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Two contrasting HP/LT and UHP metamorphic belts:Constraint on Early Paleozoic orogeny in Qilian-Altun orogen
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2010
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EngineeringPrecambrian GeologyTectonic EvolutionEarth ScienceMetamorphic ProcessMetamorphic PetrologyQilian-altun OrogenIgneous PetrogenesisEast Asian LanguagesGeologyUhp Metamorphic BeltsHigh Pressure GranuliteTectonics-North AltunNorth QilianEconomic GeologyEarly Paleozoic OrogenyEarth SciencesGeochemistryOrogenyPetrology
North Qilian(NQL)-North Altun(NAT)HP/LT metamorphic and North Qaidam(NQD)-South Altun(SAT)UHP metamorphic are located in the northern and southern sides of Qilian-Altun orogen,respectively.NQL-NAT HP/LT metamorphic mainly consists of blueschist,low-temperature eclogite and high pressure metasedimentary rocks.Eclogites were formed under the metamorphic conditions of 420~570℃ and 2.1~2.6GPa,and at 510~440Ma.The lawsonite-bearing eclogites and carpholite-bearing high pressure metasdimentary rocks indicate the existence of cold subduction zone gradient and cold subduction brought a large amount of H2O to the deep mantle in the Early Palaeozoic times.HP/LT metamorphic is associated with ophiolite,subduction-accretion complex,island arc and back-arc basin,suggesting that the NQL is a typical Early Paleozoic acccretion orogenic belt.In contrast,the NQD-SAT UHP metamorphic is composed of eclogite,garnet peridotite,high pressure granulite and continental orogneiss and paragneiss.These rocks were subjected to UHP metamorphism at T700℃ and P2.7GPa.Geochronology suggests that the UHP metamorphism occurred at 500 to 420Ma,and the protolith age of eclogite at 850~750Ma,related to a Neoproterozic continental rift origin.Field relationship,petrology and geochronology suggest that the NQD-SAT UHP metamorphic was resulted from the deep subduction of continental crust rocks.In the NQD-SAT UHP metamorphic belt,UHP eclogite and HP granulite produced in different thermal environments at same time,suggesting a paired metamorphic belt related to continental subduction and collision,and also implying that the NQD-SAT is a typical Early Paleozoic collision orogen.These data indicate accretion orogeny and collision orogeny developed synchronously in two side of Qilian-Altun orogenic belt,and formed a composite orogen as a result of different type of orogeny.HP/LT metamorphic and HP/UHP metamorphic control the orogenic type and mechanism of Qilian-Altun orogen in Early Paleozoic time.