Hf Depleted Mantle Model Ages Research Articles

Late Neoproterozoic tholeiitic plutonism in far-east Avalonia (the Dirgine Batholith, Istanbul Zone): Exposed mid-crustal section of an oceanic arc

Avalonia, a large composite terrane, extends from the northeastern regions of America and Canada to southern Britain and Belgium/Netherlands, continuing on to Poland. It was accreted to Laurentia and Baltica during early Paleozoic. The late Neoproterozoic basement of the Istanbul Zone (NW Turkey), which is considered the far-east extension of Avalonia, is poorly known. This study deals with the petrology and age of the Dirgine Batholith, the largest component of the late Neoproterozoic basement. The batholith, ∼70 km long and ∼12 km wide, was emplaced into Neoproterozoic low-grade mafic to felsic metavolcanic and metapyroclastic rocks. Tonalite and quartz diorite form the bulk of the batholith. Trondhjemite, granodiorite, and anorthositic gabbro occur in minor amounts. Overall, rock types resemble low-Al tonalite-trondhjemite-granodiorite series and trondhjemites in ophiolite sequences. The batholith formed over a narrow time interval (562–574 Ma, late Ediacaran), as inferred by U–Pb zircon geochronology. In-situ Lu-Hf analyses of igneous zircons reveal a limited range of significantly positive initial εHf values (9.47 ± 1.46, 2σ) and young Hf depleted mantle model ages (0.78 ± 0.06 Ga), indicating the relatively juvenile character of original magmas. All rock types have typical subduction-related trace-element signatures such as depletion in high-field-strength elements and enrichment in several fluid-mobile elements as well as nearly flat chondrite-normalized REE patterns. Geochemical characteristics indicate that crustal assimilation was insignificant, and the batholith formed by fractional crystallization of mantle-derived low-K tholeiitic magmas involving mainly clinopyroxene, hornblende and ± plagioclase. The Dirgine Batholith and metamorphic country rocks probably represent the mid-crustal section of a late Neoproterozoic oceanic arc. The interpretation of an oceanic rather than continental arc is based on the significantly lower concentrations of incompatible elements relative to continental arcs, nearly flat REE patterns of the rock types, and the juvenile nature of the primary magmas. These data, in concert with those in the literature, reveal that late Neoproterozoic oceanic arcs constitute an important component of far-east Avalonian terranes.

Extreme geochemical fractionation during mantle melting: Insights from Hf-Nd isotopically ultra-depleted eclogite

Extreme fractionation of elements and isotopes in mafic igneous rocks is abnormal in deciphering the source nature and melting conditions of mafic magmatism. Especially, identification of geochemically ultra-depleted mafic melts and their mantle sources has great bearing on the property of crust-mantle differentiation at plate margins. This is illustrated by extreme Hf-Nd isotope fractionation in ultrahigh-pressure eclogites from the Sulu orogen in east-central China. In addition to the previous finding of ultrahigh εNd(0) values, we report here new data of whole-rock trace elements and Lu-Hf isotopes in eclogites and related rocks from the same region. The present results show extremely high Lu/Hf ratios and abnormally high εHf(0) values of up to 576 for the eclogites, significantly different from the garnet amphibolites and other eclogite-facies metamorphic rocks in the same orogen. This feature is coupled with the ultrahigh εNd(0) values as well as the severe depletion of light rare earth elements (LREE) and high field strength elements (HFSE). Because HFSE and LREE are immobile in aqueous solutions and the effect of melt extraction is insignificant during the continental deep subduction, the extreme fractionation of Lu/Hf and Sm/Nd indicate their origination from a geochemically ultra-depleted mantle source. These eclogites have the depleted mantle Hf model ages of 1.27 Ga to 1.61 Ga, similar to the depleted mantle Nd model ages of 1.39 Ga to 1.67 Ga as previously reported. This suggests that the protolith of the extremely high εHf-εNd eclogites was a kind of mafic igneous rocks derived from fractional crystallization of geochemically ultra-depleted mafic melts, which were produced by partial melting of the highly refractory lithospheric mantle during a series of seafloor spreading initiation-failure cycles at a divergent plate margin after the breakup of supercontinent Columbia in the Early Mesoproterozoic. The mafic igneous rocks were located in a passive continental margin in the Late Paleozoic and experienced deep subduction and exhumation in the Triassic, giving rise to the presently studied eclogites. The ancient geochemical signatures were retained without considerable influence by mantle convection, providing insights into the nature of crust-mantle differentiation during the tectonic transition from supercontinental breakup to seafloor spreading beneath the sub-ridge lithospheric mantle.

Zircon within chromitite requires revision of the tectonic history of the Eoarchean Itsaq Gneiss complex, Greenland

The Ujaragssuit Nunât layered (UNL) unit in the Itsaq Gneiss Complex, west Greenland, has been considered to contain one of the oldest chromitites on Earth based on ∼ ca. 4.1 Ga Hadean whole rock Pt–Os model ages and ca. 3.81 Ga zircon U-Pb age of the surrounding orthogneiss. This study obtained zircon from the chromitite within this unit as well as granitoid sheets that intruded into the UNL unit. In–situ U–Pb–Hf–O isotope measurements were made on the zircons. Zircons from both the chromitite and the intrusive granitoids show concordant U–Pb ages of ca. 2.97–2.95 Ga. In contrast, Hf and O isotopic analyses indicate that zircons in the chromitites have a different origin from those in the intrusive granitoids. Zircons from granitoids yielded Th/U ratios higher than 0.2, initial Hf isotope ratios of 0.2805 – 0.2807 (i.e., initial εHf value of − 11 to − 5), and δ18O values of mostly 6.0‰–7.0‰, which are typical for felsic igneous rocks in Archean continental crust. The least altered zircons from a chromitite exhibited initial Hf isotope ratios of 0.28078 – 0.28084 (i.e., initial εHf value of − 1.1 to − 0.4), close the chondritic value at ca. 3.0 Ga and the depleted mantle at ca. 3.2 Ga. These zircons also have δ18O values of 4.2‰–6.1‰ which correspond to typical mantle values. The other chromitite zircons yielded Th/U ratios lower than 0.1, and Hf and O isotopic compositions ranging between the least altered zircons and the intrusive granitoid zircons. These results indicate that the zircons in the chromitites crystallized before or during the 2.97–2.95 Ga granitoid intrusion and most of the zircons were altered by subsequent metasomatism. Furthermore, the present results suggest that zircons in the chromitites originally had depleted Hf isotopic compositions at ca. 3.2–3.0 Ga. This can be explained by two different models of the evolution of the UNL unit. One is that if the UNL unit was formed at > 3.81 Ga as previously thought, with the zircons in the chromitites subsequently being precipitated by ca. 3.2–2.95 Ga during metamorphism or metasomatism. The other model is that the UNL unit itself was actually formed at ca. 3.2–3.0 Ga, with zircon in the chromitite representing the crystallisation age of the unit, which was then tectonically incorporated into the ca. 3.81 Ga orthogneiss prior to the 2.97–2.95 Ga granitoid intrusion event. In either case, our zircon analyses reveal significant evolutionary history prior to depleted mantle Hf model ages of 3.2–2.95 Ga. Revision of the geotectonic evolution of the UNL unit and the Itsaq Gneiss Complex is therefore required.

Petrogenesis and geodynamic implications of the Baimashan granitic complex in central Hunan, South China

The Baimashan granitic complex in central Hunan is a giant multiphase complex pluton within the South China Plate, which contains three phases (the Shuiche, Longtan‐Xiaoshajiang, and Longcangwan superunits). It is located in the Jiangnan Orogenic Belt and is of great significance for the study of the regional tectonic magmatic evolution. Herein, we utilized zircon U–Pb dating, major and trace element geochemistry, and zircon Hf isotope geochemistry for three of the Baimashan granitic complexes; data for the Longcangwan superunit were obtained from previous studies. The new zircon U–Pb ages of 418 ± 3.2 Ma and 223 ± 2.6 Ma for the Shuiche and Longtan‐Xiaoshajiang superunits, respectively, suggest the existence of Late Silurian and Late Triassic magmatism. Both superunits exhibited weak peraluminosity and high‐K calc‐alkaline and weak negative Eu anomalies, which are similar to those of I‐type granites. The Longcangwan superunit showed strong peraluminosity and high‐K calc‐alkaline and stronger negative Eu anomalies; Hf isotope and mineralogical evidence suggest that it is a fractionated I‐type granite. The zircon Hf(t) values and two‐stage depleted mantle Hf model ages revealed that the magma of the Baimashan granitic complex originates from the partial melting of metamorphic igneous rocks; the crust–mantle differentiation age in the source area was 1.5–1.7 Ga for Longtan‐Xiaoshajiang and Longcangwan and 1.7–1.9 Ga for Shuiche. The superunits formed in a post‐collisional extensional tectonic setting. In the case of the Shuiche supersuit this is related to the remote effect of the interaction of the South China Plate with the Australian‐Indian margin of Eastern Gondwana. The formation of the Longtan‐Xiaoshajiang and Longcangwan superunits is linked to the remote effect of the Indosinian and South China Plate collision and the superimposition of the subduction of the Palaeo‐Pacific Plate. The weakly fractionated I‐type granite that formed during the Caledonian in South China is closely related to scheelite mineralisation.

Zircon xenocrysts in Late Cretaceous magmatic rocks in the Kermanshah Ophiolite: link to Iran continental crust supports the subduction initiation model

ABSTRACT Geochronological and geochemical data show that Late Cretaceous Zagros ophiolites were generated by forearc seafloor spreading during a subduction initiation event along the southwestern margin of Iran. These ophiolites define a ~ 3000 km long belt passing from Cyprus to Turkey, Syria, Iran, the UAE, and Oman. Ten samples of plagiogranites, amphibole gabbros and diorites of the Kermanshah ophiolite yield U-Pb zircon magmatic ages indicating ophiolite formation at 98 to 96 Ma. Some Kermanshah rocks also contain abundant zircon xenocrysts with age peaks (in order of decreasing size): Triassic (~247 Ma), mid-Paleoproterozoic (~2150 Ma), Late Paleoproterozoic (~1850 Ma), Late Ediacaran (~560 Ma), Early Cretaceous (~140 Ma), and Tonian (~850 Ma). Similar age peaks are seen in magmatic and detrital rocks of Iranian crust to the north. The xenocrysts have εHf(t) values from +6.3 to – 21.6, corresponding to depleted mantle Hf model ages (TDM) of 0.6 to 3.6 Ga. The xenocryst age peaks are similar to those of zircons in Iranian continental crust. The occurrence of zircon xenocrysts in the Kermanshah ophiolite suggests that this formed adjacent to the Iranian continent, as expected for Late Cretaceous subduction initiation.

Evolution of the Neoarchean lower crust beneath Western Liaoning of the North China Craton: Evidence from U-Pb-Hf-O isotopes in zircon

A comprehensive analysis of U-Pb geochronology and Hf-O isotopes of zircons from granulite xenoliths hosted in the Cretaceous basalt and zircon xenocrysts in the Jurassic andesitic volcanic rocks of Western Liaoning within the North China Craton (NCC) provides an opportunity to understand the evolution of the lower crust in the region. Zircons from these granulites yield 207Pb/206Pb ages of 1678–2646 Ma and consistent upper intercept ages of 2514–2544 Ma. In contrast, zircon xenocrysts yield two periods of 207Pb/206Pb ages of Neoarchean (2500–2591 Ma) and Paleoproterozoic (1733–2499 Ma), which produce consistent upper intercept ages of 2543–2549 Ma and lower intercept ages of 1761–1789 Ma, respectively. The Neoarchean zircons from the granulites and xenocrysts generally have similar positive εHf(t) values resulting in Hf depleted mantle model ages (TDM) of 2.5–2.9 Ga with a mode of 2.7 Ga and variable δ18O (5.05‰ to 8.81‰) values. However, the δ18O values of zircons from the xenocrysts are a little bit higher than those of zircons from granulites (peak of 8.00‰ for the xenocrysts and 6.00‰ for the xenoliths). Some Paleoproterozoic zircon xenocrysts have negative εHf(t) values (−12.8 to −9.39) and similar 176Hf/177Hf ratios and δ18O values to those of the Neoarchean zircons, whereas the other Paleoproterozoic zircons show chondritic εHf(t) (−4.34 to 1.16) and higher δ18O (7.70‰ to 10.19‰) values. All these data demonstrate that the primordial lower crust beneath Western Liaoning originally formed in the Neoarchean, perhaps around 2.6–2.7 Ga. The ubiquitous presence of ca. 2.5 Ga magmatic zircons illustrates that an important melting event indeed happened in the lower crust beneath Western Liaoning, similar to the thermal event throughout the entire NCC. In addition, the Neoarchean lower crust underwent another important thermal event during the Paleoproterozoic, which was considered to be associated with the final amalgamation of the Western and Eastern blocks of the NCC. Thus, we can conclude that there indeed existed an Archean lower crust beneath the Western Liaoning until to the Late Mesozoic, and this Archean lower crust underwent a similar multiple-stage (Neoarchean and Paleoproterozoic) evolutionary history as those throughout the NCC.

Hf isotopic constraints and detrital zircon ages for the Austroalpine basement evolution of Eastern Alps: Review and new data

The Alps, as part of the Alpine-Mediterranean Mountain chain, are one of the classical localities for orogenic studies, where the Mesozoic-Cenozoic tectonic evolution is well known, and many classical models have been proposed to explain the tectonic evolution from Mesozoic rifting and breakup to Late Mesozoic-Cenozoic subduction, plate collision, and exhumation. However, the pre-Mesozoic tectonic evolution of the pre-Alpine basement remains poorly known because of the lack of sufficient age data due to complex polyphase deformation and multiple metamorphic overprints. New data from the mainly amphibolite-facies pre-Alpine basement of the Austroalpine mega-unit indicates that this basement is composed of a heterogeneous series of continental units, island arcs, ophiolites, subduction mélanges, accretionary wedges, and seamounts affected by different metamorphic grades. This study presents new results of LA-ICP-MS U-Pb dating and MC-ICP-MS Lu-Hf isotopic tracing of zircons from three key areas of Austroalpine basement, including the: i) Wechsel Gneiss and Waldbach Complexes, and Wechsel Phyllite Unit, (ii) Saualpe-Koralpe-Pohorje, and (iii) Schladming-Seckau areas. We determine the Wechsel Gneiss Complex to be a continental magmatic arc formed during 500–560 Ma in the proximity to a continental block with a ‘memory’ of Late Archean to Early Proterozoic continental crust. The Wechsel Gneiss Complex has U-Pb ages of 2.1 to 2.2 Ga and 2.5 to 2.8 Ga that indicate a close relationship to northern Gondwana, with depleted mantle Hf model ages as old as 3.5 Ga. The Wechsel Phyllite Unit structurally overlying the Wechsel Gneiss Complex has partly different sources, including juvenile crust formed at ca. 530 Ma. In contrast, the Waldbach Complex constantly added new crustal material during the 490–470 Ma period and bears considerably more positive εHf(t) values than the underlying Wechsel Gneiss Complex and gives relatively young, depleted mantle model ages of 700 to 500 Ma. The Waldbach Complex is, therefore, interpreted to be part of a magmatic arc that formed during closure of the Prototethys and was metamorphosed during Variscan orogenic events at ca. 350–330 Ma. The Schladming-Seckau and Wechsel Complexes represent a Cambro-Ordovician magmatic arc system formed by Prototethys subduction processes with the associated Late Neoproterozoic to Early Ordovician ophiolitic Speik complex having formed in its back-arc basin or as Prototethyan lithosphere. The Plankogel Complex and structurally overlying micaschist and amphibolite units represent accreted ocean, ocean island, and continent-derived materials, interpreted to be an accretionary complex formed during the Permo-Triassic closure of the Paleotethys. Many granites of the Austroalpine mega-unit with Permian ages (e.g., porphyric granite called Grobgneiss and other granite gneisses and associated pegmatites) were likely formed in an extensional environment that culminated in the opening of the Middle-Late Triassic Meliata oceanic rift. These granites formed by partial remelting of crust with mainly Middle Proterozoic Hf model ages. Taken all these data together, we find that the Austroalpine basement is heterogeneously composed and includes complexes of different ages, different tectonic evolutionary histories and different remolten sources representing different locations before final accretion. The composite of pre-Alpine complexes in the Austroalpine mega-unit likely assembled not earlier than Late Permian or Early Triassic.

Episodic Archean crustal accretion in the North China Craton: Insights from integrated zircon U-Pb-Hf-O isotopes of the Southern Jilin Complex, northeast China

Recent advances in high-precision in-situ analysis of zircon U-Pb-Hf-O isotopes have significantly improved our approach to studying the origin and evolution of Archean crust. This study presents high-precision in-situ zircon U-Pb-Hf-O isotopes for the Southern Jilin Complex (SJC) on the northeastern margin of the North China Craton (NCC) and explores the potential of such an integrated approach in understanding the Archean crust evolution and its tectonic implications. The SJC consists mainly of Archean granitoid gneisses hosting minor supracrustal rock xenoliths. Zircon U-Pb data from representative lithologies reveal three episodes of magmatism at ~2.78, ~2.69 and ~2.55 Ga, respectively. The former two granitoid magmatic events are recognized for the first time in NE NCC, while the latest one is widespread over the NCC. All metamorphic zircon domains document similar metamorphic ages of 2.51–2.48 Ga, consistent with the widespread metamorphism of ~2.50 Ga in the NCC. Most concordant magmatic zircons preserve “mantle-like” O isotopic signatures, indicating the magma sources were not contaminated by large amounts of high δ18O supracrustal sediments. Hf depleted mantle model ages of magmatic zircons display one predominant peak at 2.9–2.8 Ga and two subordinate peaks at 3.3–3.2 Ga and 2.7–2.6 Ga, indicating possible episodic crust-mantle differentiation in Southern Jilin. Integrated zircon U-Pb-O-Hf data from the SJC suggest that magma sources were mainly derived from multiple episodes of reworking of juvenile or ancient crust with mantle component addition, while recycling of sediments was trivial during the Neoarchean in this region. The spatial–temporal relations of magmatism, lack of supracrustal addition to magma sources, and other geological evidence from the NCC are in favor of a bottom-up geodynamic regime possibly related to plume activities, arguing against a widespread top-down mechanism involving modern-style subduction in the NCC during the Neoarchean.

Silurian to Early Devonian arc magmatism in the western Sakarya Zone (NW Turkey), with inference to the closure of the Rheic Ocean

The Rheic Ocean is the most significant Paleozoic ocean that detached peri-Gondwana terranes from the northern Gondwana margin throughout the closure of the Iapetus Ocean. The suture of the Rheic Ocean spreads from Mexico to the Middle East, and the timing of its final closure is well-documented by the rocks formed in the Variscan-Alleghanian-Ouachita orogeny which led to the formation of the supercontinent Pangaea. However, as robust paleomagnetic and quantitative data are mostly lacking, the onset and evolution of the subduction of the Rheic Ocean are highly speculative, and they require further confirmation. Recently, the well-preserved metagranitoids along the western Sakarya Zone (SZ) in Anatolia have been identified, and they provide new data that improve our knowledge on the evolution of the Early Paleozoic Rheic Ocean along the northern Gondwana. Here, we present new geochronological, in situ zircon Hf isotope, and whole-rock geochemical analyses of these metagranitoids from the western SZ to enhance our understanding of the subduction processes of the Rheic Ocean. LA-ICP-MS zircon U-Pb dating demonstrated that the Bozüyük and Borçak metagranitoids from the western SZ were emplaced during the Silurian to Early Devonian (431 ± 2.7 to 403 ± 3.5 Ma). The both granitoids have medium- to high-K, calc-alkaline magmatic character, and exhibit peraluminous to slightly metaluminous geochemical signature. They show a typical arc pattern in terms of trace elements and have a uniform, moderate negative εHf (t) of −3.2 to −9.7, with Mesoproterozoic Hf depleted mantle model ages (TDM1 = 1.2 to 1.4 Ga). The geochemical and isotopic characteristics are not consistent with those of depleted mantle melts and melts derived from the crustal rocks in an intracontinental environment. Instead, the parental magma is likely generated from the partial melting of a homogeneous and enriched mantle wedge source. We propose that the Silurian to Early Devonian arc-related magmatism is associated with a northward subduction episode of the Rheic Oceanic lithosphere beneath the peri-Gondwana terranes. Hence, we consider that the opening of Paleotethys Ocean formed in a back-arc basin of subduction in Andean style to the north rather than a continental rift to the south in response to south-directed and short-lived supra-subduction zone (SSZ)-type subduction during the Silurian to Early Devonian.

Bimodal magmatism in the Eastern Dharwar Craton, southern India: Implications for Neoarchean crustal evolution

Archean cratons provide windows to the crustal evolution history in the early Earth. The Dharwar Craton is one of the major cratonic blocks in Peninsular India and is composed of several ancient microcontinents where multiple continental growth and recycling occurred from 3.8 to 2.5 Ga. Here we investigate a suite of magmatic rocks (including granitoids, mafic magmatic enclaves, ultramafic inclusions and synplutonic mafic dyke) from the western margin of the Eastern Dharwar Craton which provide insights on bimodal magmatism in a subduction-related arc setting and crustal growth during Neoarchean. Field evidence and geochemical features suggest mixing and mingling of mafic and felsic magmas. Geochemical features including incompatible and high field strength element features of the enclaves indicate heterogeneous sources involving dominantly enriched mantle reservoirs with minor depleted mantle whereas the host granitoids display depleted to enriched source. We present zircon UPb age data from the various rock types and the combined results display unimodal distribution with a single peak of 207Pb/206Pb age at 2531 Ma (n = 159) suggesting a major Neoarchean bimodal magmatic pulse. The zircon UPb ages of the mafic enclaves indicate either similar ages or slightly younger ages as compared to their host granitoids implying that enclaves represent late stage injections into crystallizing host granitoid magmas. The trace and rare earth element data on zircon from these rocks are consistent with magmatic crystallization. Zircons from most of the rocks in our study display uniformly positive εHf(t) (0.3–8.6), suggesting depleted mantle (juvenile) source for the Neoarchean arc building, and formation of new crust. Some of the mafic magmatic and ultramafic enclaves, as well as the mafic dyke and charnockite display mixed positive and negative εHf(t) values (−7.0 to 3.8), implying mixed depleted mantle and reworked crustal components of Neoarchean to Mesoarchean age. Most of the Hf depleted mantle model ages (TDM; 2520–3115 Ma) and crustal residence ages (TDMC; 2507 Ma to 3452 Ma) fall within the region bounding CHUR and depleted mantle lines, suggesting that the dominant magma source involved juvenile (depleted mantle) components. Thus, latest Neoarchean marks a major phase of continent building in the Eastern Dharwar Craton, and the results presented in our study suggest that bimodal magmatism in a subduction-related setting identical to those in modern convergent margins contributed to crustal growth.

Downward rejuvenation of the continental lower crust beneath the southeastern North China Craton

Felsic to mafic granulite xenoliths and zircon xenocrysts from the Cenozoic (~1 Ma) Nushan basalts were analyzed to construct a lower-crustal column illustrating the fine structure and evolution of the deep crust beneath the southeastern North China Craton. Felsic granulites, sampled from the lowermost middle to the upper lower crust have a calculated Vp of 6.32 km/s. Relict magmatic zircons from them yield an upper intercept age of 2716 ± 32 Ma and depleted-mantle Hf model ages (TDM) of 3.1–2.9 Ga. Metamorphic zircons with ages of ca. 2100 Ma have initial 176Hf/177Hf ratios similar to the magmatic ones, implying in situ recrystallization of the older zircons. The intermediate granulites with Paleoproterozoic ages (2.1–1.9 Ga; whole-rock Nd TCHUR = 3.4 Ga and zircon Hf TDM = 3.1–2.8 Ga; Huang et al., 2004; Zheng et al., 2012) have a calculated Vp of 6.53 km/s, being considered that they underlie the felsic granulites. The lowermost crust is represented by mafic granulites (Vp = 6.86 km/s). With the exception of three inherited zircons from the mafic granulites, which yielded an upper intercept age of 2455 Ma and TDM of ca. 2.7 Ga, most zircons formed in the late Mesozoic (ca. 115 Ma). Zircon xenocrysts have concordant ages, including the Neoproterozoic, early Paleozoic, late Paleozoic, early Mesozoic and Cenozoic. Our data, integrated with published data, suggest that the lower crust is younger at the bottom and dominantly composed of felsic, intermediate and mafic granulites. This trend reflects the vertical accretion of the crust, including the addition of juvenile material during the Mesoarchean and Neoarchean, and repeated reworking during the Neoarchean, Paleoproterozoic and late Mesozoic. The Neoproterozoic, Paleozoic and early Mesozoic zircon xenocrysts may record the subduction events between the craton and the Yangtze Block, which also affected the nature of the lower crust beneath the southeastern part of the craton.

Dual Geochemical Characteristics for the Basic Intrusions in the Yangtze Block, South China: New Evidence for the Breakup of Rodinia

Neoproterozoic intraplate magmatic rocks are widespread in the Yangtze Block (YZB). The contrasting interpretations on their petrogenesis and tectonic evolution induce stimulating discussions on the coeval tectonic setting, including the two competing models of rift-related (R-model) and arc-related (A-model). Their main evidence is dominantly from felsic magmatic rocks. In contrast, the less evolved basic rocks are more suitable for tectonic setting discrimination. Here we study the Longtanqing basic intrusions (LTQ) that are exposed to the central part of the N–S trending Kangdian rift in the western YZB, by detailed geochemical and geochronological investigations. Zircon U–Pb dating of the two diabases from LTQ yield identical ages within error of 777 ± 17 Ma and 780 ± 5.3 Ma, respectively. LTQ rocks are characterized by low SiO2 (49.83–50.71 wt %), high MgO (5.91–6.53 wt %), and Cr (140–150 ppm) contents, supporting the significant mantle affinity. They also display dual geochemical characteristics, including a series of features of continental within-plate basalts (WPB, Ti/V = 37.3–47.5, Zr/Y = 3.4–3.8, Ta/Hf = 0.19–0.23), and the typical signatures of island arc basalt (IAB), such as highly depleted in HFSE and HREE, and enriched in LREE and LILE. Most zircon εHf(t) values are positive (1.6–9.4) while the corresponding Hf depleted mantle model ages (TDM1) range from 1.0 Ga to 1.3 Ga. In combination with the occurrence of inherited zircons (991–1190 Ma), it is suggested that their sources are dominantly derived from the lithospheric mantle that was reconstructed in the late Mesoproterozoic. Thus, LTQ is mainly formed by partial melting of the enriched lithospheric mantle, and subsequently assimilated by a juvenile crust during upwelling. The melt compositions are controlled by different degrees of the crystal fractionation of the dominant clinopyroxene and plagioclase with minor amphibole under high fO2 conditions. Combined with previous geochronological and geochemical data in the YZB, our new results support the theory that the R-model can be responsible for the petrogenesis of Neoproterozoic magmatic rocks in South China.

Origin of zircon-bearing mantle eclogites entrained in the V. Grib kimberlite (Arkhangelsk region, NW Russia): Evidence from mineral geochemistry and the U-Pb and Lu-Hf isotope compositions of zircon

The concentrations of major and trace elements in minerals, reconstructed whole-rock compositions of zircon-bearing equigranular eclogites from the V. Grib kimberlite pipe located within the Arkhangelsk Diamondiferous Province (North-Western Russia), and results of the U-Pb and Lu-Hf isotope analyses of zircon grains from eclogites and granulite xenoliths are reported. These data suggest that the equigranular eclogites could represent the fragments of mid-ocean-ridge basalt that were metamorphosed during Paleoproterozoic subduction at 1.7–1.9 Ga. The Hf isotope compositions of the eclogitic zircon display uniformity and indicate corresponding Hf-depleted mantle model ages of 2.2–2.3 Ga. The formation of zircon in eclogites could have resulted from interactions with metasomatic/subduction-related fluids just prior to, but associated with, Paleoproterozoic eclogite formation. A link between eclogitic zircon formation and continental lower-crustal rocks can be excluded based on differences in the Hf isotope compositions of eclogitic and granulitic zircon grains. The U-Pb upper intercept age of granulitic zircon of 2716 ± 61 Ma provides a new minimum age constraint for zircon crystallisation and granulite formation. The U-Pb ages obtained from granulitic zircon show two stages of Pb loss at 2.2–2.6 Ga and 1.7–2.0 Ga. The late Paleoproterozoic stage of Pb loss recorded in granulitic zircon is due to the intensive reworking of basement crustal rocks, which was caused by a tectonic process/subduction event associated with equigranular eclogite formation. Our data, along with evidence previously obtained from the V. Grib pipe coarse-granular eclogites, show at least two main subduction events in the lithospheric mantle of the Arkhangelsk region: the Archean (2.8 Ga) and Paleoproterozoic (1.7–1.9 Ga) subductions, which correspond to major magmatic and metamorphic events in the Baltic Shield.

Geochronology and petrogenesis of the granites in Malanyu Anticline in eastern North China Block

Widespread Mesozoic magmatism and intracontinental deformation is recorded in the North China Block (NCB), but few studies have considered the link between the emplacement of the granite and deformation. The Malanyu Anticline is a large-scale east-trending fold exposed in the eastern part of NCB and is intruded by many plutons. Whole rock geochemistry, LA-ICP-MS zircon U–Pb ages, Sr–Nd isotopic composition, as well as zircon Hf isotopic data for eight granitic bodies in the anticline have been studied in detail and are reported in this contribution. Fourteen zircon LA-ICP-MS U-Pb ages have been obtained ranging from ca. 218 to 126 Ma spanning the Triassic to Cretaceous, with a major population at ca. 220–160 Ma. This age range is different from rest of the Mesozoic igneous rocks in NCB, indicating the emplacement of the granites in the anticline was structurally controlled, although the regional tectonic setting during the Mesozoic played an important role in magmatism in the region. All of the granites in the Malanyu Anticline are enriched in light Rare Earth Elements relative to heavy Rare Earth Elements, and have similar trace elements patterns, with depletions in Ba, Nb, Sr, P and Ti, and enrichments in Rb, Th, U, K, La, Ce, Sm and Tb. The Triassic to Jurassic granites' two-stage depleted mantle Nd model (T2DM) ages are both Paleoproterozoic, with similar εNd(t) values and two-stage depleted mantle Nd model ages. Their in-situ zircon two-stage depleted mantle Hf model ages (tDM2) range from ca. 2861 to 1406 Ma. Integrated with the Sr-Nd-Hf data from previous studies, the Triassic Dushan Granite in the study area has high Sr and low Y assays derived from the partial melting of Archean TTGs in the lower crust with a minor contribution from a depleted mantle source. The emplacement was during a post-collisional setting following the closure of the Paleo-Asian Ocean along the Suolun Suture in the north. The emplacement of most of the Jurassic granites involved the superimposition of the initial subduction of the Paleo-Pacific Ocean plate beneath NCB, while the emplacement of the minor Jurassic and Cretaceous granites are probably related to the partial melting of the Precambrian crust due to widespread extensional tectonics at the northern margin of the NCB.

Triassic magmatism on the transition from Variscan to Alpine cycles: evidence from U–Pb, Hf, and geochemistry of detrital minerals

New data on the U–Pb geochronology and Hf-isotopic composition of detrital zircons, along with the geochemistry of garnets, chromian spinel and feldspars from Permian and Triassic volcano-sedimentary sequences in the Southern and Eastern Alps reveal similarities in the nature and tectonomagmatic setting of the Early Permian and the Middle Triassic magmatic events. Detrital zircon U–Pb geochronology displays magmatic activity at 290–280 Ma (Early Permian) and at 245–235 Ma (Middle Triassic). The two-stage Hf-depleted mantle model ages obtained from Permian and Triassic zircons cluster around 1.7–1.6 and 1.3–0.8 Ga. Epsilon Hafnium analysis on dated detrital zircons suggests mixing of mantle and crustal components into the parental magmas of Permian and Triassic magmatism, the latter showing less crustal contribution. The composition of detrital feldspars from the Permian and Triassic volcaniclastic sandstones reveals provenance from intermediate to acidic igneous rocks. Garnet in Middle and Late Triassic beds suggests the incorporation of material comparable to the mafic rocks of the Ivrea-Verbano Zone into the parental melts of the Triassic magmatism. Chromian spinel in syn-magmatic Carnian sandstones indicates an origin associated with MORB-type peridotites and therefore, corroborates a mantle contribution to the Middle Triassic magmatism. The integration of detrital zircon U–Pb ages, Hf isotopes and composition of detrital minerals suggests that continuous lithospheric stretching at the transition from the post-Variscan to the Alpine orogenic cycle, with periods of strike-slip movements, presumably led to an asymmetrical continental rifting process, in which the Middle Triassic magmatic episode represents the earliest stage of the Alpine Tethys oceanic domain.

The fate of zircon during UHT-UHP metamorphism: isotopic (U/Pb, δ18 O, Hf) and trace element constraints

Garnet–clinopyroxene ultra-high-pressure (UHP) rocks from the northern Bohemian Massif contain zircon with micro-diamond inclusions. Trace element concentrations, oxygen and hafnium isotopic composition and U–Pb age of distinct textural domains in zircon characterize their growth conditions and temporal evolution. Diamond-bearing zircon mantle domains with relicts of oscillatory zoning have uniform Th/U ratios (~0.1–0.2), high-Ti contents (110–190 ppm, corresponding to temperatures of at least 1100 °C), and some (two of 17 mantle analyses) preserve steep heavy rare earth element (HREE) patterns with YbN/GdN = 10–11, with a weak negative Eu anomaly. These signatures are consistent with crystallization from a melt under UHP/ultra-high-temperature (UHT) conditions. Some of the bright-cathodoluminscence (CL) rims preserve Th/U and Ti values characteristic of the zircon mantles, but others show elevated Th/U ratios of ~0.3–0.4 and lower Ti contents (20–40 ppm; only 13 ppm in a rare low-CL outer rim). As they feature flat HREE patterns and negative Eu anomalies and commonly make embayments and truncate the mantle zoning, we suggest that they have formed through recrystallization in the solid state during exhumation of the rock, when both garnet and plagioclase were stable. The three zircon domains, that is, cores, mantles and rims, yield U–Pb concordia ages of 340.9 ± 1.5, 340.3 ± 1.5 and 341.2 ± 3.4 Ma respectively. When linked to the previously reconstructed P–T path of the rock, the error limits of the zircon mantle and rim ages constrain the exhumation of the rocks from depth of ~140 km (UHP) to ~80 km (HP) to a minimum rate of 1.5 cm yr−1. The zircon cores are heterogeneous in terms of Th/U ratio (below 0.1 but also above 0.2) and REE characteristics, and their eHf values scatter between −15.7 and +4.8 with similar values for individual domains within a single zircon grain suggesting a very localized control on hafnium isotope composition on a grain scale. The non-equilibrated eHf values as well as a large range of the Hf-depleted mantle model ages possibly reflect the presence of a heterogeneous population of old zircon. Consequently, the uniform and young 238U/206Pb ages may represent (near-)complete resetting of the U–Pb geochronometer during the UHP–UHT event at c. 340 Ma through dissolution–reprecipitation process. In contrast to Hf, the oxygen isotope composition of zircon is homogeneous, ranging between 7.8‰ and 9.6‰ VSMOW, reflecting a source containing upper crustal material and homogenization at UHP–UHT conditions. Our study documents that continental crust was subducted to mantle depths at c. 340 Ma during the Variscan orogeny and was subsequently very rapidly exhumed, implying that the sequence of events was faster than can be resolved by the secondary ion mass spectrometry technique.

Melt-fluid infiltration in Archean suprasubduction zone mantle wedge: Evidence from geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes from Wynad, southern India

The southern Peninsular India preserves a number of Archean cratonic nuclei among which the Coorg Block is considered to represent a Mesoarchean microcontinent. Here we investigate the southern margin of this block where remnants of rare ultramafic rocks in association with arc magmatic rocks and accreted metasediments occur in the Wynad region. We present petrology, mineral chemistry, whole rock geochemistry and zircon U–Pb and Lu–Hf data from a suite of meta ultramafics and associated diorite, orthopyroxene-bearing felsic charnockite, and quartz-mica schist. The peak P–T conditions of metamorphism of these rocks are estimated at 720–840°C and 6.5–8.7kbar. The geochemical features of the ultramafic suite provide evidence for melting and metasomatism of lithospheric mantle in a suprasubduction zone (SSZ) setting with the involvement of both silicate melts and slab-dehydrated fluids. The parent melts were generated at varying depth levels extending from spinel to garnet stability fields. A subduction-modified, LREE replenished, metasomatically enriched mantle source endorses arc-like chemical signatures in the melts that were acquired through interaction between the depleted sub-arc lithospheric mantle and subduction components.We report the common presence of magmatic zircons in the meta ultramafic rocks with crystallization ages of 3.3–3.0Ga. Some of these rocks also carry another younger group of magmatic zircons with crystallization ages down to ca. 2.5Ga. Zircons in the orthopyroxene-bearing felsic charnockite also formed at 3.2–3.0Ga whereas those in the metadiorite crystallized at around 2.6–2.5Ga. The detrital zircons in the quartz mica schist show upper intercept age of 3.3Ga, and lower intercept at 2.5Ga. The Mesoarchean ages from the Wynad suite are consistent with the ages reported from the Coorg Block, whereas the Neoarchean ages correspond with those in the Nilgiri Block to the south. The Lu–Hf data from zircon grains in the various samples show both negative and positive εHf(t) values ranging from −18.23 to 13.98. The data yield Hf depleted mantle model ages (TDM) of 2562–3837Ma and Hf crustal residence model ages (TDMC) ranging from 2576Ma to 4009Ma, indicating magma derivation from both juvenile and reworked components of Neohadean to early-mid Archean age. Our results reveal a suprasubduction zone mantle wedge that has been repeatedly replenished through multiple events of subduction zone magmatism in the Mesoarchean and Neoarchean.

Triassic magmatism in the eastern part of the South China Block: Geochronological and petrogenetic constraints from Indosinian granites

The granitic dykes in the Badu Group, Zhejiang Province, South China provide important insights on tectonic setting and crustal evolution of the South China Block (SCB) and the Indochina Block during Triassic. Here we report LA-ICP-MS U–Pb data of granitic rocks from the Hucun and Kengkou which show early Triassic ages of 242 ± 2 and 232 ± 3 Ma, respectively, representing their timing of emplacement. The dyke rocks are enriched in K, Al, LREE, Rb, Th, U, and Pb, and are depleted in Nb, Ta, Sr, and Ti. The rocks are characterized by highly fractionated REE patterns with (La/Yb)N ratios of 28.46–38.07 with strong negative Eu anomalies (Eu/Eu* = 0.65–0.73). In situ Hf isotopic analyses of zircons from the Hucun granite yielded ɛHf(t) values of −13.9 to −6.4 and two-stage depleted mantle Hf model ages of 1.68–2.15 Ga, which indicate that the magma was formed by partial melting of the Paleoproterozoic metasedimentary protoliths in the Cathaysia Block. The zircons from the Kengkou granite have ɛHf(t) values ranging from −40.7 to 31.5 and yield two-stage depleted mantle Hf model ages of 0.99–2.49 Ga, indicating magma origin from a mixed source. The Hucun and Kengkou dykes, together with the Triassic A-type granites in SE China were probably generated during magmatism associated with crust–mantle decoupling along the convergent plate boundary between SCB and the Indochina Block.

Neoarchean convergent margin tectonics associated with microblock amalgamation in the North China Craton: Evidence from the Yishui Complex

Archean tectonic history of the North China Craton (NCC) involved complex processes of amalgamation of microcontinents along multiple subduction zones prior to the consolidation of the major crustal blocks and their assembly into unified cratonic architecture. Here we report a suite of granitoids, diabase, metabasalts, volcanic tuff, banded iron formations and quartzite from the Yishui Complex along the southern margin of the Jiaoliao microblock within the Eastern Block of the NCC. The geochemical features of the magmatic suite are consistent with calc-alkaline magmatism in a convergent margin setting. In tectonic discrimination diagrams, the mafic suite shows variable IAB, MORB and OIB affinities typical of rocks formed in an arc-related subduction environment. Zircon grains in most of the rocks from Yishui Complex display core–rim texture with the cores showing magmatic crystallization and the narrow structureless rims corresponding to metamorphic overgrowth. The 207Pb/206Pb ages of magmatic zircons show 2504±19Ma for the volcanic tuff, 2581±21Ma for the granitoid, 2501±19Ma for the metavolcanics, 2537±38Ma for the pyroxenite, and 2506±13Ma for the diabase. Metamorphism is constrained from the 2451±18Ma and 2466±23Ma age groups in the metavolcanics and (meta-) pyroxenites. Zircons from BIF show multiple population with the oldest showing a spot age of 2503Ma, followed by a number of distinct groups of Paleoproterozoic zircons corresponding to later thermal events. The oldest population of magmatic zircons from the quartzite shows 207Pb/206Pb mean age of 2495±24Ma. The dominantly positive εHf(t) values of the magmatic zircons from the Yishui suite are broadly consistent with a depleted mantle source with only minor input of crustal components. Their Hf crustal residence ages (TDMC) range from 2586 to 3181Ma and Hf depleted mantle model ages (TDM) are in the range of 2548–2927Ma. The data indicate that magma production involved Meso- to Neoarchean juvenile sources within a continental arc setting, suggesting the Jiaoliao microblock as one of the ancient continental nuclei in the NCC. We trace the continuity of a Neoarchean subduction system along the western and southern margins of the Jiaoliao microblock with convergence of the Qianhuai and Xuhuai microblocks towards the Jiaoliao microblock with subduction–accretion–collision during the Archean–Proterozoic transition.

Zircon U–Pb ages, geochemical and Sr–Nd–Pb–Hf isotopic constraints on petrogenesis of the Tarom-Olya pluton, Alborz magmatic belt, NW Iran

A petrological, geochemical and Sr–Nd–Pb isotopic study was carried out on the Tarom-Olya pluton, Iran, in the central part of the Alpine–Himalayan orogenic belt. The pluton is composed of diorite, monzonite, quartz-monzonite and monzogranite, which form part of the Western Alborz magmatic belt. LA–ICP–MS analyses of zircons yield ages from 35.7±0.8Ma to 37.7±0.5Ma, interpreted as the ages of crystallization of magmas. Rocks from the pluton have SiO2 contents ranging from 57.0 to 69.9wt.%, high K2O+Na2O (5.5 to 10.3wt.%) and K2O/Na2O ratio of 0.9 to 2.0. Geochemical discrimination criteria show I-type and shoshonitic features for the studied rocks. All investigated rocks are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), depleted in high-field strength elements (HFSEs), and show weak or insignificant Eu anomalies (Eu/Eu*=0.57–1.02) in chondrite-normalized trace element patterns. The Tarom-Olya pluton samples also show depletions in Nb, Ta and Ti typical of subduction-related arc magmatic signatures. The samples have relatively low ISr (0.7047–0.7051) and positive εNd(36Ma) (+0.39 to +2.10) values. The Pb isotopic ratios show a (206Pb/204Pb)i ratio of 18.49–18.67, (207Pb/204Pb)i ratio of 15.58–15.61 and (208Pb/204Pb)i ratio of 38.33–38.77. The εHf(t) values of the Tarom-Olya pluton zircons vary from −5.9 to +8.4, with a peak at +2 to +4. The depleted mantle Hf model ages for the Tarom-Olya samples are close to 600Ma. These isotope evidences indicate contribution of juvenile sources in petrogenesis of the Tarom-Olya pluton. Geochemical and isotopic data suggest that the parental magma of the Tarom-Olya pluton was mainly derived from a sub-continental lithospheric mantle source, which was metasomatized by fluids and melts from the subducted Neotethyan slab with a minor crustal contribution. Subsequent hot asthenospheric upwelling and lithospheric extension caused decompression melting in the final stage of Neotethyan subduction through slab rollback or shortly after the cessation of arc magmatism.