Search

The North Qaidam Belt (NQDB) is an Early Paleozoic continental ultrahigh-pressure (UHP) metamorphic belt between the Qaidam Block and South Qilian Belt (SQB) in northwest China, and the UHP metamorphic rocks recorded differential/diachronous exhumation and anatexis histories. To constrain the multi-stage exhumation and the associated magmatism of the NQDB and its adjacent areas, an integrated study of petrology, geochemistry, zircon U Pb and Lu Hf isotopes was conducted on the OIB-like dolerite and A-type dacite porphyry from the SQB. The OIB-like dolerite is enriched in light rare-earth elements (LREEs), large ion lithophile elements (LILEs), and high field strength elements (HFSEs), and depleted in heavy rare-earth elements (HREEs), with a crystallization age of 425 ± 7 Ma and zircon ε Hf (t) values ranging from −4.2 to 0.3. Given that the oceanic/continental deep subduction occurred in this area during Early Paleozoic, the fertile mantle source of OIB-like dolerite was likely formed by the crust-mantle interaction between the oceanic crust and juvenile lithospheric mantle at the instability field of rutile above the subduction zone. The A-type dacite porphyry is enriched in Al, Fe, LILEs and LREEs, depleted in Ca, Mg, Ba, Sr, Eu and HFSEs, and is characterized by high Zr concentrations (231–271 ppm), 10,000*Ga/Al ratios (2.51–2.73) and crystallization temperatures (738–836 °C). Zircon U Pb and Lu Hf isotopic data reveal the dacite porphyry crystalized at 423 ± 5 Ma with zircon ε Hf (t) values ranging from −8.9 to −3.3. The inherited magmatic zircons gave a formation age of 472 ± 8 Ma with ε Hf (t) values ranging from −3.9 to −1.9, contemporaneous with the oceanic subduction-related magmatism in the NQDB, indicating the dacite porphyry may be derived from the Early Paleozoic arc. Combined with the previous studies on the UHP metamorphism (ca. 458–420 Ma), retrograde metamorphism (ca. 432–397 Ma), and anatexis (ca. 436–417 Ma) in the NQDB, we conclude that the breakoff of the oceanic lithosphere from the continental lithosphere triggered the thermal perturbation from the asthenosphere upwelling at ca. 425 Ma and the subsequent partial melting of the fertile metasomatized mantle and Early Paleozoic arc, forming the OIB-like dolerite and A-type dacite porphyry in the SQB. Moreover, the coeval mafic and felsic magmatism indicates an important tectonic transformation that the subducted continental crust experienced a localized detachment/exhumation before ca. 425 Ma and entire exhumation after slab breakoff. • Coeval (425–423 Ma) dolerite and dacite porphyry in the South Qilian Belt. • These magmas were caused by the slab breakoff. • The subducted continental plate experienced two stages of exhumation in the NQDB. [ABSTRACT FROM AUTHOR]

The North Qaidam orogenic belt (NQOB) lies at the northeast margin of Tibet Plateau, between the Qilian massif to the north and Qaidam massif to the south. It consists of predominantly gneisses and marbles with minor eclogite and garnet peridotite, and is known as an Early Paleozoic continental deep subduction ultrahigh pressure (UHP) metamorphic belt in northwest China. Recently some studies suggest that the NQOB experienced oceanic deep subduction prior to the continental deep subduction. However, the records of ocean events are rarely preserved because of the complex multi‐stage transformation. Therefore, whether the Early Paleozoic ocean exists and the formation, nature and evolution of the Early Paleozoic ocean in North Qaidam are still in controversies. In this contribution, we report a new discovered SSZ‐type ophiolite suite in Kaipinggou area, the western segment of the NQOB. The ophiolite is mainly composed of peridotite, gabbro, basalt, boninite, andesitic porphyrite and plagiogranite. In which, the basalt and homogeneous gabbro are both enriched in LILE and depleted in HFSE, and display flat or LREE‐depleted REE patterns, similar to the MORB‐like geochemical characteristics of fore‐arc basalts; the boninite exhibits characteristics of fore‐arc boninites with high Si and Mg, but low Ti and Ca contents; the andesitic porphyrite is enriched in LILE but depleted in HFSE, similar to island arc magmatic rocks. Rock assemblage and their geochemical characteristics suggest that this SSZ‐type ophiolite was formed in fore‐arc setting of intra‐oceanic arc. The plagiogranite is a low‐potassium tholeiite metaluminous granite with low trace elements contents, slight enrichment of LREE, depletion of HREE and positive Eu anomaly; combining with ∊Hf (t) values (13.8–17.1) of zircons, the plagiogranite could be originated from partial melting of gabbros. LA–ICP–MS zircon U–Pb dating yielded the crystallization ages of 535±2 Ma, 513±3 Ma and 510±3 Ma for two cumulate gabbros and one homogeneous gabbro, respectively, and a formation age of 492±8 Ma for the plagiogranite. All these data suggest the existence of an Early Paleozoic ocean in the North Qaidam, and the initial subduction of the oceanic slab occurred earlier than 535Ma. [ABSTRACT FROM AUTHOR]

The North Qaidam is an Early Paleozoic UHP metamorphic belt located at the north margin of the Tibet plateau. Eclogites in this belt contain both continental‐and oceanic‐type ones. In which, the continental‐type eclogites have protolith ages of 750–850 Ma and WPB or CFB geochemical signatures and are believed to have formed in a continental rift or an incipient oceanic basin setting related to the breakup of the Rodinia supercontinent, their metamorphic ages (421–458 Ma) and P–T paths are comparable to their host gneisses; oceanic‐type eclogites have cumulate gabbro or E‐MORB geochemical signatures, their protolith and metamorphic ages are 510–516 Ma and 425–450 Ma, respectively(Zhang et al., 2008). Therefore, the North Qaidam UHP belt was thought to record the whole Neoprotoerozoic–Paleozoic Wilson cycle (Song et al., 2014). In this study, we reported three new kinds of eclogites: kyanite‐bearing eclogite, lawsonite pseudomorph‐bearing eclogite and double mineral eclogite. They occur as big lentoid blocks in regional granitic gneiss in the western part of the belt. Phase equilibrium modelling and zircon LA‐ICPMS U‐Pb dating show that all these three eclogites experienced a clockwise P–T path with peak metamorphic conditions close to or fall in the coesite stability field, and their peak metamorphic age were around 436‐439 Ma, similar to those continental‐type eclogites in this belt. But their protolith ages are between 1273 and 1070 Ma, and some of them recorded an amphibolite facies metamorphic age of 927 Ma, and geochemical data and zircon Lu‐Hf and O isotope analysis indicate these eclogites have features of present day N‐MORB. Combined with the existing results, we propose that the North Qaidam is a polycyclic composite orogenwhich recorded tectonic evolution of Mesoproterozoic ocean floor spreading, assembly and breakup of Rodinia supercontinent, Early Paleozoic oceanic deep subduction and subsequently continental deep subduction. [ABSTRACT FROM AUTHOR]