Zircon Hf Isotopic Compositions Research Articles

The Mesoarchean Tiejiashan-Gongchangling potassic granite in the Anshan-Benxi area, North China Craton: Origin by recycling of Paleo- to Eoarchean crust from U-Pb-Nd-Hf-O isotopic studies

Mesoarchean and older potassic granites are important indicators of recycling of ancient continental crust early in Earth's history. This study of integrated whole rock and zircon geochemistry and geochronology reports the age and identification of the source materials of the >200km2 Mesoarchean Tiejiashan-Gongchangling granite in the Anshan-Benxi area, North China Craton, the largest pre-Neoarchean granite domain in the craton. SHRIMP U-Pb zircon dating on 15 samples indicates the magmatic crystallization of the granites between 2.95 and 3.0Ga and reveals a superimposed tectonothermal event at ~2.91Ga. The granites are characterized by high SiO2 and K2O, low CaO, FeOt, MgO and TiO2 with peraluminuous features. They show large variations in (La/Yb)n and strong negative Eu and Ba anomalies and Nb, P and Ti depletions. Whole rock Nd and magmatic zircon Hf isotopic compositions show large variations, but with most having εNd(t) and εHf(t) values <0, with tDM(Nd) and tDM(Hf) values varying from 3.3 to 3.9Ga and 3.3 to 4.0Ga, respectively. Magmatic zircons without very strong lead loss (discordance ≤20%) have δ18O values of +3.14 to +8.39. 3.3–3.7Ga xenocrystic zircons occur in some samples. The granite formed as a result of recycling of Paleo- to Eoarchean continental material in an intracontinental environment, with little if any contribution from Mesoarchean mantle sources. The sources could be predominantly unaltered ancient gneisses, together with yet to be identified Paleo- to Eoarchean materials affected by early low temperature alteration (weathered rocks or clastic sediment).

Petrogenesis of an early Eocene gabbro–granite complex in Kachang (SW Yunnan) and its implications for Eocene magmatism in the Tengchong terrane of SW China

ABSTRACTMajor and trace element, zircon U–Pb and Hf-isotope data are reported for mafic intrusions and host granite from the Kachang pluton (North Yingjiang of SW Yunnan) in the Tengchong Terrane, in order to investigate their sources, petrogenesis and tectonic implications. The zircon U–Pb age of the mafic rocks (~55 Ma) is identical to that of the host granite (56.7 ± 0.6 Ma). The mafic rocks have high MgO concentrations (up to 13.43 wt.%) at low SiO2 contents (low to 42.73 wt.%) and slight negative to positive εHf(t) values (−2.26 to +0.59). They are enriched in LILE and LREEs and depleted in HFSEs, which can be explained as melts derived from a enriched mantle, with some crustal contamination. The host granite have high SiO2 contents (69.18–72.65 wt.%), highly negative εHf(t) values (−9.08 to −5.14), suggesting mainly derived from an ancient crustal source. Field observations, geochronology, geochemistry and zircon Hf isotopic compositions point to a complex petrogenesis, where enriched mantle- and crust-derived magma mixing was coupled with crystal fractionation, thus explaining the genetic link between mafic and felsic magmas, result of mafic magma upwelling triggered by the subduction rollback of the Neotethyan slab. Our new data, along with the data reported (especially zircon U–Pb dating and Lu–Hf isotope data) in the Tengchong Terrane, indicate that the spatial and temporal variations and changing magmatic compositions over time in the Tengchong Terrane closely resemble those of the Lhasa Terrane in southern and central Tibet.

Hf isotope evidence for effective impact melt homogenisation at the Sudbury impact crater, Ontario, Canada

We report on the first zircon hafnium-oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets. The differentiated melt sheet at the 1.85Ga, originally ca. 200km in diameter Sudbury impact crater, Ontario, Canada, yields a tight range of uniform zircon Hf isotope compositions (εHf(1850) of ca. −9 to −12). This is consistent with its well-established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised. We propose that the heterogeneity in other isotopic systems, such as Pb, in early-emplaced impact melt at Sudbury is associated with volatility-related depletion during the impact cratering process. This depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post-impact assimilation of country rock, whereas the systems of more refractory elements preserve initial homogeneities. Zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks. However, they display a marked offset approximately one-third up the melt sheet stratigraphy that is interpreted to be a result of post-impact assimilation of 18O-enirched rocks into the base of the cooling impact melt.Given that impact cratering was a more dominant process in the early history of the inner Solar System than it is today, and the possibility that impact melt sheets were sources of ex situ Hadean zircon grains, these findings may have significance for the interpretation of the early zircon Hf record. We speculate that apparent εHf-time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust.We also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at Sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself.

Origin of the mafic microgranular enclaves (MMEs) and their host granitoids from the Tagong pluton in Songpan–Ganze terrane: An igneous response to the closure of the Paleo-Tethys ocean

The Songpan–Ganze terrane is mainly composed of a Triassic sedimentary sequence and late Triassic–Jurassic igneous rocks. A large number of plutons were emplaced as a result of tectono-magmatic activity related to the late stages of Paleo-Tethys ocean closure and ensuing collision. Granitoids and their hosted mafic enclaves can provide important constraints on the crust–mantle interaction and continental crustal growth. Mesozoic magmatism of Songpan–Ganze remains enigmatic with regard to their magma generation and geodynamic evolution. The Tagong pluton (209Ma), in the eastern part of the Songpan–Ganze terrane, consists mainly of monzogranite and granodiorite with abundant coeval mafic microgranular enclaves (MMEs) (ca. 208–209Ma). The pluton comprises I-type granitoid that possesses intermediate to acidic compositions (SiO2=61.6–65.8wt.%), high potassium (K2O=3.2–4.1wt.%), and high Mg# (51–54). They are also characterized by arc-type enrichment of LREEs and LILEs, depletion of HFSEs (e.g. Nb, Ta, Ti) and moderate Eu depletions (Eu/Eu*=0.46–0.63). Their evolved zircon Hf and whole-rock Nd isotopic compositions indicate that their precursor magmas were likely generated by melting of old lower continental crust. Comparatively, the MMEs have lower SiO2 (53.4–58.2wt.%), higher Mg# (54–67) and show covariation of major and trace elements, coupled with field and petrographic observations, such as the disequilibrium textures of plagioclase and amphibole, indicating that the MMEs and host granitoids were originated from different magma sources but underwent mafic–felsic magma mixing process. Geochemical and isotopic data further suggest that the precursor magma of the MMEs was formed in the continental arc setting, mainly derived from an ancient metasomatized lithospheric mantle wedge.The Triassic granitoids from the Songpan–Ganze terrane show remarkable temporal–spatial-petrogenetic affinities to the counterparts of subduction zones in the Yidun and Kunlun arc terranes, plausibly support a double-sided subduction of the Paleo-Tethys ocean. The mixing mechanism for the formation of the Tagong pluton was likely associated with the break-off of a subducted slab of the Paleo-Tethys ocean, which triggered subsequent upwelling of hot asthenosphere beneath accreted arc fragments and induced lithospheric mantle-derived magmas suffice to underplate and mix with the lower crust-derived felsic magma. Collectively, the late Triassic igneous rocks record significant crustal growth and continental development as response to the final demise of the Paleo-Tethys ocean (ca. 210Ma), and marks the last episode of orogenic magmatism in the Songpan–Ganze terrane after which the region entered into post-orogenic phase of evolution.

A mixed source for the Late Triassic Garzê-Daocheng granitic belt and its implications for the tectonic evolution of the Yidun arc belt, eastern Tibetan Plateau

Many Late Triassic granitic plutons are present in the eastern Yidun arc belt (EYAB), and seven have been investigated in this study. From west to east, they are: the Sucuoma (235±2Ma), Ajisenduo (224±2Ma), Jiaduocuo (218±1Ma), Cuojiaoma (219±1Ma), Maxionggou (225±2Ma), Dongcuo (222±3Ma) and Daocheng (220±2Ma) plutons. Most of the plutons have granitic compositions and contain high SiO2, Al2O3 and K2O+Na2O, but low MgO, FeO* and CaO contents. They have similar trace element patterns, with depletion in high field strength elements (HFSE, e.g., Nb and Ta) and enrichment in large-ion lithophile elements (LILE, e.g., Rb, Th and U). Samples from the Sucuoma, Jiaduocuo and Dongcuo plutons have similar zircon Hf isotopic compositions (εHf(t)=−1.8 to −0.3, −3.1 to 0.4 and −4.6 to −1.4, respectively), whereas those from the Ajisenduo pluton exhibit more unradiogenic Hf (εHf(t)=−11.9 to −4.8). Additionally, the Sucuoma pluton has the lowest initial 87Sr/86Sr values (0.7060–0.7090) but the least negative εNd(t) (−4.9 to −3.3) values, whereas samples from the Ajisenduo pluton have the highest initial 87Sr/86Sr values (0.7111–0.7160), but the most negative εNd(t) values (−7.9 and −11.3). All the samples have similar high radiogenic Pb isotopic compositions (206Pb/204Pb=18.6–19.4; 207Pb/204Pb=15.7–15.8; 208Pb/204Pb=39.1–40.4). Based on the new geochemical data, it is determined that most of the granitoids are I-type granites and the magma source was a mixture of the metamorphic Kangding Complex, which is considered to be the basement of the western Yangtze Craton, and metasediments from the basement. However, the samples from the Ajisenduo pluton are S-type granites that were derived from partial melting of basement metasediments with only limited components from the Kangding Complex. Together with an evaluation of previously-published work, these new data indicate a dominant magmatic peak at ca. 216Ma for the Late Triassic granitoids of the YAB and that the ages of Late Triassic magmatism become younger eastward towards the Garzê-Litang suture zone. We consider that slab roll-back, with subsequent slab break-off, best explains the origin of these granitic plutons.

Petrological and geochemical features of the early Paleozoic granitic gneisses and iron ores in the Tianhu iron deposit, Eastern Tianshan, NW China: Implications for ore genesis

This paper reports whole-rock geochemical, zircon U–Pb and Hf isotopic data for ore-hosted granitic gneisses, mineral compositions of oxides, and sulfur isotopic data for sulfides in iron ores from the Tianhu deposit, central part of the Eastern Tianshan. Our results can provide crucial constraints on the genesis of granitic gneisses and early Paleozoic tectonic setting of the Eastern Tianshan. LA-ICP-MS U–Pb dating on magmatic zircons yielded weighted mean 206Pb/238U ages of 463 to 438Ma, interpreted as the crystallization ages of the granitic protoliths and the formation ages of the Tianhu Group. Zircon U–Pb age of ore-hosted granitic gneiss (ca. 459Ma) can provide reliable constrains on upper limit for iron mineralization age in the Tianhu deposit. Geochemical characteristics suggest that the protoliths of the Tianhu granitic gneisses are metaluminous to weakly peraluminous high-K calc-alkaline granitic rocks, exhibiting typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Zircon Hf isotopic compositions show a positive trend from ~463 to ~438Ma, indicating that ~460Ma magmas came from both ancient and juvenile sources, whereas ~438Ma magmas involved more juvenile material. Some early Paleozoic granitoids were recently identified in the Eastern Tianshan with the ages between ca. 475 and ca. 425Ma. The formation of these early Paleozoic granitoids was in response to subduction processes, suggesting that subduction of Junggar Ocean probably began in the Early Ordovician and lasted until Late Silurian.Pyrite and pyrrhotite in iron ores have δ34SCDT values from +4.6 to +15.7‰, which are consistent with the marine source, but inconsistent with the magmatic source or those involved evaporites in skarn iron deposit. Geological, geochemical, and isotopic data suggest that the Tianhu iron ores were formed by volcano-sedimentary processes in a subduction environment during the early Paleozoic time, and Tianhu is a metamorphosed volcano-sedimentary iron deposit, rather than a skarn iron deposit as previously proposed.

Tonian emplacement of ophiolites in the southern Brasiliano Orogen delimited by U-Pb-Hf isotopes of zircon from metasomatites

Key parameters in the evolution of the oceanic crust in the little-known Tonian period of the Brasiliano Orogen are presently established through the finding and characterization of zircon in chloritite and tourmalinite enclosed in serpentinite. The Ibaré and Palma ophiolites were selected for the study and both are from the juvenile São Gabriel terrane, Rio Grande do Sul state, Brazil. U-Pb and Lu-Hf isotopic analyses of zircon from metasomatites and a volcanoclastic rock with laser ablation indicate similar ages in both ophiolites, respectively 726±2Ma and 722±3Ma (metasomatites) and 758±4Ma (volcanoclastic rock). Other three representative ages recorded in the metasomatites are 880±12, 836±6 and 780±5Ma, interpreted as successive serpentinization events. Mantle Hf isotope compositions of zircon in the metasomatites range from εHf=+12.1 to +1.4 and in the volcanoclastic rock the sample displays εHf=+11.7 to +6.7. Zircon geochemistry is close to the mantle array. This direct dating of ophiolites sets new limits to the evolution of the Brasiliano Orogen during formation of Gondwana. The youngest possible age for ophiolite accretion is 722Ma.

Early Cretaceous Na-rich granitoids and their enclaves in the Tengchong Block, SW China: Magmatism in relation to subduction of the Bangong–Nujiang Tethys ocean

The Na-rich intermediate-to-felsic granitic rocks provide insights into the generation of magmas in subduction zones. This paper presents zircon LA-ICP-MS U–Pb ages as well as whole-rock geochemical, mineral chemical, and in situ zircon Hf isotopic data on Na-rich granitic rocks from the Tengchong Block, SW China. The granodiorites and associated mafic magmatic enclaves (MMEs) from the Menglian batholith yield zircon U–Pb ages of 116.1±0.8 to 117.8±0.6Ma and 117.7±0.7Ma, respectively. Both host granodiorites and enclaves show calc-alkaline and sodium-rich nature, enrichment in large-ion lithophile elements (LILEs), and variable depletion in zircon Hf isotopic compositions. Euhedral amphiboles in both granodiorites and associated enclaves are magnesian-hornblende with high Mg and Ca and contain euhedral plagioclase inclusions of labradorite to andesine (An36–57) composition. The granodiorite was most likely derived through the mixing of partial melts derived from juvenile basaltic lower crust and a minor evolved component of ancient crustal sources. The quartz monzodiorite–granodiorites and associated MMEs from the Xiaotang-Mangdong batholith yield zircon U–Pb ages of 120.3±1.3 to 122.6±0.8Ma and 120.7±1.5Ma. These rocks are also sodium-rich and show calc-alkaline trend with negative zircon Hf isotopic compositions (−5.55 to +0.58). The MMEs in the host intrusions are monzogabbro with variable and depleted zircon Hf isotopic compositions. The amphiboles in the both host intrusions and the enclaves show Al-rich ferro-tschermakite composition. We infer that the quartz monzodiorite–granodiorites were derived from magmas generated by the melting of ancient basaltic rocks in the lower arc crust induced by the underplating of mantle-derived mafic magmas. The formation of the different types of Na-rich granitic rocks is correlated to the subduction of Bangong–Nujiang Tethyan ocean. A comparison with magmatism in the northern magmatic belt suggests that mantle-derived magmas played an important role in the genesis of Early Cretaceous intrusions from Tengchong to Lhasa Blocks, although crustal melting is the dominant contributor.

Comparison of metamorphic zircons from granulite xenoliths and granulite terrain in northern North China Craton

We conduct a study of petrography, geochronology, trace element and Hf isotope compositions on zircons from granulite terrain and granulite xenoliths in the Zhangjiakou region, northern North China Craton. Most zircons from both types of granulite have core-rim structure in their CL images. All the magmatic zircon cores yield essentially identical ∼2.5Ga ages, whereas metamorphic zircons yield ages of ∼1.85Ga and 120–140Ma, respectively. Most the ∼1.85Ga metamorphic zircons have Th/U ratios lower than 0.4 with a range of 0.01 to 0.63. However, the range of Th/U ratios of Mesozoic (120–140Ma) metamorphic zircons (0.12–3.48) overlaps that of the magmatic zircons (0.12–2.44). The Mesozoic and ∼1.85Ga metamorphic zircons have REE patterns consistent with magmatic zircons, except that the ∼1.85Ga ones show less enrichments and much less negative Eu anomalies than those of magmatic zircons. Therefore, Th/U ratios and the REE patterns cannot be used to distinguish metamorphic zircons from magmatic ones. The ∼1.85Ga metamorphic zircons, however, have lower formation temperatures, oxygen fugacity and 176Lu/177Hf ratios than the Mesozoic ones. We speculate that the ∼1.85Ga metamorphic zircons represent newly grown zircons coexisting with garnets and melt/fluid while the Mesozoic metamorphic zircons were formed quickly without the presence of garnet. The Mesozoic metamorphic zircons vary in their origins; some of these zircons were metamorphic newly grown while the others experienced recrystallization from a U-depleted melt/fluid. Our results suggest that P-T conditions, the melt/fluid and oxygen fugacity play important roles in controlling the trace element distribution and Hf isotope compositions of metamorphic zircons in the granulites.

Early crustal evolution of the eastern Yangtze Block: Evidence from detrital zircon U-Pb ages and Hf isotopic composition of the Neoproterozoic Huashan Group in the Dahongshan area

The Yangtze Block is separated into the eastern Yangtze Block (EYB) and western Yangtze Block (WYB) by the Chongqing–Huaying line (CHL). An integrated study of detrital zircon Lu-Hf isotopes and U-Pb ages from two sandstones in the Huashan Group together with previous studies reveals six major tectonothermal events in the EYB occurring at about 2.87Ga, 2.67Ga, 2.5Ga, 2.0Ga, 1.84Ga and 0.8Ga. Combined with paleogeographic map, this study reveals distinct detrital zircon U-Pb ages and Hf isotopic characteristics in the western EYB and eastern EYB. The former has most zircon U-Pb ages grouped at about 2.0Ga, 2.67Ga and 2.87Ga with sporadic 2.5Ga ages. The ∼2.5Ga zircons show negative to slightly positive εHf(t) values. However, the latter has significant ages grouped at ∼2.0Ga and ∼2.5Ga with negligible 2.67Ga and older zircon ages. The ∼2.5Ga zircons show negative to high positive εHf(t) values. The Early Neoarchean (2.7–2.6Ga) was a significant period for production of mature continental crust in the western EYB and the majority of crustal materials were extracted from the depleted mantle until then. Zircon Hf isotopic compositions indicate that the Late Neoarchean to Early Paleoproterozoic event (∼2.5Ga) involved both reworking of old crust and juvenile crust growth. With previous studies of sedimentary facies, lithochemical compositions, zircon morphology and zircon Hf isotopes, it is proposed that there are unexposed ∼2.5Ga basement rocks in the EYB. During the 2.2–1.8Ga, subduction processes, collisional and post-collisional events were recorded in the north and west of the EYB, which can be correlated to the assembly and breakup of the supercontinent Columbia. Two changes in εHf(t) value at ∼840Ma and ∼790Ma have been found from detrital zircons in the northern margin of the EYB, which probably represent arc–continent collision and continent–continent collision events, respectively. This long-term subduction process occurring at 1100–790Ma in the EYB appeals for a peripheral position for the Yangtze Block in the supercontinent Rodinia.

Provenance, age, and tectonic implications of Neoproterozoic strata in the Jiamusi Massif: Evidence from U–Pb ages and Hf isotope compositions of detrital and magmatic zircons

The Jiamusi Massif forms part of the eastern Central Asian Orogenic Belt (CAOB) and the Pacific oceanic tectonic domain, but its tectonic affinity is highly controversial. Here, we present U–Pb ages and Hf isotope compositions of detrital zircons from five metasedimentary rocks and one monzogranite within the Jiamusi Massif, and our aim was to constrain the depositional ages and provenance of the strata, as well as the tectonic affinity of the massif. Two specimens of two-mica schist from the Upper and Lower Kimkanskaya groups (Russian Far East) yielded zircon age populations at 572, 826, 891, 958, and 1776Ma, and 682, 752, 835, and 925Ma, respectively; a sericite schist and muscovite schist from the Majiajie Group (NE China) yielded zircon age populations at 951, 1232, 1419, 1643, and 2687Ma, and 1151, 1276, 1444, 1623, 1805, and 2652Ma, respectively; and a two-mica schist from the Ximashan Formation of the Mashan Group (NE China) yielded zircon age populations at 994, 1281, 1439, 1647, and 1805Ma. Taking into account the general geological relationships in the area, zircon age populations, and Hf isotopic compositions, combined with an age of 492Ma for a monzogranite that intrudes the Ximashan Formation, we conclude that the Lower Kimkanskaya Group formed in the Neoproterozoic and was sourced mainly from a Neoproterozoic terrane, whereas the Majiajie Group and Ximashan Formation formed in the Neoproterozoic but were sourced dominantly from a Mesoproterozoic terrane. Taking into account previous paleomagnetic data, our dating results and comparisons with zircon age populations in Precambrian rocks from northeastern Gondwana and the southern Siberia, North China, and Tarim cratons, suggest that the Jiamusi Massif has an affinity with the Tarim Craton.

U-Pb Age and Hf Isotopic Compositions of Magmatic Zircons from a Rhyolite Flow in the Porcellanite Formation in the Vindhyan Supergroup, Son Valley (India): Implications for Its Tectonic Significance

AbstractThe Porcellanite Formation in the basal Semri Group of the Proterozoic Vindhyan Supergroup, exposed along the Son River Valley, India, primarily consists of intercalated silicified felsic tuff beds (porcellanite), rhyolitic breccia, and shale. Lensoid patches of very coarse-grained rhyolite occur in a stretch of about 10 km near the eastern end of the valley. These are newly interpreted as rhyolite flows, with flow structures including aligned volcanic fragments and phenocrysts, a few of which are ∼10-cm-long feldspars. Microphenocrysts consist of fragments of patchy perthite and quartz, with subordinate Fe-oxide minerals set in a very fine-grained groundmass of quartz and feldspar. Volcanic rock fragments and fiamme are common. Bulk chemical compositions of nine samples also identify them as rhyolite (SiO2 = 75.24%; Na2O + K2O = 7.06%, CaO = 1.66%). Rare earth element distribution is typical of granitic rocks, with a negative Eu anomaly of 0.57. The samples are enriched in U, Th, La, and Rb and d...

Crustal Accretion and Reworking within the Khanka Massif: Evidence from Zircon Hf Isotopes of Phanerozoic Granitoids

The Central Asian Orogenic Belt (CAOB) is one of the largest Phanerozoic accretionary orogen. (Windley et al., 1990, 2007; Jahn et al., 2000a, b, c; Yakubchuk, 2002, 2004; Xiao et al., 2003, 2004). It is the optimal study area for revealing the accretion and reworking processes of the continental crust. The Khanka Massif is located in the most eastern part of the CAOB, and mainly crops out in the territory of Russia, with a small segment in NE China. In addition, a large number of multi‐stage granitic rocks are formed in geological evolution in this area, recording amounts of information about crustal accretion and reworking processes (DePaolo et al., 1991; Rudnick, 1995; Wu et al., 2011). In view of this, this paper uses the spatial‐temporal variations of trace elements and zircon Hf isotopic compositions of phanerozoic granitoids within the Khanka Massif as a case to reveal the crustal accretion and reworking processes of micro continental massifs from the orogenic belt, further to understand the formation and evolution processes and mechanisms of the global continental crust.According to the statistics of zircon U‐Pb ages of granitoids in the Khanka Massif, indicate that the granitic magmatisms in the Khanka Massif have eleven peaks: 492 Ma, 460 Ma, 445Ma, 430Ma, 425Ma, 302Ma, 287Ma, 258Ma, 249 Ma, 216Ma and 213Ma, it can be divided into eight main stages: Late Cambrian, Middle‐Late Ordovician, Middle Silurian, Late Carboniferous, Early Permian, Middle‐Late Permian—Early Triassic, Late Triassic‐Early Jurassic, Early Cretaceous.The Phanerozoic granitoids in Khanka massif are selected in this paper as a suite of granodiorite‐monzogranite‐syenogranite. The SiO2 contents of the Phanerozoic granitoids exceed 65%, and has high Al2O3, low Mg#, TFe2O3, Cr, Co and Ni contents. This suggests that mixture with mantle‐derived magma did not occur, and it should be a typical crustal source (Lu and Xu, 2011). Combined with evident characteristics of light rare‐earth elements (LREEs) and large ion lithophile elements (LILEs) enrichment, and heavy rare‐earth elements (HREEs) and high field‐strength elements (HFSEs) loss, we suggest that the primary magma was derived by partial melting of lower crustal material (Xu et al., 2009), and geochemical properties of the Phanerozoic granitoids essentially reflect the nature of the magmatic source region.According to the temporal variation of zircon Hf isotopic data of Phanerozoic granitioids, zircon Hf isotopic compositions of Phanerozoic granitoids have a obvious correlation with age. With the decrease of formation time of the Phanerozoic granitoids (Late Cambrian∼Middle‐Late Ordovician∼Middle Silurian∼Early Permian∼Middle‐Late Permian–Early Triassic∼ Late Triassic‐Early Jurassic), εHf (t) values of zircons gradually increase, whereas their TDM2 ages gradually decrease (Paleoproterozoic–Neoproterozoic), suggesting that the generation of granitic magmas from the Khanka Massif could have experienced the change from the melting of the ancient crust to the juvenile crust during Paleozoic to Mesozoic.According to the sample location, it can be found that εHf (t) values of Phanerozoic granitoids have the tendency to decrease with latitude increase, showing that components of the ancient continental crust gradually increase from south to north. However, at the same latitude range, the εHf (t) values of Phanerozoic granitoids also inconsistent. Taken together, these differences reveal the horizontal and vertical heterogeneity of the lower continental crust within the Khanka Massif.According to the relative probability of two‐stage model (TDM2) ages of zircon Hf isotope from Phanerozoic granitoids within the Khanka massif, it could be divided into three stages: (1)Late Paleoproterozoic (2) Mesoproterozoic (3) Neoproterozoic. It reveals that the main part of the continental crust within the Khanka Massif were formed in Late Paleoproterozoic–Neoproterozoic. The Phanerozoic granitoids in the Khanka Massif reworked from the source rocks with different ages(Paleoproterozoic–Mesoproterozoic–Neoproterozoic).

Genesis of the Tianbaoshan Polymetallic Ore District, Yanbian, NE China: Constraints from Geochronology and Isotopic Analysis

AbstractThe large‐scale Tianbaoshan polymetallic (Cu, Pb, Zn, and Mo) ore district in the Yanbian area of China hosts the Xinxing Pb–Zn, Lishan Pb–Zn, Dongfeng Zn–Cu (Pb), and Dongfengbeishan Mo deposits. We report zircon U–Pb and Re–Os age data, geochemical data, and zircon Hf isotope compositions of granitic rocks associated with the Tianbaoshan ore district. Zircon U–Pb dating of the Xinxing, Lishan, Dongfeng, and Dongfengbeishan granitic rocks, all of which are associated with polymetallic mineralization, yielded weighted mean 206Pb/238U ages of 272.1 ± 2.1, 272.7 ± 1.8, 272.5 ± 2.7, and 199.6 ± 1.4 Ma, respectively. The Dongfengbeishan Mo deposit yields molybdenite Re–Os model ages of 200.3 and 190.3 Ma, an isochron age of 196.5 ± 3.1 Ma, and a weighted mean model age of 194.6 ± 2.6 Ma. This suggests that mineralization took place in the ore district in two stages during the middle Permian and Early Jurassic. The intrusive rocks of these two stages are identified as medium‐ to high‐K calc‐alkaline I‐type granites. They are enriched in large ion lithophile elements, depleted in high field‐strength elements, and contain positive zircon εHf(t) values (+4.6 to +13.9), indicating the primary magma originated from the partial melting of juvenile lower crust. The granitic rocks show geochemical characteristics similar to those of coeval rocks in NE China. Based on the new geochemical data and other regional geological studies, we propose that the mineralization of the two stages of granitic rocks in the Tianbaoshan polymetallic ore district occurred in active continental margin settings, but was controlled by different geotectonic mechanisms: the subduction of the Paleo‐Asian Oceanic Plate in the middle Permian and the subduction of the Paleo‐Pacific Plate in the Early Jurassic.

Zircon U–Pb ages and Hf isotope compositions of the Chencai migmatite, central Zhejiang Province, South China: constraints on the early Palaeozoic orogeny

AbstractU–Pb ages and Hf isotope compositions of zircons from the Chencai complex in Zhejiang Province have been determined to provide constraints on mechanisms of migmatization and tectonic evolution related to the early Palaeozoic orogeny in the Cathaysia Block, South China. Zircons from leucosome samples of migmatites are characterized by nebulous overgrowths enclosing inherited cores or occur as newly formed grains with weak zoning. Five samples gave weighted mean ages ranging from 438±3 Ma to 432±4 Ma, which are interpreted as recording the time of anatexis of a regional tectono-thermal event. TheirεHf(t) values range from −21.4 to −4.8 (with peak at −11), with correspondingTDM2ages of 1.73–2.77 Ga (with peak atc.1.9–2.3 Ga), suggesting that the protoliths formed by reworking of ancient crust evolved from Late Palaeoproterozoic – early Archaean crust–mantle differentiation. The migmatization was spatially and temporally associated with reported 460–435 Ma metamorphism with a clockwise pressure–temperature (P–T) path and was most likely controlled by crustal thickening driven by the early Palaeozoic orogenesis. TheTDM2ages of the Chencai complex are consistent with those of the Wuyi–Yunkai structural belt in the Cathaysia Block, but distinct from those (with peak at 2.7–3.0 Ga) of the Badu complex which lacks early Palaeozoic tectono-thermal records. The data support the suggestion that a postulated geological entity, instead of the east domain (the Badu complex being its main part) of the Cathaysia Block, was probably involved in the early Palaeozoic orogeny.

U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U–Th–Pb and Lu–Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472–465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (−0.3 to −8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc–alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes

Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since >4Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle.Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.

Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

ABSTRACTWe report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite，the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have εHf(t) values of magmatic 139–147 Ma zircons from −8.1 to −25.6, with two-stage model ages (tDM2) of 1.71–2.67 Ga, whereas εHf(t) values of inherited 582–844 Ma zircons range from 5.4 to −9.5, with tDM2 of 1.39–2.22 Ma, younger than tDM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, εHf(t) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the tDM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J3–K1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.

Zircon U–Pb geochronology and geochemistry of two types of Paleoproterozoic granitoids from the southeastern margin of the North China Craton: Constraints on petrogenesis and tectonic significance

Two types of Paleoproterozoic granitoids, i.e., K-feldspar/albite granites and granitic gneisses, were recognized from the Wuhe Complex of the Precambrian basement and neighboring areas in the southeastern margin of the North China Craton (NCC). They share the identical 207Pb/206Pb ages of 2096±9Ma and 2096±8Ma defined by zircon SHRIMP U–Pb dating. These granitic rocks are commonly enriched in HREE, Zr, Nb, Ga and Y, and depleted in V, Cr and Ni, most of which have high TFeO/MgO ratios and Zr-saturation temperatures over 850°C, suggestive of an affinity with A-type granite. In addition, both have consistent whole-rock εNd(t) values (−3.0 to −0.9) and zircon Hf isotopic compositions (εHf (t)=−3.3 to +1.9). The latter are similar with ones of the Neoarchean TTG gneisses in the Eastern Block of the NCC. Thus these ∼2.1Ga granitoids in the region were most likely derived from the partial melting of ∼2.5Ga TTG gneisses with minor involvement of coeval mafic crustal materials.The zircon grains from the granitic gneisses can be categorized into core, mantle and rim domains with distinct CL images and trace-element characteristics. The grey mantles exhibit steep REE patterns, low LREE, Th, Pb contents and remarkably low Th/U ratios (<0.1), and define mean U/Pb ages of ∼1.85Ga. The dark rims display relatively higher LREE, U, Th, Pb contents and Th/U values (0.1–0.3) with slightly younger ages of 1.81–1.83Ga than mantles. Accordingly, the zircon mantles and rims are interpreted as metamorphic zircons formed during granulite facies metamorphism and anatectic zircons crystallized from limited melts, respectively. The comparable 2.1Ga magmatism, ∼1.85Ga high grade metamorphism and subsequent anataxis between the granitic rocks in this study and the Jiao-Liao-Ji Belt (JLJB) suggest that the Wuhe Complex and the JLJB share the same tectonic evolutional processes. These two types of granitoids both formed at an extensional background at ∼2.1Ga, perhaps in a subduction-related rifting environment, subsequently suffered from granulite-facies metamorphism and selected partial melting during the Paleoproterozoic collisional orogeny.

The Central Tianshan Block: A microcontinent with a Neoarchean-Paleoproterozoic basement in the southwestern Central Asian Orogenic Belt

The Central Tianshan Block (CTB) is one of the major continental constituents of the western Central Asian Orogenic Belt (CAOB). Its nature and origin is essential for understanding the accretionary processes and the reconstruction of the architectural framework of the CAOB. The Wuwamen Metamorphic Complex (WMC), which is believed to represent the basement of the CTB, was incorporated into the Wuwamen ophiolitic mélange during the Paleozoic evolution of the South Tianshan Ocean. The WMC is mainly composed of amphibolite-facies volcano-sedimentary rocks, including migmatites, orthogneisses, amphibolites, schists and locally marbles. Zircon U–Pb geochronology suggests that the protoliths of the interlayered biotite-two feldspar and biotite-plagioclase-hornblende gneisses in the Yangjuan area were formed between 2529 and 2513Ma and overprinted by amphibolite-facies metamorphism between 1815 and 1809Ma. Subsequently, the gneisses were intruded by ∼1724Ma meta-mafic dykes and ∼787Ma leucogranite dykes/veins. Geochemical data reveal that the biotite-two feldspar gneisses have high-silica adakite affinities with high SiO2 and Na2O contents as well as high Sr/Y ratios (61–117) and slightly evolved zircon Hf isotopic compositions. Consequently, they are interpreted to be the products of partial melting of relatively old subducted oceanic crust. In contrast, the interlayered biotite-plagioclase-hornblende gneisses have high MgO, Cr, Co and Ni contents and prominent negative Nb–Ta–Ti anomalies, indicating derivation from partial melting of an ancient sub-continental lithospheric mantle in a continental arc setting. The ∼1.72Ga meta-mafic dykes have geochemical similarities of typical Fe-Ti basalts and are characterized by incompatible element enrichments and primitive Sr–Nd–Hf isotopic signatures ((87Sr/86Sr)i=0.703446−0.703535); ɛNd(t)=+6.49 to +6.90; zircon ɛHf(t)=+0.57 to +10.6), which is consistent with partial melting of a relatively fertile, isotopically depleted asthenospheric mantle in a post-collisional extensional setting. In addition, the ∼787Ma leucogranites are trondhjemitic in composition and display extremely low total REE contents and highly evolved Nd–Hf isotopic signatures (ɛNd(t)=−4.71 to −4.53; zircon ɛHf(t)=−19.4 to −13.0). They are thought to have no petrogenetic relationship with the host orthogneisses and to be products of a foreign melt injection. Our new data provide robust evidence for a Neoarchean–Paleoproterozoic basement of the CTB. Comparison of the tectono-thermal-magmatic events reveals that the CTB and the Tarim Craton have a partly overlapping Precambrian crustal evolution, and thus the CTB could have originated from the Tarim Craton. However, a major difference between both is the presence of juvenile Mesoproterozoic magmatism in the CTB and its absence in the Tarim Craton and, therefore, further geological and geochronological research is required to determine the precise origin of the CTB and its tectonic affinity with adjacent terranes in the western CAOB.