Genesis of Young Lithospheric Mantle in Southeastern China: an LAM–ICPMS Trace Element Study

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Abstract
Geological and geophysical evidence indicates that at least 100 km of Archaean to Proterozoic lithospheric mantle has been removed from beneath large areas of eastern and southeastern China during late Mesozoic to Cenozoic time. Mantle-derived xenoliths in Tertiary basalts from several localities across this region have been studied by X-ray fluorescence, electron microprobe and laser ablation microprobe–inductively coupled plasma-mass spectrometry to characterize this thinner lithosphere. Trace element patterns of clinopyroxenes in the peridotites from southeastern China can be divided into four groups: fertile garnet lherzolites, fertile spinel (± garnet) lherzolites, and depleted and enriched peridotites. The addition of Nb, Sr, light rare earth elements, but not of Ti and Zr, suggests a metasomatizing agent containing both H2O and CO2. This study also demonstrates that the negative Ti anomaly commonly observed in clinopyroxene from mantle peridotites cannot be balanced by the Ti in coexisting orthopyroxene, but can be explained by small degrees of partial melting, using appropriate distribution coefficients. Most of the peridotites from southeastern China, whether spinel or garnet facies, are highly fertile in terms of Al2O3 and CaO contents and mg-number; many resemble commonly used primitive mantle compositions. Modelling of trace element patterns in clinopyroxene indicates that most spinel and garnet peridotites from the Nushan, Mingxi and Niutoushan localities experienced less than 5%, and many less than 2%, partial melting. A few depleted spinel peridotites from Nushan, and all spinel peridotites from Mingxi, require 10–25% fractional partial melting; almost all spinel peridotites from the Qilin locality show evidence of higher degrees (6–25%) of fractional partial melting. At both Nushan and Mingxi, the more depleted compositions occur in the upper part of the lithospheric mantle, which now is ∼100 km thick. Garnet peridotites are essentially undepleted, and Y–Ga–Zr relationships of the garnets are typical of Phanerozoic mantle. The overall highly fertile nature of the existing lithosphere requires that the Archaean and Proterozoic mantle that existed beneath the region in Palaeozoic times has been largely or completely removed, and replaced by younger, hotter and more fertile material. This probably occurred by upwelling of asthenospheric material during late Mesozoic to Cenozoic time, underplating to form new lithosphere. The occurrence of rare depleted xenoliths may show that some older mantle material is residual and coexists with younger material beneath southeastern China.