IGNEOUS ROCKS AND MAGMA SOURCES OF THE OZERNAYA ZONE OPHIOLITES, MONGOLIA

Ophiolites of the Ozernaya zone, western Mongolia, which are a fragmen t of the oceanic crust of the Vendian-Early Paleozoic ocean of Central Asia, comprise two mineralogically and geochemically distinct associa tions: (1) a basitic association with siliceous sediments and (2) a di fferentiated basalt-andesite-dacite association with terrigenous (gray wacke) and carbonate sediments. The most primitive rocks of both assoc iations have similar chemistries, distribution of trace elements (for example, they show similar Ta-Nb minima in their spider diagrams), Nd isotope composition, age, and are chemically intermediate between MORE , basalts of primitive island arcs, and GIB. The processes of metamorp hism (at greenschist to amphibolite-facies conditions) and spilitizati on overprinted on these rocks affected first and foremost the distribu tion of Na2O and CaO. The basite association formed in a marginal-basi n environment and affiliates to the tholeiite series of magnetite-unde rsaturated and relatively ''dry'' magmas. The differentiated associati on developed in an environment of a primitive island are from a source that was richer in water (judging from hornblende phenocrysts) and mo re oxidized. Spider diagrams of these rocks often have Ti, Zr, and Hf minima. The mantle sources of both associations were compositionally c lose, with elevated water contents and oxidation degrees typical of is land-are magmas. The regionally occurring Ta-Nb minima of the rocks of both associations are accounted for by the earlier depletion of the m antle from which the magmas were melted out or by the involvement of s ubducted sediments in the mantle sources. The variations in the conten ts of trace elements in both associations were controlled by the fract ionation of the Pl(0.5)Cpx(0.3)Ol(0.2) assemblage in the tholeiites of the spreading-related basite association and the Pl(0.5)Amph(0.2)Cpx( 0.2)Mt(0.05) assemblage in the island-are magmas of the differentiated association. Both of the trends result in an enrichment of the residu al magmas in most trace elements, but the island are rocks become prog ressively depleted in Ti and Fe. The evolution trends of both associat ions are characterized by decreasing Cr and Ni.