Zircon Hf Isotopic Compositions Research Articles

Petrogenesis of Early Jurassic (ca. 181 Ma) dacitic–rhyolitic volcanic rocks in the Amdo ophiolite mélange, central Tibetan Plateau: Low‐pressure partial melts of Bangong–Nujiang Tethys oceanic crust?

The identification of oceanic‐crust–derived melts is critical to improving our understanding of oceanic lithosphere subduction in various geodynamic settings. However, low‐pressure melts derived from the oceanic crust can be misidentified because their compositions are often indistinguishable from those of typical arc‐related rocks. In this study, we report geochronological and geochemical data for the Amdo volcanic rocks in the central Tibetan Plateau. Zircon U–Pb dating indicates that they were generated in the Early Jurassic (ca. 181 Ma), simultaneous with spatially associated MORB‐like rocks of the Amdo ophiolite. The Amdo volcanic rocks are characterized by high SiO2 and Na2O concentrations, low MgO, K2O, and Fe2O3T concentrations, and low Mg# values. They are enriched in light rare earth and large‐ion lithophile elements, display negative Eu, Nb, and Ta anomalies, and yield low Sr (<360 ppm) and high Y (up to 57.6 ppm) concentrations with low Sr/Y ratios (<15.4). Whole‐rock Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i = 0.7040–0.7058, εNd(t) = +3.9 to +6.8, and zircon εHf(t) = +13.9 to +17.9) are different from those of regional continental‐crust–derived granitoids but similar to those of contemporary MORB‐like rocks that occur in regional ophiolites. Furthermore, zircon from the Amdo volcanic rocks yields U/Yb ratios similar to those of oceanic zircon. These characteristics, combined with regional geological observations, indicate that the Amdo volcanic rocks are a component of the Amdo ophiolite mélange and represent low‐pressure partial melts of oceanic crust, generated during incipient northward subduction of the Bangong–Nujiang oceanic lithosphere beneath the Qiangtang terrane. Our study helps to constrain the history of the Bangong–Nujiang Ocean and provides a case study on the low‐pressure melting of oceanic crust.

Zircon U–Pb ages and Hf isotope compositions of the Neoproterozoic magmatic rocks in the Helan Mountains, North China

AbstractThe periodic dispersal and assembly of continental fragments has been an inherent feature of the continental crust. Based on the discovery of large-scale supercontinent cycle and the theory of plate tectonics, several supercontinents have been identified, such as Columbia/Nuna, Rodinia, Gondwana and Pangaea. Neoproterozoic magmatic events related to the break-up of Rodinia are globally well preserved. Although Neoproterozoic magmatic events were very weak in the North China Craton (NCC), they are crucial in reconstructing the geometries of the NCC and could facilitate the completion of the Neoproterozoic configuration of the supercontinent. In this study, c. 853–835 Ma magmatic rocks are identified in the western margin of the NCC. Precise zircon U–Pb age determination yields 206Pb/238U average ages of 835.5 ± 5.3 Ma (HL-39) and 853.7 ± 4.5 Ma (HL-30). In situ zircon Hf isotope compositions of the samples reveal that their parental magma was formed by the reworking of ancient crust evolved from Mesoproterozoic mantle. In summary, the discovery of Neoproterozoic magmatic rocks in the western margin of the NCC, and reported synchronous rocks in other parts of the NCC indicate that the NCC might be conjoined with the supercontinent Rodinia during the Neoproterozoic. This discovery is of significant help in unravelling the early Neoproterozoic history of the NCC and the evolution of the supercontinent Rodinia.

Two-stage magmatic events of the early Paleozoic in the eastern Hexi Corridor Belt, NW China: petrogenesis and tectonic implications

The petrogenesis and geodynamic implications of the early Paleozoic magmatic events in the eastern Hexi Corridor Belt, NW China, remain poorly constrained. In this study, we present high-quality zircon U–Pb ages as well as geochemical and zircon Lu–Hf isotopic data for three early Paleozoic intrusive rocks from the Mafuxiazi area in the eastern Hexi Corridor Belt. The LA-ICP-MS U–Pb dating results suggest that there are two stages of magmatic activities during the late Ordovician (447 Ma) and late Silurian (426–423 Ma). The late Ordovician and late Silurian granitoids are predominated by metaluminous and belong to medium-K calc-alkaline and high-K calc-alkaline, respectively. Both two-stage granitoids have been identified as I-type granites. Geochemical characteristics show high Sr, low Y and Yb contents, and high La/Yb and Sr/Y ratios, with weakly negative or no significant Eu anomalies. However, the two-stage granitoids display different zircon Hf isotopic compositions, implying different source materials. The late Ordovician granitoids show negative to slightly positive zircon eHf(t) values of − 2.93~1.68 with crustal Hf model ages (TCDM) of 1318–1610 Ma, suggesting a mixed source from ancient crustal and mantle materials, whereas the late Silurian granitoids have a wide range of zircon eHf(t) values from − 27.34 to 3.00 (only three zircon grains show positive values) with older crustal Hf model ages (TCDM) of 1219–3117 Ma, indicating significantly ancient crustal contribution with limited mantle materials. Based on geochemical and geochronological data, accompanied with consideration of the regional geological setting, we suggest that the late Ordovician granitoids were most likely formed in a back-arc basin, whereas the late Silurian granitoids developed in a post-collision setting.

Pre-Variscan tectonic setting of the south margin of Armorica: Insights from detrital zircon ages distribution and Hf isotopic composition of the St-Georges-sur-Loire Unit (S. Armorican Massif, France)

The Armorican Massif experienced a poly-orogenic evolution, represented by the Neoproterozoic Cadomian and the Devonian-Carboniferous Variscan tectonothermal events. In the southernmost part of the Armorican Domain (or Armorica microcontinent), the St-Georges-sur-Loire Unit consists of a block-in-matrix subunit, and a turbiditic subunit in the South and North, respectively. The St-Georges-sur Loire unit is interpreted as a remnant back-arc basin formed in Silurian-Early Devonian (ca. 440–400 Ma), and was deformed during the Variscan orogeny. Most of the sedimentary and magmatic blocks enclosed in clastic matrix are olistoliths derived from an assumed southern area called the “missing domain”, presently exposed as the Variscan Mauges Nappe composed of the Cadomian rocks and Early Paleozoic sedimentary cover. The “missing domain” was subducted beneath the Armorican massif during the Variscan collision. Detrital zircon age spectra from sandstones and siltstones from the olistostrome matrix reveal Neoproterozoic and Early Paleozoic age clusters, and less abundant Paleoproterozoic and Archean ages. Hf isotopic compositions of detrital zircons indicate dominantly negative εHf (t) values which are in agreement with a recycled continental source. Minor positive values correspond to a juvenile component that was formed during the Neoproterozoic Cadomian orogeny or Archean-Paleoproterozoic events. The Cambrian and Early Ordovician ages can be related to the pre-Variscan rifting of the Pannotia megacontinent that separated the Armorica microcontinent from Gondwana. Neoproterozoic ages imply a source of the magmatic rocks exposed in the Cadomian belt of the northern part of the Armorica microcontinent, and in the Mauges Nappe.

“Ages and Hf isotopic compositions of detrital zircons in the Pinal schist, southern Arizona, USA: Provenance, tectonic setting, and evidence for pre-1.7 Ga crust in SW Laurentia”

The Pinal basin of southern Arizona is best known for the widely distributed Paleoproterozoic Pinal schist. Most detrital zircons from the Pinal schist yield ages in the 1.68–1.77 Ga range, suggesting provenance in the proximal Mazatzal and Yavapai Provinces. Hf isotopic compositions of these zircons yield εHf values that are positive, but less than estimated depleted mantle at this time, and yield single-stage depleted mantle model ages that indicate a minimum age of 1.85–2.0 Ga for an older crustal component in the source. New Sm-Nd data from Pinal schist whole rocks are compatible with the source(s) of the pelitic component of Pinal sediment being derived from a terrane with a component older than the 1.68–1.77 Ga granitoids in the Mazatzal and Yavapai Provinces, as suggested by the Lu-Hf data. If the Pinal sediments were derived from the Mazatzal and/or Yavapai arcs, these arcs should thus be considered continental arcs. The composition and age of the older crustal component(s) indicated by the Sm-Nd and Lu-Hf systematics is not uniquely defined by these data, but options on a Laurentian margin include the cryptic older crust of the Colorado Province, e.g., an extension of Trans-Hudson-Penokean age crust. If the arcs were built on non-Laurentian crust and accreted as exotic terranes several possibilities are viable in the SWEAT, AUSWUS, and other models for conjugates to southwestern Laurentia in the Nuna supercontinent.

The 2.73 Ga I-type granites in the Lengshui Complex and implications for the Neoarchean tectonic evolution of the Yangtze Craton

ABSTRACT We report the oldest I-type granites in the Lengshui Complex of the Yangtze Craton, providing new insights for its tectonic evolution during the Neoarchean. An approach-combined study of zircon U-Pb dating and Lu-Hf isotopes, as well as whole-rock element geochemistry and Nd isotopes, were employed. LA-ICP-MS zircon U-Pb dating for the monzogranite sample LSG03 and LSG16 yielded ages of 2732 ± 13 Ma and 2738 ± 25 Ma, respectively. The more precise age of 2732 ± 13 Ma for the sample LSG03 was taken as the crystallization age of the monzogranite. These rocks have high SiO2 (73.11–74.01 wt%), K2O (3.93–5.48 wt%), Na2O (3.93–4.86 wt%) and low CaO (0.30–0.69 wt%), MgO (0.17–0.30 wt%), TiO2 (0.14–0.17 wt%), P2O5 (0.01–0.06 wt%), Al2O3 (14.11–14.37 wt%) content with weakly peraluminous affinity (A/CNK = 1.04–1.11). Geochemically, they belong to I-type granites, indicating partial melting of a thickened lower crust. Their relatively high Nb/Ta (15.2–34.8) ratios further suggest they formed under eclogite-facies conditions. The consistent whole-rock Nd and zircon Hf isotopic compositions indicate a homogeneous source. According to their εHf(t) values (−2.0 to 0.8), two-stage Hf model ages (3.1 to 3.2 Ga) and positive εNd(t) (1.4 to 2.1), we argue that they were probably generated by partial melting of a juvenile lower crust with little ancient materials. Monzogranites formed in a late-orogenic or collisional compressive tectonic regime, whereas subsequent ca. 2.73 Ga and 2.67–2.62 Ga A-type granites in the Zhongxiang Uplift (including the Lengshui Complex) may represent a prolonged extensional setting. Thus, Archean subduction (probably unlike modern subduction) likely occurred prior to ca. 2.73 Ga. Similar magmatism in the Kongling Complex implies that the Zhongxiang Uplift may have accrete to the Kongling Complex during the early Neoarchean. The transition from I-type to A-type magmatism may have resulted from a change in the geodynamic regime from the late-orogenic or collisional compressive environment to an extensional environment caused by the subsequent lithospheric collapse and mantle upwelling, suggesting an early Neoarchean orogenic event in the eastern Yangtze Craton.

Differentiating continental and oceanic arc systems and retro-arc basins in the Jiangnan orogenic belt, South China

AbstractThe Neoproterozoic Jiangnan orogenic belt records the accretion and collision between the Yangtze and Cathaysia blocks in South China. The orogen is divisible into three units: a northeastern domain (also referred to as the Huaiyu or Shuangxiwu domain), a central domain (Jiuling domain) and an undifferentiated southwestern domain. Detrital zircons from the oldest sequences (Shuangqiaoshan, Lengjiaxi, Fanjingshan and Sibao groups) in the central and southwest domains yield similar age spectra with major age populations at c. 875–820 Ma, along with minor Palaeo- to Mesoproterozoic and Archaean ages. The dominance of detrital ages close to the deposition ages of the units, along with juvenile zircon Hf isotopic compositions and arc-like whole-rock compositional data, indicate the sedimentary units accumulated adjacent to a convergent plate margin magmatic arc. The presence of Mesoproterozoic and older zircons, both as detritus in the units and as xenocrysts within igneous rocks displaying a subduction-related signature, along with the compositional data, place the magmatic arc along a continental margin. In the northeastern domain, the oldest coeval sequence (Shuangxiwu and Qigong groups) and arc igneous suites are dated at c. 970–850 Ma, and lack older detritus and xenocrysts, indicating they represent an accreted oceanic arc system.

Geochronology and Petrogenesis of Mafic-Intermediate Intrusions on the Northern Margin of the Central Tianshan (NW China): Implications for Tectonic Evolution

Late Paleozoic mafic-intermediate intrusions are widespread in the central Tianshan belt, but their tectonic settings remain controversial. Petrogenesis of these intrusions can provide insights into the tectonic evolution of the belt and its adjacent areas. This study presents new whole-rock geochemical and zircon U-Pb geochronology and Hf isotopic data for the Yaxi diorite and Qianzishan gabbro on the northern margin of the central Tianshan (NW China). Zircon U-Pb dating yielded the magma crystallization ages of 313±4 and 295±4 Ma for the Yaxi diorite and Qianzishan gabbro, respectively. They have lower Mg# values, Ni and Cr contents than typical mantle-derived primary melt, with negative correlations between MgO, TFeO and SiO2 contents, indicating clinopyroxene and olivine fractionation during magma evolution. They are characterized by enrichment of large ion lithophile elements (e.g., Rb, Ba and Sr) and depletion of high field strength elements (e.g., Nb, Ta and Ti) with high Ba/Th and Rb/Y, suggesting that their mantle sources had been metasomatized by slab-derived fluids. In addition, the Qianzishan gabbro has high Al2O3 contents (19.54 wt.%-20.88 wt.%) and positive Eu anomalies (Eu/Eu*=1.09-1.42), which can be attributed to accumulation of plagioclase. Geochemical and zircon Hf isotopic compositions reveal that both the Yaxi diorite and Qianzishan gabbro were derived from depleted lithospheric mantle in the spinel stability field with insignificant crustal contamination. In association with previous investigations, we suggest that the Yaxi and Qianzishan intrusions were emplaced in a subduction-related environment, which means that the subduction of the Junggar Ocean lasted at least to the earliest Permian.

Detrital zircon ages and Hf isotopic compositions of metasedimentary rocks in the Wuqia area of Southwest Tianshan, NW China: implications for the early Paleozoic tectonic evolution of the Tianshan orogenic belt

ABSTRACTThe Chinese Southwest Tianshan Orogenic Belt is located along the boundary between the Central Asian Orogenic Belt (CAOB) and the Tarim Block (TB), NW China. It records the convergence of the Tarim Block and the Middle Tianshan, and is, therefore, a crucial region for understanding the Eurasia continental growth and evolution. The Wulagen (geographical name) metasedimentary rocks of the Wuqia area (mainly metamorphic sandstones and mica schists) form one of the metamorphic terranes in the Southwestern Tianshan Orogenic Belt. The geochronology of these rocks is poorly known, which hampers our understanding of the tectonic evolution of the belt. We analyzed 517 zircon grains for detrital zircon U–Pb dating and 93 zircon grains for in situ Lu–Hf isotopic compositions from the Wulagen metasedimentary rocks. The analyzed zircon grains yield Neoarchean to late Paleozoic U–Pb ages with major age peaks at ~2543 Ma, 1814 Ma, 830 Ma, 460 Ma, and the youngest cluster of zircon (magmatogene) ages is 395 Ma. The zircon U–Pb data show that the late Paleozoic (Early Devonian) is the maximum depositional age of the Wulagen metasedimentary rocks, rather than the previously considered Precambrian period. The zircons with Paleozoic ages yield εHf(t) values of −22.0 to +11.3 and two-stage model ages (TDM2) of 3.95 to 1.30 Ga, suggesting that the parental magmas were formed from partial melting of pre-existing crustal rocks. Our zircon U–Pb geochronology and Hf isotopic data indicate the major source regions for the Wulagen metasedimentary rocks was the Kyrgyzstan North Tianshan. The zircon age population of 600–400 Ma (peak at ~460 Ma) has negative εHf(t) values (−15.0 to −0.6) and Mesoproterozoic two-stage model ages, suggesting that the early Paleozoic magmatism resulted mainly from the melting of ancient crust, which played an important role in crustal evolution in the southern CAOB.

Detrital zircon U–Pb geochronology and geochemistry of late Neoproterozoic – early Cambrian sedimentary rocks in the Cathaysia Block: constraint on its palaeo-position in Gondwana supercontinent

AbstractWe present updated U–Pb ages and Hf isotopic compositions of detrital zircons and whole-rock geochemical data to investigate the provenance and tectonic setting of late Neoproterozoic and early Cambrian sandstones from the Cathaysia Block, in order to offer new constraints on its tectonic evolution and its palaeo-position within the supercontinent. The source rocks for the studied sandstones were dominated by felsic–intermediate materials with moderate weathering history. U–Pb dating results show major populations atc. 2500 Ma, 1000–900 Ma and 870–716 Ma with subordinate peaks at 655–532 Ma, consistent with the global Neoarchean continental crust growth, assembly and break-up of Rodinia, and Pan-African Event associated with the formation of Gondwana. Zircon U–Pb ages and Hf isotopic data suggest that most derived from exotic terranes once connected to the Cathaysia Block. Using whole-rock geochemical analysis, it was determined that the studied sedimentary rocks were deposited in a passive continental margin and the Cathaysia and Yangtze blocks were part of the same continent; no Cambrian ocean existed between them. Compiling a detrital zircon dataset from Qiangtang, northern India, the Lhasa Terrane and Western Australia, the Cathaysia Block seems to be more similar to the Qiangtang and western part of the northern India margin, instead of having a direct connection with the Lhasa Terrane and Western Australia in the Gondwana reconstruction during the late Neoproterozoic and Cambrian eons.

Spatial and temporal variations in the geochemistry of Cretaceous high-Sr/Y rocks in Central Tibet

Recent studies based on low-temperature chronology and sedimentology have proposed the existence of a proto-Tibetan Plateau (p-TP); however, the timing and mechanisms of its formation and evolution remain ambiguous. High-Sr/Y rocks are an important petrological indicator of thickening. Here, we compile geochemical data of Cretaceous rocks to interpret their petrogenesis and to constrain deep geodynamic processes. Geochemical characteristics, in combination with zircon Hf isotopic compositions, indicate that the high-Sr/Y rocks were derived from the partial melting of thickened juvenile lower crust, with or without contamination by mantle peridotite. Comparing geochronological and geochemical data, we observe a correlation between magma migration and the composition of high-Sr/Y rocks. Based on these observations, we propose a revised tectonomagmatic evolution model for central Tibet, involving crustal thickening, retreating delamination, and breakoff. Our research suggests that the rapid uplift of the p-TP was a consequence of the removal of isostatic load during the Mesozoic.

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.

Petrogeochemical characteristics, zircon SHRIMP U—Pb ages and Lu—Hf isotopic compositions of Late Carboniferous A‐type granitoids, Yili area, Inner Mongolia (China)

A large Late Palaeozoic A‐type granite belt lies along the Hegenshan‐Nenjiang‐Heihe belt in the Central Asian Orogenic Belt (CAOB). The Kuleqi, Naimanhe, and Gejiadian plutons at Yili are developed within this A‐type granite belt. The genesis and formation environment of these A‐type granites are controversial. The results of zircon SHRIMP UPb dating analyses indicate that the A‐type granites at Yili formed during the Late Carboniferous (302–307 Ma), and their geochemical data indicate that they principally represent a weakly peraluminous (A/CNK 1.01–1.21), high‐K calc‐alkaline series. These granites are enriched in HFSEs, Th, Rb, and K and depleted in LILEs, Ba, Nb, Sr, P, and Ti, which suggests that they record signatures of evolution of continental magma arc. The Nb/Ta ratios of these granites range from 6.84 to17.51, with an average value of 11.65, indicating that they represent crustal‐sourced magma. Their Zr/Hf ratios range from 37.05 to 88.70, with an average value of 59.72 (all values >39), suggesting that these rocks are dominated by mantle‐sourced components. The zircon Hf isotopic compositions of these rocks show εHf(t) values ranging from 6.8 to 12.5 (all positive), with an average value of 9.9, and depleted mantle two‐stage model ages ranging from 518 to 887 Ma, implying that these zircons may contain Early Palaeozoic–Neoproterozoic depleted mantle material. Based on these data, combined with their trace element characteristics, we conclude that the A‐type granitoids at Yili were mainly produced by the mixing of crust and mantle materials; together with their regional tectonic characteristics, these data indicate that these granitoids are related to the collision of the Xing'an and Songliao blocks, which was triggered by the subduction of the Palaeo‐Asian Ocean.

The Sources of Paleoproterozoic Collisional Granitoids (Sharyzhalgai Uplift, Southwestern Siberian Craton): from Lithospheric Mantle to Upper Crust

Abstract —The paper presents the geochemical and isotope characteristics of rocks and the U–Pb age and Lu–Hf isotope composition of zircons from three plutons of Paleoproterozoic granitoids in the Sharyzhalgai uplift (southwestern Siberian craton). The age of granitoids of the Toisuk (1838 ± 6 and 1827 ± 9 Ma), Nizhnii Kitoi (1846 ± 7 Ma), and Malaya Belaya (1863 ± 16 Ma) plutons corresponds to the Late Paleoproterozoic collision stage and is correlated with the time of mafic magmatism. The studied rocks have a wide range of silica contents. The Toisuk pluton is composed of a range of rocks from monzodiorites to granodiorites (granosyenites) and granites; the Nizhnii Kitoi pluton, of granodiorites and granites; and the Malaya Belaya pluton, of leucogranites. The rocks of the three plutons are highly ferroan, enriched in LREE, Th, and HFSE, and correspond in composition to A-type granitoids. A characteristic feature of melanocratic granitoids of the Toisuk and Nizhnii Kitoi plutons is extremely high contents of Ba: 4080–1500 ppm and 1560–990 ppm, respectively. Based on analysis of experimental data on the melting of various substrates and the results of numerical simulation, it is assumed that monzodiorite–granodiorites of the Toisuk pluton and granodiorites of the Nizhnii Kitoi pluton resulted from the differentiation/melting of a mafic source similar in Ba and Sr contents to intraplate continental basalts. The isotope compositions of zircon and melanocratic granitoids of the Toisuk (εHf from –6.0 to –10.7 and εNd from –5.3 to –10.2) and Nizhnii Kitoi (εHf from –5.0 to –8.1 and εNd = –4.0 and –5.1) plutons argue for the generation of their mafic sources from the enriched lithospheric mantle formed as a result of Neoarchean subduction processes. Vein granites of the Toisuk pluton and leucogranites of the Malaya Belaya pluton formed through the melting of quartz–feldspar (granodiorite) substrate. The contrasting isotope parameters of the Toisuk vein granites (εHf from –6.7 to –10.1, zircons, and εNd = –5.5, rock) and Malaya Belaya leucogranites (εHf from 2.9 to 5.9, zircons, and εNd from +0.7 to –1.9, rocks) indicate melting of the Archean and Paleoproterozoic crust, respectively. The more radiogenic Hf isotope composition of zircons from vein granites as compared with rocks of the Archean crust of the Irkut terrane is evident of the contribution of juvenile material to the granite formation.

Recurrent Quaternary magma generation at Baekdusan (Changbaishan) volcano: New zircon U-Th ages and Hf isotopic constraints from the Millennium Eruption

It is well known that Baekdusan (Changbaishan), an active 2744-m-high stratovolcano located on the border between North Korea and China, initiated trachytic/commenditic eruption in the Pleistocene but the petrogenetic and temporal evolutions of this alkaline magma system remain insufficiently constrained. The so-called “Millennium Eruption” (ME) in the mid-10th century, which resulted in the formation of the world's highest caldera lake (Cheonji, or Tianchi) atop the mountain, was one of the largest and most violently explosive volcanic eruptions to have occurred on Earth in the past 2000 years. Here, we present U–Th disequilibrium ages and Hf isotopic compositions of zircons from trachydacitic pumices ejected during the ME. Taken together with the previous U–Th isotope data, our laser ablation-multicollector ICPMS analyses of the zircons yielded multiple age components consisting of an earliest Holocene cluster (11.3 ± 1.3 ka) and Middle–Late Pleistocene populations (ca. 230 and 110 ka), which indicates that Baekdusan magma has been generated episodically at intervals of ca. 120–100 ky. If the eruption ages assigned for the samples analyzed in the previous isotopic studies are correct, the consistent age pattern of zircon may suggest the persistent existence of the same magmatic plumbing system beneath Baekdusan since the ME. The wide range of zircon εHf values (+5.8 to −3.5) reflects an interaction between the EM1-type primitive magma and pre-existing crustal materials, evidenced by the presence of a xenocrystic zircon core showing a significantly negative εHf value (=−21). A broad negative correlation observed between zircon εHf and the published δ18O values can be ascribed to the progressive assimilation of weathered supracrustal rocks, likely in the course of magma chamber expansion.

Tracing the origin of Southwest Japan using the Hf isotopic composition of detrital zircons from the Akiyoshi Belt

AbstractThe Permian Akiyoshi Belt, which is the oldest unmetamorphosed accretionary complex in Southwest Japan, plays an important role in understanding the origin of the Japanese Islands; however, its tectonic affinity is still not well constrained. Herein, we present the first Hf isotopic data for U–Pb dated detrital zircons of the Late Permian sandstones in the Akiyoshi Belt. Almost all the late Palaeozoic detrital zircons yielded highly positive ɛHf (t) values compatible with a juvenile source, suggesting that the major provenance of the trench sediments was from the South China block and not from the Bureya‐Jiamusi‐Khanka massif. We propose that in the late Palaeozoic, a block of juvenile lower crust existed outboard of the South China block, and it was being melted during the westward subduction of the Palaeo‐Pacific oceanic plate to produce voluminous volcanic and plutonic rocks with positive ɛHf (t) values.

Cretaceous (~100 Ma) high-silica granites in the Gajin area, Central Tibet: Petrogenesis and implications for collision between the Lhasa and Qiangtang Terranes

High-silica granites that occur as isolated plutons are widely distributed in collisional orogenic belts and are considered a hallmark of continental collision processes. The late Mesozoic collision between the Lhasa and Qiangtang Terranes along the Bangong–Nujiang Suture Zone was key event in the evolution of the Tibetan Plateau; however, the timing of this collision remains debated. Here, we report the ages and geochemical characteristics of granites from the Gajin area in the north-central Lhasa Terrane, central Tibet. Zircon UPb dating reveals that the Gajin granites formed at ~100 Ma. These granites exhibit uniformly high SiO2 contents (74.9–78.4 wt%), are weakly peraluminous compositions, and have high K2O, moderate Na2O and Al2O3, and low Fe2O3T, MgO, and CaO contents. They are characterized by enrichments in Rb, Cs, and U, and by depletions in Zr, Hf, Nb, and Ti. These characteristics, in combination with high Rb/Sr ratios and pronounced negative Eu anomalies, indicate that the Gajin granites are highly fractionated rocks. The granites have enriched whole-rock Nd (εNd[t] = −4.3 to −2.9) and relatively depleted and slightly variable zircon Hf (εHf[t] = −1.0 to +5.5) isotope compositions. The Gajin granites were probably generated by high degrees of crystal fractionations of magma produced by partial melting of mature crustal sources. The Cretaceous magmatic rocks in the north-central Lhasa Terrane record a magmatic lull between ~100 and 85 Ma, which corresponds to a transition in both geochemical composition (e.g., HfNd isotopes) and rock association at ~100 Ma. Combining the above characteristics with the development of a regional angular unconformity around the time of the formation of the granites, we suggest that the initial continental collision between the Lhasa and Qiangtang Terranes probably occurred between 110 and 90 Ma. Therefore, the ~100 Ma high-silica granites in the Gajin area were formed during the continuing continental convergence that followed the initial collision between the Lhasa and Qiangtang Terranes. The generation of discrete and monotonous high silica granites may constitute evidence for a significant tectonothermal transition in evolution of the continental crust.

Sub-arc mantle heterogeneity in oxygen isotopes: evidence from Permian mafic–ultramafic intrusions in the Central Asian Orogenic Belt

Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ18O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~ 5.3‰) and are apparently at odds with the mantle-like eNd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like eHf(t) values from 5.1 to 11.9 and crust-like δ18O values from 7.6 to 10.1‰. The observed increase of δ18O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ18O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (> 90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.

Mesozoic-Cenozoic tectonic evolution and metallogeny in Myanmar: Evidence from zircon/cassiterite U–Pb and molybdenite Re–Os geochronology

It is well known that there are many Sn–W–Mo and Cu–Au metal resources in Myanmar. However, the absence of precise mineralization ages for these deposits significantly hinders to understand the close genetic relationship among metallogeny, ore-bearing intrusions, and tectonic evolution. In this study, two groups of consistent molybdenite Re–Os and zircon U–Pb ages indicate that the Dapingba Mo–W deposit in northern Myanmar (Tengchong terrane) formed at Early Cretaceous (∼119 Ma and ∼114 Ma). The Dapingba ore-bearing granitic magmas were mainly derived from melting of ancient Tengchong crust on the basis of zircon Hf isotopic compositions (εHf(t) = −9.3 to 2.5). Moreover, the first reported cassiterite U–Pb ages of ∼60 Ma for the Bawapin and Kalonta Sn–W deposits in southern Myanmar (western Sibumasu terrane) suggest that the two deposits formed at Paleocene. Whereas the precise molybdenite Re–Os age for the Shangalon porphyry Cu–Au deposit in the West Burma terrane indicates that this deposit formed at Eocene (∼39 Ma). Importantly, combined with previous studies on tectonic evolution and magmatic petrogenesis in Myanmar and southwest China, a comprehensive Mesozoic-Cenozoic tectono-magmatic and metallogenic model are proposed. Two periods of Early Cretaceous (∼120–114 Ma) and Late Cretaceous (∼75–70 Ma) crust-derived felsic melts and related Mo–Sn–W deposits from the Tengchong and western Sibumasu terranes likely formed in a continental arc during subduction of the Meso- and Neo-Tethys oceanic lithosphere, respectively. Subsequently, Paleocene-Eocene (∼60–50 Ma) Sn–W deposits and coeval ore-bearing granites in the western Sibumasu terrane possibly formed during roll-back of the Neo-Tethys oceanic slab. After ∼50 Ma India–Asia collision, Eocene (∼41–39 Ma) porphyry Cu–Au (West Burma terrane) and Sn–W deposits (western Sibumasu terrane) likely formed during the Neo-Tethys oceanic slab tear and break-off in a collisional setting. Whereas Miocene (∼19–14 Ma) high sulfidation epithermal Cu deposit in the West Burma terrane possibly formed in an arc setting during oblique subduction of the Indian oceanic lithosphere.

Zircon U–Pb geochronology and Hf isotopic constraints on the petrogenesis of the Late Silurian Shidonggou granite from the Wulonggou area in the Eastern Kunlun Orogen, Northwest China

ABSTRACTThe Wulonggou area in the Eastern Kunlun Orogen (EKO) in Northwest China is characterized by extensive granitic magmatism, ductile faulting, and orogenic gold mineralizations. The Shidonggou granite is located in the central part of the Wulonggou area. This study investigated the major as well as trace-element compositions, zircon U–Pb dates, and zircon Hf isotopic compositions of the Shidonggou granite. Three Shidonggou granite samples yielded an average U–Pb zircon age of 416 Ma (Late Silurian). The Late Silurian Shidonggou granite is peraluminous, with high alkali contents, high Ga/Al ratios, high (K2O + Na2O)/CaO ratios, and high Fe2O3T/MgO ratios, suggesting an A-type granite. The Shidonggou granite samples have zircon εHf(t) values ranging from −7.1 to +4.4. The Hf isotopic data suggest that the Late Silurian granite was derived from the partial melting of Palaeo- to Mesoproterozoic juvenile mantle-derived mafic lower crust. Detailed geochronological and geochemical data suggest that the Late Silurian granite was emplaced in a post-collisional environment following the closure of the Proto-Tethys Ocean. Combining data of other A-type granitic rocks with ages of Late Early Silurian to Middle Devonian, such post-collisional setting related to the Proto-Tethys Ocean commenced at least as early as ~430 Ma (Late Early Silurian), and sustained up to ~389 Ma (Middle Devonian) in the EKO.