RETROGRADE DEFORMATION WITH RETROGRADE METAMORPHISM IN VARIOUS BLOCKS IN MINEOKA OPHIOLITIC MELANGE: COMPARATIVE STUDY TO THE ANKARA AND FRANCISCAN MELANGES

Origin and setting of melanges are one of the eternal topics. Among the various characteristics, we noticed that most of the blocks in melanges in the subduction zone settings have systematic series of deformation, that is told in a word as retrograde. The last stage is characterized by shearing and brecciation during the final exhumation after the first emplacement from the depths. The best examples of melange in the world, the Ankara melange, as far as mapped, are composed of various kinds of blocks, large and small, ranging from pelagic limestone to amphibolite or greenschist through basalt, chert and sandstone. Those are more or less largely deformaed into breccia. Matrix is of a little amount, and even the serpentinite is not always surrounding the blocks, but is juxtaposed as blocks side by side with other blocks, so that the total mass are said not to be a block-in-matrix relation but rather a kind of a fault belt.

The Mineoka belt in central Japan is situated on the extension of the Izu forearc, and is composed of Cretaceous to Miocene basaltic rocks and pelagic limestone/chert with considerable amount of terrigeous, some volcaniclastic some continental-arc derived fragments as well as ophiolitic materials. Although a large amount of serpentinite (harzburgite in origin) are associated, they are rarely sheared (limited only in a narrow zone around the blocks), but mostly inside is only brecciated. Brecciation is common in metamorphic rocks (2 GPa at maximum) and basaltic or gabbroic rocks of several stages of deformation and metamorphism. Deformation series is from the early mylonitic to later shearing and the last brecciation. Veining of zeolite, prehnite and calcite is also common in basaltic rocks of MORB in chemistry, suggesting various stage hydrothermal deformation since the first emplacement around a spreading ridge setting to strike-slip deformation at later stages. Several stages of such deformation are attributable to strike-slip or transpressional fields, further suggesting most of the multi-stage emplacements are of either transform along ridge-ridge setting, or obudction or even flow mélange-like exhumation along a forearc sliver fault setting.

The Franciscan melange in the San Simeon area on the middle California coast is the classical type locality for the melange study history, and is characterized by various kinds of large to small blocks in muddy matrix. However, our detail mapping indicates that fish or turtle-like shape of blocks suggests Poiseuille flow within muddy (lawsonite-bearing) matrix. Another important mode of deformation is brecciation. Even blueschist, greenschist, basalt and so on are largely brecciated as the last stage of deformation.

In these representative melanges, such modes of shearing, shape of blocks, and inside deformation from ductile to brittle conditions suggests retrograde metamorphism and associated deformation, probably during when those blocks are exhumed along the return flow into flow melanges, some may be within a subduction zone complexes (part of an accretionary prism), some in forearc sliver fault zones.