Petrogenesis of deformed tourmaline leucogranite in the Gurla Mandhata metamorphic core complex, Southwestern Tibet

Abstract

The Himalayan leucogranites, which were derived from crustal anatexis during the India-Asia continental collision, have recorded the tectono-magmatic evolution process of the Himalayan orogenic belt. In the Gurla Mandhata area of the western Himalaya, tourmaline leucogranites accompanied by variable deformation are observed along the Gurla Mandhata detachment fault. Whole-rock compositions show that these leucogranites have high SiO2, alkaline elements and low CaO, MgO, FeO, TiO2 contents, and are strongly peraluminous with A/CNK value of 1.13–1.27. For trace elements, these leucogranites are characterized by low Ba Sr, Nb, Nd concentrations and relatively high Rb, U concentrations with significant negative Eu anomalies. Zircon UPb dating on the tourmaline leucogranites reveals a crystallization age of 11–12 Ma. Field observations, mineral assemblage, and geochemical features indicate that the Gurla Mandhata tourmaline leucogranites experienced high-degree fractional crystallization, which mainly induced by the long-distance migration from generation to emplacement, and the enrichment of volatile component (B, F, Cl, and H2O) facilitates the transportation ability of the leucogranite magma. Two types of tourmaline, Tur I and Tur II, are identified in the tourmaline leucogranites, enclosing relationships and mineral compositions suggest that these tourmalines should have formed at late and early stage in terms of magma evolution, respectively. The occurrence of early-stage tourmaline suggests that the primitive magma of the Gurla Mandhata tourmaline leucogranites is enriched in boron and H2O, which is most likely derived from fluid-flux melting of metasedimentary rocks from the Greater Himalayan Crystalline. The existence of the inherited zircon with ages of Paleoproterozoic and Neoproterozoic in combination with previous whole-rock SrNd isotopic study implies that the source region of the Gurla Mandhata tourmaline leucogranites is a two-component mixture between the Greater Himalayan Crystalline and the Lesser Himalayan Sequence.