Metamorphism in collision zones
Chapter 6 described the characteristic types of metamorphism which occur around subduction zones, where oceanic crust is thrust downwards into the mantle. This process of destruction of oceanic crust will eventually change or end, by changes in the distribution of plates (e.g. when an oceanic ridge is subducted) or by the arrival of continental crust at the subduction zone in the overridden oceanic plate. When such continental fragments are small, they may be incorporated into the evolving orogenic belt above the subduction zone as allochthonous terranes. When the continental fragments are large, there will be a continental collision. Continental collisions of this type are occurring at the present day in the Himalayan mountain chain, where the Indian Plate is colliding with the Asian Plate, and in the Middle East, where the Arabian Plate is colliding with the Asian Plate. The existence of mountain chains formed by continental collision within what are now stable continental plates (e.g. the Ural Mountains of the USSR) shows that eventually such collisions ‘lock’ subduction zones.
KeywordsMetamorphic Grade Collision Zone Continental Collision Plagioclase Feldspar Garnet Porphyroblasts
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