Granitic continental crust is a unique feature of the Earth and, as such, the origin and growth of granitic continental crust is a major focus of research in the earth sciences. This special issue focuses on petrochemistry and isotope petrology, geology petrography of granites, U-Pb zircon geochronology, K-Ar geochronology, crust-mantle structure beneath Northeast Japan, and evolution of the continental crust. These topics provide usuful information on petrogenesis and tectonic evolution of the arc crust of the Japanese Islands.
The Yatsushiro area in Kyushu Island, southwest Japan, forms the western tip of the Kurosegawa tectonic belt. This area is underlain by a serpentinite melange with complicated tectonic block assemblage of granitic rocks, high-temperature (HT)-type metagabbro (Grt-Cpx granulite, Grt-amphibolite and Cpx-bearing amphibolite), and high-pressure (HP)-type metagabbro (Jd-Gln rock and Gln-bearing metagabbro). The granitic rocks and the gabbros are the protoliths of the HT-type metagabbro (HT-gabbro), and supposed to be derived from volcanic arc at an active continental margin based on their major, trace, and rare earth element (REE) chemistry, while geochemistry of the gabbro protoliths of the HP-type metagabbro (HP-gabbro) indicates mid oceanic ridge basalt (MORB)-like precursors. The laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating of the granitic rocks and the HT-gabbro gave the Ordovician magmatic ages (∼ 450 Ma). On the other hand, the HP-gabbro showed the igneous age of the Late Cambrian (∼ 490 Ma). Similar rock suites collected from the Kurosegawa tectonic belt in Shikoku and in the Kii-peninsula have the same chemical and age affinities according to their rock types. These results indicate the strong geological and tectonic similarities within the Kurosegawa tectonic belt. The similar assemblage is present in the South-Kitakami belt suggesting similarities in their origin.
The Kitakami Belt of northeast Japan is composed of the North Kitakami Belt (a Jurassic accretionary complex) and the South Kitakami Belt (a microcontinental block). Early Cretaceous plutonic rocks with adakitic, calc-alkaline, and shoshonitic affinities intruded into both the terranes. Previous studies proposed that partial melting of a subducted oceanic slab produced the Kitakami adakitic plutonic rocks. This study presents a new view of the constituent rocks in the crust and mantle beneath the Kitakami Belt based on the P- and S-wave velocities obtained by seismic tomography and comparisons with the experimentally determined velocities of rock-forming minerals. We infer that large volumes of felsic rock exist in the lower crust beneath the Kitakami Belt, underlain by orthopyroxenite in the uppermost mantle. These inferences suggest that (1) the Early Cretaceous adakitic magmatism produced the orthopyroxenite via peritectic reactions between slab-derived felsic melts and mantle peridotite; and (2) the reacted slab melt formed thick adakitic felsic rock in the lower and upper crust. Considering the regional distribution of these unusual mantle and crustal rocks, Early Cretaceous slab melting is inferred to have occurred over a wider area than previously thought.