The petrology of the lower crust and upper mantle beneath southeastern Chihuahua, Mexico: a progress report

Abstract

Granulite facies metamorphic rocks and ultramafic nodules, of lower crustal and upper mantle origin, respectively, occur as xenoliths in a host alkali basalt at the La Olivina peridot mine, approximately 150 km southeast of Ciudad Chihuahua. Toe La Olivina ultramafic xenoliths are of three compositional groups that are distinguished by clinopyroxene chemistry: a high magnesian (Mg/Mgi-Fe= 0.90), high Cr2 03 group (Group I); a moderatelymagnesian (Mg/Mg+Fe= 0.76), high AI2O3, high TiO2 group (Group II); anda group compositionally intermediate between these two (Transitional Group ). Analyses of primary minerals show that these xenoliths are similar to ones from San Carlos, Arizona (USA), from Xalapasco de La Joya, San Luis Potosí (Mexico), and from Kilbourne Hale, New Mexico (USA). Toe La Olivina mantle xenoliths are of three textural groups: allotriomorphic ranular (igneous?), porphyroclastic (metamorphic?), and granoblastic (metamorphic?). All Group I xenoliths are granoblastic, and most nodules of Group II and the Transitional Group are allotriomorphic granular. Porphyroclastic texture is less common than the other textures in the rocks studied, but it is found in sorne Transitional Group rocks. Toe La Olivina lower crust xenoliths are predominantly pelitic gneisses and pyroxene granulites. Toe pelitic gneisses are uniform in mineral assemblage with garnet+ quartz+ plagioclase+ sanidine+ sillimanite+ rutile+ graphite. Most of the pyroxene granulites are plagioclase-bearing, but sorne contain scapolite in place of plagioclase. These lower crustal samples are identical in mineral assemblage and very similar in mineral chemistry to xenoliths described from Kilbourne Hale. The granulites and pelitic gneisses have model Nd ages of 1.1 to 1.2 b.y. using a chondritic initial ratio, or 1.6 b.y. using a depleted so urce model. Toe latter age is in excellent agreement with those from Kilbourne Hale. Thus it appears that Precambrian craton, similar in age, metamorphic history, and protolith composition to that which underlies Kilbourne Hale, extends into Mexico at least as far southeast as La Olivina.