Structure and composition of the subcontinental lithospheric mantle beneath the Sangilen Plateau (Tuva, southern Siberia, Russia): Evidence from lamprophyre-hosted spinel peridotite xenoliths

摘要

We present new data on spinel peridotite xenoliths hosted in Agardag alkaline lamprophyres from the Sangilen Plateau (Tuva, South Siberia, Russia), sampling at -450 Ma the subcontinental lithospheric mantle of the Tuva-Mongolian micro-continent that belongs to the accretionary Central Asian orogenic belt at the southern edge of the Siberian craton. Xenoliths are spinel lherzolites principally showing poikilitic and sub-ordinately coarse granular and coarse equigranular textures. Geothermobarometric calculations for pyroxene yield a narrow range of equilibration temperature (ca. 1000-1100 °C) that corresponds to lithospheric depths from 43 to 53 km (1.3-1.6 GPa) along a hot intracontinental geotherm. Variation of mean Mg# [100*Mg/ (Mg + Fe)] of olivine (87.9-90.9) with mean Cr# [100*Cr/(Cr + Al)] of spinel (9.5-45.7) indicates that spinel lherzolites are mostly residues of up to 10% melting of a fertile peridotite source. In terms of normalized REE (Rare Earth Element) and incompatible trace element patterns of clinopyroxene, the Sangilen xenoliths can be classified into three types: Type I characterized by convex-upward REE patterns depleted in LREE (0.10≤La/Yb_N≤0.49), and with relative negative anomalies of Rb, Pb, Hf, Zr and Ti and positive spikes of U and Sr; Type II displaying variable LREE/HREE ratios (0.53≤La/Yb_N≤2.17) but generally flatter REE patterns) and similar abundances of other trace elements compared to Type I; and Type III showing a LREE enriched pattern [(La/Sm)_N = 2.22; (La/Yb)_N = 8.42], high REE contents and no relative anomalies of U and Sr. The elevated Yb_N concentration of one Type II clinopyroxene and the variable fractionation of LREE-MREE relative to HREE in most xenolith types indicate Sangilen xenoliths underwent variable metasomatic enrichment. This enrichment is well accounted by percolation-reaction between depleted peridotite and small-melt fractions of alkaline mafic melts precursor to the Agardag alkaline lamprophyres. The lack of correlation with depth of modal variations, textural types, inferred degrees of melting and trace element patterns in xenoliths indicates the absence of a texturally or compositionally layered lithospheric mantle sampled by Ordovician lamprophyres beneath the Sangilen plateau. The observed compositional variations are better accounted by depleted lithosphere variably metasomatized along a network of percolating alkaline mafic melts heterogeneously distributed throughout the Sangilen lithospheric mantle section.