Numerical modeling of the formation of Indo-Sinian peraluminous granitoids in Hunan Province: Basaltic underplating versus tectonic thickening

The genesis of Indo-Sinian granitic plutons with peraluminous and potassium-rich affini-ties from Hunan Province, China has been investigated by numerical modeling using the numericalcode FLAC. On the basis of the regional geological evolution in South China, we employed a real-istic numerical model in an attempt to unravel the influences of basaltic underplating and tectoniccrustal thickening on the crustal anatexis. Heat production derived from basaltic underplating (e.g.ca. 220 Ma gabbro xenoliths) can result in dehydration melting of fluid-bearing minerals in crustalrocks such as gneisses and metapelites, but its effect is limited in a relatively short time span (5-15 Ma) and on a small scale, Accordingly, it is very difficult for basaltic underplating to generate thelarge-scale Indo-Sinian granitic bathliths unless voluminous mafic magmas had been underplatedat the lower/middle crust during this period. Alternatively, crustal thickening induced by tectoniccompression can also lead to geothermal elevation, during which the temperature at the boundarybetween lower and middle crusts can be up to or greater than 700℃, triggering dehydration melt-ing of muscovite in gneiss and metapelite. The proportion of melts from muscovite-induced dehy-dration melting is close to critical melt percentage (≥20%) once the thickening factor reaches 1.3.These melts can be effectively transferred to the crust-level magma chamber and form large-scalegranitic batholiths. In combination with the Indo-Sinian convergent tectonic setting in South Chinaas well as sparse outcrops of contemporary mafic igneous rocks, we consider that tectonic crustalthickening is likely to be the predominant factor controlling the formation of the Indo-Sinian per-aluminous and potassium-rich granitoids in Hunan Province.