Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

Abstract

Garnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.

The versatility of petrological modeling: Thermobarometry of high‐pressure metabasites from the Renge and Sanbagawa belts and phase evolution during warm subduction at Nankai

“…On the other hand, to explore the HP evolution of Sanbagawa belt, we selected a kyanitebearing quartz eclogite from the Gongen area (Besshi area). The bulk-rock composition used for calculating the phase equilibrium models correspond to sample GE1501a from Miyamoto et al (2007) and Enami et al (2019). All bulk-rock compositions are given in Table 1.…”

“…In the northeastern limit of the Higashi-Akaishi, a small quartzrich eclogitic body separates the Higashi-Akaishi peridotite from the Iratsu HP metabasites (Enami et al, 2004). This body has been described in detail by Miyamoto et al (2007) and Enami et al (2019).…”

“…The peak paragenesis field expands over a large P-T range, extending up to ultra-HP conditions. Complex garnet zoning in the sample (Enami et al, 2019) precludes constraining the peak P-T conditions using garnet compositional isopleths. Thus, omphacite and phengite 3.3 | P-T evolutions of the HP metabasites from the Renge (Omi) and Sanbagawa (Besshi) belts revisited…”

Rutile exsolution lamellae of garnet in quartz eclogite from the Sanbagawa Belt, Mt. Gongen, central Shikoku, Japan

“…Gongen in the Besshi region, quartz-eclogite occurs between the ultramafic and mafic eclogitic bodies. The constituent minerals of quartz-eclogite are quartz, garnet, omphacite, phengite, epidote, kyanite, hornblende, rutile, and apatite (e.g., Ota et al, 2004;Miyamoto et al, 2007;Enami et al, 2018). The peak metamorphic conditions in this region are estimated to be 2.3-2.4 GPa and 675-750°C (Miyamoto et al, 2007).…”

Petrological and mineralogical contrasts of basic lithologies between eclogite and non–eclogite units along the Kokuryo River of the Sanbagawa belt, Central Shikoku, Japan

“…These eclogite facies lithologies might be grouped into kyanite-eclogite, common mafic eclogite, and associated schistose rocks of basic and pelitic lithologies. The kyanite-eclogite, in which protoliths might be mixtures of pelitic and basic volcaniclastic rocks (Takasu, 1989;Takasu et al, 1994;Utsunomiya et al, 2011;Enami et al, 2019), records P/T conditions of 2.3-2.5 GPa/570-740°C for the eclogite facies stage (Miyamoto et al, 2007;Endo and Tsuboi, 2013). However, common mafic eclogite retains a slightly lower P/T equilibrium of 1.8-2.1 GPa/520-590°C (e.g., Endo, 2010;Kabir and Takasu, 2010b).…”