GEOLOGY OF HORAISAN SERPENTINITE MELANGE IN HOKKAIDO, JAPAN AND INITIAL STRUCTURE OF DEEP SUBDUCTION CHANNEL

Serpentinite melange with high-pressure (HP) metamorphic blocks can be regarded as exhumed parts of deep subduction zone (serpentinite channel). Their original structures are however left poorly understood, because mineralogy and structure of serpentinite can be very easily modified in later events. We present geology of a serpentinite melange, in which macroscopic structures presumably of primary subduction stage are identifiable.

Pre-Neogene rocks in Horaisan area comprise three parallel zones: Horai-san serpentinite melange (HRS Melange), Gunkan-yama (GKY) ophiolite, and a blueschist and semi-schist of the Iwashimizu Complex (IWC). The GKY ophiolite consists of fault-bounded slices of partly serpentinized harzburgite, ultramafic cumulates, and gabbro-diabase complex with island arc chemical features (including boninitic rocks). Zircon U-Pb ages of the associated diorite and tonalite were determined as late Middle Jurassic.

HRS melange contains blocks mainly of peridotite, massive serpentinite, and amphibolites. Parts of HRS melange underwent mixing with adjacent units at shallow levels. Whereas in unmixed parts, slabs of harzburgite and its pseudomorphic low-T serpentinite are intervened by narrow zones (matrix) of sheared serpentinite with small blocks of amphibolite and antigorite serpentinite. The matrix consists of scaly low-T serpentinite with minor foliated antigorite serpentinite.

Low-T serpentinites might have been peridotite in the antigorite stage, because low-T serpentinization postdated antigorite crystallization. The original structure is therefore reconstructed so as that peridotite slabs were bounded by a network of narrow shear zones consisting of amphibolite and antigorite serpentinite. Early matrix of amphibolites and antigorite rocks now occur as blocks. Present-day matrix of low-T serpentinites originated from parts of early-stage peridotite slabs. Such transposition can be explained if we consider expansion of peridotite slabs during late-stage low-T hydration, which disconnected a network of amphibolite - antigorite shear zones, and separate them into blocks. Whereas newly formed, more incompetent low-T serpentinite took place of matrix role. Serpentinite melanges we observe today could thus be reversal of ancient subduction channels.