GEOCHEMICAL VARIABILITY OF BONINITE MAGMAS - EVIDENCE FROM MAGMATIC INCLUSIONS IN HIGHLY MAGNESIAN OLIVINE FROM LAVAS OF SOUTHWESTERN CYPRUS

Detailed geochemical, mineralogical, and experimental studies of an Ol -phyric boninite from the Mavrokolimbos area, southwestern Cyprus are presented. Olivine phenocrysts have a primitive composition (Fo(87-92) ). They contain crystalline inclusions of magnesian orthopyroxene (Mg# = 0.90-0.92), chromian spinel of variable composition [Cr/(Cr + Al) = 0.79-0.60], and syngenetic melt and fluid inclusions. The melt inclus ions, which were quenched experimentally, were analyzed for major elem ents (using electron microprobe), trace elements, and H2O (by ion micr oprobe). The detected compositional variations of the melt inclusions suggest a wide chemical diversity of the primary boninitic magmas and the involvement of at least four independent components in their compo sition. According to the proposed model, the boninite melts formed by partial mantle melting in an open system, at an effective restite perm eability of approximately 2%, a fact evident from the occurrence of an ultradepleted melt [(Ce/Yb)(n) = 0.07]. The incipient mantle melting began under the effect of the aqueous component. The primary fluid was characterized by a low content of the most incompatible elements (H2O /K2O > 150). Its origin was related to the dehydration of a subducted slab. The enrichment of the boninite in LREE and LILE was due to the p resence of an independent component [(La/Sm)(n) > 2, (K/La)(n) approxi mate to 5, (K/Sr)(n) approximate to 5, (Sr/Ce)(n) approximate to 1], w hich could be introduced in the mantle source by the aqueous component percolating through the heterogeneously metasomatized mantle. Alterna tively, the enriched component may have The genesis of this component is related to potassic mantle metasomatism. The boninitic magmas inter acted with silicic rocks/melts during the crystallization and produced hybrid melts with anomalously high contents of HREE [Yb > 2.5 ppm,(Dy /Ti)(n) > 5],Y, and Na, and low CaO/Al2O3 ratios (<0.75).