Zircon Saturation Research Articles

Widespread ca. 800 Ma granitoids in the southern Dabie Orogen: Petrogenesis and implications for Neoproterozoic accretion-type orogeny in the northern Yangtze Block

Large-scale Neoproterozoic granitoids occur widely in the Dabie Orogen of the northern Yangtze Block and provide significant clues to understanding the geological evolution of the Block within the Rodinia supercontinent. We present evidence for a prominent episode of granitoid intrusion at ca. 800 Ma in the southern Dabie Orogen (SDO), verified by new zircon LA-ICP-MS ages from four separate granitoid intrusions ranging from 807 Ma to 795 Ma in age. The granitoids are dominated by monzogranite and granodiorite with bulk-rock A/CNK and zircon δ18O values of 0.90–1.10 and 5.03–6.45‰, respectively, corresponding to I-type affinity. This is also supported by negative correlations of SiO2 vs. P2O5 and Sr vs. P2O5, and positive correlations of SiO2 vs. A/CNK, Rb vs. Y and Rb vs. Th. Low zircon εHf(t) values (−15.03 to −3.84) and relatively old two-stage Hf model ages (mainly between 1900 and 2200 Ma) for these granitoids are consistent with primary derivation of magma from partial melting of Paleoproterozoic metabasites. Relatively low Ga/Al ratios, Zr contents, and zircon saturation temperatures and consistently negative zircon εHf(t) and bulk-rock εNd(t) values are distinct from those of 780–720 Ma A-type granites in the Dabie Orogen that formed in a rift setting. We propose that the ca. 800 Ma granitoids in the SDO were products of variable degrees of partial melting of ancient mafic crust, promoted by heating of the upwelled asthenosphere due to oceanic slab breakoff in an accretion-type orogeny. Our observations suggest that the formation of the unified Yangtze Block was achieved by diachronous assembly of multiple micro-blocks during the Neoproterozoic, consistent with a peripheral location of the Yangtze Block in the Rodinia supercontinent.

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.

Water content drives distinct evolution trajectory of Early Cretaceous granitoids in inland and coastal southeast China

Water plays a crucial role in determining the crystallization sequence of magma, which subsequently influences the chemical compositions of magmatic rocks across different tectonic settings. In this study, we compared the evolutionary features of granitic rocks along the coast and inland areas of southeast China, aiming to identify the key factors influencing their evolution. Our work indicates that multiple granitic intrusions formed between 126 and 142 Ma in the coastal region of southeastern China, which is consistent with the formation ages of large-scale granitoids in the inland Gan-Hang Belt. Isotopic characteristics suggest that the granitic rocks in southern Fujian originated from the melting of juvenile crust, while those in northern Fujian and eastern Zhejiang were formed from the partial melting of ancient crustal rocks, incorporating mafic magma evolved from the mantle. Most of the granitoids from the coastal region of southeastern China exhibit low zircon saturation temperatures (680–800 °C) and Zr/Sr ratios (<1), suggesting their origin from a cold, wet magma reservoir. The porphyritic quartz diorite and porphyritic monzogranite represent the residual cumulate rocks of this hydrous magma reservoir, whereas the granitic porphyry and high-silica equigranular alkali feldspar granite evolved from the felsic melts extracted from the same reservoir. In contrast, most of the Early Cretaceous granitoids in the Gan-Hang Belt, located in the inland areas of southeast China, display high zircon saturation temperatures (800–900 °C) and Zr/Sr ratios (>1), indicating their origin from hot, water-poor magma reservoirs. The porphyritic granites in this region represent residual cumulate rocks formed in water-poor magma reservoirs, whereas the high-silica equigranular granites evolved from hot felsic melts extracted from similar magma reservoirs. In the Early Cretaceous, the coastal region of southeastern China was closer to the Late Mesozoic paleo-Pacific subduction zone, where crystal-melt segregation within cold, wet magma reservoirs predominantly influenced magma evolution. Conversely, the granitoids in the Gan-Hang Belt in the inland region, located farther from the Late Mesozoic paleo-Pacific subduction zone, were associated with a rift tectonic setting and formed through crystal-melt segregation within hot, water-poor magma reservoirs. Our study underscores the critical role of water content in magma reservoirs in shaping the chemical composition of granitic rocks through crystal-melt segregation, thereby deepening our understanding of crustal formation processes across diverse tectonic environments.

A zircon case for super-wet arc magmas

Arc magmas have higher water contents (2-6 wt.% H2O) than magmas generated in other tectonic environments, with a growing body of evidence suggesting that some deep arc magmas may be ‘super-wet’ (>6 wt.% H2O). Here, we use thermodynamic modelling to show that the behaviour of zirconium during magmatic differentiation is strongly sensitive to melt water contents. We demonstrate that super-wet magmas crystallise zircon with low, homogeneous titanium concentrations (75th percentile <10 ppm) due to a decrease in zircon saturation temperatures with increasing melt H2O. We find that zircon titanium concentrations record a transition to super-wet magmatism in Central Chile immediately before the formation of the world’s largest porphyry copper deposit cluster at Río Blanco-Los Bronces. Broader analysis shows that low, homogeneous zircon titanium concentrations are present in many magmatic systems. Our study suggests that super-wet magmas are more common than previously envisaged and are fundamental to porphyry copper deposit mineralisation.

Geochronology and Geochemical Characteristics of Granitoids in the Lesser Xing’an–Zhangguangcai Range: Petrogenesis and Implications for the Early Jurassic Tectonic Evolution of the Mudanjiang Ocean

This article focuses on zircon U-Pb isotope dating and a whole-rock elemental analysis of granodiorites, monzonitic granites, granodioritic porphyries, and alkali feldspar granites in the Yangmugang area of the Lesser Xing’an–Zhangguangcai Range. The zircon U-Pb isotope-dating results revealed that these granitic rocks formed during the late Early Jurassic period (182.9–177.2 Ma). Their geochemical characteristics and zircon saturation temperatures suggest that the granodiorites are moderately differentiated I-type granites and the monzonitic granite, granodioritic porphyries, and alkali feldspar granites are highly differentiated I-type granites. The degree of magma differentiation progressively increased from granodiorites to alkali feldspar granites. By combining the regional Nd and Hf isotope compositions, it was inferred that the magma source involved the melting of lower crustal material from the Mesoproterozoic to the Neoproterozoic eras. By integrating these findings with contemporaneous intrusive rock spatial variations, it was indicated that the late Early Jurassic granitoids in the Lesser Xing’an–Zhangguangcai Range formed within an extensional tectonic setting after the collision and closure of the Songnen–Zhangguangcai Range and Jiamusi blocks. Additionally, this study constrains the closure of the Mudanjiang Ocean to the late Early Jurassic period (177.2 Ma).

Geochemistry and geochronology of 1.7-1.8 Ga peraluminous A-type granites and granite-gneiss from the Mahakoshal Basin, Central Indian Tectonic Zone (CITZ): implications for an accretionary orogen for the evolution of CITZ

ABSTRACT Here, we report petrography, whole-rock geochemistry, and zircon U-Pb geochronology to understand the origin, source, geodynamic setting, and evolution of A-type granite and granite gneiss as well as establish their emplacement age from Mahakoshal Basin. Mineralogically, Sidhi granite gneiss and Madanmahal granite of the Mahakoshal Basin dominantly consist of quartz, K-feldspar, and plagioclase. Geochemically, Sidhi granite gneiss and Madanmahal granite are ferroan in nature. The studied rocks are akin to peraluminous A-type granites and have average zircon saturation temperatures of 955°C and 977°C, respectively. The studied rock types are derived from partial melting of pre-existing crust indicated by (Y/Nb)N >0.18, (Th/Nb)N >2, (Ce/Pb)N <1, and (La/Nb)N >2 accompanied by the Eu, Ba, Sr, Ti, and Nb negative anomalies. The zircon U-Pb geochronology yielded emplacement ages of 1723 ± 16 Ma for the Sidhi granite gneiss and 1801 ± 64 Ma for the Madanmahal granite. Based on similar geochemical characteristics such as rare earth element patterns, Eu anomalies, and Ce/Pb, La/Nb, and Th/Nb ratios, we propose that the Sidhi granite gneiss and Madanmahal granite are derived from the same source i.e. Bundelkhand gneisses and granites, and formed in a post-orogenic rift environment of the accretionary orogen setting.

Were South India and North China Craton attached during the Paleoproterozoic (Orosirian) Nuna Assembly? Novel geochemical and isotopic investigations of A2-type granites from the Khammam Schist Belt, Eastern Dharwar Craton, India

The paper presents novel geochemical and geochronological data from granites in Khammam, Eastern Dharwar Craton (EDC), India. The studied granites contain major mineral phases like quartz, alkali-feldspar, plagioclase, biotite, and muscovite in decreasing order of abundance. The accessory phases are epidote, titanite, and zircon. The samples comprise 70 ‐–77 wt% SiO2 and 12–15 wt% Al2O3. The K2O and Na2O concentrations range from 2.57 to 5.65 wt% and 1.17 to 2.69 wt%, respectively. They are enriched in Rb, Th, and Pb and depleted in Nb, Ta, and Ti. On a chondrite-normalized plot, the samples exhibit a rightward trend with a negative Eu anomaly. Zircon saturation in silicate melts yields a temperature (TZr) of 859 ‐ 978 °C. The microstructure and U - Pb isotopic analysis of zircon grains (n = 79) reveals the presence of magmatic and polymetamorphic grains with 207Pb/206Pb age clusters at 1844 Ma (number of analyses, n = 7) - 1858 Ma (n = 8), 1737 Ma (n = 5) -1768 Ma (n = 4), 1619 Ma (n = 7) - 1634 Ma (n = 6), and 1554 Ma (n = 5), respectively. The magmatic zircons exhibit ε(Hf) values between 3 and 18.9 with a two-stage model age of 2.03 Ga. In contrast, the metamorphic zircons exhibit ε(Hf) values between −5.6 and 18, yielding a two-stage model age of 1.97 Ga. The geochemical and geochronological studies indicate that the rocks are A2-type granite emplaced during the accretion of the eastern block of the North China Craton (NCC) and EDC between 1844 Ma and 1858 Ma. The zircons from 1737–1768 Ma and 1620 Ma show the time of metamorphic growth during Antarctica-Nellore Schist Belt (EDC) accretion. Finally, the U-Pb zircon ages from 1554 Ma represent Nuna's final amalgamation. The results of this study posit an association between EDC and NCC during Nuna assembly.

Post-orogenic magmatic-hydrothermal system in the southern Songpan-Ganze Orogen, eastern Tibetan Plateau

ABSTRACT Post-orogenic magmatism is uncommon in the Songpan-Ganze Orogen (SGO), and could provide deep insights into geodynamic evolution and related metallogenesis. Here we present zircon U-Pb-Hf isotopes and whole-rock geochemical data, for post-orogenic granitoids newly identified in the southern SGO. The Longxigou and Yangfanggou granites yield crystallization age of ca. 165 Ma and εHf(t) values of +3.8 to −9.1. This age post-dates regional ca. 220–200 Ma orogenic magmatism and ca. 205–190 Ma peak metamorphism, and corresponds to ca. 200–150 Ma post-orogenic magmatism which is exemplified by neighbouring and coeval Wenjiaping and Wulaxi felsic plutons. They are crosscut by ca. 133 Ma granitic pegmatites, with εHf(t) values of −4.3 to −14.2. These post-orogenic granites show high SiO2 contents, as well as low Mg# values, (Zr + Nb + Ce + Y) concentrations and zircon saturation temperatures. They have continental crust-like REE patterns and trace element profiles, and exhibit negative Eu anomalies, enrichment in large ion lithophile elements and depletion in high field strength elements. Their mineral assemblages lack typical high-temperature (e.g. pyroxene and fayalite) and peraluminous (e.g. garnet and cordierite) varieties. All these features indicate an I-type granite affinity, and a dominant derivation from thickened lower crust. In comparison, these post-orogenic granites are compositionally distinct from regional late-orogenic (>200 Ma) categories, which have more positive εHf(t) values and higher Mg# values, with subduction-related signatures involving significant mantle components. Further, several high-grade stratiform Cu deposits and a tungsten deposit occur to the south of these granitic plutons, and have mineralization ages of 164–151 Ma together with biotite alteration age of ca. 134 Ma. Hence, ca. 165–133 Ma magmatic-hydrothermal system was most likely responsible for Cu-W mineralization and effective upgrading of metal deposits in the southern SGO.

Early Jurassic A-type granite and monzodiorite from the Baoji batholith: Implication for tectonic transition from post-collision to post-orogenic extension in the Qinling Orogenic Belt, China

Introduction: The early Jurassic granitoids in the Qinling Orogenic Belt (QOB) play a crucial role in understanding the tectonic implications for the geological evolution of China. To elucidate the early Jurassic tectonic setting of QOB, we performed a comprehensive analysis of zircon U-Pb ages, whole-rock geochemistry, and in situ zircon Lu-Hf isotopes from early Jurassic monzodiorite and Kfeldspar granite within the Baoji batholith in western QOB.Geochronology Method and Results: The intrusions yielded zircon U-Pb ages of 186 ± 2 Ma and 188 ± 2 Ma, respectively.Geochemistry Results: The monzodiorites are characterized by relatively high MgO, Rb, Th, U, and LREE contents, as well as low P, Ti, and HREE contents. They also exhibit high Nb/Ta ratios (20.6–23.4). The zircon εHf(t) values for the monzodiorite sample range from −4.36 to 6.47, indicating significant contributions from a fertile continental lithospheric mantle with the involvement of crustal components. The K-feldspar granites are enriched in K2O+ Na2O, Rb, Zr, Hf, and Nb, and lower Ba, Sr, Ti, and P. They exhibit high Nb/Ta and Ga/Al ratios but low Y/Nb and Yb/Ta ratios. Their geochemical characteristics reveal an A-type granite affinity with elevated zircon saturation temperatures (848°C–900 °C). Additionally, the K-feldspar granite exhibits REE and trace element patterns similar to those observed in the monzodiorite. However, a wide range of zircon εHf(t) values (−4.72 to 3.98), differing from those of the monzodiorite, indicate that the parental magma of the K-feldspar granite experienced magma mixing between a monzodioritic magma and a crustal-derived felsic magma.Discussion: These findings suggest that both A-type K-feldspar granite and monzodiorite likely formed during post-orogenic processes. Additionally, the QOB commenced its postorogenic evolution as an extensional tectonic environment during the early Jurassic period.

Evaluation of garnet phenocrysts in volcanic rocks (Görece/İzmir – Western Türkiye) for their usability as a semi-precious gemstone by multi-analytical methods

Garnets are observed as phenocrysts in blocky rhyolites of Cumaovası Volcanites and are reddish-blackish in color, show vitreous luster, and have dodecahedron forms varying between 0.5 and 2.0 cm in size. In optical microscope investigations, they exhibit fractured structures and contain quartz, hematite, chlorite, and acicular rutile inclusions. Garnet phenocrysts are highly consistent with spessartine-type garnet end members in the Raman spectrum. This result is supported by the mineral chemistry results determined as spessartine (Sps)0.50–0.52 almandine (Alm)0.47–0.48 grossular (Grs)0.01–0.02. In the chondrite, primitive mantle, and Mid-Oceanic Ridge Basalt normalized multi element variation diagrams, garnet phenocrysts show enrichment in all elements (except Sr, K, P, and Ti). In contrast, in the upper continental crust normalized multi element variation diagram, garnet phenocrysts show depletion in large ion lithophile elements (Rb, Ba, K, and Sr) relative to high field strength elements. They exhibit slight enrichment in light rare earth elements ((La/Sm)N = 0.11–1.07) relative to heavy rare earth elements ((Sm/Yb)N=0.78–0.97). Furthermore, garnet phenocrysts have significant negative europium (Eu) ((Eu/Eu*)N = 0.01–0.02) and mildly positive cerium ((Ce/Ce*)N = 2.43–13.77) anomalies indicating hot and reduced magma conditions. The Y/Ho ratios have ranged from 18.9 to 20.3 and are close to the chondrite value. The zircon saturation temperatures (TZr) of the garnet’s host rock was determined to be 756 °C. As a result of mineralogical, geochemical and gemological studies, it was concluded that Görece garnet phenocrysts have glassy luster and semi-transparent dodecahedral crystal shapes, so they can be classified as semi-precious gemstones. In addition, the rare earth element content of the garnet phenocrysts is similar to that of industrial garnet sands and it is thought that they may have potential.

Petrogenesis of A-type leucocratic granite magmas: An example from Delbegetei massif, Eastern Kazakhstan

The genesis of leucogranite magmas is important issue of geodynamics, petrology and ore geology because leucogranites are associated with collisional belts, partial melting of sedimentary source rocks, and may host rare metal (Sn, W, Li, NbTa, and Be) mineralization. To establish the petrogenesis of large leucogranite intrusions, detailed studies of petrography, mineralogy, fluid regime, and the material and isotopic composition of rocks are required. The paper reports results of the studies of the Delbegetei massif of the intrusion in Eastern Kazakhstan. The massif is composed predominantly of leucocratic granites, while syenogranites are subordinate. Rocks of the massif belong to shoshonitic and high-K calc-alkaline series; demonstrate a predominance of K over Na, high ferroan and high contents of LREE and HFSE; which allows them to be classified as A-type granites. The age of Delbegetei massif, estimated by the UPb zircon dating, varies in the range 249–240 Ma, which correspond the Early-Middle Triassic. The differences in rock composition and in temperatures of zircon saturation allow supposing that syenogranites and leucogranites formed from different parental magmas. Syenogranite magma formed as a result of partial melting of metamorphosed volcanic rocks (andesidacites or dacites) with possible influence of mafic magmas. Leucogranite magma formed as a result of fluid-present partial melting of metaterrigenous sedimentary rocks. Leucogranite magma underwent the feldspars differentiation in the fluid-present conditions. This led to composition variations of leucogranites. Analysis of the geological position, age and composition of the rocks allows concluding that the Delbegetei massif formed at the Early Triassic in an intraplate geodynamic setting and that the activity of the Siberian mantle plume is the most probable reason for their formation.

New discovery of 3.84–3.64 Ga diverse granitoids in eastern Hebei, North China Craton: Petrogenesis and significance

Abstract Eoarchean rocks are scarce worldwide, limiting our understanding of Earth’s early history. This contribution presents the field geology, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of Eoarchean granitoids newly discovered in the Labashan area, eastern Hebei, North China Craton. Eoarchean rocks, including 3838–3754 Ma trondhjemitic gneisses (4 samples), 3786–3773 Ma granodioritic gneisses (4 samples), 3775 Ma quartz monzonite gneiss (1 sample), and 3786–3640 Ma potassic granite gneisses (3 samples), are identified in a tract 1 km long. They are in tectonic contact with 3.4–3.1 Ga supracrustal rocks, with both occurring as enclaves in 2.5 Ga potassic granite. The trondhjemitic gneisses show low (La/Yb)N values (23.4–56.8) and insignificant Eu anomalies (Eu/Eu* = 0.86–1.24). Compared with the trondhjemitic gneisses, the granodioritic gneisses and quartz monzonite gneiss have lower (La/Yb)N ratios (9.2–11.3) and lower Eu/Eu* values (0.62–0.79). The potassic granites have (La/Yb)N ratios of 20.7–55.2 and Eu/Eu* values of 0.75–1.84. Zircon saturation temperatures of the granitoids range from 667 °C to 878 °C. They have whole-rock εNd(t) of −4.6 to 1.0 and tCHUR(Nd) of 4.3–3.6 Ga, and zircon εHf(t) values of −6.9 to 0.4 and tCHUR(Hf) ages of 4.1–3.5 Ga (mainly 4.1–3.8 Ga) and δ18O of 4.6‰–7.5‰ (CHUR—chondritic uniform reservoir). It is speculated that different amounts of low-K mafic rocks and tonalite-trondhjemite-granodiorite rocks with prior crustal residence times were the source of the spectrum of trondhjemitic to granitic compositions at Labashan. Two possible models, magma underplating and convergent plate boundary processes, are explored for the tectonic regime forming these Eoarchean granitoids.

Reinterpretation of the post-26 ka Taupō Rhyolitic Magmatic System (New Zealand) as Deep and Vertically Extensive Based on Isotope Thermometry and Measured and Modeled Zircon Destinies

Abstract Taupō volcanic zone, the site of the 26 ka Oruanui supereruption, produced ~70 km3 of new rhyolites since 11 ka, culminating in 50 km3 Taupō eruption 1.8 ka. Major phenocrysts decrease from 4 to 1 vol%, and Oruanui and post-Oruanui ignimbrites all have identical high-δ18Omelt values of 7.39 ± 0.1‰ and lack low-δ18O values despite overlapping calderas. The Δ’17O values are −0.07‰, lower than the mantle and indicate source contamination of high-δ18O, low-Δ’17O metasediments, and limited interaction with high-Δ’17O hydrothermally altered crust. Previously published U-Th-Pb zircon ages demonstrate their diversity spanning 104–105 years for each unit. Zircon crystal size distribution shows a decrease in abundance and the mean size, and some units lack small (&amp;lt;~10 um) zircons suggesting that zircons were both growing and dissolving in the coexisting magma generation areas. Isotope thermometry indicates heating of the system from ~812 ± 35°C to 874 ± 36°C past zircon saturation in 1.8 ka eruption. We advocate that a deep vertically continuous and laterally discontinuous silicic magma system at the base of the Taupō rift, rather than a shallow batholith or an evolving mush, drives volcanism at Taupō. To explain the post-Oruanui magma production, rift-base silicic magma origin and moderate (~2 km3/1000 years) rhyodacitic magma flux from a growing and heating liquid magma body creates a sufficient solution for the most recent magmatism.

Genesis of Rare Metal Granites in the Nubian Shield: Tectonic Control and Magmatic and Metasomatic Processes

The Igla Ahmr region in the Central Eastern Desert (CED) of Egypt comprises mainly syenogranites and alkali feldspar granites, with a few tonalite xenoliths. The mineral potential maps were presented in order to convert the concentrations of total rare earth elements (REEs) and associated elements such as Zr, Nb, Ga, Y, Sc, Ta, Mo, U, and Th into mappable exploration criteria based on the line density, five alteration indices, random forest (RF) machine learning, and the weighted sum model (WSM). According to petrography and geochemical analysis, random forest (RF) gives the best result and represents new locations for rare metal mineralization compared with the WSM. The studied tonalites resemble I-type granites and were crystallized from mantle-derived magmas that were contaminated by crustal materials via assimilation, while the alkali feldspar granites and syenogranites are peraluminous A-type granites. The tonalites are the old phase and are considered a transitional stage from I-type to A-type, whereas the A-type granites have evolved from the I-type ones. Their calculated zircon saturation temperature TZr ranges from 717 °C to 820 °C at pressure &lt; 4 kbar and depth &lt; 14 km in relatively oxidized conditions. The A-type granites have high SiO2 (71.46–77.22 wt.%), high total alkali (up to 9 wt.%), Zr (up to 482 ppm), FeOt/(FeOt + MgO) ratios &gt; 0.86, A/CNK ratios &gt; 1, Al2O3 + CaO &lt; 15 wt.%, and high ΣREEs (230 ppm), but low CaO and MgO and negative Eu anomalies (Eu/Eu* = 0.24–0.43). These chemical features resemble those of post-collisional rare metal A-type granites in the Arabian-Nubian Shield (ANS). The parent magma of these A-type granites was possibly derived from the partial melting of the I-type tonalitic protolith during lithospheric delamination, followed by severe fractional crystallization in the upper crust in the post-collisional setting. Their rare metal-bearing minerals, including zircon, apatite, titanite, and rutile, are of magmatic origin, while allanite, xenotime, parisite, and betafite are hydrothermal in origin. The rare metal mineralization in the Igla Ahmr granites is possibly attributed to: (1) essential components of both parental peraluminous melts and magmatic-emanated fluids that have caused metasomatism, leading to rare metal enrichment in the Igla Ahmr granites during the interaction between rocks and fluids, and (2) structural control of rare metals by the major NW–SE structures (Najd trend) and conjugate N–S and NE–SW faults, which all are channels for hydrothermal fluids that in turn have led to hydrothermal alteration. This explains why rare metal mineralization in granites is affected by hydrothermal alteration, including silicification, phyllic alteration, sericitization, kaolinitization, and chloritization.

Two episodes of extension induced by slab rollback and root foundering in the Eastern Tianshan, southwestern Altaids: Insights from granites and intermediate dykes

The widespread Late Paleozoic-Early Mesozoic magmatism in the Eastern Tianshan provides critical insights into geodynamic evolution of the southern Altaids. In this study, we conduct an integrated study of field geology, petrology, U-Pb zircon geochronology and major-trace elemental and Sr–Nd–Hf isotopic geochemistry on the Late Carboniferous monzogranite and diorite dyke and the Late Triassic gabbroic diorite dyke in the central part of the Eastern Tianshan. Geochronological data indicate that the monzogranite, diorite dyke and gabbroic diorite dyke formed at ∼318 Ma, ∼317 Ma and ∼235 Ma, respectively. The Late Carboniferous monzogranite shows transitional characteristics between highly fractionated and A-type granites, with relatively high zircon saturation temperatures (800–836 °C). It was derived by partial melting of juvenile infracrustal rocks, followed by plagioclase-dominated fractional crystallization. The Late Carboniferous diorite dyke has high Al2O3 contents (mostly >17 wt%) and Ba/La ratios, indicating an origin from a hydrous lithospheric mantle source that had been metasomatized by slab-derived fluids. The Late Triassic gabbroic diorite dyke, with relatively high Mg# values (57–58) and Th contents (3.28–3.46 ppm), originated from a less hydrous lithospheric mantle source containing significant sediment-derived components. Our new results, combined with previous studies, reveal an end-Early Carboniferous to middle Late Carboniferous magmatic flare-up in the Dananhu arc. The magmatic flare-up occurred in a long E-W trending banded zone parallel to the strike of the arc and was accompanied by significant normal arc magmatism, which widely migrated to the south of the Dananhu arc in comparison to prior magmatism. These together with coeval extension-related volcanic, sedimentary, structural and metamorphic events in the Dananhu and adjacent areas were attributed to southward rollback of the subducted Kanguer oceanic slab. The long-time arc magmatism and regional crustal thickening before the Late Triassic led to the formation of an arclogite-bearing root beneath the Dananhu arc. The Late Triassic extension-related magmatism in the Dananhu arc, which was synchronous with or slightly younger than regional thickened crust-derived magmatism, was linked to the local root foundering of the long-lived Dananhu arc at the late stage of orogenesis. The episodic occurrence of geodynamic processes such as slab rollback, arc root foundering and ridge subduction/slab tearing, which were accompanied by massive input of juvenile materials into continental crust, contributed to multi-stage significant crustal growth in southern Altaids.

Eocene rhyolites in the Shanglaxiu-Xialaxiu area of north-eastern Qiangtang Block, Tibet: Partial melting of juvenile crust?

Many granitoids and their eruptive equivalents are substantially derived from the uppermost proportion of continental crust, and so their petrogenesis and source are significant in understanding the evolution and characteristics of continental crust. The Eocene felsic rocks in the Qiangtang block (central Tibet) commonly have enriched Nd isotope compositions and have been considered to be related to the recycling of ancient continental crustal material. In this paper we report new data on early Eocene felsic rocks with positive εNd(t) values in the northeastern Qiangtang block, which can be used to help understand Cenozoic evolution and characteristics of continental crust in the Qiangtang block. These rocks consist of rhyolites and occur in the Shanglaxiu and Xialaxiu areas in the north−eastern Qiangtang block. Zircon U−Pb dating yields ages of 47.8 ± 0.6 and 44.2 ± 0.7 Ma for the Shanglaxiu and Xialaxiu rhyolites, respectively, and so they represent the earliest record of Eocene magmatism in the Qiangtang block. The Shanglaxiu−Xialaxiu rhyolites have relatively low calculated zircon saturation temperatures (719–819 °C), along with high SiO2 (74.8–83.3 wt%) and K2O (5.1–7.8 wt%) contents, and low MgO (0.2–0.3 wt%), Cr and Ni contents. They are enriched in light rare earth elements (LREEs), Rb, K, Th, Zr, and Hf, and are depleted in Nb, Ta, Ti, Sr, and P, with moderately negative Eu anomalies. The Shanglaxiu rhyolites have positive whole−rock εNd(t) (0.3–0.4) and zircon εHf(t) (1.8–10.4), and variable zircon δ18O (6.3–7.3‰). The Xialaxiu rhyolites have slightly different whole−rock εNd(t) (−1.5 to −1.6), but similar zircon εHf(t) (2.2–7.7) and δ18O (6.7–7.2‰). We propose that the Shanglaxiu−Xialaxiu rhyolites were most probably generated by partial melting of the pre−existing juvenile crust with the addition of minor sedimentary material. This partial melting was possibly triggered by the south−dipping continental subduction of the Songpan−Ganzi block. The pre−existing juvenile crust was probably formed during the subduction of Paleo−Tethyan oceanic slab in Paleozoic−Early Mesozoic.

Early Devonian Post‐collisional Granitic Magmatism in the North Qilian Orogenic Belt, Western China: Insights into Lithospheric Delamination and Orogenic Collapse

AbstractPost‐collisional magmatism contains important clues for understanding the reworking and growth of continental crust, as well as lithospheric delamination and orogenic collapse. Early Devonian magmatism has been identified in the North Qilian Orogenic Belt (NQOB). This paper reports an integrated study of petrology, whole‐rock geochemistry, Sm‐Nd isotope and zircon U‐Pb dating, as well as Lu‐Hf isotopic data, for two Early Devonian intrusive plutons. The Yongchang and Chijin granites yield zircon U‐Pb ages of 394–407 Ma and 414 Ma, respectively. Both of them are characterized by weakly peraluminous to metaluminous without typical aluminium‐rich minerals, LREE‐enriched patterns with negative Eu anomalies and a negative correlation between P2O5 and SiO2 contents, consistent with geochemical features of I‐type granitoids. Zircons from the studied granites display negative to weak positive εHf(t) values (–5.7 to 2.1), which agree well with those of negative εNd(t) values (−6.4 to −2.9) for the whole‐rock samples, indicating that they were derived from the partial melting of Mesoproterozoic crust. Furthermore, low Sr/Y ratios (1.13–21.28) and high zircon saturation temperatures (745°C to 839°C, with the majority being &gt;800°C) demonstrated a relatively shallow depth level below the garnet stability field and an additional heat source. Taken together, the Early Devonian granitic magmatism could have been produced by the partial melting of ancient crustal materials heated by mantle‐derived magmas at high‐temperature and low‐pressure conditions during post‐collisional extensional collapse. The data obtained in this study, when viewed in conjunction with previous studies, provides more information about the tectonic processes that followed the closure of the North Qilian Ocean. The tectonic transition from continental collision to post‐collisional delamination could be constrained to ∼430 Ma, which is provided by the sudden decrease of Sr/Y and La/Yb ratios and an increase in zircon εHf(t) values for granitoids. A two‐stage tectonic evolution model from continental collision to post‐collisional extensional collapse for the NQOB includes (a) continental collision and crustal thickening during ca. 455–430 Ma, characterized by granulite‐facies metamorphism and widespread low‐Mg adakitic magmatism; (b) post‐collisional delamination of thickened continental crust and extensional collapse of orogen during ca. 430–390 Ma, provided by coeval high‐Mg adakitic magmatism, A‐type granites and I‐type granitoids with low Sr‐Y ratios.

Petrogenesis and Tectonic Implications of the Oligocene Dalongtan Shoshonitic Syenite Porphyry in Central Yunnan, Southeastern Tibetan Plateau: Constraints from Geochronology, Geochemistry and Sr-Nd-Hf Isotopes

Shoshonitic rocks are widely distributed in post-collisional settings and provide key information on deep geodynamic mechanisms and magmatic evolution. In this paper, we present petrographic, zircon U-Pb age-related, trace elemental, Hf isotopic, bulk-rock elemental, and Sr-Nd isotopic data of the Dalongtan shoshonitic syenite porphyries (DSSPs) in central Yunnan, southeastern Tibet. The DSSPs formed at 33.2 ± 0.3 Ma in a post-collisional setting. They define linear trends on Harker diagrams, and they display similar trace element patterns and enriched bulk-rock Sr-Nd isotopes [(87Sr/86Sr)i = 0.70964–0.70968, εNd(t) = −12.9 to −12.7] and zircon Hf isotopes (εHf(t) = −15.7 to −13.1) to the coeval mantle-derived potassic mafic rocks. This suggests that the DSSPs were fractionated from the lithospheric mantle-derived mafic magmas. The DSSPs, along with the coeval felsic and mafic magmatic rocks (37.2–32.3 Ma), exhibit a planar distribution on the SE Tibet and predate the left-lateral shearing of the Ailaoshan–Red River shear zone (ARSZ) (32–22 Ma), suggesting that there are no genetic relationships between them. The DSSPs have geochemical characteristics similar to those of A-type granites, with high total alkalinity (10.39–11.17 wt.%), HFSE concentrations (Zr + Nb + Ce + Y = 890.2–1054.3 ppm), Ga/Al ratios (10,000 × Ga/Al = 2.95–3.46), whole-rock zircon saturation temperatures (906–947 °C), and oxygen fugacity (ΔFMQ = +3.30–+4.65), indicating that they are products of the high-temperature melting of the lithosphere as a result of asthenosphere upwelling in extensional settings. Based on our data and regional observations, it is proposed that the generation of the DSSPs may be linked to the convective thinning of the thickened lithospheric mantle following the India–Asia collision.

Whole-rock geochemical and zircon U-Pb-Hf isotopic compositions of the Yunlougang and Wucun granitic plutons in the Hetai area of South China: Implications for petrogenesis and geodynamic setting

In the Nanling tectono-magmatic belt of South China, the geodynamic settings and tectonic style of Indosinian and Yanshanian magmatism remain debatable. Hereby, an integrated study of zircon U-Pb ages and Lu-Hf isotopes as well as whole-rock elemental and Sr-Nd isotopic compositions was carried out on the Yunlougang (Late Permian) and the Wucun (Late Jurassic) granitic plutons in the Hetai area, located to southwest of the Nanling region. Zircon U-Pb dating yielded emplacement ages of 255 Ma for the Yunlougang biotite monzogranite, 240 Ma for the Yunlougang granodiorite as dyke crosscutting the former, and 155 Ma for the Wucun two-mica granite. The biotite monzogranite and the two-mica granite are strongly peraluminous S-type granites (A/CNK ratios of 1.06–1.21) with a relatively low zircon saturation temperature (631–741 °C). The high initial 87Sr/86Sr ratios (0.71733 to 0.73383) and low εNd(t) values (−14.6 to −8.69) suggest a crustal protolith. However, the Wucun biotite granite is highly fractionated I-type one (most samples are metaluminous with A/CNK < 1.1) with a relatively high zircon saturation temperature (762–770 °C). Additionally, they exhibit less radiogenic 87Sr/86Sri ratios (0.70724–0.70786) and higher εNd(t) values (−5.34 to −4.94) as well as zircon εHf(t) values of −5.67 to −2.44, suggesting an origin of a crustal source with extra input from mantle. Based on the tectonic history of South China, we infer that the Yunlougang granite occurred in a compressive setting and might be produced by partial melting of a thickened ancient crust during the collision between the South China and the Indochina blocks, while the Wucun complex granites generated in an extensional environment that was influenced by the paleo-Pacific tectonic regime. Combined with previous studies, a NE-oriented I-type granite belt probably existed along the juncture belt of the Yangtze and Cathaysia blocks, which was respond to the foundering rather than rollback of the paleo-Pacific plate that led to the rapid change of the temperature of granite precursor and the involvement of mantle components into the Hetai region.

Petrogenesis and geodynamic implications of the Late Devonian dioritic and granitic intrusive rocks in the Dananhu Belt, Eastern Tianshan Orogenic Belt

The Late Devonian magmatism of the Dananhu arc belt in the Central Asian Orogenic Belt provides a critical geological record. This study provides new comprehensive geochronological, geochemical, and Sr–Nd isotopic data of granodioritic and dioritic intrusions in the Dananhu belt. These findings contribute to unraveling the regional tectonic history and constraining the geodynamic processes involved. Zircon U–Pb dating indicates that the granodiorites and diorites were formed at 382.7 ± 3.8Ma and 363.1 ± 4.3Ma, respectively. The granodiorites show characteristics similar to I-type granites with high SiO2, low MgO and Mg# and aluminium saturation index (<1.1), negligible Eu anomalies, low (K2O + Na2O)/CaO ratios and zircon saturation temperatures (average = 696 °C). Granodiorites also show depleted isotope signatures (εNd(t) = +5.85 to +6.27 and low (87Sr/86Sr)i = 0.704082–0.704583) and juvenile TDM ages (741–793Ma), indicating their origin from a juvenile crust. The diorites are characterized by enrichments in large ion lithophile elements, U and Pb, but depleted in Nb and Ta displaying typical geochemical features of a subduction-related origin, together with high εNd(t) (+6.10 to +6.84) and low initial Sr isotopes (0.703745–0.704601), suggesting that they originated from a subduction fluid modified depleted mantle. The petrogenesis of both granodiorites and diorites in the Dananhu arc provides evidence that they formed through magmatic processes in a subduction tectonic setting. Considering the adakites associated with slab melting from previous studies in the Dananhu arc, it is plausible that the north-dipping subduction of the North Tianshan oceanic lithosphere have contributed to the Dananhu arc magmatism during the Late Devonian.