PETROLOGICAL AND GEOCHEMICAL CHARACTERIZATION OF THE SAN JUAN DE OTATES ULTRAMAFIC COMPLEX, LEÓN, GUANAJUATO: TECTONIC IMPLICATIONS

The San Juan de Otates ultramafic-mafic complex is located on the eastern part of the Guerrero Composite Terrene (Central Mexico), and is mainly constituted by: i) peridotites, including serpentinized dunites with pseudomorphic textures and wehrlites with lobulate textures; ii) pyroxenites, comprising olivine-clinopyroxenites with protogranular, polymodal inequigranular, lobulate and porphyroclastic textures and clinopyroxenites with mosaic equigranular texture; iii) gabbros with cumulated textures; and iv) plagiogranite dykes.

Compositionally, peridotites have higher Mg# (83.7-85.1) than pyroxenites (75.6-82.8) and gabbros (#Mg =57.7-67.5), showing an increase of SiO₂, Al₂O₃, CaO, TiO₂ y Na₂O coeval with decrease of Mg# and MgO contents. Moreover, peridotites and pyroxenites show a convex-up REE patterns, whereas gabbros have a LREE-enriched pattern, having positive anomalies in Pb, Ba and Cs, but negative anomalies in Nb, Ta, Hf and Zr. The olivine crystals from pyroxenites are Fo_0.78-0.83 and show an increase of MnO but decrease of NiO coeval with Fo decreasement. Likewise, clinopyroxene from peridotites has higher Mg# (86.3-91.0) than pyroxenites (Mg#= 80.3-90.6), and their TiO₂ and Al₂O₃ contents increase with #Mg decreases.

The geochemical and petrological variations displayed by peridotites, pyroxenites and gabbros from the San Juan de Otates ultramafic-mafic complex suggest that these rocks are magmatic cumulates formed by fractional crystallization of basaltic magmas on the transitional arc-root in a suprasubduction zone. Additionally, some pyroxenites (i.e., clinopyroxenites) could have formed by: i) interaction of secondary melts that percolated through the ultramafic sequence, or ii) magma mixing between mantle-derived melts that formed ultramafic sequence and more evolved mantle melts. Therefore, the peridotites, pyroxenites and gabbros from the San Juan de Otates ultramafic-mafic complex were formed in a root of transitional arc during Early Cretaceous, modified by secondary fluids and/or melts in a suprasubduction zone.