Petrogenesis And Tectonic Implications Of Leucogranite In Luozha-cuona Area,Southern Tibet

In the early Cenozoic,the Himalayan orogenic belt was formed by the Continental collision between the Indian plate and the Eurasian plate.Along with this collision and orogeny,the Himalayan area underwent extensive crustal anatexis and formed the Himalayan leucogranite.The leucogranite is mainly distributed in parallel with the high Himalayan leucogranite belt?HHG?along the northern Himalaya?Tethys Himalaya?leucogranite belt?NHGD?.The leucogranite in the Minjiuma and Kuju are distributed in the northern Himalayan leucogranite belt.The geotectonics is located in the Himalayan block in the Middle East of the Tethys Himalayan orogenic belt,It is adjacent to the south Tibetan detachment system?STDS?in the south,and the yarlung zangbo suture zone?YZSZ?is developed in the north.Based on the analysis of the geochemical and isotopic characteristics of granites,this paper discusses the genesis,magma source area,tectonic environment and tectonic significance of granites.The main lithology of Minjiuma leucogranite is muscovite monzogranite,tourmaline monzogranite and monzogranite;the main lithology of Kuju leucogranite is monzogranite,muscovite monzogranite,garnet muscovite granite and garnet Muscovite monzogranite.Both of them are characterized by high Si O2,rich Al2O3 and K2O,and poor Cao,Mg O and Ti O₂.They are peraluminous rocks of high potassium calc alkaline series,with high degree of crystallization differentiation of magma,indicating that the magma source is crust.The total content of REE in the two leucogranite is relatively low,which is obviously lower than the average abundance of acid rocks in the world,with the characteristics of light REE enrichment.The?Eu of minjiuma leucogranite is0.32?0.41,and the?Eu of kuju leucogranite is 0.38?0.74,which reflects that both of the two leucogranite have obvious negative Eu anomaly and strong magmatic crystallization differentiation.The curve in the spider web map of the original mantle normalized ratio of trace elements generally tilts to the right,the large ion lithophile elements Rb,K and the high field strength elements Th are relatively rich,and the large ion lithophile elements Sr and the high field strength elements Ti present low-lying negative anomalies.At the same time,most of the high field strength elements Hf have no negative anomaly,and the rare earth and trace elements reflect that there may be plagioclase residues or crystal differentiation of plagioclase in the source area.The zircon U-Pb ages of Minjiuma leucogranite are 22±1Ma and 23±0.7Ma.The zircon U-Pb ages of Kuju leucogranite are 21±0.3Ma,It reflects that the formation time is Miocene.It belongs to the new Himalayan stage?26ma?13Ma?,which coincides with the active age of STDS.It was formed under the background of extensional thinning after the start of STDS,and it is the product of partial melting of mudstone in the crust during the collision of Indian plate and Eurasia continent.It shows that the activity of STDS promoted the rapid uplift of the High Himalaya region and produced leucogranite,which makes the transition in Himalaya region from the compression background to the extensional collapse background.