Subduction Record Of Mongol-okhotsk Ocean: Constrains From Badaguan Metamorphic Complexes In The Erguna Massif, NE China

The basement rocks in parts of NE China constitute a khondalitic sequence ofsillimanite-and garnet-bearing gneisses, hornblende-plagioclase gneiss and various felsicparagneisses. Zircon U–Pb dating of garnet-sillimanite gneiss samples from the Erguna,Xing’an, Jiamusi and Khanka blocks all indicate that high-grade metamorphism occurredat~500Ma. Evidence from detrital zircons in Paleozoic sediments from the SongliaoBlock also indicates the former presence of a~500Ma component.LA-ICP MS zircon U-Pb dating and geochemical data are reported from theBadaguan complex at the western margin of the Erguna massif in NE China, with the aimof constraining formation time and the regional tectonic setting. The Badaguan complex,which was originally defined as the Jiageda Formation, is mainly composed ofbiotite-bearing amphibolitic gneiss and granitic mylonite. The zircon LA-ICP-MS U-Pbanalysis shows that the zircons are of typical magmatic origin with oscillatory zoning,higher concentrations of Th, U, and REE. Th/U ratios of zircons are between0.13and1.57. Chondrite-normalized REE were loss of light rare earth elements and significantenrichment of heavy rare earth elements. Zircons have a strong positive Ce anomaly andnegative Eu anomalies strongly, which show that these zircons are magmatic in origin.The dating results indicate that the protolith ages of the two biotite-bearingamphibolitic gneisses are210±2Ma and214±2Ma, respectively. The protolith age of the granitic mylonite yielded weight mean age of203±3Ma, existing~501Ma and~795Macapture zircon ages. Thus the formation age of the Badaguan complex is at Late Triassicrather than Late Proterozoic as previously considered. Furthermore, the capture zirconages display that the same tectonic evolution history between the Erguna massif and otherNE massifs. Geochemical studies have shown that the Badaguan complex, characterizedby high Na2O, Al2O3, A/CNK=0.86~1.05, A/NK=1.53~1.97, suggesting aluminum to theweakly peraluminous calc-alkaline series, enrichment in light REE and depletion in heavyREE with （La/Yb）Nratios of6~31. They have weakly negative Eu abnormality（Eu/Eu*=0.50~1.01）, high Sr content （378×10-6~598×10-6） and low Yb content（0.71×10-6~3.50×10-6）. Their primitive mantle normalized trace element spider diagramsshow they are enrich in large ion lithophile element Rb、Ba、K and Sr, and are stronglydepleted in high field-strength element Nb、Ta、P、Ce and Ti. This geochemical dataindicates that the Badaguan complex formed in an active continental margin or island arcsetting. We suggest that the tectonic setting of the Badaguan complex wasMongol-Okhotsk ocean subduction beneath the Erguna massif, which provides keyevidence for the subduction of Mongol-Okhotsk ocean at Triassic.