TRACE-ELEMENT AND SR-ND-PB ISOTOPIC CONSTRAINTS ON A 3-COMPONENT MODEL OF KAMCHATKA ARC PETROGENESIS

The Kamchatka are (Russia) is located in the northwestern Pacific Ocea n and is divided into three segments by major sub-latitudinal fault zo nes (crustal discontinuities). The southern (SS) and central (CS) segm ents are associated with the subduction of old Pacific lithosphere, wh ereas the northern, inactive segment (NS) was formed during westward s ubduction of young (<15 Ma) Komandorsky Basin oceanic crust. Further s egmentation of the are is outlined by the development of the Central K amchatka Depression (CKD) intra-are rift, which is oriented parallel t o the are and is splitting the CS into the active Eastern Volcanic Fro nt (EVF) and the largely inactive, rear-are Sredinny Range. The NS vol canics (15-5 Ma) include calc-alkaline lavas, shoshonites, adakites, a nd Nb-enriched are basalts. Isotopically all magma types share high Nd -143/Nd-144 ratios of 0.512976-0.513173 coupled with variable Sr-87/Sr -86 (0.702610-0.70356). NS lavas plot within or slightly above the Pac ific MORB field on the Pb isotopic diagrams. The EVF volcanoes have mo re radiogenic Nd-143/Nd-144 (0.51282-0.513139) and Pb-208/Pb-204 (38.0 11-38.1310) than the NS lavas. CKD lavas display MORB-like Nd isotope ratios at slightly elevate. Sr-87/Sr-86 values accompanied by a slight ly less radiogenic Pb composition. Kamchatka lavas are thought to be d erived from a MORB-like depleted source modified by slab-derived silic eous melts (adakites) and fluids (NS), or fluids alone (CS and SS). Th e NS and EVF lavas may have been contaminated by small fractions of a sedimentary component that isotopically resembles North Pacific sedime nt. Petrogenesis in the Kamchatka are is best explained by a three-com ponent model with depleted mantle wedge component modified by two slab components. Slab-derived hydrous melts produced incompatible element characteristics associated with northern segment lavas, while hydrous slab fluids caused melting in the depleted mantle below the southern a nd central segments of the Kamchatka are. Trace element characteristic s of Kamchatka lavas appear to be controlled by slab fluids or melts, while radiogenic isotope ratios which are uniform throughout the are r eflect depleted composition of sub-are mantle wedge. Copyright (C) 199 7 Elsevier Science Ltd.