Petrology, Mineralogy And Metamorphic Evolution Of The Dulan UHP Terrane In North Qaidam, NW China, And Its Tectonic Implications

The Dulan UHP metamorphic terrane, a major terrane of the east part of the north Qaidameclogite—gamet-peridotite—gneiss belt, was first discovered in 1999. Eclogites are intercalated aslens or layers in both granitic and pelitic gneisses. The largest eclogite body is about 200×300meters in size. Two distinguished sub-belts have been recognized on the basis of petrology,mineralogy and P-T conditions. Geochemical analysis suggest that protoliths of eclogites from thenorth belt of Dulan (NBD) are characterized by E-type MORB and within plate basalt, and, fromthe south belt of Dulan (SBD), by island arc tholeiite and high-Al basalt. The mineral assembalgeof eclogite from the NBD is Grt (Alm_41～60Pyr_10～27Gross_13～38)+Omp(Jd_27-42)+Phe(Si_{3.28～3.53}+Ep+Arg/Cal+Rt+Apt+Zir. Quartz pseudomorphs after coesites and polycrystalline quartz+sanidine aggregation with radial cracks are commonly seen in both garnets and omphacites ineclogites. Quartz lamellae, the excluded SiO₂ rods from a previous supersilicic clinopyroxenewhich stabilize at UHP conditions, have been widespreadly observed in omphasites. All suggestUHP metamorphism of the NBD eclogites. The peak metamorphic assemblage of eclogites fromthe SBD is Grt (Alm_32-35.4Pyr_{43.6～50.5} Gross_{16.8～20.7})+Omp (Jd_26～46)+Ky+Rt+Phe(Si_3.44).Typical coesite pseudomorphs and SiO₂ lamellae were only observed in a few omphaciticclinopyroxene grains.4 coesite inclusions have been first discovered in zircons from paragneisses in the NBD,which give the critical evidence of UHPM terrane for this region. The regular change of inclusionsfrom quartz in the core of zircon to coesite on the rim suggests a process of pressure-increasingprogressive metamorphism during plate subduction. The coexisting graphite inclusions confinethat the maximum pressure is lower than 3.5 GPa. Mineral inclusions in zircons from eclogites areGrt, Omp, Phe, Arg, Ky (SBD) etc.. Quartz inclusions are sparse in the core and no ceositeinclusions were found in zircons from both the NBD and SBD. Zircons from granitic gneissesshow typical magmatogenetic features without any sigh of HP-UHP mineral inclusions andmetamorphic rims, suggesting that orthogneisses in this area have not undergone the UHPM event.Sm-Nd and Zircon SHRIMP geochronological studies reveal that metamorphic ages ofeclogites from both the NBD and SBD are 443-496 Ma, in accord with the ages of Daqaidam andAltun eclogites, the west part of Qaidam-Altun eclogite belt. The forming ages of granitic gneissare 932-1011 Ma, which is associated with the construction of Rodinia supercrust. Ar-Ar of whitemica gives a plateau age of 401±1.2 Ma. This age is associated with the ductile shearingdeformation prevailed widely in the North Qaidam region.The P-T paths are much different between the N-BD and the SBD, suggesting that theirsubduction and exhumation processes are of the great distinction. The peak metamorphicconditions of the NBD are T=580-770℃and P=2.9 GPa. After the peak metamorphism abovethe stability of coesite, UHP rocks have undergone a retrogressive process induced by sharpdecrease of pressure of fast exhumation. P-T path of the SBD shows a clockwise circle, and thehighest pressure is about 2.6 GPa at 600℃. With the decrease of pressure from 2.6 GPa to 1.8-2.0GPa, temperature increases from about 600℃to 900℃. It suggests that UHP rocks haveexperienced another high-pressure granulite facies of metamorphism during the slow exhumationto the Earth's surface.The Dulan ultrahigh-pressure metamorphic terrane plays an important role in defining the North Qaidam—Altun HP-UHP belt, which extends 1000 km long in NW from Dulan UHPterrane, via Xietieshan eclogite-gneiss terrane, Luliangshan garnet-peridotite terrane, Yukaeclogite-gneiss terrane, to the Altun eclogite-gneiss terrane. This super-belt represents aCaledonian continental convergent border between Qaidam Massif and Qilian terrain. Theestablishment of this super-belt is very significant in reconsidering the construction and tectonicevolution of the Caledonian continent in not only the northwestern China but all over the world.