonomagmatic discrimination diagrams and batch-partial melting modeling, show that the Rincón Murillo Gabbro unit was generated in a continental intraplate tectonic setting, under moderate degrees of partial melt (5-13 %) of a partially metasomatized mantle region with garnet-rich lithologies (peridotites / pyroxenites) with amphibole, in a post-orogenic stage after the formation of the Mexican Orogenic Belt.