MOJAVE ENRICHED SUBCONTINENTAL LITHOSPHERIC MANTLE (SCLM) AND IMPLICATIONS FOR THE CENOZOIC CONTINENTAL BASALTIC VOLCANISM IN THE MOJAVE PROVINCE

Previous studies on the Cenozoic basalts across the Mojave Desert have shown their enriched Sr and Nd isotopic compositions, which suggests the existence and involvement of an enriched SCLM in their petrogenesis. To better characterize the nature of the SCLM underlying the Mojave province, we have conducted Sr-Nd isotopic systematics on a suite of mantle and crustal xenoliths from the Cima volcanic field (CVF) and Dish Hill(DH). CVF mantle xenoliths are depleted peridotites and dunites, whereas DH mantle xenoliths are more fertile lherzolites.

Gabbros from the CVF with a limited range of Nd and Sr isotope compositions with ⁸⁷Sr/⁸⁶Sr from 0.70395 to 0.70440, and e_Ndfrom +6.43 to +7.46, which is similar to previous published results on the erupted host basalts and mafic crustal xenoliths. In contrast, Nd and Sr isotope compositions for mantle xenoliths from the CVF have unradiogenic Nd but radiogenic Sr isotope compositions with e_Nd -7.28 to -24.42 and ⁸⁷Sr/⁸⁶Sr 0.70717 to 0.71097, respectively. DH mantle xenoliths have e_Nd values ranging from +0.80 to -18.72, and ⁸⁷Sr/⁸⁶Sr from 0.70535 to 0.73517, and higher Rb/Sr ratios than those from CVF. Nd and Sr data from this preliminary study show that (1) The Mojave SCLM has low Sm/Nd ratios and a wide range of Nd and Sr isotope compositions. (2) Nd isotope compositions are negatively correlated with Nd concentrations, and linearly correlated with the model age (T_CHUR or T_DM). These observations suggest that the Mojave SCLM either formed from an ancient (Proterozoic?) continent differentiation event and more recently (Mesozoic or Cenozoic?) perturbed to variable degrees by infiltration of melt or fluid with a depleted mantle signature, or the SCLM is relative young (Mesozoic or Cenozoic?) but metasomatismed by melts or fluids with an enriched Nd signature.

This study suggests that the enriched SCLM in the Mojave province may not be an important reservoir for continental basaltic volcanism of the Mojave Desert region. If the Mojave SCLM played a significant role in such basaltic volcanism, then the basalts should display more variable and wider range of negative e_Nd values, which is not generally observed.