Evolution of Mojavian mantle lithosphere influenced by Farallon plate subduction: Evidence from Hf and Nd isotopes in peridotite xenoliths from Dish Hill, CA

Abstract

A major issue in the assembly of continents is the role of subduction in building and reworking the continental mantle lithosphere. Spinel lherzolite xenoliths from Dish Hill, CA represent Mojavian sub-continental lithospheric mantle (SCLM) that existed along an off-craton continental edge during late Cretaceous Farallon plate subduction. The Dish Hill locale is well situated for recording any Farallon plate influence, be it as oceanic lithosphere accretion or for its role in providing metasomatic agents to the Mojavian SCLM. The 176Hf/177Hf and 143Nd/144Nd isotopic compositions of clinopyroxenes from these xenoliths are radiogenic with εHf from +12.9 to +134.4 and εNd from +2.2 to +26.1, indicative of ancient Proterozoic melt depletion. Four out of the sixteen samples lie on a 2.1Ga reference line for melt extraction from primitive mantle for both 176Hf/177Hf and 143Nd/144Nd, confirming their position on the 2.1Ga 187Os/188Os aluminachron from previous work on these peridotites (Armytage et al., 2014). A second Re-depletion age obtained from an 187Os/188Os aluminachron of 1.3Ga is also observed in the 176Lu–176Hf and 147Sm–143Nd systematics. The 176Hf/177Hf–143Nd/144Nd data from Dish Hill do not provide strong evidence for the existence of a duplex of oceanic lithosphere and SCLM, or for these peridotites being sourced from modern asthenospheric mantle upwelling after lithospheric removal. However, subchondritic 176Lu/177Hf and 147Sm/144Nd ratios and trace element compositions in some of the peridotites point to the influence of metasomatic processes. In seven of the peridotites 176Hf/177Hf ratios are not complemented by similarly radiogenic 143Nd/144Nd ratios. Such decoupling, relative to the mantle array, indicates that the 176Hf/177Hf record in these peridotites is more robust to resetting by these local metasomatic processes than 143Nd/144Nd. The 87Sr/86Sr ratios measured in these samples fall into two distinct groups based on (Ce/Yb)PM, with the less depleted 87Sr/86Sr ratios occurring in samples with (Ce/Yb)PM>0.8, consistent with post melt extraction LREE-enrichment. Geochemical modeling of melt addition to variably depleted peridotite rules out silicate melt compositions such as MORB (mid-ocean ridge basalt) or arc basalt as primary metasomatic agents. Instead the trace element profile of the modeled melt is closest to a carbonatite melt. In particular, the melt composition is most consistent with carbonatites found in subduction zone settings rather than those associated with stable intra-plate continental lithosphere. The timing of this metasomatic event is constrained at <200Ma based on 176Lu–176Hf and 147Sm–143Nd isotopic data. Both the timing and the composition of the melt points to Farallon plate subduction as producing the metasomatic agents that locally modified the Mojavian mantle.