regional metamorphism

• Orogenesis is the formation of mountain ranges by folding and faulting of the earths crust. 
• Regional metamorphism is associated with convergent plate margins during subduction and continental collision. It involves thousands of cubic km of rock. 
• At convergent plate boundaries there are high T and P acting over a long period of time causing broad and complex organic belts that are affected by all grades of metamorphism. At the deepest points of the belt the tension and pressure will be the greatest so high grade metamorphism takes place. The further from the collision point and the shallower the depth, the lower the grade of metamorphism. 
• Regional metamorphic rocks show foliation, a banding/layering/alignment of crystal long axes as they crystalised under directed stress. 
• In regional metamorphism of argillaceous rocks that are rich in clay minerals and include siltstone, clay and mudstone, metamorphosize in to different metamorphic rocks under increasing pressure and temperature. 
• Low grade regional metamorphic rocks (slate) form between depths of 5-15km, at low temperature and pressure. Clay mineral recrystalise into chlorite and biotite mica and are coarser grained than the original mineral. 
• Chlorite and biotite mica are platy minerals and have their long axes aligned at right angles to the principle stress direction to form slaty cleavage. At low grade, some pre-existing sedimentary structures may be preserved, but may be deformed, compressed or stretched. 
• Medium grade metamorphic rocks (schist) is formed under medium temperatures and depth. A higher temperature will result in larger crystals and the growth of minerals like garnet, quartz and micas. 
• High grade regional metamorphic rocks (gneiss) are formed under high temperatures and pressures. High temperatures result in larger crystals over 2 mm and the growth of minerals like feldspar, quartz and mica. 
• in high grade The minerals are separated into discontinuous layers to produce a coarse foliation called gneissose banding; the minerals composition is now similar to granite.