Tectonothermal History Of High Pressure Granulite Unit In Dulan Area, The North Qaidam Mountains:Implications For Paleozoic Collisional Orogeny

The North Qaidam Mountains, which is located at the northern margin of Tibet Plateau, is a typical ultra-high pressure metamorphic belt related to continent deep subduction, characterized by the occurrence of eclogite, garnet peridotite and related gneiss. On the basis of field work and large-scale mapping, a small high-pressure (HP) granulite unit was recognized in the Dulan area, which is presumed to be in faulted contact with the eclogite-bearing unit. The HP granulite unit comprises HP granulite, granitic gneiss, pelitic schist (gneiss), garnet amphibolite and tonalite. The HP granulite, which occurs as lense or ribbon in garnet-bearing mica-schist and gneiss, varies from ultramafic, mafic, to intermediate and acid in composition. Field relationship shows that the intermediate and acid (felsic) granulite was derived from the leucosome of partial melting, and the mafic granulite was the residual of partial melting.Textural relationships and mineralogical data indicate that the HP granulite experienced five metamorphism characterized by:(1) pre-peak epidote-amphibolite facies assemblage represented by amphibole, plagioclase and epidote inclusions within the core of zoned garnet porphyroblasts representing P-T conditions of 0.9-1.15GPa and 550-595℃; (2) The peak mineral assemblages of HP granulite are as followings:Grt+Cpx+P1+Qtz±Ky±Rt (mafic),Grt+Ky+Kfs+PI+ Qtz (felsic), Grt+ternary-feldspar+Ky+Qtz (paragneiss), and yielded P-T conditions of 1.45-1.85GPa and 800-950℃. (3) Initial decompressional retrogression begining under a transitional stage between HP granulite-facies and high amphibolite-facies (1.2-1.4GPa and 735-800℃) characterized by clinopyroxene+plagioclase symplectite. (4) Subsequent retrogression under amphibolite facies is characterized by the occurrence of amphibole and biotite, with P-T conditions estimated at 0.86-1.05GPa and 600-675℃. (5) The final retrogression under greenschist-facies conditions (P<0.8GPa and T<550℃) developed at shallow crustal levels, characterized by the occurrence of hydrous minerals including actinolite, biotite, epidote, chlorite, muscovite, margarite and albite. The combination of petrographic textures, mineral compositions and thermobarometric data define a single clockwise P-T path for the Dulan HP granulite, suggesting the following dynamic processes:initial continent-continent collision lead to thickening of crust and HP granulite-facies metamorphism, and then returned rapidly with near-isothermal decompression or slight cooling and decompression, finally relifted to shallow crustal circumstance with amphibolite-facies and green-schist-facies overprinting.Bulk-rock geochemistry, Sm-Nd isotope and Lu-Hf isotope of zircon indicate that the felsic HP granulite (leucosome) and tonalite have a marked chemical resemblance to adakitic rocks. They are probably derived from partial melting of thickened basaltic lower crust or newly underplated basaltic lower crust with a depth more than 50km (T>900℃). The granite in Dulan area shows geochemical character similar to S-type granite, and was suggested to be derived from partial melting of Middle Proterozoic crust in the post-orogenic extension stage.Geochronology of zircon U-Pb indicates that the peak metamprphic age of the mafic HP granulite ranges from 434 to 446Ma, and the felsic HP granulite (leucosome) experienced partial melting and related metamorphism between 428 and 442 Ma, the emplacement age of tonalite ranges from 432 to 443Ma, which are in consistent with the time of eclogite-facies UHP metamorphism in adjacent eclogite unit. Therefore, the existence of contemporaneous eclogite-facies UHP metamorphism, HP granulite-facies metamorphism and anatexis define a tectonothermal event in the early time of collisional orogenesis. Moreover, in combination with field relationship, microtexture and geochemistry, the the mafic HP granulite and adakitic rocks including felsic granulite and tonalite are further suggested to form contemporaneously in a single dynamic process in the thickened lower crustal criumstance. The 40Ar-39Ar of 423-432 Ma for amphibole recorded the cooling time of HP granulite or amphibolite-facies overprinting. This implies a rapid cooling and exhumation rate for high pressure granulite. The S-type granite was emplaced at 382～391 Ma, which reflect relaxation or the transformational time from compression to entension after collision orogeny.