We examine the petrological nature of the mantle-crust transition zone (MCTZ) based on detailed field observations of the northern Oman ophiolite. Two kinds of MCTZ, early-gabbroin-dunite and late-dunite-in-gabbro transition zones, can be recognized between the residual peridotite and the layered gabbro sequence. They are distinguished by an intrusive relationship between gabbro and dunite. In the early-gabbro-in-dunite transition zone, gabbro forms network-like sills and has intrusive contact with dunite. The frequency of the gabbro sills gradually increases from the top of residual peridotite to the base of layered gabbro, which itself has a sharp boundary with the underlying dunite. All constituents of the earlygabbro-in-dunite transition zone are deformed, and lithological boundaries are parallel to foliation of the rocks. On the other hand, in the late-dunite-in-gabbro transition zone, dunite has intrusive contact with gabbro sills and layered gabbro. Clinopyroxenite produced by reaction/partial melting occurs frequently along the intrusive contact. The late-dunite-ingabbro transition zone is of secondary origin, being modified from the primary layered gabbro to the early-gabbro-in-dunite transition zone by later dunite intrusion.
Degree of serpentinization is irregularly distributed, and antigorite, a high-temperature serpentine species, is not found in the peridotite portion. This indicates the Hess model that the oceanic Moho is placed within peridotite as a serpentinization front is not deduced from observations of the Oman ophiolite.
We propose a model for the formation of two kinds of MCTZ. The gabbro sills in dunite were originally formed beneath a mid-oceanic ridge as a network of upward-moving melt within residual harzburgite. Dunite was produced by a reaction between melt and harzburgite. The network-like gabbro and dunite were deformed to become elongated by the horizontal mantle flow with leaving the spreading ridge. The boundary between the melt-rich part (center of paleo-melt flow) and melt-poorer part later became the layered gabbro/dunite boundary. The early-gabbro-in-dunite transition zone is the part between the layered gabbro/dunite boundary and the residual harzburgite. An off-ridge magmatism formed socalled late-intrusive plutonic bodies including dunite, cutting the primary rocks formed at the spreading ridge. The dunite formed intrusive contacts throughout the pre-existing crustal sequence, that is, the secondary late-dunite-in-gabbro transition zone. The intrusive bodies have island-arc geochemical signatures and are genetically linked to effusive rocks of islandarc type including picrite. The late-dunite-in-gabbro transition zone probably formed at an island-arc setting during detachment and obduction of a slice of oceanic lithosphere as an ophiolite suite. The early-gabbro-in-dunite transition zone may occur frequently beneath the ocean floor, especially that of the fast-spreading ridge system. We predict a common latedunite-in-gabbro transition zone beneath some oceanic island arcs and back-arc basins.