Oceanic-arc subduction, stagnation, and exhumation: zircon U–Pb geochronology and trace-element geochemistry of the Sanbagawa eclogites in central Shikoku, SW Japan

Abstract

In the Iratsu and the quartz-bearing eclogite bodies of the Sanbagawa high-pressure type metamorphic belt, southwest Japan, zircon U–Pb dating and trace-element analysis of the mafic gneiss combined with its geologic structure revealed that the protolith basaltic rock constituted the topographic high on a seafloor in relation to intra-oceanic arc magmatism at ca. 195 Ma. Moreover, the metamorphic zircon U–Pb data and the rare-earth element patterns obtained from the subordinated metasedimentary rocks of the Iratsu and the quartz-bearing eclogite bodies indicate that both bodies were subducted from a trench at ca. 120 Ma and underwent the eclogite facies metamorphism between ca. 120 and 90 Ma. This study, combined with previous studies for the Sanbagawa rocks and the Jurassic-Cretaceous accretionary complexes in Japan, identifies the following constraints that led to the tectonic evolution of the Sanbagawa eclogites: 1) the metamorphic unit including the Iratsu and the quartz-bearing eclogite bodies (the Besshi unit) was subducted from a trench at ca. 120 Ma. 2) This unit was stagnated at the depth of the eclogite–facies condition between ca. 120 and 90 Ma. 3) The eclogites in the Besshi unit was exhumed with the younger metamorphic rocks which were subducted at ca. 100–90 Ma (Asemi-gawa unit). 4) The Besshi unit is a high-pressure metamorphic equivalent of the non- or weakly metamorphic Sanbosan accretionary complex and the Mikabu greenstones from a standpoint of age similarity on accretion. The probable mechanism for the stagnation of the Besshi unit at the depth of the eclogite–acies condition needs 1) the detachment of oceanic-arc material from the subducting slab, driven by the resistance against the subduction of the topographic-high part underneath the forearc, and 2) the oceanward movement of the entire arc-trench system, which might have depressed the subduction of the Besshi unit into a deeper depth than its eclogite depth.