Petrological Relationships between Mantle Xenolths and their Host Magmas in the Alkaline Province Mexico

Along the periphery of Sierra de San Carlos-Cruillas and Sierra de Tamaulipas outcrops Tertiary-Quaternary volcanic rocks containing ultramafic xenoliths from the Mantle. The host rocks are alkaline and basaltic, and have characteristics of primary magmas from subcontinental zones (SiO2 = 40.6–49.7%, MgO = 5.62–11.6%, Mg-v = 59.2–69.7).

Primitive Mantle normalized patterns are similar to extension-related mafic magmas. These patterns indicate that host magmas were generated in a highly-incompatible elements enriched Mantle zone. This is supported by an enrichment in light REE ([La/Yb]N = 10.8–27.1) and the behavior of immobile trace elements. The tectonomagmatic discrimination diagrams indicate that host magmas were generated in an intraplate environment.

Using a REE partial melting model, it was pointed out that Mantle represented by the xenoliths cannot reproduce the REE-patterns of the host magmas. After the application of an inverse melting model, the host magmas were generated in the spinel lherzolite stability field, in a more enriched zone (highly incompatible elements).

The xenoliths are spinel-lherzolites, harzburgites and rare dunites. They have are protogranular, nevertheless, a few xenoliths displayed protogranular- porphyroclastic transitional textures, indicating that the xenoliths come from stable zones with little or no deformation. The mineralogy (olivine + orthopyroxene + clinopyroxene ± spinel) is typical of unaltered Mantle nodules. The core and rim compositions in olivine range from Fo86 to Fo92, whereas orthopyroxenes are characterized by En87–91 and clinopyroxenes by En43–45-Fs2.74–9.31-Wo49–51. Chromiferous spinels have Mg/(Mg + Fe+2) = 0.70–0.81 and Cr/(Cr + Al) = 0.10–0.35.

Primitive Mantle-normalized multielements diagrams and Chondrite-normalized REE patterns for xenoliths, pointed out that they represent an enriched Mantle for the SSCC area, and a lesser enriched Mantle for the ST.

The xenoliths equilibrated at 820-1190°C and from 10-25 kbar.