Metasomatic origin of garnet orthopyroxenites in the subcontinental lithospheric mantle underlying Pali Aike volcanic field, southern South America

摘要

Garnet-bearing orthopyroxenites occur as both discrete xenoliths and as veinlets in peridotite xenoliths that were brought to the surface by Quaternary Pali Aike alkali basalts in the southernmost of the Patagonian plateau lava fields in southern South America. Orthopyroxenites commonly contain Ti-rich minerals and also relict grains of olivine (01) and clinopyroxene (Cpx) occurring as inclusions in secondary orthopyroxene (Opx). The secondary Opx contains high TiO_2 (0.20-0.59 wt. percent ), moderate Al_2O_3 (2.87-5.10 wt. percent ) and low magnesium numbers (Mg#, 84.5-89.2) compared with Opx in garnet-bearing perido-tites. This suggests that secondary Opx formed at the expense of 01 and Cpx during metasomatism by an evolved Ti-rich melt. The orthopyroxenites contain bulk-rock concentrations of Cr, Ni and platinum group elements similar to peridotites, suggesting that these metals were essentially immobile during metasomatism, and that the metasomatiz-ing melt did not introduce these elements. Instead, the metasomatizing melt contributed alkalis, Ti, Si, Al, and S to the orthopyroxenites based on increased concentrations of these elements and mineralogy. Secondary Opx has also been documented in mantle peridotite xenoliths derived from beneath both active arcs and ancient cratons. In comparison with such Opx in these mantle samples, the secondary Opx in the Pali Aike xenoliths contains relatively high Ti and Al and low Mg. High Ti and low Mg in these samples reflect the evolved nature of the metasomatizing melt that originated from the underlying asthenospheric mantle. The type of secondary Opx, typified by these samples, may be common elsewhere in subcontinental mantle litho-sphere (SCLM) affected by asthenospheric upwelling.