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

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 Cent ral Asian Fold Belt, in northeastern China. Mantle xenoliths found in the p otassic rocks are mainly spinel peridotite (some phlogopite-bearing) associ ated with minor Fe-rich peridotitz (some phlogopite-bearing) and pyroxenite . Clinopyroxene in the spinel peridotite xenoliths is generally low in Al2O 3, Sc, Y and heavy rare earth elements and along with orthopyroxene and spi nel has high Cr/(Cr+Al). The bulk rock has low basaltic components (Al2O3, CaO and TiO2), demonstrating their refractory nature. However, the relative ly low forsterite contents (<0.915) of olivine and high bulk rock Ca/Al (1. 3-3.1) and Mg/Si (1.2-1.6) ratios at given Mg/(Mg + Fe) distinguish them fr om either Archaean lithospheric mantle or the residue of melt extraction fr om Phanerozoic oceanic mantle. Clinopyroxenes from all the xenoliths are en riched in light rare earth elements, Th and Pb and depleted in high field s trength elements, with similar incompatible element and ran earth element p atterns. Clinopyroxenes from the spinel peridotite xenoliths have (87)Sd/Sr -86 (0.70480-0.70516) higher than primitive mantle, but variable <epsilon>( Nd) values (+6.9 to -2.0). Petrological and geochemical evidence from the x enoliths 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 fractiona l melting) and then metasomatised extensively by SiO2-undersaturated potass ic magma that was likely generated from low-degree partial melting of eithe r deep asthenosphere or metasomatised lower part of lithospheric mantle. Bo th the melt extraction and metasomatic event(s) may have occurred during th e Proterozoic. Geochemical signatures of the spinel peridotite xenoliths an d the inferred garnet peridotites at the base of the subcontinental lithosp heric mantle from chemistry of the host potassic rocks (Zhang ct al., J. Pe trol., 36 (1995) 1275; Zhang et al., AGU Geodynamic Series, 27 (1998) 197) indicates the preservation of a Proterozoic lithospheric mantle section wit hin the Phanerozoic Central Asian Fold Belt. (C) 2000 Elsevier Science B.V. All rights reserved.