Nature of the lithospheric mantle beneath the eastern part of the Central Asian fold belt: mantle xenolith evidence

Abstract

The late Cenozoic potassic volcanic provinces of Wudalianchi, Erkeshan and Keluo are located at the boundary between the Great Xing'an Mountains and the northwestern margin of the Songliao Basin, the eastern part of the Central Asian Fold Belt, in northeastern China. Mantle xenoliths found in the potassic rocks are mainly spinel peridotite (some phlogopite-bearing) associated with minor Fe-rich peridotite (some phlogopite-bearing) and pyroxenite. Clinopyroxene in the spinel peridotite xenoliths is generally low in Al 2O 3, Sc, Y and heavy rare earth elements and along with orthopyroxene and spinel has high Cr/(Cr+Al). The bulk rock has low basaltic components (Al 2O 3, CaO and TiO 2), demonstrating their refractory nature. However, the relatively low forsterite contents (<0.915) of olivine and high bulk rock Ca/Al (1.3–4.1) and Mg/Si (1.2–1.6) ratios at given Mg/(Mg+Fe) distinguish them from either Archaean lithospheric mantle or the residue of melt extraction from Phanerozoic oceanic mantle. Clinopyroxenes from all the xenoliths are enriched in light rare earth elements, Th and Pb and depleted in high field strength elements, with similar incompatible element and rare earth element patterns. Clinopyroxenes from the spinel peridotite xenoliths have 87Sr/ 86Sr (0.70480–0.70516) higher than primitive mantle, but variable ϵ Nd values (+6.9 to −2.0). Petrological and geochemical evidence from the xenoliths supports a common two-stage evolution model for the upper part of the lithospheric mantle in the region. The mantle protolith was stabilised after moderate to high degrees of melt extraction (e.g. 8–13% for fractional melting) and then metasomatised extensively by SiO 2-undersaturated potassic magma that was likely generated from low-degree partial melting of either deep asthenosphere or metasomatised lower part of lithospheric mantle. Both the melt extraction and metasomatic event(s) may have occurred during the Proterozoic. Geochemical signatures of the spinel peridotite xenoliths and the inferred garnet peridotites at the base of the subcontinental lithospheric mantle from chemistry of the host potassic rocks (Zhang et al., J. Petrol., 36 (1995) 1275; Zhang et al., AGU Geodynamic Series, 27 (1998) 197) indicates the preservation of a Proterozoic lithospheric mantle section within the Phanerozoic Central Asian Fold Belt.