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°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 (≤ 50 km). Crustal anatexis was related to post-collisional lithospheric delamination and upwelling of hot asthenosphere, forming large-volume, hot (≥ 875°C) SP granite melts with low Al₂O₃/TiO₂ ratios. Both clay-rich, plagioclase-poor (< 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.