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.     

Determination of the stress conditions of the ductile-to-brittle regime along the Asuke Shear Zone,SW Japan

The stress conditions of the ductile-to-brittle regime have been assessed along the Asuke Shear Zone (ASZ), which strikes NE–SW in the Cretaceous Ryoke granite terrain in SW Japan. Along the ASZ, pseudotachylyte and mylonitized pseudotachylyte are locally developed together with cataclasite. The simultaneous operation of dislocation creep and grain-size-sensitive creep, as indicated by the coexistence of the Z-maximum and relatively random c-axis lattice preferred orientations as well as the sizes of dynamically recrystallized quartz grains (6.40–7.79 μm) in the mylonitized pseudotachylyte, suggest differential stresses of 110–130 MPa at ∼300 °C. The e-twin morphology, twinning ratio, and distribution of the glide direction on the e-twin plane of the twinned calcite in the amygdules of the pseudotachylyte suggest the stress conditions of the σ₁ and σ₃ axes trend 228° and 320° and plunge 55° and 1°, respectively, and indicate differential stresses of 40–80 MPa at 150–200 °C. Based on kinematic indicators in the fault rocks, the stress conditions estimated from calcite twins, and the cooling history of the granitic protolith, the ASZ is inferred to have been activated under a stress state that caused sinistral normal movements before and after pseudotachylyte formation at 70–50 Ma.     

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.     

U–Pb and Re–Os Ages of the Huaheitan Molybdenum Deposit,NW China

The Huaheitan molybdenum deposit in the Beishan area of northwest China consists of quartz‐sulfide veins. Orebodies occur in the contact zone of the Huaniushan granite. LA‐ICPMS U–Pb zircon dating constrains the crystallization of the granite at 225.6 ± 2.2 Ma (2σ, MSWD = 4.5). Re–Os dating of five molybdenite samples yield model ages ranging from 223.2 ± 3.5 Ma to 228.6 ± 3.4 Ma, with an average of 225.2 ± 2.4 Ma. The U–Pb and Re–Os ages are identical within the error, suggesting that the granite and related Huaheitan molybdenum deposit formed in the Late Triassic. Our new data, combined with published geochronological results from the other molybdenum deposits in this region, imply that intensive magmatism and Mo mineralization occurred during 240 Ma to 220 Ma throughout the Beishan area.     

Tectonics of the southern Asian Plate margin along the Karakoram Shear Zone: Constraints from field observations and U–Pb SHRIMP ages

New geological observations, recent published data and U–Pb SHRIMP zircon dating from the Karakoram Mountains along the Nubra and Shyok Rivers reveal that the initial subduction of the Tethyan oceanic lithosphere took place ~ 110 Ma beneath the Paleozoic–Mesozoic platform of the southern edge of the Asian Plate. This has produced the I-type plutons within the Karakoram Batholith Complex, well before the juxtaposition of the Asian Plate along the Karakoram Shear Zone. Within this shear zone, U–Pb zircon crystallisation ages of ~ 75 Ma from mylonitised granitoids and 68 Ma from undeformed Tirit granodiorite constrain the timing of suturing of the Karakoram terrain with the Trans-Himalaya between 75 and 68 Ma. Post-shearing leucogranite was episodically generated within frontal migmatised Karakoram Metamorphic Belt and emplaced between 20 and 13 Ma within the shear zone. Presence of a low resistivity zone as a possible indication of mid-crustal partial molten crust underneath the Higher Himalaya–Ladakh–Karakoram terrains manifests the impingement of the Indian Plate along the Main Himalayan Thrust at depth.

Physical continuity of the Baltoro granite belt into the Karakoram Batholith is established as well as the continuity of the Shyok suture as the Shiquanhe Suture Zone in western Tibet through the Chushul–Dungti sector. The Karakoram Shear Zone, therefore, displays a complex geological history of movements since ~ 75 Ma and plays a very significant role in the overall India–Asia convergence, rather than merely being a strike-slip fault for eastward extrusion of a segment of Asia in Tibet.     

Geologic evolution of the Serrinha nucleus granite–greenstone terrane (NE Bahia, Brazil) constrained by U–Pb single zircon geochronology

U–Pb single zircon crystallization ages were determined using TIMS and sensitive high resolution ion microprobe (SHRIMP) on samples of granitoid rocks exposed in the Serrinha nucleus granite–greenstone terrane, in NE Brazil. Our data show that the granitoid plutons can be divided into three distinct groups. Group 1 consists of Mesoarchaean (3.2–2.9 Ga) gneisses and N-S elongated TTG (Tonalite-Trondhjemite-Granodiorite) plutons with gneissic borders. Group 2 is represented by ca. 2.15 Ga pretectonic calc-alkaline plutons that are less deformed than group 1. Group 3 is ca. 2.11–2.07 Ga, late to post-tectonic plutons (shoshonite, syenite, K-rich granite and lamprophyre). Groups 2 and 3 are associated with the Transamazonian orogeny. Xenocryst ages of 3.6 Ga, the oldest zircon yet recorded within the São Francisco craton, are found in the group 3 Euclides shoshonite within the Uauá complex and in the group 2 Quijingue trondhjemite, indicating the presence of Paleoarchaean sialic basement.Group 1 gneiss-migmatitic rocks (ca. 3200 Ma) of the Uauá complex constitute the oldest known unit. Shortly afterwards, partial melting of mafic material produced a medium-K calc-alkaline melt, the younger Santa Luz complex (ca. 3100 Ma) to the south. Subsequent TTG melts intruded in different phases now exposed as N-S elongated plutons such as Ambrósio (3162 ± 26 Ma), Araci (3072 ± 2 Ma), Requeijão (2989 ± 11 Ma) and others, which together form a major part of the Archaean nucleus. Some of these plutons have what appear to be intrusive, but are probably remobilized, contacts with the Transamazonian Itapicuru greenstone belt. The older gneissic rocks occur as enclaves within younger Archaean plutons. Thus, serial additions of juvenile material over a period of several hundred m.y. led to the formation of a stable micro-continent by 2.9 Ga. Evidence for Neoarchaean activity is found in the inheritance pattern of only one sample, the group 2 Euclides pluton.Group 2 granitoid plutons were emplaced at 2.16–2.13 Ga in a continental arc environment floored by Mesoarchaean crust. These plutons were subsequently deformed and intruded by late to post-tectonic group 3 alkaline plutons. This period of Transamazonian orogeny can be explained as a consequence of ocean closure followed by collision and slab break-off. The only subsequent magmatism was kimberlitic, probably emplaced during the Neoproterozoic Braziliano event, which sampled older zircon from the basement.     

Petrology and zircon U–Pb dating of well‐preserved eclogites from the Thongmön area in central Himalaya and their tectonic implications

The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1₍₁₎, (b) in the peak eclogite facies [stage M1₍₂₎], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1₍₁₎ is constrained to 19–20 kbar and 640–660°C and M1₍₂₎ occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene.     

Geochronology and Genesis of the Tiegelongnan Porphyry Cu(Au) Deposit in Tibet: Evidence from U–Pb,Re–Os Dating and Hf,S, and H–O Isotopes

The Tiegelongnan Cu (Au) deposit is the largest copper deposit newly discovered in the Bangong–Nujiang metallogenic belt. The deposit has a clear alteration zoning consisting of, from core to margin, potassic to propylitic, superimposed by phyllic and advanced argillic alteration. The shallow part of the deposit consists of a high sulphidation‐state overprint, mainly comprising disseminated pyrite and Cu–S minerals such as bornite, covellite, digenite, and enargite. At depth porphyry‐type mineralization mainly comprises disseminated chalcopyrite, bornite, pyrite, and a minor vein molybdenite. Mineralization is disseminated and associated with veins contained within the porphyry intrusions and their surrounding rocks. The zircon U–Pb ages of the mineralized diorite porphyry and granodiorite porphyry are 123.1 ± 1.7 Ma (2σ) and 121.5 ± 1.5 Ma (2σ), respectively. The molybdenite Re–Os age is 121.2 ± 1.2 Ma, suggesting that mineralization was closely associated with magmatism. Andesite lava (zircon U–Pb age of 111.7 ± 1.6 Ma, 2σ) overlies the ore‐bodies and is the product of post‐mineralization volcanic activity that played a critical role in preserving the ore‐bodies. Values of ?4.6 ‰ to + 0.8 ‰ δ³⁴S for the metal sulfides (mean ? 1.55 ‰) suggest that S mainly has a deep magmatic source. The H and O isotopic composition is (δD = ?87 ‰ to ?64 ‰; δ¹⁸O_H2O = 5.5 ‰ to 9.0 ‰), indicating that the ore‐forming fluids are mostly magmatic‐hydrothermal, possibly mixed with a small amount of meteoric water. The zircon ε_Hf(t) of the diorite porphyry is 3.7 to 8.3, and the granodiorite porphyry is 1.8 to 7.5. Molybdenite has a high Re from 382.2 × 10^?6 to 1600 × 10^?6. Re and Hf isotope composition show that Tiegelongnan has some mantle source, maybe the juvenile lower crust from crust–mantle mixed source. Metallogenesis of the Tiegelongnan giant porphyry system was associated with intermediate to acidic magma in the Early Cretaceous (~120 Ma). The magma provenance of the Tiegelongnan deposit has some mantle‐derived composition, possibly mixed with the crust‐derived materials.     

Further evidence for 1.85 Ga metamorphism in the Central Zone of the North China Craton: SHRIMP U–Pb dating of zircon from metamorphic rocks in the Lushan area, Henan Province

This paper reports SHRIMP zircon U–Pb dating of Precambrian supracrustal and granitic rocks from the Lushan area, Henan Province, in the southern portion of the Central Zone (also referred to as the Trans-North China Orogen) of the North China Craton. A graphite–garnet–sillimanite gneiss (Sample TW0006/1) of the Shangtaihua ‘Group’ gives a range of inherited zircon ages from 2.73 to 2.26 Ga and a metamorphic zircon age of 1.84 ± 0.07 Ga. A garnet-bearing gneissic granitoid (Sample TWJ358/1), which is considered to intrude the Shangtaihua ‘Group’, gives a magmatic zircon age of 2.14 ± 0.02 Ga and a metamorphic zircon age of 1.87 Ga. The metamorphic zircon ages of 1.87–1.84 Ga obtained in this study indicate that an important tectonothermal event occurred at the end of the Paleoproterozoic in the Lushan area. This supports the southern continuation of a Central Zone in the North China Craton that workers have recently considered to result from continent–continent collision. It is also evident that the Shangtaihua ‘Group’ was formed during the Paleoproterozoic (between 2.26 and 2.14 Ga), and not during the Archean, as previously considered.     

U–Pb and Re–Os Geochronology of the Tongcun Molybdenum Deposit and Zhilingtou Gold‐Silver Deposit in Zhejiang Province,Southeast China,and Its Geological Implications

Mesozoic ore deposits in Zhejiang Province, Southeast China, are divided into the northwestern and southeastern Zhejiang metallogenic belts along the Jiangshan–Shaoxing Fault. The metal ore deposits found in these belts are epithermal Au–Ag deposits, hydrothermal‐vein Ag–Pb–Zn deposits, porphyry–skarn Mo (Fe) deposits, and vein‐type Mo deposits. There is a close spatial–temporal relationship between the Mesozoic ore deposits and Mesozoic volcanic–intrusive complexes. Zircon U–Pb dating of the ore‐related intrusive rocks and molybdenite Re–Os dating from two typical deposits (Tongcun Mo deposit and Zhilingtou Au–Ag deposit) in the two metallogenic belts show the early and late Yanshanian ages for mineralization. SIMS U–Pb data of zircons from the Tongcun Mo deposit and Zhilingtou Au–Ag deposit indicate that the host granitoids crystallized at 169.7 ± 9.7 Ma (2σ) and 113.6 ± 1 Ma (2σ), respectively. Re–Os analysis of six molybdenite samples from the Tongcun Mo deposit yields an isochron age of 163.9 ± 1.9 Ma (2σ). Re–Os analyses of five molybdenite samples from the porphyry Mo orebodies of the Zhilingtou Au‐Ag deposit yield an isochron age of 110.1 ± 1.8 Ma (2σ). Our results suggest that the metal mineralization in the Zhejiang Province, southeast China formed during at least two stages, i.e., Middle Jurassic and Early Cretaceous, coeval with the granitic magmatism.     

Structure and U–Pb zircon geochronology in the Variscan foreland of SW Sardinia,Italy

New field mapping, U–Pb zircon geochronology and structural analysis of the southernmost Sardinia metamorphic basement, considered a branch of the Variscan foreland, indicate that it is, in part, allochthonous and was structurally emplaced within the foreland area, rather than being older depositional basement beneath the foreland succession. The Bithia Formation, classically considered part of the ‘Southern Sulcis metamorphic Complex’ (and here termed the Bithia tectonic unit, or BTU), is a greenschist facies metamorphic unit commonly interpreted as Precambrian in age. New geochronology of felsic volcanic rocks in the BTU, however, yield a U–Pb zircon age of 457.01 ± 0.17 Ma (Upper Ordovician). Thus, the depositional age of the unit is younger than the weakly metamorphosed Lower Cambrian rocks of the adjacent foreland succession. New detailed mapping and analysis of the field relationships between the BTU and foreland succession indicates that their contact is a mylonitic shear zone. The metamorphic character, general lithology, and deformational history of the BTU are similar to those of units in the Variscan Nappe Zone located northeast of the foreland area. We reinterpret the BTU as a synformal klippe of material related tectonically to the Variscan Nappe Zone. We infer that it was thrust over and became infolded into the foreland during late stages of the Variscan contractional deformation. Copyright © 2012 John Wiley & Sons, Ltd.     

Laser ablation coupled with ICP-MS applied to U–Pb zircon geochronology: A review of recent advances

Improvements in the technology of laser ablation and ICP-MS instruments make LA-MC-ICPMS a rapid, precise and accurate method for U–Pb zircon geochronology. In this review we describe the main stages of the evolution of this in situ approach from the early 1990s to the present time. Some key points have been progressively improved. The crater size has been reduced to achieve real in situ measurements. The laser wavelength has been reduced as well as the duration of each pulse in order to lower inter-element fractionation. The blank from the gas has to be lowered as far as possible. Double focusing instruments and magnetic field sectors allow flat-topped peaks required for precise isotope ratio measurement to be obtained. The use of a multi-ion counting system significantly improves the sensitivity of the method and the static mode of integration favours the precision of measurement of the transient signal originating from a noisy laser ablated particle beam.Combining the use of a 213 nm UV laser and a MC-ICPMS equipped with a multi-ion counting system operating in static mode, the common precisions achieved for the key ratios ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁶Pb/²³⁸U are better than 1% and 3% (2σ) respectively, including error propagation associated with standard normalization. Until now, the use of a zircon standard has remained necessary to ensure the accuracy of the calculated age. A strategy for common-Pb correction is proposed according to the age of the zircon and according to the Th/U ratio of the grains. After recording sixteen to twenty spot analyses the precision usually achieved on the age is about 1% and even significantly better for Proterozoic samples.In order to show the performance achieved by modern LA-MC-ICPMS geochronology, we tested four zircon samples covering a wide age range from 290 to 2440 Ma. These new age determinations can be compared in term of precision and accuracy since they have already been dated by reference methods (ID-TIMS and SHRIMP). Further developments in the technology of ion counters equipping modern MC-ICPMS and in laser systems will certainly be applied to a large field of geochronology studies in the near future as an alternative to SIMS for in situ age determination.     

Zircon U–Pb Dating and Geochemistry of the Linong Granitoid and Its Relationship to Cu Mineralization in the Yangla Copper Deposit,Yunnan, China

The Yangla copper deposit (Cu reserves: 1.2 Mt) in the Jinshajiang–Lancangjiang–Nujiang region in China is spatially associated with the Linong granitoid. Zircon U–Pb dating shows the granitoid formed at 234.1 ± 1.2 to 235.6 ± 1.2 Ma, and the KT2 ore body of the deposit yields a molybdenite Re–Os model age of 230.9 ± 3.2 Ma. The ages of mineralization and crystallization of the granitoid are identical within the measurement uncertainties, suggesting the Yangla deposit is genitically related to the Indosinian Linong granitoid.     

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.     

Zircon U–Pb and Wolframite Sm–Nd Dating of the Bayinsukhtu Tungsten Deposit in Southern Mongolia and its Geological Significance

The Bayinsukhtu tungsten deposit is a newly discovered quartz‐vein tungsten deposit in the Xing'an–Mongolia Orogenic Belt (XMOB) in southern Mongolia, hosted by the Bayinsukhtu granite porphyry. The granite porphyry is located mainly south of the study area, over 3 km². The rock consists of quartz and feldspar phenocrysts in a fine‐grained matrix, also mainly composed of feldspar and quartz. The granite porphyry samples demonstrate high SiO₂ and high alkalinity. All samples also straddle the high‐potassium calc‐alkaline series. In a plot of the molar ratios of A/NK versus A/CNK, the granites are metaluminous. The chondrite‐normalized REE patterns are characterized by large negative Eu anomalies and fractionated LREEs. The U–Pb age of zircons from the granite porphyry is 298.8 ± 1.8 Ma, and the Sm–Nd age of the five wolframite samples from the tungsten deposit is 303 ± 19 Ma. The cooling age of the granite porphyry and tungsten mineralization is within the error of measurement and is of the Late Carboniferous age. Geological and geochronological evidence shows that the tungsten mineralization and the granite porphyry at Bayinsukhtu are genetically closely related and that they are results of Carboniferous magmatism. Their tectonic setting is related to the accretion of the Central Asian Orogenic Belt during the late Paleozoic era.     

Re–Os and U–Pb Geochronology of the Dawan Mo–Zn–Fe Deposit in Northern Taihang Mountains,China

The Dawan Mo–Zn–Fe deposit located in the Northern Taihang Mountains in the middle of the North China Craton (NCC) contains large Mo‐dominant deposits. The mineralization of the Dawan Mo–Zn–Fe deposit is associated with the Mesozoic Wanganzhen granitoid complex and is mainly hosted within Archean metamorphic rocks and Proterozoic–Paleozoic dolomites. Rhyolite porphyry and quartz monzonite both occur in the ore field and potassic alteration, strong silicic–phyllic alteration, and propylitic alteration occur from the center of the rhyolite porphyry outward. The Mo mineralization is spacially related to silicic and potassic alteration. The Fe orebody is mainly found in serpentinized skarn in the external contact zone between the quartz monzonite and dolomite. Six samples of molybdenite were collected for Re–Os dating. Results show that the Re–Os model ages range from 136.2 Ma to 138.1 Ma with an isochron age of 138 ± 2 Ma (MSWD = 1.2). U–Pb zircon ages determined by laser ablation inductively coupled plasma mass spectrometry yield crystallization ages of 141.2 ± 0.7 (MSWD = 0.38) and 130.7 ± 0.6 Ma (MSWD = 0.73) for the rhyolite porphyry and quartz monzonite, respectively. The ore‐bearing rhyolite porphyry shows higher K₂O/Na₂O ratios, ranging from 58.0 to 68.7 (wt%), than those of quartz monzonite. All of the rock samples are classified in the shoshonitic series and characterized by enrichment in large ion lithophile elements; depletion in Mg, Fe, Ta, Ni, P, and Y; enrichment in light rare earth elements with high (La/Yb)_n ratios. Geochronology results indicate that skarn‐type Fe mineralization associated with quartz monzonite (130.7 ± 0.6 Ma) formed eight million years later than Mo and Zn mineralization (138 ± 2 Ma) in the Dawan deposit. From Re concentrations in molybdenite and previously presented Pb and S isotope data, we conclude that the ore‐forming material of the deposit was derived from a crust‐mantle mixed source. The porphyry‐skarn type Cu–Mo–Zn mineralization around the Wanganzhen complex is related to the primary magmatic activity, and the skarn‐type Fe mineralization is formed at the late period magmatism. The Dawan Mo–Zn–Fe porphyry‐skarn ores are related to the magmatism that was associated with lithospheric thinning in the NCC.     

U–Pb zircon geochronology of ferrodiorites and quartz diorites from the Turkel Anorthosite Complex: a Neoarchaean convergent margin in eastern India

The Turkel anorthosite Complex (TAC) in the Eastern Ghats Belt in India is composed of anorthosites and leuconorites at the centre and ferrodiorites and quartz diorites at the periphery. Here we report whole‐rock geochemistry, and zircon U–Pb data and REE geochemistry from a co‐spatial ferrodiorite and two quartz diorites from the TAC. The diorites have low abundance of High Field Strength Elements (HFSE) and REE, exhibit a flat chondrite‐normalized pattern with slight LREE enrichment and negligible or no Eu anomaly. Our results show weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 2433 ± 33 Ma for the ferrodiorite. Two quartz diorite samples from Turkel yield mean²⁰⁷Pb/²⁰⁶Pb ages of 2419 ± 32 Ma and 2505 ± 31 Ma. The zircons from all the analysed samples show high REE contents, prominent HREE enrichment and a conspicuous positive Eu anomaly, suggesting a common magmatic source. The prominent Neoarchaean to early Palaeoproterozoic magmatic ages from the anorthosite complex deviate from the late Neoproterozoic ages reported from other anorthosite suites in the Eastern Ghats Belt, and suggest an active convergent margin along SE India during Archaean–Proterozoic transition. Copyright © 2014 John Wiley & Sons, Ltd.     

SHRIMP U–Pb zircon dating of the Neoproterozoic Penglai Group and Archean gneisses from the Jiaobei Terrane, North China, and their tectonic implications

Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.     

U–Pb and Re–Os Geochronology of Karamay Porphyry Mo–Cu Deposit in Western Junggar,NW China

The Karamay porphyry Mo–Cu deposit, discovered in 2010, is located in the West Junggar region of Xinjiang of northwest China. The deposit is hosted within the Karamay granodiorite porphyry that intruded into Early Carboniferous sedimentary strata and its exo‐contact zone. The LA‐ICPMS U–Pb method was used to date the zircons from the granodiorite samples of the porphyry. Analyses of 12 spots of zircons from the granodiorite samples yield a U–Pb weighted mean age of 300.8 ± 2.1 Ma (2σ). Re–Os dating for five molybdenite samples obtained from two prospecting trenches and three outcrops in the deposit yield a Re–Os isochron age of 294.6 ± 4.6 Ma (2σ), with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.0 ± 1.1. The isochron age is within the error of the Re–Os model ages, demonstrating that the age result is reliable. The Re–Os isochron age of the molybdenite is consistent with the U–Pb age of the granodiorite porphyry, which indicates that the deposit is genetically related with an Early Permian porphyry system. The ages of the Karamay Mo–Cu deposit and the ore‐bearing porphyry are similar to the ages of intermediate‐acid intrusions and Cu–Mo–Au polymetallic deposits in the West Junggar region. This consistency suggests the same geodynamic process to the magmatism and related mineralization.     

U–Pb Dating of Hydrothermal Zircons from the Neoproterozoic Liushanyan VMS Cu–Zn Deposit,Central China: Evidence for a Triassic Deformation Event

The Liushanyan deposit is an important volcanic‐host massive sulfide (VMS) Cu–Zn deposit in the Qinling‐Tongbai‐Dabie orogenic belt, central China, with reserve of 2.38 Mt Cu and 16.11 Mt Zn. Orebodies occur in the meta‐quartz keratophyre of the Liushanyan formation. In this paper, we present textural features and laser ablation ICP‐MS U–Pb dating results of zircons from the ore‐bearing mylonitized meta‐quartz keratophyre. The hydrothermal zircons are distinct from metamorphic zircons in this rock, showing low cathodoluminescence (CL) response and hydrothermal rims (black in CL images). They have relatively flat light rare earth element patterns and high La content and low (Sm/La)_N and Ce/Ce* values. These features are typical of hydrothermal zircons. The cores of metamorphic zircons yield a weighted mean ²⁰⁶Pb/²³⁸U age of 900 ± 26 Ma, interpreted as the volcanic and related VMS mineralizing age. Two much younger events are also recorded by zircons in this rock: (i) the Early Silurian amphibolites–greenschist facies metamorphism at 435 ± 26 Ma; and (ii) the growth of hydrothermal zircons at ca. 241 ± 1 Ma, associated with the ductile shear deformation. The Silurian metamorphic event is probably associated with the arc–continent collision, while the Triassic ductile deformation event formed in the final continent–continent collision setting.