Permalink : http://hdl.handle.net/2115/36730

In an effort to examine the natures and origin of tonalitic intrusive masses lying between Takanuki and Gozaisho metamorphic units in the Abukuma axial metamorphic belt a study has been made of the Usuki tonalite complex, which crops out in the Takanuki district, as the typical one of the intrusive masses. Two-thirds of the compelx is represented by the lithofacies of tonalite proper, with which migmatitic and granodioritic lithofacies are intimately associated. The host rocks of the migmatitic facies are biotite schist and amphibolite of both metamorphic units. The plagioclases formed in the neosomes of the migmalitic facies are fairly similar to those shown in tonalite proper; they are indicalive of having been under overwhelming conditions of plagioclase formation la king place in the facies of tonalite proper, the conditions prevailing in the whole extent of the complex. Meanwhile, the granodioritic facies is of metasomatic replacement origin, which is considered to be a stable lithofacies according to the behaviour of its plagioclase. An examination of chemical characters of these lithofacies indicates that the lithofacies of granodiorite and tonalite proper are in a stable state of equilibrium. On the other hand, most of the neosomes seem to represent an unstable or transitional state of the lithofacies which deviated from the standard rock species. Based on the geological and petrological evidence obtained through the detailed survey of the whole Takanuki district, the writer has arrived at the conclusion that the present tonalite which derives from crustal materials, did not come about from the partial melting of basic rocks. He dose not agree to the currently accepted view concerning the origin of the early Precambrian voluminous trondhjemite-tonalite suite. Initially, it was formed beneath the central zone of the Takanuki metamorphic belt by the granilization of the Takanuki metamorphics at a deep level during the movement of the main stage of the Abean orogeny. Succeedingly, the granitized materials at that level were rejuvenated and mobilized into the fracture zone formed between the Takanuki and the Gozaisho metamorphics during the subsequent stage of the orogeny. Measured data on lead isotope ratios of rocks of all types in the district also support the above conclusion.