Zircon Hf Isotopic Compositions Research Articles

Petrogenesis and tectonic implication of the alkaline ferroan granites from Ropp complex, north‐central Nigeria: Clues from zircon chemistry, U–Pb dating, and Lu–Hf isotope

Zircon holds crucial information for deciphering the origin of granitic rocks, enabling robust scrutiny of their magmatic evolution. Herein, we report an integrated whole‐rock and zircon U–Pb and Lu–Hf isotope chemistry of Ropp alkaline granites in north‐central Nigeria. The rock units include riebeckite‐biotite granite porphyries (161.1 ± 1.7 Ma), aegirine‐biotite granites (162.4 ± 1.6 Ma), biotite granites (162.3 ± 5.1 Ma), and albite riebeckite granites. They show diagnostic characteristics of ferroan alkaline granites such as high FeOt/(FeOt + MgO), low Y/Nb (<1.2), and high Nb/Y (>1.0). Their zircon Hf isotopic compositions (εHf(t) = −8.08 to −4.38; 176Hf/177Hf = 0.282492–0.282543), major and trace element compositions such as high SiO2 >70% and incompatible trace elements (e.g., Rb, Ba, and La), low ratios of whole rock Th/U, Hf/Ta. Ni/Co and Th/Ta = 2.82, and zircon U/Yb, Gd/Yb, and Th/Yb ratios as well as oxygen fugacity (logfO2 = −26.09 to −12.32) reflect protracted fractional crystallization of a highly reduced mantle‐derived oceanic island basalt (OIB) magma from EM2 with considerable crustal involvement. Magma evolution was fuelled by fractionation of calcic‐pyroxene, calcic‐amphibole, Mg‐olivines, feldspars, titanite, and apatite, as corroborated by the marked depletions of CaO, MgO, TiO2, and P2O5. Intense crustal assimilation, particularly in the biotite granite masked their enriched mantle (EM) ancestry. This is attested by the presence of inherited Pan‐African (ca. 582 Ma) zircon, elevated crustal‐like zircon U/Yb, low zircon Gd/Yb, and Th/Yb ratios as well as high whole‐rock Rb/Nb ratios and relatively low estimated temperatures, which mimic crustal‐derived granites. Reactivation of pre‐existing megashear zones of weaknesses and opening of related transcurrent faults following periodic plate movements generated intense stress field which invoked the release of pressure and hot metasomatic fluids that partially melted the lower EM, giving rise to the Ropp OIB magmas.

Petrogenesis and tectonic implications of Late Triassic granitoids in the Alananshan, East Kunlun belt: evidence from geochemistry, geochronology, and zircon Hf isotopic compositions

Petrogenesis and tectonic implications of Late Triassic granitoids in the Alananshan, East Kunlun belt: evidence from geochemistry, geochronology, and zircon Hf isotopic compositions

Tracing remnants of Ediacaran S-type granitoids from beach placers in SE Madagascar

Southeast Madagascar hosts several major deposits of beach placers, the provenance of which is little understood. To redress this imbalance, we present new laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages, and trace elements of zircons and monazites, as well as Hf isotopes of zircons from heavy mineral beach sands at Taolagnaro (Fort Dauphin) in SE Madagascar. The ages of detrital zircon grains range from 650 Ma to 450 Ma and from 1900 Ma to 1700 Ma (one grain has an age of 2633 Ma), and detrital monazite ages range from 574 Ma to 484 Ma. The Ediacaran−Cambrian zircons (650−450 Ma) and monazites (574−484 Ma) record a major Pan-African orogenic event in SE Madagascar. Geologically, the 650−450 Ma ages correlate well with coeval granitic gneisses in southern Madagascar, and a few 1900−1700 Ma ages can be linked to putative Paleoproterozoic protoliths of metasedimentary rocks from the Anosyen Domain and from the Itremo Group of central Madagascar. The zircon mineral chemistry indicates that some of the grains (P>25 μmol g−1) were sourced from S-type granitic rocks. The monazite chemistry points toward a metamorphic provenance from garnet-bearing amphibolite facies rocks, such as migmatitic gneisses in southern Madagascar. The Hf isotopic compositions of detrital zircons indicate reworking of their probable Paleoproterozoic−Archean source rocks. We conclude that the beach sand zircons and monazites were largely sourced from the uplifted and eroded Anosyen and Androyan Domains in SE Madagascar, and with minor contributions from central Madagascar, which were all transported by rivers to the ambient ocean to the southeast.

Homogenization of zircon Hf isotopes during late-stage granite crystallization

Zircon Hf isotope compositions provide valuable constraints on granite petrogenesis and the evolution of continental crust. However, the causes of variations in zircon Hf isotope ratios during magma evolution are poorly understood. Here we compiled zircon UPb ages and LuHf isotope compositions of the Permian-Triassic granites in South China to demonstrate that Hf isotope variability is common in the early stages of granitic magma evolution, but Hf isotope ratios are homogenized during the final stages of granite crystallization. Late-grown zircons have relatively high concentrations of U and have εHf(t) values approximating the median values of multiple analyses for the low-U zircon in the same samples. Hafnium isotope variations measured in low-U zircon could be related to incremental assembly of multiple isotopically distinct magma batches derived from partial melting of heterogeneous crustal rocks; these discrete magma batches may have remained isolated from each other during segregation and extraction in the source and during ascent through the crust. After emplacement, these magma batches may have experienced variable degrees of mixing resulting in homogenization of Hf isotopes in late crystallized zircon. Coupled with whole-rock geochemical data of the Permian-Triassic granites, we argue that the Hf isotope homogenization is related to crystallization of zircon from late-stage magmas or precipitation of hydrothermal zircon from exsolved hydrothermal fluids during the final stages of magma solidification. Hafnium isotope homogenization in late-stage melts/fluids may have been driven by an increased concentration of fluorine and the efficient diffusion of hafnium-fluoride complexes through silicic melt. These results provide a deeper understanding of the mechanisms that contribute to Hf isotope heterogeneity in zircon during granite crystallization.

A combination of plume and subduction tectonics contributing to breakup of northern Rodinia: Constraints from the Neoproterozoic magmatism in the Dunhuang-Alxa Block, northwest China

Neoproterozoic igneous rocks are widely distributed in the Dunhuang-Alxa Block, northwest China, and record geodynamic processes caused by the assembly and break-up of Rodinia. In this study, we present new petrological, geochemical, and zircon U-Pb-Hf isotope data for the Langshan gabbros in the northeastern Alxa Block. Langshan gabbros (827−819 Ma) have depleted whole-rock Nd (+3.2 to +4.9) and zircon Hf (+4 to +21) isotopic compositions and weak enrichments in large-ion lithophile elements. These rocks were probably derived by partial melting of a depleted mantle wedge metasomatized by fluids released from a subducted slab in a back-arc basin environment. Geochemical and zircon Hf isotope data for early Neoproterozoic igneous rocks are consistent with the presence of a long-lived (ca. 930−810 Ma) retreating subduction zone along the northern margin of the Dunhuang-Alxa Block, indicating it was located at the northern margin of Rodinia. Some mantle plume-related magmatism, including A1-type silicic volcanic rocks in the Langshan Group and Jinchuan ultramafic-mafic intrusions, occurred in the interior of the Dunhuang-Alxa Block, which was similar to early Neoproterozoic plume-related magmatism (ca. 850−820 Ma) in the interior of northern Rodinia. The temporal link between subduction and extension suggests that this retreating subduction zone along the northern margin of Rodinia, in combination with a mantle plume beneath its interior, resulted in lithospheric extension and break-up of Rodinia during ca. 850−810 Ma.

Geochemical and geochronological constraints on the Mesoproterozoic Red Granite Suite, Kunene AMCG Complex of Angola and Namibia

Anorthosite-mangerite-charnockite-granite (AMCG) suites form large batholiths emplaced during the Proterozoic Eon. Magma source(s), emplacement mechanisms, and the tectonic setting of AMCG suites remain poorly understood. We present new major and trace element geochemistry combined with U-Pb zircon geochronology and Lu-Hf isotopes for the Red Granite Suite, part of the Mesoproterozoic Kunene AMCG Complex (Angola and Namibia), and its Paleoproterozoic host rocks, to elucidate the petrogenesis and tectonic setting of the Kunene Complex granitoids. The studied samples are mostly granite with minor syenite and quartz monzonite. LA-MC-ICPMS U-Pb zircon dates indicate that felsic magmatism was active for at least 90 Myr (1450–1360 Ma) and shows spatial and temporal similarities with emplacement of the main anorthositic body. The geochemistry of the granitoids did not change substantially over time, with predominant alkali-calcic, A-type granites. The εHf(t) of zircon grains in the Red Granite Suite ranges between −11.3 and + 1.6. These results, when considered together with Hf isotopic compositions of zircon from the Paleoproterozoic host rocks (εHf(t) = -11.3 to + 0.4) and xenocrysts in the Red Granite, suggest the suite is derived from the mixing of a Paleoproterozoic crustal component with juvenile mantle-derived material. Syn-magmatic east–west shortening has been documented in both the anorthosites and Red Granites at the southwestern margin of the Kunene Complex in the period 1400–1380 Ma. The results from the Hf isotopes on zircon, combined with the long-lived nature of magmatism, the syn-contractional magmatism, at least locally, and the requirement for extensive crustal melting, suggest that the Red Granite Suite was formed in a convergent margin setting. This challenges the traditional view that A-type granitoids are restricted to anorogenic, extensional settings, and is in agreement with recent views on the petrogenesis of massif-type anorthosites and their associated MCG magmatism.

Provenance of pre- and Carboniferous sequences of the Esquel-Arroyo Pescado-Tepuel regions (Argentine Patagonia): A combined U–Pb and Hf isotope study of detrital zircon and constraints on depositional setting

Provenance and stratigraphic age of pre-Carboniferous or pre-glacial (Esquel and Arroyo Pescado formations) and Carboniferous glacial (Valle Chico Formation) metasedimentary units exposed in the southwestern sector of the North Patagonian Argentine region are analyzed by sedimentary petrology and combined U–Pb and Lu–Hf isotope on detrital zircons. Additionally, a sample of the Jaramillo Formation (Tepuel Group) allowed comparison and correlation to the Carboniferous deposits in the type area of the Tepuel-Genoa basin.Despite differences, samples of all units studied indicate deposition linked to an arc tectonic environment using ternary petrological diagrams. U–Pb detrital zircon age distribution of the Esquel Formation show a dominance of Famatinian, Pampean-Brazilian and Mesoproterozoic ages, whereas the Arroyo Pescado Formation displays a predominant Mesoproterozoic data cluster. On the other hand, the Valle Chico Formation records a main input of post-Famatinian or Chanic (Devonian) ages and minor Gondwanic (Carboniferous) ages. The Jaramillo Formation is unique due to the remarkable unimodal concentration of Gondwanic ages (Carboniferous) and the scarcity of older than Famatinian detrital zircon ages. The maximum sedimentation age is Lochkovian (Lower Devonian) for the Esquel Formation and, Tournaisian and Tournaisian-Visean (lower Carboniferous) for the Valle Chico and Jaramillo formations, respectively; the youngest records for the Arroyo Pescado Formation indicate a middle Cambrian to Middle Ordovician age. Most Mesoproterozoic zircon grains show a remarkable juvenile Hf isotopic character and a slight age difference between U–Pb and Hf TDM data. These records are equivalent to those identified in different units of the Paleozoic of the Patagonian region and the Malvinas/Falkland Islands, where the Mesoproterozoic detrital zircons with positive εHf (t) values indicative of a juvenile character are present. The conspicuous participation of Mesoproterozic zircons ages within the pre-Carboniferous units supports the probable existence of an unexposed Mesoproterozoic crust in the North Patagonian region, as mentioned by previous authors.The allochthonous or autochthonous character of South Patagonia is tested using combined U–Pb ages and initial Hf-isotope compositions of single zircons. The allochthonous alternative of South Patagonia in the pre-Carboniferous stage, favours contributions from eastern Antarctica, South Africa, Malvinas/Falkland Islands and local sources such as the Deseado Massif and the suspected Chaitenia terrane. An autochthonous scenario suggests that the North Patagonian, Deseado Massifs, Malvinas/Falkland Islands and local areas (Chaitenia terrane) were the main source for the pre-Glacial and Glacial studied units, with minor influence from distant regions of South America.

Geochronology and petrogenesis of the Early Palaeozoic Fuxi magnesian granodiorite in southern Zhuguangshan, South China Block and its geodynamic significance

This study provides relevant data on the granodiorite in southern Zhuguangshan, South China Block (SCB), including mineralogical information, zircon U–Pb ages, zircon Hf isotope data, whole‐rock geochemical data, and Sr–Nd–Pb isotopic data. These data indicate that the granodiorite in southern Zhuguangshan crystallized at approximately 448.7 Ma. The hornblende and micas of the granodiorites are magnesio‐hornblende and Mg‐rich biotite, respectively. The granodiorite samples are characterized by relatively high SiO2 contents of 61.72–66.19 wt.%, K2O contents of 3.77–5.18 wt.%, Na2O contents of 2.42–3.16 wt.%, low FeOT contents of 3.93–5.79 wt.%, and relatively high MgO contents of 1.78–2.55 wt.%, with Mg# values (molar Mg/[Mg + Fe] × 100) of 43–50. The contents of rare earth elements (REEs) in the granodiorite specimens are 166–283 ppm, and the REEs show both a chondrite‐normalized REE pattern that inclines rightward and slightly negative Eu anomalies (0.62–0.90). The Fuxi granodiorite is rich in large‐ion lithophile elements (LILEs; Rb, Th, U, and K) but is depleted in high‐field‐strength elements (HFSEs; Nb, Ta, and Ti). All the samples have similar Sr–Nd–Pb and zircon Hf isotopic compositions and have a (87Sr/86Sr)i ratio of 0.707319–0.710888, a (143Nd/144Nd)i ratio of 0.511705–0.511760, a (206Pb/204Pb)t ratio of 17.918–18.459, a (207Pb/204Pb)t ratio of 15.711–15.813, and a (208Pb/204Pb)t ratio of 37.953–38.563. The εNd(t) and εHf(t) values range from −7.12 to −5.88 and from −9.81 to −4.45, respectively, with corresponding two‐stage Nd model ages of 1.66–1.76 Ga and two‐stage Hf model ages of 1.55–1.78 Ga. These findings indicate that the granodiorite is magnesian andesite and that the magmas are derived from a mixture of crustal and mantle materials. Based on these findings in combination with previous research results, it can be concluded that the mantle beneath the Cathaysia Block was moderately depleted during the Early Palaeozoic and that Early Palaeozoic magmatism may be related to Early Palaeozoic oceanic crust subduction.

Petrogenesis of Paleozoic–Early Mesozoic Granites and Pegmatites in the Yuechengling Pluton of South China

Abstract Although pegmatites are volumetrically minor in the upper continental crust, these rocks host abundant rare metal deposits (e.g., Li, Be, Rb, Ta, and Nb). Pegmatites can be formed either by extensive fractional crystallization of granitic magmas or by low-degree partial melting of metasedimentary rocks. The Mao’ershan–Yuechengling composite batholith in the Nanling Range in the South China Block (SCB) is of early Paleozoic–Triassic age (440–381 and 236–204 Ma, respectively). Recently, hundreds of pegmatites associated with Nb, Ta, Be, Rb, and Li mineralization have been identified in this batholith. These pegmatites are hosted by granitic wall rocks. However, the relationships between the pegmatites and granitic wall rocks are not well constrained. To address this, we investigated the Mao’antang (MAT) and Tongzuo (TZ) pegmatites and their biotite granite wall rocks in the middle part of the Yuechengling pluton. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb ages revealed that the MAT pegmatites formed during the Permian (269 Ma) and Triassic (231 Ma) and that the MAT biotite granite wall rock records two stages of magmatic activity (271 and 231 Ma) that are coeval with the pegmatites. The TZ pegmatites probably formed during the Triassic (235 Ma), and the TZ biotite granite wall rock formed during the Silurian (435 Ma). The MAT biotite granite and pegmatites (εNdt=−12.0 to −10.6; εHft=−8.0 to −1.0), TZ pegmatites (εNdt=−10.4 to −6.1), and TZ biotite granites (εNdt=−9.1 to −8.7; εHft=−7.7 to −4.1) have enriched whole-rock Nd and zircon Hf isotopic compositions that are similar to those of early Paleozoic and Triassic S-type granites in the SCB. In addition, the whole-rock Pb isotopic compositions of the MAT and TZ pegmatites and granites are distributed along the upper crust evolution line. We suggest that the MAT and TZ biotite granites were mainly derived from Paleoproterozoic metasedimentary rocks in the middle crust. The MAT pegmatites are fractional crystallization products of the MAT biotite granites, whereas the TZ pegmatites were formed by fractional crystallization of hidden parental S-type granites. We propose that the MAT and TZ pegmatites have potential for rare metal (Nb, Ta, Be, and Li) mineralization, as they record high degrees of fractional crystallization. The MAT and TZ areas in the middle of the Yuechengling pluton are promising targets for rare metal exploration.

On the Statherian–Calymmian palaeogeography of northwestern Laurentia

The c. 1.75 to 1.27 Ga Hornby Bay intracontinental basin, exposed in northwestern Canada, NE of Great Bear Lake, includes the Big Bear, Mountain Lake and Dismal Lakes groups. The Big Bear group comprises mainly immature clastic rocks deposited by high-energy rivers in restricted fault-bounded basins formed during the late-stage assembly of supercontinent Nuna. Detrital zircon analysis of the Big Bear group suggests provenance from a combination of local sources including recycling of sedimentary rocks, which were originally derived from sources further east, such as the Thelon orogen. The overlying Mountain Lake group was deposited by westerly flowing rivers over a much broader region, with less evidence for syndepositional faulting, suggesting thermal subsidence. It displays provenance via recycling of strata from the proximal Coronation Supergroup and the more distal and correlative Goulburn Supergroup of Kilohigok Basin. Following tectonic uplift and erosion, basal clastic rocks of the Dismal Lakes Group were deposited in fluvial and then shallow-marine to paralic environments. Detrital zircon from these strata displays prominent late Archean peaks demonstrating provenance from adjacent rocks of the Slave craton and recycling of the Mountain Lake group. Detrital zircon geochronology of sandstones from the Hornby Bay Basin supports stratigraphic correlation with the Wernecke Supergroup in the Wernecke Mountains and the Muskwa assemblage, located further southward along the ancient western margin of Laurentia. Our data, along with Nd and C isotopes from corresponding shale and carbonate units, provide robust evidence for a palaeogeographical model whereby these successions represent the terrestrial and marine components of a west-facing, passive-margin clastic wedge, considered the westward extension of an Amazon-scale, braided fluvial system that originated in the foreland of the Trans-Hudson orogen, to the east. Elements of this system have been identified from inliers in northeastern Australia, which was attached to northwestern Laurentia prior to the break-up of supercontinent Nuna. Supplementary material: U–Pb and Hf results, and Hf isotopic compositions of reference zircons are available at https://doi.org/10.6084/m9.figshare.c.5980940

Mid-Cretaceous drainage reorganization and exorheic to endorheic transition in Southeast Tibet

To understand the early topographic growth of the Tibetan Plateau and the causes and effects of climate change in this region, it is important to reconstruct the drainage evolution across Southeast Tibet. Based on age constraints provided by detrital zircons in the fluvial sandstones in the Khorat Plateau Basin, we identified a major Early Cretaceous drainage disruption and reorganization event. The ages and Hf isotope compositions of the detrital zircons in the Upper Jurassic-Lower Cretaceous Phu Kradung to Phu Phan formations (166–122 Ma) suggest the existence of an exorheic continental-scale paleo-Mekong River with headwaters in the Songpan-Garze and Qiangtang terranes. This paleo-river flowed southwards across the Khorat Plateau to what is presently the offshore Phuquoc Basin, which was then part of the Neo-Tethys Ocean. In contrast, the U-Pb-Hf compositions of the detrital zircons in the Lower Cretaceous Khok Kruat Formation (122–113 Ma) indicate that a prominent detrital supply from the Sibumasu magmatic rocks was transported by the paleo-Salween River. This major change reflects the occurrence of ancestral transcontinental river disruption at around 122 Ma, which was driven by the final collision between the Lhasa and Qiangtang and Woyla Arc and Sibumasu terranes. The tectonic uplift and ensuing arid climate corresponded to the reorganization of the ancestral Mekong drainage system during the Mid-Cretaceous (113–94 Ma).

Geochronology, petrogenesis and metallogenic implications of granitoids in the Xiaotuergen Cu deposit, Northern Chinese Altai Orogen, NW China: Constraints from zircon, apatite and whole-rock geochemistry

The Xiaotuergen Cu deposit is the first Cu deposit described in the Northern Chinese Altai Orogen, Xinjiang, NW China. The multiple phases of granitic intrusions occur in the Xiaotuergen region, including the intrusions of granodiorite porphyry, granite porphyry, and biotite monzogranite. Among these types of intrusions, Cu mineralization is considered to be closely related to the early Hercynian granodiorite porphyry. However, the precise emplacement ages, petrogenesis, redox states, and tectonic setting of the three Xiaotuergen granitoids and the relationship between these granitoids and mineralization was unclear. To resolve these issues, zircon U–Pb dates, zircon in situ trace element concentrations, Hf isotopic compositions, apatite in situ major element concentrations, and whole-rock geochemistry were determined in this study. Three different intrusive phases are recognized within the district; in order of deceasing age, these phases are biotite monzogranite, granodiorite porphyry, and granite porphyry, which have the ages of 400.2 ± 1.6 Ma to 399.6 ± 2.4 Ma, 390.3 ± 1.6 Ma to 377.1 ± 1.6 Ma, 381.0 ± 2.0 Ma to 378.7 ± 1.2 Ma, respectively, suggesting that they were emplaced during the Early-Middle Devonian. Geochemical data show that these intrusions are metaluminous to weakly peraluminous (A/CNK = 0.91–1.28), calc-alkalic to shoshonitic I-type granitoids. The two Harker plot trends reveal derivation from two magma sources (biotite monzogranite vs granodiorite porphyry and granite porphyry) that experienced different evolutionary processes. Multiple indicators, including the zircon Ce4+/Ce3+ ratio and oxygen fugacity (logfO2) values, apatite Mn oxybarometer, and whole-rock Fe2O3/FeO ratio, consistently indicate that the parental magma of the fertile granodiorite porphyry was more oxidized than that of the two barren intrusions (biotite monzogranite and granite porphyry). Furthermore, apatite grains from the fertile granodiorite porphyry have higher Cl contents and Cl/F ratios than those of the ore-barren biotite monzogranite and granite porphyry, which may have resulted from the higher water content of the parental magma and increased lithospheric mantle-derived fluid involvement in their generation. Thus, higher Cl/F ratios of apatite can be used to discriminate fertile granitoid intrusions that host Cu mineralization in the Northern Altai orogen. Zircon Hf isotopic compositions show widely ranging and mostly positive εHf (t) values (−4.2 to +11.4, mainly concentrated between +2 and +8). These isotope characteristics in combination with young zircon TDMC (Hf) ages (655–1,659 Ma; mainly concentrated between 850 Ma and 1250 Ma) indicate that the Xiaotuergen granitoids were most likely derived from the partial melting of juvenile lower crust, with some interaction of mantle materials from the underlying mantle wedge. The three Xiaotuergen granitoids were enriched in light rare earth elements and large ion lithophile elements, depleted in heavy rare earth elements, and showed negative Nb, Ta, Sr, P, and Ti anomalies, displaying typical arc geochemical affinities. Combined with the regional tectonic evolution, we propose that the Xiaotuergen granitoids were generated in a continental arc setting associated with subduction processes, which were the products of syn-accretion orogenic processes.

Late cretaceous magmatism in the Northern Lhasa subterrane, central Tibet: geodynamic implications

ABSTRACT Compositional changes of successively emplaced magmatites can be used to infer changes in their petrogenetic evolution and geodynamic setting. The central-northern Lhasa terrane hosts extensive Late Cretaceous magmatism, however, the mechanism that triggered the magmatism and its relationship to the directly preceding Late Cretaceous tectonic transition in the central Tibetan Plateau is still unresolved. Therefore, we conducted a detailed petrological, geochronological, geochemical, and isotopic study on recently identified Late Cretaceous doleritic and granodioritic rocks in the Xiongmei area, northern Lhasa subterrane. The potassic (K2O/Na2O ratios >1) dolerites (ca. 80 Ma) display a post-collisional basaltic shoshonitic signature with uniform moderately enriched whole-rock Sr–Nd and zircon–Hf isotopic compositions ((87Sr/86Sr)t = 0.7070 to 0.7095, whole-rock εNd(t) = – 2.4 to – 1.3, and zircon εHf(t) = – 5.9 to +1.9), suggesting that they evolved from a similar parental magma and were likely derived by partial melting of an enriched lithospheric mantle metasomatized by hydrous fluids. The granodiorites are geochemically divided into medium-K and high-K calc-alkaline rocks. The medium-K samples (ca. 80 Ma) are generally characterized by low K2O/Na2O ratios <1, whereas the high-K samples (78–76 Ma) display high K2O/Na2O ratios >1. The medium-K samples have positive zircon εHf(t) values from +4.8 to +6.5 and uniform whole-rock ɛNd(t) values from – 2.4 to – 1.3, which significantly differ from those of the high-K samples that have negative to positive zircon εHf(t) values from – 1.9 to +2.8 and negative whole-rock ɛNd(t) values from – 8.4 to – 5.1. The medium-K samples are characterized by high Sr contents and Sr/Y ratios, coupled with low Y and HREE contents, similar to adakitic compositions, and were interpreted to have been derived from deep partial melting of the foundering juvenile garnet-bearing mafic lower crust at relatively low temperatures. Compared to the medium-K samples, the high-K samples have lower Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and distinct negative Eu, Sr and Ti anomalies and are most likely derived from shallower partial melting of the K-rich juvenile lower crust at relatively high temperatures. Considering the occurrence of variable magmatism in the central-northern Lhasa subterrane during the Late Cretaceous, the change in magma composition and lower crustal melting conditions (cold-deep vs. hot-shallow) suggest that the lithospheric mantle keel was partially detached by small-scale mantle convection in response to the low-angle Neo-Tethys-associated subduction between 85 and 70 Ma.

Reassessing the intrusive tempo and magma genesis of the late Variscan Aar batholith: U–Pb geochronology, trace element and initial Hf isotope composition of zircon

The Variscan orogeny was responsible for the formation of a significant volume of igneous basement throughout present-day Europe. Detailed understanding of these rocks has, however, been obfuscated by significant overprinting during younger geologic events. In order to better understand the formation of this basement, we present U–Pb dates, trace element concentrations and Hf isotope compositions of zircon from 17 intrusions of the Variscan Aar batholith, located in the Aar Massif, Central Alps, Switzerland. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) was used to generate a large set of U–Pb dates, trace element and Hf isotope compositions on untreated zircon, as well as zircon pretreated by chemical abrasion. Furthermore, a subset of samples was also analyzed for high-precision U–Pb geochronology using chemical abrasion, isotope dilution, thermal ionization mass spectrometry (CA-ID-TIMS). The U–Pb dates of both dating techniques are significantly dispersed, indicating that they are influenced by multiple forms of complexity, including inheritance, domains of secondary alteration likely related to Alpine overprint or growth, decay damage related Pb-loss, and potentially protracted magmatic growth. Decay-damage related Pb-loss is likely a subordinate source of age scatter within the data, therefore chemical abrasion pretreatment is not capable of completely mitigating the observed analytical scatter. After rejection of outliers, the remaining data still exhibit excess scatter of several percent among 206Pb/238U dates in individual samples, however it is possible to interpret reasonable geologic ages from these data. These new U–Pb zircon age interpretations indicate the Aar batholith grew incrementally through four major magmatic pulses, which occurred at approximately 348, 333, 309 and 298 Ma. Based on the trace element and Hf isotope geochemistry, the melt source(s) of the Aar batholith evolved throughout the duration of batholith formation and growth. The transitioning from (i) melting of depleted mantle at 348 Ma during a stage of active continental arc magmatism (εHf = + 12 to + 10), (ii) melting of metasomatically enriched lithospheric mantle, possibly contaminated by crust during the 333 Ma pulse (εHf = − 10 to − 3), followed by (iii) an increasing incorporation of a juvenile mantle components during the 309 and 298 Ma pulses (εHf = − 3 to + 6). Finally, these new U–Pb ages yield a more detailed understanding of the Variscan Aar batholith by integrating the new detailed mapping of Aar Massif for the Geological Atlas of Switzerland, allowing for more accurate characterization and categorization of variably deformed heterogeneous intrusive bodies.

U(‐Th)–Pb age and petrogenesis of Shigar leucogranites, Karakoram Metamorphic Complex, North Pakistan

The Shigar valley of northern Pakistan is a well‐known region for holding a variety of gemstones. As the gemstone‐bearing rock, post‐collisional leucogranites, and pegmatites in Shigar valley have been sporadically researched but the detailed and updated timing record along with the source rocks were not adequate. Here, we present comprehensive U(‐Th)Pb ages, mineral, and whole‐rock isotopic analyses to investigate the petrogenetic evolution of these granitoids. The zircon, xenotime, and monazite U(‐Th)–Pb ages show crystallization of leucogranites that spanned as recent as ~7–3 Ma. The REEs with nearly smooth patterns and distinct negative Eu anomalies in all the leucogranites and pegmatites with contrasting negative Eu anomalies illustrate significantly plagioclase fractionation. These leucogranites and pegmatites have high SiO2 content showing peraluminous geochemistry. Zircon Hf isotopic composition of leucogranites, pegmatites and gneisses have εHf(t) values of −28.4 to +7.4, −24.0 to −16.5, and −13.2 to +1.8, respectively, which shows crustal origin along with the probable contamination from Kohistan‐Ladakh Arc at depths. Furthermore, monazite Nd isotopic composition of leucogranites, pegmatites, and gneisses have εNd(t) values of −22.9 to −9.5, −25.1 to −22.8, and −26.3 to −24.8, which seems purely from crustal source. The whole‐rock Sr‐Nd‐Hf isotopic composition shows that leucogranites and gneisses have 87Sr/86Sr(i) values of 0.71603 to 0.92518, εNd(t) values of −22.2 to −2.7, and εHf(t) values of −44.1 to −0.2, respectively, that also confines it to the metapelitic origin. Our results overall reveal that the leucogranites are produced mainly by the partial melting of Dassu felsic gneiss from the Karakoram Metamorphic Complex, showing signatures of the upper crustal rocks of the subducting Indian Plate and hybridized juvenile Asian crust. The input of heat for partial melting is likely to be derived either from the lithospheric mantle which followed the slab break‐off or from shear heating along Main Karakoram Thrust. Furthermore, a leucogranite sample yielded a zircon U–Pb age of 84.9 Ma, and monazite U(‐Th)–Pb age of 10.8 Ma. The in‐situ zircon Hf and monazite Nd isotopes yielded εHf(t) values of +3.6 to +10.7, and εNd(t) values of +1.0 to +2.4, whereas the whole‐rock 87Sr/86Sr(i) value is 0.70522, εNd(t) is +1.1, and εHf(t) is +4.8, respectively, which confirms it to be sourced from the nearby Kohistan‐Ladakh Arc.

Large zircon age spans record multi-stage history of batholith assembly: Insights from the Late Triassic Dongcuo batholith in the eastern Tibetan Plateau

Large and homogeneous plutons can assemble incrementally over millions of years, yielding large age spans. Determination of the age span is crucial to constrain the timing and nature of the batholith assembly. This paper reports comprehensive whole-rock geochemical data, LA-ICP-MS zircon U-Pb age, and zircon Hf isotopic data to constrain the timing and assembly process of the Late Triassic Dongcuo batholith, the largest batholith in the Yidun Terrane, eastern Tibetan Plateau. Samples from the batholith consist of metaluminous to weakly peraluminous granitoids (SiO2 = 69.91–79.85 wt%). These granitoids show large variation of zircon Hf isotopic compositions with εHf(t) of -17.15 to +0.13. The multiple zircon populations in single hand specimens revealed by kernel density estimation spread over 20 Myr (ca. 225–204 Ma), which is also confirmed by core-rim ages of single zircon grains, suggesting prolonged magmatism of the Dongcuo batholith. Our data indicate that the assembly of the batholith resulted from emplacement of discrete magma batches which derived from spatially varied sources and underwent variable differentiation. The assembly mainly involved three stages, including an early episode of magmatism (ca. 228 Ma), a high-flux magmatism between ca. 220 Ma and ca. 216 Ma, and a late-stage thermal event (ca. 211–204 Ma) possibly related to high temperature and localized remelting of the batholith. This study highlights the complexity of zircon populations in composite granitic batholith, showing that the search for single crystallization ages for a sample is fraught with difficulties and may mask rather than elucidate the crystallization history of the magmatic body.

Tectonic switch from a lithospheric rift to an active continental margin in the Paleoproterozoic: Evidence from low δ18O granites from the Trans-North China Orogen in the North China Craton

Tectonic switch from rift zones to subduction zones is common along convergent plate boundaries. While this process is susceptible to retrieving from Phanerozoic rock records, the difficulty has been encountered for Precambrian rock records because of the relative lack of characteristic geological signatures. Nevertheless, such a difficulty can be overcome by finding of specific geochemical signatures in ancient rock records. This paper reports our finding of low δ18O zircons from the Trans-North China Orogen (TNCO) in the North China Craton (NCC), where the tectonic switch would occur during the early to middle Paleoproterozoic in association with the amalgamation of supercontinent Columbia. A combined study of zircon U-Pb ages and Hf-O isotopes as well as whole-rock major-trace elements and Nd isotopes were carried out for magmatic rocks from the Taiyue complex in the southern part of the TNCO. Zircon U-Pb dating yields two episodes of magmatism at ca. 2.34–2.31 Ga and ca. 2.2–2.1 Ga. These magmatic rocks are dominated by the ca. 2.18–2.16 Ga granites, 2.18–2.17 Ga diorites and 2.17–2.11 Ga mafic–ultramafic cumulates that intruded the 2.34–2.30 Ga granites and diorites. The 2.2–2.1 Ga diorites, mafic-ultramafic cumulates, and the regional mafic dykes/intrusions exhibit continuously varying major element compositions, arc-like trace element patterns, and consistent zircon Hf and whole-rock Nd isotope compositions, indicating their derivation from the same suite of continental arc magmas. Such primitive arc magmas would evolve through fractionation and accumulation of pyroxenes and plagioclase into dioritic magmas. The two episodes of granites are similar in major and trace element compositions, generally belonging to alkali-calcic or calc-alkalic A-type granitoids. Although both groups show a small difference in their zircon Hf isotope compositions, they exhibit a big difference in their zircon O isotope compositions. The 2.31 Ga granites show variably low zircon δ18O values of 3.4–5.5‰, mostly lower than normal mantle zircon values. The 2.18 Ga granites also exhibit variable zircon δ18O values from 3.6 to 6.0‰, but mostly mantle-like values. It is inferred that the 2.31 Ga granites would acquire their low δ18O signatures from partial melting of the high-T seawater-hydrothermally altered Archean crust in an early Paleoproterozoic continental rift. This rift would extend for at least 300 km along the TNCO. The 2.18 Ga granites are closely associated with the 2.2–2.1 Ga diorites and mafic–ultramafic cumulates. They would be most likely to form through differentiation of the 2.2–2.1 Ga continental arc magma, with their low 18O signatures being contaminated by the low δ18O 2.3 Ga granites. The low 18O signatures in the 2.3 Ga and 2.18 Ga granites indicate that the southern part of the TNCO would have probably evolved from a lithospheric rift to an active continental margin during ca. 2.3–2.1 Ga.

Magmatic response to arc-arc amalgamation: Insights from latest Paleozoic igneous rocks from the Gangou section of the Eastern Tianshan

Multiple arc systems were developed in response to Neoproterozoic to Mesozoic consumption of the Paleo-Asian Ocean and their magmatic evolution is crucial for understanding the arc-arc amalgamation in the Central Asian Orogenic Belt. Here, we report whole-rock geochemical data as well as simultaneous in situ zircon U-Pb and Lu-Hf isotope results for the late Paleozoic magmatic rocks associated with the shear zones in the Gangou section of the Eastern Tianshan to constrain their petrogenesis and arc-arc amalgamation processes. We obtained ∼307 Ma and ∼270 Ma zircon U-Pb ages for the Late Carboniferous diorites and Middle Permian andesites, respectively. These rocks have high Mg# (∼65 and ∼47, respectively) values, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), with depleted zircon εHf(t) values (+12.1 to +18.1), suggesting an origin from partial melting of the depleted mantle with various degree of differentiation. Furthermore, our results also provided ∼276 Ma zircon U-Pb ages for the Middle Permian granitic porphyries and ∼256 Ma zircon U-Pb ages for the Late Permian diabase rocks. The granitic porphyries are geochemically similar to A2-type granites, i.e., high K2O + Na2O, FeOT/MgO, Ga/Al and low CaO, Sr and Ba. They have high zircon εHf(t) (+9.0 to +13.7) values, indicating a possible derivation from juvenile lower crust. The diabase rocks show depletion of light rare earth elements (LREEs), resembling normal mid-ocean ridge basalt. These rocks have depleted zircon Hf isotopic compositions (εHf(t) = +8.9 to +15.8), demonstrating a possible derivation from partial melting of the depleted mantle. The available geochemical data from the Eastern Tianshan show that the Permian mafic rocks along the shear zones possess higher Nb/La and lower Ba/La ratios than their counterparts in the Carboniferous. These contrasting features imply that the Carboniferous mafic rocks originated from a metasomatic mantle wedge, while the Permian mafic rocks were likely derived from the depleted mantle with addition of enriched asthenosphere components. The high zircon saturation temperatures (mostly > 800 °C) of the Permian A-type granitoids and diverse magma sources of coeval mafic rocks also imply possible asthenosphere upwelling. Therefore, we propose that the Late Carboniferous magmatic rocks were likely resulted from subduction of the North Tianshan oceanic plate, while the spatial and temporal evolution of the Permian magmatic rocks were probably attributed to large-scale dextral transcurrent tectonics associated with arc-arc amalgamation.

Petrogenesis and geodynamic significance of Late Triassic mafic microgranular enclaves and its host granodiorite in the Shuanghu area, central Qiangtang

The genesis of Late Triassic granitoids in the central Qiangtang is significant for deducing the evolution of the Palaeo‐Tethys Ocean in Qiangtang Block. Herein, we present a comprehensive study of zircon U–Pb ages, whole‐rock geochemistry and zircon Lu–Hf isotopes for the mafic microgranular enclaves (MMEs) and host granodiorites from the Shuanghu area, in order to evaluate their petrogenesis and geodynamic implications. New zircon U–Pb dating show identical formation ages for host granodiorites (207.1 ± 2.2 Ma and 206.6 ± 3.8 Ma) and MMEs (205.4 ± 2.9 Ma and 209.2 ± 2.2 Ma). The Shuanghu granodiorites contain amphibole and biotite and are characterized by moderate SiO2 (64.74–66.40 wt%) and K2O contents (2.48–3.62 wt%) as well as low A/CNK (0.69–1.00), indicating I‐type granite affinity. Its enriched zircon Hf isotopes (εHf(t) = −12.04 to −5.09) indicate that they were derived from the ancient mafic lower crust. The MMEs have a similar mineral assemblage, emplacement ages, trace elemental and zircon Hf isotopic compositions with the host granodiorites, indicating the cognate origin. These MMEs represent autoliths captured by latter ascending host magmas in the middle‐upper magma chamber. The highly variable Mg# (37–53), Cr (16.9–190 ppm) and Ni (5.71–67.2 ppm) in the Shuanghu MMEs and host granodiorites indicate that the mantle‐derived magma provided not only the heat but also the mafic components. In combining with previous studies, we suggest that the Shuanghu granodiorites and MMEs were formed in the post‐collision stage, which were caused by the asthenosphere upwelling related to the slab breakoff.

Late Paleoproterozoic magmatism in North Hengshan: Final collapse of the Trans-North China Orogen

Post-collisional magmatism carries important information for understanding when and how an ancient continental collisional orogen unrooted and collapsed. However, the post-collisional magmatic response to the final collapse of the Trans-North China Orogen, one of the 2.1–1.8 Ga collisional orogens that formed in coincidence with the assembly of the Paleo–Mesoproterozoic Columbia supercontinent, remains controversial. Here we present the results of a combined geochronological and geochemical investigation on a suite of Late Paleoproterozoic potassic granites in the North Hengshan terrane of the Trans-North China Orogen, the North China Craton. In situ zircon U-Pb dating shows that these potassic granites, as the last phase of Precambrian magmatism in this terrane, were emplaced between 1790 Ma and 1774 Ma. Their enriched bulk-rock Nd and zircon Hf isotopic compositions point to an ancient crustal source, and their zircon O isotopic variation indicates the involvement of weathered supracrustal rocks in their sources. Considering their bulk-rock major and trace elements, the Late Paleoproterozoic potassic granites in North Hengshan are best explained to be produced by melting of immature metagraywacke that belonged to the Neoarchean–Early Paleoproterozoic sedimentary sequences, at middle–lower crustal depths. The 1.79–1.77 Ga potassic granites in North Hengshan postdated the main phase of the Late Paleoproterozoic collisional orogeny, and were a magmatic response to the final collapse of the Trans-North China Orogen. These 1.79–1.77 Ga potassic granites, along with other 1.80–1.75 Ga magmatism in the Trans-North China Orogen, were most likely generated in a post-collision extensional setting, possibly superimposed by an Andean-type continental arc at its southern margin. The 1.80–1.75 Ga magmatism marked the end of collision between the Western and Eastern Blocks that gave rise to the final consolidation of the North China Craton and the assembly of the Columbia supercontinent, and recorded the outgrowth of this supercontinent through accretion at its continental margins.