The poorly studied Douling Complex is a crystalline basement that developed in the Neoproterozoic-Paleozoic weakly metamorphosed to non-metamorphosed strata at the South Qinling tectonic belt.Five banded dioritic-granitic gneiss samples from the Douling Complex were chosen for LA-MC-ICPMS U-Pb zircon dating,which yielded protolith emplacement ages of 2469±22Ma,2479±12 Ma,2497±21 Ma,2501±17 Ma and 2509±14 Ma,respectively.An important peak age of^2.48 Ga was also obtained for a metasedimentary rock in the same region.These discoveries suggest the occurrence of magmatic activity of2.51–2.47 Ga at the northern margin of the Yangtze craton.The age-correctedεHf(t)values obtained from in situ zircon Hf isotopic analysis are mainly between 5.5 and+0.3,and the two-stage zircon Hf model ages range from 3.30 to 2.95 Ga.Considering two important periods of^3.3–3.2 Ga and^2.95–2.90 Ga for the continental crustal growth in the Yangtze craton,we infer that the dioritic-granitic gneisses from the Douling Complex are the products of reworking of Paleo-to Mesoarchean crust at the northern margin of the Yangtze craton at^2.5 Ga.In addition,metamorphic ages of 837±8 Ma and 818±10 Ma were obtained for zircon overgrowth rims from a dioritic gneiss and a metasedimentary rock,indicating that the main phase amphibolite facies metamorphism of the Doulng Complex occurred during the Neoproterozoic,although its geological meaning remains ambiguous.
HU JuanLIU XiaoChunCHEN LongYaoQU WeiLI HuaiKunGENG JianZhen
The Tongbai-Hong'an orogen is located in a key tectonic position linking the Qinling orogen to the west and the Dabie-Sulu orogen to the east. Because the orogen preserves a Paleozoic accretionary orogenic system in the north and a latest PaleozoicMesozoic collisional orogenic system in the south, it may serve as an ideal place to study the tectonic evolution between the North and South China Blocks. The available literature data in the past 20 years indicate that the tectonic processes of the Tongbai-Hong'an orogen involved four stages during the Phanerozoic:(1) Early Paleozoic(490–420 Ma) oceanic subduction, arc magmatism and arc-continent collision created a new Andean-type active continental margin on the North China Block;(2) Late Paleozoic(340–310 Ma) oceanic subduction and accretion generated separated paired metamorphic belts: a medium P/T Wuguan-Guishan complex belt in the south of the Shandan-Songpa fault and a high P/T Xiongdian eclogite belt in the northern edge of the Mesozoic HP metamorphic terrane;(3) Latest Paleozoic-Early Mesozoic(255–200 Ma) continental subduction and collision formed the Tongbai HP terrane in the west and the Hong'an HP/UHP terrane in the east as a consequence of deep subduction towards the east and syn-subduction detachment/exhumation of the down-going slab;(4) Late Mesozoic(140–120 Ma) extension, voluminous magma intrusion and tectonic extrusion led to the final exhumation of the Tongbai-Hong'an-Dabie HP/UHP terrane and the wedge-shaped architecture of the terrane narrowing towards the west. However, many open questions still remain about the details of each evolutionary stage and earlier history of the orogen. Besides an extensive study directly on the Tongbai-Hong'an orogen in the future, integrated investigation on the "soft-collisional" Qinling orogen in the west and the "hard-collisional" Dabie-Sulu orogen in the east is required to establish a general tectonic model for the whole Qinling-TongbaiHong'an-Dabie-Sulu orogenic belt.
