Zircon Hf Isotopic Compositions Research Articles

Tracing Archaean terranes under Greenland's Icecap: U–Th–Pb–Hf isotopic study of zircons from melt-water rivers in the Isua area

The Archaean gneisses of the Nuuk area (southern West Greenland) are partitioned into tectonostratigraphic terranes – blocks of arc-like crust that evolved independently until they coalesced by collisional orogeny. To ‘map’ terranes to the east under the Inland Ice, sand samples were taken from rivers issuing from the edge of the Icecap; three from the Isua area and one from ∼20km to the south. Bedrock along this part of the ice front consists of ∼3820–3600Ma amphibolite facies rocks. 40km south of Isua (Kapisilik terrane in the Ivisaartoq area) and also from ∼10km to the north there are Mesoarchaean amphibolite facies gneisses (3070–2950Ma) exposed at the ice front. In the moraine fields in the Isua area there are erratic blocks of granulite facies gneisses. These were sourced from a hidden terrane to the east under the ice because no such rocks are exposed in the Isua area.The majority of the zircons from the sand samples yielded close to concordant U–Pb ages. Apart from one 2414Ma grain, all are Archaean. The Isua sands show ∼2695, 2710 and 2730Ma; then 2790, 2805 and 2840Ma clusters with a few grains back to 2950Ma and then a complex 3440–3960Ma spectrum. Less than 1% of the grains have ages between 3000 and 3400Ma. In the Isua sands, Neoarchaean and late Mesoarchaean zircons form >50% of the population. The sand 20km to the south shows a similar span of zircon ages, but there is a 2960Ma peak not seen in the Isua sands.As the Eoarchaean and late Mesoarchaean–Neoarchaean cycles progress, the spread of Th/U and ɛHf values seen in each zircon age population increases. This is interpreted as repeated addition of juvenile material to the crust, but also with increasing amounts of high temperature tectonothermal recycling of older materials within a package. However, there is no isotopic evidence for any contribution of Eoarchaean crust to the later ≥2750Ma Archaean cycle(s), showing evolution in disparate terranes. Furthermore, no evidence for Hadean material beneath the ice is seen in either the U–Pb ages or Hf isotopic compositions of the detrital zircons. On the other hand, 2750–2500Ma metamorphic zircons show a spread of initial ɛHf values of zero to −25, with the most negative indicating zircon growth in Eoarchaean rocks. Overall these results closely mirror those obtained from direct sampling of rocks of the Archaean terranes exposed beyond the Icecap.The zircon data indicate that Mesoarchaean rocks equivalent to the Kapisilik terrane are absent under the ice. Instead, zircons were sourced from a late Mesoarchaean to Neoarchaean terrane affected by high grade metamorphism, whose age spectra perfectly matches the Tasiusarsuaq terrane to the southeast of Nuuk. Guided by the zircon results and aeromagnetic signatures, a terrane map is presented that extrapolates the Tasiusarsuaq terrane northwards under the Inland Ice towards the Tuno terrane in the north of the craton. Implications for different syntheses on the evolution of Archaean crust in southern West Greenland are discussed.

Ordovician appinites in the Wugongshan Domain of the Cathaysia Block, South China: Geochronological and geochemical evidence for intrusion into a local extensional zone within an intracontinental regime

Abstract Palaeozoic mafic igneous rocks are potentially significant in constraining the tectonic nature and evolution of the Kwangsian Orogeny in the eastern South China Block, yet they have received little attention because of their limited outcrop. Geochemistry and geochronology was carried out on newly identified Ordovician ultramafic–mafic appinites in the Wugongshan Domain of the Cathaysia Block. Seven appinite samples yielded 206Pb/238U crystallisation ages ranging from 452 ± 4 Ma to 473 ± 3 Ma. Abundant 480–500 Ma zircon xenocrysts and/or inherited zircons were found in the appinites, possibly indicating an earlier magmatism episode in the early Palaeozoic period. The Wugongshan appinites are ultramafic to mafic in composition, and the ultramafic rocks display features of cumulates (high concentrations of MgO, Fe2O3t, Cr, Ni, and low concentrations of total alkali and total rare earth elements [REE]). The appinite geochemistry displays: relatively flat chondrite normalised REE patterns with slight enrichment in light REE and weak negative Eu anomalies; enrichment in large-ion lithophile elements (such as Rb, K), and weak depletion in Nb–Ta in primitive mantle normalised trace element patterns. We suggest that the Wugongshan appinites likely originated from an ancient metasomatised mantle, and that crustal assimilation, fractional crystallisation (AFC), magma mingling and hydration were involved in the petrogenetic process, based on the combination of geochemistry, crust-like bulk Sr, Nd and zircon Hf isotopic compositions (eNd (t) = − 8.2 to − 3.2, initial 87Sr/86Sr ratios of 0.7067–0.7144, zircon eHf (t) values peaking at − 9 to − 3) and regional geological data. Further considering the alignment and chronology of the appinites, we suggest that the appinitic magmas probably were emplaced along the Jiangshan–Shaoxing Fault in a local extensional zone in an intracontinental regime in the early Palaeozoic.

Geochronology, geochemistry, and zircon Hf isotopic compositions of Mesozoic intermediate–felsic intrusions in central Tibet: Petrogenetic and tectonic implications

Abstract Mesozoic intermediate–felsic intrusions are widely distributed in the southern Qiangtang terrane, central Tibet. Zircon U–Pb dating shows that these intrusions formed in two periods in the Jurassic (169–150 Ma) and Cretaceous (127–113 Ma). They mostly belong to the high-K calc-alkaline series, and show strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb, Ta, and Ti, and negative Ba anomalies on primitive mantle-normalized diagrams. P2O5 contents decrease with increasing SiO2 content, and Th contents increase with increasing Rb content, consistent with the evolution trend of I-type magmas. These intrusions show a wide range of zircon eHf(t) values from − 19.4 to 11.2, including in rocks with similar SiO2 contents, suggesting variable contributions from mantle and Qiangtang crustal sources. Fine-grained mafic to intermediate igneous enclaves in Jurassic intrusions have similar zircon U–Pb ages and similar or slightly higher zircon eHf(t) values to the host rocks, suggesting that the enclave magmas were derived from mixed magmas at depth and injected into more evolved magmas in upper crustal magma chambers. Magma mixing is also supported by the wide range of zircon Hf isotopic compositions (eHf(t) = − 19.4 to 2.5) from within individual Jurassic and Cretaceous intrusions, and Jurassic enclaves. The Jurassic–Cretaceous magmas likely formed in a continental arc setting during subduction of the Bangong–Nujiang ocean between 170 and 110 Ma, and evolved in the upper plate crust by MASH processes.

Geochronology of magmatism and mineralization of the Daheishan giant porphyry molybdenum deposit, Jilin Province, Northeast China: constraints on ore genesis and implications for geodynamic setting

Daheishan giant porphyry Mo deposit is located in the Lesser Xing’an–Zhangguangcai Ranges, Jilin Province, NE China. Mineralization is closely related to the Daheishan intrusive complex, which can be divided into Changganglin biotite granodiorite, Qiancuoluo biotite granodiorite, and Qiancuoluo granodioritic porphyry. Four stages of mineralization are distinguished, based on the cross-cutting relationships of mineralized veins. LA-ICPMS zircon U-Pb analysis yields 206Pb/238U ages of 177.9 ± 2.3 Ma for the Changganglin biotite granodiorite, 169.9 ± 2.3 Ma for the Qiancuoluo biotite granodiorite, and 166.6 ± 4.0 Ma for the Qiancuoluo granodioritic porphyry. Hydrothermal fluids responsible for mineralization evolved from different magmas. Six molybdenite samples yield Re-Os model ages of ~167 Ma. Muscovite from the last mineralization stage gives a 40Ar/39Ar plateau age of 163.6 ± 0.9 Ma. Geochronology data indicate that the entire magmatic system lasted for about 10 million years, and the total duration of hydrothermal activity was less than 4 million years. The εHf(t) values of zircons obtained from the Changganglin biotite granodiorite, Qiancuoluo biotite granodiorite, and Qiancuoluo granodioritic porphyry range from 4.5 to 9.1, 5.7 to 10.9, and 4.4 to 7.1, respectively, indicating that they were mainly derived from the depleted mantle, although contaminated by crustal materials to a greater or lesser extent. The formation of the Daheishan porphyry Mo deposit was temporally and spatially related to the amalgamation of Jiamusi Massif and Songliao terrane in the Palaeo-Pacific Ocean regime. Regional Hf isotopic compositions of zircon suggest an episode of crustal growth in the Phanerozoic in the Lesser Xing’an–Zhangguangcai Ranges. Regional Mo mineralization ages suggest a peak of porphyry Mo mineralization in the Jurassic in the Lesser Xing’an-Zhangguangcai Ranges.

Geochronological, geochemical, and Sr–Nd–Hf isotopic characteristics of Cretaceous monzonitic plutons in western Zhejiang Province, Southeast China: New insights into the petrogenesis of intermediate rocks

We present comprehensive petrological, geochemical, and Sr–Nd–Hf isotopic data for the Matou and Dalai plutons in western Zhejiang Province, Southeast China, with the aim of constraining the petrogenesis of monzonites and to offer new insights into the deep processes of interaction between crustal- and mantle-derived magmas beneath SE China. The Matou pluton comprises quartz monzonite, whereas the Dalai pluton consists of quartz monzodiorite. Zircon U–Pb ages obtained by laser ablation-inductively coupled plasma-mass spectrometry show that both plutons were emplaced at 99–101Ma. Rocks of both plutons are intermediate to silicic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. Samples of the plutons are enriched in large ion lithophile (e.g., Rb, K, and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and have small negative or no Eu anomalies. In addition, the rocks have high Mg# values (up to 53.9), high zircon εHf(t) values (up to −1.4), and low Nb/U and Ta/U ratios. Geochemical evidence suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of these monzonitic rocks. The presence of inherited zircons with Palaeoproterozoic ages and zircons with unusually low εHf(t) values (−12.9) in the Matou quartz monzonites indicates that ancient crustal materials were also involved in their petrogenesis. In combination with the presence of abundant mafic microgranular enclaves (MMEs) with spheroidal to ellipsoidal–ovoidal shapes and xenocrysts within the more diffused enclaves, and the results of trace element modelling, we suggest that the Matou quartz monzonites were generated by mixing between mantle-derived mafic magmas and crustally derived silicic magmas. The Dalai pluton is relatively homogeneous and contains fewer MMEs than the Matou pluton. Zircons from the Dalai pluton show no inherited components, indicating that crustal materials have played a limited role in the petrogenesis of the quartz monzodiorites. The Dalai quartz monzodiorites have lower SiO2 contents, higher Mg# values, and considerably higher and variable Cr, Co, and Ni concentrations than the Matou quartz monzonites. Zircon Hf isotopic compositions of the Dalai pluton are relatively homogeneous (εHf(t)=−5.2 to −3.2). The combined petrological, geochemical, and isotopic features indicate that the Dalai monzodiorites were generated by olivine- and pyroxene-dominated fractional crystallisation from basaltic magmas, which were in turn produced by mixing between melts from depleted asthenosphere and subduction-enriched mantle. Our interpretation implies that Late Mesozoic monzonitic rocks in Southeast China require a significant input of mantle melts, and some may have been generated solely by fractionation of basaltic magmas. This petrogenetic model may be applicable to other monzonitic rocks in Southeast China, and to similar tectonic settings and sites of monzonitic magma generation worldwide.

Zircon U-Pb age and Hf isotopic characteristics of the Huangtuliang monzonitic granite, North Hebei Province, China

The Huangtuliang monzonitic granite outcrops on the northern side of the Huangtuliang gold mining district, Chicheng, North Hebei Province. Our predecessors only made isotopic age determination using the K-Ar method. Through LA-MC-ICP-MS zircon U-Pb dating and zircon Hf isotopic composition determination, this study acquired the age of 244.8±2.0 Ma (MSWD=0.57) on the basis of the weighed mean 206 Pb/ 238 U ratio, indicating that the Huangtuliang monzonitic granite was formed during the Middle Triassic period, which is the product of Early Indosinian magmatic activities in the region of North Hebei. eHf(t) values vary relatively evenly, within the range of -10.65− -14.03, with an average of -12.14. The two-stage evolution model ages, tDM2, vary between 1943 and 2144 Ma, implying that the rock-forming materials of the Huangtiliang monzonitic granite mainly came from the Paleo- proterozoic recirculated crustal materials, though a small quantity of enriched-mantle materials would have been involved.

Generation of Cenozoic granitoids in Hokkaido (Japan): Constraints from zircon geochronology, Sr-Nd-Hf isotopic and geochemical analyses, and implications for crustal growth

The island of Hokkaido is a young accretionary terrane, basically built with a Jurassic accretionary complex and Cretaceous arc in the west (= NE Japan arc terrane), a Cretaceous-Paleogene forearc basin and accretionary complex with the Hidaka metamorphic belt in the center, and a Cenozoic island arc with Cretaceous basement in the east (= Chishima or Kuril arc terrane). Though volumetrically small, Paleogene and Neogene granitoids are widespread in central Hokkaido (Hidaka Belt). Granitoids are the most representative component of the continental crust, so in this work we aimed to study the mode of generation and source characteristics of these granitoids in order to assess the crustal composition of Hokkaido and examine the general problem of continental growth. New zircon geochronology on nine granitic and one gabbroic rocks from the Hidaka Belt reveals three distinct magmatic episodes, two in the Eocene at 45–46 Ma (3 granites), and 37.0 ± 0.5 Ma (1 granite), and one in the Miocene at 18 to 19 Ma (5 granites and 1 gabbro). The Miocene episode represents the most important granitic emplacement in Hokkaido. The early Eocene zircon ages of 45 to 46 Ma are identified for the first time for granitoids that occur in the northern part of the Hidaka Belt. The zircon age of 37 Ma for a granite from Shirataki is rather rare in Hokkaido, but similar ages had been reported for a tonalite and a granite from the Hidaka metamorphic belt. Geochemically, all granites are slightly peraluminous but not S-type, and they possess volcanic arc granitoid characteristics. Their REE distribution patterns are typically “granitic,” showing fractionated patterns with LREE enrichment and distinct negative Eu anomaly. The whole-rock isotopic signatures [ISr = 0.7044 to 0.7061; eNd(t) = +1.0 to +4.7; TDM-1 = 400-1000 Ma] reveal their largely juvenile characteristics. This is corroborated by the zircon Hf isotopic compositions [eHf(t) = +8 to +19]. The Eocene granites were most probably generated by melting of subducted accretionary complex in a prolonged period from 46 to 37 Ma in supra-subduction zone; whereas the Miocene granites were also generated by melting of accretionary complex in a back-arc rifting setting. In both cases, the involved accretionary complex was probably dominated by the mantle-derived lithological component with little Paleozoic or older crustal material. Hokkaido provides an excellent example of juvenile crust addition to the continental crust.

Contrasting Lu–Hf isotopes in zircon from Precambrian metamorphic rocks in the Jiaodong Peninsula: Constraints on the tectonic suture between North China and South China

Zircon Lu–Hf isotopes, together with zircon U–Pb ages, trace elements and O isotopes were analyzed for various types of metamorphic rocks from the Jiaodong and Jiaobei terranes in the Jiaodong Peninsula, east-central China. The results show contrasts in the crustal nature between the two terranes and thus provide constraints on the tectonic suture between the North China Craton and the South China Block in this region. Zircon U–Pb dating for the Jiaodong terrane indicates protolith emplacement mainly in the middle Neoproterozoic and subordinately in the middle Paleoproterozoic. The protolith zircon of Neoproterozoic U–Pb ages exhibits a wide range of ɛHf(t) values from −16.8±1.0 to 5.9±1.0, indicating reworking of both ancient and juvenile crustal rocks during the Neoproterozoic magmatism. Low zircon δ18O values (<5‰) are a diagnostic feature of the Neoproterozoic protolith in the Jiaodong terrane. Growth of the juvenile crust in the middle Paleoproterozoic is also recorded by a few samples. In contrast, zircon U–Pb dating for the Jiaobei terrane yields three episodes of magmatism at ∼2.9, ∼2.7 and ∼2.5Ga, respectively. Zircon Hf isotope compositions suggest growth of the juvenile crust at the two earlier episodes but reworking at the later episode. The crustal reworking is particularly intensive in the middle Paleoproterozoic, resulting in the diagnostic high-pressure granulite-facies metamorphism at 1.90–1.85Ga and migmatization at 1.75Ga. At the same time, remelting of the Archean crust gave rise to the 1.90–1.85Ga S-type granitic rocks. The zircon of Paleoproterozoic U–Pb ages usually exhibits high δ18O values (7–10‰). Taken together, the crustal nature of the Jiaodong and Jiaobei terranes are generally similar to that of the South China Block and the North China Craton, respectively. This lends support to the contention that the suture between the two collided continents is located along the Wulian–Yantai fault marking the boundary between the Jiaodong and Jiaobei terranes.

Petrogenesis of Late Mesozoic granitoids and coeval mafic rocks from the Jiurui district in the Middle–Lower Yangtze metallogenic belt of Eastern China: Geochemical and Sr–Nd–Pb–Hf isotopic evidence

Large-scale Cu–Au mineralization is associated with the Late Mesozoic granitoids in the Jiurui district of the Middle–Lower Yangtze Mineralization Belt in Eastern China. To constrain the petrogenesis of these granitoids and coeval mafic rocks, a detailed geochemical and Sr–Nd–Pb–Hf isotopic study was performed. The Jiurui granitoids are made up primarily of granodiorite porphyry and quartz diorite porphyry. These granitoids are characterized by SiO2 and K2O contents of 58.8wt.% to 68.6wt.% and 1.9wt.% to 5.7wt.%, respectively. These granitoids show relatively high MgO contents (1.0wt.% to 3.1wt.%, average 2.1wt.%) and high Mg# values (39 to 70, average 54). The Jiurui granitoids are enriched in light rare earth elements (LREE), large ion lithophile elements (LILE), and compatible trace elements (Cr, Ni and V) but are relatively depleted in Nb, Ta, Y and Yb. These rocks show a negligible negative Eu anomaly (Eu/Eu⁎=0.76–1.13, average 0.91) and nearly no negative Sr anomaly. The whole-rock initial 87Sr/86Sr ratios range from 0.7060 to 0.7092, and the εNd(t) values vary from −5.4 to −2.0. The granitoids show radiogenic Pb isotopic ratios with values of 206Pb/204Pb (17.93–18.21), 207Pb/204Pb (15.55–15.58), and 208Pb/204Pb (38.16–38.56) for the plagioclases. The zircon Hf isotopic compositions show εHf(t) values from −11.8 to 2.4. The coeval mafic rocks consist of lamprophyre, diabase and fine-grained mafic dyke. These rocks are characterized by SiO2 contents ranging from 47.6wt.% to 54.8wt.%, with a negative Eu anomaly and a positive to negative Sr anomaly. The whole-rock initial 87Sr/86Sr ratios range from 0.7059 to 0.7071, and the εNd(t) values vary from −3.8 to −1.4. By comparing the geochemical and isotopic compositions of the Jiurui granitoids and the coeval mafic rocks, we conclude that the granitoids are similar to adakites that were likely related to the delamination processes, and the coeval mafic rocks may have originated directly from enriched lithospheric mantle. The thickened lower continental crust and underlying enriched lithospheric mantle may have been simultaneously delaminated into the asthenospheric mantle. This process led to the partial melting of the thickened lower crust and the enriched lithospheric mantle, and the melts experienced an interaction with the asthenospheric mantle during ascension. Mafic melts could be generated from the enriched lithospheric mantle, which was heated by the upwelling asthenospheric mantle. The geochemical and isotopic compositions of mafic rocks were most likely controlled by the AFC process. A geochemical and isotopic comparison of the granitoids in the Jiurui district with those of two other districts (e.g., Angui and Tongling) in the same mineralization belt indicates that a significant crust–mantle interaction played a crucial role in their petrogeneses and metallogeneses. From the southwest to northeast in this metallogenic belt, the proportion of mantle component that was involved in the granitic magma shows a decreasing trend from the Jiurui to the Angui and Tongling districts. It is suggested that the Juirui district has the highest potential for a significant amount of Cu–Au accumulation because the mantle is an important supply for these metals.

Accurate Hf isotope determinations of complex zircons using the “laser ablation split stream” method

The “laser ablation split stream” (LASS) technique is a powerful tool for mineral‐scale isotope analyses and in particular, for concurrent determination of age and Hf isotope composition of zircon. Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus, the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a single‐collector (for U‐Pb age determination) and a multicollector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC‐ICPMS). The ability of the LASS for concurrent Hf + age technique to extract meaningful Hf isotope compositions in isotopically zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components can be a significant cause of excess scatter in Hf isotope analyses and demonstrate that the LASS approach offers a robust solution to these issues. The veracity of the approach is demonstrated by accurate analyses of 10 reference zircons with well‐characterized age and Hf isotopic composition, using laser spot diameters of 30 and 40 µm. In order to expand the database of high‐precision Lu‐Hf isotope analyses of reference zircons, we present 27 new isotope dilution‐MC‐ICPMS Lu‐Hf isotope measurements of five U‐Pb zircon standards: FC1, Temora, R33, QGNG, and 91500.

Petrogenesis of Cretaceous adakite-like intrusions of the Gangdese Plutonic Belt, southern Tibet: Implications for mid-ocean ridge subduction and crustal growth

We have conducted a whole-rock geochemical, U–Pb zircon geochronological, and in situ zircon Hf–O isotopic compositional study of rocks in southern Tibet from the Langxian igneous suite (including a lamprophyre dyke, mafic enclaves, a granodiorite, and a two-mica granite) and the Nuri igneous suite (a quartz–diorite). U–Pb zircon dating indicates that the timing of crystallization of the mafic enclaves and host granodiorite of the Langxian suite are ca. 105Ma and 102Ma, respectively, that the Langxian lamprophyre dyke and the two-mica granite were emplaced at ca. 96Ma and 80–76Ma, respectively, and that the Nuri quartz–diorite was emplaced at ca. 95Ma. With the exception of the lamprophyre dyke and mafic enclaves in the Langxian area, felsic rocks from the Langxian and Nuri igneous suites all show signs of a geochemical affinity with adakite-like rocks. The high Mg-numbers, high abundance of compatible elements, high εNd(t) (2.7 and 2.8) and δ18O (8.9 and 9.2‰) values, elevated zircon εHf(t) (11.0–17.0) values, and low 87Sr/86Sr(i) ratios (0.7040), collectively indicate that the Nuri adakite-like quartz–diorite was derived from partial melting of the low temperature altered Neo-Tethyan oceanic crust, and that these dioritic magmas subsequently interacted with peridotite as they rose upwards through the overlying mantle wedge. The observation of identical differentiation trends, similar whole-rock Sr–Nd and zircon Hf isotopic compositions, and consistently low (Dy/Yb)N ratios among the Langxian igneous suite rocks, indicates that the adakite-like granodiorite was produced by low-pressure fractional crystallization of precursor magmas now represented by the (relict) mafic enclaves. However, relatively high Al2O3 contents, low MgO, Cr and Ni contents, and low (La/Yb)N and (Dy/Yb)N values indicate that the two-mica granite was derived from partial melting of the southern Tibetan mafic lower crust in the absence of garnet, while isotopic data suggest that at least 70% of the magma source region was juvenile materials. Combined with the presence of HT (high temperature) charnockitic magmatism, HT granulite facies metamorphism, and large volumes of Late Cretaceous batholiths, the oceanic-slab-derived Nuri adakitic rocks indicate a substantial high heat flux in the Gangdese batholith belt during the Late Cretaceous, which may have been related to subduction of a Neo-Tethyan mid-ocean ridge system. According to this model, hot asthenosphere would rise up through the corresponding slab window, and come into direct contact with both the oceanic slab and the base of the overlying plate. This would cause melting of both the oceanic slab and the overlying plate by the addition of heat that was ultimately linked with peak magmatism and the significant growth and chemical differentiation of juvenile crust in southern Tibet during the Late Cretaceous (105–76Ma). In addition, the petrogenesis of the Langxian adakite-like two-mica granite indicates that the southern Tibetan crust was still of normal thickness prior to the emplacement of these intrusions at ca. 76Ma. This probably means that large parts of southern Tibet were not very highly elevated prior to the Indian–Asian collision.

Tectonic development from oceanic subduction to continental collision: Geochemical evidence from postcollisional mafic rocks in the Hong'an–Dabie orogens

Continental collision is generally developed from oceanic subduction to continental subduction. Recycling of both oceanic and continental crustal materials is expected to occur in the same subduction zone. This is demonstrated by an integrated study of major-trace elements and stable-radiogenic isotopes in postcollisional mafic igneous rocks from the Hong'an–Dabie orogens, east-central China. Mafic dykes in the Hong'an orogen exhibit OIB-like trace element distribution patterns, high Nb/U and TiO2/Al2O3 ratios, relatively depleted radiogenic isotope compositions with consistently high εNd(t) values of −1.8 to 4.5 and low initial 87Sr/86Sr ratios of 0.7040 to 0.7050. They are significantly different from mafic intrusives in the Dabie orogen that exhibit arc-like trace element distribution patterns, low Nb/U and TiO2/Al2O3 ratios, relatively enriched radiogenic isotope compositions with low εNd(t) values of −2.3 to −20.7 and high initial 87Sr/86Sr ratios of 0.7061 to 0.7114. In addition, whole-rock and zircon Hf isotope compositions are decoupled in the Hong'an mafic dykes but coupled in the Dabie mafic intrusives. Nevertheless, zircon O isotope compositions exhibit similar variations for the two occurrences of mafic rocks. These observations are interpreted as indicating significant differences in the nature of mantle sources between the Hong'an and Dabie mafic rocks. Two types of melt-peridotite reaction are assumed to occur in oceanic and continental subduction channels, respectively, during continental collision. Reaction of juvenile subcontinental lithospheric mantle (SCLM) peridotite with felsic melt derived from subducting oceanic basalt and sediment before continental subduction would generate the mantle source for the Hong'an mafic dykes, whereas reaction of ancient SCLM peridotite with felsic melt from subducting continental crust would generate the mantle source for the Dabie mafic intrusives. While such contrasting types of melt-peridotite reaction at the slab-mantle interface are responsible for the systematic differences in the geochemical compositions of mantle sources, the incorporation of crust-derived felsic melts into the mantle wedge result in the similar lithology of non-peridotite such as pyroxenite and hornblendite. Therefore, the two types of slab-mantle interaction record the tectonic transition from oceanic subduction to continental subduction in the course of continental collision. The nature of slab–mantle interaction in subduction channel is a key to the origin of mantle sources for postcollisional mafic igneous rocks in continental collision orogens.

Paleoproterozoic tectonic transition from collision to extension in the eastern Cathaysia Block, South China: Evidence from geochemistry, zircon U–Pb geochronology and Nd–Hf isotopes of a granite–charnockite suite in southwestern Zhejiang

The Badu complex and associated Paleoproterozoic granitoids are among the oldest known rocks in the Cathaysia Block in South China. The Paleoproterozoic units of the Badu complex are dominantly composed of metapelitic rocks and meta-greywackes. Here we report LA-SS-ICP-MS (laser ablation-split-stream inductively coupled plasma-mass spectrometry) zircon U–Pb data from a newly discovered garnet-bearing granite which show an emplacement age of 1929±15Ma and metamorphism at 1872±34Ma. We also report U–Pb ages of 1886±16Ma, 1858±7Ma, 1848±11Ma from a gneissic granodiorite, and two charnockites respectively. The garnet-bearing granite is peraluminous with A/CNK range from 1.1 to 1.3. The rock shows relatively high SiO2, K2O and Rb contents, and low total REE, Sr, CaO and ferromagnesian components, typical of leucogranites. The whole rock Nd two-stage model age(TDM2(Nd)) of this rock is ca. 2.7Ga, zircon Hf crustal model ages(TDMC(Hf)) peak at about 2.7Ga, and abundant inherited zircons occur with U–Pb ages in the range of 2044 to 2803Ma. Evidences from zircon U–Pb age and Hf isotope compositions, whole rock Nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith, and that the garnet-bearing granite (leucogranite) was derived by partial melting in a thickened crust at about 1.93Ga. The gneissic granodiorite and charnockites show indistinguishable major and trace element features, as well as zircon Hf and whole rock Nd isotope compositions, indicating that they were generated from the same source rocks. The gneissic granodiorite and charnockites are ferroan, calc-alkalic and metaluminous with A/CNK range from 0.80 to 0.98. They display relatively low SiO2 contents and Ga/Al ratios, suggesting their A-type affinity. Their zircon TDMC(Hf) age-peak is 2.9Ga and whole rock TDM2(Nd) ages range from 2.8 to 2.9Ga. These high temperature rocks were generated possibly through the partial melting of ancient amphibolites (2.9Ga), with heat input from asthenosphere upwelling and magmatic underplating in an extensional setting at about 1.85–1.88Ga.Combined with data from previous studies, we suggest three episodes of major crustal growth in this region at 1.8–1.9Ga, ~2.7Ga and ~2.9Ga. During the 1.8–1.9Ga event, extensive reworking also took place. From the 1.93Ga syn-collision leucogranite (garnet-bearing granite), the 1.88–1.85Ga post-collision A-type granites and charnockites (the granite–charnockite suite), and the1.85–1.77Ga intraplate basaltic rocks (Li 1997 and Xiang et al. 2008) in the region, a Paleoproterozoic orogenic cycle can be deciphered in the Cathaysia Block. The rapid transition of tectonic regime from collision to post-collisional extension can be correlated to the assembly of the supercontinent Columbia.

Zircon geochronology and Hf isotopes of Mesozoic intrusive rocks from the Yidun terrane, Eastern Tibetan Plateau: Petrogenesis and their bearings with Cu mineralization

Zircon U–Pb ages and Hf isotopic composition of Mesozoic intrusive rocks from the Yidun terrane were systematically investigated in order to decipher the geodynamic setting of magmatism and their relationship with Cu-polymetallic mineralization in the Eastern Tibetan Plateau. Zircon U–Pb dating results on seven granitoid rocks and four diorite samples indicate that the Mesozoic plutons from the Yidun terrane were emplaced in the period of 224–214Ma with a peak at ∼216Ma. Zircon Hf isotopic compositions show that these intrusive rocks were derived from different source regions. The dioritic porphyries with the formation ages of ∼216Ma in the Zhongdian area of the southern Yidun terrane, together with the granodiorite of ∼216Ma from the Maxionggou batholith, originated from the remelting of the late Mesoproterozoic (∼1.2Ga) juvenile continental lower crust, whereas most granitic rocks were derived from the reworking of the early Mesoproterozoic (∼1.6 to 1.5Ga) middle-lower crust. Our results, together with previous data, indicate that the collision between the Yidun and Songpan–Ganzi terranes and subsequent orogeny had been completed by the middle Triassic. A post-orogenic collapse model is proposed for the Meosozoic magmatism and related Cu-polymetallic mineralization in the Yidun terrane.

Guidelines for reporting zircon Hf isotopic data by LA-MC-ICPMS and potential pitfalls in the interpretation of these data

Technical Paper High quality analytical data are essential for the development of sound scientific interpretations. To ensure the quality of the data published in Chemical Geology, a new type of contribution has been introduced - the Invited Technical Paper. When a particular technical issue needing discussion or clarification is identified, specialists in the technique will be invited to share their expertise. As is true for all articles published in the journal, these contributions will be subject to peer review before publication. The first topic to be treated concerns in situ Hf isotopic measurements in zircon by laser ablation inductively coupled plasma mass spectrometry. This is an extremely powerful and promising technique that has been adopted by many laboratories throughout the world. However, if insufficient care is taken during analysis and data processing, inaccurate results can be obtained, notably because of the existence of very large isobaric interferences on the isotope of interest. To address this issue, we have asked Christopher Fisher, Jeffery Vervoort and John Hanchar to provide a set of guidelines that can be adopted to assure that reliable Hf isotopic data are obtained by this technique. Abstract Over the past decade, the Hf isotope composition of zircon, as determined in situ by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS), has been applied increasingly to a wide range of geological problems and has proven to be a valuable analytical tool. There has been no uniformity, however, in the reporting of in situ Hf isotopic data and, quite often, reviewers and readers of papers are not provided with sufficient information to assess data quality. The goal of this invited contribution is to provide the non-specialist with a brief outline and explanation of what is required for proper presentation and documentation of in situ Hf isotopic data from zircon, including details of how the challenging large isobaric interference corrections have been made. In addition, we discuss a number of potential pitfalls vis-a-vis the assignment of the incorrect age to the measured Hf isotope composition. Non-specialists should be aware of these important issues when doing their own analyses and evaluating the analyses done by others.

Mineral ages and zircon Hf isotopic composition of the Andong ultramafic complex: implications for the evolution of Mesozoic subduction system and subcontinental lithospheric mantle beneath SE Korea

AbstractThe Phanerozoic subduction system of the Korean peninsula is considered to have been activated by at least Middle Permian time. The geochemically arc-like Andong ultramafic complex (AUC) occurring along the border between the Precambrian Yeongnam massif and the Cretaceous Gyeongsang back-arc basin provides a rare opportunity for direct study of the pre-Cretaceous mantle wedge lying above the subduction zone. The tightly constrained SHRIMP U–Pb age of zircons extracted from orthopyroxenite specimens (222.1±1.0 Ma) is indistinguishable from the Ar/Ar age of coexisting phlogopite (220±6 Ma). These ages represent the timing of suprasubduction zone magmatism likely in response to the sinking of cold and dense oceanic lithosphere and the resultant extensional strain regime in a nascent arc environment. The nearly coeval occurrence of a syenite-gabbro-monzonite suite in the SW Yeongnam massif also suggests an extensional tectonic setting along the continental margin side during Late Triassic time. The relatively enriched ɛHf range of dated zircons (+6.2 to −0.6 at 222 Ma) is in contrast to previously reported primitive Sr–Nd–Hf isotopic features of Cenozoic mantle xenoliths from Korea and eastern China. This enrichment is not ascribed to contamination by the hypothetical Palaeozoic crust beneath SE Korea, but is instead attributable to metasomatism of the lithospheric mantle during the earlier subduction of the palaeo-Pacific plate. Most AUC zircons show a restricted core-to-rim spread of ɛHf values, but some grains testify to the operation of open-system processes during magmatic differentiation.

Origin of volcanic ash beds across the Permian–Triassic boundary, Daxiakou, South China: Petrology and U–Pb age, trace elements and Hf-isotope composition of zircon

Abundant volcanic ash layers, largely altered to clay, characterize the Permian–Triassic boundary beds of the Daxiakou section, South China. Volcanism may have had an important role in the Permian–Triassic mass extinction, so it is important to understand the origin of these ash beds. The lithology of ash bed 260, as constrained by modes of crystal fragments (66.7% plagioclase, 7.2% alkali feldspar, and 26.1% quartz), suggests that the ash represents dacitic volcanism. The U–Pb ages, trace elements and Hf-isotope compositions of zircons from ten ash beds have been analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons can be divided into two groups: magmatic zircons (~97%, 227–279Ma) and inherited zircons (~3%, 678–2424Ma). Magmatic zircons have Y, Hf, Th and U contents and Nb/Ta ratios typical of zircons from silicic calc-alkaline volcanism. εHf(t) values of magmatic zircons vary from −10.9 to +5.3, implying magmatic mixing between juvenile crustal material (probably Neoproterozoic in age) and ancient crustal material (probably Archean). Beds 252–259-b have relatively depleted Hf-isotope compositions with εHf(t) values of −10.9–+5.3 and averages of −5.0–−2.5, while other beds have εHf(t) values of −10.8–+4.4 (mostly −10.8–−3.5) and averages of −8.9–−6.8, implying more input of juvenile crustal material in the volcanism of Beds 252–259-b. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggests that the volcanism producing the ashes took place along the convergent continent margins during the formation of the Pangea supercontinent. This intense silicic volcanism may have played an important role in causing the Permian–Triassic mass extinction in South China.

The Namuskluft and Dreigratberg sections in southern Namibia (Kalahari Craton, Gariep Belt): a geological history of Neoproterozoic rifting and recycling of cratonic crust during the dispersal of Rodinia until the amalgamation of Gondwana

This paper presents combined U/Pb, Th/U and Hf isotope analyses on detrital and magmatic zircon grains together with whole-rock geochemical analyses of two basement and eight sedimentary rock samples from the Namuskluft and the Dreigratberg in southern Namibia (Gariep Belt). The sedimentary sections evolved during the Cryogenian on the SW part of the Kalahari Craton and where therefore deposited in an active rift setting during the break-up of Rodinia. Due to insufficient palaeomagnetic data, the position of the Kalahari Craton within Rodinia is still under discussion. There are possibilities to locate Kalahari along the western side of Australia/Mawsonland (Pisarevski et al. in Proterozoic East Gondwana: supercontinent assembly and break-up, Geological Society, London, 2003; Evans in Ancient Orogens and modern analogues. Geological Society, London, 2009; and others) or together with the Congo-Sao Francisco and Rio de la Plata Cratons (Li et al. in Prec Res 45: 203–2014, 2008; Frimmel et al. in Int J Earth Sci (Geol Rundsch) 100: 323–354, 2011; and others). It is sill unclear which craton rifted away from the Kalahari Craton during the Cryogenian. Although Middle to Upper Cryogenian magmatic activity is known for the SE Kalahari Craton (our working area) (Richtersveld Suite, Rosh Pinah Fm), all the presented samples show no U/Pb zircon ages younger than ca. 1.0 Ga and non-older than 2.06 Ga. The obtained U/Pb ages fit very well to the exposed basement of the Kalahari Craton (1.0–1.4 Ga Namaqua Province, 1.7–2.0 Ga Vioolsdrif Granite Suite and Orange River Group) and allow no correlation with a foreign craton such as the Rio de la Plata or Australia/Mawsonland. Lu–Hf isotopic signatures of detrital zircon point to the recycling of mainly Palaeoproterozoic and to a smaller amount of Archean crust in the source areas. eHf(t) signatures range between −24 and +14.8, which relate to TDM model ages between 1.05 and 3.1 Ga. Only few detrital zircon grains derived from magmas generated from Mesoproterozoic crustal material show more juvenile eHf(t) signatures of +14, +8 to +4 with TDM model ages of 1.05–1.6 Ga. During Neoproterozoic deposition, only old cratonic crust with an inherited continental arc signature was available in the source area clearly demonstrated by Hf isotope composition of detrital zircon and geochemical bulk analysis of sedimentary rocks. The granodiorites of the Palaeoproterozoic basement underlying Namuskluft section are ca. 1.9 Ga old and show eHf(t) signatures of −3 to −5.5 with TDM model ages of 2.4–2.7 Ga. These basement rocks demonstrate the extreme uplift and deep erosion of the underlying Kalahari Craton at its western margin before general subsidence during Cryogenian and Ediacaran time. The sedimentary sequence of the two examined sections (Namuskluft and Dreigratberg) proposes the presence of a basin and an increasing subsidence at the SW part of the Kalahari Craton during the Cryogenian. Therefore, we propose the initial formation of an intra-cratonic sag basin during the Lower Cryogenian that evolved later to a rift basin at the cratonic margin due to increasing crustal tension and rifting together with the opening of the Adamastor Ocean. As the zircons of the sedimentary rocks filling this basin show neither rift-related U/Pb ages nor an exotic craton as a possible source area, the only plausible sedimentary transport direction providing the found U/Pb ages would be from the E or the SE, directly from the heart of the Kalahari Craton. Due to subsidence and ongoing sedimentation from E/SE directions, the rift-related magmatic rocks were simply covered by the input of old intra-cratonic material that explains the absence of Neoproterozoic zircon grains in our samples. The geochemical analyses show the erosion of a continental arc and related sedimentary rocks with an overall felsic provenance. The source area was a deeply eroded and incised magmatic arc that evolved on continental crust, without any evidence for a passive margin. All of this can be explained by the erosion of rocks related to the Namaqua Belt, which represents one of the two major peaks of zircon U–Pb ages in all analysed samples. Therefore, the Namaqua Belt was well exposed during the Cryogenian, available to erosion and apart from the also well-exposed Palaeoproterozoic basement of the Kalahari Craton one potential source area for the sedimentary rocks in the investigated areas.

Half a billion years of reworking of Hadean mafic crust to produce the Nuvvuagittuq Eoarchean felsic crust

The Nuvvuagittuq greenstone belt is dominated by mafic rocks, called the Ujaraaluk unit, that are mostly composed of cummingtonite–plagioclase–biotite with variable amounts of garnet. While the oldest zircons contained in thin intrusive trondhjemitic bands are ∼3.8 Ga, 146Sm–142Nd systematics suggest that the Ujaraaluk unit is as old as 4.4 Ga. The Nuvvuagittuq greenstone belt is surrounded by Eoarchean TTGs that have geochemical and isotopic compositions consistent with their derivation by partial melting of a source similar in composition and age to the Ujaraaluk unit. New zircon dates reported here show the Nuvvuagittuq TTGs to consist at least of four distinct age units of 3.76 Ga, 3.66 Ga, 3.5–3.4 Ga and 3.35 Ga. The Hf isotopic compositions of zircons from the TTG are consistent with derivation from Hadean mafic crust. The 3.66 Ga to 3.35 Ga TTGs appear to have been formed primarily from melting of a source compositionally similar to the 4.4 Ga Ujaraaluk unit, whereas the more radiogenic Hf of the zircons from the 3.76 Ga TTGs may suggest derivation from melting of a source compositionally similar to 4.1 Ga intrusive gabbros. Alternatively, the distinct rare earth element patterns of the 3.76 Ga and 3.66 Ga TTGs suggest their derivation from sources with variable amounts of residual garnet and hence formation at different depths. The composition of the older TTGs is indicative of a deeper source that may have involved a greater interaction between the melt and the mantle to explain the more radiogenic Hf isotopic compositions of their zircons. Sources compositionally similar to the Ujaraaluk unit and intrusive gabbros appear to be the most likely candidates for the Hadean precursor of the Nuvvuagittuq TTGs.

Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust with mixing of mafic magma: Insights from integrated zircon U–Pb ages, petrography, geochemistry and Nd–Sr–Hf isotopes

Whole rock major and trace element, Nd–Sr and zircon Hf isotopic compositions and secondary-ion mass spectrometry zircon U–Pb ages of eleven granitoid intrusions and dioritic rocks from the East Junggar (NW China) were analyzed in this study. The East Junggar granitoids were emplaced during terminal Early to Late Carboniferous (325–301Ma) following volcanic eruption of the Batamayi Formation. Zircons from the East Junggar granitoids yielded 210 concordant 206Pb/238U ages which are all younger than 334Ma and exhibit εHf(t) values distinctly higher than Devonian arc volcanic-rocks. Seismic P-wave velocities of deep crust of the East Junggar proper resemble those of oceanic crust (OC). These characteristics suggest absence of volcanic rock and volcano-sedimentary rock of Devonian and Early Carboniferous from the source region. The East Junggar granitoids show εNd(t) and initial 87Sr/86Sr values substantially overlapping those of the Armantai ophiolite in the area. The Early Paleozoic OC with seamount-like composition as the Zhaheba–Armantai ophiolites remained in the lower crust and formed main source rock of the East Junggar granitoids. Based on petrography and geochemistry, the East Junggar granitoids are classified into peralkaline A-type in the northern subarea, I-type (I1 and I2 subgroups) mainly in the north and A-type in the south of the southern subarea. The perthitic or argillated core and oligoclasic rim with an argillated boundary of feldspar phenocrysts and inclusion of perthites or its overgrowth by matrix plagioclase, in the monzogranites (northern subarea), suggest mixing of peralkaline granitic magma with mafic magma. In the north of the southern subarea, the presence of magmatic microdioritic enclaves (MMEs) in the I1 subgroup granitoids, transfer of plagioclase phenocrysts and hornblendes between host granodiorite and the MME across the boundary and a prominent resorption surface in the plagioclase phenocrysts indicate mixing of crustal magma (I2 subgroup granitoids) with mafic magma. Magma mixing shifted (87Sr/86Sr)i of the I1 subgroup granitoids towards the mantle array. Two generations of hornblende with zonal distribution and similar mineral and geochemical compositions of quartz monzodiorite and hosted MME with unfractionated rare earth elements (REE) suggest extended magma mixing with onset probably at or near source region. These observations imply concurrency of mantle input and the crustal melting and, hence, a causal relationship between underplating/intraplating and the lower OC/upper OC melting. The I-type granitoids experienced plagioclase and hornblende fractionations, whereas fractionated phases of the two groups of A-type granites were alkali feldspar and albite–oligoclase with significant involvement of F−-rich fluid. Granodioritic parent magmas of the I2 subgroup granitoids stemmed from the hydrous upper OC. Parent magmas of the two A-type groups possess syenogranitic or quartz syenitic compositions. The peralkaline A-type granites stemmed from the lower OC, whereas the A-type granites from dehydrated upper OC left behind after extensive partial melting and extraction of I-type granitoids. Based on comparison in the ternary system Mg2SiO4–CaAl2SiO6–SiO2, most of the Batamayi volcanic rocks with affinity to ocean-island basalts were derived from asthenospheric upwelling. The gabbro–dioritic rocks with higher light to heavy REE ratios stemmed from metasomatized lithospheric mantle. Both of the above mafic rocks contain subducted slab component.Loci of ponding of mafic magma and crustal melting migrated upward and southeastward from the lower OC in the northern subarea (yielding the peralkaline A-type granites) to the upper OC in the southern subarea. The latter evolved from hydrous (yielding the I-type granitoids in the north of the subarea) to dehydrated (yielding the A-type granites in the south of the subarea). Sequence of these events resulted in increasingly-later formation of the East Junggar granitoids southeastwards with an age span of ~20Ma. Minor contribution from Late Paleozoic OC to the source of the southern granitoid subarea caused the latter showing distinctly higher εNd(t) than the northern subarea. In conclusion, underplating caused OC remelting which, together with significant magma mixing, produced the East Junggar granitoids in a non-subduction zone environment.