Polybaric petrogenesis of Neogene alkaline magmas in an extensional tectonic environment: Viliga Volcanic Field, northeast Russia

Neogene alkaline intraplate volcanic rocks from the Viliga Volcanic Field (NE-Russia) were studied both to precisely characterize their geochemical composition and to unravel their petrogenetic history. The sampled volcanic rocks crop out within the voluminous calc-alkaline sequences of the Cretaceous Okhostk–Chukokta Volcanic Belt, an Andean-type arc formed during subduction of the Paleo-Pacific Plate beneath modern far-east Asia. The mantle xenolith- and xenocryst-bearing basanites and nephelinites have intraplate ocean island basalt-type geochemical features. Sr and Nd isotopes combined with major and trace element systematics and rare-earth element modeling suggest polybaric melt generation of these alkaline magmatic rocks from a homogeneous garnet peridotite facies source with minor involvement of spinel peridotite facies partial melts. The basanite samples indicate segregation PT conditions around 1500 °C at 33–38 kbar whilst the nephelinites reflect smaller melt fractions segregated at over 40 kbar. During ascent, olivine (up to 7%) was the main fractionating phase in the basanites; whereas in the nephelinites, both olivine and minor clinopyroxene fractionation occurred. Crustal contamination during ascent was insignificant. We argue that the melt generation of these alkaline magmas from the Viliga Volcanic Field was triggered by an extending lithosphere resulting in upwelling asthenosphere and decompression melting, analogous to geodynamic models of the coeval alkaline volcanic rocks along the adjacent North Pacific continental margins, rather than by subduction- or plume-related processes.