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An integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust. [ABSTRACT FROM PUBLISHER]

• A clockwise P–T path was established for Chachahe eclogites in North Qaidam UHP belt, NW China. • The Chachahe eclogites experienced a 1.92 Ga metamorphic event prior to eclogitization at ∼ 450 Ma. • Overriding plate material can enter subduction channel via tectonic erosion and be carried to mantle depths of ≥ 85 km. Although numerical modelling and marine seismic data show that overriding plate material can enter subduction channel and even recycle to mantle, and finally contribute to mantle heterogeneity and arc magmas, protoliths of the majority of high pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks on the earth's surface originate from the subducting plate. Whether overriding plate material can enter subduction channel and their fate in subduction channel remain enigmas. The North Qaidam UHP Metamorphic Belt (NQUB), located in northwestern China, was formed by north-dipping subduction of the Qaidam Block beneath the Quanji block in the Early Paleozoic. In this paper, we report that some eclogites in Chachahe area, NQUB experienced a 1.92 Ga low-grade metamorphism prior to eclogitization at ∼ 450 Ma. This Paleoproterozoic metamorphic event is not reported in the subducted Qaidam Block but widespread in the overriding Quanji Block, indicating that protoliths of Chachahe eclogites originated from the overriding plate. A clockwise P–T path from 12 kbar/520 °C to near UHP conditions of 25.5 kbar/690 °C, followed by isothermal decompression, was estimated for the Early Paleozoic metamorphism using a combination of garnet major and trace element zoning, phase equilibrium modelling and thermobarometry. This P–T path pattern is similar to other eclogites in this area, protoliths of which originated from the subducted plate. These results clearly show that overriding plate material can enter subduction channel and be carried to mantle depths of at least 85 km and exhume fast along subduction channel with other slices from subducted slab. [ABSTRACT FROM AUTHOR]

The article presents abstracts on geological topics which include the origin and structure of zircon in metamorphic rocks, the role of fluids during processes involving the continental crust, and geochemical characteristics of granites found in the Dabie Mountains, China.

A combined study of CL imaging, LA-ICP-MS U-Pb dating and trace element analysis were carried out for zircon grains from high-pressure basic granulite at Songshugou in North Qinling. The zircons display core-rim internal texture. The cores are made up of sector or planar domains, partially displaying weak zoning or misty texture, and have the feature of low content of Th, U, REE, HREE, low Th/U ratio and high (Nd/Yb)N ratio, and the REE pattern typical for zircon coexisting with garnet. These features indicate the metamorphic genesis for the zircon cores. The rims have narrow light margins (<30 μm in width) around the cores and boundaries between core and rim are rounded or lough-like, indicating that the rims were formed during retrograde metamorphism. U-Pb isotopic data of 24 spots from the cores of 11 zircon grains yield a 206Pb/238U weighted average age of 485±3.3 Ma, which is interpreted as the peak metamorphic age of high-pressure (HP) basic granulite. This age is well consistent within error with that of ultrahigh-pressure (UHP) eclogite and its country rock gneiss (507±37 Ma) that occur at the north side of the Qinling Group, possibly indicating the occurrence of two contemporaneous Early-Paleozoic HP to UHP metamorphic rock belts on the north and south sides of the Qinling Group. Combining with the regional geological studies and the distribution of Early-Paleozoic granites related to subduction and collision, the two HP to UHP metamorphic belts may be the products of continental crust bi-directional deep subduction toward Qinling micro-block caused by the subduction and collision of Paleozoic Qinling ocean and Erlangping ocean, respectively. [ABSTRACT FROM AUTHOR]

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 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]

[Display omitted] • Magmatism formed during the initial stage (ca. 480Ma) and advanced stage (ca. 450Ma) of exhumation of subducted continental crust have been recognized in South Altyn western China. • Two stages of magmatism exhibits evidence for the partial melting of subducted continental crust during differential exhumation process of South Altyn in Palaeozoic. The early Palaeozoic continental subduction–collisional belt in South Altyn (SA) western China distributes a large amount of (ultra)-high-pressure ((U)HP) metamorphic and granitic rocks associated with subduction, collision, and exhumation. In this study, four granitic rocks were collected from Bashenwake located in eastern SA. The ages of the garnet-bearing alkali-feldspar granite (GAFG), garnet-bearing granitic gneiss (GGG), alkali-feldspar granite (AFG), and syenogranite (SG) are 458.0 ± 4.1, 483.0 ± 4.1, 450.0 ± 2.4, and 482.0 ± 4.9 Ma, respectively. Whole-rock major and trace elements and Sr–Nd–Pb isotope, and zircon Hf isotope compositions for all samples exhibit characteristics of a continental crustal source. Based on partial melting modeling, the source temperature of the GGG, GAFG, AFG and SG was concluded as 750–780 °C, 730–760 °C, 730–770 °C and ca. 800 °C, respectively. The source pressure was estimated as ca. 0.8 GPa for the GGG, GAFG and AFG, and ca. 1.2 GPa for SG. Combined with studies on (U)HP metamorphic rocks in SA, this study reveals that GGG is formed by metamorphism while SG is formed from partial melting of upper crustal rocks heated by upwelling mantle during the initial stage of exhumation (ca. 480 Ma) of subducted continental crust in SA. During further stage of exhumation of the subducted continental crust at ca. 450 Ma, the GAFG and AFG were formed from partial melting of GGG as a result of decreased pressure and further mantle upwelling. Consequently, the exhumation age of subducted continental crust in eastern SA is ca. 30 Myr earlier than that in western SA. Thus, the geochronology and petrogenesis of four granitic samples in this study exhibit one of very few evidence for partial melting of subducted continental crust during the exhumation and also provide magmatism exhibits evidence for the differential exhumation model of eastern SA in Palaeozoic. [ABSTRACT FROM AUTHOR]

The North Qinling orogenic belt is an important component of Qinling composite orogen in Central China. High-pressure and ultra-high pressure (HP-UHP) rocks occur as lenses or layers surrounded by gneissic rocks in the northern, central, and southern Qinling Complex of the North Qinling belt, considered to have formed through deep continental subduction. The magmatic protoliths of the HP-UHP metamorphic rocks show formation ages (ca. 800 Ma), geochemical characteristics, and Pb-Nd isotopic compositions similar to those of the Neoproterozoic igneous rocks of the South Qinling belt. Continental materials of the South Qinling belt were dragged down to mantle depths by the Shangdan oceanic crust subducted and subjected to HP-UHP metamorphism at ca. 500 Ma. The Erlangping (and Kuanping) backarc basin formed in response to subduction of the Shangdan ocean and might have developed into a limited small ocean basin at ca. 500 Ma. The HP-UHP rocks yielded retrograde metamorphic ages of ca. 470–450 Ma and ca. 420–400 Ma. These ages are identical to the age of magmatic events in the North Qinling HP-UHP belt at ~ 500 Ma, ~ 450 Ma and ~ 420 Ma, related to deep subduction/collision, slab-breakoff and crustal thinning during post-collisional extension. The dominant ca. 500–400 Ma ages of detrital zircons from the Liuling Group of the South Qinling belt match well with those from the three stages of magmatic rocks and HP-UHP rocks in the Qinling Complex. This correlation suggests that the magmatic rocks and HP-UHP metamorphic rocks in the North Qinling belt initially exhumed to the surface, eroded and were then deposited in the Liuling basin in a post-orogenic extensional setting during middle to late Devonian. New evidence suggests that the Qinling Complex is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent as previously believed, and is mainly composed of the exhumed HP-UHP metamorphic rocks, deep subduction- exhumation-related magmatic rocks and the early Neoproterozoic granites together with the host rocks from the over-riding plate at an active continental margin. The early Paleozoic tectonic history of the NQB includes oceanic slab subduction and formation of arc, backarc spreading, continental deep subduction, arc-continent collision, break off, and multi-stage exhumation of the deep subducted slab, as well as extension and thinning and associated erosion and deposition. [ABSTRACT FROM AUTHOR]