Separating the Qiangtang terrane in the north and the Lhasa block in the south, the Bangong-Nujiang suture zone is an important tectonic boundary within Central Tibet. However, subduction of the Bangong-Nujiang oceanic crust remains obscure. Here we report several garnet granulites in the vicinity of Dong Tso area, western Bangong-Nujiang suture zone, contributing to understand the tectonic evolution of Bangong-Nujiang suture zone. The garnet granulites as lenses in size of 5. 0 similar to 10mm are distinguished in amphibolites, which occur as tectonic slices in the Bangong-Nujiang suture zone and structurally contacting with ultramafic and mafic rocks. The garnet grains within garnet granulites present as porphyroblasts with irregularly compositional zonation. The composition of garnet core is more homogeneous and characterized by higher Ca and Mg but lower Fe and Mn than the garnet rim. Moreover the rims and cracks of garnet show paragenetic relationship with fine veins composed of chlorite and clinozoisite indicating that the garnet grains suffered metasomatism by later fluids. Fine-grained early garnets and amphiboles were found as inclusions in diopside. These garnets are smaller than 10 pan and characterized by higher Fe and Mn but lower Ca and Mg than the garnet core. All the amphiboles are calcic amphiboles. The early amphiboles are different from the late amphiboles by their enrichment in Na, Fe3+, as well as trace amount of Ba. The early amphiboles normally as inclusions in garnet and diopside porphyroblasts have been transformed into diopsides (rich in Ca but poor in Fe3+ and Na) and melt pockets dominanted by albite. Inclusions/exsolutions composed of plagioclases and Ba-rich hyalophanes are widely distributed in the melt pockets. The late amphiboles occur as fine-grained symplectite with plagioclases surrounding the rim of garnet or present as porphyroblast in matrix, suggesting that they are retrograde products of garnet granulites. In addition, sphenes (rich in Al) are found in the matrix and rutile grains have been replaced by fine and irregular sphene (poor in Al) and ilmenite. Therefore the mineral assemblage of prograde stage is consisting of early garnet (Grt,), early amphibole (Amp,), Al-rich sphene (Spn,) and so on. The peak metamorphic stage is documented by garnet core (Grt(2)) + diopside + plagioclase (Pl(1),) + quartz + rutile + Na-rich and Ba-rich melts, which formed at 870 degrees C, 11. 7kbar on the basis of traditional geothermobarometer. Garnet rim/crack (Grt) + late amphibole (Amp(2)) + clinozoisite + chlorite + plagioclase (Pl(2)) + white mica + Al-poor sphene (Spn(2)) + ilmenite are regarded as the retrograde stage assemblage. In combination with petrographic and geochemical investigations and estimation of metamorphic P-T condition indicates that Dong Tso garnet granulites are products of warm subduction of oceanic crust, providing petrological evidence for the view that 'the Bangong-Nujiang oceanic basin contains many sub-oceanic basin '. The significant decrease of Fe3+ (a. p. f. u) in amphibole and marked increase of almandine from peak to retrograde stage reveals that the Dong Tso garnet granulites suffered reduction reaction and the amphiboles were redoxed during dehydration, generating adakite magma with high oxygen fugacity which promotes the activation from subducted warm oceanic crust and migration to arc magma of ore-forming elements, such as Cu and Au. Therefore, our work provides a case study of the chemical interactions and material migration between subducted warm oceanic slab and overlying subarc mantle wedge, and also contributes to intensively understanding the formation of numerous porphyry-type copper and gold deposits within the Bangong-Nujiang metallogenic belt.
