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

Abstract

AbstractTourmaline 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 δ11B 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 δ11B 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.