The Dur’ngoi ophiolite in East Kunlun, northern Qinghai-Tibet Plateau: a fragment of paleo-Tethyan oceanic crust

Abstract:The Dur’ngoi ophiolite in the north Qinghai-Tibetan Plateau consists of meta-peridotite, mafic-ultramafic cumulate, sheeted dikes and basaltic lavas. The meta-peridotite consists of dunite, harzburgite, lherzolite, feldspar-bearing lherzolite and garnet-bearing lherzolite and contains residual spinel with Cr#s [100*Cr/(Cr+Al)] of 30-57 and Mg#s [100*Mg/(Mg+Fe2+)] of 50-75, indicating an Al- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg#s of 89.2-92.6, Al2O3 contents of 1%-4 % and slightly depleted REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the lherzolite are andradite enriched in Ca and Fe and depleted in Mg and Al (And 95-97, Pyr 0.27-5.06,Gro 0-2.62), indicating that they formed by metamorphism. Cumulates consist mainly of dunite, wehrlite, pyroxenite and gabbro. The dunite contains minor feldspar, mostly replaced by chlorite and clay minerals. The wehrlite consists of olivine, diopside and minor plagioclase. A well-layered gabbro-pyroxenite complex is defined by variations in modal plagioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotite. Garnets in the cumulates are grossular (Gro 69.19-89.93；And 9.12-18.84；Br 0.73-11.63), formed by metamorphism. Diabase dikes are depleted in LREE with (La/Sm)N ratios of 0.49-0.75 and have flat HREE patterns with positive Eu anomalies (δEu =1.14-1.27). Basalts have REE patterns similar to those of MORB with (La/Sm)N ratios of 0.46-0.95 and small negative Eu anomalies. The basalt compositions, particularly their consistent REE patterns, suggest no contamination in their magma source and little differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basalts yielded 206Pb/238U ages of 276-319 Ma (average 308.2±4.9 Ma). The Dur’ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust formed at a fast-spreading mid-ocean ridge. Intense serpentinization and carbonization reflect hydrothermal alteration on the seafloor associated with formation of the Dur’ngoi Cu-Co-Zn massive sulfide deposit. The ophiolite was probably emplaced during closure of the paleo-Tethyan ocean basin and is believed to mark the suture between Gondwana and Eurasia.