Ancient Continental Crust Research Articles

Generation of Pre-Caldera Qixiangzhan and Syn-Caldera Millennium Rhyolites from Changbaishan Volcano by Shallow Remelting: Evidence from Zircon Hf–O Isotopes

The Changbaishan volcano is well known for its major caldera-forming Millennium Eruption (ME) in 946 CE (Common Era). We report Hf–O isotopes of zircon grains from pre-caldera Qixiangzhan (QXZ) and syn-caldera eruptions of the Changbaishan (Baitoushan) volcano to constrain magma chamber processes. Zircon grains from the pre-caldera QXZ comendite lavas have δ18O ranging from 4.46 to 5.16 (lower than mantle values) and εHf ranging from −4.47 to +4.37. Zircon grains from the syn-caldera ME1 charcoal-bearing non-welded comendite pyroclastic flow deposits have δ18O ranging from 2.25 (lower than mantle values) to 5.51 and εHf from −3.75 to +3.31. By comparison, zircon grains from the ME2 welded trachytes have δ18O ranging from 5.66 to 6.20 (higher than mantle zircon values) and εHf from −1.97 to +6.23. There are no correlations between O and Hf isotopes for all zircon grains in QXZ and ME1 comendites and ME2 trachyte. The ubiquitous occurrence of low-δ18O zircon grains in QXZ and ME1 comendites indicates shallow remelting of hydrothermally altered low-δ18O juvenile rocks. By contrast, ME2 trachyte zircons (except for two zircon grains) have normal δ18O (5.66 to 6.10) values, indicating a lack of remelting processes. Similar zircon Hf–O isotopes between pre-caldera QXZ comendites and syn-caldera ME1 comendites indicate tapping of the upper portion of a zoned magma chamber. Higher δ18O in ME2 trachyte zircons indicate tapping of the deeper portion of a zoned magma chamber free from shallow remelting. The lack of significant correlations between zircon O and Hf isotopes, and the relatively high εHf values for all Changbai zircon grains, argue against partial melting of ancient continental crust or significant contaminations by ancient crustal rocks as an origin for these felsic magmas. The QXZ and ME1 comendites were formed by shallow remelting of hydrothermally altered juvenile volcanic rocks, and ME2 trachytes were formed by evolution of mantle-derived basaltic magmas free of hydrothermal assimilations. A proto-caldera likely formed prior to the generation of QXZ lavas at 10 ka.

Jurassic magmatism in the Garzón region of the Colombian Andes: insights from whole-rock geochemistry and zircon isotope analysis of granites and subvolcanic rocks

ABSTRACT New whole-rock geochemical and zircon U – Pb dating and Lu – Hf isotope data are presented for the Garzón granites and related subvolcanic rocks, which formed during widespread Jurassic arc magmatism in the Colombian Andes. This magmatism resulted from the subduction of a proto-Pacific plate under the NW South American margin. Our data indicate that the granitic and related rocks of the Algeciras, Altamira, and Sombrerillo Plutonic Massifs in the Garzón region have crystallization ages ranging from 179 to 169 Ma. These rocks are characterized as high-K alkali-calcic or calc-alkaline, magnesian, with 0.66 ≤ Fe# ≤ 0.94, and metaluminous to slightly peraluminous, with 0.67 ≤ ASI ≤ 1.01, exhibiting geochemical signatures. The trace element patterns normalized to MORB reveal pronounced negative anomalies for Nb, P, and Ti, alongside positive anomalies for Pb, Rb, Sr, Y, Zr, and Ba. The REE patterns display strong LREE fractionation over HREEs, with moderate to absent negative Eu anomalies in mafic to intermediate rocks (0.5 ≤ Eu/Eu* ≤ 1.0) and greater significance in felsic rocks (Eu/Eu* up to 0.6). Most granitic rocks display negative εHf(t)-in-zircon values (−7.7 to −1.2), indicating substantial contributions from ancient continental crust. In contrast, the subvolcanic rocks have a mix of positive and negative εHf(t) values (from −7.9 to + 11.5), reflecting significant input from both juvenile mantle and ancient crustal sources (221 Ma ≤ TDM ≤1516 Ma). After 167 Ma, the magmatic activity diminished and ceased in the Garzón region, while it continued to the west in the Central Cordillera until ca. 129–110 Ma, with a gradual increase in contributions from the mantle, as indicated by systematic zircon Hf isotopes, arguably due to changes in the subduction regimes.

Generation of Neoproterozoic granites of the Huangling batholith in the northern Yangtze Block, South China: Implications for the evolution of the Precambrian continental crust

Granites may contain critical information about composition and reworking of continental crust and are widely used to investigate continental crust evolution. Extensive Neoproterozoic granites in the northern Yangtze Block are significant to the Neoproterozoic continental evolution of the Yangtze Block. We investigated the petrology, mineralogy, whole-rock geochemistry, and zircon U–Pb–Hf isotopes of the Neoproterozoic Wuduhe biotite granites and Xiabaoping K-feldspar granites of the Huangling batholith in the northern Yangtze Block to determine their source characteristics and geological implications for the northern Yangtze Block. The Wuduhe biotite granites yielded zircon U–Pb ages of ca. 818–815 Ma. They displayed high SiO2 contents (70.19–74.23 wt%) and Na2O/K2O ratios (1.38–3.35) but low MgO (0.37–0.90 wt%) and CaO (1.49–2.97 wt%) contents, representing calc-alkaline Na-rich I-type granites. They exhibited enriched whole-rock εNd(t) (−19.8 to − 18.8) and zircon εHf(t) values (−41.3 to − 24.2), and had low CaO/Na2O (0.34–0.63), Al2O3/TiO2 (46.6–89.7), Rb/Ba (0.02–0.08), and Rb/Sr (0.04–0.12) ratios, as well as low zircon saturation temperatures (739 − 770 °C), indicating derivation from Na-rich metabasalts of the Archean Kongling complex. The Xiabaoping K-feldspar granites had zircon U − Pb crystallization ages of ca. 816–812 Ma. They displayed high SiO2 (70.82–74.80 wt%), K2O (4.41–6.35 wt%), and Al2O3 (13.46–16.31 wt%) contents, suggesting A2-type affinities. They showed enriched zircon εHf(t) (−44.3 to − 18.4) and whole-rock εNd(t) values (−23.4 to − 18.4) and high zircon saturation temperatures (869–912 °C), indicating formation by anhydrous melting of K-rich felsic rocks from the Archean Kongling complex. The geochemical diversity of these granites from the Huangling batholith was primarily controlled by source heterogeneity of the Kongling complex and distinct melting temperatures. We infer that the northern Yangtze Block underwent significant remelting of ancient continental crust in a subduction background during the Neoproterozoic, induced by continuous upwelling of mantle-derived mafic magmas.

Amphibole crystallization as a tool for monitoring complex mixing between basaltic and latitic magmas: The Fazenda Lagoas monzonitic stock, Borborema Province, Brazil

Amphibole is the dominant mafic in the quartz monzonite and microgranular dioritic enclaves of the Fazenda Lagoas stock (623 ± 5 Ma), located in the Macururé Domain, in the central region of the Sergipano Orogenic System. Structures and textures in the rocks studied indicate the presence of several pulses of basaltic magma (diorite enclaves) in the latitic magma (quartz monzonites). The magmatic amphibole crystals correspond to pargasite, edenite, tschermakite and Mg-hornblende. The chemical evolution of amphibole crystals occurs through an increase in Si, Mg, vacancy in the A site, and a decrease in Altotal, K, Na, Fe3+ Ca, Ti (substitutions of edenite, tschermakite, and pargasite). Reduction in temperature, pressure, water solubility in magmas, and increase in oxygen fugacity indicate the evolution of the Fazenda Lagoas plutonic system. Amphibole thermobarometry made it possible to calculate the depth and temperature of the earliest interaction between basaltic and latitic magmas at a depth of 27 km, with latitic magma temperatures between 967 and 940 °C. Subsequently, there were several contributions of basaltic magma until the magma chamber at Fazenda Lagoas was ca. 17 km deep. Fazenda Lagoas stock is one of the shoshonite intrusions of the Macururé Domain, the one that has a record of the deepest hydrated basaltic magma, probably at the base of the ancient continental crust of the Sergipano Orogenic System.

Constraints on the growth and evolution of continental crust in the NE Cathaysia Block: insights from detrital zircon U–Pb ages and Hf–O isotopic analyses in Tingjiang sediments

Fluvial detrital zircons provide crucial insights into crustal evolution in the early Precambrian. We used U–Pb geochronology and the Hf–O isotopic compositions of detrital zircons from the Tingjiang River to determine the provenance and continental crustal evolution of the NE Cathaysia Block. The zircon U–Pb ages show seven major populations at 177–105, 262–200 and 520–376 Ma, with striking peaks at c. 422, 775–606, 1457–836, 2042–1710 and 2661–2300 Ma, corresponding to documented tectono-thermal events in the South China Block. Episodic magmatism was the response to the formation and break-up of the Columbia and Rodinia supercontinents. The episode at 422 Ma matches with the ages of widespread early Paleozoic intraplate orogenic events in the South China Block. The isotopic signatures of Hf and O in the detrital zircons indicate that the majority of the Precambrian zircons likely originated from remelted ancient continental crust, with contributions from juvenile mantle-derived magmas. The Phanerozoic zircons, by contrast, were formed through the partial melting of recycled crustal material. Analysis of crustal growth rates, based on the two-stage Hf model ages, suggests a significant presence of juvenile crustal growth during the Paleoproterozoic era. Specifically, it is estimated that c. 40 and 90% of the present crust within the NE Cathaysia Block was formed at c. 2.3 and 1.6 Ga, respectively.

Late Paleozoic potassic igneous rocks of the Kensu and Dzholkolot plutons in the eastern Kyrgyz Tien Shan: Petrology, geochemistry, U-Pb zircon geochronology, and related skarn-porphyry W-Mo-Cu-Au mineralization

The Kensu and Dzholkolot multiphase intrusions are situated in the eastern part of the “Main Structural Line of Tien Shan”, or the “Nikolaev Line”. The plutons comprise mafic to intermediate (monzogabbro, monzonite, syenite, quartz syenite) and silicic (quartz monzonite, monzogranite, and leucogranite-alaskite) members, together with the interim (camptonite) and final monzodiorite-porphyry dikes. Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions, with a strong A-type affinity, emplaced in post-collisional setting. Magmatic evolution included a generation of shoshonitic magma by a low-degree partial melting of the metasomatically-enriched upper mantle, followed by amphibole fractionation in a deep (lower crustal?) magma chamber. This was followed by a generation of mantle-induced granitic magmas in a crustal protolith, with possible mixing/mingling of various magmas. The rocks crystallized at shallow levels, in an oxidized environment, under decreasing pressure and temperature toward the younger (granitic) intrusion phases.Isotopic U-Pb (LA-ICP-MS) zircon dating of the igneous rocks indicates their Late Carboniferous age (ca. 325–302 Ma). The magma emplacement included a number of intrusion phases from monzogabbro (321 ± 4 Ma), monzonite (319 ± 4 Ma), and camptonite (306 ± 4 Ma), to quartz syenite (305.5 ± 2 Ma to 302 ± 3.7 Ma), quartz monzonite (305 ± 3 Ma) and subsequent granitic phases. Although the ages identified correspond to those of the subduction-related magmatism in the western part of the Middle Tien Shan, geochemical characteristics of the rocks supporting rather post-collisional setting of the plutons are in agreement with a “scissor-like” closure of the Turkestan paleoocean, starting from the east. Zircon xenocrysts dated at ca. 1.9 Ga suggest the presence of an underlying old continental crust.The skarn-porphyry W-Mo(−Cu-Au) mineralization complements the group of similar deposits associated with high-K calc-alkaline to shoshonitic series intrusions in the Middle Tien Shan and globally. The high endowment in W and Mo can be related to the fertilization of subduction-modified subcontinental lithospheric mantle in these metals, together with more common fertilization in Cu and Au, with its subsequent involvement in the magma generation in post-collisional setting. The endowment in W and Mo can also reflect a greater involvement of an ancient continental crust as the magma source(s). The oxidized paragenesis of hydrothermal alteration assemblages and mineralization corresponds to the respective affinities of the igneous rocks.

Geochemical and petrological diversity of a transcrustal magmatic system driven by mushy magma mixing: Insights from the Triassic dike swarms in East Kunlun orogen, northern Tibetan Plateau

Abstract Geochemical and petrological diversity within transcrustal magmatic systems usually reflects the magma properties and magmatic processes and thus is critical to understanding the origin of magmatic complexes and the evolution of continental crust. Herein, we present an integrated study on the petrology, mineralogy, geochronology, geochemistry, and Sr-Nd-Hf isotopes of Triassic mafic-felsic dikes in the East Kunlun orogenic belt, northern Tibetan Plateau, to elucidate the nature and evolution of the transcrustal magmatic system. The studied dikes intruding into the granodiorite pluton (ca. 235–233 Ma) comprise coeval ca. 220–218 Ma gabbroic diorite porphyry, diorite porphyry, granodiorite porphyry, and alkali-feldspar granite, resembling composite dike swarms. The macrocrysts in these dikes show various zoning patterns, indicating episodic magma recharge and crystal resorption. The compositional gap between the intermediate-mafic dikes (SiO2 = 52.9–67.8 wt%) and the granitic dikes (SiO2 > 75 wt%), as well as their homogeneous whole-rock Sr-Nd isotopes, with (87Sr/86Sr)i = 0.708387–0.710995 and εNd(t) = −5.83 to −4.34, but variable zircon Lu-Hf isotopes, i.e., εHf(t) = −7.67 to −0.36, demonstrates that magma mixing rather than cogenetic fractional crystallization accounts for their origin. In combination with thermobarometric insights, these results suggest that the mafic and felsic parental magmas originating from an enriched lithospheric mantle and ancient continental crust, respectively, were ultimately emplaced and stagnated at varying crustal depths (~22–30 km and 8–17 km). Subsequently, the felsic magma mush was replenished and rejuvenated by the underplated mafic magma, leading to varying degrees of crystal-melt and/or melt-melt mixing. This mush-facilitated crust-mantle magma mixing is an important mechanism accounting for the compositional diversity of the transcrustal magmatic system.

Reworking and maturation of continental crust in collision zones: Insights from Early Cretaceous compositionally diverse magmatic rocks in central Tibet

The reworking of continental crust in collision zones is a key process in the chemical maturation of crustal rocks. However, the mechanisms that cause crustal reworking remain highly controversial. The compositionally diverse magmatic rocks along the Lhasa–Qiangtang collision zone provide a favorable opportunity for studying the mechanism of crustal reworking and maturation in continental collision zones. Here we report zircon UPb ages and Hf isotopes, mineral major elements, and whole-rock geochemistry and SrNd isotopes for the Momu intrusive complex in the Lhasa–Qiangtang collision zone, central Tibet. Zircon UPb ages reveal that the intrusive complex formed during the late Early Cretaceous (115–113 Ma). Diorites from the intrusive complex have high Mg# values (49.7–51.2) and initial 87Sr/86Sr ratios (0.7082 to 0.7083), and low εNd(t) values (−7.3 to −6.9), with negative to slight positive zircon εHf(t) values (−3.8 to +0.8), suggesting that these rocks were likely generated by the partial melting of the enriched lithospheric mantle. Granodiorites from the intrusive complex have high SiO2 (64.4–67.9 wt%) and Al2O3 (14.8–16.1 wt%) and low MgO (1.4–1.8 wt%) contents. These rocks are also characterized by high initial 87Sr/86Sr ratios (0.7091–0.7098) and whole-rock negative εNd(t) (−9.9 to −8.4) and zircon εHf(t) (−7.9 to −2.2) values. The associated granites have the highest SiO2 (68.7–71.5 wt%) contents among the intrusive complex rocks, high initial 87Sr/86Sr (0.7089–0.7093) ratios, and negative whole-rock εNd(t) (−10.1 to −7.7) and zircon εHf(t) (−5.3 to −0.2) values. These geochemical characteristics indicate that the granodiorites were most likely derived from the partial melting of ancient continental crust and that the coeval granites were formed by the fractional crystallization of granodioritic magmas. Integrating our new results with data for the coeval magmatic rocks of the Lhasa–Qiangtang collision zone, we suggest that the Momu intrusive complex formed in a post-collisional slab breakoff setting. Asthenosphere upwelling caused by slab breakoff could have provided a substantial source of heat for crustal reworking, which gradually contributed to the maturation of continental crust in the Lhasa–Qiangtang collision zone.

Crustal evolution of Paleozoic-Mesozoic granitoid in Dakrong-A Luoi area, Truong Son belt, central Vietnam: evidence from zircon U-Pb geochronology, geochemistry, and Hf isotope composition

ABSTRACT The Dakrong-A Luoi area is located in the South Truong Son belt in the Indochina block and has been affected by multiple stages of magmatic-metamorphic events. In this paper, Paleozoic-Mesozoic granitoids from the Dakrong-A Luoi area were studied using LA ICP-MS U-Pb zircon geochronologies, Hf isotopes on zircon and whole-rock geochemistry of the Paleozoic to examine the tectonic-magmatic history of the Southern Truong Son belt. The magmatism history of the area can be divided into three phases, at ~ 452 Ma (Ordovician), ~259 Ma (Permian), and ~ 243 Ma (Triassic). The Ordovician granites were exposed in the narrow belt of NW-SE trending, while the later two granite phases were exposed in E-W trending. The Ordovician granites show consistent LA-ICP-MS zircon ꜪHf(t) values from + 6.2 to + 8.3. Their corresponding Hf model ages (TDM2) from 0.9 Ga to 1.0 Ga suggest that the Ordovician granite was derived from the partial melting of ancient crustal rocks, which is related to the subduction of the Tam Ky-Phuoc Son (TKPS) ocean beneath the Truong Son belt. The Permian granites have Hf isotopic natures (ꜪHf(t)=-1.6 to 1.3, TDM2 = 1.2–1.3 Ga) of partial melting from ancient mantle components with minor Paleoproterozoic ancient continental crust The Triassic granites have negative ɛHf(t) values ranging from −11.9 to −7.0 and Hf model ages of 1.7–2.0 Ga suggesting that they were a product of partial melting from the Paleoproterozoic crust, corresponding to subduction/collision of the Paleo-Tethys ocean beneath the Indochina block along the Song Ma suture.

Trace element geochemistry of sediments in the Pearl River Estuary and its implication for provenance

The Pearl River system is China’s third longest river and the Pearl River Estuary is a major estuary in China. In the paper, a sediment core collected from the coastal zone in the Pearl River Estuary was analyzed for trace element concentrations and grain size to identify provenance during the past decades. In addition, the tectonic framework of the study area has also been studied using reliable geochemical tracers, including rare earth elements (REEs) and trace elements. The results reveal a significant contribution of reworked sediment from multiple sources. Discrimination diagrams suggest that the sediments display felsic characteristics with recycled components, and therefore, the source rock types of the sediments are presumed to be granitic rocks. The REE patterns observed in the samples primarily reflect the influence of an ancient continental crust that contains felsic materials on the sediments with an LREE/HREE ratio of 7.94–9.56 (mean 8.46) and a δ Eu value of 0.61–0.70 (mean 0.65). The sediments are plotted into a zone of continental island arcs, which was formed during the late Mesozoic subduction of the ancient Pacific. The study on the source of sediment cores shows that the main source of sediments in this area excludes the possibility of the upper middle river from the Pearl River Watered, which reflects the importance of the lower Pearl River Basin, especially the Dongjiang River, for material transport in the study area. Our results demonstrate that the geochemical characteristics of trace elements in sediment could provide an efficient approach to identify sediment provenance in estuaries worldwide.

СТРУКТУРЫ ЦЕНТРАЛЬНОГО ТИПА В ЮЖНОМ СИХОТЭ-АЛИНЕ В ВЕРОЯТНОСТНЫХ ГРАВИТАЦИОННЫХ МОДЕЛЯХ: ГЛУБИННОЕ СТРОЕНИЕ, РЕОЛОГИЯ, ИЕРАРХИЯ, ФРАКТАЛЬНОСТЬ И МЕТАЛЛОГЕНИЯ

As a result of the analysis of 3D-distributions of density contrast in the crust of the South Sikhote-Alin, connections are established between ore districts, ore clusters, and ore fields, on the one hand, and hierarchically subordinate structures of the central type of three taxonomic levels, on the other. Based on the coincidence of the areas of spatial distribution of ore mineralization with the projections of deep density heterogeneities, the probable depth of occurrence of fluid-magmatic sources of ore mineralization and vertical extent of ore-magmatic systems of different specialization were determined: tin (depth of 2‒6 km), tin-polymetallic (4‒12 km), polymetallic (6‒20 km), and gold-tungsten (14‒24 km). The spatial distribution of central-type structures and the associated areas of ore mineralization are coincident with viscous shear zones, within which subsidence of the crustal upper layer is observed (Samarkinsky terrane, Kavalerovsky district and Soboliny ore cluster), as well as displacement of the contours of density heterogeneities in different depth sections (Dalnegorsky region). Deep boundaries of the ancient continental crust and Mesozoic island-arc wedges subducted under accretionary-fold complexes were determined. The joint of these structures is characterized by distinct features of a transform fault.

Inherited zircons in Early Cretaceous dyke: Implication of growth and reworking of continental crust in the Jiaodong Peninsula

Inherited zircons from Early Cretaceous dykes across the Jiaodong Peninsula provide a means of exploring magmatic episodes. Negative Eu anomalies, high Th and U contents, and high U/Yb ratios confirmed a zircon source of ancient continental crust. Differences in mineralogy, chronology, geochemistry, and Hf isotopes showed that these inherited zircons were not brought in by the Triassic Yangtze Craton collision. Zircons within 2938–2800 Ma confirmed that the ancient crust in the Jiaobei Terrane originated from the Mesoarchean crustal growth. This crustal growth continued until the Early Paleoproterozoic and was accompanied by crustal transformation activities in the later period. After the collision of the Jiao–Liao–Ji Belt, the Jiaodong Peninsula entered a typical craton period until it was reactivated at the end of the Neoproterozoic, which was characterised by the Archean crustal transformation. During the Palaeozoic, smooth crustal thickness change with limited addition of oceanic crust material indicated the Qinling–Dabie–Sulu Orogenic Belt as a distal island arc system that slowed the impact of Paleo-Tethys subduction. There was no record of positive-εHf(t) zircons in the Jiaodong Peninsula during the Mesozoic, but the crust thickened rapidly. This confirmed that the Paleo-Pacific Plate may have directly subducted beneath the lithospheric mantle and had little contact with the lower crust. This subduction pattern released little hydrous fluid but much heat, which prompted the remodelling of the lower crust in the Mesozoic. Furthermore, the large-scale lithospheric thinning event around 120 Ma was identified as a result of the delamination of the lithospheric mantle together with lower crust.

Petrogenesis of Jurassic granitic plutons in the Yanbian area, NE China: Implications for the subduction history of the Paleo-Pacific Plate

It is widely accepted that the subduction of the Paleo-Pacific Plate beneath the Eurasian continent began during the early Mesozoic; however, the details of the subduction process remain uncertain. This paper presents new zircon U–Pb–Hf and whole-rock geochemical data for Jurassic intrusive rocks from the Yanbian area of NE China to provide a better understanding of the subduction history of the Paleo-Pacific Plate. Zircon U–Pb dating results reveal that three stages of Jurassic magmatism are recorded in the Yanbian area, namely, Early Jurassic (188–179 Ma), Middle Jurassic (172–163 Ma), and Late Jurassic (158–156 Ma). Early Jurassic intrusive rocks from the Yanbian area are predominantly calc-alkaline gabbro, gabbro-diorite, quartz monzonite, granodiorite, and syenogranite. These geochemical features and rock associations are indicative of formation in an active continental margin setting related to the initial subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The Early Jurassic granitoids were formed mainly by the partial melting of heterogeneous lower crust. The Middle Jurassic granitoids are geochemically similar to adakites and have εHf(t) values of + 2.0 to + 10.1. These rocks are inferred to have been derived from magmas generated by the partial melting of thickened juvenile lower-crustal material. Their formation was related to the subduction of the Paleo-Pacific Plate. Late Jurassic adakites in the Yanbian area formed in a compressional tectonic setting. Their negative εHf(t) values (−17.2 to − 8.6) and TDM2 ages (2025–1552 Ma) indicate that the primary magma was derived from the partial melting of ancient continental crust. Combining our data with the distribution of Late Jurassic igneous rocks in the NE Asian continental margin and the northward migration of Jurassic accretionary complexes along the margin, we conclude that the subduction of the Paleo-Pacific Plate beneath the Eurasian continent during the Late Jurassic–earliest Early Cretaceous had a low angle of obliquity.

Geochronology and geochemistry of early Paleozoic magmatism in the Qilian orogen: Constraints on closure of the Proto-Tethys Ocean

The Proto-Tethys Ocean was a Neoproterozoic to early Paleozoic ocean that opened upon the breakup of Rodinia and closed by Paleozoic oceanic and continental subduction. The Qilian orogenic belt is the northernmost orogen of the Proto-Tethys domain and consists of the North Qilian belt, the Central Qilian block and the South Qilian belt. The processes responsible for extensive early Paleozoic magmatism in the Qilian orogen are crucial to understanding the closure of the ocean. This paper presents a systematic investigation of early Paleozoic mafic and felsic rocks in the Qilian orogen. Gabbroic dikes from the Central Qilian block have LA-ICP-MS zircon U-Pb ages of 499–497 Ma with a wide range of εHf(t) values of −9.94 to + 4.28 and TDM1 model ages of 1557–929 Ma, and dioritic dikes have a LA-ICP-MS zircon U-Pb age of 489 ± 3 Ma, positive εHf(t) values of +7.61 to +10.53, TDM1 model ages of 684–790 Ma, and an amphibole 40Ar/39Ar age of 486.3 ± 4.3 Ma. Both sets of dikes were derived from partial melting of a heterogeneous spinel-facies lherzolite lithospheric mantle that was modified by fluids and melts from a subducted slab. The protolith of amphibolites from the Hualong block in the South Qilian belt was a mafic intrusion of 454 ± 2 Ma with negative εHf(t) values of −9.69 to −7.79 and TDM1 model ages of 1390–1478 Ma. Its trace element signature indicates that sediment-derived melts were involved in the magma formation. An amphibole 40Ar/39Ar age of 418.7 ± 3.8 Ma from an amphibolite records the timing of amphibolite facies metamorphism. Felsic volcanic tuffs and granitic intrusions from the Central Qilian block have zircon U-Pb ages of 507–440 Ma with a broad range of zircon εHf(t) values of −9.60 to + 11.87 and TDM2 model ages of 2037–584 Ma. They have geochemical characteristics similar to I-type and S-type granites that were sourced from partial melting of juvenile and ancient continental crust with minor input of mantle materials. The formation of the felsic rocks indicates the transition from an arc-related setting to a post-collisional setting. Combining our results with existing data, we suggest that the Central Qilian block was affected by both southward subduction of the North Qilian ocean and the northward subduction of the South Qilian ocean, resulting in the formation of an Andean-type continental arc along the northern margin of the Central Qilian block and a subduction-accretion system along the southern margin of the Hualong block at ca. 530–440 Ma. Collision of the Central Qilian, Hualong, Quanji and Qaidam blocks at ca. 440–418 Ma marks the final closure of the Proto-Tethys Ocean.

Early Neoproterozoic magmatism in the Central Tianshan Block, China: Implications for its tectono-magmatic evolution and relationship to Rodinia

As an important ancient continental constituent of the Central Asian Orogenic Belt (CAOB), the Central Tianshan Block (CTB) is indispensable to improving our understanding of the early evolution and architectural framework of this orogenic belt. Here, we present new zircon U–Pb ages and Hf isotopes, as well as bulk-rock geochemistry, for amphibolite, gneissic granite, and augen granite from several localities in the CTB. Zircon U–Pb ages of ten samples demonstrate that the protolith of the amphibolite, gneissic granite, and augen granite formed in the early Neoproterozoic (from 905.9 ± 4.8 Ma to 940.0 ± 4.5 Ma). Geochemically, the early Neoproterozoic amphibolite is depleted in Nb, Ta, P, and Ti, but enriched in light REE (LREE) and large ion lithophile elements (LILE), similar to arc-related basalt. The gneissic granites from the Shardland area and the Alatage area, and gneissic granites and augen granites from the Xingxingxia area are enriched in SiO2, Na2O + K2O, LREE, and LILE, and depleted in heavy REE (HREE) and high field strength elements (HFSE), exhibiting an I-type granite affinity. Zircons from the amphibolite in the Shardland area exhibit positive εHf(t) values of +1.7 to +4.9 and TDM1 model ages of 1.27–1.39 Ga, indicating that the protolith crystallized from a mantle-derived magma. Zircons from the gneissic granites in the Shardland area, the Alatage area and the Xingxingxia area have εHf(t) values that range from –5.9 to +7.3 and TDM2 model ages of 1.33–2.17 Ga, indicating that the granites may have been derived from partial melting of ancient continental crust that contained compositionally variable juvenile material. These new results, combined with previous studies, indicate that early Neoproterozoic magmatic rocks are well-developed throughout the CTB and formed along a subduction-related continental arc. The CTB and other microcontinents of the southwestern CAOB may have been located proximal to the northern margin of the Tarim Craton during the early Neoproterozoic, thereby representing the active continental margin of the circum-Rodinia subduction–accretion system.

Tracing the Early Crustal Evolution of the North China Craton: New Constraints from the Geochronology and Hf Isotopes of Fuchsite Quartzite in the Lulong Area, Eastern Hebei Province

Understanding the composition, formation and evolution of the oldest continental crust is crucial for comprehending the mechanism and timing of crustal growth and differentiation on early Earth. However, the preservation of the ancient continental crust is limited due to extensive reworking by later tectonothermal events. In the Lulong area of eastern Hebei, abundant ca. 3.8–3.4 Ga detrital zircons of the fuchsite quartzite have been previously identified. Nonetheless, the provenance and Hf isotopic compositions of the fuchsite quartzite remain unclear. In this study, we present new detrital zircon ages and Hf isotopic for the fuchsite quartzite in the Lulong area to establish the timing of deposition, the provenance and the regional stratigraphic relationship. Zircon U-Pb dating indicates that the fuchsite quartzite was deposited between 3.3–3.1 Ga and most grains were sourced from the 3.8 Ga TTG gneisses and Paleoarchean magmas. Field investigations and regional correlations reveal that the fuchsite quartzite from the Lulong area is equivalent to that of the Caozhuang area. Zircon Hf isotopic data from eastern Hebei Province (Lulong and Caozhuang areas) and Anshan and Xinyang areas indicate that the oldest crustal growth event of North China Craton occurred in the Hadean.

Paleoproterozoic U Mineralization in Huayangchuan Deposit, Xiaoqinling Area: Evidence from the U–Rich Granitic Pegmatite

The Huayangchuan uranium deposit, located in the west of the Xiaoqinling belt on the southern margin of the North China Craton, is a large U–Nb–Pb deposit accompanied with rare–earth elements. The Huayangchuan uranium deposit, discovered in the 1950s, has long been known as a carbonatite–type uranium deposit. Recently, new geological work has found uranium mineralization in many granitic pegmatite veins in the Huayangchuan deposit and adjacent areas. Here, we report a systematic investigation of the petrography, whole–rock geochemistry, zircon U–Pb ages, and in situ Lu–Hf isotopic characteristics of newly discovered U–rich granitic pegmatite veins in the west of Huayangchuan deposit. The petrological results showed that the lithology of the samples is granite pegmatite. The U–Pb ages of zircon were 1826.3 ± 7.9 and 1829 ± 11 Ma. Microscopically, the paragenetic characteristics of zircon, betafite, and uraninite exist in the intergranular fissures of K–feldspar and quartz, reflecting metallogenic phenomena in the rock formation process. Almost all whole–rock samples were rich in SiO2 (64.37−70.69 wt.%), total alkalis (K2O + Na2O = 8.50–10.30 wt.%), and Al2O3 (12.20–14.41 wt.%) but poor in TiO2 (0.23–0.73 wt.%), MgO (0.38–0.90 wt.%), CaO (1.23–2.22 wt.%), P2O5 (0.14–0.83 wt.%), and MnO (0.04–0.57 wt.%). Additionally, they showed enrichment of LILEs (such as Rb, Ba, Th, U, and K), depletion of HFSEs (such as Ta, P, Ti, and Hf), and no alkaline dark minerals, and the characteristics are intraplate A1–type granite. The A1–type granite displayed low zircon εHf(t) values (−19.42–−15.02) with zircon two–stage Hf model aged 3.10–2.76 Ga, indicating that the U–rich granitic pegmatite was derived predominantly from partial melting of the ancient continental crust (such as the early Taihua group formed in Archean–Neoarchean). Combined with the above results and regional geological data, the U–rich granitic pegmatite discovered in the Huayangchuan deposit was formed in a post–collisional regime after the Luliang movement in the late Paleoproterozoic. This study suggests that future uranium prospecting work in this area should focus on late Paleoproterozoic U–rich granitic pegmatites.

In Situ LA-ICP-MS U-Pb Geochronology, Sr-Nd-Hf Isotope and Trace Element Analysis of Volcanic Rocks from the Gacun Volcanic-Hosted Massive Sulfide Deposit in Sichuan, China

The Gacun deposit is a typical Volcanic Hosted Massive Sulfide (VHMS) associated with Late Triassic seafloor calc-alkaline felsic volcanics. Studies of zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf isotope geochemistry of volcanic rocks from the Northern Yidun arc were undertaken in this paper. We reshaped the Gacun magmatic system activity time, defined the origin of magma evolution, and proposed a metallogenic model of the deposit. Whole-rock major element compositions of the magmatic rocks in the Northern Yidun island arc indicate that they are a complete basalt–andesite–dacite–rhyolite assemblage, showing three obvious stages of composition evolution. They are enriched in large-ion lithophile and light rare earth elements, but depleted in high field-strength and heavy rare earth elements, with weak-to-negligible Eu anomalies (obvious in rhyolite). These geochemical features indicate that the Northern Yidun island arc is a magmatic arc based on ancient continental crust. The Ganzi–Litang oceanic subduction induced mantle melting and produced calc-alkaline basaltic magma, while the MASH processes at the bottom of the crust produced andesitic magma. Part of the andesite magma erupted to form andesite lava. The remaining part was mixed with magma produced via anatexis of ancient crust (approximately 20%–40% of the ancient crustal component), forming the ore-bearing rhyolite. Zircon U-Pb age data defines Gacun magmatic–hydrothermal mineralization sequence of events: At 238 Ma, arc magmatism led to the formation of andesite in the eastern part of the deposit. At 233 Ma, in the arc zone (the western part of Gacun deposit), a large-scale bimodal magmatism formed the main ore-bearing rock series of Gacun deposit, rhyolitic volcanic rocks. At 221 Ma, volcanic eruptions tended to end and sub-volcanic intrusion occurred, forming a lava dome, which was located under the ore-bearing rhyolitic volcanic rocks. The lava dome acted as a thermal engine and promoted hydrothermal circulation. The hydrothermal activity reached a peak at 217 ± 1 Ma, and the Gacun VHMS deposit was formed.

Zircon U–Pb ages and Hf–O isotopes of Xiaomasa Miocene granite in the Ximeng area, western Yunnan, southwest China: Constraints on petrogenesis and tectonic setting

Characteristic granites of the East Tethyan Tectonic Domain (ETTD) are primarily exposed along the Himalayan orogenic belt and Gangdese batholith in the north, extend from western Yunnan in the centre, and through the Burma batholith to Peninsular Malaysia in the south. Western Yunnan is an important part of the ETTD magmatic arc, featuring characteristic granites that mainly developed between the Jurassic and Paleogene. Nonetheless, compared to other regions of the ETTD, Miocene magmatism has been poorly documented in western Yunnan. The Xiaomasa granite occurs in the Ximeng area of western Yunnan. In this study, we present U–Pb ages, and Hf–O isotope data for zircon crystals from the Xiaomasa granite pluton. Laser Ablation–Multicollector–Inductively Coupled Plasma–Mass Spectrometry (LA–MC–ICP–MS) zircon U–Pb dating yields precise emplacement ages of 15.8–16.1 Ma for the Xiaomasa granite, indicating that Miocene granitic magmatism occurred in western Yunnan. Zircons from the Xiaomasa granite have negative εHf(t) values ranging from −6.6 to −11.6, relatively high δ18O values (6.8%–10.2‰), and two‐stage Hf isotopic model ages (1.51–1.83 Ga). Zircon Hf–O isotopic characteristics indicate that the Xiaomasa granite was mainly derived from partial melting of ancient continental crust. The Ximeng Precambrian metamorphic basement is a probable material source for the granite. The Xiaomasa granite was probably produced by partial melting of the thickened Ximeng ancient metamorphic basement under a reduced pressure environment, reflecting a cumulative effect of the escape of Sundaland caused by the dynamics of the continuous India–Asia collision.

Petrogenesis of Early Triassic Felsic Volcanic Rocks in the East Kunlun Orogen, Northern Tibet: Implications for the Paleo-Tethyan Tectonic and Crustal Evolution

The felsic volcanic rocks in orogenic belts are vital probes to understand the tectonic evolution and continental crust growth. Here, we present a comprehensive study on the zircon U–Pb geochronology, whole-rock geochemistry, and zircon Lu-Hf isotopes of Early felsic volcanic rocks from the Hongshuichuan Formation, East Kunlun Orogen, Northern Tibet, aiming to explore their petrogenesis and implications for the Paleo-Tethyan orogeny and crustal evolution. The studied felsic volcanics comprise rhyolite porphyry and rhyolite, exhibiting coeval zircon U–Pb ages of ca. 247–251 Ma. Rhyolite porphyries show metaluminous to peraluminous nature (A/CNK = 0.88–1.24) with high SiO2 contents (72.1–78.9 wt%) and moderate Mg# values (22–40), and they display enrichment of LREE with (La/Yb)N ratios of 6.02–17.9 and depletion of high field strength elements. In comparison, the rhyolites are strongly peraluminous (A/CNK = 1.09–1.74) with high SiO2 contents (71.7–74.3 wt%) and high Mg# values (43–52) and are also enriched in LREE ((La/Yb)N of 6.65–18.4) and depleted in HFSE (e.g., Nb, Ta, Ti). Combining with their different zircon Lu-Hf isotopes, i.e., enriched isotopes for the rhyolite porphyries (εHf(t) = −7.3 to −3.8) and depleted Hf isotopes for the rhyolites (ɛHf = −0.6 to +3.0), we interpret that the studied rhyolite porphyries and rhyolites were derived by partial melting of Mesoproterozoic metagreywacke sources followed by plagioclase-dominated fractional crystallization, but the latter shows the significant contribution of crust–mantle magma mixing. The mixed mantle-derived magma comes from an enriched lithospheric mantle source that had been metasomatized by subduction-related fluids. Combining with other geological evidence, we propose that the studied Early Triassic felsic volcanic rocks were formed in a subduction arc setting, and the reworking of ancient continental crust with crust–mantle magma mixing is the major mechanism of crustal evolution in the East Kunlun Paleo-Tethyan orogenic belt.