Zircon Hf Isotopic Compositions Research Articles

U-Pb ages and Hf isotope composition of zircon from the Shimanto accretionary complex: Evidence for heterogeneous sources

U-Pb ages and Hf isotope composition of zircon from the Shimanto accretionary complex: Evidence for heterogeneous sources

Petrogenesis and geodynamic implications of the late Triassic bojites in Yajiangqiao area, Hunan Province, South China

AbstractThe mafic igneous activities in the late Triassic play an important role in the tectonic evolution of the South China Block (SCB). In this paper, relevant data of the bojites in Yajiangqiao area, eastern Hunan Province, South China were provided, including zircon U–Pb ages, whole‐rock geochemical data, Sr–Nd–Pb isotopic data, and zircon Hf isotope data. These data indicate that the bojites were crystallized at ~ 215.7 Ma, and thus are coeval with biotite monzonitic granites of Yajiangqiao pluton. The bojite samples feature a low SiO2 content of 48.84–49.94 wt%, a low ALK (K2O + Na2O) content of 5.13–6.13 wt%, a relatively high Al2O3 content of 17.64–21.22 wt%, a moderate MgO content of 4.32–7.07 wt%, and a moderate FeOT content of 5.66–7.33 wt%. In addition, the samples have a total rare earth element (REE) content of 108–163 ppm. They display right‐declined REE patterns, enriched in light rare earth elements and large‐ion lithophile elements (such as Rb, K, Th, and U) and depleted in high field strength elements (such as Nb, Ta, P, and Ti). All the samples have homogeneous Sr–Nd–Pb and zircon Hf isotopic compositions, with the (87Sr/86Sr)i ratio varying from 0.708 202 to 0.709 515, (143Nd/144Nd)i ratio from 0.512 156 to 0.512 229, (206Pb/204Pb)t ratio from 18.185 to 18.264, (207Pb/204Pb)t ratio from 15.667 to 15.672, (208Pb/204Pb)t ratio from 38.510 to 38.587, and initial 176Hf/177Hf ratio ranging from 0.282 428 to 0.282 530. Meanwhile, the calculated εNd(t) and εHf(t) values range from −4.0 to −2.6 and from −7.9 to −4.1, respectively, with two‐stage Nd model ages of 1.20–1.32 Ga and two‐stage Hf model ages of 1.35–1.56 Ga, respectively. As indicated by geochemical data, the primitive magmas of the bojites in Yajiangqiao area were derived from lithospheric mantle in post‐collisional extensional environment, which underwent low‐degree partial melting (1–5 %) of spinel‐garnet lherzolites (spinel > garnet) and was subjected to metasomatism of slab‐derived fluids. Furthermore, it can be concluded from these data and previous data that the SCB was transformed from syn‐collisional compression to post‐collisional extension in the late Triassic (~ 234 Ma) and the post‐collisional extension lasted until 215 Ma.

Sedimentary response to the non-synchronous closure of Paleo-Asian Ocean: evidence from U-Pb ages and Hf isotopic compositions of detrital zircons from the Linxi Formation, NE China

ABSTRACT The Linxi Formation is widely deposited in eastern Inner Mongolia (NE China), especially along northern parts of the Solonker-Xar Moron Suture (SXMS) in the eastern Central Asian Orogenic Belt, yet its depositional age and provenance remain ambiguous. We studied the petrography, geochronology, and Hf isotopes of two sandstone samples from this formation, and estimated the depositional age and the provenance of the Linxi Formation, and further constrained the evolution of the Paleo-Asian Ocean (PAO). Based on detailed field observations and measurements, we. Zircons from rocks of the Linxi Formation are mainly composed of detrital zircons of magmatic origin. The determined ages of detrital zircons range from 233 ± 2 Ma to 1365 ± 14 Ma, and indicate that the provenance of the Linxi Formation experienced at least four tectonic-thermal events between 1365 and 233 Ma: Neoproterozoic (ca. 1365–543 Ma), Early Palaeozoic (ca. 512–426 Ma), Late Palaeozoic (ca. 418–251 Ma), and Early Mesozoic (ca. 249–233 Ma). The youngest detrital zircon, with a U-Pb age of 233 ± 2 Ma, defines the maximum depositional age of the Linxi Formation. The εHf (t) values of dated zircons from the sandstone samples have a large range (−7.8–15.0). The four dominant age groups and the range in εHf (t) values are widely found in the Xing’an Block and Songnen-Zhangguangcai Range, which imply that the source of the Linxi Formation had an affinity with northeastern China. These data together with previous geochronological data (from west to east along the SXMS) on the crystallization and depositional ages of detrital zircon suggest that the Linxi Formation was deposited in different tectonic settings: convergent, collisional, and extensional. This study provides further evidence and constraints for the non-synchronous closure of the PAO.

Ore genesis of Axi post-collisional epithermal gold deposit, western Tianshan, NW China: Constraints from U–Pb dating, Hf isotopes, and pyrite in situ sulfur isotopes

The Axi gold deposit has long been regarded as a typical Paleozoic low sulfidation epithermal deposit, located in the western Tianshan, in the Central Asian Orogenic Belt (CAOB). SIMS zircon U–Pb dating of the ore-bearing andesite and dacite yielded concordia ages of 350.8 ± 2.7 Ma and 351.3 ± 3.1 Ma, respectively. In situ SIMS hydrothermal rutile U-Pb dating yielded two 207Pb corrected ages as 306.1 ± 16.9 Ma and 303.8 ± 14.6 Ma, respectively. These ages are considerably younger than the zircon SIMS U-Pb ages of ore-bearing volcanic rocks, which precludes a genetic link between the epithermal gold mineralization and the subduction-related Late Devonian-Early Carboniferous magmatism in the western Tianshan. Instead, the gold mineralization is interpreted to be genetically related to a buried ~300 Ma pluton that formed in a post-collisional environment. Combined with previously published zircon Hf isotopic compositions of ~300 Ma felsic intrusive rocks from western Tianshan, the results show that ~300 Ma felsic rocks have higher zircon εHf(t) values than those from ~351 Ma felsic rocks in the Axi deposit. These elevated εHf(t) values suggest that mantle material inputs of ~300 Ma Axi epithermal deposit in a post-collisional environment. Pyrite, the most common sulfide in the Axi ores, displays heterogeneous textures and has a large variation of As contents and sulfur isotopes, suggesting complex ore-forming processes in the Axi deposit. This study highlights the importance of in situ isotopes of hydrothermal minerals to decode the ore-forming histories of epithermal systems.

Geochemical discrimination between fertile and barren Eocene potassic porphyries in the Jinshajiang Cu–Au–Mo metallogenic belt, SW China: Implications for petrogenesis and metallogeny

The Jinshajiang potassic porphyry metallogenic belt represents one of the most important Chinese intrusive belts that hosts Eocene Cu–Au–Mo porphyry systems including the Narigongma Mo, Yulong Cu, and Beiya Au deposits. This metallogenic belt also contains several uneconomic potassic intrusions, such as the Jianchuan, Shigu and Binchuan granite porphyries. Bulk-rock geochemical compositions together with Sr–Nd–Pb isotope ratios, and zircon Hf isotope and trace element compositions are compiled and interpreted to reveal the fundamental differences leading to the different fertility of these potassic porphyries at Jinshajiang. The economic Narigongma, Yulong, Beiya, and barren Jianchuan, Binchuan, Shigu granite porphyries are all formed in the same post-collisional tectonic setting, and all exhibit high SiO2 (61–65 wt%), and total alkali contents (K2O + Na2O > 8 wt%). Based on low Ce contents (<10), FeOT/MgO ratios (<100) ratios and A/CNK values (<1.1), these granite porphyries are classified as potassic I-type granites. High (87Sr/86Sr)i ratios (0.70669 to 0.70786), low bulk-rock εNd(t) values (−8.7 to −0.7), MgO (0.2–1.4 wt%), Cr (0.1–36.4 ppm) and Ni (0.4–26.8 ppm), and radiogenic Pb isotopic compositions suggest that the granite porphyries, both fertile and barren, were most likely generated by partial melting of thickened lower crust with the addition of mantle-derived magmas. However, the fertile Narigongma, Yulong and Beiya granite porphyries have low Y (7.8–16.0 ppm) and Yb (0.6–1.5 ppm) contents, high (La/Yb)N ratios (31.3–98.9) and overall steeper REE profiles, reflecting residual garnet during partial melting of thickened lower crust, whereas the barren Jianchuan, Shigu and Binchuan granite porphyries have high Y (12.3–19.4 ppm) and Yb (1.1–2.5 ppm) contents, low (La/Yb)N ratios (25.2–44.3) and flat HREE patterns, indicating a garnet-poor magma source. Garnet that remains as a residual mineral during partial melting of thickened lower crust can drive magma oxidation, as shown by higher zircon Ce4+/Ce3+ ratios (averaging 75.6) and high fO2 values (averaging ΔFMQ = +3.9) of the fertile Yulong and Beiya granite porphyries than those of the barren Shigu and Jianchuan granite porphyries with average Ce4+/Ce3+ = 18.2 and ΔFMQ = +0.6. Therefore, magma source played an important role in determining the fO2 of the granite porphyry, thus controlling its fertility for Cu–Au–Mo mineralization.

Mesozoic crustal growth in Mainland Southeast Asia: Zircon U-Pb and Hf isotopic evidence from the Late Cretaceous Luyingtang granitic pluton in the northernmost SE Asian granite Province, SW China

The Changning-Menglian suture in the Sanjiang region of SW China, the Chiang Rai line in Thailand, and the Bentong-Raub suture in Peninsular Malaysia are marked by remnants of the Paleo-Tethys oceanic lithosphere in Mainland Southeast Asia. We present U-Pb ages and Hf isotopic compositions of zircon from Luyingtang granites east of the Changning-Menglian suture, with the aim of identifying their sources and petrogenesis, which are significant for understanding the crustal evolution of the Mainland Southeast Asia. The Luyingtang granitic pluton is located in the northernmost SE Asian granite Province, SW China. New zircon U-Pb ages indicate that the granites were emplaced in the Late Cretaceous, in accordance with the regional scale inland magmatic event in Mainland Southeast Asia. Petrographically, the samples range from granodiorite, biotite granite, two-mica granite to pegmatite. The biotite granite, two-mica granite and pegmatite have low contents of TiO2, Al2O3, FeO*, MgO, CaO and P2O5, but high values of SiO2 and K2O, compared to the granodiorite. The granodiorite, biotite granite and two-mica granite invariably show enrichment in light rare earth elements (LREE). The biotite granite and two-mica granite show negative Eu anomalies, but the granodiorite displays weak Eu anomaly. The pegmatites show a clear tetrad effect and contain pronounced negative Eu anomalies. All the rocks demonstrate characteristic negative Nb, Ta, Sr and Ti anomalies, and a positive Pb anomaly, when plotted on spidergrams. Most of the granites are characterized by positive εHf (t) values (0.3–8) and fairly young TDM2 ages (612–1094 Ma), suggesting they contain a significant proportion of juvenile component. Hf isotopic modelling shows that the Luyingtang granites are likely to have been produced by partial melting of mixed sources, which were probably of basalts in proportions of 50–70% and Precambrian continental crust. The emplacement of granites with such a large proportion of mantle-derived component indicates crustal growth in Mainland Southeast Asia during Mesozoic. In contrast, one of the two-mica granite samples displays negative εHf (t) values (−7.6 to −3) and older TDM2 ages (1305–1599 Ma), which suggest a more crustal affinity. The arc-continent collision during the Cretaceous may have induced foundering of an over-thickened lithospheric mantle keel beneath the Paleo-Tethys orogenic belt, and then resulted to localized upwelling of asthenosphere mantle and basaltic underplating. The parental magmas of the hinterland I-type granites formed by melting of a mixed source, containing both Precambrian crust and mantle-derived basalt, followed by extensive magmatic differentiation.

Petrogenesis of granitoids from the Lachlan Fold Belt, southeastern Australia: The role of disequilibrium melting

S- and I-type granites from the Lachlan Fold Belt, southeastern Australia, have been investigated to assess the role of disequilibrium melting in their petrogenesis. Differences between the median initial εHf compositions of magmatic zircon populations and the host bulk-rock (ΔεHfblk-zrc) range from −0.6 to +2.5 ε units, providing evidence for intra-sample (and hence inter-phase) Hf-isotopic heterogeneity. Linear variations on Harker diagrams and O and Hf isotope compositions of magmatic zircon preserved in many I- and S-type granites are inconsistent with assimilation or simple mixing hypotheses. In contrast, isotopic disequilibrium between the melt and a restite assemblage can explain the bulk-rock versus zircon differences observed in these samples.Assuming that magmatic zircon records the melt composition, differences between the bulk-rock εHf and εHf of magmatic zircon (ΔεHfblk-zrc values) measured for I-type granites (0.4–2.5) can largely be explained by disequilibrium amphibole dehydration melting of meta-igneous protoliths that were either isotopically heterogenous at the time they were formed, or perfectly homogeneous before being aged in the crust for 0.4–1.0 billion years prior to partial melting. The Currowong Suite exhibits petrographic features and preserves geochemical and isotopic compositions that do not lend themselves to simple restite model or magma mixing explanations; however, these observations could be explained by the restite unmixing of magma batches generated from a single source rock if, as modelling has suggested, separate batches contain different melt compositions.By investigating the application of disequilibrium melting to granite genesis, this study demonstrates that isotopic heterogeneity at various sampling scales should actually be expected for the production of granites from a single source, rather than necessitating the involvement of multiple sources and mixing processes. As a result great care should be taken in the interpretation of isotope data from granitic bulk-rocks or their zircons.

Petrogenesis of Late Triassic high-Mg diorites and associated granitoids with implications for Paleo-Tethys evolution in the northeast Tibetan Plateau

Abstract Recent research on Paleo-Tethys tectonics has identified a huge late Paleozoic to Mesozoic igneous belt that extends more than 2500 km in the northeast Tibetan Plateau. However, the magma genesis and evolution in this belt remains a subject of considerable debate. This paper presents a combination of zircon U-Pb ages, mineral compositions, major and trace element concentrations, and Sr-Nd-Hf isotopic data for the plutons across the Zhiduo arc belt that marks the site connecting different tectonic-magmatic units. The studied rocks from one quartz diorite, two granodiorite plutons, and their mafic enclaves define a continuous compositional evolution varying from high- to medium-K calc-alkaline gabbroic diorite to granodiorite. Laser ablation–inductively coupled plasma–mass spectroscopy U-Pb analyses of zircons from these three plutonic suites and one mafic enclave yield Late Triassic ages of 222–217 Ma, establishing that the mafic and felsic magmas were nearly coeval. All these rocks are featured by zoned hornblende and plagioclase with Mg- and Ca-rich mantles or oscillatory change in compositions. They exhibit high and variable MgO (up to 4.88–5.66 wt%), Cr, and Ni contents except that one granitoid pluton (Dangjiangrong) possesses high Co (up to 145.0 ppm). They are characterized by subduction-type trace element patterns, with prominent positive Rb, Th, Pb, and K anomalies and negative Ba, Nb, P, and Ti. Together with continuous and heterogeneous Sr-, Nd-, and zircon Hf-isotopic compositions, it suggests that these Late Triassic high-Mg diorites and associated granitoids were generated through magma mixing and fractional crystallization accompanied by chemical exchange. Taking into account the magmatic record from nearby regions, we suggest that double-sided subduction and rollback of the subducting Paleo-Tethys oceanic slab is the main mechanism to generate geochemically-varied magmatism in the northeast Tibetan Plateau, and eventually close the Paleo-Tethys Ocean during much of the Late Triassic.

Interaction between oceanic slab and metasomatized mantle wedge: Constraints from sodic lavas from the Qilian Orogen, NW China

Generation of arc volcanic rocks is a complicated process, potentially involving some or all of oceanic slab subduction, mantle wedge metasomatism, and melting of both the subducted slab and the mantle wedge. Interaction between melts of different sources plays important roles in the subduction factory and the formation of magmatic arcs and is still controversial. To elucidate the mantle-melts interaction process, we present a study of the sodic adakite-like lavas (445-444 Ma) and basalt-basaltic andesite (basaltic) enclaves (∼453 Ma) in the early Paleozoic intra-oceanic arc system from the South Qilian Accretionary Belt, Qilian Orogen. Basaltic enclaves in the Muli adakite-like lavas have high Mg# (62–69) high Cr and Ni, and radiogenic Sr and unradiogenic Nd isotopic compositions (ISr = 0.706327–0.706497; εNd(t) = −3.2∼ − 2.0). They are considered to represent primitive magmas derived from a highly metasomatized mantle wedge. The adakite-like lavas are dacitic with adakite-like compositions, enriched SrNd isotopic compositions (ISr = 0.705597–0.706747; εNd(t) = −1.7∼ + 0.4) and significantly variable zircon Hf isotopic compositions (εHf(t) = +0.9∼ + 17.8), most likely a hybrid magma produced by mixing primitive basaltic melts of metasomatized mantle wedge and melts of subducted oceanic crust.The Muli adakite-like lavas and basaltic enclaves occurred in an intra-oceanic island arc setting with an unusually hot thermal structure, which induced melting of subducted slab (including sediments and mafic oceanic crust) and metasomatized mantle wedge. The geodynamic mechanism associated with slab melting is plausibly either or both of the juvenile nature of South Qilian Oceanic Crust, and/or the heating of a slab edge by upwelling hot asthenospheric mantle during subduction initiation.

Turbidite record of a middle Neoproterozoic active continental margin in the West Cathaysia terrane, South China: Implications for the relationships between the Yangtze and Cathaysia blocks and their positions in Rodinia

South China is the largest block of Rodinia in southeastern Asia. It consists of the Yangtze and Cathaysia blocks, separated by the Jiangshan-Shaoxing-Pingxiang Fault. Most studies suggest that the two blocks were amalgamated in the early Neoproterozoic, and the middle Neoproterozoic vocano-sedimentary sequences on both blocks were deposited in a post-collisional rift basin (the Nanhua rift basin). Here, we conducted a geochemical and geochronological study of a turbidite sequence at the western margin of the Cathaysia Block that was previously considered to have been deposited in the Nanhua rift basin. The sedimentary rocks are characterized by bathypelagic to bathyal facies. They are composed of siliciclastic sedimentary rocks interlayered with tuffaceous materials and have geochemical compositions between felsic to intermediate arc igneous rocks. Abundant euhedral detrital zircons define a major age peak at ca. 790–770 Ma, indicating that the detrital grains were derived mostly from locally distributed syn-sedimentary middle Neoproterozoic arc igneous rocks. Predominance of zircons with ages close to the time of deposition and the whole-rock trace element compositions of the turbidite sequence also imply a convergent plate margin setting for the turbidite. The Hf isotopic compositions of the Neoproterozoic detrital zircons suggest that their provenance consisted of largely reworked crustal materials, inconsistent with a potential interpretation that similarly aged igneous rocks in the Jiangnan Orogen were the provenance. These lines of evidence, combined with other available data, suggest that the western margin of the Cathaysia Block (the West Cathaysia terrane) bears more similarities to the western and northern margins of the Yangtze Block and northern India and formed part of a Neoproterozoic subduction-accretion system located along the margin of Rodinia. We suggest that early Paleozoic strike-slip movement along the Jiangshan-Shaoxing-Pingxiang Fault played a significant role in shaping the current tectonic framework of South China by bringing the West Cathaysia terrane to its present position with respect to the Yangtze Block.

Geochemistry and detrital zircon records of the Ruyang-Luoyu groups, southern North China Craton: Provenance, crustal evolution and Paleo–Mesoproterozoic tectonic implications

Paleo- to Mesoproterozoic sedimentary rocks in the southern margin of the North China Craton (NCC) are represented by the Ruyang and Luoyu groups. We studied the sedimentary rocks from the Yunmengshan and Beidajian formations of the Ruyang Group and the Cuizhuang and Sanjiaotang formations of the Luoyu Group. Detrital zircon grains from these formations have U–Pb age populations of 3.64–3.31 ​Ga, 2.96–2.86 ​Ga, 2.72–2.59 ​Ga, 2.56–2.47 ​Ga, 2.45–2.0 ​Ga, 1.99–1.85 ​Ga and 1.84–1.65 ​Ga. The geochemical features of the sedimentary rocks suggest that some of the sediments were sourced from intermediate to felsic magmatic rocks. The age groups of the detrital zircon are roughly consistent with the tectono-thermal events in the southern margin of the NCC. The Hf isotopic compositions of detrital zircon from the sedimentary rocks in Ruyang and Luoyu groups suggest that significant crustal growth and reworking of the NCC took place during the Neoarchean and early- to mid-Paleoproterozoic, while crustal reworking at the Paleoarchean and late-Paleoproterozoic, and crustal growth at the Mesoarchean. We suggest the depositional times of the Ruyang Group and Luoyu Group are constrained to no older than 1.75–1.7 ​Ga and 1.7–1.65 ​Ga, respectively. Formation of late-Paleoproterozoic basins related to the strike slip and extrusion tectonics that cross-cut the NCC during the late Paleoproterozoic (<1.75 ​Ga), and the late Paleoproterozoic sedimentation once isochronous developed in the southern margin of the NCC through the Taihang region of the interior NCC and linked the Yanshan–Liaoxi regions of the northern NCC.

Neoproterozoic peraluminous granites in the western margin of the Yangtze Block, South China: Implications for the reworking of mature continental crust

Significant widespread melting of the mantle and juvenile mafic lower crust is known to have occurred along the western margin of the Yangtze Block during the Neoproterozoic, but melting of the mature continental crust remains poorly understood. Peraluminous granites can provide vital insights on the reworking of mature continental crustal materials. We present zircon U-Pb-Hf isotopic, whole-rock geochemical, and Sr-Nd isotopic data from the Neoproterozoic peraluminous granites in the western Yangtze Block, South China. Zircon U-Pb dating displays concordant crystallization ages of ca. 840 Ma for the Kuanyu granites and ca. 835 Ma for the Cida granites. These peraluminous granites have high SiO2 (66.9–75.6 wt%), K2O (4.61–7.29 wt%) contents, as well as high K2O/Na2O (1.44–3.25) and A/CNK (molar ratio of Al2O3/(CaO + Na2O + K2O)) (1.04–1.18) ratios. The samples are enriched in Rb, K, Th, U, and Pb, and depleted in Nb, Ta, Ba, Sr, and Ti, indicating a middle-upper crustal affinity. They are also characterized by high (87Sr/86Sr)i (0.7099–0.7217) and negative εNd(t) values (−5.1 to −2.9), which resemble the isotopic features of an evolved continental crust source. Furthermore, these peraluminous granites possess variable CaO/Na2O (0.09–0.65) and Al2O3/TiO2 (25.3–88.4) ratios, moderate Rb/Ba (1.68–3.86) and Rb/Sr (0.32–0.85) ratios, as well as high molar Al2O3/(MgO + FeOT) (2.04–5.23) and low molar CaO/(MgO + FeOT) (0.15–0.48), implying that they predominantly originate from heterogeneous metasedimentary sources (metapelites + metagreywackes). Scarce evidence of hybridization processes between crust- and mantle-derived components indicates that their heterogeneous zircon Hf isotopic compositions (εHf(t) = −7.75 to +3.31) may be caused by disequilibrium partial melting of heterogeneous metasedimentary sources, similar to peraluminous granites from the Altai area and Jiangnan orogen in China. In combination with previously reported results, we suggest that the Kuanyu and Cida peraluminous granites represent melting of mature crustal material in an evolved middle-upper crust source during the early stages of the Neoproterozoic subduction process. The western margin of the Yangtze Block underwent not only melting of the juvenile mafic lower crust but also reworking of the mature continental crust during the Neoproterozoic.

Petrogenesis and tectonic implications of 2.45 Ga potassic A-type granite in the Daqingshan area, Yinshan Block, North China Craton

An Archean–Paleoproterozoic (A–P) A-type granite 15 km wide and 25 km long has been identified in the Daqingshan area south of the Yinshan Block (YB), North China Craton (NCC). Known as the Jinpen pluton, this body is the largest individual granite pluton in the Daqingshan area. Its crystallization ages of 2460–2450 Ma has been determined precisely using the secondary ion mass spectrometer (SIMS) zircon U–Pb method. The Jinpen pluton is lithologically homogeneous, consisting mainly of gneissic monzogranites with metaluminous affinity and high K2O/Na2O (1.08–2.06), Ga/Al, and Zr + Nb + Ce + Y values with high zircon saturation temperatures (Tzr) of 889–837 °C; these features are typical of A-type granite. Most samples contain enriched large ion lithophile elements (LILEs) and light rare earth elements (LREEs), high Y and Yb abundances with pronounced Eu negative anomalies, low Sr/Y and (La/Yb)N ratios, and low MgO, Cr, and Ni contents, implying a shallow crustal source for the pluton. Its whole-rock Nd isotope composition (εNd(t) = 2.5–0.6) and zircon Hf isotope composition (εHf(t) = 5.2–1.7, TDMC = 2.88–2.65 Ga) indicate that it mainly originated from pre-existing crustal materials. The zircon δ18O values of 6.58–5.71‰ suggest a crustal source of orthogneiss with little metasedimentary material. Previous studies have demonstrated that the YB experienced significant crustal growth at 2.8–2.7 Ga and further crustal reworking at about 2.5 Ga. This study reveals that the Jinpen potassic A-type granite was generated at 2.46–2.45 Ga by partial melting of pre-existing crustal materials at relatively low pressure (P) and high temperature (T) conditions symbolizing the first and essential cratonization of the lithospheric stabilization of the NCC.

Lu-Hf isotopic evidence of a deep mantle plume source for the ∼2.06 Ga Bushveld Large Igneous Province

The Bushveld Large Igneous Province (B-LIP) comprises a diverse array of >30 magma bodies that intruded the Kaapvaal Craton at ∼2.06 Ga. In this paper we use zircon and bulk-rock Lu-Hf isotope data to determine whether the B-LIP formed in response to the arrival of a plume(s) from the deep mantle or by melting of the depleted upper mantle during foundering of an eclogitized residue at the base of the lithosphere. New zircon Hf isotope compositions for four B-LIP bodies yield intrusion-specific average εHf(2.06Ga) values that range from −20.7 ± 2.8 to −2.7 ± 2.8, largely consistent with literature zircon data for other B-LIP intrusions. Bulk-rock solution εHf(2.06Ga) values for a variety of B-LIP intrusions range from −2.1 ± 0.2 to −10.6 ± 0.2. Because the most radiogenic Hf isotope compositions across the entire B-LIP are nearly primordial, having an εHf(2.06Ga) close to 0, it is likely that the heat source of the B-LIP was a plume(s) from deep mantle. The Hf isotope data further suggests that individual intrusions in the B-LIP were produced by melting of three distinct source reservoirs (in addition to melts derived from the plume itself): 1) Subduction and plume modified continental lithospheric mantle; 2) Older (∼2.7 Ga) mafic-ultramafic plume-related material trapped in the Kaapvaal lithosphere; and 3) Mid- to upper Kaapvaal crust. The presence of ∼2.7 Ga mafic-ultramafic material in the Kaapvaal lithosphere may have acted to strengthen the lithosphere so that it was able to resist being dispersed by the arrival of the B-LIP plume at ∼2.06 Ga.

Early Carboniferous High Ba‐Sr Granitoid in Southern Langshan of Northeastern Alxa: Implications for Accretionary Tectonics along the Southern Central Asian Orogenic Belt

Voluminous granitoids are widely distributed in the Langshan region, northeast of the Alxa block, and record the evolutionary processes of the southern Central Asian Orogenic Belt. The Dabashan pluton was emplaced into the Paleoproterozoic Diebusige complex. Early Carboniferous zircon LA‐ICP MS U‐Pb ages were from 327 Ma to 346 Ma. The Dabashan pluton can be classified as monzogranite and syenogranite, and exhibits high K2O contents and K2O/Na2O ratios, which reveal a high‐K calc‐alkaline nature. The samples display strongly fractionated REE patterns, and are enriched in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The Dabashan plutons display unusually high Ba (823–2817 ppm) and Sr (166–520 ppm) contents and K/Rb ratios (315–627), but low Rb/Ba ratios (0.02–0.14), and exhibit fertile zircon Hf isotopic compositions [εHf(t)=–14 to –20], which are comparable to those of typical high Ba–Sr granitoids. Based on the geochemical compositions of the samples, we suggest that subducted sediments and ancient crustal materials both played important roles in their generation. Basaltic melts were derived from partial melting of subcontinental lithophile mantle metasomatized by subducted sediment‐related melts with residual garnet in the source, which caused partial melting of ancient lower crust. Magmas derived from underplating ascended and emplaced in the middle–upper crust at different depths. The resultant magmas experienced some degree of fractional crystallization during their ascent. Given these geochemical characteristics, together with regional tectonic, magmatic, and structure analysis data, an active continental margin environment is proposed for the generation of these rocks.

Periodic Paleoproterozoic calc-alkaline magmatism at the south eastern margin of the Yilgarn Craton; implications for Nuna configuration

The age and composition of magmas provide fundamental information to chart the tectonic setting of crustal development through time and hence refine paleogeographic reconstructions. The Biranup Zone of the Albany-Fraser Orogen in southwestern Australia preserves a protracted record of magmatism associated with the formation and subsequent break-up of the Paleoproterozoic supercontinent Nuna. Yet, the configuration of Proterozoic Australia within Nuna is not well constrained. New U-Pb zircon geochronology on four samples of mafic intrusions in the north eastern Biranup Zone yield U-Pb crystallisation ages of 1809 ± 17 Ma, 1798 ± 12 Ma, 1796 ± 12 Ma, and 1755 ± 12 Ma coeval with known pulses of felsic magmatism elsewhere in the orogen. The Lu-Hf isotope composition of zircon crystals from these mafic intrusions, and the metasedimentary rocks they intrude, reveal juvenile magmatic input into the Archean Yilgarn Craton with this magmatism commencing as early as c. 1.90 Ga. Following this juvenile magmatism, the margin of the craton was affected by at least three pulses of calcic to calc-alkaline magmatism between 1.81 and 1.65 Ga. Secular changes in zircon Hf isotope composition are comparable in both duration and periodicity to 50–100 Ma changes in magma composition and zircon chemistry observed in modern volcanic arcs of the American Cordillera and in Cambrian to Carboniferous accretionary complexes of eastern Australia. Isotopic patterns and whole-rock chemistry are apparently consistent with the 1.81–1.70 Ga Paleoproterozoic igneous rocks of the Albany-Fraser Orogen forming in a magmatic arc above a Pacific-type subduction zone which extended along the south eastern margin of the Yilgarn Craton. This interpretation is consistent with paleomagnetic data which places the Yilgarn Craton on the periphery of the supercontinent Nuna at 1.90–1.60 Ga.

Late Jurassic I-type rhyolites from the Duobuza region, north-central Tibet: evidence for the occurrence of juvenile lower crust and crustal growth in the Southern Qiangtang Terrane

ABSTRACT Previous studies of the Southern Qiangtang (SQ) Terrane in Tibet have shown significant growth of continental crust during the late Mesozoic (ca. 170–120 Ma) through underplating of mantle-derived melts beneath ancient lower crust. However, the mechanism of crustal growth remains defective during the Late Jurassic. In this contribution, we report new zircon U–Pb ages, whole-rock major and trace element concentrations, and zircon Hf isotopic compositions of Late Jurassic rhyolites from the Duobuza area in the SQ Terrane. Zircon LA–ICP–MS U–Pb dating yields concordant ages and weighted mean 206Pb/238U ages of 151–149 Ma, indicating that the rhyolites were erupted during the Late Jurassic. The rhyolite samples yield high SiO2 (69.1–73.4 wt.%) and Na2O (3.82–6.49 wt.%) concentrations, high values of the differentiation index (95.4–98.5), moderate Al2O3 (13.1–15.5 wt.%) and K2O (2.55–3.26 wt.%) concentrations, and relatively low TiO2 (0.34–0.44 wt.%), MgO (0.14–0.67 wt.%), and MnO (0.01–0.10 wt.%) concentrations and Mg# values (8.9–33.5). They are enriched in Rb, Th, Ba, and Pb, depleted in Nb, Ta, Sr, Ti, and P, and yield negative Eu anomalies. These characteristics suggest that the Duobuza rhyolites represent medium- to high-K calc-alkaline, highly fractionated I-type granitoids. Variations in major and trace element concentrations indicate that the rhyolites underwent significant fractionation of Fe–Ti oxides, plagioclase, K-feldspar, hornblende, biotite, and apatite. The samples yield positive zircon εHf(t) values (+0.08 to +10.9) and young TCD M model ages (816–386 Ma), indicating that they were generated through partial melting of juvenile lower crust of the SQ Terrane. These data are distinct from those of Late Jurassic intermediate–felsic plutons in the SQ Terrane that generally yield negative εHf(t) values and ancient TC DM model ages. We suggest that the occurrence of the Duobuza rhyolites is the significant witness. The presence of juvenile lower crust beneath the SQ Terrane during the Late Jurassic (ca. 150 Ma) rather than during the Early Cretaceous (ca. 120Ma). The transition from ancient to juvenile lower crust has already began in the SQ Terrane during the Late Jurassic.

Bulk rock and zircon geochemistry of granitoids from the Chotanagpur Granite Gneissic Complex (CGGC): implications for the late Paleoproterozoic continental arc magmatism in the East Indian Shield

The north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) exposes a vast expanse of felsic orthogneisses (~ 1450 Ma magmatic crystallization age) containing a variety of metamorphosed supracrustal rocks and augen gneiss. However, origin and timing of emplacement of the protolith of the augen gneiss are not well constrained. Owing to lack of detailed study and paucity of exposures, the antiquity of the basements of the CGGC also remains poorly understood. To address this issue, we present bulk rock geochemical data and the U–Pb and Lu–Hf isotope composition of zircon from augen gneisses from two adjoining areas within the CGGC. Our obtained data suggest that the augen gneiss have been derived from hybrid source involving older crustal rocks and metasomatised mantle. The bulk rock and zircon geochemistry also suggest that the protolith of the augen gneiss was emplaced in an early Paleoproterozoic basement in a continental arc setting. U–Pb ages of the magmatic zircon grains constrain the age of intrusion at ca. 1710 Ma and ca. 1635 Ma. The timing of this late Paleoproterozoic felsic magmatism overlaps with the timing of the regional high-to-ultrahigh-temperature granulite facies metamorphism in the CGGC.

Geochronology, geochemistry and Sr–Nd–Hf isotopic compositions of Late Cretaceous–Eocene granites in southern Myanmar: Petrogenetic, tectonic and metallogenic implications

The geochronology, petrogenesis and tectonic setting of granites associated with tin–tungsten mineralization in southern Myanmar remain unclear. This work presents whole-rock geochemical, Sr–Nd isotopic, and zircon U–Pb and Hf isotopic data for felsic intrusions in the Sibusima terrane. Zircon U–Pb dating indicates formation in the Late Cretaceous–Early Eocene (~84–48 Ma) and suggests that these intrusions likely represent a southward extension of the coeval magmatic belt in the Tengchong terrane. Primarily classified as high-K calc-alkaline, the granites show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, P, and Ti, and negative Ba and Sr anomalies on primitive mantle-normalized diagrams. P2O5, CaO, Al2O3, MgO, TiO2 contents, and Nb/Ta ratios decrease with increasing SiO2 contents, possibly consistent with evolution trends in fractionated I-type granitic magmas. In addition, these intrusions display a wide range of negative εNd(t) (−14.6–−5.5) and zircon εHf(t) values (–22.7–5.7), suggesting a dominant Sibusimacrustal source with a minor mantle contribution. Notably, the Eocene and a few Paleocene granites show more contribution of mantle material based on higher εNd(t) (−8.5–−5.5) and zircon εHf(t) (−11.0–−3.2) values. Large variations in zircon Hf isotopic compositions within an individual Eocene granitic intrusion (e.g., −15.2–5.7) and corresponding biotite-rich enclave (−17.8–0.2) likely indicate magma mixing. Considering the spatial-temporal distribution of magmatism in the West Burma and Sibusima terranes, the studied Late Cretaceous–Eocene felsic magmas likely formed in a continental arc setting during normal subduction (~100–60 Ma) and subsequent roll-back of the Neo-Tethyan oceanic lithosphere (~60–50 Ma). Additionally, most of granites have the elevated Sn with increasing Rb/Sr ratios, and decreasing TiO2 contents and Nb/Ta ratios, suggesting Sn enrichment is primarily controlled by magmatic fractionation. Meanwhile, Sn and Li depletions in some granites deviate from the magmatic evolution trend, likely as a result of fluid exsolution. The exsolved Sn-rich fluids could have made a genetic contribution to hydrothermal Sn mineralization in southern Myanmar.

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 (&lt;360 ppm) and high Y (up to 57.6 ppm) concentrations with low Sr/Y ratios (&lt;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.