Geochemical and Boron Isotopic Evidence that Tourmaline Records Country Rock Assimilation of Leucogranites in the Himalayan Orogen

Tourmaline geochemical and boron(B)isotopic compositions in two-mica granites(TMG),tourmaline-bearing leucogranites(Tou-LG),tourmalites and metapelites from the Gyirong-Malashan areas of the Himalayan orogen provide evidence for country rock assimilation during the intrusion of Himalayan leucogranite.The schorls in Gyirong leucogranitic plutons show low contents of MgO(0.238%-1.160%)and δ^(11)B values(-12.1‰--11.2‰),while dravites gathered in the contact zone between the leucogranitic veins and metapelites show high contents of MgO(4.815%-6.755%)and δ^(11)B values(-10.7‰--9.3‰).This geochemical and isotopic variation of tourmalines can also be identified in the Malashan gneiss dome.As a result,three types of tourmaline were identified in the Himalayan orogen:(1)Tou-Ⅰ in the TMG and Tou-LG,which is the most common tourmaline type of schorl;(2)Tou-Ⅱ(dravite and high-Mg schorl)in the Tou-LG and tourmalite at the margins of the leucogranite;and(3)Tou-Ⅲ(mainly dravite,with minor high-Mg schorl)in metapelites of the High Himalayan Crystalline Sequence.The lenses and veins of Tou-LG may have experienced metasomatism and assimilation as a result of interaction with the High Himalayan Crystalline Sequence metasedimentary country rocks,which can be traced by the geochemical and isotopic characteristics of the tourmaline therein.