Post-collisional strongly peraluminous granites

Strongly peraluminous (SP) granites have formed as a result of post-collisional processes in various orogens. In 'high-pressure' collisions such as the European Alps and Himalayas, post-collisional exhumation of overthickened crust (> 50 km), heated by radiogenic decay of K, U and Th during syn-collisional thickening, produced small- to moderate-volume, cool (< 875 degrees C) SP granite melts with high Al2O3/TiO2 ratios. In 'high-temperature' collisions such as the Hercynides and Lachlan Fold Belt (LFB), there was less syn-collisional crustal thickening(less than or equal to 50 km). Crustal anatexis was related to post-collisional lithospheric delamination and upwelling of hot asthenosphere, forming large-volume, hot (greater than or equal to 875 degrees C) SP granite melts with low Al2O3/TiO2 ratios. Both clay-rich, plagioclase-poor (less than or equal to 5%) pelitic rocks and clay-poor, plagioclase-rich (> 25%) psammitic rocks have been partially melted in high-pressure and high-temperature collisional orogens, with the pelite-derived SP granites tending to have lower CaO/Na2O ratios (<0.3) than their psammite-derived counterparts. The predominance of pelite-derived SP granites in the Himalayas and psammite-derived SP granites in the LFB suggests that mature continental platforms made up more of the accreted crust in the Himalayan collision than in the LFB. (C) 1998 Elsevier Science B.V. All rights reserved.