Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico

Abstract

The trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma‐mass spectrometer to constrain the composition and evolution of the Mexican lower crust. Most xenoliths found are pyroxeneplagioclase orthogneisses, although quartzo‐feldspathic paragneisses form up to 50% of the xenolith population in some areas. Lithologies in the exposed terrains consist of pelitic and carbonate‐rich paragneisses, charnockites, and less abundant basic, intermediate, and felsic orthogneisses. Both xenoliths and exposed granulites generally have low abundances of incompatible elements such as K, Rb, Th, and U. The xenoliths, however, often have lower abundances of these elements than the exposed granulites. High K/Rb ratios, coupled with an inverse correlation between K2O and K/Rb, indicate that most samples from the Mexican lower crust are depleted in both K and Rb. Furthermore, Th/U ratios similar to upper crustal materials and high La/Th ratios suggest depletion in both Th and U relative to average crustal material. Removal of a melt phase, either after the crystallization of cumulates or following a partial melting event, can explain many of the chemical features observed and may be responsible for the elemental depletions. Some samples, however, appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths. Available geobarometric data suggest that xenoliths equilibrated near the crust/mantle boundary (∼10 kbar) and represent the lowermost crust, while the exposed granulites equilibrated at shallower crustal levels (∼7 kbar). Thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the Mexican lower crust. The lowermost crust in Mexico consists of crystallized basaltic liquids, cumulates, and/or residue after the removal of a partial melt as well as garnet‐rich metasediments.