Late Paleozoic Magmatism And Its Tectonic Significance In The Northern Great Xing'an Range, Northeastern China

The Great Xing'an Range, located within the eastern part of the Central Asian Orogenic Belt and between Xar Moron River-Yanji suture and Mongolia-Okhotsk suture, is composed of Ergun, Xing'an and Songnen blocks from north to south. Recent studies show that the history of the Central Asian Orogenic Belt is closely related to the closing of Paleo-Asian Ocean. However, the Late Paleozoic tectonic development of the northern Great Xing'an Range has long been a subject of controversy, including rift valley or aulacogen, the northern continental margin of Jiameng block and arc-basin system, etc. Until now, only a few detailed chronological and geochemical data were available and thus the tectonic setting of magmatic rocks and marine sediments was poorly constrained. In this paper, we condected petrology, U-Pb chronology and geochemistry on representative Late Paleozoic magmatic rocks from the eastern Great Xing'an Range, meanwhile, performed U-Pb chronological analyses on detrital zircon obtained from six sedimentary rock samples.The results shed light on the tectonic evolution of Late Palaeozoic in the eastern Great Xing'an Range.Late Demonian volcanic rocks, along Yakeshi-Xinglong belt in the northern Great Xing'an Range, extend as a long strip in NE direction. The volcanic rocks in Yakeshi area consist of calc-alkaline series basalt, andesite, dacite, rhyolite and tuff. Zircon U-Pb istopic dating reveals that the basalt and tuff were formed at 373.2±5.3Ma and 383.1±2.1Ma respectively. The rocks are rich in LILE, LREE, and poor in HFSE, and have the subduction zone volcanic geochemistry characteristics. The zirconεHf(t) values of basalt are within a range from+14.67 to +18.67, which are similar to the Hf isotope features of mid-oceanic ridge basalt and modern subduction zone basalt. These characteristics suggest that the basalts were derived probably from a depeleted mantle wedge modified by predominant slab fluids and formed in an active continental margin.Early Carboniferous magmatic rocks also extend as a long strip in NE direction in study area and present Nengjiang-Arongqi and Wuerqihan-Tahe magmatic rock zones. The volcanic rocks in Nengjiang area, belonged to Nengjiang-Arongqi magmatic rock zone, mainly consist of calc-alkalic-high-K calc-alkalic series medium acidic lava and tuff, and grow together with marine sediments, have been proven to be formed during Early Carboniferous(355~352Ma) based on zircon U-Pb dating. Arongqiyinhe rock mass consists of calc-alkalic cumulate gabbro and quartzdiorite. Quartzdiorites yield age of 351.4±1.4Ma, enrichment in LILE, LREE and depletion in HFSE, and similar to the geochemistry characteristics of active continental margin granite. Wuerqihan rock mass, belonged to Wuerqihan-Tahe magmatic rock zones, mainly consists of granodiorite.The U-Pb age indicates that the pluton was emplaced at 331.2±3.7Ma.The rocks show adakitic geochemical features, marked with high Sr, Sr/Yr ratios, enrichment in LREE and strong depletion in HREE, Y and HFSE, either with no obvious negative or positive Eu anomaly. The positiveεHf(t) values(+12.78~+14.54) indicate that the granodiorites were derived from an underplated, basalt-andesite crust under the early-stage island arc. Combined with sedimentary characteristics, we think the formation of Arongqi-Nengjiang magmatic rocks were probably related to the underthrusting of oceanic slab, and the formation of Wuerqihan granite body was related to the upwelling of the under asthenosphere of early-stage thicken island arc area and the mantle-derived magma underplating.Late Carboniferous magmatic rocks present surface shape in the northern Great Xing'an Range. The volcanic rocks, situated in the northwestern Lesser Xing'an Range, consist plenty of high-K calc-alkalic-shoshonite series medium and acidic lava and tuff whose zircon U-Pb ages are from 307 Ma to 306Ma, and grow together with continental sediments. The granites are manily of high-K calc-alkalic series, including a few of shoshonite series, widely distribut in the northern Great Xing'an Range, have been proven to be emplaced at 320~300Ma. The geochemistry and regional stratigraphy demonstrates that these granites and studied volcanic rocks were probably formed in the post-collision environment.The detrital zircon chronology of Devonian sedimentary rocks in the northern Great Xing'an Range shows that detrital zircons from sandstone of Niqiuhe Formation in Handaqi area yield four age intervals: 382~407Ma(n=42), 440~508Ma(n=14), 820~939Ma(n=7) and 1295~2447Ma(n=5). Zircons from slate of Daminshan Formation in Gentouhelinchang are yield age populations of 361~383Ma(n=31) and 432~527Ma(n=6). The smallest peak age values of two samples are 384Ma(n=16) and 373Ma(n=31) respectively, identical to those of the Late Devonian arc magmas from the northern Great Xing'an Range.These age date indicate that rocks from the Niqiuhe Formation and Daminshan Formation were deposited after 384Ma and 373Ma, and these rocks were sourced mainly from the Late Devonian volcanic arc, including a few of Early Paleozoic magmatic rocks and partial naked Precambrian continental crust.The clasolite samples of late Devonian probably represented forearc sediments.The detrital zircon chronology of sedimentary rocks from Early Carboniferous Hongshuiquan Formation and Honghutuhe Formation in the northern Great Xing'an Range shows that zircons from sandstone of Hongshuiquan Formation in Erguna area yield four age intervals: 353~379Ma(n=16),428~449Ma(n=14),492~514Ma(n=9) and 1235~2594Ma(n=13). Zircons from sandstone of Hongshuiquan Formation in Tenihesumu area yield four age populations of 353~379Ma(n=16),428~449Ma(n=14), 492~514Ma(n=9) and 1235~2594Ma(n=13). Zircons from sandstone of Hongshuiquan Formation in Gentouhelingchang area with age results between 456 Ma and 1876Ma(n=23). Zircons from phyllite of Honghutuhe Formation in Heihe area are age dated between 334 and 393Ma(n=24), with the peak age values 353Ma and 385Ma. These age date suggest that deposition of four samples had started by 364Ma, 339Ma, 366Ma and 353Ma respectively. In Ergun area, zircons show several modal age peaks,and in Hehei area, zircons show a unimodal age peak. Associated with sedimentary characteristics, we believe that Hongshuiquan Formation may represented backarc and interarc basin sediments, Honghutuhe Formation represented forearc sediments.In summary, the Late Paleozoic tectonic evolution in the northern Great Xing'an Range is as follows. In the Late Devonian, a north-dipping subduction zone induced the development of the Yakeshi-Xinglong arc volcanic rock belt and forearc sequence. In the Early Carboniferous, the suducted slab migrated laterally and thus the magmatic arc zone moved from Yakeshi-Xinglong to Arongqi-Heihe belt, a wide magmatic rock zone and back-arc, interarc and forearc basins were formed in this process. The collision between the Ergun-Xing'an block and Songnen block took place in the late of Early Carboniferous along the Nenjing-Hegenshan suture zone.Then post-collisional magmatic rocks were widely developed, while the sedimentary strata recorded continental deposit during the Late Carboniferous. In the Early-Middle Permian, the occurrence of A-type granites means that the northern Great Xing'an Range entered the extensional stage of post-orogeny. Our studies also show that the late Paleozoic is also an important period in continental crust growth in the northern Daxinganling, and the accretion mechanism includes the lateral related to subduction and the vertical related to underplating of mantle-derived magma.