Paleozoic Ophiolites And Tectonic Evolution Of The North Qinling

The giant Central Orogenic Belt (COB) of central China extends over 4000 km from West Kunlun on the west, through Altun, Qinlian, Qinling, and Dabie to Sulu on the east. This feature has played an important role in the formation and tectonic evolution of the Asian continent. Two major periods of deep subduction and collision have been identified thus far; one in the early Paleozoic and one in the Indosinian. The early Paleozoic North Qinling orogenic belt, the focus of this study, lies in the central part of the COB. It is a component of the Qinling orogenic belt, which is located between the Shangdan suture and the Luonan-Luanchuan-Fangcheng rupture. It extends more than 1000 km, in an E-W direction and is 80-150 km wide. The primary purpose of this paper is to describe the basic geology and tectonic framework of this important. The new discoveries and interpretation obtained by this study are given.(1) The Yinggezui ophiolite and Heihe mafic lavas, located in the east-west Qinling integration area, record the whole history of subduction of the early Paleozoic Qinling ocean.The Yinggezui ophiolite and Heihe mafic lavas, which crop out in the middle of the North Qinling orogen belt, preserve relatively complete sequence. The tectonic setting of Yinggezui ophiolite is similar to that of the Troodos ophiolite, which was formed in a fore-arc basin setting. LA-ICP-MS U-Pb dating of zircons from two gabbros yielded ages of 523.8±1.3 Ma for formation of oceanic lithosphere and 474.3±1.4 Ma for initiation of subduction. We suggest the 523.8±1.3 Ma gabbro marks the age of intra-ocean subduction. LA-ICP-MS U-Pb analyses of zircon from a diabase yielded an age of 454±2.5 Ma for the Heihe lavas, which is interpreted as the time of ocean-continent subduction.(2) The mafic to intermediate-silicic volcanic rocks that crop out in the Houchuan-Chenjiahe area of western Qinling represent a back-arc bimodal suite formed in the late Ordovician. LA-ICP-MS U-Pb analyses of zircon grains from the mafic lavas of Hongtubao in western Qinlng yielded an age 423±1.4 Ma, indicating that the Erlangping back-arc basin existed until Late Silurian.Mafic lavas in the Houchuan-Chenjiahe area of western Qinling occur as beds and lenses in the intermediate-silicic volcanic rocks. The mafic and intermediate-silicic volcanic rocks have different geochemical characteristics and were probably derived from totally unrelated sources. The intermediate-acidic volcanic rocks probably formed by anatexis of the crust. LA-ICP-MS U-Pb dating of zircons from Hongtubao mafic lavas yielded an age of 423±1.4Ma for initial opening of the back arc basin. It is inferred that the Erlangping back-arc volcanism was likely still in existence until Late Silurian.(3) The Erlangping back-arc basin developed in the Qinling Group, which formed the basement of North Qinling. It closed before the Late Silurian (～420Ma).Strontium and Nd isotopic compositions of the mafic lavas of the Erlangping back-arc basin indicate that a mixed source involving both Proto-Tethyan ocean crust and Qinling Group paragneiss. Thus, we infer that the Erlangping back-arc basin developed when Proto-Tethys was subducted northwards beneath the Qinling Group. However, more Paleo-Tethyan oceanic crust was consumed by subduction than was generated in the back-basin. The Erlangping back-arc basin closed before development of the Qinling ocean.(4) The early Paleozoic ocean basin in Qinling was subducted from the Middle Cambrian (～514 Ma) until the Late Devonian (～350 Ma). However, it is quite likely subduction in the west ended before that in the east because of oceanic ridge subduction.Based on newly published isotopic age data for the mafic lavas and gabbros, bounded by longitude 108°E, we have identified a likely bimodal subduction-related geochronology. There is a significant difference in timing of subduction between the eastern and western parts of the area, with subduction continuing for 25 m.y. longer in the east, probably due to different subduction mechanics in the two areas. Based on the available geochronological data for the Yinggezui ophiolite, and the known metamorphic and magmatic events, we suggest subduction of the Paleo-Tethyan ocean continued from ca.514 Ma until 350 Ma.(5) Subduction of an oceanic ridge occurred in the North Qinling belt in the early Paleozoic. The presence of this oceanic ridge suggests that there was a fairly wide ocean between the Yangtze block and North China Craton in the early Paleozoic.The early Paleozoic ophiolite and subduction-related volcanic melange are mainly exposed along the Shangdan suture in the North Qinling. The North Qinling orogenic belt is divided into an eastern and western part at longitude 108°E, called "the West Qinling" and "the East Qinling". The subduction-related rock assemblages in the West Qinling are composed of mafic lavas, boninites and adakitic rocks. The compositional characteristics of these mafic lavas are similar to those of the Chile Ridge basalts which reflect oceanic ridge subduction, suggesting that the rock assemblages in West Qinling may have formed in a subducted oceanic ridge-affected forearc basin setting. The rock assemblages in the East Qinling are predominantly composed of island-arc volcanic rocks similar to those formed in oceanic crust subduction-related settings.The existence of an oceanic ridge suggests that a fairly broad ocean was already established in North Qinling before the early Paleozoic, contrary to previous views that envisaged only a limited oceanic basin between the Yangtze block and the North China craton.Our study suggests that there were crucial differences in the history and tectonic evolution along the North Qinling in the Paleozoic because of different subduction dynamics between the eastern and western parts of the Qinling ocean. This new discovery provides strong evidence for the existence of a heterogeneous tectonic evolution in the region.