U–Pb SHRIMP and Sm–Nd geochronology of granite–gneiss complexes and implications for the evolution of the Central Brazil Archean Terrain

The paper is a first attempt to unravel the Archean multi-stage metaplutonic assemblage of the Meso/Neoarchean terrane of the State of Goiás, Central Brazil, by means of the U–Pb SHRIMP zircon and Sm–Nd techniques. Two stages of granitic plutonism, spanning ca. 140 m.y., were precisely established for the accretion of the gneiss protoliths. The earliest stage embraces tonalitic to granodioritic and minor granitic orthogneisses with Nd juvenile signature, emplaced from ca. 2845 to ca. 2785 Ma, interpreted as the roots of an early arc. Inherited zircon xenocrysts and Nd isotopic data indicate that the juvenile magmas underwent contamination from a sialic crust as old as 3.3 Ga, from which there are, so far, no recognizable exposures. The second stage comprises granodioritic to granitic gneisses and lasted from ca. 2711 to 2707 Ma. Based on their Nd isotopic signatures and on inherited zircon crystals, their protoliths are interpreted as dominantly crustal-derived. The SHRIMP data from zircon crystals did not depict a Paleoproterozoic overprinting on the Archean gneisses, which is due to geological processes with prevailing temperatures below the isotopic stability of the U/Pb/Th system in the mineral. These processes comprise crustal extension and intrusion of a mafic dike swarm at ca. 2.3 Ga, followed by low grade events mostly related to shear zones between ca. 2.15 and 2.0 Ga. The study also revealed the extent of the Pan- African tectono-thermal overprinting on the Archean orthogneisses. Most of the zircon populations show morphological evidence of metamorphic peripheral recrystallization dated between ca. 750 and 550 Ma. One of the banded gneisses with a crystallization age of ca. 2700 Ma (2σ) has a more complex zircon population including magmatic new grains, which yielded a precise 206Pb/238U crystallization age of 590 ± 10 Ma (2σ). These new grains are interpreted to have grown in anatectic veins injected within strongly sheared gneiss.The data characterize a widespread Pan-African-aged metamorphic overprinting, culminating with localized anatexis of the Archean orthogneisses.     

Petrology,phase equilibria modelling and zircon U–Pb geochronology of Paleoproterozoic mafic granulites from the Fuping Complex,North China Craton

The Fuping Complex is one of the important basement terranes within the central segment of the Trans‐North China Orogen (TNCO) where mafic granulites are exposed as boudins within tonalite–trondhjemite–granodiorite (TTG) gneisses. Garnet in these granulites shows compositional zoning with homogeneous cores formed in the peak metamorphic stage, surrounded by thin rims with an increase in almandine and decrease in grossular contents suggesting retrograde decompression and cooling. Petrological and phase equilibria studies including pseudosection calculation using thermocalc define a clockwise P–T path. The peak mineral assemblages comprise garnet+clinopyroxene+amphibole+quartz+plagioclase+K‐feldspar+ilmenite±orthopyroxene±magnetite, with metamorphic P–T conditions estimated at 8.2–9.2 kbar, 870–882 °C (15FP‐02), 9.6–11.3 kbar, 855–870 °C (15FP‐03) and 9.7–10.5 kbar, 880–900 °C (15FP‐06) respectively. The pseudosections for the subsequent retrograde stages based on relatively higher H₂O contents from P/T–M(H₂O) diagrams define the retrograde P–T conditions of <6.1 kbar, <795 °C (15FP‐02), 5.6–5.8 kbar, <795 °C (15FP‐03), and <9 kbar, <865 °C (15FP‐06) respectively. Data from LA‐ICP‐MS zircon U–Pb dating show that the mafic dyke protoliths of the granulite were emplaced at c. 2327 Ma. The metamorphic zircon shows two groups of ages at 1.96–1.90 Ga (peak at 1.93–1.92 Ga) and 1.89–1.80 Ga (peak at 1.86–1.83 Ga), consistent with the two metamorphic events widely reported from different segments of the TNCO. The 1.93–1.92 Ga ages are considered to date the peak granulite facies metamorphism, whereas the 1.86–1.83 Ga ages are correlated with the retrograde event. Thus, the collisional assembly of the major crustal blocks in the North China Craton (NCC) might have occurred during 1.93–1.90 Ga, marking the final cratonization of the NCC.     

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.     

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.     

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.     

Paleozoic tectonism on the East Gondwana margin: Evidence from SHRIMP U–Pb zircon geochronology of a migmatite–granite complex in West Antarctica

The Fosdick Mountains migmatite–granite complex in West Antarctica records episodes of crustal melting and plutonism in Devonian–Carboniferous time that acted to transform transitional crust, dominated by immature oceanic turbidites of the accretionary margin of East Gondwana, into stable continental crust. West Antarctica, New Zealand and Australia originated as contiguous parts of this margin, according to plate reconstructions, however, detailed correlations are uncertain due to a lack of isotopic and geochronological data. Our study of the mid-crustal exposures of the Fosdick range uses U–Pb SHRIMP zircon geochronology to examine the tectonic environment and timing for Paleozoic magmatism in West Antarctica, and to assess a correlation with the better known Lachlan Orogen of eastern Australia and Western Province of New Zealand.NNE–SSW to NE–SW contraction occurred in West Antarctica in early Paleozoic time, and is expressed by km-scale folds developed both in lower crustal metasedimentary migmatite gneisses of the Fosdick Mountains and in low greenschist-grade turbidite successions of the upper crust, present in neighboring ranges. The metasedimentary rocks and structures were intruded by calc-alkaline, I-type plutons attributed to arc magmatism along the convergent East Gondwana margin. Within the Fosdick Mountains, the intrusions form a layered plutonic complex at lower structural levels and discrete plutons at upper levels. Dilational structures that host anatectic granite overprint plutonic layering and migmatitic foliation. They exhibit systematic geometries indicative of NNE–SSW stretching, parallel to a first-generation mineral lineation. New U–Pb SHRIMP zircon ages for granodiorite and porphyritic monzogranite plutons, and for leucogranites that occupy shear bands and other mesoscopic-scale structural sites, define an interval of 370 to 355 Ma for plutonism and migmatization.Paleozoic plutonism in West Antarctica postdates magmatism in the western Lachlan Orogen of Australia, but it coincides with that in the central part of the Lachlan Orogen and with the rapid main phase of emplacement of the Karamea Batholith of the Western Province, New Zealand. Emplaced within a 15 to 20 million year interval, the Paleozoic granitoids of the Fosdick Mountains are a product of subduction-related plutonism associated with high temperature metamorphism and crustal melting. The presence of anatectic granites within extensional structures is a possible indication of alternating strain states (‘tectonic switching’) in a supra-subduction zone setting characterized by thin crust and high heat flow along the Devonian–Carboniferous accretionary margin of East Gondwana.     

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.     

Age of the Wirgane granodiorite intrusions (Western High-Atlas, Morocco): New U–Pb constraints

U–Pb geochronological data, obtained on single zircon grains using the ion microprobe Cameca IMS 1270 (CRPG-CNRS, Nancy), indicate a Neoproterozoic age (625 ± 5 Ma) for the two intrusions of Wirgane granodiorite. This age is in variance with the previously published conclusions where the intrusions were supposed Variscan. Consequently, the Wirgane intrusions are interpreted as remnants of the Neoproterozoic basement of the Variscan belt. They are similar to the Anti-Atlas calc-alkaline intrusions of the collisional to post-collisional stages of the Pan-African orogeny. At that time, the Wirgane region was an area of continental growth to the north of the major South Atlasic Fault.     

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.     

U–Pb geochronology of the southern Brasília belt (SE-Brazil): sedimentary provenance, Neoproterozoic orogeny and assembly of West Gondwana

The Brasília belt borders the western margin of the São Francisco Craton and records the history of ocean opening and closing related to the formation of West Gondwana. This study reports new U–Pb data from the southern sector of the belt in order to provide temporal limits for the deposition and ages of provenance of sediments accumulated in passive margin successions around the south and southwestern margins of the São Francisco Craton, and date the orogenic events leading to the amalgamation of West Gondwana.Ages of detrital zircons (by ID–TIMS and LA-MC-ICPMS) were obtained from metasedimentary units of the passive margin of the São Francisco Craton from the main tectonic domains of the belt: the internal allochthons (Araxá Group in the Áraxá and Passos Nappes), the external allochthons (Canastra Group, Serra da Boa Esperança Metasedimentary Sequence and Andrelândia Group) and the autochthonous or Cratonic Domain (Andrelândia Group). The patterns of provenance ages for these units are uniform and are characterised as follows: Archean–Paleoproterozoic ages (3.4–3.3, 3.1–2.7, and 2.5–2.4 Ga); Paleoproterozoic ages attributed to the Transamazonian event (2.3–1.9 Ga, with a peak at ca. 2.15 Ga) and to the ca. 1.75 Ga Espinhaço rifting of the São Francisco Craton; ages between 1.6 and 1.2 Ga, with a peak at 1.3 Ga, revealing an unexpected variety of Mesoproterozoic sources, still undetected in the São Francisco Craton; and ages between 0.9 and 1.0 Ga related to the rifting event that led to the individualisation of the São Francisco paleo-continent and formation of its passive margins. An amphibolite intercalation in the Araxá Group yields a rutile age of ca. 0.9 Ga and documents the occurrence of mafic magmatism coeval with sedimentation in the marginal basin.Detrital zircons from the autochthonous and parautochthonous Andrelândia Group, deposited on the southern margin of the São Francisco Craton, yielded a provenance pattern similar to that of the allochthonous units. This result implies that 1.6–1.2 Ga source rocks must be present in the São Francisco Craton. They could be located either in the cratonic area, which is mostly covered by the Neoproterozoic epicontinental deposits of the Bambuí Group, or in the outer paleo-continental margin, buried under the allochthonous units of the Brasília belt.Crustal melting and generation of syntectonic crustal granites and migmatisation at ca. 630 Ma mark the orogenic event that started with westward subduction of the São Francisco plate and ended with continental collision against the Paraná block (and Goiás terrane). Continuing collision led to the exhumation and cooling of the Araxá and Passos metamorphic nappes, as indicated by monazite ages of ca. 605 Ma and mark the final stages of tectonometamorphic activity in the southern Brasília belt.Whilst continent–continent collision was proceeding on the western margin of the São Francisco Craton along the southern Brasília belt, eastward subduction in the East was generating the 634–599 Ma Rio Negro magmatic arc which collided with the eastern São Francisco margin at 595–560 Ma, much later than in the Brasília belt. Thus, the tectonic effects of the Ribeira belt reached the southernmost sector of the Brasília belt creating a zone of superposition. The thermal front of this event affected the proximal Andrelândia Group at ca. 588 Ma, as indicated by monazite age.The participation of the Amazonian craton in the assembly of western Gondwana occurred at 545–500 Ma in the Paraguay belt and ca. 500 Ma in the Araguaia belt. This, together with the results presented in this work lead to the conclusion that the collision between the Paraná block and Goiás terrane with the São Francisco Craton along the Brasília belt preceded the accretion of the Amazonian craton by 50–100 million years.     

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.     

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.     

Zircon U–Pb geochronological and geochemical constraints on the petrogenesis of the Taishan sanukitoids (Shandong): Implications for Neoarchean subduction in the Eastern Block, North China Craton

Elemental, Sr–Nd–Pb isotopic and geochronological data are presented for the Taishan high-mg dioritic rocks (western Shandong) from the Eastern Block of the North China Craton in order to better understand the Archean tectonic evolution and crustal growth of the Craton. The rocks gave the zircon U–Pb age of 2536–2540 Ma. They show low SiO₂ and Al₂O₃ contents, high MgO, mg-number, Cr, Ni, Y, Yb, Sr and Ba, enriched LILEs and LREEs, depleted HFSEs and HREEs with (Nb/La)_N of 0.07–0.12. They exhibit _Nd(t) values of 1.53–3.30, (²⁰⁶Pb/²⁰⁴Pb)_i of 11.20–15.30, (²⁰⁷Pb/²⁰⁴Pb)_i of 14.14–14.83 and (²⁰⁸Pb/²⁰⁴Pb)_I of 31.10–33.93. Such geochemical features with an affinity to both a mantle- and crust-like source for the Taishan dioritic rocks are similar to those of the typical Archean sanukitoids, suggesting an origination from a sub-arc mantle wedge variably metasomatized by the slab-derived dehydration fluids and melts before 50–100 Ma of the emplacement of the Taishan sanukitoid plutons. It is proposed that the Taishan sanukitoids resulted from the sudden change of the downgoing slab from a flat subduction to subsequently steeper subduction in an active continental margin regime during Neoarchean time.     

U–Pb SHRIMP zircon geochronology and T–t–d history of the Kampa Dome, southern Tibet

Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia ²⁰⁶Pb/²³⁸U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D₁), predominately top-to-N ductile shearing and crustal extension (D₂), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D₃), and continued N–S contraction, E–W extension and doming (D₄). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.     

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.     

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.     

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.     

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.     

Geochemistry,and zircon U–Pb and molybdenite Re–Os geochronology of Jilongshan Cu–Au deposit,southeastern Hubei Province,China

The Jilongshan skarn Cu–Au deposit is located at the Jiurui ore cluster region in the southwestern part of the Middle–Lower Yangtze River valley metallogenic belt. The region is characterized by NW‐, NNW‐ and EW‐trending faults and the mineralization occurs at the contact of lower Triassic carbonate rocks and Jurassic granodiorite porphyry intrusions. The intrusives are characterized by SiO₂, K₂O, and Na₂O concentrations ranging from 61.66 to 67.8 wt.%, 3.29 to 5.65 wt.%, and 2.83 to 3.9 wt.%, respectively. Their A/CNK (A/CNK = n(Al₂O₃)/[n(CaO) + n(Na₂O) + n(K₂O)]) ratio, δEu, and δCe vary from 0.77 to 1.17, 0.86 to 1, and 0.88 to 0.96, respectively. The rocks show enrichment in light rare earth elements ((La/Yb)_N = 7.61–12.94) and large ion lithophile elements (LILE), and depletion in high field strength elements (HFSE), such as Zr, Ti. They also display a peraluminous, high‐K calc‐alkaline signature typical of intrusives associated with skarn and porphyry Cu–Au–Mo polymetallic deposits. Laser ablation inductively coupled plasma spectrometry (LA‐ICP‐MS) zircon U–Pb age indicates that the granodiorite porphyry formed at 151.75 ± 0.70 Ma. A few inherited zircons with older ages (677 ± 10 Ma, 848 ± 11 Ma, 2645 ± 38 Ma, and 3411 ± 36 Ma) suggest the existence of an Archaean basement beneath the Middle–Lower Yangtze River region. The temperature of crystallization of the porphyry estimated from zircon thermometer ranges from 744.3 °C to 751.5 °C, and 634.04 °C to 823.8 °C. Molybdenite Re–Os dating shows that the Jilongshan deposit formed at 150.79 ± 0.82 Ma. The metallogeny and magmatism are correlated to mantle–crust interaction, associated with the subduction of the Pacific Plate from the east. Copyright © 2013 John Wiley & Sons, Ltd.