Abstract:

Garnetbearing orthopyroxenites occurred as both discrete xenoliths and veinlets in peridotite xenoliths that were brought to the surface by Quaternary Pali Aike alkali basalts in southern South America. Orthopyroxenites commonly contain Tirich minerals and also relict grains of olivine and clinopyroxene occurring as inclusions in secondary orthopyroxene. The secondary orthopyroxene contains high TiO2, moderate Al2O3 and low Mg＃ compared with orthopyroxene in garnetbearing peridotites. This suggests that the secondary orthopyroxene formed at the expense of olivine and clinopyroxene during the metasomatism by an evolved Tirich melt. Compared with peridotites, the orthopyroxenites contain slightly low Co and Ni, and similar Cr and PGE, suggesting that these metals were essentially immobile during metasomatism. The components introduced by the metasomatizing melt are mainly alkalis (K+Na), Ti, Si, Al and S. Secondary orthopyroxene has also been documented in mantle peridotite xenoliths derived from beneath both active arcs and ancient cratons. In comparison with such orthopyroxene in these mantle samples, the secondary orthopyroxene in the Pali Aike xenoliths contains relatively high Ti and Al and low Mg. High Ti and low Mg, which reflects the evolved nature of the metasomatizing melt that originated from the underlying asthenospheric mantle. The type of secondary orthopyroxene, typified by these samples, may be common elsewhere in subcontinental mantle lithosphere affected by asthenospheric upwelling. The study may provide a new clue for understanding the thinning mechanism of lithosphere beneath North and Northeast China since the Mesozoic Era.

Key words: asthenospheric melt, mantle metasomatism, metasomatic orthopyroxenite, platinumgroup elements (PGE), southern South America, thinning of lithospheric mantle