Lower crustal xenoliths from Queensland, Australia: Evidence for deep crustal assimilation and fractionation of continental basalts

Abstract

A suite of mafic, granulite facies xenoliths from north Queensland possesses petrographic and geochemical features of basaltic cumulates crystallized at lower crustal pressures. Negative correlations between incompatible trace elements and Mg# and positive correlations between compatible trace elements and Mg# suggest the xenoliths are genetically related and crystallized from a continuously evolving melt. Zr, Hf, Y, HREE, Ti and V do not correlate with Mg#, but show excellent negative correlations with Al2O3 content, reflecting the proportion of cumulate plagioclase to clinopyroxene. These chemical trends also suggest the trace element concentrations have not been affected by subsolidus recrystallization. The xenoliths have a large range in Sr and Nd isotopic compositions (87Sr86Sr = 0.70239 to 0.71467, ϵNd = +9.5 to −6.1) which cannot be produced by crystal fractionation alone, and excellent correlations between isotope ratios and Mg# suggest the variable isotope compositions are not due to mantle source heterogeneities.These mafic xenoliths are proposed to be cumulate products from a melt undergoing simultaneous assimilation and fractional crystallization (AFC). The data illustrate that only a few percent AFC in lower crustal environments can dramatically change the Sr and Nd isotopic composition of a basaltic melt, and suggest the use of caution when inferring mantle source isotopic compositions from continental basalts. Additionally, the Nd isotopic data plot on a positive trend on an Sm-Nd isochron diagram with an age of ~570 Ma. However, if these xenoliths formed by AFC, the positive trend reflects mixing between two isotopic end members and has no age significance. The correlations between Sr and Nd isotopic compositions with Mg# degrade as the isotopic ratios are back-calculated to earlier times, suggesting the xenoliths are relatively young; the xenoliths may be related to the Cenozoic igneous activity which occurs throughout eastern Australia.