Comparison of New England Orogen and Japanese terranes

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Main results obtained by this project are as follows :(1) Although Paleozoic New England orogen in eastern Australia and Mesozoic terranes in Southwest Japan formed at different time, they have many geological characteristics in common. For example, the lower Paleozoic Djungati terrane in Australia and Tamba-Mino-Ashio terrane in Japan have a similar lithological sequence changing from radiolarian bedded chert through siliceous shale to turbidite, which seems to show oceanic basin stratigraphy accreting to continental margins.(2) Both the New England orogen and Japan are mainly composed of accretion complexes. Detailed field observation, however, revealed that there are some differences ; accretion complexes in Japan include large amount of melanges, while Australian counterpart has little of them. It may be related to the processes of subduction and accretion and/or state of exposure (heavy weathering in eastern Australia might easily weathered relatively weak melanges).(3) Dikes in the Port Macquarie area are divided into high-Mg ultrabasic (SiO2 : 35 - 45%) and low-Mg basic (SiO2 : 45 -55%) dikes. The high-Mg dikes are characterized by the occurrence of pargasitic brown hornblende. The dikes with low SiO2 contents include olivine and chromspinel phenocrysts. Although the high-Mg dikes have been regarded as boninite (e.g.Aitchison, 1994), chemical compositions, mineralogical characteristics and field evidence indicate that the dikes formed by intrusion of high-Mg komatiite magma.(4) Preliminary radiolarian biostratigraphic study on Devonian Gamilaroi terrane rocks shows that the Radiolaria with three-bladed spines previously thought to be rare in the Devonian were abundant and diverse by this time. Comparative study with the Devonian radiolarians in Japan has possibility to make clear the radiolarian biogeographic provinces of the "Pacific" in Devonian time.