Lu Hf Isotope Analyses Research Articles

Mesoarchean to Paleoproterozoic crustal evolution of the Taihua Complex in the southern North China Craton

The Taihua Complex is exposed mostly in the central-western part along the southern margin of the North China Craton (NCC). In this study, field geological, geochemical, LA-ICP-MS zircon U-Pb age dating and Lu-Hf isotopic analysis are applied to the orthorgneisses, felsic leucosomes, and granites of the Taihua Complex, in order to provide constraints on its crustal evolution from the Mesoarchean to Paleoproterozoic (ca. 2.9–1.8 Ga). The ca. 2.90–2.73 Ga magmatism was an important episode of contemporaneous crustal growth and crustal reworking. Among them, the ca. 2.90 Ga granodioritic enclaves have features similar to those of the Archean sanukitoids, and were generated by the direct partial melting of peridotitic mantle wedge in the subduction processes, representing the early continental nucleus. The ca. 2.84 Ga high Mg# TTGs were derived from the partial melting of subducted oceanic crust after interaction with mantle wedge, whereas the ca. 2.82–2.77 Ga low Mg# TTGs were originated from the partial melting of underplated basaltic crust, pointing to a series of continuous arc magmatic activities. The formation of the ca. 2.75–2.73 Ga crust-derived granites and felsic leucosomes in association with the contemporaneous metamorphism indicate the presence of a crustal anatexis, which probably was the result of an important magmatic underplating event. The magmatism between 2.9 Ga and 2.73 Ga indicates that the increased crust-mantle interaction and crustal differentiation stimulated the crust in the Taihua Complex to become more matural. The ca. 2.57–2.43 Ga magmatism represents another prominent magmatism in the Taihua Complex. The TTG gneisses and dioritic gneisses aged at ca. 2.57–2.50 Ga were derived from partial melting of the hydrated basalts under eclogite to garnet amphibolite facies conditions with interaction between initial melts and mantle wedge, indicating that incessant subduction led to the amalgamation of micro-blocks. The intrusion of granitic rocks, felsic leucosomes, TTG-like gneisses accompanied by metamorphism during ca. 2.50–2.43 Ga indicate that a dominant crustal anatexis event occurred in association with the cratonic stabilization processes. The ca. 2.36–2.24 Ga magmatism was also characterized by reworking of the basement rocks, which are represented by the intrusion of calc-alkaline monzogneisses, A-type granitic gneisses and mafic dykes. This scenario was coeval with the formation of volcanic-sedimentary sequences, indicating the presence of an extension event in the Taihua Complex. The ca. 1.96–1.85 Ga tectono-thermal event represented the last phase of compressional deformation and consequent metamorphism, leading to reworking of the pre-existed basement rocks. This event formed a WNW-ESE trending metamorphic belt that extends over 1000 km along the southern margin of the NCC and was well concordant with the world-wide orogeny associated with the amalgamation of Nuna supercontinent. Subsequently, widespread A-type granites, mafic dykes and volcanic rocks aged of ca. 1.84–1.74 Ga formed, marking the final consolidation of the crystalline basement at the Taihua Complex. Since then, the southern NCC became stable and entered into the stage of development of platform.

Latest Permian–early Triassic arc amalgamation of the Eastern Tianshan (NW China): Constraints from detrital zircons and Hf isotopes of Devonian–Triassic sediments

The temporal–spatial framework of arc amalgamation is an important key for understanding the anatomy of orogenic collages, present and past. The Junggar‐Balkhash Ocean was a major branch of the southern Paleo‐Asian Ocean where several arcs were amalgamated. Among them, the Dananhu arc with its characteristic juvenile magmatism and lack of Precambrian basement is an efficient recorder of the evolutional history. From our U–Pb and Lu–Hf isotopic analyses of Devonian to Triassic sediments in the Dananhu arc and Permian sediments in the Yamansu‐Central Tianshan arc, we discovered two major changes in detrital zircon provenance of the Dananhu arc: (a) An increasing input of Precambrian ages from the Mongolia collage at 850, 1,850, and 2,500 Ma, together with Phanerozoic zircons ranging from 420 to 480 Ma with negative εHf(t) value since ca. 288 Ma; (b) the presence of the Yamansu‐Central Tianshan arclike Precambrian age cluster of 1,400–1,600 Ma around 243 Ma. In combination with regional tectonothermal events, these two changes correspond to the formation of the Harlik‐Dananhu composite arc in the latest Carboniferous–early Permian followed by its collision with the Yamansu‐Central Tianshan arc in the latest Permian–early Triassic, which marks the termination of the eastern Junggar‐Balkash Ocean. Analysis of the sedimentary successions within the intraoceanic arcs sheds light on the amalgamation history of the Central Asian Orogenic Belt.

Genesis of the giant Shizhuyuan W–Sn–Mo–Bi–Pb–Zn polymetallic deposit, South China: Constraints from zircon geochronology and geochemistry in skarns

The giant Shizhuyuan W–Sn–Mo–Bi–Pb–Zn polymetallic skarn-type deposit, which lies in the world-famous Nanling W–Sn metallogenic belt, has a close affinity with the Qianlishan granite complex. Previous studies focused on the Qianlishan granitoids rather than relevant skarns, where the evolutionary processes of skarn-forming fluids, mineralized fluids and sources of metals are debated. This study carried out systematic EPMA and LA-ICPMS zircon U–Pb dating, major- and trace-element and Lu–Hf isotopic analyses for three types of skarns: ore-free, greisen stockwork and sulfide-bearing skarns at Shizhuyuan. The whole skarn zircons were classified into six age clusters (125–187 Ma, ∼235 Ma, 345–509 Ma, 677–772 Ma 811–921 Ma and >1000 Ma) and three genetic types (detrital, magmatic and hydrothermally-altered). Among them, the majority of the skarn zircons are of detrital origin (345 to 2225 Ma, peaking at ∼410 Ma, ∼710 Ma and ∼905 Ma) and showing a wide range of Th/U ratios (0.05–7.09). They share similar characteristics of zircons hosted by Devonian strata in South China, suggesting the inheritance from wall-rock. The magmatic zircons in skarns can be further subdivided into two clusters: Triassic (∼235 Ma) and Jurassic (∼150 Ma), which are derived from the corresponding granite. Ore-free skarn contains only detrital zircons that have experienced the weakest alteration, indicating the minor participation of granitic fluid. The greisen stockwork skarn consists of both detrital zircons and Jurassic magmatic–hydrothermal zircons, implying that granitic and stratigraphic materials may both contribute greatly to the W–Sn polymetallic mineralization. Sulfide-bearing skarn includes abundant detrital zircon, a few of Triassic zircons but no Jurassic zircons, indicating that the latest sulfide mineralization was dominated by sediments rather than granitic rocks. The two mineralized skarn zircons display elevated concentrations of most trace elements (e.g., U, REE, Ti, Y, P, Nb and Ta) coupled with decreased Si and Zr, which reflects strong fluid-rock reaction. The lowest ratios of Ce/Ce* (1.22–89.78, mean = 24.13) in greisen stockwork skarn zircons reveal the most reducing environment for the main W–Sn mineralizing event. Jurassic zircons are homogeneous in 176Hf/177Hf (0.282352–0.282454) and εHf (t) (−11.58 to −7.27), in well accord with local granitic rocks. Triassic zircons have variable εHf (t) (−9.5 to −1.6) values, suggesting more mantle input of precursor pluton before the Jurassic main mineralization event. Based on the zircon geochronology in skarns, we speculate that concealed Triassic magmatism exists in the deep Shizhuyuan area. Consequently, a three-stage ore-forming model was established for the giant Shizhuyuan deposit. This study further illustrates that zircon can be used as an important indicator for ore genesis in complex metallogenic systems.

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane, central Appalachian Piedmont Province

ABSTRACTThe Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.

Eoarchean to Mesoarchean crustal evolution in the Dharwar craton, India: Evidence from detrital zircon U-Pb and Hf isotopes

The formation and evolution of continental crust in the Early Earth are of fundamental importance in understanding the emergence of continents, their assembly into supercontinents and evolution of life and environment. The Dharwar Craton in southern India is among the major Archean cratons of the world, where recent studies have shown that the craton formation involved the assembly of several micro-continents during Meso- to Neoarchean through subduction-accretion-collision processes. Here we report U-Pb-Hf isotope data from detrital zircons in a suite of metasediments (including quartz mica schist, fuchsite quartzite and metapelite) from the southern domain of the Chitradurga suture zone that marks the boundary between the Western and Central Dharwar Craton. Morphology and internal structure of the zircon grains suggest that the dominant population was derived from proximal granitic (felsic) sources. Zircon U-Pb data are grouped into Paleo-Mesoarchean and Neoarchean to Paleoproterozoic with peaks at 3227 Ma and 2575 Ma. The age spectra of detrital zircon grains, in combination with the Lu-Hf isotopic analyses indicate sediment provenance from magmatic sources with model ages in the range of ca. 3.67 to 2.75 Ga. A transition from dominantly juvenile to a mixture of juvenile and recycled crustal components indicate progressive crustal maturity. The results from this study suggest major crustal growth events during ca. 3.2 Ga and 2.6 Ga in Dharwar. Our study provides insights into continental emergence, weathering and detrital input through river drainage systems into the trench during Eoarchean to Mesoarchean.

Deciphering the Mesoarchean to Neoarchean history of crustal growth and recycling in the Caochang region of the Eastern Hebei Province, North China Craton using combined zircon U–Pb and Lu-Hf isotope analysis

In Archean cratons, the end of the Archean aeon (3.0–2.5 Ga) is marked by a considerable diversification in both the nature and petrogenesis of granitoid rocks. We reported the studies of a series of Mesoarchean-Neoarchean diorite and tonalite–trondhjemite–granodiorite (TTG) and granitic gneisses in the Caochang area, Eastern Hebei region of the North China Craton. Combined zircon U-Pb and Lu-Hf isotope analyses and CL imaging analysis reveal three major magmatic events (3.2-3.0 Ga, 2.9-2.8 Ga, ~2.5 Ga) and one major metamorphic overprint (~2.5 Ga). The close temporal relationship for different zircon domains reveals that the late-Neoarchean crustal materials were recycled or reworked immediately after their formation, immediately followed by ~2.5 Ga recrystallized metamorphism.The voluminous mantle-derived mafic rocks indicate that the 2.5-2.6 Ga is an important period of crustal addition in the Eastern Hebei region. The peak of Hf model age and the widespread Mesoarchean or Paleoarchean crustal materials in Eastern Hebei may point to another important period of crustal addition before the Neoarchean.

Early Neoproterozoic gneissic granitoids in the southern Yili Block (NW China): Constraints on microcontinent provenance and assembly in the SW Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) was built by amalgamation of diverse geologic units including several microcontinents. The nature and geodynamic evolution of the Precambrian basement of these units are essential to constrain their origin, and thus the geodynamic and paleogeographic reconstruction of the accretionary system. The Yili Block, located in the west of the Chinese Tianshan Orogen, is one of these Precambrian-based continental constituents. Field investigations, zircon U-Pb dating and Lu-Hf isotopic analysis, whole-rock geochemical and Sr-Nd isotopic studies were conducted on the gneissic granitoids from the Giergelang area, southern Yili Block. These rocks yielded early Neoproterozoic zircon U-Pb ages (962–934 Ma), containing substantial number of Paleo- to Mesoproterozoic inherited zircons (2403–1268 Ma); their protoliths belong to peraluminous high-K granite series displaying right-declining REE patterns and Eu negative anomalies together with LILE (K, Rb, U, Th) enrichment and depletions in Ba, Nb, Sr, P, Ti; they have negative εNd(t) values ranging from −2.5 to −4.7, and old Nd model ages varying from 1650 Ma to 1910 Ma; they also exhibit highly variable zircon εHf(t) values (−12.1 to 4.4), and Hf-in-zircon model ages (1558–2574 Ma). Integrating all these results, it is suggested that the primary magma of gneissic granitoids was generated by partial melting of supracrustal rocks with involvement of old crustal materials.A comparison of data from the Yili Block and its neighboring continental units reveals that the Neoproterozoic magmatic event is an important episode of crustal reworking for the major continental blocks within the CAOB. Similar Neoproterozoic magmatic events and comparable upper Neoproterozoic to lower Paleozoic stratigraphic sequences of the Yili, Chinese Central Tianshan and Tarim blocks, suggest that they had a similar crustal evolution and close tectonic and paleogeographic relation during Neoproterozoic, and were likely derived from one single continental plate.

Petrogenesis of Late Cretaceous mafic enclaves and their host granites in the Nyemo region of southern Tibet: Implications for the tectonic-magmatic evolution of the Central Gangdese Belt

Late Triassic-Early Eocene Gangdese magmatic belt is over 2000 km long forming a significant component of the Lhasa Terrane, and is regarded as the ideal place to study the tectonic-magmatic evolution of the Neo-Tethys Ocean and the collision of the Indian and Asian continental plates. Recent studies elucidate the major Late Cretaceous magmatic events in southern Tibet, but the evolution of the region is still equivocal. In this paper, we report new results from systematic field work as well as new geochronological, petrological, geochemical, and zircon Lu-Hf isotope analyses of granite and contemporaneous mafic microgranular enclaves (MMEs) from the Nyemo region in southern Tibet. Both the host granite and MMEs are dated at ca. 87–84 Ma, suggesting that they are related to a coeval magmatic event. Zircon Hf isotopes (εHf(t) = +11.0 – +14.6) and Mg# numbers (50–52) indicate that the host granite is derived from partial melting of juvenile crust with the contribution of material from the mantle, whereas the MMEs are derived from partial melting of depleted mantle (refractory peridotite melts) as deduced from their zircon Hf isotopes (εHf(t) = +6.12 – +13.79). Geochemically, both the granite and MMEs are characterized by enriched light rare earth elements (LREE) and large-ion lithophile elements (LILE), but depleted high field-strength elements (HFSE), indicative of arc-related geochemical compositions. The host granite also has high Sr/Y (111–140), which are interpreted as proxies for magmas with high water contents, and their elevated (La/Yb)N ratios of 22–33 are proxies for a thickened continental crust. We propose that the Late Cretaceous (ca. 100–80 Ma) magmatism dated some 30 million years before the collision of the Indian and Eurasian plates is possibly related to the interaction between upwelling asthenospheric mantle and metasomatized lithospheric mantle during subduction and possible rollback of the Neo-Tethys Oceanic plate. The subducted slab presumably led to mantle magma underplating and potentially caused crustal thickening in southern Tibet. Hence, the rollback of the Neo-Tethys Oceanic plate represents a key feature in the tectonic evolution of southern Tibet during the early Late Cretaceous.

Petrogenesis of Ore-Hosting Diorite in the Zaorendao Gold Deposit at the Tongren-Xiahe-Hezuo Polymetallic District, West Qinling, China

The newly discovered Zaorendao gold deposit is in the Tongren-Xiahe-Hezuo polymetallic district in the westernmost West Qinling orogenic belt. The estimated pre-mining resource is approximately 13.6 t of Au at an average grade of 3.02 g/t. Mineralization is predominantly controlled by NW-trending and EW-trending faults within diorite intrusions and surrounding sedimentary rocks. In the present study, in situ zircon U–Pb geochronology and Lu–Hf isotopic analyses of the ore-hosting diorite at Zaorendao were measured using LA-ICP-MS. The data suggest that the diorite was emplaced at ca. 246.5 ± 1.9 Ma. The large variation of zircon Hf isotopic composition (ɛHf(t) values ranging from −12.0 to −1.8) indicates a two-stage model age (TDM2) that ranges from 1.4 Ga to 2.0 Ga. Such Lu–Hf isotopic compositions indicate that the diorite was dominantly derived from a Paleo- to Meso-Proterozoic continental crust. The wide range of εHf(t) and the presence of inherited zircon can be interpreted to suggest the mixing of Paleo- to Meso-Proterozoic continental crust with a mantle component. Combining such characteristics with the geochemistry of coeval rocks that are associated with the diorite, we therefore proposed that the gold-hosting Triassic diorite in the Zaorendao gold deposit formed in an active continental margin that was associated with the northward subduction of the paleo-Tethyan ocean.

U-Pb detrital zircon constraints on the depositional age and provenance of the dinosaur-bearing Upper Cretaceous Wadi Milk formation of Sudan

Cretaceous continental deposits in Sudan have long been recognized as important archives of continental vertebrate fossils in central Africa. A number of different sedimentary units including the Wadi Milk, Shendi and Kababish formations from northern and central Sudan are known to yield dinosaur and other vertebrate fossils. The ages of these deposits are poorly constrained, and traditionally assumed to be of Albian to Santonian age based on biostratigraphic evidence. However, recent palynological analyses suggest a Campanian–Maastrichtian age for the Shendi Formation. Not only are the ages of these units poorly resolved; but stratigraphic correlations within and between them and similar aged units in central Africa, remains tenuous. To address these issues a detailed sedimentary fingerprinting and provenance analysis was conducted on the putatively correlative Shendi and Wadi Milk formations using sandstone petrography and a multifaceted detrital zircon investigation combining U-Pb geochronology, Lu-Hf isotope analysis and trace element geochemistry. The objective was to provide constraints on the age of deposition, sediment sources and paleofluvial drainage patterns. Based on laser ablation ICP-MS U-Pb detrital zircon geochronology of 18 sandstone samples (>1400 detrital zircons), we identified a population of six Late Cretaceous zircons from the Wadi Milk Formation yielding a maximum depositional age of 79.2 ± 2.4 Ma (MSWD = 0.65, probability 0.62). Specifically, these data restrict the depositional age to Campanian or younger for the Wadi Milk Formation, which significantly refines our understanding of the age of this unit and its fauna. The Lu-Hf-isotope and trace element analyses of the remainder of the dated zircons show a provenance dominated by Neoproterozoic crustal sources and minor Paleoproterozoic and Archean sources for both units, which we interpret as being sourced from the Arabian-Nubian Shield to the south and southeast of the study area. These findings, coupled with the recently updated biostratigraphic age of the Shendi Formation, confirm that both units are correlative to one another and were probably deposited synchronously by north-north westerly flowing fluvial system draining into the Tethys Sea during the Late Cretaceous.

Zircon U-Pb-Hf isotope studies of the early Precambrian metasedimentary rocks in the Kongling terrane of the Yangtze Block, South China

Some metamorphosed pelites (commonly called as kondalites) expose over the Archean crystal basement in the Kongling terrane of the Yangtze Block. These sedimentary rocks mainly contain biotite – plagioclase gneisses, garnet bearing biotite – plagioclase – silimanite gneisses, garnet bearing biotite – plagioclase – silimanite – cordierite gneisses, graphite schists, and some marbles and quartzites. Amphibolites and mafic granulites always occur as lenses or layers in these metasedimentary rocks. In this study, three kinds of metasedimentary rocks, which are Grt-Bi-Pl gneiss, Grt-Sil-Crd-Bi-Pl gneiss and Grt-Sil-Bi-Pl gneiss, are chosen for petrographic, zircon U-Pb, and Lu-Hf isotopic analysis. The mineral assemblages of these three rocks are Grt + Bi + Pl + Q, Grt + Sil + Crd + Bi + Pl + Q + Spl + Kf and Grt + Sil + Bi + Pl + Q respectively, which imply granulite facies metamorphism. In addition, garnet porphyroblasts in both of these metasedimentary rocks and the mafic granulites which are enclaves in these meta-sedimentary rocks develop decompressional coronas, indicating isothermal decompression (ITD) processes during retrograde metamorphism. Core-Rim structures in zircons can be observed in all these three samples based on cathodoluminescence images. Morphological characteristics, Th/U ratios and trace element compositions indicate that zircon cores are of magmatic origin while zircon rims and some single grain zircons are of metamorphic origin. Ages of magmatic zircon cores in Grt-Bi-Pl gneiss are between 3532 ± 20 Ma to 2286 ± 47 Ma while those of metamorphic zircons are 2022 ± 22 Ma. Most zircons in the Grt-Sil-Crd-Bi-Pl gneiss are of metamorphic origin and they have a weighted mean 207Pb/206Pb age of 2017 ± 15 Ma. Magmatic zircon cores in Grt-Sil-Bi-Pl gneiss have a weighted mean 207Pb/206Pb age of 2160 ± 31 Ma and metamorphic zircons have a weighted mean 207Pb/206Pb age of 2028 ± 43 Ma. One zircon core (3199 ± 25 Ma) in Grt-Bi-Pl gneiss possesses the oldest two-stage Hf model age of 3676 ± 85 Ma. Two-stage Hf model (TDM2) ages of all the other analyzed zircon cores in this sample range from 3452 ± 40 to 3277 ± 43 Ma, and the metamorphic zircons give a weighted mean TDM2 age of 3471 ± 14 Ma. Metamorphic zircons in the Grt-Sil-Crd-Bi-Pl gneiss have a weighted mean two-stage Hf model (TDM2) age of 3468 ± 16 Ma. These TDM2 ages indicate that the sources of these metasedimentary rocks are obviously older than those documented by previous studies and they also suggest that protolith of these metasedimentary rocks belong to Archean crustal materials of the Kongling terrane, but with obviously different sedimentary sources. The widespread decompressional textures around garnet in various samples in the study area suggest that the Kongling area underwent a high-pressure granulite facies metamorphism and an isothermal decompression process at ∼2.0 Ga, corresponding to the assemblage of the Paleoproterozoic Nuna Supercontinent.

Origin of zircon-bearing mantle eclogites entrained in the V. Grib kimberlite (Arkhangelsk region, NW Russia): Evidence from mineral geochemistry and the U-Pb and Lu-Hf isotope compositions of zircon

The concentrations of major and trace elements in minerals, reconstructed whole-rock compositions of zircon-bearing equigranular eclogites from the V. Grib kimberlite pipe located within the Arkhangelsk Diamondiferous Province (North-Western Russia), and results of the U-Pb and Lu-Hf isotope analyses of zircon grains from eclogites and granulite xenoliths are reported. These data suggest that the equigranular eclogites could represent the fragments of mid-ocean-ridge basalt that were metamorphosed during Paleoproterozoic subduction at 1.7–1.9 Ga. The Hf isotope compositions of the eclogitic zircon display uniformity and indicate corresponding Hf-depleted mantle model ages of 2.2–2.3 Ga. The formation of zircon in eclogites could have resulted from interactions with metasomatic/subduction-related fluids just prior to, but associated with, Paleoproterozoic eclogite formation. A link between eclogitic zircon formation and continental lower-crustal rocks can be excluded based on differences in the Hf isotope compositions of eclogitic and granulitic zircon grains. The U-Pb upper intercept age of granulitic zircon of 2716 ± 61 Ma provides a new minimum age constraint for zircon crystallisation and granulite formation. The U-Pb ages obtained from granulitic zircon show two stages of Pb loss at 2.2–2.6 Ga and 1.7–2.0 Ga. The late Paleoproterozoic stage of Pb loss recorded in granulitic zircon is due to the intensive reworking of basement crustal rocks, which was caused by a tectonic process/subduction event associated with equigranular eclogite formation. Our data, along with evidence previously obtained from the V. Grib pipe coarse-granular eclogites, show at least two main subduction events in the lithospheric mantle of the Arkhangelsk region: the Archean (2.8 Ga) and Paleoproterozoic (1.7–1.9 Ga) subductions, which correspond to major magmatic and metamorphic events in the Baltic Shield.

Precambrian Hongqiyingzi Complex at the northern margin of the North China Craton: Its zircon U-Pb-Hf systematics, geochemistry and constraints on crustal evolution

The Precambrian Hongqiyingzi Complex (HQC) in the middle segment of the northern margin of the North China Craton (NCC) is a crucial magmatic intrusive complex offering insights into the early evolutionary history of the northern margin of the NCC. The HQC, exposed from Congli to Longhua towns in the northern Hebei Province, is mainly composed of Neoarchean quartz dioritic-tonalitic-granodioritic (DTG) gneisses and Paleoproterozoic granodiorite-monzogranite-syenogranite (GMS) intrusions. SHRIMP and LA-ICP-MS zircon dating results show that the DTG gneisses formed at 2546–2532 Ma, while the GMS intrusions were emplaced between 1870 and 1819 Ma. LA-MC-ICP-MS zircon Lu-Hf isotope analyses indicate that the Neoarchean DTG magmas were mainly derived from partial melting of juvenile crust, but the Paleoproterozoic GMS magmas originated from partial melting of the metamorphic basalts and greywackes from Neoarchean to Paleoproterozoic volcanic or related sedimentary materials.Lithological assemblages, chronological and geochemical data, petrogensis and geodynamic settings of the Neoarchean DTG and Paleoproterozoic GMS magmatism show that the HQC is most likely at the northernmost part of the Trans-North China orogen of the NCC. The Precambrian HQC thus recorded Neoarchean subduction-related arc magmatism and Paleoproterozoic post-collisional magmatism that were associated with subduction and collision at the northern margin of the NCC. These Precambrian geological units were intensively overprinted by Late Paleozoic calc-alkaline magmatism, which may be caused by southward subduction and collision of the Central Asian Orogenic Belt (CAOB) with the NCC.

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

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 ϵHft range from −8.8 to +8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic ϵHft (∼−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 ϵHft (∼−9 to −6) but more subdued δ18O values (∼7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic ϵHf 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 (ϵHft 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 (ϵHft 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.

Significant Carboniferous magmatism and continental growth in the northern West Tianshan orogen, NW China: Revealed by detrital zircon U-Pb and Lu-Hf analyses for turbidites from the North Tianshan Accretionary Complex

We conducted an integrated study including U-Pb dating and Lu-Hf isotope analysis for detrital zircons from the turbidites of the North Tianshan Accretionary Complex, northern Chinese West Tianshan, to constrain their depositional age, source characteristics and tectonic setting. Almost all the detrital zircons display well developed oscillatory zoning textures and have high Th/U ratios, reflecting an igneous origin. Based on the analytical results, the depositional ages of the four samples were 303 Ma–312 Ma, 310 Ma and 321 Ma. The studied samples all display unimodal detrital zircon spectra, with few Precambrian zircons, coinciding with the age spectra of sediments deposited in convergent basins, most likely a forearc basin. Most of the zircons and other mineral grains have euhedral to subhedral shapes, reflecting a relatively short transportation distance. Considering the occurrence of volcanic lithic fragments in thin-sections of the sandstones, we propose that the northern edge of Yili Block, which is widely accepted as a Paleozoic continental arc formed by subduction of the North Tianshan Ocean, was the most likely source area for the North Tianshan turbidites. There are 137 Lu-Hf isotope analyses on zircons with concordant U-Pb ages, all showing positive εHf(t) values (εHf(t) = +2.9–+15.8), and young TCDM ages (328–1152 Ma, peak at 400–800 Ma). The Hf isotope characteristics indicate important growth of juvenile crust and minor crustal contamination during the formation of the zircons’ host rocks. When we examine the Hf isotope of reported magmatic rocks from the northern Yili Block, we found an increasing trend of εHf(t) values from the Early Paleozoic to Late Paleozoic. This indicates that there was a large amount of juvenile material extracted from the depleted mantle in the Carboniferous. Taking into account of previous research, we conclude that the northern West Tianshan was a subduction-arc system during the Carboniferous, and the studied turbidites from the North Tianshan Accretionary Complex were sourced from the magmatic arc of the northern Yili Block and most likely deposited in a fore-arc basin in the Late Carboniferous.

Paleozoic granitic magmatism and tectonic evolution of the South Altun block, NW China: Constraints from zircon U-Pb dating and Lu-Hf isotope geochemistry

The South Altun orogenic belt (SAOB), consisting of the South Altun block (SAB) and the South Altun continental block (SACB), is one of several important orogenic belts in NW China. Numerous granite bodies are widely distributed in the SAB. However, the petrogenesis and tectonic environment of these granites, and their relationships to those of the SACB, Central Altun Continental block (CACB), North Altun block (NAB), Qilian block (QLB), and North Qaidam block (NQB) remain unclear. In this investigation, we selected the SAB granites for detailed study, and explore the petrogenesis employing bulk-rock geochemistry, zircon U-Pb dating, and Hf isotopic analysis. Zircon U–Pb LA-ICP-MS dating of the granites from the SAB identified seven spatially distinct Paleozoic plutonic suites: (a) Yigan—352–343 Ma; (b) Baiganhu—448–444 Ma; (c) west Mangya—444–435 Ma; (d) Aketishan—265 Ma; (e) Chaishuigou—406 Ma; (f) Changchungou—469 Ma; and (g) north Mangya—462 Ma. Based on previous research combined with our new U-Pb dating and bulk-rock geochemical investigations in the SAB, we identdified six distinct episodes of granitoid emplacement: episodes 1 and 2, quartz diorite + granodiorite + monzogranite with I-type affinities, and ages of 503–497 Ma and 483–458 Ma, respectively; episode 3, granodiorite + monzogranite + syenogranite with I- and S-type affinities, and an age of 456–432 Ma; episode 4, monzogranite + syenogranite + alkali feldspar granite with A-type affinities, and an age of 424–385 Ma; episode 5, granodiorite + monzogranite with S-type affinities, and an age of 352–338 Ma; and episode 6, quartz diorite + monzogranite + syenogranite with I-type affinities, and an age of 265–264 Ma. Zircon Lu-Hf isotope analyses from various episodes show that εHf (t) values are positive in most cases, but negative for a small number of inherited (xenocrystic) zircons. Source rocks were mainly juvenile crust, evidently mixed with minor ancient continental material. Based on our new results integrated with regional geological data, we conclude that episodes 1 and 2 granitic magma generations were related to the subduction of oceanic crust, episode 3 to post-collisional partial melting, episode 4 to the isostatic adjustment of orogenic blocks after plate suturing, episode 5 to petrotectonic thickening of the Altun crust, and episode 6 to Altun fault transcurrent slip attending Kunlun Paleo Tethys subduction.

Jurassic ca. 160 Ma crustal remelting and Paleoproterozoic intrusive rock residues in the Liaodong Peninsula, East China: Evidence from in situ zircon U–Pb dating and Lu–Hf isotopic analysis

The Liaodong Peninsula is an important component of the eastern North China Craton which underwent a transition from an orogenic stage to extension and lithospheric thinning in the Mesozoic era. This transition resulted in large‐scale magmatism and gold mineralization. In this paper, we report findings on Paleoproterozoic intrusive rock residues in the Mesozoic Wulong pluton. To analyse the chronology, magma source, and petrogenetic relationships between the Mesozoic and Paleoproterozoic intrusives, we performed SHRIMP U–Pb isotopic dating and in situ micro‐area Lu–Hf isotopic analysis of zircons. The results show that the crystallization age of the large‐scale Mesozoic intrusive rock is 158 ± 2 Ma, which is in the Late Jurassic. The εHf(t) values of the zircons are between −29.3 and −22.1, and their tDM2 values are between 2,606 and 3,064 Ma, which indicates that the magma source of materials is the Archean supracrust. The zircons of the Paleoproterozoic intrusive rock residues have a typical core–mantle–rim texture. The cores of the zircons have an age range from 1,851 to 2,528 Ma, εHf(t) from −12.2 to +4.0, and tDM2 between 2,519 and 3,437 Ma, reflecting characteristics of multiple sources for the inherited zircons. The mantles of the zircons have an upper intercept age of 1,862 ± 13 Ma, εHf(t) values between −11.9 and −2.2, and tDM2 between 2,644 and 3,241 Ma representing the age of the source rocks. The new growth rims of the zircons have a concordia age of 157.1 ± 2.2 Ma, εHf(t) values from −30.8 to −26.1, and tDM2 between 2,852 and 3,150 Ma representing the metamorphic recrystallization age and characteristics of the upper crustal magma source. The findings of this work suggest that the Wulong pluton, which occupies a large area in the Liaodong Peninsula, recrystallized from Paleoproterozoic granites that were affected by Late Jurassic (ca. 160 Ma) crustal remelting. The incompletely remolten residues are the direct result of the delamination and remelting during the Mesozoic era of Palaeoproterozoic intrusive rocks in the northern margin of the eastern North China Craton.

TAPHONOMY, GEOLOGICAL AGE, AND PALEOBIOGEOGRAPHY OF LOTOSAURUS ADENTUS (ARCHOSAURIA: POPOSAUROIDEA) FROM THE MIDDLE-UPPER TRIASSIC BADONG FORMATION, HUNAN, CHINA

Lotosaurus adentus is an unusual sail-backed, edentulous poposauroid pseudosuchian primarily known from a single, nearly monospecific bonebed discovered and excavated in the 1970s in the Middle-Upper Triassic Badong Formation of Sangzhi County, Hunan Province, South China. Renewed interest in this unique taxon prompted exposure of an additional 90 m2 of the bonebed. Almost 1000 new L. adentus bones, 28% of which were articulated, were discovered during this excavation. The bones lack evidence of tooth marks, trample marks, or insect modification, and display minimal weathering. The site is reinterpreted as a pedogenically modified floodplain pond (and overlying fluvial channel) within a warm, semi-arid sub-tropical region (paleolatitude ∼ 34°N), contrasting with previous interpretations of the locality as a tidal flat deposit. The occurrence of mudcracks, conchostrachan fossils, and vertic paleosol development with calcium carbonate accumulations in both overlying and underlying facies indicates periodic aridity and ephemeral conditions. The bonebed is characterized by partial disarticulation and minor transport, and is confined to a thin, < 30 cm-thick interval. Considered together, these features are most consistent with a mass mortality event, possibly drought related considering the sedimentological context, followed by minor transport during a rapid burial event. U-Pb detrital zircon geochronology and Lu-Hf isotope analysis were utilized to reassess the provenance and age of the deposit, and suggest that L. adentus was likely Ladinian or possibly even Carnian in age, rather than Anisian as previously reported. Paleocurrent data, detrital zircon age spectra, and Lu-Hf isotopes indicate that fluvial sediments were partially derived from sources in the North China craton and Qinling-Dabieshan tectonic belt, implying that faunal interchange between the North and South China blocks was possible by this time.

New evidence for an old idea: Geochronological constraints for a paired metamorphic belt in the central European Variscides

New geochronological data reveal a prolonged tectonothermal evolution of the Variscan Odenwald-Spessart basement, being part of the Mid-German Crystalline Zone in central Europe. We report the results from (i) secondary ion mass spectrometry (SIMS) U-Pb dating of zircon, rutile and monazite, (ii) SIMS zircon oxygen isotope analyses, (iii) laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon Lu-Hf isotope analyses and, (iv) LA-ICPMS zircon and rutile trace element data for a suite of metamorphic rocks (five amphibolite- and eclogite-facies mafic meta-igneous rocks and one granulite-facies paragneiss). The protoliths of the mafic rocks formed from juvenile as well as depleted mantle sources in distinct tectonic environments at different times. Magmatism took place at a divergent oceanic margin (possibly in a back-arc setting) at 460 Ma, in an intraoceanic basin at ca. 445 Ma and at a continental margin at 329 Ma. Regardless of lithology, zircon in eclogite, amphibolite and high-temperature paragneiss provide almost identical Carboniferous ages of 333.7 ± 4.1 Ma (eclogite), 329.1 ± 1.8 to 328.4 ± 8.9 Ma (amphibolite), and 334.0 ± 2.0 Ma (paragneiss), respectively. Rutile yielded ages of 328.6 ± 4.7 and 321.4 ± 7.0 Ma in eclogite and amphibolite, and monazite in high-temperature paragneiss grew at 330.1 ± 2.4 Ma (all ages are quoted at the 2σ level). The data constrain coeval high-pressure eclogite- and high-temperature granulite-facies metamorphism of the Odenwald-Spessart basement at ca. 330 Ma. Amphibolite-facies conditions were attained shortly afterwards. The lower plate eclogite formed in a fossil subduction zone and the upper plate high-temperature, low-pressure rocks are the remains of an eroded Carboniferous magmatic arc. The close proximity of tectonically juxtaposed units of such radically different metamorphic conditions and thermal gradients is characteristic for a paired metamorphic belt sensu Miyashiro (1961). Thus, the Odenwald-Spessart basement represents the first recognised paired metamorphic belt in the European Variscides.

U–Pb and Hf isotope analysis of detrital zircons from Devonian–Permian strata of Kotel’ny Island (New Siberian Islands, Russian Eastern Arctic): Insights into the Middle–Late Paleozoic evolution of the Arctic

U–Pb and Lu–Hf isotope analyses of detrital zircons collected from the Devonian–Permian strata of Kotel’ny Island (New Siberian Islands, Eastern Russian Arctic) provide vital information about provenance areas and history of the poorly studied Paleozoic basement of the Laptev and East Siberian shelves. Based on detrital zircon signatures, our study reveals that the studied succession can be divided into two distinct groups: Devonian–Lower Carboniferous and uppermost Carboniferous–Permian. U–Pb and Lu–Hf isotope signatures of detrital zircons from the Devonian–Lower Carboniferous deposits correspond to well-known Precambrian–Early Paleozoic magmatic and metamorphic events within the northern part of Baltica, indicative of a peri-Baltican affinity of the New Siberian Islands during this time. A lack of zircon ages close to the depositional age of the formation, along with a mature composition of the sandstones, suggest that the Devonian–Lower Carboniferous succession formed as a result of extensive sediment reworking from a distal provenance. By contrast, uppermost Carboniferous–Permian sandstones have an immature composition, with numerous young detrital zircons close to the age of sedimentation and Hf signatures typical of a continental arc environment. We therefore suggest that the provenance for these younger deposits was located within a coeval orogeny and based on previous studies, we conclude that these deposits were sourced from the north-western part of the Uralian Orogen.