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The Wujiaping Sn deposit is located at the northern Dayishan ore field, South China, whose ore veins are mainly hosted in the Dayishan pluton. LA–ICP–MS zircon U–Pb dating for the medium-fine grained- and medium-coarse grained-biotite monzogranite of Dayishan pluton yields emplacement ages of 154.5 ± 1.6 Ma (MSWD = 2.0) and 155.5 ± 0.5 Ma (MSWD = 1.9), respectively, which are consistent with the muscovite 40Ar-39Ar plateau age of 150.4 ± 0.9 Ma for the quartz vein type ore veins. It is indicated that the Sn mineralization in the Wujiaping deposit is related to the Late Jurassic granitic magmatism. These granites show the geochemical features of highly fractionated S-type granite: 1) low Zr + Nb + Ce + Y contents ( 1.1); 3) low crystallization temperature (mean = 688 °C); 4) high Rb/Sr ratios (mostly > 48); 5) high differentiation index (DI > 90); and 6) low CaO, P2O5, Sr, and Eu contents. Whole rocks isotopes show that these granites shows variable initial 87Sr/86Sr ratios (0.70251–0.71208), negative eNd(t) values (−7.94–5.62) and old two stage Nd model ages of 1491–1563 Ma. LA–MC–ICP–MS zircon Lu–Hf isotopes show that the medium-fine-grained monzogranite have positive eHf(t) values of 0.37–8.5 and two stage Hf model ages of 662–1180 Ma, whereas the medium-coarse grained-biotite monzogranite have negative eHf(t) values of −6.42–3.55 and two stage Hf model ages of 1431–1599 Ma. It is proposed of that these granites are originated from melts mixed by crustal- and mantle-constituents and are formed in an extensional setting caused by the subduction of the Palaeo-pacific plate. The low LogfO2 values calculated through zircons (−21.2–13.1) and high F contents (3630–5120 ppm) indicate the granites derived from reduced and F-rich melts. Therefore, the reduced melt is highly fractionated and enriched in Sn and F, resulting in the large scale Sn mineralization in the Dayishan ore field.

The Maozaishan Sn deposit, located south of the Dayishan ore field in the Nanling Range, is a newly explored greisen-type Sn deposit. Two muscovite samples from tin-bearing ores yielded 40Ar/39Ar plateau ages of 154.7 &plusmn, 1.1 Ma (Mean standard weighted deviation (MSWD) = 0.48) and 152.6 &plusmn, 0.7 Ma (MSWD = 0.25), respectively. Zircon U&ndash, Pb dating result of fine-grained biotite monzogranite in the Maozaishan mining area shows that these zircon grains can be subdivided into two populations, with ages of 154.2 &plusmn, 2.0 Ma (MSWD = 0.51) and 159.6 &plusmn, 1.9 Ma (MSWD = 0.09), respectively, indicating that the monzogranite is formed by a multi-stage magmatic event. It is indicated that formation of the Maozaishan Sn deposit is closely related to the Middle Jurassic granitic magmatism. Based on the trace element compositions of zircon grains, the calculated magma temperatures and oxygen fugacity (log(fO2)) values range from 638 &deg, C to 754 &deg, C (mean = 704 &deg, C) and from &minus, 18.9 to &minus, 15.8 (mean = &minus, 17.1), respectively. In addition, these intrusive rocks in the Dayishan ore field belong to highly fractionated granites and are characterized by low oxygen fugacity and crust&ndash, mantle origin, which are consistent to these tin-bearing granites in the Nanling Range and in favor of the Sn mineralization.

The roles of the mantle in generating granites and the extent of crust-mantle interaction are fundamental topics to our understanding of Sn mineralization. In this study, we report geochronological, geochemical, and Sr-Nd–Hf isotopic data for the granites, and muscovite 40Ar/39A isotopic data of the greisen type Sn ores, in the Dashunlong Sn deposit, Dayishan orefield, middle Qinzhou-Hangzhou metallogenic belt. LA–ICP–MS zircon U–Pb dating for the medium-fine grained- and coarse grained-biotite monzogranite of Dayishan pluton yields emplacement ages of 154.6 ± 1.2 Ma (MSWD = 0.73) and 154.7 ± 1.6 Ma (MSWD = 1.9), respectively, which are consistent with the muscovite 40Ar/39Ar plateau age of 151.1 ± 0.9 Ma (MSWD = 0.09) for the greisen-type Sn ores. It is indicated that the Sn mineralization in the Dashunlong deposit is related to the Late Jurassic Dayishan granite. The granite shows the geochemical features of highly fractionated A-type granite: 1) high 10,000 Ga/Al ratios of 2.95–3.88; 2) high Zr + Nb + Ce + Y contents of 292–637 ppm; 3) high TFeO/(TFeO* + MgO) ratios of 0.86–0.97; and 4) high Rb/Sr ratios of 2.80–18.8. Whole rocks isotopes show that this granite has variable initial 87Sr/86Sr ratios (0.71607–0.72437), negative eNd(t) values (−7.20 to −5.46), and two-stage Nd model ages of 1530–1388 Ma. LA–MC–ICP–MS zircon Lu-Hf isotopes show that the medium-fine-grained monzogranite has eHf(t) values of −0.94–2.20 and two-stage Hf model ages of 1298–1059 Ma, whereas the coarse-grained biotite monzogranite has relatively high eHf(t) values of −0.84–5.95 and two-stage Hf model ages of 1257–823 Ma. Different Hf and Nd isotopes indicate that the Nd-Hf isotopic decoupling occurs in the Dayishan granite, which was likely caused by the partial melting of the rocks containing abundant high Lu/Hf minerals. The magma of the Dayishan granite is reduced, highly fractionated, and enriched in Sn and F, resulting in large-scale Sn mineralization in the Dayishan orefield. It is proposed that these granites are originated from melts mixed by crustal- and mantle constituents and are formed in an extensional setting caused by the subduction of the Palaeo-pacific plate. A two end-member modeling indicates that ca. 10–20 percent of mantle-derived melts were likely involved in the formation of the Dayishan granites.

Ascertaining the petrogenesis of granitoid rocks in subduction zones holds the key for understanding the processes of how continental crust is produced. The synchronous Taoyuan and Huanggang plutons occur in two different geological units of the Paleozoic Tongbai orogen of central China. They provide an optimal opportunity for a study to address the role of the crustal basement in generating voluminous granitoid magmatism in subduction zones. The Taoyuan and Huanggang plutons have identical U–Pb zircon crystallization ages of 440–444 Ma, which are temporally related to northward subduction of the Paleotethyan Ocean. The Taoyuan samples show high SiO 2 (73.36–79.16%) and low Al 2 O 3 (12.00–13.45%) contents, Mg numbers (20.6–38.2), and Sr/Y (2.04–10.1) and (La/Yb) N (2.34–7.32) ratios with negative Eu anomalies (Eu/Eu* = 0.33–0.93). They yielded positive e Nd (t) (+ 3.0 to + 6.7) and e Hf (t) (+ 11.8 to + 13.2) values, elevated initial Sr isotopic ratios (0.7040–0.7057) and relatively low zircon δ 18 O values of 4.62–5.39‰. These suggest that they were produced through partial melting of hydrothermally altered lower crust of the accreted Erlangping oceanic arc. In contrast, the Huanggang samples exhibit variable whole-rock geochemical and isotopic compositions with SiO 2 contents of 57.01–64.42 wt.%, initial Sr isotopic ratios of 0.7065–0.7078, and e Nd (t) values of − 5.7 to − 9.4. Additionally, they have high zircon δ 18 O values of 7.57–8.45‰ and strongly negative zircon e Hf (t) values of − 14.4 to − 10.5. They were suggested to have been mainly derived from ancient continental crust of the Kuanping crustal unit with the addition of 20–40% juvenile, mantle-derived material. Accordingly, the granitoids in both oceanic and continental arcs are likely to be mainly derived from intracrustal melting of their crustal basement. It is revealed by the Huanggang pluton that little net continental crust growth occurs in continental arcs, and addition of new volume of continental crust may be balanced by the loss of old mafic lower continental crust. Continental growth was likely achieved through the accretion and subsequent differentiation of oceanic arcs, as manifested by the Taoyuan pluton.

An integrated study of petrography, zircon U-Pb ages, Hf-O isotope compositions, as well as whole rock geochemistry was carried out for a Paleoproterozoic granite complex in the northern Yangtze Block, to investigate its petrogenesis and geological significances. Petrographically, the Huashanguan complex is a granite complex comprised of monzogranite in the marginal facies and K-feldspar granite in the central facies. U-Pb dating of magmatic zircons from the monzogranite and K-feldspar granite yielded ages of 1816 ± 50 Ma and 1843 ± 9 Ma respectively. The more precise age of 1843 ± 9 Ma is suggested to represent the formation age of the Huashanguan complex. The granites have uniform negative zircon e Hf (t) values of −20.2 to −15.8, with corresponding T DM2 ages of ca. 3.6–3.5 Ga. The δ 18 O values average at 6.80 ± 0.09‰. These indicate that the granites were generated by partly melting of Archean rocks, implying the widespread of Archean basement in the Yangtze Block. The monzogranites display low FeO T and MgO contents and evolved trace element features, suggesting their formation by more fractional crystallization from similar parent magma of the K-feldspar granites. The occurrence of the Huashanguan granite complex indicates that the Yangtze Block has experienced a tectonic transition from tectonic convergence to extension at ca. 1.85 Ga, possibly due to the collapse of the ca. 2.0 Ga orogeny.

Adakitic rocks are often considered as a key to deciphering the genesis of Archean TTGs and the early crustal growth. Granites from the Huichizi pluton in the North Qinling (NQ) unit have high Sr/Y and (La/Yb) N ratios similar to adakites. Their relatively high SiO 2 , K 2 O, and Na 2 O and very low MgO, Cr, and Ni contents are in the range of high-SiO 2 adakites and early Archean TTGs and are compositionally similar to experimental melts derived from metabasalt sources. New SIMS zircon U–Pb dating constrains the emplacement age of the Huichizi pluton at 422 ± 5 Ma. Rock samples from the Huichizi pluton have e Nd (t) and zircon e Hf (t) values similar to the Neoproterozoic metabasalts in the NQ unit. In combination with their normal mantle-like δ 18 O zir values, these adakites are best explained by partial melting of the Neoproterozoic mafic crustal root due to subduction of the Shangdan ocean. Regional geological data suggest that the crust was probably thickened by a ca. 490 Ma arc-collision process prior to the emplacement of the Huichizi pluton. Our results confirm that underplating of mafic magma and its subsequent fusion triggered by slab subduction under high pressure conditions could be an important mechanism for the formation of early continental crust.

The presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution. Due to the poor exposure of Archean rocks, the early evolution of the Yangtze Craton has not been well constrained yet. In this study, a combined study of zircon U–Pb and Hf–O isotopes, as well as whole rock major and trace elements and Nd isotopes have been conducted for a granite pluton at Huji town, in the northern part of the Yangtze Craton. U–Pb dating for magmatic zircons yielded a weighted mean age of 2656 ± 6 Ma, revealing another Archean outcrop in the Yangtze Craton besides the Kongling terrane. The granite samples show relatively evaluated (K 2 O + Na 2 O)/CaO, TFeO/MgO ratios, and Zr, Nb, Ce, Ga and Y contents, sharing the same features as the A-type granite. They have pronounced Eu, Sr and Ba negative anomalies on the primitive mantle-normalized trace element spidergram, implying the residual of plagioclase in the magma sources. Their high Zr contents of 440–529 ppm correspond to initial magma temperatures of 930–962 °C. The zircons have negative ɛ Hf ( t ) values of −2.7 to 0.1, and Hf model ages of ca. 3.2 Ga, respectively. The δ 18 O values of the zircons range between 4.94 and 5.72‰ and give an average of 5.26 ± 0.14‰, suggesting no contribution of supracrustal rock into the host magma. Therefore, the Hf model ages indicate the generation of the Huji granite by reworking of ca. 3.2 Ga juvenile crustal rocks. Combined with previous studies, the finding of the Neoarchean Huji granites with ca. 3.2 Ga Hf model ages suggests the widespread of Archean basement in the Yangtze Craton. The occurrence of the ca. 2.65 Ga Huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the Yangtze Block.

The Yangtze Craton is one of the most important cratonic blocks in eastern Asia. However, its early evolution has not been well constrained yet, because of the poor exposure of Archean basement rocks. In this contribution, a combined study of zircon U–Pb age, Hf–O isotope compositions, as well as whole rock element compositions was carried out for gray gneiss rocks from the Douling complex in the northwestern part of the Yangtze Craton. Zircon U–Pb dating for two gneiss samples yielded weighted mean ages of 2509 ± 21 and 2496 ± 15 Ma. The gray gneisses have high SiO2 (63.64–73.74%), Na2O (3.66–6.21%), but low K2O (0.76–2.93%) contents, and belong to the trondhjemite, tonalite and granodiorite series rocks. They are characterized by enrichments of LILEs and LREEs, but notable depletions in Nb–Ta and Ti, as well as heavy REEs and Y. All the samples plot into the fields of typical Archean TTG rocks in the (La/Yb)N vs. (Yb)N and Sr/Y vs. Y diagrams. Therefore, the gray gneisses in the Douling complex are Neoarchean TTG rocks. The relatively high Nb/Ta and Zr/Sm ratios of 19.6–46.0 and 14.5–143.4 suggest their formation under eclogite-facies conditions. They have low ɛHf(t) values of −3.21 and −3.39 and Hf model ages of 3194 ± 26 and 3205 ± 24 Ma, indicating their derivation from ca. 3.2 Ga juvenile mafic rocks. Their δ18O values of 6.41 ± 0.12 and 5.44 ± 0.12‰ suggest that surface rocks were differently recycled into their sources. Collectively, the TTG rocks in the Douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary. A compilation of available zircon U–Pb and Hf isotope compositions of the Yangtze Craton reveals that there are quite differences of Hf isotope compositions in Archean. The ca. 3.8–3.4 Ga zircons exhibit ɛHf(t) values of −5.8 to 0.0. These chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra-crustal reworking of ancient protoliths without juvenile input. By contrast, zircons with ages younger than ca. 3.2 Ga register both negative and positive ɛHf(t) values up to the depleted mantle, providing clear evidence for the addition of juvenile materials. These imply that the subduction-driven plate tectonics might have been operated since ca. 3.2 Ga in the Yangtze Craton.

S-type peraluminous granite can crop out in various tectonic settings. The Paleozoic Piaochi granitic intrusion is a large S-type peraluminous intrusion located in the North Qinling orogen, and has important implication for its tectonic evolution. Though intensively studied, the age and tectonic setting of this intrusion are still controversial. In this study, we carried out an integrated study of zircon U–Pb age, major and trace elements, and Sr–Nd–Hf–O isotope compositions for the Piaochi intrusion, with aims to constrain its petrogenesis and tectonic significance. SIMS and LA-ICPMS U–Pb zircon data obtained from magmatic zircons indicate that the Piaochi intrusion was emplaced at 473 ± 4 Ma, and contained ca. 490 Ma inherited zircons. The granite samples from the Piaochi intrusion show typical S-type characteristics, such as the relatively moderate aluminum saturation index (1.00–1.08), low zirconium saturation temperatures, occurrences of muscovite, and high zircon δ 18 O values (8.70 to 9.54‰). The relatively low Sm/Nd (0.13–0.20) and FeO T /MgO ratios (2.21–3.9) suggest that the granites are unfractionated. The geochemical compositions reveal that they were derived from partial melting of a clay-poor, but plagioclase- and biotite-rich psammitic source, similar to metasedimentary rocks outcropping in the North Qinling unit. Moreover, the Piaochi granites are moderately depleted in Eu and heavy rare earth elements, and have relatively high (La/Yb) N values (22.8–56.8), implying a high-pressure, plagioclase-poor and garnet-rich residual assemblage. The Piaochi granites have high and variable initial 87 Sr/ 86 Sr ratios (0.72167–0.72511) and low e Nd (t) (from − 10.23 to − 8.71) values with two-stage Nd model ages (T DM2 ) of 1.91–2.03 Ga, which are comparable with those of supracrustal materials in the North Qinling unit. In addition, our results show that the ca. 470 Ma magmatic zircons display a narrow e Hf (t) array within those of the inherited cores. These imply that the Piaochi granites inherited their isotopic characteristics wholly from their sources, and the Hf isotopes might have been differently homogenized during the partial melting. Our new zircon U–Pb ages indicate that the Piaochi granites were emplaced after the climax of the UHP metamorphism, coeval with later retrograde metamorphism during exhumation. Therefore, we suggest that the formation of the early Paleozoic Piaochi intrusion was triggered by the exhumation of the UHP rocks.

Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c. 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c. 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c. 455–400 Ma. The migmatization was c. 50 Ma later than the c. 490 Ma ultra-high-P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.

Eclogites in an orogen, serving as a credible marker for deep subduction of oceanic or continental crust, are a key to decipher the process and fate of plate subduction. The nature of eclogite precursors bears important information on orogenic mechanism and pre-subducted geological history. Despite their importance, the nature of ultra-high pressure (UHP) eclogites in the North Qinling (NQ) terrane is still poorly constrained, which leads to conflicting tectonic scenarios for the evolution of the Qinling orogenic belt. In this paper, we provide a combined study of zircon U–Pb ages, major and trace element data, and Sr–Nd–Pb–Hf isotope compositions for eclogites from the NQ terrane. U–Pb age determinations on magmatic zircon cores of two eclogite samples yield similar ages of ca. 800 Ma, indicating that the protoliths of the NQ UHP eclogites formed in the Neoproterozoic. The NQ eclogites have tholeiitic compositions and are characterized by high TiO 2 (1.40–2.40%), and low P 2 O 5 (0.09–0.22%), Na 2 O (1.10–2.49%) and K 2 O (0.17–1.04%) contents. They are moderate fractionation in REEs (La N /Yb N = 2.31–4.20) and show negligible Nb, Ta, Ti, and Eu anomalies (Eu/Eu* = 0.82–1.10). The eclogites have high positive whole-rock ɛ Nd ( t ) (2.81–5.53) and zircon ɛ Hf ( t ) (10.5–11.9) values, indicating their derivation from a relatively depleted mantle source. On geochemical discriminant diagrams based on immobile element ratios, the NQ eclogites fall in the field of continental basalts. Moreover, both the geochronological and geochemical features of the eclogite protoliths are quite different from those of the Erlanpging basaltic rocks, but comparable to those of the coeval continental basalts in South Qinling (SQ) orogen and the northern margin of the South China Block (SCB). As the mid-Neoproterozoic basaltic rocks in the SQ orogen and the northern margin of the SCB were well documented to form in a continental rift setting in response to the global-scale breakup of the Rodinia supercontinent, it is inferred that the NQ eclogite precursors might also form in a continental rift setting at ca. 800 Ma, which resulted in the splitting of the NQ terrane from the SCB. As the protoliths of the NQ eclogites have a continental property, we suggest that the ca. 490 Ma UHP metamorphism of the NQ unit might result from its northward subduction beneath the Erlangping arc and thus manifested an arc-continent collision process.

The Dabie–Sulu orogen in east-central China is the largest UHP belt in the world. Many studies have dealt with detrital minerals from sedimentary basins surrounding the Dabie belt in order to constrain the tectonic unroofing of this terrane. On the other hand, still few are the works focused on detritals from the basins associated with the Sulu belt. The Jiaolai basin, adjacent to the Sulu belt, is made of Mesozoic sedimentary rocks. Two sandstone samples collected from the Early Cretaceous Laiyang Group in the northern and southern margins of the Jiaolai basin were selected for zircon U–Pb dating and trace element analysis. The zircons show main U–Pb age clusters at ca. 2500–2400, 2000–1700, 850–700, and 330–130 Ma, with a minor peak at ca. 1100 Ma and a few Archean ages. Most zircons show texture features reflecting a magmatic origin, whereas several late Paleoproterozoic and Triassic zircons show homogeneous CL images and low Th/U ratios, indicating a metamorphic origin. The Neoproterozoic ages are consistent with the protolith ages of the metamorphic rocks in the Sulu terrane, implying that the Sulu tarrane may be a major source for this component. Whereas most Neoproterozoic zircons are euhedral to subhedral or angular in shape and are not surrounded by overgrowths, this implies that low-grade metamorphic Neoproterozoic metaigneous rocks ocurring along the northern margin of the Sulu terrane or metasediments overlying the HP–UHP rocks in the Sulu terrane might provide source materials to these detrital zircons. The late and early Paleoproterozoic age peaks are consistent with the thermal events occurred in the Jiaobei terrane, slightly younger than those in the Yangtze Block. These suggest that the Jiaobei terrane was a potential source for the Paleoproterozoic to Archean detrital zircon components. Only two early Paleozoic zircon grains were found in the studied samples, indicating the early Paleozoic arc did not extend into the Sulu terrane. According to the distribution of different kinds of rock adjacent to the Sulu terrane, the Early Cretaceous detrital zircons may be potentially derived from Cretaceous plutons from the Sulu belt. Eight detrital zircons including six metamorphic grians and two metamorphic overgrowth rims yield the Triassic ages, which might be derived from HP–UHP rocks from the Sulu terrane. A metamorphic overgrowth rim yields a metamorphic age of 246 ± 3 Ma, probably recording the timing of the prograde metamorphism of the Sulu terrane. One metamorphic grain yields an age of 212 ± 2 Ma, which is interpreted to record a retrograde metamorphic event. Based on the REE patterns, the remaining six Triassic analyses can be subdivided into two groups. Four of them record a weighted mean age of 230 ± 6 Ma and show nearly flat HREE patterns and slight negative or positive Eu anomalies. The last two grains are enriched HREE; considering their ages and the REE patterns, it is suggested that they may originate from other diverse UHP rocks. The Triassic metamorphic zircons were detected in the Laiyang Group, indicating that the Sulu UHP rocks were exposed to the surface before the Early Cretaceous.

The Xitieshan terrane is one of four metamorphic terranes in the North Qaidam metamorphic belt, which is an early Paleozoic high-pressure to ultrahigh-pressure (HP-UHP) metamorphic belt in NW China. However, conclusive evidence and precise timing of UHP metamorphism in the Xitieshan terrane have not been well documented. In this study, we report an integrated study of zircon grains from an amphibolite in the Xitieshan terrane in terms of mineral inclusions, trace elements, and U-Pb age systematics. The first record of coesite is reported as an inclusion in a metamorphic zircon, which provides unambiguous evidence for the UHP metamorphism of the Xitieshan terrane. The metamorphic zircon domains have weak or no zoning, very low Th/U ratios, insignificant Eu anomalies, and flat HREE patterns. Zircon grains contain inclusions of garnet, omphacite and rutile, in addition to the coesite inclusion. These inclusions indicate that the metamorphic zircon grains formed under UHP metamorphic conditions. The metamorphic zircon grains were dated by the SIMS and LA-ICPMS methods and yield weighted mean ages of 432 ± 14 and 441 ± 9 Ma, respectively. Combined with previous results, the HP-UHP metamorphism of the Xitieshan terrane may have lasted 460–440 Ma with the peak UHP metamorphism at 441 ± 9 Ma. A compilation of the reported geochronological data reveals that all four terranes of the North Qaidam metamorphic belt might have experienced coeval UHP metamorphism during the early Paleozoic (420–450 Ma), and thus may have suffered a coherent subduction, UHP metamorphism, and exhumation cycle.

The tectonic properties of the Erlangping unit and the subduction polarity of oceanic basins in the North Qinling–Tongbai orogen have been the focus of debate for more than twenty years. The resolution of these controversies hinges on the refined constraints on the location and nature of subduction-related magmatism. In this contribution, we provide a combined study of zircon U–Pb ages, major and trace element geochemistry, and Sr–Nd–Hf isotope compositions of gabbroic rocks from the Erlangping unit in the Tongbai orogen. U–Pb dating of zircons from two gabbroic diorite samples yielded two weighted mean ages of 443 ± 3 and 440 ± 4 Ma. The Erlangping gabbroic rocks are tholeiitic in composition with variable MgO (4.48–8.17%), and low K 2 O (0.25–1.01%) and TiO 2 (0.60–1.79%) contents. They are characterized by enrichment of LILEs, depletion of HFSEs, and highly positive whole-rock e Nd (t) (3.76–5.68) and zircon e Hf (t) (12.0–12.1) values. The geochemical features indicate that the Erlangping gabbroic rocks were derived from a depleted lithospheric mantle that had been exposed to metasomatism by slab-derived fluid shortly before their generation. Their occurrence, coupled with coeval granitoids with highly positive e Nd (t) values, manifest significant crustal growth in the North Qinling–Tongbai orogen during the late Ordovician–Silurian. In addition, the Erlangping gabbroic rocks have low Sr/Y (7.32–26.2) and (La/Yb) N (1.34–4.07) ratios, but high HREEs, Y and Sc contents. This implies that their mantle source had no residual garnet, and thus partial melting of the lithospheric mantle occurred under relatively low-pressure conditions. The generation of the Erlangping gabbroic rocks might result from incipient rifting of the Erlangping back-arc basin induced by the northward subduction of the Paleotethyan Shangdan Ocean. Taking into account the ca. 490 Ma collision between the Erlangping arc and North Qinling microcontinent, we suggested that the evolution of the North Qinling belt from an arc to a back-arc realm involved a prolonged period of underplating of basaltic magma, granulite-facies metamorphism, and anatexis of precursor rocks. Consequently, the Qinling–Tongbai orogenic belt provides a key place for better understanding the Phanerozoic crustal growth and differentiation.