Post-collisional strongly peraluminous granites

Strongly peraluminous (SP) granites have formed as a result of post-collisi onal processes in various orogens. In 'high-pressure' collisions such as th e European Alps and Himalayas, post-collisional exhumation of overthickened crust (> 50 km), heated by radiogenic decay of K, U and Th during syn-coll isional thickening, produced small- to moderate-volume, cool (< 875 degrees C) SP granite melts with high Al2O3/TiO2 ratios. In 'high-temperature' col lisions such as the Hercynides and Lachlan Fold Belt (LFB), there was less syn-collisional crustal thickening(less than or equal to 50 km). Crustal an atexis was related to post-collisional lithospheric delamination and upwell ing 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-ri ch, plagioclase-poor (less than or equal to 5%) pelitic rocks and clay-poor , plagioclase-rich (> 25%) psammitic rocks have been partially melted in hi gh-pressure and high-temperature collisional orogens, with the pelite-deriv ed SP granites tending to have lower CaO/Na2O ratios (<0.3) than their psam mite-derived counterparts. The predominance of pelite-derived SP granites i n the Himalayas and psammite-derived SP granites in the LFB suggests that m ature continental platforms made up more of the accreted crust in the Himal ayan collision than in the LFB. (C) 1998 Elsevier Science B.V. All rights r eserved.