A COMPOSITIONALLY STRATIFIED MANTLE BELOW THE NORTHERN MEXICAN BASIN AND RANGE: TRACE ELEMENT AND ISOTOPIC EVIDENCE FROM MANTLE XENOLITHS FROM THE PINACATE VOLCANIC FIELD, SONORA, MEXICO

The Pinacate volcanic field (31° 46’ N, 113° 30’ W) is a Quaternary (>1.7 Ma to 12 ka) alkali basaltic lava field located in the southern Basin and Range of NW Mexico, ~100 km east of the northern end of the Gulf of California. Ultramafic nodules from Pinacate tuff breccias include Group I (Cr-diopside-bearing) xenoliths with metamorphic fabrics and spinel lherzolite phase assemblages. Equilibration temperatures of these xenoliths are 930-1020° C (Gutmann, 1986), far below the liquidi of Pinacate alkali basalts, which themselves were derived from an enriched, garnet-bearing asthenospheric source. In order to examine mantle stratification in this area, whole-rock powders of spinel-bearing xenoliths were leached in 2N HCl and both residues and leachates were analyzed for trace elements by ICP-MS. Residues generally have LREE- and LILE-depleted trace element patterns, whereas leachates exhibit “spoon-shaped” REE profiles, LILE enrichment, and strong HFSE depletion. Such signatures are characteristic of fluid metasomatism of mantle xenoliths. In addition, leached clinopyroxene separates also have LREE-depleted patterns with elevated Sm/Nd ratios (0.41-0.47). Radiogenic isotope ratios show that leached solid residues and clinopyroxenes from Pinacate xenoliths are more depleted (⁸⁷Sr/⁸⁶Sr = 0.7030-0.7038; ε_Nd = 6-10, ²⁰⁶Pb/²⁰⁴Pb = 18.02-18.91) than whole-rock leachates (⁸⁷Sr/⁸⁶Sr = 0.7056-0.7079; ε_Nd = 1-4, ²⁰⁶Pb/²⁰⁴Pb = 18.64-19.11). The isotopic signature of Pinacate xenoliths and their clinopyroxenes is similar to that of Group I xenoliths from other alkali basalt fields in the southern Basin and Range and resembles a Pacific MORB-source depleted mantle that has been overprinted by an enriched metasomatic agent. However, mixing between these endmembers cannot explain the HIMU-like isotopic signature of Pinacate alkalic lavas (²⁰⁶Pb/²⁰⁴Pb up to 19.54). This, as well as the depleted trace element patterns in Pinacate xenoliths, confirms that the xenoliths do not represent the mantle source of Pinacate lavas. Instead, these xenoliths likely reflect a shallow wedge of depleted mantle (DMM) metasomatized by arc fluids, fragments of which were entrained in upwelling Pinacate lavas originating from greater depth within the mantle.