GEOCHEMISTRY AND SR-ND ISOTOPE COMPOSITIONS OF PERIDOTITE MANTLE XENOLITHS FROM NORTHWESTERN MÉXICO (BAJA CALIFORNIA, SINALOA, AND NAYARIT)

The geochemical and isotopic composition of the mantle is accessible through peridotite xenoliths in alkali basalts, outcrops of peridotite massifs, and ophiolitic complexes. In Mexico, several volcanic complexes transported spinel lherzolite xenoliths to the surface. These Pliocene-Quaternary alkali-basaltic volcanic fields are located along different tectonostratigraphic terranes in the north-central part of Mexico. Here, we concentrate on three localities: the San Quintín Volcanic Field (SQVF), Baja California, the Mesa Cacaxtla (MC), Sinaloa, and the Isla Isabel (ISA), Nayarit, all of them located in the Mesozoic Guerrero-Alisitos-Vizcaino Superterrane.

Preliminary results of two mantle xenoliths from MC show Mg# of 89. In a chondrite-normalized REE diagram, these xenoliths define subfractionated REE patterns (La_N/Yb_N = 0.3). The depletion in the light REE in these xenoliths suggests they were affected by some degree of partial melting. These xenoliths yield average isotopic ratios of ⁸⁷Sr/⁸⁶Sr = 0.703830, ¹⁴⁷Sm/¹⁴⁴Nd = 0.2568, and ¹⁴³Nd/¹⁴⁴Nd = 0.513059. In comparison, one alkali basalt that hosts the xenoliths shows a Mg# of 65, isotopic ratios of ⁸⁷Sr/⁸⁶Sr = 0.702974, ¹⁴⁷Sm/¹⁴⁴Nd = 0.116, and ¹⁴³Nd/¹⁴⁴Nd = 0.513979. This basalt shows a fractionated REE pattern (La_N/Yb_N = 8.9), with a relative enrichment in the LREE regarding the HREE, suggesting a low degree of partial melting in its genesis. Based on the correspondence on the REE patterns between the xenoliths and the alkali basalt (residue vs. product of partial melting, respectively), we propose that mantle xenoliths from MC might be restites from the mantle source that generated the alkali basalts.