PETROLOGY AND GEOCHEMISTRY OF CRETACEOUS ULTRAMAFIC VOLCANICS FROM EASTERN KAMCHATKA

Abstract

The origin, evolution and primary melt compositions of late Cretaceous high-K ultramafic volcanics and associated basalts of Eastern Kamchatka are discussed on 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 meltsare 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). εNd values lie in a narrow range (+10·7 to +9·1), typical of N-MORB (mid-ocean ridge basalt), but 87Sr/86Sr (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-arc magmatic spectrum; basaltic members of the suite resemble arc shoshonites. The primary melts were produced under high-pressure (30–50 kbar) and high-temperature(1500–1700°C) conditions by partial melting of a refractory peridotitic mantle.