The age and source of late Hercynian magmatism in the central Alps: evidence from precise U−Pb ages and initial Hf isotopes

This study presents U–Pb ages for zircon, titanite, allanite and epidote, and initial Hf isotopic compositions for zircon of Upper Carboniferous granites, diorites and syenites from the Aar massif, central Alps. The rocks were emplaced during three magmatic pulses after Hercynian collisional tectonics: (A) a shoshonitic-ultrapotassic series at 334±2.5 Ma; (B) scattered diorites and granites at 308–310 Ma; and (C) a high-K cale-alkaline granite batholith at 298±2 Ma. Inheritance of old zircons is negligible among all three groups. The Southern Aar granite, in contrast, is a syn-tectonic, probably ca. 350 Ma old granite that contains large amounts of inherited Precambrian zircons. Alpine metamorphism caused weak lead loss in many analyzed zircon fractions, but left the titanite U–Pb system undisturbed: thorites were almost completely reset by Alpine and recent lead loss. Mineral isochrons defined by titanite, allanite, epidote and apatite yield initial Pb isotopic compositions that are in agreement with the model values of Stacey and Kramers. Initial Hf isotopic compositions range from _Hf=–8 to +3.5. The data follow a trend of increasing _Hf with decreasing age. The _Hf versus element concentration relationships suggest mixing between a mantle and a crustal component. These relationships can be explained in terms of generation of the melts from a subcontinental mantle that had been enriched during subduction events at about 1 Ga and by 300 Ma had developed an isotopic signature distinct from that of MORB-type mantle. Further contamination of the melts occurred during ascent and differentiation in the crust. This late Hercynian magmatism can be related to post-collisional strike-slip tectonics.     

Origin of the Alxa Block,western China: New evidence from zircon U–Pb geochronology and Hf isotopes of the Longshoushan Complex

The NW–SE trending Longshoushan is in the southwestern margin of the Alxa Block, which was traditionally considered the westernmost part of the North China Craton (NCC). Precambrian crystalline basement exposed in the Longshoushan area was termed the “Longshoushan Complex”. This complex's formation and metamorphism are significant to understand the geotectonics and early Precambrian crustal evolution of the western NCC. In this study, field geology, petrology, and zircon U–Pb and Lu–Hf isotopes of representative orthogneisses and paragneisses in the Longshoushan Complex were investigated. U–Pb datings reveal three Paleoproterozoic magmatic episodes (ca. 2.33, ca. 2.17 and ca. 2.04 Ga) and two subsequent regional metamorphic events (ca. 1.95–1.90 Ga and ca. 1.85 Ga) for metamorphic granitic rocks in the Longshoushan Complex. U–Pb dating of the detrital magmatic zircons from two paragneisses yields concordant ²⁰⁷Pb/²⁰⁶Pb ages between 2.2 Ga and 2.0 Ga, and a small number of metamorphic zircon rims provide a ca. 1.95 Ga metamorphic age, suggesting that the depositional time of the protolith was between 2.0 and 1.95 Ga and that the sedimentary detritus was most likely derived from the granitic rocks in the Longshoushan Complex itself. Zircon Lu–Hf isotopic analyses indicate that nearly all magmatic zircons from ca. 2.0 Ga to ca. 2.17 Ga orthogneisses have positive εHf(t) values with two-stage Hf model ages (T_DMC) ranging from 2.45 to 2.65 Ga (peak at ca. 2.5 Ga), indicating that these Paleoproterozoic granitic rocks were derived from the reworking of the latest Neoarchean–early Paleoproterozoic juvenile crust. Detrital magmatic zircons from two paragneisses yield scattered ¹⁷⁶Hf/¹⁷⁷Hf ratios, εHf(t) and T_DMC values, further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded Paleoproterozoic igneous rocks. The ca. 2.15 Ga detrital magmatic zircons from one paragneiss have negative εHf(t) values with T_DMC ranging from 2.76 to 3.04 Ga, indicating another important crustal growth period in the Longshoushan region. These data indicate that the Longshoushan Complex experienced Neoarchean–Early Paleoproterozoic crustal growth, approximately ca. 2.3–2.0 Ga experienced multiphase magmatic events, and approximately ca. 1.95–1.90 Ga and ca. 1.85 Ga experienced high-grade metamorphic events. The sequence of tectonothermal events is notably similar to that of the main NCC. Together with the datasets from an adjacent area, we suggest that the western Alxa Block was most likely an integrated component of the NCC from the Neoarchean to the Paleoproterozoic.     

Detrital zircon U–Pb ages of Late Triassic–Late Jurassic deposits in the western and northern Sichuan Basin margin: constraints on the foreland basin provenance and tectonic implications

Upper Triassic to Upper Jurassic strata in the western and northern Sichuan Basin were deposited in a synorogenic foreland basin. Ion–microprobe U–Pb analysis of 364 detrital zircon grains from five Late Triassic to Late Jurassic sandstone samples in the northern Sichuan Basin and several published Middle Triassic to Middle Jurassic samples in the eastern Songpan–Ganzi Complex and western and inner Sichuan Basin provide an initial framework for understanding the Late Triassic to Late Jurassic provenance of western and northern Sichuan Basin. For further understanding, the paleogeographic setting of these areas and neighboring hinterlands was constructed. Combined with analysis of depocenter migration, thermochronology and detrital zircon provenance, the western and northern Sichuan Basin is displayed as a transferred foreland basin from Late Triassic to Late Jurassic. The Upper Triassic Xujiahe depocenter was located at the front of the Longmen Shan belt, and sediments in the western Sichuan Basin shared the same provenances with the Middle–Upper Triassic in the Songpan–Ganzi Complex, whereas the South Qinling fed the northern Sichuan Basin. The synorogenic depocenter transferred to the front of Micang Shan during the early Middle Jurassic and at the front of the Daba Shan during the middle–late Middle Jurassic. Zircons of the Middle Jurassic were sourced from the North Qinling, South Qinling and northern Yangtze Craton. The depocenter returned to the front of the Micang Shan again during the Late Jurassic, and the South Qinling and northern Yangtze Craton was the main provenance. The detrital zircon U–Pb ages imply that the South and North China collision was probably not finished at the Late Jurassic.     

Tracing the position of the South China block in Gondwana: U–Pb ages and Hf isotopes of Devonian detrital zircons

U–Pb detrital zircon geochronology from Lower Devonian quartz arenites of the northwestern margin of the Yangtze block yields dominant early Neoproterozoic (0.85–1.0 Ga), Pan-African (0.5–0.65 Ga) and middle Neoproterozoic (0.68–0.8 Ga) age populations and minor Mesoproterozoic to middle Mesoarchean (1.0–3.0 Ga) ages. Middle Mesoarchean to Mesoproterozoic rocks, however, are widespread in the South China block. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all the age groupings, the crust growth, essentially mantle-derived, occurred mainly around 3.1 Ga, 1.9 Ga and 1.0 Ga, respectively. Zircon typology and youngest grain ages indicate that this suite of quartz arenites was the product of multiphase reworking. Abundant magmatic zircon detritus with concordant U–Pb Grenvillian and Pan-African ages, together with accompanying various ε_Hf(t) values, indicate an exotic provenance for the quartz arenite external to the South China block. Qualitative comparisons of age spectra for the late Neoproterozoic sediments of the Cathaysian Block, early Paleozoic sediments of pre-rift Tethyan Himalaya sequence in North India and lower Paleozoic sandstone from the Perth Basin in West Australia, show that they all have two the largest age clusters representing Grenvillian and Pan-African orogenic episodes. The resemblance of these age spectra and zircon typology suggests that the most likely source for the Lower Devonian quartz arenites of the South China block was the East African Orogen and Kuunga Orogen for their early Grenvillian and Pan-African populations, whereas the Hannan–Panxi arc, Jiangnan orogen, and the Yangtze block basements might have contributed to the detrital zircon grains of the Neoproterozoic and Pre-Grenvillian ages. Hf isotopic data indicate that the crustal evolution of the drainage area matches well with the episodic crust generation of Gondwana. These results imply that the previously suggested position of the SCB in Gondwana should be re-evaluated, and the South China block should be linked with North India and West Australia as a part of East Gondwana during the assembly of Gondwana, rather than a discrete continent block in the paleo-Pacific.     

Late Palaeozoic and early Mesozoic tectonic and palaeogeographic evolution of central China: evidence from U–Pb and Lu–Hf isotope systematics of detrital zircons from the western Qinling region

The western Qinling region of central China is situated centrally in the Kunlun, Qilian, Qinling, Longmenshan, and Songpan–Ganzi orogens. Late Palaeozoic and Early Mesozoic sediments deposited here may provide keys to understanding the tectonic evolution of the Palaeo-Tethys and collision of the North China and Yangtze Cratons. We conducted in situ U–Pb and Lu–Hf isotope analyses of 568 detrital zircons collected from Upper Palaeozoic to Mesozoic sandstones in the central Qinling block, Taohe depression, and Bailongjiang block in western Qinling to constrain the sources of these sandstones. Our results reveal that the Bailongjiang block has affinities with the Yangtze Craton, from which it may have been rifted. Therefore, the Palaeo-Tethyan Animaqen suture between the two cratons lies north of the Bailongjiang block. We identified the North China Craton as the main source for Triassic flysch in central China. It is possible that the Bailongjiang block could have blocked detritus shed from the North China Craton into the main depositional basins in the Songpan–Ganzi area. The dominance of 300–200 Ma detrital zircons of metamorphic origin in Lower Jurassic sandstones indicates that the Dabie–Qinling orogen was elevated during Early Jurassic time. In addition, our Lu–Hf isotopic results also reveal that Phanerozoic igneous rocks in central China were mostly products of crustal reworking with insignificant formation of juvenile crust.     

Contrasting sources of Late Paleozoic rhyolite magma in the Polish Lowlands: evidence from U–Pb ages and Hf and O isotope composition in zircon

International Journal of Earth Sciences - The Polish Lowlands, located southwest of the Teisseyre–Tornquist Zone, within Trans-European Suture Zone, were affected by bimodal, but dominantly...     

Petrology,geochemistry and zircon U–Pb and Lu–Hf isotopes of the Cretaceous dykes in the central North China Craton: Implications for magma genesis and gold metallogeny

The Trans-North China Orogen (TNCO), a Paleoproterozoic suture that amalgamates the Western and Eastern Blocks of the North China Craton (NCC), witnessed extensive magmatism and metallogeny during Mesozoic, associated with intraplate tectonics and differential destruction of the cratonic lithosphere. Here we investigate a suite of porphyry dykes surrounding the Mapeng batholith in the Fuping Complex within the TNCO in relation to the Mesozoic gold and molybdenum mineralization. The major element chemistry of these dykes show a range of SiO₂ (57.92 to 69.47 wt.%), Na₂O (3.20 to 4.77 wt.%), K₂O (3.12 to 4.60 wt.%) and MgO (0.51 to 3.67 wt.%), together with high concentration of LREE and LILE, and relatively low contents of HREE and HFSE. The rocks display (La/Yb)_N = 13.53–48.11, negative Nb, Ta, Th, U and Zr anomalies, and distinctly positive Ba, K and Sm anomalies. The mineralogy and geochemistry of the porphyry dykes indicate the rocks to be high-K calc-alkaline, and I-type, with adakitic features similar to those of the adjacent Mapeng batholith. The source magma for these rocks was derived from a mixture of reworked ancient continent crust and juvenile mantle materials. The zircon U–Pb data from these rocks show ages in the range of 124 to 129 Ma, broadly coinciding with the emplacement age of the Mapeng intrusion. The inherited zircons of ca. 2.5, 2.0 and 1.8 Ga in the dykes represent capture from the basement rocks during melting. The zircon Lu–Hf isotopic compositions show negative ε_Hf(t) values varying from − 27.8 to − 11.3, with Hf depleted model ages (t_DM) ranging from 1228 Ma to 1918 Ma and Hf crustal model ages (t_DM^C) of 1905 Ma to 2938 Ma, suggesting that the Mesozoic magmatism and associated metallogeny involved substantial recycling of ancient basement rocks of the NCC. We present an integrated model to evaluate the genesis of the porphyry systems and their relation to mineralization. We envisage that these dykes probably acted as stoppers (impermeable barriers) that prevented the leakage and run-off of the ore-bearing fluids, and played a key role in concentrating the gold and molybdenum mineralization.     

Zircon U–Pb ages and Hf isotopic compositions from the Sin Quyen Formation: the Precambrian crustal evolution of northwest Vietnam

The location of the suture zone between the South China and Indochina blocks in northwest Vietnam has been under debate for decades. Generally, the boundary between these blocks has been placed along (1) the Ailaoshan–Red River zone or (2) the Song Ma zone. The Sin Quyen Formation, lying between these zones, was previously regarded as a Palaeo- and Mesoproterozoic sequence. It comprises its provenance and tectonic affinity. We analysed detrital zircons from two paragneisses and one migmatite of the Sin Quyen Formation employing laser ablation inductively coupled plasma mass spectrometry U–Pb dating techniques. U–Pb ages of these zircons show three main periods of zircon formation: ～2.7–3.0, ～2.2–2.5, and ～1.8 Ga, suggesting that Sin Quyen rocks were mainly derived from Palaeoproterozoic and Archaean basement units. Inasmuch as the South China basement comprises rocks of similar ages, we conclude that the Sin Quyen Formation belongs to that block. Our new data strengthen the view that the suture between the South China and Indochina blocks is located within the Song Ma zone. In addition, zircons with U–Pb ages >3.0 thousand million years represent the oldest minerals reported in northwest Vietnam so far, indicating the existence of Mesoarchaean crustal remnants in this region.     

U–Pb ages and Lu–Hf isotopes of detrital zircons from the southern Qinling Orogen: Implications for Precambrian to Phanerozoic tectonics in central China

The Qinling Orogen, central China, was constructed during the Mesozoic collision between the North China and Yangtze continental plates. The orogen includes four tectonic units, from north to south, the Huaxiong Block (reactivated southern margin of the North China Craton), North Qinling Accretion Belt, South Qinling Fold Belt (or block) and Songpan Fold Belt, evolved from the northernmost Paleo-Tethys Ocean separating the Gondwana and Laurentia supercontinents. Here we employ detrital zircons from the Early Cretaceous alluvial sediments within the Qinling Orogen to trace the tectonic evolution of the orogen. The U–Pb ages of the detrital zircon grains from the Early Cretaceous Donghe Group sediments in the South Qinling Fold Belt cluster around 2600–2300 Ma, 2050–1800 Ma, 1200–700 Ma, 650–400 Ma and 350–200 Ma, corresponding to the global Kenorland, Columbia, Rodinia, Gondwana and Pangaea supercontinent events, respectively. The distributions of ages and εHf(t) values of zircon grains show that the Donghe Group sediments have a complex source comprising components mainly recycled from the North Qinling Accretion Belt and the North China Craton, suggesting that the South Qinling Fold Belt was a part of the united Qinling–North China continental plate, rather than an isolated microcontinent, during the Devonian–Triassic. The youngest age peak of 350–200 Ma reflects the magmatic event related to subduction and termination of the Mian-Lue oceanic plate, followed by the collision between the Yangtze Craton and the united Qinling–North China continent that came into existence at the Triassic–Jurassic transition. The interval of 208–145 Ma between the sedimentation of the Early Cretaceous Donghe Group and the youngest age of detrital zircons was coeval with the post-subduction collision between the Yangtze and the North China continental plates in Jurassic.     

Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238U–230Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine

The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare ²³⁸U–²³⁰Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in ¹⁷⁶Hf/¹⁷⁷Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.     

Petrogenesis of the Late Jurassic Laomengshan rhyodacite (Southeast China): constraints from zircon U–Pb dating,geochemistry and Sr–Nd–Pb–Hf isotopes

Extensive magmatism occurred in southeast China during Late Jurassic time, forming large-scale granitic and volcanic rocks associated with non-ferrous, rare earth and rare, radioactive metal deposits. The Shuikoushan Pb–Zn–Au orefield is a typical example located in Hunan Province. This study reports LA-ICP-MS zircon U–Pb ages, whole-rock chemistry, and Sr–Nd–Pb isotopic compositions, and in situ Hf isotopic geochemistry of zircons from the Laomengshan rhyodacite in the Shuikoushan Pb–Zn–Au orefield. Zircon U–Pb dating yields a weighted average age of 156.7 ± 1.6 Ma for the intrusion of the rhyodacite. The rhyodacite samples are mainly shoshonitic series, having metaluminous to weakly peraluminous A/CNK values ranging from 0.96 to 1.09, with moderately high magnesium content (Mg# = 42.4–47.5). Samples display high (⁸⁷Sr/⁸⁶Sr)_i values (0.71165–0.71176), low ε_Nd(T) values (?10.7 to ?10.3), old Nd model ages (T_DM = 1.73–1.86 Ga), and relatively homogeneous Pb isotopic compositions [(²⁰⁶Pb/²⁰⁴Pb)_i = 18.365–18.412, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.663–15.680, and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.625–38.666]. The zircons exhibit enriched ε_Hf(T) values (?16.22 to ?9.86) and old two-stage Hf model ages (T_DM2 = 1.82–2.22 Ga). All the above data indicate that the Laomengshan rhyodacite originated from melting Palaeoproterozoic basement, perhaps contaminated by subordinate mantle melts. Intense extension and thinning of the continental lithosphere during Late Jurassic time resulted in melting of upwelling asthenosphere, and mafic mantle melts interacted with and melted Palaeoproterozoic lower crust, thus forming the Laomengshan rhyodacite.     

Mid- to lower-crustal architecture of the northern Lachlan and southern Thomson orogens: evidence from O–Hf isotopes

Abstract

The nature of the substrate below the northern Lachlan Orogen and the southern Thomson Orogen is poorly understood. We investigate the nature of the mid- to lower-crust using O and Lu–Hf isotope analyses of zircons from magmatic rocks that intrude these regions, and focus on the 440–410 Ma time window to minimise temporal effects while focussing on spatial differences. Over the entire region, weighted mean δ¹⁸O values range from 5.5 to 9.8‰ (relative to VSMOW, Vienna Standard Mean Oceanic Water), and weighted mean ?Hf_t range from ?8.8 to +8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic ?Hf_t (～?7 to ?4) and elevated δ¹⁸O values (～9 to 10‰) reflect a supracrustal source component that may be common to both orogens. Magmatic rocks intruding the Warratta Group in the western part of the Thomson Orogen also have unradiogenic ?Hf_t (～?9 to ?6) but more subdued δ¹⁸O values (～7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic ?Hf and mantle-like δ¹⁸O values, indicative of either a contribution from arc-derived rocks or a direct mantle input. In the northeast Lachlan Orogen Hermidale Terrane, magmatic rocks record mixing of the supracrustal source component with input from a infracrustal or mantle source component (?Hf_t as high as +8.5, δ¹⁸O values as low as 5.5‰), possibly of Macquarie Arc affinity. Samples in the west-southwestern Thomson Orogen also record some evidence of radiogenic input (?Hf_t as high as ?0.5, δ¹⁸O values as low as 6.4‰), possibly from the Mount Wright Arc of the Koonenberry Belt. Overall, our results demonstrate a strong spatial control on isotopic compositions. We find no isotopic differences between the bulk of the Lachlan Orogen and the bulk of the Thomson Orogen, and some indication of similarities between the two.     

Geochronology,geochemistry and Sr–Nd–Hf–S–Pb isotopes of the Early Cretaceous Taoxihu Sn deposit and related granitoids,SE China

The Taoxihu deposit (eastern Guangdong, SE China) is a newly discovered Sn polymetallic deposit. Zircon U-Pb dating yielded 141.8 ± 1.0 Ma for the Sn-bearing granite porphyry and 145.5 ± 1.6 Ma for the biotite granite batholith it intruded. The age of the granite porphyry is consistent (within error) with the molybdenite Re–Os isochron age (139.0 ± 1.1 Ma) of the Sn mineralization, indicating a temporal link between the two. Geochemical data show that the granite porphyry is weakly peraluminous, contain high Si, Na and K, low Fe, Mg, Ca and P, and relatively high Rb/Sr and low K/Rb values. The rocks are enriched in Rb, Th, U, K, and Pb and depleted in Ba, Sr, Ti and Eu, resembling highly fractionated I-type granites. They contain bulk rock initial ⁸⁷Sr/⁸⁷Sr of 0.707371–0.707730 and εNd(t) of −5.17 to −4.67, and zircon εHf(t) values from −6.67 to −2.32, with late Mesoproterozoic T_DM2 ages for both Nd and Hf isotopes. This suggests that the granite porphyry was likely formed by the partial melting of the crustal basement of Mesoproterozoic overall residence age with minor mantle input.δ³⁴S_CDT values of the Taoxihu chalcopyrite and pyrite range from 0.1 to 2.1‰ (average: 0.9‰), implying a dominantly magmatic sulfur source. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of the Taoxihu sulfide ores are 18.497–18.669, 15.642–15.673 and 38.764–38.934, respectively, indicating a mainly upper continental crustal lead source with minor mantle contribution. The highly fractionated and reduced (low calculated zircon Ce⁴⁺/Ce³⁺ and Eu_N/Eu_N¹ values) nature of the ore-forming granitic magma may have facilitated the Sn enrichment and played a key role in the Sn mineralization. We propose that the ore-forming fluids at Taoxihu were of magmatic-hydrothermal origin derived from the granite porphyry, and that both the granite porphyry and the Sn mineralization were likely formed in an extensional setting, possibly related to the subduction slab rollback of the Paleo-Pacific Plate.     

Petrogenesis of synorogenic high-temperature leucogranites (Damara orogen,Namibia): Constraints from U–Pb monazite ages and Nd,Sr and Pb isotopes

Two suites of leucogranites were emplaced at 508 ± 5.9 Ma in the Okombahe District of the Damara belt (Namibia) synchronous with the peak of regional high-temperature metamorphism. The Sr (⁸⁷Sr/⁸⁶Sr_init: 0.707 to 0.711), Nd (εNd_init: − 4.5 to − 6.6), and Pb isotopic (²⁰⁶Pb/²⁰⁴Pb: 18.51–19.13; ²⁰⁷Pb/²⁰⁴Pb: 15.63–15.69; ²⁰⁸Pb/²⁰⁴Pb: 38.08–38.66) compositions indicate that these peraluminous S-type granites were derived from mid- to lower-crustal rocks, which are slightly different to the metapelitic rocks into which they intruded. Since the leucogranites are unfractionated and show no evidence for assimilation or contamination, they constrain the temperature and pressure conditions of their formation. Calculated Zr and LREE saturation temperatures of ca. 850 °C indicate high-temperature crustal melts. High Rb/Sr and low Sr/Ba ratios are consistent with biotite dehydration melting of pelitic source rocks. Qz–Ab–Or systematics reveal that melting and segregation for the least fractionated samples occurred at ca. 7 kbar corresponding to a mid-crustal level of ca. 26 km. However, there is no evidence for a mantle component that could have served as a local heat source for crustal melting. Therefore, the hot felsic magmas that formed close to the time of peak metamorphism are the result of long-lasting high temperature regional metamorphic conditions and intra-crustal collision.     

Late Permian–Triassic siliciclastic provenance,palaeogeography, and crustal growth of the Songpan terrane,eastern Tibetan Plateau: evidence from U–Pb ages,trace elements,and Hf isotopes of detrital zircons

In order to constrain the detrital provenance of the siliciclastic rocks, palaeogeographic variations, and crustal growth history of central China, we carried out simultaneously in situ U–Pb dating and trace element and Hf isotope analyses on 368 detrital zircons obtained from upper Permian–Triassic sandstones of the Songpan terrane, eastern Tibetan Plateau. Two groups of detrital zircons, i.e. magmatic and metamorphic in origin, have been identified based on cathodoluminescence images, zircon Ti-temperatures, and Th/U ratios. Our data suggest that the derivation of siliciclastic rocks in the Songpan terrane was mainly from the Qinling, Qilian, and Kunlun orogens, whereas the Yangtze and North China Cratons served as minor source areas during late Permian–Triassic times. The detrital zircons from Middle–Late Triassic siliciclastic rocks exhibit wide age spectra with two dominant populations of 230–600 Ma and >1600 Ma, peaking at ~1.8–1.9 Ga and ~2.4–2.5 Ga, suggestive of a derivation from the Qinling, Qilian, and Kunlun orogens and the Yangtze Craton being the minor source area. The proportions of detrital zircon populations from the northern Qinling, Qilian, and Kunlun orogens distinctly decreased during Middle–Late Triassic time, demonstrating that the initial uplift of the western Qinling occurred then and it could have blocked most of the detritus from the Qilian–northern Qinling orogens and North China Cratons into the main Songpan–Ganzi depositional basin. The relatively detrital zircon proportions of the Yangtze Craton source decreased during Early-Middle Late Triassic time, indicating that the Longmenshan orogen was probably being elevated, since the early Late Triassic and gradually formed a barrier between the Yangtze Craton and the Songpan terrane. In addition, our Lu–Hf isotopic results also reveal that the Phanerozoic magmatic rocks in central China had been the primary products of crustal reworking with insignificant formation of a juvenile crust.     

Sedimentological characteristics and new detrital zircon SHRIMP U–Pb ages of the Babulu Formation in the Fohorem area,Timor-Leste

Sedimentological characteristics and zircon provenance dating of the Babulu Formation in the Fohorem area, Timor-Leste, provide new insights into depositional process, detailed sedimentary environment and the distribution of source rocks in the provenance. Detrital zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages range from Neoarchean to Triassic, with the main age pulses being Paleozoic to Triassic. In addition, the maximum deposition ages based on the youngest major age peak (ca 256–238 Ma) of zircon grains indicate that the basal sedimentation of the Babulu Formation occurred after the early Upper Triassic. The formation consists predominantly of mudstone with minor sandstone, limestone and conglomerate that were deposited in a deep marine environment. These deposits are composed of six lithofacies that can be grouped into three facies associations (FAs) based on the constituent lithofacies and bedding features: basin plain deposits (FA I), distal fringe lobe deposits (FA II) and medial to distal lobe deposits (FA III). The predominance of mudstone (FA I) together with intervening thin-bedded sandstones (FA II) suggest that the paleodepositional environment was a low energy setting with slightly basin-ward input of the distal part of the depositional lobes. Discrete and abrupt occurrences of thick-bedded sandstone (FA III) within the FA I mudstone suggests that sandstone originated from a collapse of upslope sediments rather than a progressive progradation of deltaic turbidites. This combined petrological and geochronological study demonstrates that the Babulu Formation in the Fohorem area of the Timor-Leste was initiated as a submarine lobe system in a relatively deep marine environment during the Upper Triassic and represents the extension of the Gondwana Sequence at the Australian margin.     

The first U–Pb isotope age data of island arc and continental–margin magmatism in the central part of the Koryak Highlands and U–Pb age of ore-controlling granitoids of the talyayigin ore field

Precision U–Pb (SHRIMP-II) isotope geochronological data, obtained for the first time, make it possible to suggest that sediments of the Neocomian primitive island arc sequence are missed or poorly developed in the South-Western part of the Mainitskii terrane of the Koryak Highlands. However, Late Albian mature island arc tuff and tuffaceous–turbidite formations are common. This enables us to extend the age range of the Mainitskii island arc from the Early Neocomian to the Late Albian and to suggest a two-stage pattern of its development. The isotope-geochronological data obtained for plagiogranite and moderately acid subvolcanics, previously attributed to the Koryak–Western Kamchatka volcanoplutonic belt, indicate that it is possible to combine them into the Middle Miocene postsubduction? polygenic complex. In addition, owing to modern high-precision isotope–geochronological methods, it has become possible to determine the age of gold–sulfide mineralization of the Talyaigin ore field, paragenetically related to the manifestations of the Middle Miocene Vilyuneiveem complex.     

Geochemistry and zircon U–Pb–Hf isotopes of the 780 Ma I-type granites in the western Yangtze Block: petrogenesis and crustal evolution

ABSTRACT

Neoproterozoic I-type granites could provide vital insights into the crust–mantle interaction and the crustal evolution along the western Yangtze Block, South China. This paper presents new zircon U–Pb ages, bulk-rock geochemistry, and in situ zircon Lu–Hf isotope on the Dalu I-type granites from the southwestern Yangtze Block. Zircon U–Pb dating show the crystallization ages of 781.1 ± 2.8 Ma for granodiorites and 779.8 ± 2.0 Ma for granites, respectively. The Dalu granodiorites are Na-rich, calc-alkaline, metaluminous to slightly peraluminous (A/CNK = 0.94–1.08). Zircons from granodiorite have positive εHf(t) values (+2.16 to +7.39) with crustal model ages of 1.21–1.54 Ga, indicating juvenile mafic lower crust source. The Dalu granites are high-K calc-alkaline, peraluminous rocks. They have variable zircon εHf(t) values (?4.65 to +5.80) with crustal model ages of 1.31–1.97 Ga, suggesting that they were derived from the mature metasediment-derived melts by the mixing of newly formed mafic lower crust-derived melts. The geochemical variations in Dalu pluton is dominated not only by the different source rocks but also by the different melting temperatures. Combining with the geochemistry and isotopic compositions of I-type granitoids and tectonic setting in the western Yangtze Block, we propose that the Dalu I-type granodiorites–granites associations are the magmatic response from different crustal levels, which were induced by the heat anomaly due to the asthenosphere upwelling in the subduction-related setting.     

U–Pb ages of detrital zircons within the Inthanon Zone of the Paleo-Tethyan subduction zone,northern Thailand: New constraints on accretionary age and arc activity

U–Pb dating of detrital zircons was performed on mélange-hosted lithic and basaltic sandstones from the Inthanon Zone in northern Thailand to determine the timing of accretion and arc activity associated with Paleo-Tethys subduction. The detrital zircons have peak ages at 3400–3200, 2600–2400, 1000–700, 600–400, and 300–250 Ma, similar to the peaks ages of detrital zircons associated with other circum-Paleo-Tethys subduction zones. We identified two types of sandstone in the study area based on the youngest detrital zircon ages: Type 1 sandstones have Late Carboniferous youngest zircon U–Pb ages of 308 ± 14 and 300 ± 16 Ma, older than associated radiolarian chert blocks within the same outcrop. In contrast, Type 2 sandstones have youngest zircon U–Pb ages of 238 ± 10 and 236 ± 15 Ma, suggesting a Middle Triassic maximum depositional age. The youngest detrital zircons in Type 1 sandstones were derived from a Late Carboniferous–Early Permian ‘missing’ arc, suggesting that the Sukhothai Arc was active during sedimentation. The data presented within this study provide information on the development of the Sukhothai Arc, and further suggest that subduction of the Paleo-Tethyan oceanic plate beneath the Indochina Block had already commenced by the Late Carboniferous. Significant Middle Triassic arc magmatism, following the Late Carboniferous–Early Permian arc activity, is inferred from the presence of conspicuous detrital zircon U–Pb age peaks in Type 2 sandstones and the igneous rock record of the Sukhothai Arc. In contrast, only minimal arc activity occurred during the Middle Permian–earliest Triassic. Type 1 sandstones were deposited between the Late Permian and the earliest Triassic, after the deposition of associated Middle–Late Permian cherts that occur in the same mélanges and during a hiatus in Sukhothai Arc magmatism. In contrast, Type 2 sandstones were deposited during the Middle Triassic, coincident with the timing of maximum magmatism in the Sukhothai Arc, as evidenced by the presence of abundant Middle Triassic detrital zircons. These two types of sandstone were probably derived from discrete accretionary units in an original accretionary prism that was located along the western margin of the Sukhothai Arc.     

Late Permian to Early Triassic crustal evolution of the Kontum massif,central Vietnam: zircon U–Pb ages and geochemical and Nd–Hf isotopic composition of the Hai Van granitoid complex

The Hai Van granitoid complex constitutes an important part of the Indosinian batholith of the northern Kontum massif in central Vietnam. The Kontum massif is a key region for understanding the palaeo-Tethyan accretionary history and the crustal evolution of the Indochina terrane. In this study, zircon U–Pb ages and geochemical data on the Hai Van complex are reported to shed more light on these geological processes. Zircon U–Pb isotopic analyses of six representative samples by the LA-ICP-MS technique show that granitoids from this complex crystallized between 242 and 224 Ma, indicating a prolonged period of magmatism in a late orogenic stage. Rocks from the Hai Van complex have S-type geochemical characteristics with high SiO₂, Na₂O + K₂O, and Zr contents as well as typical S-type minerals such as cordierite and muscovite. Trace elements of the samples are depleted in Ba, Nb, P, Sr, and Ti and enriched in Rb, K, Th, U, and Pb. Negative Eu-anomalies indicate feldspar fractionation during magma crystallization. The Hai Van granitoids exhibit Nd–Hf isotopic features similar to Proterozoic rocks exposed in South China, with low initial ?_Hf and initial ?_Nd values and two-stage Hf model ages of 1.9–1.7 Ga. It is suggested that during the palaeo-Tethyan orogeny, central Vietnam experienced strong early Mesozoic magmatism that originated from partial melting of crustal material similar in composition to the basement of the South China block.