Asthenospheric signature in fertile spinel lherzolites from the Viliga Volcanic Field in NE Russia

Mantle xenolith bearing olivine melanephelinites from the Okhotsk sector of the Okhotsk–Chukotka Volcanic Belt (OCVB), northeastern Russia, occur as small isolated volcanoes emplaced within massive late Early to Late Cretaceous subduction-related calc-alkaline rocks. The xenoliths are typical mediumto fine-grained anhydrous mainly spinel lherzolites that are strongly to weakly foliated with intensive to minor recrystallization to equigranular texture. The primitive mantle normalized whole-rock REE have flat patterns or patterns with slightly elevated light REE (LREE) ((La/Yb)N 1⁄4 0.48–1.38). The REE in clinopyroxenes have systematically decreasing normalized abundances from Sm to La, implying that the LREE enrichments in the whole-rock REE patterns are attributed to circulation of minor intergranular fluids or melts. Equilibration temperatures and pressures calculated for the Viliga samples are in the range of 1050–1160 8C and 15–21 kbar, respectively. Ca diffusion rates in olivine reveal a rapid transport to the surface (2–6 days) of these peridotites. Model calculations have shown that the fertile lherzolites can be produced by 2–9% batch melting, whereas the depleted peridotites require 15% batch melting of a primitive source. The cessation of the interaction between the palaeo-Pacific plate and the NE Russian margin at c. 87 Ma apparently caused a ‘piecemeal’ collapse of the former followed by intrusion and ascent of olivine melanephelinitic magma, which entrained xenoliths from the asthenospheric mantle of the subducted plate during the Pliocene through the generated window(s). Moreover, clinopyroxenes that have low Sr/Sr and high Nd/Nd and plot in and above the mid-ocean ridge basalt (MORB) field are consistent with an upwelling asthenospheric mantle through the window(s) created by the ‘piecemeal’ collapse of the palaeo-Pacific plate. Mantle xenoliths in alkali basalts emplaced in massive subduction-related calc-alkaline rocks are important rocks, as they could provide information about the nature of the lithospheric mantle wedge and the geochemical and metasomatic processes that occur in such tectonic settings. Fluids released from sediments of a downgoing subduction slab could cause modal metasomatism in the lithospheric mantle wedge, forming hydrous phases such as amphibole and phlogopite (Peacock 1993). Migration of melts produced by melting of a downgoing subduction slab can also affect the lithospheric mantle wedge, causing crypticmetasomatism, as has been demonstrated by Kilian & Stern (2002) for the Cerro Fraile mantle xenoliths in southernmost Patagonia and by Kepezhinskas et al. (1996) for the Valovayam Volcanic Field in the northern segment of Kamchatka arc xenoliths. The mantle xenoliths from the Viliga River volcanic field in the northern Okhotsk region, NE Siberia (Fig. 1), are hosted by olivinemelanephelinites and basanites and occur at two localities (Kananyga River and Aliki River areas) on top of calc-alkaline rocks up to 1550 m above sea level, which are related to the subduction of the palaeo-Pacific slab (Zonenshain et al. 1990). The age of the undersaturated lavas ranges from 10.7 to 7 Ma (Akinin 1997, 2005; Apt et al. 1998). A limited number of mantle xenoliths from the region have been studied by Akinin et al. (1997) and Apt et al. (1998). Here we present a summary of the petrology, mineralogy and geochemistry of the ultramafic xenoliths from both localities and discuss the implications of these data with respect to the subduction slab and mantle wedge processes.