A Preliminary Study On The Paleozoic Evolution Of The Nenjiang-Heihe Tectonic Belt In The Northern Part Of Great Xing’an Range

Northeast China, located in the eastern part of Xing’an Mongolian orogenic belt, is composed of the Erguna, Xing’an and Songnen massifs, from northwest to southeast,separating by the Xinlin-Xiguitu suture zone and the Hegenshan-Heihe suture zone,respectively. The previous study inferred that the Hegenshan-Heihe suture zone extend to the northern Great Xing’an Range. However, for the lack of evidence related to residual ocean crust, the collision position of Xing’an and Songnen massifs is always in debate. In this study, the petrography, chronology, and geochemistry studies on the Magmatic rock and volcanic rock exposed in northern Great Xing’an range have been carried out.Meanwhile, this study also combined with previous sedimentological studies to explore the evolution history of the Heihe-Nenjiang suture zone in Paleozoic.In early Paleozoic, there exist a northward subduction of the oceanic plate in the northern Great Xing’an range, and a large number of arc volcanic rocks are developed. The previous studies regarded the island arc volcanic rocks of Ordovician and the diorite-granodiorite assembly of island-type of Caledonian(456~475 Ma)in the Xing’an block as the sign of the beginning of the subduction of the Nenjiang-Heihe ocean. The geochronological data of the rhyolitic tuff in Duobaoshan formation and the ultrabasic rocks show that the zircon U-Pb ages are respectively as 491.1±5.3 Ma and 497.1±5.9Ma, and the geochemical characteristics indicate that the two rocks are both formed in the tectonic settings of subduction. While the gabbro from the Balin area with zircon U–Pbage of 426.7 ± 6.8 Ma, has similar Chondrite-normalized REE patterns curve with N-MORB. On the Hf/3 vs Th vs Nb/16 and Ti O2 vs Mn O*10 vs P2O5*10 diagrams,samples are all plotted in the island-arc tholeiite fields, which reflects a tectonic setting of subduction. We thus consider that since 497 Ma there existence the subduction of Nenjiang-Heihe ocean, and the subduction was still ongoing until 427 Ma.The results of zircon U-Pb age of the Nenjiang sodic amphibole-bearing mylonites and the Moguqi gabbros are respectively 352.4±2.3 Ma and 352.6±2.6 Ma, and in the Nenjiang sodic amphibole-bearing mylonites, the riebeckite was found, which shows that it might be influenced by subduction process, and its geochemical characteristics are also reflected this character. The Moguqi gabbros has similar Chondrite-normalized REE patterns curve with E-MORB, and it shows negative Nb、Ta、Zr、Hf anomalies on the PM-normalized plot. On the 2Nb vs Zr/2 vs Y diagram, samples are all plotted in both normal mid-ocean-ridge basalt and island-arc basalt fields, and on the Hf/3 vs Th vs Nb/16 diagram, samples are all plotted in the calc-alkaline basalt fields, which suggested that the magma source of the gabbros was related to volcanic arc, and the formation of the magma may be closely related to the subduction and the magma may be developed from the subduction of Oceanic crust to Xing’an block.The discovery of Nenjiang sodic amphibole-bearing mylonites and the Moguqi gabbros suggested that the paleo subduction zone has gone throughout both the Nenjiang and Moguqi areas. Although the typical ophiolitic melange and blueschist have not yet been found, the protolith of the sodic amphibole-bearing mylonites formed in the active continental margin environment, which might represent a subduction environment. The Nenjiang sodic amphibole-bearing mylonites and the Moguqi gabbros recorded the position between Xing’an and Songnen suture, and the collision was later than Early Carboniferous.In summary, the Paleozoic evolution of the Nenjiang-Heihe tectonic belt in the northern part of Greater Xing’an Range is summarized as follows: ~497 Ma, the Nenjiang-Heihe ocean, one of the branches of paleo-asia ocean, initially subducted. With the subduntion process, the Early Paleozoic arc volcanic belt were generated. Also, aseries of volcanic-magmatic events related to subduction(485~350Ma) happened(Balin gabbro, 427Ma; Nenjiang sodic amphibole-bearing mylonites, 352 Ma and the Moguqi gabbros, 352 Ma). ~320 Ma is the peak age of the Late Paleozoic tectonic-magmatic events in the area, which corresponds to the closure of Heihe-Nenjiang ocean. Meanwhile,Xing’an and Songnen massifs finished assembling.