Crust Anatexis in Collisional Orogenic Belt: Constraints from Natural Observations and Experimental Investigation on Nanogranite Inclusions

Crustal anatexis during the tectonic evolution of collisional orogenic belts has a fundamental impact on physical properties and chemical compositions of the deep crust. As the direct witness to crustal anatexis, nanogranite inclusions, which formed during partial melting of hosted crustal rocks, are key to determine the compositions of the natural melts as well as melting mechanism. Nanogranite inclusions within garnet and zircon were identified in pelitic gneisses and felsic gneisses from the Namche Barwa complex of the eastern Himalayan syntaxis. They have a typical granitic mineral assemblage of K-feldspar, plagioclase, and quartz with or without biotite. These minerals represent the former melt as a result of dehydration melting of biotite in gneisses, which is captured by peritectic minerals (like garnet). Homogenization experiments on the nanogranite inclusions were conducted under high temperature and pressure and high temperature and room pressure conditions in order to obtain homogenized glasses. Chemical analyses show that these homogenized glasses are dominated by peraluminous granites. The major-and trace-element compositions of these glasses can be used to trace the melting processes of the host rocks. A combination of natural observation and experimental investigation of nanogranite inclusions from collisional orogenic belts is crucial to gain further insights into the crustal anatexis and associated melt compositions.