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

摘要

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-pressu