Post-collisional granitoids from the Dabie orogen in China: Zircon U–Pb age, element and O isotope evidence for recycling of subducted continental crust

While recycling of subducted oceanic crust is widely proposed to be associated with oceanic island, island arc, and subduction-related adakite magmatism, it is less clear whether recycling of subducted continental crust takes place in continental collision belts. A combined study of zircon U–Pb dating, major and minor element geochemistry, and O isotopes in Early Cretaceous post-collisional granitoids from the Dabie orogen in China demonstrates that they may have been generated by partial melting of subducted continental crust. The post-collisional granitoids from the Dabie orogen comprise hornblende-bearing intermediate rocks and hornblende-free granitic rocks. These granitoids are characterized by fractionated REE patterns with low HREE contents and negative HFSE anomalies (Nb, Ta and Ti). Although zircon U–Pb dating gives consistent ages of 120 to 130 Ma for magma crystallization, occurrence of inherited cores is identified by CL imaging and SHRIMP U–Pb dating; some zircon grains yield ages of 739 to 749 Ma and 214 to 249 Ma, in agreement with Neoproterozoic protolith ages of UHP metaigneous rocks and a Triassic tectono-metamorphic event in the Dabie–Sulu orogenic belt, respectively. The granitoids have relatively homogeneous zircon δ¹⁸O values from 4.14‰ to 6.11‰ with an average of 5.10‰ ± 0.42‰ (n = 28) similar to normal mantle zircon. Systematically low zircon δ¹⁸O values for most of the coeval mafic–ultramafic rocks and intruded country rocks preclude an AFC process of mafic magma or mixing between mafic and felsic magma as potential mechanisms for the petrogenesis of the granitoids. Along with zircon U–Pb ages and element results, it is inferred that the granitic rocks were probably derived from partial melting of intermediate lower crust and the intermediate rocks were generated by amphibole-dehydration melting of mafic rocks in the thickened lower crust, coupled with fractional crystallization during magma emplacement. The post-collisional granitoids in the Dabie orogen are interpreted to originate from recycling of the subducted Yangtze continental crust that was thickened by the Triassic continent–continent collision. Partial melting of orogenic lithospheric keel is suggested to have generated the bimodal igneous rocks with the similar crustal heritage. Crustal thinning by post-collisional detachment postdated the onset of bimodal magmatism that was initiated by a thermal pulse related to mantle superwelling in Early Cretaceous.     

Zircon U–Pb geochronology,geochemistry and Hf isotopes of the Late Cretaceous Hongshan intrusion,western Yunnan,SW China

The Hongshan quartz monzonite porphyry is one of the Yanshanian intrusions in the southern part of the Yudun Arc. Detailed zircon U–Pb data of four samples yielded ages of 78.8–80.7 Ma, indicating that the Hongshan intrusion was emplaced during the late stage of Late Cretaceous. The Hongshan intrusion shows shoshonitic and high‐K calc‐alkaline, with A/CNK = 0.64–1.14. The rocks show an obvious fractionation between light and heavy rare‐earth elements (average [La/Yb]_N = 38.85), with negative Eu anomalies (Eu/Eu* = 0.60–0.87), enrichment in large‐ion lithophile elements (Rb, Th, U and K) and depletion in high field‐strength elements (Nb, Ta and P). Rocks have high Sr and low Y content which are characteristics of adakitic rocks, suggesting magma derivation from thickened lower crust. In order to evaluate the nature of the source region, Hf isotope data of zircons were acquired through LA‐MC‐ICPMS. The negative and variable ε_Hf(t) values demonstrate that the Hongshan intrusion was derived from ancient crust, without mantle‐derived components and is significantly different from the Triassic intrusions in the southern part of the Yudun Arc. The three Yanshanian intrusions in Hongshan, Relin and Tongchangou are remarkably similar in terms of geochronology, geochemistry and Hf isotopes. We therefore infer that these intrusions had the same magmatic source and we correlate the tectonics with northward subduction of Tethys underneath the Asian continent. Copyright © 2015 John Wiley & Sons, Ltd.     

Zircon U–Pb and Hf evidence for coupled subduction of oceanic and continental crust during the Carboniferous in the Huwan shear zone,western Dabie orogen,central China

Both oceanic and continental HP rocks are juxtaposed in the Huwan shear zone in the western Dabie orogen, and thus provide a window for testing the buoyancy‐driven exhumation of dense oceanic HP rocks. The HP metamorphic age of the continental rocks in this zone has not been well constrained, and hence it is not known if they are of the same age as the exhumation of the HP oceanic rocks. In situ laser ablation (multiple collector) inductively coupled plasma mass spectrometry (LA‐(MC‐)ICP‐MS), U–Pb, trace element and Hf isotope analyses were made on zircon in a granitic gneiss and two eclogites from the Huwan shear zone. U–Pb age and trace element analysis of residual magmatic zircon in an eclogite constrain its protolith formation at 411 ± 4 Ma. The zircon in this sample displays ε_Hf (t) values of +6.1 to +14.4. The positive ε_Hf (t) values up to +14.4 suggest that the protolith was derived from a relatively depleted mantle source, most likely Palaeotethyan oceanic crust. A granitic gneiss and the other eclogite yield protolith U–Pb ages of 738 ± 6 and 700 ± 14 Ma, respectively, which are both the Neoproterozoic basement rocks of the Yangtze Block. The zircon in the granitic gneiss has low ε_Hf (t) values of ?14.2 to ?10.5 and old T_DM2 ages of 2528–2298 Ma, suggesting reworking of Palaeoproterozoic crust during the Neoproterozoic. The zircon in the eclogite has ε_Hf (t) values of ?1.0 to +7.4 and T_DM1 ages of 1294–966 Ma, implying prompt reworking of juvenile crust during its protolith formation. Metamorphic zircon in both eclogite samples displays low Th/U ratios, trace element concentrations, relatively flat heavy rare earth element patterns, weak negative Eu anomalies and low ¹⁷⁶Lu/¹⁷⁷Hf ratios. All these features suggest that the metamorphic zircon formed in the presence of garnet but in the absence of feldspar, and thus under eclogite facies conditions. The metamorphic zircon yields U–Pb ages of 310 ± 3 and 306 ± 7 Ma. Therefore, both the oceanic‐ and continental‐type eclogites share the same episode of Carboniferous eclogite facies metamorphism. This suggests that high‐pressure continental‐type metamorphic rocks might have played a key role in the exhumation and preservation of oceanic‐type eclogites through buoyancy‐driven uplift.     

Zircon U–Pb ages and Hf isotope compositions of migmatites from the North Qinling terrane and their geological implications

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.     

Zircon U–Pb SHRIMP ages of weakly to unmetamorphosed granitoids of the Yangtze basement outcrop in Dabieshan, central China

The Qichun granitoids exposed in the Dabie Orogen of China are composed of two types of rocks: porphyritic monzogranite (with variable schistosity) and syenogranite (without schistosity). The two types show large differences in geochemical characteristics. The porphyritic monzogranite is characterized by high Al₂O₃ content (15.73%), relatively high CaO (2.46%) and Na₂O contents (Na₂O/K₂O=1.27), strong depletion in HREE and strong fractionation between LREE and HREE ((La/Yb)_N=46.8), similar to some high Al₂O₃ Archaean TTG gneisses. Conversely, the syenogranite is characterized by relatively low Al₂O₃ (14.05%) and CaO (0.82%) contents, and higher K₂O than Na₂O (Na₂O/K₂O=0.81). The degree of fractionation between LREE and HREE is minor. The U–Pb SHRIMP zircon age of the porphyritic monzogranite is 841±15 and 824±27 Ma for the syenogranite. These ages are similar to the protolith emplacement ages of granitic gneisses in the Dabie Orogenic Belt. The existence of weakly to unmetamorphosed granitoids in the Dabie Orogen shows that the granitoids were situated in the back part of the subducted plate during collision and subduction between the Yangtze and the North China cratons, and may represent outcrops of the Yangtze basement.     

Coupled garnet Lu–Hf and monazite U–Pb geochronology constrain early convergent margin dynamics in the Ross orogen,Antarctica

The Ross orogen of Antarctica is an extensive (>3000 km‐long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo‐Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well‐established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser‐ablation split‐stream ICP‐MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock‐forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock‐forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian‐style prograde path, reaching temperatures of ~610–680 °C. Despite near‐complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low‐Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low‐Y and low‐HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High‐Y monazite inclusions within pristine, crack‐free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high‐Y and high‐HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana‐wide collisional Pan‐African orogenies.     

Petrogenesis of the Yangchang Mo‐bearing granite in the Xilamulun metallogenic belt,NE China: geochemistry,zircon U–Pb ages and Sr–Nd–Pb isotopes

The Yangchang granite‐hosted Mo deposit is typical of the Xilamulun metallogenic belt, which is one of the important Mo–Pb–Zn–Ag producers in China. A combination of major and trace element, Sr, Nd and Pb isotope, and zircon U–Pb age data are reported for the Yangchang batholith to constrain its petrogenesis and Mo mineralization. Zircon LA‐ICPMS U–Pb dating yields mean ages of 138 ± 2 and 132 ± 2 Ma for monzogranite and granite porphyry, respectively. The monzogranites and granite porphyries are calc‐alkaline with K₂O/Na₂O ratios of 0.75–0.92 and 1.75–4.42, respectively. They are all enriched in large‐ion lithophile elements (LILEs) and depleted in high‐field‐strength elements (HFSEs) with negative Nb and Ta anomalies in primitive‐mantle‐normalized trace element diagrams. The monzogranites have relatively high Sr (380–499 ppm) and Y (14–18 ppm) concentrations, and the granite porphyries have lower Sr (31–71 ppm) and Y (5–11 ppm) concentrations than those of monzogranites. The monzogranites and granite porphyries have relatively low initial Sr isotope ratios of 0.704573–0.705627 and 0.704281, respectively, and similar ²⁰⁶Pb/²⁰⁴Pb ratios of 18.75–18.98 and 18.48–18.71, respectively. In contrast, the ε_Nd(t) value (−3.7) of granite porphyry is lower than those of monzogranites (−1.5 to −2.7) with Nd model ages of about 1.0 Ga. These geochemical features suggest that the monzogranite and granite porphyries were derived from juvenile crustal rocks related to subduction of the Paleo‐Pacific plate under east China. Copyright © 2013 John Wiley & Sons, Ltd.     

Zircon U–Pb age and trace element evidence for Paleoproterozoic granulite-facies metamorphism and Archean crustal rocks in the Dabie Orogen

Zircon U–Pb ages and trace elements were determined for granulites and gneiss at Huangtuling, which are hosted by ultrahigh-pressure metamorphic rocks in the Dabie Orogen, east-central China. CL images reveal core–rim structure for most zircons in the granulites. The cores show oscillatory zoning, relatively high Th/U ratios, and HREE enriched patterns, consistent with a magmatic origin. They gave a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2766 ± 9 Ma, interpreted as dating magma emplacement of the protolith. The rims are characterized by sector or planar zoning, low Th/U ratios, negative Eu anomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphic conditions. Zircon U–Pb dating yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2029 ± 13 Ma, which is interpreted to record a metamorphic event, possibly during assembly of the supercontinent Columbia. The gneiss has a protolith age of 1982 ± 14 Ma, which is younger than the zircon age of the granulite-facies metamorphism, suggesting a generally delay between HT metamorphism and the intrusion of post-collisional granites. A few inherited cores with igneous characteristics have ²⁰⁷Pb/²⁰⁶Pb ages of 2.90, 3.28 and 3.53 Ga, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants in the Yangtze Craton. A few Cretaceous metamorphic ages were also obtained, suggesting the influence of post-collisional collapse in response to Cretaceous extension of the Dabie Orogen. It is inferred that the recently discovered Archean basement of the Yangtze Craton occurs as far north as the Dabie Orogen.     

U–Pb ages of Neoarchean granitoids from the Black Hills, South Dakota, USA: implications for crustal evolution in the Archean Wyoming province

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent ²⁰⁷Pb/²⁰⁶Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.     

Ion microprobe U–Pb dating of zircon with a 15 micrometer spatial resolution using NanoSIMS

We developed a ²³⁸U–²⁰⁶Pb and ²⁰⁷Pb–²⁰⁶Pb zircon dating method using a Cameca NanoSIMS NS50 ion microprobe. A 7-to 9-nA O⁻ primary beam was used to sputter a 15-μm crater, and secondary positive ions were extracted for mass analysis using the Mattauch–Herzog geometry. The multicollector system was modified to detect ⁹⁰Zr⁺, ²⁰⁴Pb⁺, ²⁰⁶Pb⁺, ²³⁸U¹⁶O⁺, and ²³⁸U¹⁶O₂⁺ ions simultaneously. A mass resolution of about 4000 at 10% peak height and with a flat peak top was attained, and the sensitivity of Pb was about 4 cps·nA^− 1·ppm^− 1. A multicrystal zircon standard (QGNG) from South Australia with a U–Pb age of 1842 Ma was used as a reference for Pb⁺/UO⁺–UO₂⁺/UO⁺ calibration, and on the basis of the positive correlation between these ratios, we determined the sample ²⁰⁶Pb/²³⁸U ratios. ²⁰⁷Pb/²⁰⁶Pb ratios were measured by magnetic scanning in single-collector mode. The standard zircons 91500, from Canada, and SL13, from Sri Lanka, were analyzed against QGNG. Observed ²³⁸U–²⁰⁶Pb and ²⁰⁷Pb–²⁰⁶Pb ages agreed well with published ages within experimental error. Then, 16 zircon grains in a metamorphic rock from Nagasaki, Japan, were analyzed. Observed ages were compatible with SHRIMP ages, suggesting that the NanoSIMS with a 15-μm probe diameter is suitable for ion microprobe U–Pb zircon dating.     

Petrogenesis of metatexite and diatexite migmatites determined using zircon U–Pb age,trace element and Hf isotope data,Higo metamorphic terrane,central Kyushu,Japan

Metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the Higo low‐P/high‐T metamorphic terrane. Here, we report data from an outcrop in the highest grade part of the granulite facies zone, in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic‐structured metatexite within pelitic gneiss. The migmatites and gneiss contain the same peak mineral assemblage of biotite + plagioclase + quartz + garnet + K‐feldspar with retrograde chlorite ± muscovite and some accessory minerals of ilmenite ± rutile ± titanite + apatite + zircon + monazite ± pyrite ± zinc sulphide ± calcite. Calculated metamorphic P–T conditions are 800–900 °C and 9–12 kbar. Zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning, but no core–rim structure. Zircon from the gneiss and metatexite forms euhedral–subhedral grains comprising inherited cores overgrown by thin rims. The overgrowth rims in the metatexite have lower Th/U ratios than zircon in the diatexite and yield a ²⁰⁶Pb/²³⁸U age of 116.0 ± 1.6 Ma, which is older than the 110.1 ± 0.6 Ma ²⁰⁶Pb/²³⁸U age derived from zircon in the diatexite. Zircon from the diatexite has variable REE contents with convex upward patterns and flat normalized HREE, whereas the overgrowth rims in the metatexite and gneiss have steep HREE‐enriched patterns; however, both types have similar positive Ce and negative Eu anomalies. ¹⁷⁶Hf/¹⁷⁷Hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite. Based on U–Pb ages, trace element and Hf isotope data, the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt, following partial dissolution of inherited protolith zircon during anatexis, whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component. By integrating zircon and petrographic data for the migmatites and pelitic gneiss, the metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived juvenile component. The Cretaceous high‐temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle‐derived basalts under a volcanic arc along the eastern margin of the Eurasian continent and advection of heat via hybrid silicic melts from the lower crust. Post‐peak crystallization of anatectic melts in a high‐T region at mid‐crustal depths occurred in the interval c. 116–110 Ma, as indicated by the difference in zircon ages from the metatexite and diatexite migmatites.     

Zircon U–Pb Geochronology,Elemental Geochemistry and Geological Implications of the Andesite of the Yixian Formation in the Tiejiangyingzi Basin in Southeastern Chifeng,China

The Early Cretaceous andesite of the Yixian Formation is an ore‐host rock in the Tiejiangyingzi basin in southeastern Chifeng, China, and bears several meso‐epithermal Au deposits, such as the Nalingou and Yaogou Au deposits. The geochronology and elemental geochemistry of the Yixian Formation were investigated to reveal its metallogenic context, petrogenesis, and mineralization contribution. The ²⁰⁶Pb/²³⁸U ages of zircons reveals that the andesite formed at 125.5 ± 0.9 Ma. This rock belongs to calc‐alkali rock series according to its elemental geochemical analysis results (i.e., SiO₂ = 63.63%–67.27%, MgO = 1.17%–1.43%, K₂O = 4.07%–4.67%, Mg^# = 35–41, Al₂O₃ = 17.16%–18.63%, Na₂O = 3.35%–4.08%, Sr = (240.1–585.6) × 10^‐6, Yb = (1.83–2.22) × 10^‐6, Y = (13.9–23.36) × 10^‐6, Na₂O/K₂O = 0.7–0.9, Eu/Eu* = (0.75–0.79) × 10^‐6, Sr/Y = 13–27, La_N/Yb_N = 11–19 ). The chondrite‐normalized trace element plot shows that the andesite is enriched in light rare earth elements (LREEs) and Yb. Thus, the residual phase of andesite volcanic rocks is a granulite suite that contains amphibole and little remaining garnet in the Tiejiangyingzi basin. It can be concluded that andesite volcanic rocks distributed in the Tiejiangyingzi basin were formed in an extensional tectonic environment during the Mesozoic orogen, and the lower crust had experienced extensive high‐temperature metamorphism and melting. This gold‐bearing lower crust is closely related to mineralization. Therefore, the conclusions of this research have an important implication for further prospecting in southeastern Chifeng, China.     

Genesis of the Weiquan Ag–Polymetallic Deposit in East Tianshan, China: Evidence from Zircon U–Pb Geochronology and C–H–O–S–Pb Isotope Systematics

The Weiquan Ag-polymetallic deposit is located on the southern margin of the Central Asian Orogenic Belt and in the western segment of the Aqishan-Yamansu arc belt in East Tianshan,northwestern China. Its orebodies, controlled by faults, occur in the lower Carboniferous volcanosedimentary rocks of the Yamansu Formation as irregular veins and lenses. Four stages of mineralization have been recognized on the basis of mineral assemblages, ore fabrics, and crosscutting relationships among the ore veins. Stage I is the skarn stage(garnet + pyroxene), Stage Ⅱ is the retrograde alteration stage(epidote + chlorite + magnetite ± hematite 士 actinolite ± quartz),Stage Ⅲ is the sulfide stage(Ag and Bi minerals + pyrite + chalcopyrite + galena + sphalerite + quartz ± calcite ± tetrahedrite),and Stage IV is the carbonate stage(quartz + calcite ± pyrite). Skarnization,silicification, carbonatization,epidotization,chloritization, sericitization, and actinolitization are the principal types of hydrothermal alteration. LAICP-MS U-Pb dating yielded ages of 326.5±4.5 and 298.5±1.5 Ma for zircons from the tuff and diorite porphyry, respectively. Given that the tuff is wall rock and that the orebodies are cut by a late diorite porphyry dike, the ages of the tuff and the diorite porphyry provide lower and upper time limits on the age of ore formation. The δ~(13)C values of the calcite samples range from-2.5‰ to 2.3‰, the δ~(18)O_(H2 O) and δD_(VSMOW) values of the sulfide stage(Stage Ⅲ) vary from 1.1‰ to 5.2‰ and-111.7‰ to-66.1‰, respectively,and the δ~(13)C, δ~(18)O_(H2 O) and δD_(V-SMOW) values of calcite in one Stage IV sample are 1.5‰,-0.3‰, and-115.6‰, respectively. Carbon, hydrogen, and oxygen isotopic compositions indicate that the ore-forming fluids evolved gradually from magmatic to meteoric sources. The δ~(34)S_(V-CDT) values of the sulfides have a large range from-6.9‰ to 1.4‰, with an average of-2.2‰, indicating a magmatic source, possibly with sedimentary contributions. The ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of the sulfides are 17.9848-18.2785,15.5188-15.6536, and 37.8125-38.4650, respectively, and one whole-rock sample at Weiquan yields~(206)Pb/~(204)Pb,~(207)Pb/~(204)Pb, and ~(208)Pb/~(204)Pb ratios of 18.2060, 15.5674, and 38.0511,respectively. Lead isotopic systems suggest that the ore-forming materials of the Weiquan deposit were derived from a mixed source involving mantle and crustal components. Based on geological features, zircon U-Pb dating, and C-H-OS-Pb isotopic data, it can be concluded that the Weiquan polymetallic deposit is a skarn type that formed in a tectonic setting spanning a period from subduction to post-collision. The ore materials were sourced from magmatic ore-forming fluids that mixed with components derived from host rocks during their ascent, and a gradual mixing with meteoric water took place in the later stages.     

Geochemistry, Zircon U–Pb Dating and Hf Isotopic Characteristics of Neoproterozoic Granitoids in the Yaganbuyang Area, Altyn Tagh, NW China

The South Altyn continental block is an important geological unit of the Altyn Tagh orogenic belt, in which numerous Neoproterozoic granitoids crop out. Granitoids are mainly located in the Paxialayidang–Yaganbuyang area and can provide indispensable information on the dynamics of Rodinia supercontinent aggregation during the Neoproterozoic. Therefore, the study of granitoids can help us understand the formation and evolutionary history of the Altyn Tagh orogenic belt. In this work, we investigated the Yaganbuyang granitic pluton through petrography, geochemistry, zircon U–Pb chronology, and Hf isotope approaches. We obtained the following conclusions:(1) Yaganbuyang granitoids mainly consist of two-mica granite and granodiorite. Geochemical data suggested that these granitoids are peraluminous calc–alkaline or high-K calc–alkaline granite types. Zircon U–Pb data yielded ages of 939±7.1 Ma for granodiorite and ~954 Ma for granitoids, respectively.(2) The εHf(t) values of two–mica granite and granodiorite are in the range of-3.93 to +5.30 and-8.64 to +5.19, respectively. The Hf model ages(TDM2) of two-mica granite and granodiorite range from 1.59–.05 Ga and 1.62–2.35 Ga, respectively, indicating that the parental magma of these materials is derived from ancient crust with a portion of juvenile crust.(3) Granitoids formed in a collisional orogen setting, which may be a response to Rodinia supercontinent convergence during the Neoproterozoic.     

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pb_tot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.     

大别山超高压榴辉岩和花岗片麻岩中锆石Lu-Hf同位素研究

对大别山超高压榴辉岩和花岗片麻岩进行了锆石Lu-Hf同位素分析,结果为原岩来源提供了制约,表明扬子陆块在Rodinia超大陆裂解时的裂谷岩浆活动中发生了显著的陆壳生长.对这些锆石的不同区域进行的U-Pb和Lu-Hf同位素分析和比较表明,不同成因的锆石在^206Pb/^238U年龄、初始Hf同位素组成、Th/U及Lu/Hf比值等方面具有明显的差异.与年龄较老的岩浆核部和幔部相比,年轻的变质增生边具有低的Th/U和Lu/Hf比值但高的εHf（t）值.不同成因锆石的Th/U和Lu/Hf比值存在着正相关性,表明变质作用对锆石的U-Th-Pb和Lu-Hf同位素体系有着相似的影响.高级变质作用有时能够引起岩浆锆石增生边^176Hf/^177Hf比值的显著升高,导致变质新生颗粒或增生边类似于新生地壳的高εHf（t）值假象.对榴辉岩和片麻岩锆石核部的分析发现,镁铁质和长英质原岩在大约750 Ma左右形成一个双峰式火山岩套,另外包含少量的年龄约为2.15 Ga的陆壳.初始Hf同位素组成可分成两组：第一组具有正的εHf（t）值,为5.9±0.9～12.9±0.7;第二组εHf（t）值在零左右,为-4.3±0.5～2.3±0.3.正的εHf（t）值与较年轻的模式年龄相对应,负的εHf（t）值与古元古代模式年龄相对应.前者表明,在扬子陆块北缘裂谷岩浆作用将亏损地幔物质加入到大陆地壳中,同时在新元古代中期的裂谷构造带中存在同时期的壳-幔相互作用.因此,在扬子陆块北缘新元古代中期裂谷岩浆活动中,既有新生地壳生长和即时再造,也有古老地壳再造.     

The timing of ultrahigh-temperature metamorphism in Southern India: U–Th–Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

We report here U–Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat–Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at 2420 Ma, while those from Mettanganam show 2300 Ma peak. Younger zircons with peak ages of 2100 and 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510–450 Ma.While the zircon populations in these rocks suggest multiple sources of Archaean and Palaeoproterozoic age, the monazite data are interpreted to date the timing of ultrahigh-temperature metamorphism in southern India as latest Neoproterozoic to Cambrian in both the Palghat–Cauvery shear zone system and the northern Madurai Block. The data illustrate the extent of Neoproterozoic/Cambrian metamorphism as India joined the Gondwana amalgam at the dawn of the Cambrian.     

Zircon megacrysts from basalts of the Venetian Volcanic Province (NE Italy): U–Pb ages, oxygen isotopes and REE data

Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ¹⁸O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma.     

Two Periods of Skarn Mineralization in the Baizhangyan W–Mo Deposit,Southern Anhui Province,Southeast China: Evidence from Zircon U–Pb and Molybdenite Re–Os and Sulfur Isotope Data

The recently discovered Baizhangyan skarn‐porphyry type W–Mo deposit in southern Anhui Province in SE China occurs near the Middle–Lower Yangtze Valley polymetallic metallogenic belt. The deposit is closely temporally‐spatially associated with the Mesozoic Qingyang granitic complex composed of g ranodiorite, monzonitic g ranite, and alkaline g ranite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones of Sinian Lantian Formation contacting with buried fine‐grained granite, and diorite dykes. There are two types of W mineralization: major skarn W–Mo mineralization and minor granite‐hosted disseminated Mo mineralization. Among skarn mineralization, mineral assemblages and cross‐cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the first W–Au period related to the intrusion of diorite dykes, and the subsequent W–Mo period related to the intrusion of the fine‐grained granite. In this paper, we report new zircon U–Pb and molybdenite Re–Os ages with the aim of constraining the relationships among the monzonitic granite, fine‐grained granite, diorite dykes, and W mineralization. Zircons of the monzonitic granite, the fine‐grained granite, and diorite dykes yield weighted mean U–Pb ages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdenite Re–Os age determinations yield an isochron age of 136.9 ± 4.5 Ma and a weighted mean age of 135.0 ± 1.2 Ma. The molybdenites have δ³⁴S values of 3.6‰–6.6‰ and their Re contents ranging from 7.23 ppm to 15.23 ppm. A second group of two molybdenite samples yield ages of 143.8 ± 2.1 and 146.3 ± 2.0 Ma, containing Re concentrations of 50.5–50.9 ppm, and with δ³⁴S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations of Re (7.23–50.48 ppm), suggesting that metals within the deposit have a mixed crust–mantle provenance. Field observation and new age and isotope data obtained in this study indicate that the first diorite dyke‐related skarn W–Au mineralization took place in the Early Cretaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine‐grained granite‐related skarn W–Mo mineralization took place a little later at 135.0–136.9 Ma, and was crust‐dominated. The fine‐grained granite was not formed by fractionation of the Qingyang monzonitic granite. This finding suggests that the first period of skarn W–Au mineralization in the Baizhangyan deposit resulted from interaction between basaltic magmas derived from the upper lithospheric mantle and crustal material at 143.0–146.3 and the subsequent period of W–Mo mineralization derived from the crust at 135.0–136.9 Ma.     

Internal structures and U–Pb ages of zircons from a tuff layer in the Meishucunian formation, Yunnan Province, South China

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U–Pb dating.We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U–Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U–Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 ± 17 Ma for the oscillatory rims and 515 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U–Pb nano-SIMS age of 536.5 ± 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone 1 (Marker A) is more appropriate for the PC/C boundary than is the top of Zone 1 (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.