Zircon Hf Isotopic Compositions Research Articles

Detrital zircons from Neoproterozoic sedimentary rocks in the Yili Block: Constraints on the affinity of microcontinents in the southern Central Asian Orogenic Belt

The Yili Block is one of the Precambrian microcontinents dispersed in the Central Asian Orogenic Belt (CAOB). Detrital zircon U–Pb ages and Hf isotopic data of Neoproterozoic meta-sedimentary rocks (the Wenquan Group) are presented to constrain the tectonic affinity and early history of the Yili Block. The dating of detrital zircons indicates that both the lower and upper Wenquan Groups have two major populations with ages at 950–880Ma and 1600–1370Ma. Moreover, the upper Wenquan Group has two minor populations at ~1100Ma and 1850–1720Ma. According to the youngest age peaks of meta-sedimentary rocks and the ages of related granitoids, the lower Wenquan Group is considered to have been deposited during the early Neoproterozoic (900–845Ma), whereas the upper Wenquan Group was deposited at 880–857Ma. The zircon εHf (t) values suggest that the 1.85–1.72Ga source rocks for the upper Wenquan Group were dominated by juvenile crustal material, whereas those for the lower Wenquan Group involved more ancient crustal material. For the 1.60–1.37Ga source rocks, however, juvenile material was a significant input into both the upper and lower Wenquan Groups. Therefore, two synchronous crustal growth and reworking events were identified in the northern Yili Block at ca. 1.8–1.7Ga and 1.6–1.3Ga, respectively. After the last growth and reworking event, continuous crustal reworking took place in the northern Yili Block until the early Neoproterozoic. Comparing the age patterns and Hf isotopic compositions of detrital zircons from the Yili Block and the surrounding tectonic units indicates that the Yili Block has a close tectonic affinity to the Chinese Central Tianshan Block in the Precambrian. The Precambrian crustal evolution of the Yili Block is distinct from that of the Siberian, North China and Tarim Cratons. Such difference therefore suggests that the Yili Block and the Chinese Central Tianshan Block may have been united in an isolated Precambrian microcontinent within the CAOB rather than representing two different blocks rifted from old cratons on both sides of the Paleo-Asian Ocean.

From source-to-sink: The Late Permian SW Gondwana paleogeography and sedimentary dispersion unraveled by a multi-proxy analysis

The Late Permian sedimentary succession of the Paraná Basin, southern Brazil, provide a valuable source of information about sediment provenance, tectonic processes and, consequently, the paleogeography of the southwestern Gondwana supercontinent. In order to understand the patterns of sedimentary dispersal and reconstruct the Late Permian source-to-sink dynamic, we report a complete series of U-Pb ages and Hf isotopic compositions of detrital zircons from the Rio do Rasto Formation sandstones allied with detailed paleocurrent and sedimentologic data. Our integrated provenance study reveals a consistent sediment transport from the south to the north and northwest. According to the evaluation of zircon ages and Hf isotopes, it was possible to determine four distinct source areas: (i) a distant Late Paleozoic active magmatic arc located in the southwestern Gondwana margin (i.e. Gondwanides Orogen), corresponding to the North Patagonian Massif; (ii) recycling of orthoquartzites from the uplifted Paleozoic Ventania Fold Belt and immature sandstones from the Claromecó Foreland Basin in central-eastern Argentina and the Silurian-Devonian successions of the southern Paraná Basin (central-northern Uruguay) and North Patagonian Massif; (iii) exhumed areas of the Archean-Paleoproterozoic basement and Neoproterozoic to Early Paleozoic mobile belts of the Damara in southwestern Africa and Ribeira Fold Belt in Uruguay and southern Brazil; and (iv) southeastward provenance of Grenvillian (1.2–1.0 Ga) zircons coming from the mafic to intermediate Mesoproterozoic igneous units of the Namaqua-Natal Belt in South Africa and Namibia. These data allow us to argue that sediments deposited in the Paraná Basin during the Late Permian come from both short- and long-distance source areas. In this context, an important population of Permian detrital zircons comes from the Gondwanides Orogen in the south, probably carried by transcontinental alluvial systems. Close to the source area, antecedent river channels would have eroded older sedimentary rocks of the Ventania Fold Belt, Claromecó and Chaco-Paraná basins and North Patagonian Massif, crosscutting a peripheral bulge and flowing towards the alluvial plain of the intraplate setting to areas further north. Concomitantly, these sediments are reworked by strong winds that even helped carrying the grains to the Paraná Basin depositional site.

Trace elements and Hf isotope composition as indicators of zircon genesis due to the evolution of alkaline-carbonatite magmatic system (Ilmeny-Vishnevogorsky complex, Urals, Russia)

We present results of investigation of the trace-element (REE, HFSE) and Hf isotope compositions and U-Pb age of single zircons crystallized from alkaline-carbonatite magmas of the Ilmeny-Vishnevogorsky complex (IVC) (Urals, Russia). It has been established that the geochemical characteristics of the early zircon (U-Pb age of 430-410 Ma) from alkaline rocks and carbonatites of this complex are determined mainly by the magmatic evolution of parental fluid-saturated alkaline-carbonatite melts and are highly associated with the cocrystallization of zircon and uranium-containing rare-metal minerals (gatchettolite and pyrochlore) at the final stages of the magmatic-system activity. The early zircons have a moderately depleted Hf isotope composition (eHf from +11.3 to +4.7), confirming the mantle nature of the magma source and indicating the participation of DM-like and enriched-source (probably, lower-crust component) substances in the magma generation. The considerable variations in the initial Hf isotope composition of the early zircons testify to the multistage zircon crystallization involving new portions of melts with different isotope compositions controlled by mixing of substances at their source. Late IVC zircons (250-350 Ma) have strongly disturbed “rejuvenated” isotope systems and a geochemical composition different from that of the magmatic zircons. They formed apparently at the metamorphic stage of the IVC evolution without a significant input of additional material.

Genesis of highly fractionated I-type granites from Fengshun complex: Implications to tectonic evolutions of South China

The South China Block is characterized by the large-scale emplacement of felsic magmas and giant ore deposits during the Yanshanian. We present zircon Hf isotopic compositions, whole-rock major and trace element compositions of the Fengshun complex, located in eastern Guangdong Province, South China. The Fengshun complex is a multi-stage magmatic intrusion. It is composed of two main units, i.e., the Mantoushan (MTS) syeno-monzogranites, alkali feldspar granites and the Hulutian (HLT) alkali feldspar granites. LA-ICPMS zircon dating shows that the complex emplaced in 166–161 and 139±2 Ma, respectively. Geochemically, the MTS granites show relatively various geochemical compositions with low REE contents (87.76×10-6–249.71×10-6), Rb/Sr ratios (1.19–58.93), pronounced Eu negative anomaly (0.01–0.37) and low Nb/Ta ratios (2.40–6.82). In contrast, the HLT granites exhibit relatively stable geochemical characteristics with high REE contents (147.35×10-6–282.17×10-6), Rb/Sr ratios (2.05–10.30) and relatively high Nb/Ta ratios (4.45–13.00). The isotopic data of the MTS granites display relatively enriched values, with I Sr varying from 0.708 2 to 0.709 7, e Nd(t) from -7.8 to -6.9 and e Hf(t) from -7.4 to -3.2, in comparison with those of the HLT which are ISr=0.703 05–0.704 77, e Nd(t)=-5–-3.4 and e Hf(t)=-0.7–1.8). The two-stage model ages of the MTS granites (T 2DM(Nd)=1.51–1.59 Ga and T 2DM(Hf)=1.26–1.48 Ga) are also higher than those of the HLT granites (T 2DM(Nd)=1.21–1.34 Ga and T 2DM(Hf)=0.96–1.10 Ga). Thus the MTS and HLT granites might originate from different sources. The former is more likely derived from partial melting of Meso-Proterozoic basement triggered by upwelling of asthenosphere and/or underplate of the basaltic magma and then extensive fractional crystallisation, similar to the genesis of Early Yanshanian granitoids of the EW-trending tectono-magmatism belt in the Nanling range. In comparison, the latter might have involved with asthenosphere component, similar to the Early Cretaceous granitoids of NE-NNE-trending granitoid-volcanic belt in coastal region, southeastern China. We propose that the MTS granites were mainly formed in Paleo-Tethyan post-orogenic extensional tectonic setting whereas the HLT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represents the initial time of roll-back of the paleo-Pacific Plate in SE-trending.

Partial melting of the South Qinling orogenic crust, China: Evidence from Triassic migmatites and diorites of the Foping dome

The Qinling orogen was ultimately formed by suturing of the South Qinling and Yangtze blocks, but the exact timing of the final amalgamation of the two blocks has not been well established so far. Partial melting of the Qinling orogenic continental crust resulted in the generation of migmatites, and such rocks may help to decipher the chronology of such event. In this paper, we report U–Pb ages, trace element, and Hf isotopic compositions of zircons from migmatites and diorite gneisses of the Foping dome, South Qinling. Zircons from migmatites form anhedral grains of variable sizes that are characterized by complex trace element compositions. Based on zircon U–Pb ages, the migmatites can be subdivided into two groups: Group 1 migmatites mainly retain Triassic zircons with U–Pb ages of 214–211Ma and Hf model ages of ~1.46Ga in core and rim domains; zircons from Group 2 migmatites record both Triassic (~210Ma) and Neoproterozoic U–Pb ages, analogous to igneous rocks of the Wudang and Yaolinghe Groups exposed in South Qinling. Zircons from the diorite gneisses yield U–Pb ages of 216–210Ma with Hf isotopic composition (TDM2 ages of ~1.46Ga) similar to the migmatites. Evidence from whole-rock Nd isotopic analyses also points to a similar genesis between migmatites and diorite gneisses. It is proposed that Group 1 migmatites were derived by melting of Triassic diorites, while Group 2 migmatites were derived from Neoproterozoic igneous rocks, a major basement lithology of South Qinling. Partial melting of the orogenic crust took place at ~214–210Ma, approximately consistent with the retrograde metamorphism of granulites exposed along the suture zone between the South Qinling and Yangtze blocks. We suggest that the collision of these two blocks occurred prior to ~215Ma and that the Foping dome resulted from rapid collapse of an overthickened crust followed by partial melting enhanced by asthenospheric influx.

Geochemistry and Hf–Nd isotope characteristics and forming processes of the Yuntoujie granites associated with W–Mo deposit, Guangxi, South China

The Yuntoujie W–Mo deposit is so far the only Indosinian granite-related deposit with an element association of W–Mo in the Nanling ore belt, South China. The granites in the Yuntoujie ore field consist of the Yuntoujie batholith and late-stage granites. The W–Mo mineralization is associated closely with the late-stage granites. Here we report major and trace element, Sr–Nd isotope and zircon Hf isotope compositions of the Yuntoujie batholith and the late-stage granites. The Yuntoujie batholith and the late-stage granites have high SiO2 (70.77–74.86wt.%) and alkaline (K2O+Na2O=6.27–8.27wt.%) concentrations and are characterized as peraluminous (A/CNK>1.1, A/NK=1.30–1.43). The Yuntoujie batholith and late-stage granites feature negative correlation between Th, Y and Rb, no positive correlation between P2O5 and SiO2 and high A/CNK values >1.1, suggesting that they could be classified as S-type granites. The Yuntoujie batholith and the late-stage granites might be sourced from the partial melting of Proterozoic crust without contribution from juvenile crust, as evidenced from the low 143Nd/144Nd values of 0.511942–0.512477, the ɛNd(t) values of −8.52 to −11.76 (falling in the field of Nd isotope evolution for the Proterozoic crust), and the zircon ɛHf(t) values of −7.19 to −14.89 with the TDM2 ranging from 1713Ma to 2187Ma. The major and trace element compositions of the Yuntoujie batholith and late-stage granites, especially the low CaO/Na2O and CaO/(MgO+FeOT) ratios and the high Rb/Ba and Rb/Sr ratios, further indicate that their sources are predominantly argillaceous sedimentary. The trace element characteristics, such as the positive correlation between La/Yb and La and between Ba and Sr and negative correlation between Ba and Rb, suggest that the late-stage granites are the product of highly fractionated melts of the Yuntoujie magma chamber. Moreover, 1) the ‘tetrad effect’ observed in the REE patterns for the late-stage tourmaline muscovite monzogranite porphyry and 2) the abundant fluid inclusions in their quartz phenocrysts imply that the processes of the volatile phase exsolution and the fluid-melt reaction had taken place in the evolved water-saturated magma. These geochemical features, together with the fact that the Yuntoujie magma chamber had survived for >10Myr, suggest that the peraluminous magma experienced a prolonged period of crystallization differentiation, which plays a key role in the formation of W–Mo mineralization associated with granites. From the observations that the granites in the Yuntoujie district were formed about 20–10Ma later than the peak age of the Indosinian collisions and that they all fall in the post-CEG area in the tectonomagmatic discrimination diagrams, we proposed that the Yuntoujie granites were formed in the post-collisional environment.

Crustal melting and magma mixing in a continental arc setting: Evidence from the Yaloman intrusive complex in the Gorny Altai terrane, Central Asian Orogenic Belt

Granitoids and their hosted mafic enclaves may retain important information on crust–mantle interaction, and thus are significant for study of crustal growth and differentiation. An integrated petrological, geochronological and geochemical study on the granitoid plutons of the Yaloman intrusive complex from the Gorny Altai terrane, northwestern Central Asian Orogenic Belt, was conducted to determine their source nature, petrogenesis and geodynamics. Mafic enclaves are common in the plutons, and a zircon U–Pb age (389Ma±4Ma) indicates that they are coeval with their granitoid hosts (ca. 393–387Ma). Petrographic observations reveal that these mafic enclaves probably represent magmatic globules commingled with their host magmas. The relatively low SiO2 contents (46.0–60.7wt.%) and high Mg# (38.9–56.5) further suggest that mantle-derived mafic melts served as a crucial component in the formation of these mafic enclaves. The granitoid hosts, including quartz diorites and granodiorites, are I-type in origin, possessing higher SiO2 contents (60.2–69.9wt.%) and lower Mg# (32.0–44.2). Their zircon Hf and whole-rock Nd isotopic compositions indicate that the magmas were dominated by remelting of Neoproterozoic (0.79–1.07Ga) crustal materials. Meanwhile, the geochemical modeling, together with the common occurrence of igneous mafic enclaves and the observation of reversely zoned plagioclases, suggests that magma mixing possibly contributed significantly to the geochemical variation of the granitoid hosts. Our results imply that mafic magmas from the mantle not only provided substantial heat to melt the lower crust, but also mixed with the crust-derived melts to form the diverse granitoids.The oxidizing and water-enriched properties inferred from the mineral assemblages and compositions imply that the granitoid plutons of the Yaloman intrusive complex were possibly formed in a continental arc-related setting, which is also supported by their geochemistry. The Devonian granitoids from the Gorny Altai terrane show remarkable temporal–spatial–petrogenetic affinities to the counterparts from the Altai-Mongolian terrane, indicating that these two terranes were possibly under subduction of the same oceanic plate (i.e., the Ob-Zaisan Ocean). The voluminous granitoids signify significant crustal recycling and growth as a response to the underplating of extensive mantle-derived basaltic melts.

Zircon U–Pb age and Sr–Nd–Hf isotope geochemistry of the Ganluogou dioritic complex in the northern Triassic Yidun arc belt, Eastern Tibetan Plateau: Implications for the closure of the Garzê-Litang Ocean

The Triassic Yidun arc belt (YAB) lies between the Jinshajiang suture zone to the west and the Garzê-Litang suture zone to the east, Eastern Tibetan Plateau. To study the YAB can not only help us to better understand the evolutionary history of the Garzê-Litang Ocean but can also provide some important information to constrain the evolution of the eastern Paleo-Tethys. In this paper, the geochronological and geochemical data of the Ganluogou dioritic complex were systematically investigated in order to decipher the geodynamic setting of the complex and to further determine the final closure time of the Garzê-Litang Ocean. The Ganluogou dioritic complex is located in the northern part of the YAB. It consists of ferrodiorite, diorite and a mixing zone between them and is the largest intermediate-mafic pluton in the YAB. The ferrodiorites were emplaced at 213±2Ma have low SiO2 and high Fe2O3* contents, whereas the diorites formed at 209±2Ma and have relatively higher SiO2, Na2O+K2O, Th, U, Zr, and Hf contents, but lower Al2O3, MgO, CaO, Co, and Sr contents than the ferrodiorites. Relative to the primitive mantle both the ferrodiorites and diorites are depleted in Nb and Ta. However, the ferrodiorites exhibit strong depletion in Zr and Hf, whereas the diorites contain relatively higher Th and U contents without negative Zr and Hf anomalies. Both rock-types have similar chondrite-normalized rare earth element patterns with (La/Yb)N ratios=4.4 to 18.2, and show weak Eu anomalies, with Eu/Eu* of 0.47 to 1. They both show narrow ranges in Sr–Nd–Hf isotopic compositions. However, the ferrodiorites contain lower initial 87Sr/86Sr ratios (0.7052–0.7057) and relatively higher εNd(t) values (−3.8 to −2.4) than the diorites, which record values of 0.7062–0.7066 and −5.5 to −5.7, respectively. For the zircon Hf isotopic composition, the ferrodiorites also exhibit higher 176Hf/177Hf ratios (0.282738–0.282804) and more depleted εHf(t) values (3.4–5.6) than those of the diorites (176Hf/177Hf ratios of 0.282690–0.282728 and εHf(t) values of 1.7–2.8). These geochemical and isotopic features suggest that the ferrodiorites and diorites were derived from the different magma sources. We propose that the ferrodiorites were produced by differentiation of mantle-derived tholeiitic arc magma, whereas the diorites were derived either from dehydration melting of meta-basaltic rocks from the Kangding Complex (the basement of the west Yangtze Craton) or young arc volcanic rocks but mixed with a small amount of sediments. Both ferrodiorites and diorites were emplaced at relatively low pressure (3.3–3.9kbar) but with quite different magma temperature (ferrodiorites: 821–960°C; diorites: 692–788°C). The Ganluogou dioritic complex was therefore formed under an extension setting that was associated with the closure of the Garzê-Litang Ocean. Previous studies on Late Triassic magmatism in the area have shown that there was slab-break off event at ~216Ma. Combined with regional geological background, we conclude that the final closure of the Garzê-Litang segment of the Paleo-Tethys Ocean occurred prior to 216Ma.

Clarifying the zircon Hf isotope record of crust–mantle evolution

Zircon has played a critically important role in our understanding of the growth and evolution of the Earth. The U–Pb isotope system as preserved in zircon, more than any other mineral or method, has provided the most precise geochronological constraints for timing of geological events and processes on the Earth. More recently, technological advances have allowed for the precise determination of the Hf isotope composition of zircon, a geochemical tracer that has provided important details on the Earth's chemical evolution, in particular the evolution of the crust–mantle system. When combined, U–Pb ages and Hf isotopes in zircons hold the promise of providing unprecedented resolution in the timing and processes of planetary differentiation. Nowhere is this more true than for the early history of the Earth, where younger tectonothermal processes have compromised the isotope information in bulk rock samples. With the promise of this integrated technique, however, lies numerous potential pitfalls in the acquisition and interpretation of these data. In this paper we review several important issues related to unraveling the complexities of integrated U–Pb age and Hf isotope datasets, especially with respect to understanding crust–mantle evolution. In particular, we address the potential difficulty of assigning accurate initial Hf isotope compositions as well as some of the inherent problems associated with so-called “depleted-mantle model ages”. Finally, we make some suggestions regarding the optimum analytical approach and presentation of the Hf (and Nd) isotope data to obtain the clearest record of Earth's chemical evolution.

Geochronology and petrogenesis of Miocene granitic intrusions related to the Zhibula Cu skarn deposit in the Gangdese belt, southern Tibet

The Zhibula Cu skarn deposit (19.5Mt ore @ 1.64%), near the Qulong porphyry Cu–Mo deposit, is located in the Gangdese porphyry copper belt in southern Tibet. The deposit is a typical metasomatic skarn that is related to the interaction of magmatic-hydrothermal fluids and calcareous host rocks. Stratiform skarn orebodies are mainly distributed in the contact between tuff and marble in the lower part of the Jurassic Yeba Formation. Endoskarn zonations for an outward trend are observed in the granodiorite, which grade from a fresh granodiorite to a weakly chlorite-altered granodiorite, a green diopside-bearing granodiorite, and a dark red-brown garnet-bearing granodiorite. The Zhibula granodiorite and monzogranite have similar secondary ion mass spectrometry (SIMS) zircon U–Pb ages of 16.9±0.3Ma and 17.0±0.2Ma, respectively. They exhibit different fractional crystallization from granodiorite (SiO2=64.8–69.3wt.%) to monzogranite (SiO2=72.3–76.8wt.%). Both the granodiorite and monzogranite are characterized by high Al2O3 (12.6–16.7wt.%) and K2O (1.5–5.5wt.%) contents, high Sr/Y (35–151) and La/Yb (19–48) ratios, and variable MgO (0.16–3.91) and Mg# (31–61) values. They display features of enrichment in large ion lithophile elements (LILEs, e.g., Rb, Ba, Sr, and K), depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and P), and moderate negative Eu anomalies (δEu=0.58–0.98). They show restricted in situ zircon Hf isotopic compositions (+6.7 to +8.8; only one sample is +4.5) and consistent δ18O values (+6.0‰ to +6.6‰). The geochemical data indicated that the Miocene Zhibula granitic intrusions formed by the magma that were characterized by high Sr/Y ratios and were derived from the partial melting of the thickened juvenile lower crust, which may have been metasomatized by the slab melts during subduction of the Neo-Tethyan oceanic crust and were induced by the convective removal of the thickened lithosphere. In addition, the Zhibula Cu skarn deposit and the Qulong Cu–Mo deposit form a significant porphyry–skarn ore system.

Early–Middle Paleozoic subduction–collision history of the south-eastern Central Asian Orogenic Belt: Evidence from igneous and metasedimentary rocks of central Jilin Province, NE China

To constrain the Early–Middle Paleozoic tectonic evolution of the south-eastern segment of the Central Asian Orogenic Belt (CAOB), we undertook zircon U–Pb dating and analyzed major and trace elements and zircon Hf isotope compositions of Late Cambrian to Middle Devonian igneous and metasedimentary rocks in central Jilin Province, NE China. LA–ICP–MS zircon U–Pb dating indicates that the Early–Middle Paleozoic magmatism in central Jilin Province can be divided into four episodes: Late Cambrian (ca. 493Ma), Middle Ordovician (ca. 467Ma), Late Ordovician–Early Silurian (ca. 443Ma), and Late Silurian–Middle Devonian (425–396Ma). The progression from subduction initiation to maturity is recorded by Late Cambrian low-K tholeiitic meta-diabase, Middle Ordovician medium-K calc-alkaline pyroxene andesite, and Late Ordovician to Early Silurian low-K tonalite, which all have subduction-related characteristics and formed in an evolving supra-subduction zone setting. Late Silurian to Middle Devonian calc-alkaline igneous rocks, with the lithological association of granodiorite, monzogranite, rhyolite, dacite, and trachydacite, show progressively increasing K2O contents from medium K to shoshonite series. Furthermore, the Early–Middle Devonian monzogranites are characterized by high K2O, Sr/Y, and [La/Yb]N values, indicating they were generated by the melting of thickened lower crust. These results suggest a transition from subduction to post-orogenic setting during the Late Silurian–Middle Devonian. Our interpretation is supported by the maximum age of molasse deposition in the Zhangjiatun member of the Xibiehe Formation. Overall, we suggest that Late Cambrian tholeiitic meta-diabase, Middle Ordovician pyroxene andesite, and Late Ordovician–Early Silurian tonalite formed above the northward-subducting and simultaneously seaward-retreating of Paleo-Asian Ocean plate. Subsequently, the northern arc collided with the North China Craton and post-orogenic extension occurred during the Late Silurian–Middle Devonian.

Post-collisional potassic magmatism in the eastern Lhasa terrane, South Tibet: Products of partial melting of mélanges in a continental subduction channel

Post-collisional, potassic magmatic rocks widely distributed in the eastern Lhasa terrane provide significant information for comprehensive understanding of geodynamic processes of northward subduction of the Indian lithosphere and uplift of the Tibetan Plateau. A combined dataset of whole-rock major and trace elements, Sr–Nd–Pb isotopes, and in situ zircon U–Pb dating and Hf–O isotopic analyses are presented for the Yangying potassic volcanic rocks (YPVR) in the eastern part of the Lhasa terrane, South Tibet. These volcanic rocks consist of trachytes, which are characterized by high K2O (5.46–9.30wt.%), SiO2 (61.34–68.62wt.%) and Al2O3 (15.06–17.36wt.%), and relatively low MgO (0.47–2.80wt.%) and FeOt (1.70–4.90wt.%). Chondrite-normalized rare earth elements (REE) patterns display clearly negative Eu anomalies. Primitive mantle-normalized incompatible trace elements diagrams exhibit strong enrichment in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and display significantly negative Nb–Ta–Ti anomalies. Initial isotopic compositions indicate relatively radiogenic Sr [(87Sr/86Sr)i=0.711978–0.712090)] and unradiogenic Nd [(143Nd/144Nd)i=0.512121–0.512148]. Combined with their Pb isotopic compositions [(206Pb/204Pb)i=18.615–18.774, (207Pb/204Pb)i=15.708–15.793, (208Pb/204Pb)i=39.274–39.355)], these data are consistent with the involvement of component from subducted continental crustal sediment in their source region. The whole-rock Sr–Nd–Pb isotopic compositions exhibit linear trends between enriched mantle-derived mafic ultrapotassic magmas and relatively depleted crustal contaminants from the Lhasa terrane. The enrichment of the upper mantle below South Tibet is considered to result from the addition of components derived from subducted Indian continental crust to depleted MORB-source mantle during northward underthrusting of the Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ~55Ma. Secondary Ion Mass Spectrometry (SIMS) U–Pb zircon analyses yield the eruptive ages of 10.61±0.10Ma and 10.70±0.18Ma (weighted mean ages). Zircon Hf isotope compositions [ƐHf(t)=−4.79 to −0.17], combined with zircon O isotope ratios (5.51–7.22‰), imply an addition of crustal material in their petrogenesis. Clinopyroxene-liquid thermobarometer reveals pressure (2.5–4.1kbar) and temperature (1029.4–1082.9°C) of clinopyroxene crystallization, suggesting that depth of the magma chamber was 11.6–16.4km. Energy-constrained assimilation and fractional crystallization (EC–AFC) model calculation indicates depth of assimilation and fractional crystallization in the region of 14.40–18.75km underneath the Lhasa terrane, which is in consistent with depth of the magma chamber as suggested by clinopyroxene-liquid thermobarometer. Based on the whole-rock major and trace elements and Sr–Nd–Pb isotope compositions, combined with EC–AFC modeling simulations and zircon Hf–O isotope data, we propose that the YPVR resulted from assimilation and fractional crystallization (AFC) process of the K-rich mafic primitive magmas, which were caused by partial melting of the Indian continental subduction-induced mélange rocks.

Different sources involved in generation of continental arc volcanism: The Carboniferous–Permian volcanic rocks in the northern margin of the North China block

New zircon U–Pb dating results on the Carboniferous–Permian volcanic rocks in the northern margin of the North China block (NCB) indicate their eruption during the Early Carboniferous to Late Permian from 347±3Ma to 258±1Ma and a slight decrease of the upper limits of the volcanic sequences from west to east. They have a main rock association of basalt, basaltic andesite, andesite, dacite, rhyolite, tuff, and tufaceous sandstone. Most of them have calc-alkaline compositions and exhibit variable SiO2 contents from 48.2wt.% to 77.1wt.%. There is no significant gap between the mafic and felsic volcanic rocks in major and trace element classification diagrams, indicating that they are not bimodal in composition. The Carboniferous–Permian volcanic rocks exhibit subduction-related geochemical features such as negative Nb and Ta anomalies of mafic to intermediate rocks on primitive mantle-normalized diagrams, indicating they were formed in an Andean-type continental arc during southward subduction of the Paleo-Asian oceanic plate beneath the northern NCB. However, their wide range of whole-rock Sr–Nd and zircon Hf isotopic compositions indicate that their source areas are very complex and different sources were involved in generation of these volcanic rocks. Geochemical and Sr–Nd–Hf isotopic results show that the basalt and some andesite were produced by fractional crystallization of mafic magma derived from partial melting of mantle wedge and subducted oceanic crust; however, most of the intermediate to felsic volcanic rocks were derived from partial melting of lower continental crust. There is an increasing input of crustal materials from the Carboniferous to Permian as indicated by increasing volumes of felsic volcanic rocks in the volcanic sequences. The results show that origin of the continental arc volcanism is very complex and both materials from the subducted oceanic crust and sediments, mantle wedge and arc continental crust could be involved in their generation. For the Permian andesite, dacite and rhyolitic volcanic rocks, there is a striking correlation between their Sr–Nd–Hf isotopic compositions and those of the underlying basement. Sr–Nd–Hf isotopic differences between the Permian intermediate to felsic volcanic rocks from either side of the northern boundary fault of the North China craton suggest that mixture and homogenization of the magmas in the melting–assimilation–storage–hybridization (MASH) zone within this continental arc was incomplete, and incomplete homogenization of magmas is likely a common feature of many MASH zones within continental arcs. Our results on the volcanic rocks also show that the final closure of the Paleo-Asian Ocean along the Solonker Suture Zone has not been finished prior to the Middle Permian and the decease of the Paleo-Asian oceanic subduction beneath the northern NCB in its middle-western parts is a little earlier than that in its eastern part.

U-Pb and Hf isotope analysis of detrital zircons from Paleozoic strata of the southern Alexander terrane (southeast Alaska)

The Alexander terrane is an unusual tectonic fragment in the North American Cordillera in that it contains a long and very complete stratigraphic record, including sedimentary or volcanic rocks representing every period and nearly every epoch between Neoproterozoic and Late Triassic time. The terrane is also unusual in that the southern portion of the terrane experienced arc-type magmatism during Neoproterozoic−early Paleozoic time, whereas the northern portion of the terrane consists mainly of Paleozoic shelf-facies strata. This long and diverse history provides opportunities to reconstruct the evolution and displacement history of the terrane, and specifically test the prevailing interpretation that the terrane formed in the paleo-Arctic realm. This study presents U-Pb geochronologic data and Hf isotopic information for detrital zircons from arc-type rocks in the southern portion of the terrane. Information has been generated from seven samples of Ordovician through Devonian age, complementing the information available from previous studies of Ordovician through Triassic strata. Together, these data sets yield a robust record of the magmatic history of the southern Alexander terrane, with dominant age groups of 640−550 Ma, 490−400 Ma, 380−340 Ma, and 310−275 Ma (dominant ages of 579, 441, 361, and 293 Ma). There are few pre−640 Ma grains in any of the samples. Hf isotope compositions of the detrital zircons are exceptionally juvenile, with most epsilon Hf (t) values between +15 and +5. Collectively, the available geologic, U-Pb geochronologic, and Hf isotopic evidence suggests that the southern Alexander terrane formed within a juvenile Neoproterozoic−early Paleozoic arc system, with little continental influence, whereas the northern portion of the terrane formed in proximity to a continental landmass that experienced similar Neoproterozoic−early Paleozoic ages of continental-affinity magmatism. Our data are consistent with previous suggestions that the Alexander terrane resided in the paleo-Arctic realm during early Paleozoic time, with the northern portion of the terrane adjacent to Baltica and the Caledonides, and the southern portion of the terrane forming further offshore as a juvenile north-facing oceanic arc.

Ages, trace elements and Hf-isotopic compositions of zircons from claystones around the Permian-Triassic boundary in the Zunyi Section, South China: Implications for nature and tectonic setting of the volcanism

A growing body of evidence shows that volcanism near the Permian-Triassic boundary (PTB) may be crucial in triggering the Permian–Triassic (P–Tr) mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin (241-279 Ma) except for two inherited/xenocrystic zircons (939 and 2 325 Ma). The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the Middle- Late Permian boundary (~261.5 Ma, MLPB), the Wuchiapingian-Changhsingian boundary (~254.5 Ma, WCB), and the PTB (~250.5 Ma), respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with eHf(t) values of -11.4 to +0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope composition (eHf(t)=-11.4- -4.8) of Bed ZY13 (~250.5 Ma) implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean.

Late Paleoproterozoic tectonic setting of the northern margin of the North China Craton: Constraints from the geochronology and geochemistry of the mangerites in the Longhua and Jianping areas

The tectonic setting of the late Paleoproterozoic suite of anorthosite, mangerite, charnockite and alkali-feldspar granite (AMCG) on the northern margin of the North China Craton (NCC) is controversial. This paper reports on the petrology, geochronology and geochemistry of the Beibaiqi, Beipuzi and Shibalitai mangerites in the Longhua area and the Shinao mangerite in the Jianping area. Zircon U–Pb dating reveals that these mangerites formed 1768–1725Ma, similar to the other lithologies (1753–1673 Ma) of the AMCG suite on the northern margin of the NCC. The mangerites have intermediate SiO2 contents (57.2–66.8wt.%) and high K2O (4.04–7.05wt.%) and Na2O (3.75–5.50wt.%) contents, which belong to alkaline series rocks. They are enriched in light rare-earth elements (REEs) and large-ion lithophile elements (LILEs) of Rb, Ba and K, and they are depleted in heavy REEs and the high field-strength elements (HFSEs) of Th, U, Nb, Ta, Ti and P. The zircon ɛHf(t) values of these mangerites are negative (−8.3 to −1.7), with depleted mantle model ages (TDM) and average crustal two-stage modal ages (T2DM) from 2250Ma to 2533Ma and 2573Mato2959 Ma, respectively. These geochemical characteristics suggest that these mangerites originated from the early Precambrian lower crust of the NCC. Compared with the mangerites, the other lithologies of the AMCG suite display different evolutionary trends of their major elements and different distribution patterns of their REEs and trace elements, indicating that those lithologies were not formed through fractional crystallization by the same primary magma. The anorthosites share similar distribution patterns of REEs and trace elements and zircon Hf isotopic composition with associated norites. The data suggest that the anorthosites were formed by noritic magmas through fractional crystallization at the base of the lower crust. The alkali-granites and rapakivi granites display the geochemical features of A-type granites, and might have originated from partial melting of the mid-upper crust. Combined with regional geological data, these findings suggest that the AMCG suite occurred in a post-collisional/post-orogenic extensional setting rather than an anorogenic continental rift setting.

The Late Cretaceous I- and A-type granite association of southeast China: Implications for the origin and evolution of post-collisional extensional magmatism

We present new geochronological, mineralogical, and geochemical data for granitic plutons that crop out within the Zhoushan archipelago, northeastern coastal Zhejiang Province, in order to constrain their origin, and the genetic relationship between the I- and A-type granites. These granites can be divided into two groups: (1) the northern I-type Putuoshan (PTS) and Dadong'ao (DDA) plutons; and (2) the southern A-type Daqingshan (DQS), Taohuadao (THD), and Xiazhidao (XZD) plutons. Zircon LA–ICP–MS U–Pb dating yielded ages of 98–96Ma for the northern I-type plutons and 89–86Ma for the southern A-type plutons. All of these granites are highly siliceous, K-rich, and have similar total alkali and total rare earth element (REE) abundances. However, there are also geochemical differences between the I-type and the A-type granites. The northern I-type alkali-feldspar granites are high-K calc-alkaline, metaluminous to mildly peraluminous, contain low concentrations of the high field strength elements (HFSE; e.g., Nb, Ta, Zr, and Hf), and have low Ga/Al ratios (2.04–2.44). In contrast, the southern A-type granites are peralkaline and F-rich, and have lower CaO and Al2O3 concentrations, and higher Fe2O3T and HFSE concentrations and Ga/Al ratios (3.25–3.86). Meanwhile, they have slightly higher heavy REE (HREE) concentrations, and are more depleted in Ba, Sr, P, Ti, and Eu than the northern I-type granites. Both the I- and A-type granites have homogeneous whole-rock Nd and highly variable zircon Hf isotopic compositions. Of note, the southern peralkaline A-type granites appear to have more radiogenic Nd and Hf isotope compositions than the northern I-type granites. The present data, together with the results of a previous study on mafic enclaves within the PTS pluton, suggest that the northern I-type alkali-feldspar granites were generated by mixing of mantle-derived material with crustal-derived magmas that formed by dehydration melting of mica-bearing basaltic rocks, leaving a granulite residue in the lower crust. Both the I- and A-type granites were emplaced during post-collisional extensional tectonism associated with rollback of the steeply subducting paleo-Pacific Plate. An increase of the dip angle of the subducted palaeo-Pacific plate between the Early and Late Cretaceous resulted in enhanced extension, further thinning the lithosphere and causing more intensive underplating of mantle-derived magmas. This generated high temperatures and caused the partial melting of the residual granulite material, producing relatively anhydrous F-bearing felsic melts. The mixing of these unusual crustal melts with more voluminous mantle-derived mafic magmas generated the parental magmas that eventually formed the peralkaline A-type granites.

The geochemistry and geochronology of the Xiarihamu II mafic–ultramafic complex, Eastern Kunlun, Qinghai Province, China: Implications for the genesis of magmatic Ni–Cu sulfide deposits

The Xiarihamu Ni–Cu deposit of Qinghai Province, China, is located in the central region of the Eastern Kunlun Orogen (EKO), which is bordered by the Qaidam block (QDM) to the north and the Bayan Har-Songanganzi Terrane (BH-SG) to the south. The Xiarihamu region contains four mafic–ultramafic complexes that have intruded Neoproterozoic garnet-bearing granitic gneiss and metamorphic rocks of the Paleoproterozoic Jinshuiko Group. Ni–Cu sulfide ores of economic interest are found within intrusions of these four complexes. The Xiarihamu I intrusion is the main host of the Ni–Cu mineralization. Evidence of strong mineralization is also found in trenches and drill-cores from the Xiarihamu II intrusion. The remaining two intrusions contain little or no mineralization. The economic Ni–Cu ores are bedded or lenticular in nature or occur as localized veins of sulfide minerals. The Hf isotopic compositions of zircons from the two complexes have positive εHf(t) values of 4.0–13.7, indicating that its parental magmas originated from depleted asthenospheric mantle. However, the Xiarihamu II intrusion contains many xenoliths of Neoproterozoic monzogranite, and the samples from both complexes show similar rare earth and trace element patterns, are strongly enriched in lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, and are moderately depleted in Nb, Ta, P, and Ti, indicating significant crustal contamination. Sulfide minerals from the two complexes have δ34S values of 3.4%–7.6% that lie outside the range of mantle values (−2%±2%), suggesting that assimilated crustal sulfur was the main contributor to S saturation at the Xiarihamu Ni–Cu deposit. The major and trace element characteristics, and Sr, Nd and Hf isotopic compositions of the two Xiarihamu mafic–ultramafic complexes suggest that they were derived from a common source. A sample of gabbro from the Xiarihamu II mafic–ultramafic complex yields an age of 424±1Ma, which is interpreted as the timing of magmatic crystallization. Considering the tectonic evolution of the region, the complex probably formed in a post-collisional extensional regime.The parental magma experienced significant crustal contamination, either during ascent or emplacement, triggering S over-saturation that resulted in the precipitation of sulfide minerals, and sulfide enrichment in the magma. The silicate magma continued to ascend due to buoyancy, eventually forming the Xiarihamu mafic–ultramafic complex and the Xiarihamu Ni-Cu sulfide deposit.

Stages and conditions of metamorphism of mafic granulites in the Early Precambrian complex of the Angara–Kan terrane (southwestern Siberian Craton)

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~1.77 Ga) and two-pyroxene granulites (~1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of 176Hf/177Hf values for the core and rims. The positive εHf values (up to +6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).

Asynchronizing paleo-Pacific slab rollback beneath SE China: Insights from the episodic Late Mesozoic volcanism

Extensive Late Mesozoic igneous rocks in SE China have been widely considered to be generated under the paleo-Pacific tectonic regime. Previous studies suggested a fundamental oceanward younging trend for the igneous rocks, further implying a NW-ward paleo-Pacific subduction model with gradual slab rollback. New LA–ICP–MS zircon U–Pb dating results show that early stage of the Late Mesozoic volcanic sequences (also known as the lower volcanic series) in Fujian Province was formed in three episodes: 160–148Ma for the Changlin Formation, 145–130Ma for the Nanyuan Formation, and 130–127Ma for the Xiaoxi Formation, among which the second episode made the volcanism climax. Thus the entire lower volcanic series in Fujian were formed earlier than those in Zhejiang (140–118Ma), displaying a NE-ward younging trend parallel to the potential subduction belt. Besides, in situ Hf isotope analyses on dated zircons yield an εHf(t) range of −19.2 to +1.7 for the lower volcanic series in Fujian. The majority of the studied volcanic rocks have more radiogenic Hf than that of the metamorphic basement, requiring the involvement of juvenile components in their origin. Moreover, the zircon εHf(t) value increases with time in each single area, as well as the igneous rocks elsewhere in SE China, except for the Changlin Formation which shows very opposite isotopic varying trend. Increasing zircon εHf(t) values imply an increasing material contribution of contemporaneous underplated mantle-derived magmas, which was plausibly induced by gradual crustal decompression. As well as the NE-ward younging trend of the lower volcanic series, it is also identified that the juvenile material contribution in the igneous petrogenesis gradually took place in the same direction along the coastal area. Thus here we propose an asynchronizing paleo-Pacific slab rollback model during 150–120Ma to account for the episodic magmatism and crustal extension in SE China. On the contrary, decreasing zircon εHf(t) values of the Changlin Formation volcanics indicate that they were formed under enhanced crustal compression probably induced by slab advance. On the other hand, the upper volcanic series in Fujian were formed during 110–88Ma coevally to those in Zhejiang, with more depleted zircon Hf isotopic compositions than the lower volcanic series as well, indicating that the entire coastal SE China was under the back-arc tectonic setting during that time.