A new perspective on metamorphism and metamorphic belts

The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolutionized the classic interpretation of (1) progressive and retrogressive metamorphism recorded on surface exposures of regional metamorphic belts, (2) geochronology of the various stages of metamorphism, (3) origin of metamorphic textures, (4) P–T–t path, (5) metamorphic facies series, (6) exhumation model, and (7) role of fluids during regional metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia, Franciscan and Sanbagawa belts, we suggest the following revolutionary paradigm shifts.The so-called mineral isograds defined on the maps of regional metamorphic belts were a misunderstanding of the progressive dehydration reaction during subduction because extensive late-stage hydration has mostly obliterated the progressive minerals in pelitic–psammitic and metabasic rocks. Progressively zoned garnet has survived as the sole progressive mineral that was unstable with the majority of matrix-forming minerals. The classic Barrovian isograds should therefore be carefully re-examined. The well-documented SHRIMP chronology of spot-dating zoned zircons with index minerals from low-P in the core, through HP–UHP in the mantle to low-P on the rim clearly shows that the slow exhumation speed of 23–40 My from mantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensive hydration of the UHP–HP unit occurred due to fluid infiltration underneath, when the UHP–HP unit intruded the low-grade to unmetamorphosed unit at a mid-crustal level. Most deformation textures such as mineral lineations, porphyroblasts, pull-apart, or boudinaged amphiboles, formed during extensive hydration at the late stage do not constrain the progressive stress regime. The P–T–time path determined by thermobarometry using mineral inclusions in garnet and forward modeling of garnet zoning, independent of the matrix minerals, indicates an anticlockwise trend in the P–T space, and follows an independent P–T change in the metamorphic facies series. This is consistent with the numerically calculated geotherm along the Wadati–Benioff plane. Collision-type orogenic belts have long been regarded as being characterized by the intermediate-pressure type metamorphic facies series. The kyanite–sillimanite is an apparent type facies series formed by the late-stage extensive hydration. In contrast, the original high-P to ultrahigh-P type facies series with an anticlockwise kink-point at around 10 kb is a progressive type. A collision-type regional metamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low-P or weakly metamorphosed units. The metamorphic belt has an aspect ratio (thickness vs width) of 1:100, and it extends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantle extending from 100 to 200 km depth into the crustal rock unit. The underlying unit is thermally metamorphosed in the andalusite–sillimanite type facies series. The major reason for the misunderstanding of the progressive metamorphism in collision-type orogenic belts is the underestimation of the role of fluids derived from the underlying low-grade metamorphic unit, when juxtaposed at a mid-crustal level. The circulation of fluids along the plate boundaries is more important than a P–T change.