THERMAL EFFECTS OF RIDGE SUBDUCTION AND ITS IMPLICATIONS FOR THE ORIGIN OF GRANITIC BATHOLITH AND PAIRED METAMORPHIC BELTS

Abstract

Distribution of temperature and H2O in subduction zones during the course of ridge subduction are investigated numerically, in order to constrain the timing and extent of metamorphism and melting within the subducting oceanic crust and the overlying arc crust. The model assumes that after ridge subduction, the two plates move together without creating a slab window. The results suggest that, before ridge subduction, temperature along the subducting slab starts to rise when the slab is younger than ~10 Myr. A drastic thermal impact occurs associated with subduction of the ridge crest, followed by a diffusion stage of the impact. The temperature elevation is limited within a few tens of kilometers from the slab-crust interface and within 30 Myr after ridge subduction, although it will last for a longer period if a slab window is formed. During the peak stage, slab melting occurs and generates a significant amount of granitic melt mainly at depths shallower than 40 km, at P-T conditions corresponding to low-P/T metamorphism. High-P/T metamorphism with a significant water flux is also achieved near the interface when a subducting slab is less than 10 Myr old before ridge subduction or 20 Myr old after ridge subduction. Consequently, the low- and high-P/T metamorphism accompanied by a significant amount of granitic melt is associated closely in time and space with the ridge subduction. This model can explain the observed characteristics of the granitic batholith and the paired metamorphic belts. Through these processes, arcs and continents can grow within the short period of time associated with the episodic subduction of ridges.