Mineral chemistry and geochemistry of mantle xenoliths from the Santo Domingo volcanic field, San Luis Potosí: Evidences of mantle metasomatic processes beneath portions of the Mesa Central, Mexico

Abstract

The Santo Domingo volcanic field consists of at least four Quaternary maars: Santo Domingo, Joya de los Contreras, El Banco and Joya Prieta, as well as a cinder cone and associated lava flows (La Pólvora). Mantle peridotites - mainly spinel lherzolite – occur as xenoliths that were carried to the surface by mafic alkalic intraplate magmas. Outstanding features of the xenoliths of Santo Domingo, compared with other xenolith localities in central and northern Mexico, are the porphyroclastic texture, abundance of kaersutite megacrysts. Composite xenoliths, wherein hornblendite veinlets cut across the foliation in the peridotite, show that the hydrated phases come from the mantle. Composite xenoliths, where hornblende veinlets cut across the mylonitic foliation in the peridotite, show that the hydrated phases come from the mantle.

Xenoliths of sheared peridotites collected at Joya de los Contreras, Santo Domingo and Joya Prieta maars display modal metasomatism evidenced by the presence, to a greater or lesser extent, of crystals of amphibole (kaersutite and/or pargasite) and phlogopite. The hydrated phases are spatially associated with spinel and/or clinopyroxene crystals. All the studied mantle xenoliths contain trace amounts of Ni-rich sulphides, which have irregular- or bleb-like shapes. Sulphides occur in the intracrystalline spaces of olivine crystals or as isolated inclusions in clinopyroxene. Sulphides in mantle xenoliths from Mexico have not been previously reported.

There is clear evidence of interaction with fluids in the mantle xeno- liths from the Santo Domingo volcanic field. However, metasomatism was not pervasive in the mantle region where the xenoliths were incorporated in the transporting magma. The most important clues about how the man- tle was metasomatized are the presence of hornblendite veinlets in some of the peridotites and the occurrence of kaersutite megacrysts. The veinlets indicate that the metasomatism was developed in the upper mantle and the occurrence of some peridotites with disseminated hydrous minerals, as well as peridotites with cryptic metasomatism, suggest the existence of zoning in a metasomatic aureole around the veinlets. Hornblendite dikes originated from magmas related to the first pulses of intraplate magmatism in the region.