Mesozoic accretion of juvenile sub-continental lithospheric mantle beneath South China and its implications: Geochemical and Re–Os isotopic results from Ningyuan mantle xenoliths

The mechanism for the widespread Mesozoic magmatism in South China has been ascribed to either the paleo-Pacific plate subduction or intra-continental lithospheric extension. Mantle xenoliths entrained in the Jurassic Ningyuan alkaline basalts from southern Hunan Province, including twelve lherzolites and one harzburgite, have been studied to constrain the composition and age of the Mesozoic sub-continental lithospheric mantle. The lherzolites contain 2.06–4.09 wt.% Al2O3 and 2.03–3.91 wt.% CaO, which are higher than the abundances of these species in the harbzurgite (1.72 wt.% and 1.17 wt.%, respectively). The Cr# of spinel varies from 0.07 to 0.24. Orthopyroxene contains 3.61–4.96 wt.% Al2O3 and 0.51–0.8 wt.% CaO, whereas clinopyroxene contains 4.78–7.32 wt.% Al2O3 and 1.04–2.01 wt.% Na2O. Both whole rock and mineral compositions suggest that the Ningyuan mantle xenoliths have been subjected to low degrees of partial melting. Modeling of Y and Yb contents of clinopyroxene indicates that the lherzolites have been subjected to 3–5% degrees of partial melting, while the harzburgite has experienced about 12% melting. Clinopyroxene in most Ningyuan mantle xenoliths shows variable depletion in incompatible elements, suggesting that they have been weakly enriched after melting. Clinopyroxene in one sample (TYS01-3) displays strong enrichment in incompatible elements, reflecting the local occurrence of melt metasomatism. Equilibrium temperatures estimated by two-pyroxene geothermometry vary from 994 to 1081 °C, indicating that the Mesozoic mantle lithosphere has a hot geotherm. All lherzolites display consistent highly siderophile element (HSE) patterns and have suprachondritic Ru/Ir and Pd/Ir ratios. The harzburgite shows remarkable depletion in Pt, Pd and Re. The lherzolites have 187Os/188Os ratios of 0.12116–0.12929, which are higher than that of the harzburgite (0.11681). A correlation between 187Os/188Os and Al2O3 exists among the Ningyuan mantle xenoliths, which if interpreted as an isochron analog yields a model age of ~ 2.2 Ga. This age is older than the Re depletion age (TRD) of the harzburgite (~ 1.8 Ga), which represents a minimum age of melt depletion. The old ages either support the existence of ancient lithosphere relics beneath the Ningyuan region or reflect the accretion of ancient mantle materials from the asthenosphere during the Mesozoic. Although the tectonic implication of the old ages is ambiguous, compositions of the Ningyuan mantle xenoliths suggest that ancient mantle lithosphere beneath South China has been thinned and replaced by hotter, younger mantle during the Mesozoic, which has led to extensive lithospheric extension and abundant magmatism.

► We report Re–Os isotopes of the Mesozoic Ningyuan mantle xenoliths from South China Block (SCB).
► The Ningyuan mantle xenoliths have fertile compositions and Paleoproterozoic Os model ages.
► The ancient lithospheric mantle beneath SCB has been replaced by juvenile asthenospheric mantle.
► Mantle replacement has caused lithosphere extension and the Jurassic magmatism in SCB.