Geochemical constraints on the nature, age structure and development of the lithosphere in Zealandia

Abstract

Zealandia is a largely submerged, proto-continental fragment in the southwest Pacific, whose origins, age structure and relationships with other major continents remains poorly understood. A suite of 40 off-cratonic mantle xenoliths, dominated by clinopyroxene-poor lherzolites and harzburgites, was assembled from 13 localities throughout the North, South, and Chatham Islands of New Zealand. Major and trace element data, including platinum group elements, and U-Th-Pb, Sr-Nd, Sm-Nd and Re-Os isotopic data were obtained from whole rock powders or clinopyroxene separates. Classical peritectic melting models using Yb concentrations indicate that xenoliths underwent 2-23% melt extraction. Previous work using solely clinopyroxene to quantify melt depletion is considered inaccurate, because sub-solidus redistribution of elements between pyroxenes is significant during cooling. A novel modelling approach, taking into account the effects of sub-solidus re-equilibration on clinopyroxene compositions, was employed and resulted in comparable estimates of 2-24% melting. The majority of xenoliths show the effects of metasomatic overprinting with samples varying from those with weak LREE enrichment and notable positive Sr and U-Th and negative Nb-Ta anomalies in clinopyroxene relative to primitive mantle, to those that have extremely high concentrations of LREE, Th-U and Nb. All of the samples are consistent with a carbonatite component contributing to the metasomatism of the lithosphere under Zealandia. Platinum group elements were not affected by this metasomatic event, instead recording simple melt depletion and the retention of primary mantle sulfides. A rapid fall in Pt/Ir observed at ca. 1 wt % Al2O3 is a direct result of the exhaustion of sulfide in the residual mantle at 20-25% melting. Re-Os isotopic systematics yield model ages between 0 and 2.3 Ga, with six samples from the newly defined Waitaha domain, South Island, having a narrow range of model ages from 1.6-1.9 Ga, and in agreement with an aluminochron melt depletion ‘age’ for this mantle domain of 1.9 Ga and a 3-point Re-Os isochron age of 2.2 Ga. These ages are >500 m.y. older than model ages preserved in adjacent regions of lithospheric mantle at the eastern margin of Gondwana and >1 b.y. older than the oldest crustal rocks exposed in New Zealand. The SCLM of Zealandia thus has a complex age structure, including a large region of Paleoproterozoic mantle. The mantle xenoliths preserve significantly greater heterogeneity in Sr-Nd-Pb isotopic space than observed in the intraplate basalt record of Zealandia, with the most highly metasomatised samples converging to a restricted range of Sr-Nd isotopic compositions reflecting pervasive overprinting. Unmetasomatised samples from the Waitaha domain possess radiogenic Nd and unradiogenic Sr isotopic compositions consistent with Paleoproterozoic melt depletion at 1.9-2.2 Ga. The coupling of ancient Os and Nd melt depletion ages demonstrates a regional melting event rather than simply preservation of refractory Os-rich phases. HIMU-like Pb isotopic compositions are uncorrelated with other isotopic data, consistent with this signature being a recent, <150 Ma, addition to the lithosphere. The radiogenic Pb signature was imparted into the lithospheric mantle by a weak mantle plume containing carbonatitic bodies, which preferentially melted and metasomatised the SCLM producing unique U-Th-Pb characteristics.