The Fate Of Subducted Oceanic Crust

Two subducted palaeo-oceanic crusts within the northwestern Tibetan region are selected, a case study, to investigate the deep geological processes of subducted oceanic crust in this study.Occurring in the Longmu Co-Shuanghu suture zone, Gemuri eclogite previously regarded as a consequence of cold subduction of oceanic crust. The mineral assemblages of the Gemuri eclogite can be divided into four stages. The first stage consists of Fe³⁺-bearing Ca-Na amphibole, paragonite, albite, chlorite, quartz, allanite enriched in LREE, Th and U, sphene and Mn-rich garnet core. The mineral assemblage of the second stage is composed of Mg-rich garnet, omphicite, clinozoisite, quartz, paragonite, rutile and pyrite. The third stage includes the Fe-rich garnet occurring in the rims of garnet porphyroblast, albite, chlorite, sphene and actinolite. The last stage is composed of magnetite, trace-element-free epidote and needlelike ilmenite. The semiquantitatively thermodynamic modeling results have revealed that the core garnet was formed at T=642℃ P=13.3 kbar, log₁₀fO₂ =-16.9^-17.1, while the mantle garnet was formed at T=695 ℃, P=16.6 kbar, log₁₀fO₂ =-19.0^-19.5, indicating that the Gemuri eclogite is an intermediate-temperature eclogite. The Gemuri eclogite has suffered reduction in the prograde stage, releasing water, REE, Th, U, and Ba in the subduction channel.,as well as transferring into the overlying mantle wedge along with the fluids. This highlights the origin of island-arc magmatic rocks with unique arc geochemical characteristics.Garnet grains in the garnet granulites in the vicinity of Dong Tso area, western Bangong-Nujiang suture zone, exhibited porphyroblasts with heterogenous compositional zonation. The garnet core is more homogeneous with higher Ca and Mg contents compared with the garnet rim. Early fine-grained garnets and amphiboles were found as inclusions in large diopside grains. All amphiboles are calcic amphiboles. The early amphiboles are distinctive from the late amphiboles by the 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 and melt pockets dominated by albite. Inclusions/exsolutions composed of Ba-rich hyalophanes are widely distributed in the melt pockets. Therefore the mineral assemblage of prograde stage consists of early garnet（Grt₁）, early amphibole（Amp₁） and Al-rich sphene（Spn₁）. The peak metamorphic stage is documented by garnet core（Grt₂） + diopside + plagioclase（Pl₁） + quartz + rutile + Na-rich and Ba-rich melts, which formed at 870℃, 11.7kbar, acquired using traditional geothermobarometers. This clearly suggests that Dong Tso garnet granulites are products of warm subduction of oceanic crust, providing a line of metamorphic evidence to support the argument that “the Bangong-Nujiang Ocean contains numerous sub-oceanic basins”. The Fe³⁺-bearing amphiboles in the garnet granulites were redoxed during dehydration, generating adakite-like magmas with high oxygen fugacity.The case studies present here has clearly illustrated that at deep subduction zone, subducted oceanic crusts experienced prograde metamorphism and reduction. This work highlights the processes responsible to the mass-transferring from the downgoing slabs to the overlying mantle wedge, driven by the prograde metamorphism and reduction together.