Hf Depleted Mantle Model Ages Research Articles

Zircon U–Pb geochronology, geochemistry and tectonic implications of Triassic A-type granites from southeastern Zhejiang, South China

The late Permian–Triassic granites in southeastern China have important tectonic significance for the evolution of East Asia. A detailed study utilizing zircon U–Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry for Dashuang and Jingju granites in Zhejiang Province, South China was performed. LA-ICP-MS zircon U–Pb analyses yielded ages of quartz-monzonite and monzogranite that are 234±3Ma and 231±3Ma, respectively, from the Dashuang pluton, while the LA-ICP-MS zircon U–Pb ages of monzogranite and K-feldspar granite from the Jingju pluton are 246±2Ma and 241±3Ma, respectively. These ages indicate that the magmatism event took place during the early Triassic to Mid-Triassic. The two granites have high contents of total alkalis (Na2O+K2O=7.73–10.24%), high-field-strength elements (e.g., Zr=215–471ppm, Y=25.8–36.5ppm, Nb=15–28ppm, and Zr+Nb+Ce+Y=293–849ppm) and rare earth elements (total REE=299–701ppm), as well as high Ga/Al ratios (10,000×Ga/Al=2.44–2.9). The zircon saturation temperatures were 800–837°C for the two granites, which suggests that they have the petrographic and geochemical characteristics of A-type granites. In-situ Hf isotopic analyses revealed that the two granites have εHf(t) values ranging from −20 to −6 and two-stage depleted mantle Hf model ages from 1.6Ga to 2.6Ga, which indicate that the two granite magmas were formed by the partial melting of Paleoproterozoic crust rocks in the Cathaysia Block. A series of A-type granites distributed in the coastal region probably defines an extensional environment in the early Triassic.

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt

The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.

Geochemistry and Zircon U–Pb–Hf Isotopic Systematics of the Sanchahe Quartz Monzonite Intrusion in the North Qinling Tectonic Zone, Central China: Implications for its Petrogenesis and Tectonic Setting

AbstractThe Sanchahe quartz monzonite intrusion is situated in the middle segment of the North Qinling tectonic belt, Central China mainland, and consists chiefly of sanukitoid–like and granodioritic‐monzogranitic rocks. The sanukitoid–like rocks are characterized by quartz monzonites, which display higher Mg# (55.0–59.0), and enrichments in Na2O+K2O (7.28–8.94%), Ni (21–2312 ppm), Cr (56–4167 ppm), Sr (553–923 ppm), Ba (912–1355 ppm) and LREE ((La/Yb)N=9.47–15.3), from negative to slightly positive Eu anomalies (δEu=+0.61 to +1.10), but also depletion in Nb, Ta and Ti. The granodioritic‐monzogranitic rocks diaplay various Mg# of 6.00–53.0, high Na2O+K2O (7.20–8.30%), Sr (455–1081 ppm) and (La/Yb)N (27.6–47.8), with positive Eu anomalies (δEu=1.03–1.57) and depleted Nb, Ta and TL Laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) zircon U‐Pb isotopic dating reveals that the sanukitoid‐like rocks were emplaced at two episodes of magmatism at 457±3 Ma and 431±2 Ma, respectively. The monzogranites were emplaced at 445±7Ma. Sanukitoid‐like rocks have their ∊Hf(r) values ranging from +0.3 to +15.1 with Hf–depleted mantle model ages of 445 to 1056 Ma, and the monzogranite shows its ∊Hf(t) values ranging from ‐21.6 to +10.8 with Hf–depleted mantle model ages of 635 to 3183 Ma. Petrological, geochemical and zircon Lu–Hf isotopic features indicate that the magmatic precursor of sanukitoid–like rocks was derived from partial melting of the depleted mantle wedge materials that were metasomatized by fluids and melts related to subduction of oceanic slab, subsequently the sanukitoid magma ascended to crust level. This emplaced mantle magma caused partial melting of crustally metamorphosed sedimentary rocks, and mixing with the crustal magma, and suffered fractional crystallization, which lead to formations of quartz monzonites. However, the magmatic precursor of the granodioritic‐monzogranitic rocks were derived from partial melting of subducted oceanic slab basalts. Integrated previous investigation for the adackitic rocks in the south of the intrusion, the Sanchahe intrusion signed that the North Qinling tectonic zone was developed in an early Paleozoic transitionally tectonic background from an island arc to back–arc.

Zircon Hf isotopes and geochemistry of the early paleoproterozoic high-Sr low-y quartz-diorite in the Quanji massif, NW China: Crustal growth and tectonic implications

The metamorphosed Early Paleoproterozoic granitoids in the Quanji massif, Northwestern China provide constraints for relationship between the Tarim Craton and North China Craton. Among granitoids batholiths, rocks of the Mohe quartz-diorite show typically adakitic geochemical characteristics, with medium K2O/Na2O ratios (0.56–1.17) and high Sr (519–619 ppm) low Y (9.37–20.40 ppm) and Yb (0.97–1.77 ppm) concentrations. The rocks have ɛ Nd(t) values between +2.4 and +4.4 and depleted mantle Nd model ages of 2.43–2.59 Ga. The magmatic zircons have positive ɛ Hf(t) values ranging from +0.40 to +7.60 and depleted mantle Hf model ages of 2.43–2.70 Ga, with major peaks at ∼2.54 and ∼2.65 Ga. The geochemical and Nd-Hf isotopic characteristics indicate that the Mohe quartz-dioritic rocks might be formed by partial melting of high-pressure metamorphosed juvenile crustal rocks in post-orogenic extensional regime in the Early Paleoproterozoic. It suggests that important crustal growth occurred in the Quanji massif and the Tarim Craton at ∼2.4 and 2.5–2.7 Ga. The Quanji massif and Tarim Craton might share a similar crustal evolution history with the North China Craton in the Neoarchean.

Early Paleozoic crustal anatexis in the intraplate Wuyi–Yunkai orogen, South China

Early Paleozoic amphibolite- to granulite-facies metamorphism, crustal anatexis and coeval magmatism are extensively developed in the Wuyi–Yunkai intraplate orogen in the South China block. However, the exact timing of granulite-facies partial melting and its link with orogenesis have not been well constrained. In this study, the charnockites, gneissic migmatites and Al-rich gneisses (Grt–Sil–Bt gneiss and Bt–Pl gneiss) from the Gaozhou Complex of the Yunkai uplift in the Cathaysia block were selected for the analysis of whole-rock major elements and zircon U–Pb dating, trace elements and Lu–Hf isotopes. The Gaozhou Complex experienced early Paleozoic regional high-temperature (up to 850°C), low- to medium-pressure (4–7kbar) metamorphism accompanied by crustal anatexis. The melts were produced through the dehydration of mica, such as biotite+quartz+plagioclase=orthopyroxene+K-feldspar+melt and biotite+quartz+plagioclase+sillimanite=garnet+K-feldspar+melt in the charnockites, and muscovite+quartz+plagioclase=sillimanite+K-feldspar+melt in the Grt–Sil–Bt gneisses. The charnockites, gneissic migmatites and gneisses are felsic with SiO2>64% and peraluminous with A/CNK>1.0, reflecting protoliths with affinities to sedimentary rocks. Zircons from these rocks partly show clear core–rim structure and yield concordant ages mainly around 440–425Ma, with minor groups at 2.8–2.4Ga, 1.5–1.25Ga, 1.2–0.9Ga, 850–540Ma and 460–450Ma. The 440–425Ma grains are euhedral, oscillatorily-zoned and have steep slopes from the LREE to the HREE with a positive Ce anomaly and clear negative Eu anomaly, suggesting they (re-) crystallized in the melts. These early Paleozoic zircons have negative εHf (t) (−34.1 to −1.5) and much older TCRUST (3.6–1.5Ga), demonstrating they were formed by re-melting of old crustal materials (>1.5Ga). The zircons with ages of 2.8–2.4Ga, 1.6–1.2Ga and 1.2–0.9Ga have relatively high εHf (t) values (up to +10.2–+15.2). The 850–540Ma zircons show variable εHf (t) values of +9.0 to −24.0 with TDM (depleted mantle Hf model ages)=2.2–1.0Ga and TCRUST (crustal Hf model ages)=3.1–1.1Ga. Combined with the published data, we suggest that the Cathaysia block contains Archean materials as old as 3.6Ga and has had a complex evolution, including the addition of juvenile materials at ca 2.7Ga, 1.6–1.2Ga and 1.2–0.9Ga. Reworking of old crustal components dominated at ca 850–750Ma, 750–540Ma, 460–450Ma and more intensively at ca 440–425Ma. Synthesizing the obtained results, we argue that the Yunkai charnockites, gneissic migmatites and Bt–Pl gneisses were formed due to the early Paleozoic high-T crustal anatexis, which may have been triggered by crustal shortening and thickening during the intraplate Wuyi–Yunkai orogeny in the South China block.

The latest Neoarchean–Paleoproterozoic evolution of the Dunhuang block, eastern Tarim craton, northwestern China: Evidence from zircon U–Pb dating and Hf isotopic analyses

The Dunhuang block, in the easternmost segment of the Tarim craton, exposes Precambrian mafic granulite and felsic gneiss. To better understand the poorly known formation and evolution of the Tarim craton, we carried out detailed field-based petrological and geochronological investigations on the major lithologies of the Dunhuang block. U–Pb dating and Hf isotopic analyses on the mafic granulite and felsic gneiss (TTG gneiss) of the Dunhuang block reveal two distinct age populations: the Latest Neoarchaean (∼2.5Ga) and the Late Palaeoproterozoic (1.82–1.85Ga). The zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of ∼2.5Ga, supporting the existence of Archean rocks in the Dunhuang block. The short time interval between Latest Neoarchean magmatism and high-grade metamorphism suggests that they were related to the same Latest Neoarchean tectonothermal event. The ∼2.5Ga zircons show Hf-depleted mantle model ages between 2.55Ga and 2.8Ga with a peak at ∼2.7Ga, suggesting that ∼2.7Ga juvenile crust of the Dunhuang block was reworked during a 200–300m.y. period at the end of the Neoarchean. The age of ∼1.85Ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high-pressure granulite facies metamorphism. Combined with the Hf isotopic data, these data suggest that the Dunhuang block experienced ∼2.7Ga crustal growth, a ∼2.5Ga magmatic–metamorphic event and a 1.82–1.85Ga (HP) granulite facies metamorphic event. This sequence of events is very similar to that of the North China Craton, implying that the evolutionary history of the Dunhuang block was consistent with that of the North China Craton from the Neoarchean to Paleoproterozoic.

Geochemistry and zircon U–Pb ages and Hf isotopic composition of Permian alkali granitoids of the Phan Si Pan zone in northwestern Vietnam

The late Permian granitoids exposed in the Phan Si Pan zone of northwestern Vietnam consist mainly of the Ye Yen Sun metaluminous granites and the Nam Xe–Tam Duong peralkaline granites. Laser ablation inductively coupled plasma mass spectrometry U–Pb zircon analysis reveals that both the granite suites were emplaced from 253Ma to 251Ma. They have a distinctive A-type geochemistry of high 10,000×Ga/Al ratios of 3.0–5.7 and are also characterized by elevated contents of high field strength elements, A/CNK values of 0.85–1.58, negative Eu-anomalies. Magmatic zircons from the granitoids exhibit positive initial ɛHf values ranging from 6.4 to 15.9 and yield single-stage depleted mantle Hf model ages of 257–663Ma. This Hf isotopic feature implies significant contribution of juvenile mantle material to the magmas of the spatially and temporally associated Ye Yen Sun metaluminous and Nam Xe–Tam Duong peralkaline granites.

Chronology, petrogenesis and tectonic setting of the Neoproterozoic Tongchang dioritic pluton at the northwestern margin of the Yangtze Block: Constraints from geochemistry and zircon U–Pb–Hf isotopic systematics

The Neoproterozoic Tongchang dioritic pluton is situated at the northwestern margin of the Yangtze Block and dominated by quartz diorite and granodiorite. Laser Ablation–ICPMS zircon U–Pb isotopic dating shows that the Tongchang dioritic pluton was emplaced at three Neoproterozoic magmatic episodes of ~879Ma, ~840Ma and ~824Ma, and the copper deposits formed following the second magmatic episode, revealing important magmatic episodes during the rarely reported “magmatic gap” between older arc magmatism (950–895Ma) and younger plume-related magmatism (740–825Ma) along the western margin of the Yangtze Block. The Tongchang dioritic pluton is characterized by high alkali (Na2O+K2O=6.26–8.75wt.%), Cr (186–561ppm) and Ni (76–292ppm), medium to high Mg# (44.71–64.21), and possesses metaluminous to weakly peraluminous features with ASI (aluminous saturation index) of 0.70 to 1.06. The chondrite-normalized REE patterns display the moderately enriched LREE ((La/Sm)N=2.41–4.55) and weakly differentiated HREE ((Gd/Yb)N=0.91–1.40) and negative to positive Eu anomalies (EuN/EuN⁎=0.68–1.86). The primitive mantle-normalized multi-element diagrams show enrichment in Ba, Th and K but depletion in Nb, Ta and Ti.The in situ zircon Lu–Hf isotopic data show that the Tongchang dioritic pluton has significantly variable Hf isotopic compositions with the εHf(t) values ranging from −12.1 to +3.3 and the Hf-depleted mantle model ages of 1.36 to 1.86Ga. Accounting the previous whole-rock Rb–Sr and Sm–Nd isotopic data, we conclude that the Tongchang dioritic pluton was produced by magma mixing, and the end-members of mixing magmas were derived from the depleted mantle, which was metasomatized by fluids fluxed from the subducted oceanic slabs and the lower continental crust, respectively. The above whole-rock geochemical data and petrogenetic studies suggest that the Tongchang dioritic pluton formed in a subduction-related arc setting.

Geochronological and geochemical constraints on the petrogenesis of late Triassic aluminous A-type granites in southeast China

The late Permian–Triassic granites in the South China Block (SCB) have important tectonic significance for the evolution of East Asia. Most of the intrusive bodies in the central SCB consist of S-type granites. Here, we report the first unambiguous discovery of Wengshan aluminous A-type granites in southwest Zhejiang Province, southeast China. SHRIMP and LA-ICP-MS zircon U–Pb analyses yielded similar ages about 224 Ma for two samples from the Wengshan pluton, in concordance with peak time of late Indosinian (234–205 Ma) tectonic magmatic activity in the SCB. The Wengshan granites are peraluminous with A/CNK values ranging from 1.09 to 1.24, enriched in K, Rb, Th, REE and Zr + Nb + Ce + Y, have elevated FeO tot/MgO and Ga/Al ratios, and are depleted in Mg, Ca, Mn, Ba, Sr, P and Ti. In addition, they show high zircon saturation temperatures (820–878 °C). Their REE patterns are highly fractionated, with (La/Yb) N ratios of 36.89–101.24 and strongly negative Eu anomalies (Eu/Eu * = 0.28 − 0.53). In-situ zircon Hf isotopic analyses indicate that the Wengshan granites have ε Hf( t) values ranging from −15.9 to −12.7 and two-stage depleted mantle Hf model ages from 2.05 to 2.26 Ga. Sr–Nd isotope systematics are characterized by a high initial 87Sr/ 86Sr ratios of 0.7092–0.7218 and a low ε Nd( t) values of −15.0 to −12.1, with two-stage depleted mantle Nd model ages of 1.99–2.22 Ga, consistent with those of Paleoproterozoic basement rocks in the area. It is inferred that the Wengshan granite magma formed by partial melting of Paleoproterozoic metasedimentary sources in Cathaysia Block. Meanwhile, the underplating of mafic magma in the lower crust may provide the heat source for the formation of the Wengshan granites. The data show that the Wengshan granites, unlike other S-type granites in the SCB, are late Triassic aluminous A-type granite. Therefore they may indicate distinct geodynamic significance. Our study of the Wengshan A-type granites (224 Ma), together with Jingju A-type granites (215 Ma) in Zhejiang Province and Gaoxi A-type granites (215 Ma), Xiaotao A-type granites (222 Ma), Tieshan (254 Ma) and Yangfang (242 Ma) alkaline syenites in Fujian Province, define important extensional events in Triassic. Considering these available data, we suggest that the late Permian–Triassic A-type granites and alkaline syenites in the coastal region of the SCB probably formed under an extensional regime related to oblique subduction of the paleo-Pacific Plate.

Zircons in the Shenglikou ultrahigh-pressure garnet peridotite massif and its country rocks from the North Qaidam terrane (western China): Meso-Neoproterozoic crust–mantle coupling and early Paleozoic convergent plate-margin processes

The North Qaidam terrane lies between the Qaidam and Qilian blocks of western China, and contains the Shenglikou ultrahigh-pressure (UHP) garnet peridotite massif. Simultaneously analyzed U–Pb ages, trace element patterns and Hf-isotope compositions of zircons from the peridotite massif and its country rocks (migmatite and gneiss) reveal the complex evolution of the terrane. Zircons in peridotite and pyroxenite from the massif document metamorphism at ca. 429 ± 2 Ma (2 σ, n = 30, MSWD = 1.03). The peridotitic and pyroxenitic zircons have similar trace-element patterns and ranges in ɛ Hf( t) (+3.5 to −7.0), suggesting that these zircons crystallized from a metamorphic fluid, sourced mainly from the surrounding garnet-bearing ultramafic rocks. The protolith of the peridotite massif is interpreted as a fragment of the metasomatized Archean subcontinental lithospheric mantle (SCLM) from the Qilian block. Hf depleted-mantle model ages ( T DM) mainly cluster in the interval 1.1–0.9 Ga, implying that the protolith of the massif probably underwent mantle metasomatism in Meso-Neoproterozoic time. Zircons from the migmatite yield concordant 206Pb/ 238U ages, with a major peak at 468 ± 4 Ma (2 σ, n = 7, MSWD = 2.2) and smaller populations of 1003 ± 6 Ma, 951 ± 8 Ma, 738 ± 4 Ma and 577 ± 14 Ma. Due to the systematically lower Th/U ratios and trace element concentrations in the early Paleozoic zircons, compared to the Proterozoic grains, we suggest the early Paleozoic (∼468 Ma) zircons were formed by metamorphic recrystallization of their Proterozoic precursors. Their variable Hf isotopic compositions, i.e., ɛ Hf( t) from +14.2 to −15.9, T DM from 1.7 Ga to 0.6 Ga and T crust (average crustal model ages) from 2.4 Ga to 0.6 Ga, imply complex origins, including the introduction of juvenile components during events at ∼1.1–0.9 Ga. Zircons from the gneiss show a major concordant population with 206Pb/ 238U ages of 430 ± 5 Ma (2 σ, n = 10, MSWD = 2.3) and minor populations with ages of 806 ± 13 Ma and 613 ± 24 Ma, indicating that metamorphic recrystallization of the protolith occurred in the early Paleozoic (∼430 Ma). The original protolith of the gneiss was ancient (ca. T crust = 2.8–1.8 Ga), with some Meso-Neoproterozoic (ca. 1.1 Ga) juvenile crustal components. These data together with published results suggest that the North Qaidam terrane is a collisional orogen with Archean crustal components, and the Qaidam and Qilian blocks underwent crust–mantle coupling events in Meso-Neoproterozoic time (∼1.1–0.9 Ga) and convergent plate-margin processes including the UHP metamorphism in early Paleozoic (∼468–430 Ma), i.e., the beginning of continental subduction at ∼468 Ma, the subsequent continental deep subduction and final continent–continent collision at ∼430 Ma.

Highly evolved Archean basement beneath the western Cathaysia Block, South China

How has the Earth’s continental crust evolved? Most of our knowledge comes from surface exposures, but zircon xenocrysts in volcanic rocks can provide samples of deeper crustal levels. The U–Pb and Hf-isotope systematics of xenocrystic zircons brought to the surface by the Cenozoic (48–49 Ma) Pingnan basaltic rocks and the Mesozoic (166 Ma) Pingle minettes in Guangxi Province (South China), suggest the presence of unexposed relict Archean basement beneath the western Cathaysia Block, where the oldest exposed rocks are Neoproterozoic–Phanerozoic in age. This basement has provided zircons with several distinct age populations: 3.85, 3.55, 3.3–3.2 and mainly 2.9–2.5 Ga. These have Hf depleted-mantle model ages ( T DM) of 2.5 to ⩾3.9 Ga. The oldest T DM (∼3.9 Ga) shows the existence of Paleo- to Eoarchean components in this area. This relict basement experienced complex modification, including the addition of juvenile mantle material (with ε Hf up to +12.7) at ca 3.6–3.2, 2.5, 1.0 and 0.5 Ga. The zircons also record thermal events that reworked (remelted) the older crustal components of the block at ca 2.0–1.8, 1.6–1.5 Ga and ∼80 Ma. Although these younger events have modified the Archean nature of the basement, it seems that they do not represent significant post-Archean crustal growth.

Zircon U-Pb geochronology, Hf isotopic composition and geological implications of the rhyodacite and rhyodacitic porphyry in the Xiangshan uranium ore field, Jiangxi Province, China

The Xiangshan uranium ore field is the largest volcanic rock hosted uranium deposit in China. The host rock is a volcanic intrusive complex, including rhyodacite, porphyroclastic lava and late stage sub-volcanic rocks. In this study, zircons from an early stage rhyodacite and a late stage rhyodacite porphyry were dated by SHRIMP and LA-ICP-MS U-Pb methods, and their Hf isotopic compositions were measured by LA-MC-ICP-MS. 206Pb/238U ages of 135.1±1.7 and 134.8±1.1 Ma were obtained for the rhyodacite and rhyodacitic porphyry, respectively. These accurate ages indicate that the Xiangshan volcanic-intrusive complex formed in the Early Cretaceous rather than in the Late Jurassic, as concluded in some previous studies. By the Early Cretaceous, the tectonic setting of the area has evolved into a back-arc extensional setting, possibly related to subduction of the paleo-Pacific plate. The close ages of the (early) eruptive rhyodacite and the (late) hypabyssal rhyodacitic porphyry shows that the Xiangshan volcanism was intensive and concentrated in a short time. Zircons from the rhyodacite show negative eHf(t) values of −5.7 to −8.5, with Hf depleted mantle model ages between 1550 and 1720 Ma, whereas zircons from the rhyodacitic porphyry yield eHf(t) values of −6.9 to −10.1 and Hf model ages between 1621 and 1823 Ma. These zircon Hf model ages are similar to the whole rock Nd model ages (1486 to 1911 Ma). Combined with other geochemical characteristics, the Xiangshan rhyodacite and rhyodacitic porphyry may have been derived from partial melting of the Paleo-Mesoproterozoic metamorphic rocks from the Xiangshan basement, without any significant addition of mantle-derived magma. Contribution of basement of this age is also supported by finding a Paleoproterozoic xenocrystic zircon core in the rhyodacite sample.

Nd–Sr–Hf–O isotope provinciality in the northernmost Arabian–Nubian Shield: implications for crustal evolution

Multi-isotope study including whole-rock Nd–Sr, single zircon Hf, and SIMS δ18O analyses of zircons sheds light on magma sources in the northernmost Arabian–Nubian Shield (ANS) during ~820–570 Ma. Reconnaissance initial Nd and Sr isotope data for the older rocks (~820–740 Ma) reaffirms previous estimates that early crustal evolution in this part of the shield involved some crustal contamination by pre-ANS material. Prominent isotope provinciality is displayed by post-collisional calc-alkaline and alkaline igneous rocks of ~635–570 Ma across a NW-SE transect across basement of the Sinai Peninsula (Egypt) and southern Israel. Silicic rocks of the NW-region are characterized by lower εNd(T)–εHf(T) and higher Sri and δ18O compared with rocks of the SE-region, and the transition between the regions is gradual. Within each region isotope ratios are independent of the extent of magma fractionation, and zircon cores and rims yield similar δ18O values. Comparison with southern segments of the ANS shows that the source for most ~635–570 Ma rocks can be modeled as the isotopically aged lower-intermediate crust in the ANS core (SE-region) and its northern, more contaminated ANS margins (NW-region). Nevertheless, Nd–Sr isotope enrichment of the lithospheric mantle is indicated by some basic magmas of the NW-region displaying the most enriched Nd–Sr isotope compositions. Comparison of Nd and Hf depleted mantle model ages for rocks of the SE-region may indicate that crustal formation events in the ANS geographical core took place at 1.1–1.2 Ga and were followed by crustal differentiation starting at ~0.9 Ga.

Timing of crust formation and recycling in accretionary orogens: Insights learned from the western margin of South America

Abstract Accretionary orogens are considered major sites of formation of juvenile continental crust. In the central and southern Andes this is contradicted by two observations: siliciclastic fills of Paleozoic basins in the central Andean segment of the accretionary Terra Australis Orogen consist almost exclusively of shales and mature sandstones; and magmatic rocks connected to the Famatinian (Ordovician) and Late Paleozoic magmatic arcs are predominantly felsic and characterized by significant crustal contamination and strongly unradiogenic Nd isotope compositions. Evidence of juvenile crustal additions is scarce. We present laser ablation (LA)-ICPMS U–Pb ages and LA-MC-ICPMS Hf isotope data of detrital zircons from seven Devonian to Permian turbidite sandstones incorporated into a Late Paleozoic accretionary wedge at the western margin of Gondwana in northern Chile. The combination with Nd whole-rock isotope data permits us to trace the evolution of the South American continental crust through several Proterozoic and Paleozoic orogenic cycles. The analyzed detrital zircon spectra reflect all Proterozoic orogenic cycles representing the step-wise evolution of the accretionary SW Amazonia Orogenic System between 2.0 and 0.9 Ga, followed by the Terra Australis Orogen between 0.9 and 0.25 Ga. The zircon populations are characterized by two prominent maxima reflecting input from Sunsas (Grenville) age magmatic rocks (1.2–0.9 Ga) and from the Ordovician to Silurian Famatinian magmatic arc (0.52–0.42 Ga). Grains of Devonian age are scarce or absent from the analyzed zircon populations. The Hf isotopic compositions of selected dated zircons at the time of their crystallization (eHf(T); T = 3.3–0.25 Ga) vary between − 18 and + 11. All sandstones have a significant juvenile component; between 20 and 50% of the zircons from each sedimentary rock have positive eHf(T) and can be considered juvenile. The majority of the juvenile grains have Hf-depleted mantle model ages (Hf TDM) between 1.55 and 0.8 Ga, the time of the Rondonia–San Ignacio and Sunsas orogenic events on the Amazonia craton. The corresponding whole-rock eNd(T) values fot these same rocks are between − 8 and − 3 indicating a mixture of older evolved and juvenile sources. Nd-depleted mantle model ages (Nd TDM⁎) are between 1.5 and 1.2 Ga and coincide broadly with the zircon Hf model ages. Our data indicate that the Paleo- and Mesoproterozoic SW Amazonia Orogenic System, and the subsequent Neoproterozoic and Paleozoic Terra Australis Orogen in the region of the central and southern Andes, developed following two markedly different patterns of accretionary orogenic crustal evolution. The SW Amazonia Orogenic System developed by southwestward growth over approximately 1.1 Ga through a combination of accretion of juvenile material and crustal recycling typical of the extensional or retreating mode of accretionary orogens. In contrast, the central Andean segment of the Terra Australis Orogen evolved from 0.9 to 0.25 Ga in the compressional or advancing mode in a relatively fixed position without the accretion of oceanic crustal units or large scale input of juvenile material to the orogenic crust. Here, recycling mainly of Mesoproterozoic continental crust has been the dominant process of crustal evolution.

Zircon Hf Isotopic Evidence for Mixing of Crustal and Silicic Mantle-derived Magmas in a Zoned Granite Pluton, Eastern Australia

Zircon Hf isotopic data from a zoned pluton of the Moonbi supersuite, New England batholith, eastern Australia, are consistent with magma mixing between two silicic melts, each derived from isotopically distinct sources. Although zircons from three zones within the Walcha Road pluton give a U^Pb crystallization age of 249 � 3 Ma, zircon populations from each zone have a range in Hf. Zircons from the mafic hornblende^biotite monzogranite pluton margin and intermediate zones have eHf �þ 5t oþ11, whereas those from the more felsic centre of the pluton have eHf �þ 7t o þ16, representing a total variation of 11 eHf units. The Lu^Hf depleted mantle model ages range from � 650 to 250 Ma, with the younger zircons present only in the felsic pluton centre.The variation in eHf indicates the involvement of silicic melts from at least two sources, one a crustal component with a Neoproterozoic model age and the other a primitive mantle-derived component with model ages similar to the U^Pb crystallization age of the pluton. The zircons reflect the isotopic compositions of the different proportions of crustal-derived silicic melt, relative to mantle-derived silicic melt, between melt generation and final pluton construction. The Walcha Road pluton is considered to have formed by incremental assembly of progressively more felsic melt batches resulting from mixing, replenishment and crystal^melt separation, with final pluton construction involving mechanical concentration as zones of crystal mush. The zoned pluton and, more broadly, the Moonbi supersuite provide examples of magma mixing by which the more silicic units have more juvenile isotopic compositions as a result of increasing proportions of residual melt from basalt fractionation, relative to crustal partial melt.

Zircon U–Pb age and Sr–Nd–Hf isotope geochemistry of the Panzhihua A-type syenitic intrusion in the Emeishan large igneous province, southwest China and implications for growth of juvenile crust

The late Permian Panzhihua syenitic stock in the Pan-Xi area, SW China, showing the typical association of mantle-derived mafic and alkaline rocks along with silicic units, is genetically related to the upwelling Emeishan plume head. This syenitic intrusion consists mainly of metaluminous syenite, metaluminous to peralkaline quartz syenite with subordinate syenite porphyry and alkali-feldspar granite. SHRIMP U–Pb zircon analysis reveal that the Panzhihua syenitic pluton was emplaced at 253.1 ± 1.9 Ma, and is slightly younger than the crystallization age of the spatially associated gabbroic intrusion. These syenitic and granitic rocks have a distinctive A-type chemistry characterized by elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios (3.95 to 6.79). Nb/Ta and Zr/Hf ratios of the syenitic and granitic rocks, gabbros and syenodiorites are similar to those typical of oceanic island basalt (OIB). These granitoids all have relatively low initial 87Sr/ 86Sr ratios (0.7037–0.7058), positive ε Nd( t) values (+ 2.4 to + 3.5). The magmatic zircons from the syenites exhibit positive ε Hf( t) values between 5.9 and 10.4, which correspond to single-stage depleted mantle Hf model ages ( T DM1) of 494 to 671 Ma. In contrast, the magmatic zircons from the syenite porphyries are characterized by positive ε Hf( t) values of 11.4 to 12.9 with younger T DM1 model ages of 382 to 449 Ma than those of the syenites. The positive ε Nd( t) and ε Hf( t) values of the Panzhihua syenitic intrusion indicate a significant contribution of plume-derived basaltic underplating to magma genesis. The similarity of the Nd isotopic compositions of the syenitic and granitic rocks, gabbros and syenodiorites suggests that their parental magmas were derived from a common reservoir. We propose that the Panzhihua syenitic intrusion was predominantly generated by the emplacement of two distinct syenitic melts from differentiation of newly underplated, mildly alkaline basaltic magmas ponded at depth, with incorporation of little Neoproterozoic lower crustal melts, and subsequent fractional crystallization of individual melts. It is therefore demonstrated that the growth of juvenile crust through mantle-derived underplating during the late Permian was significant in the inner zone of the Emeishan large igneous province.

Paleoproterozoic crustal growth in the Western Block of the North China Craton: Evidence from detrital zircon Hf and whole rock Sr-nd isotopic compositions of the Khondalites from the Jining Complex

Detrital zircons from the khondalites in the Western Block of the North China Craton were mainly derived from a 1.9 to 2.1 Ga Paleoproterozoic provenance. Most of the more than 200 detrital zircons analyzed in this study have positive initial eHf values and a wide range of Hf depleted mantle model ages (TDMC) that range from 2.08 Ga to 2.74 Ga, with a peak at 2.24 to 2.40 Ga. The whole rock Nd isotopic analyses for the khondalite samples give quite uniform 143Nd/144Nd ratios from 0.511188 to 0.511858, corresponding to initial eNd (2.10Ga) values ranging from -1.6 to +0.6. In contrast, Sr isotopic compositions of these samples are more varied, with initial 87Sr/86Sr ratios at 2.10 Ga ranging from 0.7017 to 0.7076. In the eNd versus 87Sr/86Sr correlation diagram, most samples plot near the mixing hyperbola of the depeleted mantle and crust, with the contribution of depleted mantle about 65 to 75 percent. These results suggest that a large amount of juvenile materials from the depleted mantle was accreted to the Western Block at 2.1 Ga, a period of magmatic quiescence in many cratons of the world.

Zircon U–Pb age, Hf and O isotope constraints on protolith origin of ultrahigh-pressure eclogite and gneiss in the Dabie orogen

Zircon U–Pb dating, Hf and O isotope analyses were carried out for ultrahigh-pressure eclogite and felsic gneiss from the Dabie orogen in China. The results provide constraints on their protolith origin, with significance for continental growth by rift magmatism during splitting of the Yangtze Block from the supercontinent Rodinia. Mafic and felsic protoliths formed as a bimodal volcanic suite at about 750 Ma, with incorporation of ca. 2.15 Ga crust into the felsic protolith. δ18O values of −4.1‰ to 4.4‰ suggest differential involvement of meteoric water in protolith magmas. Initial Hf isotope ratios are subdivided into two groups, with positive εHf(t) values of 12.9±0.7 to 5.9±0.9 and neutral εHf(t) values of 2.3±0.3 to −2.7±0.6, respectively. The positive εHf(t) values correspond to depleted mantle Hf model ages of 0.82 to 1.24 Ga, whereas the negative εHf(t) values correspond to crust Hf model ages of 1.82±0.07 Ga. A few zircons from the gneiss have strongly negative εHf(t) values of −22.6±0.6, which are associated not only with consistent depleted mantle and crust Hf model ages of 2.2 Ga but also with the old U–Pb ages falling on a 2147±22 Ma discordia line. Thus the felsic protolith contains Paleoproterozoic crustal relicts, some of which were originally derived from coeval depleted mantle. On the other hand, strongly positive εHf(t) values are associated with young U–Pb ages falling on the ∼750 Ma discordia lines. This suggests that both eclogite and gneiss protoliths principally formed by immediate reworking of mid-Neoproterozoic juvenile crust, with contrasting features only in petrochemistry. The highest εHf(t) values of 12.7 to 14.4 for the eclogite correspond to the youngest Hf model ages of 0.75 to 0.82 Ga relative to the depleted mantle reservoir. These Hf model ages are not only close to the timing of zircon growth from the mafic magma, but also similar to ages for bimodal magmatism in the periphery of the Yangtze Block. This demonstrates new addition of the depleted mantle material to the continental crust by rift magmatism in the northern margin of the Yangtze Block, with coeval crust–mantle interaction in the mid-Neoproterozoic rifting tectonic zone. Therefore, the bimodal magmatism at about 750 Ma transports both heat and material from the mantle to the crust, resulting in reworking of both the Paleoproterozoic old crust and the meteoric-hydrothermally altered juvenile crust along the active rifting zone.

Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U–Pb isotopic analyses of mantle zircons

In-situ Hf isotope analyses and U–Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U–Pb zircon results yield a wide range of ages (∼2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U–Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U–Pb date. The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U–Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (∼2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between ‘crustal’-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the ∼2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield ‘fictitiously’ older Hf model ages.