A high‐Tmetamorphic complex derived from the high‐PSuo metamorphic complex in the Omuta district, northern Kyushu, southwest Japan

Abstract

New U–Pb ages of zircons from migmatitic pelitic gneisses in the Omuta district, northern Kyushu, southwest Japan are presented. Metamorphic zonation from the Suo metamorphic complex to the gneisses suggests that the protolith of the gneisses was the Suo metamorphic complex. The zircon ages reveal the following: (i) a transformation took place from the high‐P Suo metamorphic complex to a high‐T metamorphic complex that includes the migmatitic pelitic gneisses; (ii) the detrital zircon cores in the Suo pelitic rocks have two main age components (ca 1900–1800 Ma and 250 Ma), with some of the detrital zircon cores being supplied (being reworked) from a high‐grade metamorphic source; and (iii) one metamorphic zircon rim yields 105.1 ±5.3 Ma concordant age that represents the age of the high‐T metamorphism. The high‐P to high‐T transformation of metamorphic complexes implies the seaward shift of a volcanic arc or a landward shift of the metamorphic complex from a trench to the sides of a volcanic arc in an arc–trench system during the Early Cretaceous. The Omuta district is located on the same geographical trend as the Ryoke plutono‐metamorphic complex, and our estimated age of the high‐T metamorphism is similar to that of the Ryoke plutono‐metamorphism in the Yanai district of western Chugoku. Therefore, the high‐T metamorphic complex possibly represents the western extension of the Ryoke plutono‐metamorphic complex. The protolith of the metamorphic rocks of the Ryoke plutono‐metamorphic complex was the Jurassic accretionary complex of the inner zone of southwest Japan. The high‐P to high‐T transformation in the Omuta district also suggests that the geographic trend of the Jurassic accretionary complex was oblique to that of the mid‐Cretaceous high‐T metamorphic field.

Metamorphic condition of a regional metamorphic complex in the Omuta district in northern Kyushu, southwest Japan

“…The current chronological constraints do not contradict the assumption. That is, the locality of the sample showing metamorphic age of 105.1 ±5.3 Ma is same as O‐23d in the sillimanite–K‐feldspar zone (Miyazaki et al, ). Tomita et al () report similar biotite and hornblende K–Ar ages of the Tamana Granodiorite, as described before.…”

“…The boundary between the Tamana Granodiorite and the metamorphic complex is, nevertheless, oblique to the isograds (Figure 1(b)). Miyazaki et al (2017) reveal that zircons in the metamorphic complex show U-Pb detrital ages of ca. 250 Ma, similar to those in the Suo metamorphic complex in the Kurume-Yame-Yamaga area.…”

“…Miyazaki et al () reveal that zircons in the metamorphic complex show U‐Pb detrital ages of ca . 250 Ma, similar to those in the Suo metamorphic complex in the Kurume–Yame–Yamaga area.…”

“…Combined with stratigraphic continuity from the Kurume-Yame-Yamaga area, Miyazaki et al (2017) consider that the protolith of the high-T metamorphic complex is the Suo metamoprhic complex.…”

“…These considerations suggest that the high-T metamorphic complex was formed not by intrusion of the Tamana Granodiorite but formed by dynamic regional metamorphism before the intrusion of Tamana Granodiorite. Miyazaki et al (2017) consider that the high-T complex in the present study is the western continuation of the Ryoke plutonometamorphic complex, based on chronological similarity of the metamorphism. This study reveals additional features similar to the Ryoke complex in the Yanai area, such as metamorphic condition and crustal thinning during metamorphism.…”