Revisiting the high temperature metamorphic field gradient of the Ryoke Belt (SW Japan): New constraints from the Iwakuni-Yanai area

Abstract

We revisit the origin of the metamorphic field gradient exposed in the Ryoke Belt (SW Japan) — the type locality for low pressure/high temperature (LP/HT) conditions — based on combined structural, petrological and geochronological data from the Iwakuni–Yanai area (W Ryoke). In this area metasedimentary rocks pass from schist in the north to partly migmatitic paragneiss in the south, and are surrounded by numerous granitoid bodies. Three deformation phases are emphasized; an accretionary-stage or burial deformation of weak intensity (D0), a vertical shortening (D1) which generated a subhorizontal, locally extensional foliation in the southern gneiss zone, and E–W upright folding (D2) which reworked both the northern schist and southern gneiss zones. Crystallization–deformation relationships indicate that, in the schist zone, most andalusite, cordierite and mica porphyroblasts formed after D0 and before D1. Syn-D1 mineral assemblages define E–W trending metamorphic zones with increasing grade from north to south, and rare syn-D2 parageneses reveal a similar pattern. Continuous magmatic activity lasted from ca. 105 to 94Ma. The oldest granitoids (Shimokuhara, Soo, Namera), found in the western part of the belt, were emplaced below schistose rocks at 105–100Ma. Subsequently, syn- to post-D1 granodiorite (Gamano) intruded concordantly with the foliation of southern, high-grade gneissic rocks from 100 to 94Ma. A large, syn-D1 pluton (Kibe) intruded the center of the belt at 98Ma, whereas a younger granite (Iwakuni) and its satellite dykes were emplaced to the northeast at 96–94Ma. Constraints on the timing of D1 (103–99Ma) and the likely diachronous D2 phase (100–94Ma) suggest a deformation continuum during magma intrusion and metamorphism.We conclude that the exposed metamorphic field gradient is an apparent, but fortuitous, alignment of P–T data resulting from spatially and temporally distinct thermal events. Peak temperature conditions in the relatively shallow schist zone were attained during a first, pre-tectonic, contact metamorphic event. A second, regional metamorphic overprint arose from the syn-tectonic intrusion of granitoids in the deeper gneiss zone. A younger contact metamorphic event locally affected the eastern part of the schist zone. The revised tectono-thermal history highlights the composite and diachronous character of the Ryoke metamorphic field gradient.