Petrology and geochemistry of Cretaceous ultramafic volcanics from Eastern Kamchatka

The origin, evolution and primary melt compositions of late Cretaceous high-K ultramafic volcanics and associated basalts of Eastern Kamchatka are discussed an the basis of a study of the mineralogy and geochemistry of the rocks and magmatic inclusions in phenocrysts. The exceptionally primitive composition of the phenocryst assemblage [olivine-Fo(88-95), Cr-spinel-Cr/(Cr + Al) up to 85] provides direct evidence of the mantle origin of primary melts, which were highly magnesian compositions (MgO 19-24 wt%). The rocks and melts are characterized by strong high field strength element (HFSE) depletion in comparison with rare earth elements, and high and variable levels of enrichment in large ion lithophile elements (LILE), P, K and H2O (0.6-1.2 wt% in picritic to basaltic melts). epsilon(Nd) values lie in a narrow range (+10.7 to +9.1), typical of N-MORB (mid-ocean ridge basalt), but Sr-87/Sr-86 (0.70316-0.70358) is slightly displaced from the mantle array. High-K ultramafic melts from Kamchatka are considered as a new magma type within the island-are magmatic spectrum; basaltic members of the suite resemble are shoshonites. The primary melts were produced under high-pressure (30-50 kbar) and high-temperature (1500-1700 oC) conditions by partial melting of a refractory peridotitic mantle.