Mexican peridotite xenoliths and tectonic terranes: Correlations among vent location, texture, temperature, pressure, and oxygen fugacity

Twenty-six spinel peridotite xenoliths were studied from five Quaternary volcanic fields that define a transect extending for similar to 1700 km from southeast to northwest across the southern Basin and Range Province in Mexico. Calculated equilibration temperatures and pressures are highest from xenoliths from Ventura-Espiritu Santo (1073 +/- 41 degrees C, 19.6 +/- 3.0 kbar; n = 9) and Durango (1082 +/- 21 degrees C, 17.0 +/- 1.7 kbar; n = 7). These fields lie near the center of the transect in the interior of the Tepehuano tectonic terrane and have xenoliths dominated by coarse-equant and coarse-tabular textures. Spinel-pyroxene clusters, interpreted as decompression reaction products of garnet + olivine, are only found at these fields, only in xenoliths with calculated pressures > 15 kbar. In contrast, calculated temperatures and pressures are lowest for the Santo Domingo (1026 +/- 19 degrees C, 14.4 +/- 2.0 kbar; n = 4) and San Quintin (1034 +/- 66 degrees C, 10.7 +/- 2.5 kbar; n = 5) fields, where xenoliths display porphyroclastic or granuloblastic textures. These two fields mark the ends of our transect and lie near tectonic terrane boundaries. Calculated oxygen fugacities, expressed relative to the fayalite-magnetite-quartz buffer (Delta FMQ), increase from southeast to northwest along the transect: Santo Domingo, -1.3 +/- 0.7 (n = 4); Ventura-Espiritu Santo, -0.6 +/- 0.4 (n = 9); Duango, -0.2 +/- 0.3 (n = 7); Mesa Cacaxta, 0.3 (n = 1); San Quintin, 0.0 +/- 0.2 (n = 5). The lithosphere closest to the Mesozoic-Cenozoic paleotrench appears to have been most strongly affected by slab-derived, oxidized fluids or melts.