Petrogenesis Of The High Himalayan Leucogranites And Geodynamic Significance

The presence of felsic continents makes Earth a unique planet in the solar system.Understanding the petrogenesis of felsic rocks is therefore crucial for determining how our planet evolved.The High Himalayan leucogranites formed by partial melting of continental crustal materials during the collision of India with Asia are ideals to unraveling the formation of felsic continental crust.The debate on the petrogenesis of the High Himalayan leucogranites is still ongoing.Some studies suggest that the High Himalayan leucogranites are formed by dehydration melting.However,more and more evidence shows that the High Himalayan leucogranites can be formed with the presence of free water,which is referred to“water-fluxed melting”.Therefore,the debate focuses on whether or not free water plays a key role in the anatexis of continental crust.This study compiled available data for the High Himalayan leucogranites.Our detailed geochemical analyses demonstrated that both dehydration and water-fluxed melting reactions should have played a key role in formation of the High Himalayan leucogranites.The consumption of plagioclase is greater than muscovite in water-fluxed melting,which causes that the leucogranites are characterized by high Sr and low Rb and Rb/Sr ratios.In contrast,dehydration melting reactions are mainly involved the breakdown of muscovite,which predicts high Rb/Sr and Nb/Ta ratios in leucogranites.Furthermore,the peritectic K-feldspar and biotite in dehydration melting would further fractionate Rb/Sr but dilute the Nb/Ta ratios in derived leucogranites.The dissolution of accessory minerals is other factors to affect the geochemical characteristics of the leucogranites.The water-fluxed melting-derived leucogranites display high Zr,?LREE and Zr/Hf and Th/U ratios,as a result of dissolution of both zircon and monazite.Moreover,the dissolution of zircon and monazite will dilute the expected positive Eu anomaly caused by melting of plagioclase.The dissolution of apatite during dehydration melting may contribute to the high P₂O₅content feature of the dehydration melting formed leucogranites.This study further estimated melts water contents by combining the thermodynamic simulation of zircon crystallization and Ti-in-zircon thermometry.The simulation results show that the crystallization of zircon is mainly controlled by the water content of the primary melt.Furthermore,the High Himalayan leucogranite can be divided into two groups:with the presence of free water,water contents of primary melts are high and commonly close to 5 wt.%,whereas the primary melt water contents of the other group do not exceed 1wt.%.This study also applied thermodynamic simulation to estimate the phase diagrams of melting reactions.The phase diagrams show that the melt generated by water-fluxed melting has a lower melting temperature,and a faster rate of generation than those formed by dehydration melting.The derivative phase relationship shows that plagioclase preferentially enters into melts rather than muscovite in water-fluxed melting reaction,which are consistent with the observed geochemical characteristics of natural leucogranites.Furthermore,the calculated melt compositions are comparable with those of the natural High Himalayan leucogranites.Therefore,the correlation between the physical properties of melts and the geochemical characteristics of the leucogranites in different melting reactions further indicates that the High Himalayan leucogranites can be both formed by dehydration melting and water-fluxed melting.Petrogenesis of the High Himalayan leucogranite demonstrates the widespread presence of free water in continental crust.The free water is mainly locked in hydrous minerals or from fast wet upwelling driven by released volatile from subduction oceanic slabs in deep mantle.The Main Central Thrust may provide a migration channel for free water,which then induced the anatexis of continental crust.The presence of free water in an orogenic belt may trigger exhumation of high-pressure metamorphic rocks,which will enhance the deep water cycling within the system.This study therefore concludes that free water is crucial for the evolution of continental crust and orogenic belts.