OS-HF-SR-ND ISOTOPE AND PGE SYSTEMATICS OF SPINEL PERIDOTITE XENOLITHS FROM TOK, SE SIBERIAN CRATON: EFFECTS OF PERVASIVE METASOMATISM IN SHALLOW REFRACTORY MANTLE

Abstract

Os-Hf-Sr-Nd isotopes and PGE were determined in peridotite xenoliths carried to the surface by Quaternary alkali basaltic magmas in the Tokinsky Stanovik Range on the Aldan shield. These data constrain the timing and nature of partial melting and metasomatism in the lithospheric mantle beneath SE Siberian craton. The xenoliths range from the rare fertile spinel lherzolites to the more abundant, strongly metasomatised olivine-rich (70-84%) rocks. Hf-Sr-Nd isotope compositions of the xenoliths are mainly within the fields of oceanic basalts. Most metasomatised xenoliths have lower 143Nd/144Nd and 176Hf/177Hf and higher 87Sr/86Sr than the host basalts indicating that the metasomatism is older and has distinct sources. A few xenoliths have elevated 176Hf/177Hf (up to 0.2838) and plot above the Hf-Nd mantle array defined by oceanic basalts. 187Os/188Os in the poorly metasomatised, fertile to moderately refractory (Al2O3 ≥ 1.6%) Tok peridotites range from 0.1156 to 0.1282, with oldest rhenium depletion ages being about 2 Ga. The 187Os/188Os in these rocks show good correlations with partial melting indices (e.g. Al2O3, modal cpx); the intercept of the Al-187Os/188Os correlation with lowest Al2O3 estimates for melting residues (~0.3-0.5%) has a 187Os/188Os of ~0.109 suggesting that these peridotites may have experienced melt extraction as early as 2.8 Gy ago. 187Os/188Os in the strongly metasomatised, olivine-rich xenoliths (0.6-1.3% Al2 O3) ranges from 0.1164 to 0.1275 and shows no apparent links to modal or chemical compositions. Convex-upward REE patterns and high abundances of heavy to middle REE in these refractory rocks indicate equilibration with evolved silicate melts at high melt/rock ratios, which may have also variably elevated their 187Os/ 188Os. This inference is supported by enrichments in Pd and Pt on chondrite-normalised PGE abundance patterns in some of the rocks. The melt extraction ages for the Tok suite of 2.0 to 2.8 Ga are younger than oldest Os ages reported for central Siberian craton, but they must be considered minimum estimates because of the extensive metasomatism of the most refractory Tok peridotites. This metasomatism could have occurred in the late Mesozoic to early Cenozoic when the Tok region was close to the subduction-related Pacific margin of Siberia and experienced large-scale tectonic and magmatic activity. This study indicates that metasomatic effects on the Re-Os system in the shallow lithospheric mantle can be dramatic. © 2005 Elsevier B.V. All rights reserved.