Genesis of the Kitami epithermal gold province, southwestern Kuril arc 千島弧南西端北見浅熱水性金鉱床区の成因

博士論文

The Kitami provice, one of the epithermal gold provinces in Japan, is on the backarc side of the present Kuril-arc volcanic front. With the assumptions that the Pachfic plate has been subducting beneath the Japanese islands in the same orientation as that revealed by the Hawaiian volcanic chain, and that the subduction angle was constant during Middle and Late Miocene time, the Miocene tectonics ofnortheastern Hokkaido is interpreted as follows: The subduction direction of the Pacific plate rotated from N33°W at 14 Ma to N90°W at 10.6 Ma. Thus, the subduction along the Kuril arc changed from normal to oblique, and the apparent dip of the slab beneath the Kuril arc steepened. This change shifted the volcanic front of the Kuril arc trenchward, and may have allowed hot asthenosphere injections from the depth into the space that previously occupied by the cold slab of lithosphere on the backarc side of the volcanic front. Between 10.6 Ma and 5.8 Ma, the subduction direction rotated clockwise from N90°W to N51°W and the oblique-subduction setting of the Pacific plate beneath southwestern Kuril arc disappeared at ca．6 Ma. This model explains the timing of the volcanism(ca. 14-5 Ma), wide distribution of the volcanic rocks across the arc (ca. 200km), activities of unimodal andesite followed by bimodal rhyolite and basalt at each locality, absence of systematic K20 variation in andesites across the arc, presence of non-arc-related volcanic rocks such as icelandite, high-Ti andesite and trachyandesite, and abundance of plagioclase-phyric rhyolite without hydrous minerals. This model also explains the genesis of arc-parallel right-lateral strike-slip faulting (Kamishiyubetsu Tectonic Zone), which occurred during 13-10 Ma due to the increasing obliquity of the subducting Pacific plate. The faulting extended trenchward with time, following to the shift of the volcanic front. The epithermal gold mineralization in the Kitami province is related to the rhyolitic intrusions and extrusions of the bimodal volcanism. The rhyolitic magmas are inferred to have occurred by partial melting of the ophiolite, which was trapped by the paleo Kuril arc during Cretaceous to Paleogene time and has occupied the lower crust of central and eastern Hokkaido. This partial melting may be due to either basaltic magma formed by the asthenosphere injections into the lower crust or the injection itself. Higher gold concentration in the ophiolite than that in terrigenous sedmentary rocks covering northeastern Hokkaido suggests that gold in the epithermal deposits in the Kitami province was derived from the ophiolite. The faults of the Kamishiyubetsu Tectonic Zone provided conduits for hydrothermal solution discharged from the rhyolitic magma, and also loci of precipitation of gold minerals. Thus, the Kitami province provided the gold with (1) the source (ophiolite), (2) the medium of extraction and transportation (rhyolite) and (3) the depo sition site (strike-slip faults; mainly faults of the Kamishiyubetsu and Saroma Tectonic Zones). Among these points, (2) and (3) owe muoh to the steepening slab and strike-slip fault zones caused by the oblique subduction. Therefore, the mo st important factor of the Kitami epithermal gold province is the rotation of the subduction direction.