PETROLOGY AND GEOCHEMISTRY OF CRETACEOUS ULTRAMAFIC VOLCANICS FROM EASTERN KAMCHATKA

The origin, evolution and primary melt compositions of late Cretaceous high-X ultramafic volcanics and associated basalts of Eastern Kamchat ka are discussed an the basis of a study of the mineralogy and geochem istry of the rocks and magmatic inclusions in phenocrysts. The excepti onally primitive composition of the phenocryst assemblage [olivine-Fo( 88-95), Cr-spinel-Cr/(Cr + Al) up to 85] provides direct evidence of t he mantle origin of primary melts, which were highly magnesian composi tions (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 io n 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 (0.7 0316-0.70358) is slightly displaced from the mantle array. High-K ultr amafic melts from Kamchatka are considered as a new magma type within the island-are magmatic spectrum; basaltic members of the suite resemb le are shoshonites. The primary melts were produced under high-pressur e (30-50 kbar) and high-temperature (1500-1700 degrees C) conditions b y partial melting of a refractory peridotitic mantle.