Petrogenesis And Geodynamic Significance Of The Leucogranite In Northern Himalayan In Saga Region, Tibet

There are mainly two leucogranite belts in southern Tibet, which are the HighHimalaya leucogranites （HHL） and the Northern Himalaya leucogranites （NHL）. Thelatter distributes with two patterns, one is the independent intrusives, the other islocated in a series of gneiss domes. The results initiate and perform high-levelresearch on leucogranites in this area. However, the questions, such as the rock typesand formation mechanism, need to deeply understand again. This paper presents U-Pbisotope chronology, petrology and geochemistry of leucogranites from Qukangyi、Qiazuweng、Malashan dome in Saga, aiming to provide more answers for the solvingof this question.The main rock types we got in Saga are two-mica leucogranite andgarnet-bearing leucogranite. The result of zircon U-Pb dating indicates that the age ofleucogranite is from24.4±2.1to21.1±0.4Ma. They have varying major elementscontent, with SiO₂=71.47～75.54%, K2O+Na2O=6.91～8.81%, K2O/Na2O=0.91～1.12and A/CNK=1.10～1.27. The research results indicate that the lithochemistry ischaracterized by high silica and alkali, rich potassium, low calcium, low iron andmagnesium, peraluminous. However，they display wide variations of trace elementcomposition with Rb=193～406ppm, Sr=28.6～238ppm, Ba=38.6～524ppm,（La/Yb）N=0.62～19.82, δEu=0.11～0.80. Their∑REE contents are low and showslightly LREE enriched patterns and a obvious to weak negative Eu anomaly. In themean time, they are enriched in LILE and depleted in HFSE. The two leucogranitetypes are alike and yet different.The leucogranites from the NHL resemble the two-mica granite from the HHL.The author think that the High Himalayan para-metamorphic rocks are the sourcerocks of the NHL, which is mainly formed by muscovite dehydration melting ofmetapelite and metagraywacke. However, the two-mica leucogranite is formed bymuscovite and biotite. At the aspect of dynamic mechanism, NHL（27.5-10Ma） isformed in the initial extensional stage after the crustal tectonic regime had changedfrom compression to extension, and closely related to decompression of the base. To consider the leucogranites as the representative of the crustal anatexis, there is acrossfeed relationship between it and large-scale extensional deformation in southernTibet. They should be closely associated, and can’t separate.