Rate of growth of the preserved North American continental crust: Evidence from Hf and O isotopes in Mississippi detrital zircons

Detrital zircons from the Mississippi River have been analyzed for U-Th-Pb, Lu-Hf and O isotopes to constrain the rate of growth of the preserved North American continental crust. One hundred and forty two concordant zircon U/Pb dates on grains mounted in epoxy, obtained by Excimer laser ablation ICP-MS method, resolved six major periods of zircon crystallization: 0-0.25, 0.3-0.6, 0.95-1.25, 1.3-1.5, 1.65-1.95 and 2.5-3.0 Ga. These age ranges match the ages of the recognized lithotectonic units of the North American continent in the hinterland of the Mississippi River. Ninety-six zircons mounted on tape, which show no age zonation and were within 7.5% of concordance, were selected to represent the six U/Pb age time intervals and analyzed for Lu-Hf and O isotope by laser ablation MC-ICP-MS and SHRIMP II, respectively. The δ¹⁸O values of the zircons show a small step increase in the maximum δ¹⁸O values at the Archean-Proterozoic boundary from 7.5‰ in the Archean to 9.5‰, and rarely 13‰, in the Proterozoic and Phanerozoic. However, the average value of δ¹⁸O in zircons changes little with time, showing that the increase in the maximum δ¹⁸O values between 2.5 and 2.0 Ga, which can be attributed to an increase in the sediment content of the source regions of younger granitoids, is largely balanced by an increase in zircons with anomalously low δ¹⁸O, which can be attributed to hydrothermally altered crust in the granitoid source region.εHf_i values for the zircons range from 13.1 to −26.9. Zircons derived from juvenile crust, which we define as having mantle δ¹⁸O (4.5-6.5‰) and lying within error of the Hf depleted mantle growth curve, are rare or absent in the Mississippi basin. The overwhelming majority of zircons crystallized from melted pre-existing continental crust, or mantle-derived magmas that were contaminated by continental crust. The average time difference between primitive crust formation and remelting for each of the recognized lithotectonic time intervals, which is defined as crustal incubation time in this study, is 890 ± 460 Myr. There is also a suggestion that the crustal incubation time increases with decreasing age in the Mississippi basin, which is consistent with the declining role of radioactive heat production in the lower crust with time.The average Hf model age (1.94 Ga), weighted by fraction of zircons in the river load is in reasonable agreement with the Nd model age (1.7 Ga) for the Mississippi River. However, if the zircons are weighted by the area of North America covered by the six recognized periods of zircon crystallization the average model age is 2.35 Ga, which compares favorably with an area weighted Nd model age of 2.36 Ga. Our preferred approach is to use the measured O isotope values to constrain variations in the ¹⁷⁶Lu/¹⁷⁷Hf ratio of the granitic source region from which the zircons crystallized, making the assumption that zircons with mantle-like O isotopic ratios have higher ¹⁷⁶Lu/¹⁷⁷Hf than zircons with higher O isotope values. This method gives an average Hf model age of 2.53 Ga, which is 180 Myr older than the constant ¹⁷⁶Lu/¹⁷⁷Hf calculation.The area weighted zircon Hf model ages show two distinct periods of crust formation for the North American continent, 1.6-2.2 and 2.9-3.4 Ga. At least 50% of the preserved North American continental crust was extracted from the mantle by 2.9 Ga and 90% by 1.6 Ga. Two similar periods of crustal growth are also recognized in Gondwana (Hawkesworth C. J. and Kemp A. I. S. (2006) Using hafnium and oxygen isotopes in zircons to unravel the record of crustal evolution. Chem. Geol.226, 144-162.), suggesting that these may be periods of global continental crustal growth. However, we stress that more data from other continents are required before the hypothesis of episodic global continental growth can be accepted with confidence.     

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

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

Destruction of ancient lower crust through magma underplating beneath Jiaodong Peninsula, North China Craton: U-Pb and Hf isotopic evidence from granulite xenoliths

Zircons from granulite xenoliths entrained in a Late Cretaceous mafic dike in the Jiaodong Peninsula, North China Craton (NCC), show three distinct U-Pb age populations. Part of the old zircon grains yield discordant data that project to ages of about 2.4 to 2.5 Ga, a few grains indicate growth at about 2.0 Ga and a third group yield Cretaceous ages with peaks at 120 and 90 Ma. The oldest zircons give Hf T_DM model ages of 2.6-2.8 Ga. These results demonstrate the existence of original Archean lower crust in the Jiaodong region. Zircons of 2.0 Ga have similar Hf T_DM model ages as the Neoarchean-Paleoproterozoic grains, suggesting that these zircons were products of metamorphic recrystallization due to thermal event without juvenile input. Early Cretaceous zircons yield εHf(t) values of − 21 to − 12 and Late Cretaceous zircons large variable εHf(t) from + 4 to − 50. These data suggest that magmatic underplating occurred in the Neoarchean to Earliest Proterozoic lower crust of the NCC, both in the Early and Late Cretaceous. It is suggested that the Mesozoic magma underplating, which also provided the heat source for the voluminous Mesozoic magmatism in the NCC, significantly modified the composition of the Archean to Paleoproterozoic lower crust of the NCC.     

U-Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of ²⁰⁶Pb/²³⁸U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ¹⁸O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ε_Hf (t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between ²⁰⁶Pb/²³⁸U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.     

Early Archean crustal evolution of the Jack Hills Zircon source terrane inferred from Lu-Hf, Pb/Pb, and δO systematics of Jack Hills zircons

Several lines of isotopic evidence - the most direct of which is from Hadean Jack Hills zircons - suggest a very early history of crust formation on Earth that began by about 4.5 Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial ¹⁷⁶Hf/¹⁷⁷Hf ratios and δ¹⁸O systematics for <4 Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic ¹⁷⁶Hf/¹⁷⁷Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5 Ga, Jack Hills zircons younger than ca. 3.6 Ga are more radiogenic than -13ε (CHUR) at 3.4 Ga in contrast to projected values at 3.4 Ga of -20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu-Hf systematics together with a narrow range of mostly mantle-like δ¹⁸O values among the <3.6 Ga zircons (in contrast to the spread towards sedimentary δ¹⁸O among Hadean samples) suggests a period of transition between 3.6 and 4 Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution.     

In situ U-Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust

The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U-Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. ²⁰⁷Pb/²⁰⁶Pb dating of oscillatory-zoned zircon using SHRIMP RG (n = 142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69 Ga.Analyses of ¹⁸O/¹⁶O compositions of olivine from a harzburgite/dunite (n = 8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle (δ¹⁸O_Ol = 6.0 ± 0.4‰) was slightly enriched in ¹⁸O, but not significantly different from that of the modern mantle. Zircon δ¹⁸O measurements from the six felsic rocks (n = 93) record mean or weighted mean compositions ranging from 4.9 ± 0.7‰ to 5.1 ± 0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ¹⁸O_Zr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite δ¹⁸O_WR compositions (6.7-6.9‰) resemble those of modern adakites.LA-MC-ICPMS zircon ¹⁷⁶Hf/¹⁷⁷Hf analyses were obtained from six samples (n = 122). Five samples record weighted mean initial ε_Hf compositions ranging from to 0.5 ± 0.6 to −0.1 ± 0.7 (calculated using λ¹⁷⁶Lu = 1.867 × 10⁻¹¹ yr⁻¹), while one sample records a composition of 1.3 ± 0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf.Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro (δ¹⁸O_WR = 5.5‰) with ∼20% hydrothermally altered gabbro/basalt (δ¹⁸O_WR = 4.0‰) would produce tonalite magmas within the average compositional range observed. ¹⁸O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high ¹⁸O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab.     

U-Pb and Lu-Hf isotope record of detrital zircon grains from the Limpopo Belt - Evidence for crustal recycling at the Hadean to early-Archean transition

Detrital zircon grains from Beit Bridge Group quartzite from the Central Zone of the Limpopo Belt near Musina yield mostly ages of 3.35-3.15 Ga, minor 3.15-2.51 Ga components, and numerous older grains grouped at approximately 3.4, 3.5 and 3.6 Ga. Two grains yielded concordant Late Hadean U-Pb ages of 3881 ± 11 Ma and 3909 ± 26 Ma, which are the oldest zircon grains so far found in Africa. The combined U-Pb and Lu-Hf datasets and field relationships provide evidence that the sedimentary protolith of the Beit Bridge Group quartzite was deposited after the emplacement of the Sand River Gneisses (3.35-3.15 Ga), but prior to the Neoarchean magmatic-metamorphic events at 2.65-2.60 Ga. The finding of abundant magmatic zircon detritus with concordant U-Pb ages of 3.35-3.15 Ga, and ¹⁷⁶Hf/¹⁷⁷Hf of 0.28066 ± 0.00004 indicate that the Sand River Gneiss-type rocks were a predominant source. In contrast, detrital zircon grains older than approximately 3.35 Ga were derived from the hinterland of the Limpopo Belt; either from a so far unknown crustal source in southern Africa, possibly from the Zimbabwe Craton and/or a source, which was similar but not necessarily identical to the one that supplied the Hadean zircons to Jack Hills, Western Australia. The Beit Bridge Group zircon population at >3.35 Ga shows a general εHf_t increase with decreasing age from εHf_3.9Ga = −6.3 to εHf_3.3-3.1Ga = −0.2, indicating that Hadean crust older than 4.0 Ga (T_DM = 4.45-4.36 Ga) was rejuvenated during magmatic events between >3.9 and 3.1 Ga, due to a successive mixing of crustal rocks with mantle derived magmas. The existence of a depleted mantle reservoir in the Limpopo’s hinterland is reflected by the ∼3.6 Ga zircon population, which shows εHf_3.6Ga between −4.6 and +3.2. In a global context, our data suggest that a long-lived, mafic Hadean protocrust with some tonalite-trondhjemite-granodiorite constituents was destroyed and partly recycled at the Hadean/Archean transition, perhaps due to the onset of modern-style plate tectonics.     

Extreme oxygen isotope signature of meteoric water in magmatic zircon from metagranite in the Sulu orogen, China: Implications for Neoproterozoic rift magmatism

Unusual ¹⁸O depletion, with δ¹⁸O values as negative as −10‰ to −4‰ relative to VSMOW, was reported in zircons from ultrahigh-pressure eclogite-facies metamorphic rocks in the Dabie-Sulu orogenic belt, China. But it is critical for the negative δ¹⁸O zircons to be distinguished between magmatic and metamorphic origins, because the ¹⁸O depletion can be acquired by high-T eclogite-facies metamorphism of meteoric-hydrothermally altered low δ¹⁸O rocks. While zircon O diffusion kinetics has placed a reasonable constraint on this, zircon trace element compositions can provide a straightforward distinction between the magmatic and metamorphic origins. This paper reports our finding of unusual ¹⁸O depletion in zircon from granitic gneiss in the northeastern end of the Sulu orogen. Zircon δ¹⁸O values vary from −7.8‰ to −3.1‰ along a profile of 50 m length at Zaobuzhen. They are close to extremely low δ¹⁸O values of −9.0‰ to −5.9‰ for metagranite at Qinglongshan and adjacent areas in the southwestern end of the Sulu orogen. CL imaging suggests that the low δ¹⁸O zircons at Zaobuzhen are primarily of magmatic origin, but underwent different degrees of metamorphic modification. Zircon U-Pb dating yields middle Neoproterozoic ages of 751 ± 27 to 779 ± 25 Ma for protolith crystallization and Triassic ages of 214 ± 10 to 241 ± 33 Ma for metamorphic resetting. However, no metamorphic modification occurs in zircon REE patterns that only indicate magmatic recrystallization and hydrothermal alteration, respectively. Thus, the negative δ¹⁸O zircons are interpreted as crystallizing from negative δ¹⁸O magmas due to melting of meteoric-hydrothermally altered negative δ¹⁸O rocks in an active rift setting at about 780 Ma. The variation in zircon δ¹⁸O values indicates considerable O isotope heterogeneity in its granitic protolith. Zircon Lu-Hf isotope analyses give positive ε_Hf(t) values of 1.6-4.1 and Hf model ages of 1.18-1.30 Ga. This suggests that the granitic protolith was derived from the mid-Neoproterozoic reworking of late Mesoproterozoic juvenile crust. The metagranites at Zaobuzhen and Qinglongshan, about 450 km apart, are two known occurrences of the unusually low δ¹⁸O zircons below −6‰ so far reported in the Sulu orogen. They are similar to each other in both protolith and metamorphic ages, so that they share the same nature of both Neoproterozoic protolith and Triassic metamorphism. Therefore, the locally negative δ¹⁸O zircons may register centers of low δ¹⁸O magmatism during the supercontinental rifting.     

Combined U-Pb SHRIMP and Hf isotope Complex, Rio Negro Province, Argentina： of the Patagonia composite terrane study of the Late Paleozoic Yaminué Implications for the origin and evolution

We have carried out zircon U-Pb SHRIMP dating and Hf isotope determinations on a biotite paraschist and on a tonalitic orthogneiss of the Yaminue Complex,and re-evaluate this complex in the broader context of the tectonic evolution of the Patagonia composite terrane.In the metasedimentary unit （msuYC）,the youngest detrital zircon dated at 318±5 Ma（Mississippian/Pennsylvanian boundary） indicates a Pennsylvanian（or younger） depositional age.The three main age populations peak at 474,454 and 374 Ma.Preliminary Hf isotope data for two detrital zircons（447 and 655 Ma） yieldedε（Hf） values of -0.32 and 0.48,indicating that their primary sources contained small amounts of recycled crustal components(of Calymmian age;T_DM 1.56 Ga).Zircons from the orthogneiss（miuYC;intrusive into msuYC） show a crystallization age of 261.3±2.7 Ma（Capitanian;late middle Permian） which is broadly coeval with deformation,and Neoarchean-Paleoproterozoic inheritance.Meaningful core-rim relationship between Neoarchean zircon cores and late Permian rims is well defined,indicating the occurrence of Archean crust in this sector of Patagonia.Hf T_DM of Permian zircons is mainly Meso-Paleoarchean（2.97-3.35 Ga）,with highly negativeε（Hf） values（ca.-33）.Hf T_DM of inherited Neoarchean zircon cores is also Meso-Paleoarchean（3.14-3.45 Ga） but more juvenile（ε（Hf） = -0.3）.Hf isotopes reinforce the presence of unexposed ancient crust in this area. Combining geological and isotope data,as well as geophysical models,we identify the Yaminue Complex within the La Esperanza-Yaminue crustal block flanked by two other,distinct crustal blocks:the Eastern block which forms part of the Patagonia terrane sensu stricto,located in the eastern Patagonian region,and the Western block forming part of the Southern Patagonia terrane.Their origins and timing of amalgamation to form the Patagonia composite terrane are also discussed.     

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~ 1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)_n ratios, which is typical of igneous zircon. The weak positive correlation between ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~ 2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of ε_Hf values (from + 9.8 to –3.3) and model age T ^C 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive ε_Hf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower ε_Hf and higher T ^C values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~ 2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (ε_Hf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.     

Temperature and Hf-O isotope correlations of young erupted zircons from Tengchong (SE Tibet): Assimilation fractional crystallization during monotonic cooling

Young zircons from crystal-poor volcanic rocks provide the best samples for the investigations of pre-eruption magmatic processes and for testing a possible relationship between zircon Eu anomalies and crustal thickness. We report trace element chemistry and Hf-O isotope compositions of young zircons from 3 Holocene volcanoes in the Tengchong volcanic field, SE Tibet, in order to provide insights into magma evolution processes and conditions for high-K calc-alkaline volcanic rocks in a post-collisional setting. As decreasing zircon Ti content and falling temperature, zircon Hf content and Yb/Sm increase whereas zircon Eu anomaly and Th/U decrease, indicating fractional crystallization of plagioclase and zircon during magma cooling. More importantly, zircon Hf isotope ratio (ε_Hf values) increases with decreasing zircon Ti content and falling temperature (T), suggesting gradually increasing incorporation of relatively high ε_Hf juvenile materials in the crystallizing zircons during magma evolution. Negative correlations between zircon ε_Hf and zircon δ¹⁸O also support open-system magma evolution. Our data suggest fractional crystallization of a magma with simultaneous contamination by high ε_Hf and low δ ¹⁸O juvenile (immature) crustal materials during monotonic cooling after zircon saturation. The low-T, high-ε_Hf and low- δ ¹⁸O zircons may indicate the involvement of the early Cretaceous juvenile granitic country rocks during shallow magma evolution. Average Eu anomalies in zircons from young Tengchong lavas yield crustal thickness of 40.7 ± 6.8 km, consistent with present crustal thickness (42.5 km) determined by geophysical methods.     

Late Neoarchean reworking of the Mesoarchean crustal remnant in northern Liaoning,North China Craton: A U–Pb–Hf–O–Nd perspective

How the earth's crust formed and evolved during the Precambrian times is one of the key questions to decipher the evolution of the early Earth. As one of the few cratons containing well-preserved Eoarchean to Neoarchean basement on Earth, the North China Craton is an ideal natural laboratory to unravel the early crustal evolution. It is controversial whether the Archean tectonothermal events in this area represents reworking or growth of the continental crust. To solve this issue, we have compelled field-based mapping, zircon U–Pb dating by SHRIMP RG and LA–ICP–MS U–Pb, zircon SHRIMP SI oxygen and LA–MC–ICP–MS Hf isotope, and whole-rock Nd–O isotope analyses from the Archean granitoids in northern Liaoning, North China Craton. On the basis of zircon U–Pb isotopic dating and measured geological section investigation, two distinct magmatic suites as enclaves in the Jurassic granites are recognized, viz. a newly discovered 3.0 Ga crustal remnant and a 2.5 Ga granitoid. The Mesoarchean zircons from the 3.0 Ga granodioritic gneisses exhibit heterogeneous Hf isotopic compositions, with the most radiogenic analysis (ε_Hf(t) = +3.8) following the depleted mantle evolution array and the most unradiogenic ε_Hf(t) extending down to −3.4. This implies that both ancient continental crust at least as old as 3.4 Ga and depleted mantle contributed to the magma source of the protoliths of the Mesoarchean gneisses. The ε_Hf(t) values of the Neoarchean zircons from these gneisses overlap the 3.4–3.0 Ga zircon evolution trend, indicating that the ancient crustal materials have been reworked during the late Neoarchean. The Neoarchean zircons from the 2.5 Ga granitoids have a relatively small variation in the Hf isotope and are mainly plotted in the 3.0–2.8 Ga zircon evolution field. However, taking all the ε_Hf(t) values of the Neoarchean zircons into the consideration, we find that the Hf model age of the Neoarchean zircon does not represent the time of crustal growth or reworking but are artifacts of magma mixing. The interaction between the magmas derived from the ancient crustal materials and the depleted mantle is also supported by zircon O isotopic data and Hf–O isotopic modeling of the Neoarchean granitoids. Both Mesoarchean and late Neoarchean tectonothermal events involved synchronous crustal growth and reworking, which may be applicable to other parts of the world.     

Central Antarctic provenance of Permian sandstones in Dronning Maud Land and the Karoo Basin: Integration of U-Pb and TDM ages and host-rock affinity from detrital zircons

In conjugate SE Africa and Antarctica, Early Permian sandstones of the Swartrant Formation of the Ellisras Basin, Vryheid Formation of the Karoo Basin, and Amelang Plateau Formation of Dronning Maud Land (DML) were deposited after Gondwanan glaciation on a westward paleoslope. We analysed detrital zircons for U-Pb ages by a laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) and attached age significance only to clusters of three or more overlapping analyses. We analysed Hf-isotope compositions by a multi-collector spectrometer (LAM-MC-ICPMS) and trace elements by electron microprobe (EMP) and ICPMS. These analyses indicate the rock type and source (whether crustal or juvenile mantle) of the host magma, and a “crustal” model age (T_DM^C). The integrated analysis gives a more distinctive, and more easily interpreted, picture of crustal evolution in the provenance area than age data alone.Zircons from the Ellisras Basin are aged 2700-2540 Ma with minor populations about 2815 Ma and 2040 Ma, which correspond with the ages of the upslope parts of the proximal Kaapvaal Craton and Limpopo Belt. Mafic rock is the dominant host rock, and it reflects the Archean granite-greenstone terrane of the Kaapvaal Craton.The three Karoo Basin samples and the two DML samples have zircons with these common properties: (1) 1160-880 Ma, host magma mafic granitoid (< 65% SiO₂) derived from juvenile depleted mantle sources (ε_Hf positive) at 1.65 Ga and 1.35 Ga, with T_DM^C of 2.0-0.9 Ga; (2) 760 to 480 Ma, host magma granitoid and low-heavy rare earth element rock (?alkaline rock-carbonatite), derived from mixed crustal and juvenile depleted mantle sources (ε_Hf positive and negative) at 1.50 Ga and 1.35 Ga, with T_DM^C of 2.0-0.9 Ga.Together with similar detrital zircons in Triassic sandstone of SE Australia, these properties reflect those in upslope central Antarctica, indicating a provenance of ∼ 1000 Ma (Grenville) cratons embedded in 700-500 Ma (Pan-Gondwanaland) fold belts. Detrital zircons in Cambrian sediments of the Ellsworth-Whitmore Mountains block and Cambrian metasediments of the Welch Mountains with comparable properties suggest that the central Antarctic provenance operated also in the ∼ 500 Ma Cambrian.     

Metamorphic growth and recrystallization of zircons in extremely O-depleted rocks during eclogite-facies metamorphism: Evidence from U-Pb ages, trace elements, and O-Hf isotopes

A combined in situ SIMS and LA-(MC)-ICPMS study of U-Pb ages, trace elements, O and Lu-Hf isotopes was conducted for zircon from eclogite-facies metamorphic rocks in the Sulu orogen. The two microbeam techniques sampled various depths of zircon domains, revealing different element and isotope relationships between residual magmatic cores and new metamorphic rims and thus the geochemical architecture of metamorphic zircons which otherwise cannot be recognized by the single microbeam technique. This enables discrimination of metamorphic growth from different subtypes of metamorphic recrystallization. Magmatic cores with U-Pb ages of 769 ± 9 Ma have positive δ¹⁸O values of 0.1-10.1‰, high Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, high REE contents, and steep MREE-HREE patterns with negative Eu anomalies. They are interpreted as crystallizing from positive δ¹⁸O magmas during protolith emplacement. In contrast, newly grown domains have concordant U-Pb ages of 204 ± 4 to 252 ± 7 Ma and show negative δ¹⁸O values of −10.0‰ to −2.2‰, low Th/U and ¹⁷⁶Lu/¹⁷⁷Hf ratios, low REE contents, and flat HREE patterns with weak to no Eu anomalies. They are interpreted as growing from negative δ¹⁸O fluids that were produced by metamorphic dehydration of high-T glacial-hydrothermally altered rocks during continental subduction-zone metamorphism. Differences in δ¹⁸O between different domains within single grains vary from 0.8‰ to 12.5‰, suggesting different degrees of O isotope exchange between the positive δ¹⁸O magmatic core and the negative δ¹⁸O metamorphic fluid during the metamorphism. The magmatic zircons underwent three subtypes of metamorphic recrystallization, depending on their accessibility to negative δ¹⁸O fluids. The zircons recrystallized in solid-state maintained positive δ¹⁸O values, and REE and Lu-Hf isotopes of protolith zircon, but their U-Pb ages are lowered. The zircons recrystallized through dissolution exhibit negative δ¹⁸O values similar to the metamorphic growths, almost completely reset U-Pb ages, and partially reset REE systems. The zircons recrystallized through replacement show variably negative δ¹⁸O values, and partially reset REE, and U-Pb and Lu-Hf isotopic systems. Therefore, this study places robust constraints on the origin of metamorphic zircons in eclogite-facies rocks and provides a methodological framework for linking the different types of metamorphic zircons to petrological processes during continental collision.     

Zircons from kimberlite: New insights from oxygen isotopes, trace elements, and Ti in zircon thermometry

Zircons found in mantle-sourced kimberlite provide probes into the isotopic chemistry of the asthenosphere and subcontinental lithospheric mantle. However, little is known about the conditions of formation of these zircons. A suite of 88 zircons found in kimberlites from Africa, Siberia, Brazil, and the United States have been analyzed for their Ti concentration and selected zircons were analyzed for their Rare Earth Element (REE) concentrations by ion microprobe. In addition, precise and accurate laser-fluorination oxygen isotope data were obtained for zircons from Brazil (5.1 ± 0.3‰, 1SD) and the Midwest United States (5.3 ± 0.3‰), yielding mantle-like δ¹⁸O values similar to published data for Africa (5.2 ± 0.3‰) and Siberia (5.3 ± 0.2‰). Most megacrysts in this study preserve fine-scale, oscillatory zoning in CL and are generally homogenous in oxygen isotopic composition, consistent with preservation of primary compositions. A few zircons from Brazil show some evidence of chemical zoning due to recrystallization. The Ti content of mantle zircon is in general low with average compositions from each locality of 13 ± 8.4 ppm (1SD, Kaapvaal craton), 12 ± 8.7 ppm (Siberian platform), 18 ± 11 ppm (Brazil), and 4.8 ±4.3 ppm (United States). The recently calibrated Ti in zircon thermometer yields an average temperature of 744 ±62 °C (1SD) for the average of 13 ± 9 ppm Ti, with no correction for pressure, a_TiO2, or a_SiO2. The Ti content of zircons found within rutile nodules from the Orapa kimberlite (Kaapvaal craton) is almost indistinguishable from those with no constraint on a_TiO2, suggesting that reduced a_TiO2 is not responsible for lower than expected mantle temperatures. The average temperature in this study corresponds to ∼3 GPa on a 40 mW/m² cratonic geotherm. If correct, this would suggest that zircon megacrysts from all four cratons formed in the shallow lithospheric mantle. However, there are several possibly confounding effects to this thermometer, including: a pressure correction and disequilibrium zircon growth. Zircons from rutile nodules have REE contents that span the range of mantle zircon REE and are similar to both zircon megacrysts and zircons from metasomatic assemblages.     

Crustal formation in the Nanling Range,South China Block: Hf isotope evidence of zircons from Phanerozoic granitoids

Granitic rocks are the principle agent of crustal differentiation, therefore their origins yield important information on crustal formation and reworking. An extensive survey of zircon Hf isotopes from granitic rocks in a large region can provide a profile of crustal characteristics that may be further linked to previous crustal evolution. In this study, we measured U–Pb ages and Hf isotope compositions of zircon grains extracted from twenty-five Jurassic, five Triassic and two Ordovician granitic plutons from the Nanling Range, South China Block (SCB). Combined with the published Lu–Hf isotopic data for the granitic rocks in the studied and adjacent areas, three domains with different crustal formation histories have been identified in the southern part of the SCB: eastern side, middle part and western side. The eastern side extends to the coastal area of the SCB, with dominant Hf crustal model ages (T_DM2) in zircons falling within the range of 2.2–1.6 Ga. The middle part is partly coincided with the low-Nd model age belt proposed by Chen and Jahn (1998), with zircon Hf T_DM2 ranging from 1.6 to 1.0 Ga. The western side covers the westernmost Nanling Range and the western end of the Jiangnan orogen, in which the granitoids have zircon Hf T_DM2 model ages spanning 2.2–1.8 Ga. The Paleo- to Meso-Proterozoic model ages of the Phanerozoic granitoids in the Nanling Range imply a long-term crustal reworking. Zircons from the western and eastern sides have an average ε_Hf(155 Ma) at around −10, about 4 epsilon units lower than the middle part (ε_Hf(155 Ma) = −6). Hf T_DM2 histogram from the western Nanling Range is similar to that of the Neoproterozoic granitoids in northern Guangxi Province to the west but much lower to the granites in the middle part to the east. The eastern side has a broader range of Hf model ages in zircons, with the main peak low to ca 1.6 Ga, suggesting the reworking of Mesoproterozoic crust. However, granitoids in the middle part have zircon Hf T_DM2 ages at 1.6–1.0 Ga, which indicates the incorporation of younger crust materials into the magma sources. The Hf model ages of granitoids, as well as four zircon xenocrysts with ages around 920 Ma within the Mesozoic granitoids in the middle part, indicate that the middle part has similar crustal features with the eastern Jiangnan orogen. We propose that this low T_DM2 granite belt is probably part of the early Neoproterozoic arc-continent collision belt between different continents (possibly Yangtze and Cathaysia) during the early assembling processes, while the granitoids in the western and eastern sides have similar crustal compositions.     

Origin of the Alxa Block,western China: New evidence from zircon U–Pb geochronology and Hf isotopes of the Longshoushan Complex

The NW–SE trending Longshoushan is in the southwestern margin of the Alxa Block, which was traditionally considered the westernmost part of the North China Craton (NCC). Precambrian crystalline basement exposed in the Longshoushan area was termed the “Longshoushan Complex”. This complex's formation and metamorphism are significant to understand the geotectonics and early Precambrian crustal evolution of the western NCC. In this study, field geology, petrology, and zircon U–Pb and Lu–Hf isotopes of representative orthogneisses and paragneisses in the Longshoushan Complex were investigated. U–Pb datings reveal three Paleoproterozoic magmatic episodes (ca. 2.33, ca. 2.17 and ca. 2.04 Ga) and two subsequent regional metamorphic events (ca. 1.95–1.90 Ga and ca. 1.85 Ga) for metamorphic granitic rocks in the Longshoushan Complex. U–Pb dating of the detrital magmatic zircons from two paragneisses yields concordant ²⁰⁷Pb/²⁰⁶Pb ages between 2.2 Ga and 2.0 Ga, and a small number of metamorphic zircon rims provide a ca. 1.95 Ga metamorphic age, suggesting that the depositional time of the protolith was between 2.0 and 1.95 Ga and that the sedimentary detritus was most likely derived from the granitic rocks in the Longshoushan Complex itself. Zircon Lu–Hf isotopic analyses indicate that nearly all magmatic zircons from ca. 2.0 Ga to ca. 2.17 Ga orthogneisses have positive εHf(t) values with two-stage Hf model ages (T_DMC) ranging from 2.45 to 2.65 Ga (peak at ca. 2.5 Ga), indicating that these Paleoproterozoic granitic rocks were derived from the reworking of the latest Neoarchean–early Paleoproterozoic juvenile crust. Detrital magmatic zircons from two paragneisses yield scattered ¹⁷⁶Hf/¹⁷⁷Hf ratios, εHf(t) and T_DMC values, further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded Paleoproterozoic igneous rocks. The ca. 2.15 Ga detrital magmatic zircons from one paragneiss have negative εHf(t) values with T_DMC ranging from 2.76 to 3.04 Ga, indicating another important crustal growth period in the Longshoushan region. These data indicate that the Longshoushan Complex experienced Neoarchean–Early Paleoproterozoic crustal growth, approximately ca. 2.3–2.0 Ga experienced multiphase magmatic events, and approximately ca. 1.95–1.90 Ga and ca. 1.85 Ga experienced high-grade metamorphic events. The sequence of tectonothermal events is notably similar to that of the main NCC. Together with the datasets from an adjacent area, we suggest that the western Alxa Block was most likely an integrated component of the NCC from the Neoarchean to the Paleoproterozoic.     

胶北太古宙早期锆石U-Pb定年及Hf同位素研究：华北克拉通古老陆壳增生及再循环的证据

古老陆壳物质的发现与鉴别是探索地球早期陆壳形成与演化历史的重要内容之一,锆石U-Pb年龄结合Hf同位素研究是该研究的重要手段。本文通过对胶北地体内一个长英质副片麻岩中的锆石开展系统的原位U-Pb定年和微量、稀土元素分析,获得了多个太古宙早期的锆石。根据这些锆石的阴极发光图像、Th/U比值及稀土元素球粒陨石标准化配分模式,它们具有典型岩浆锆石的特征,其中2个分析点给出了3413Ma和3400Ma(~3.4Ga)的锆石U-Pb年龄,7个分析点给出3547±19Ma(MSWD=1.16)的锆石U-Pb年龄,指示太古宙早期的陆壳岩浆事件;结合华北克拉通其它地区的类似研究结果,暗示华北克拉通可能曾经存在比现今出露面积更大的太古宙早期的古老陆壳。这些古老锆石的Hf同位素分析显示,它们的εHf(t)值在-6.19~0.95之间,平均为-2.54,两阶段Hf模式年龄在3737~4353Ma之间,平均值为~4.1Ga,远大于锆石的U-Pb年龄,指示华北克拉通存在~4.1Ga的地壳增生作用及古老陆壳(3.55Ga)的再循环。     

Episodic crustal growth of North China as revealed by U-Pb age and Hf isotopes of detrital zircons from modern rivers

Clastic sedimentary rocks are samples of the exposed continental crust. In order to characterize the crustal growth history of North China and its possible regional variations, 479 concordant detrital zircons in three sand samples from the lower reach of the Yellow River (which drains the Tibet-Qinghai Plateau, the Western Qinling Orogen, the Qilian Orogen and the North China Craton) and two sand samples from the Luan River and the Yongding River (which run entirely within the North China Craton) were measured for U-Pb age and Lu-Hf isotopic compositions by excimer laser-ablation ICP-MS and MC-ICP-MS. Although regional variations exist, concordant detrital zircons from the Yellow River reveal three major age groups of 2.1-2.5 Ga, 1.6-2.0 Ga, and 150-500 Ma. Detrital zircons from the smaller Luan and Yongding Rivers show three broadly similar major age groups at 2.3-2.6 Ga, 1.6-2.0 Ga, and 140-350 Ma, but with narrower age ranges. Compared to the Luan and Yongding River zircons, the Yellow River zircons are characterized by a significant number of Neoproterozoic grains. Although Hf isotopic compositions show both juvenile crustal growth and crustal reworking for all age groups, much of the crustal growth of North China occurred in the Neoarchean and Mesoproterozoic. All three rivers are characterized by a common prominent group of Hf crust formation model ages at 2.4-2.9 Ga with a peak at 2.7-2.8 Ga. A less significant age group lies between 1.4 and 1.8 Ga for the Yellow River, and between 1.6 and 1.9 Ga for the Yongding River and Luan River. Crustal growth rates based on Hf continental crust formation model ages suggest 45% and 90% of the present crustal volume was formed by 2.5 Ga and 1.0 Ga, respectively, for the drainage area of the Yellow River. In comparison, 60% and 98% of the present crustal volume of the North China Craton was generated by 2.5 Ga and 1.0 Ga, respectively, for the Luan and Yongding Rivers. The 2.7-2.8 Ga age peak observed in all river samples agrees well with the coeval major peak for global crustal growth. However, the other suggested global peaks of crustal growth at 3.4 and 3.8 Ga are insignificant in North China. Taken together with our previous studies of the Yangtze Craton, which show insignificant crustal growth at 2.7-2.8 Ga, we suggest that these advocated worldwide crust formation peaks be re-examined and treated carefully. Our results also show that Phanerozoic zircons may have been derived from crustal sources separated from the mantle up to 2.0 Ga ago before the zircons crystallized, suggesting long-term preservation, reworking and recycling of the continental crust.     

Reworking of Earth's first crust: Constraints from Hf isotopes in Archean zircons from Mt. Narryer,Australia

Discoveries of >4 Ga old zircon grains in the northwest Yilgarn of Western Australia led to the conclusion that evolved crust formed on the Earth within the first few 100 Ma after accretion. Little is known, however, about the fate of the first crust that shaped early Earth's surface. Here we report combined solution and laser-ablation Lu–Hf–U–Pb isotope analyses of early Archean and Hadean detrital zircon grains from different rocks of the Narryer Gneiss Complex (NGC), Yilgarn Craton, Western Australia. The zircons show two distinct groups with separate evolutionary trends in their Hf isotopes. The majority of the zircon grains point to separation from a depleted mantle reservoir at ∼3.8–3.9 Ga. The second Hf isotope trend implies reworking of older Hadean zircon grains. The major trend starting at 3.8–3.9 Ga defined by the Hf isotopes corresponds to a Lu/Hf that is characteristic for felsic crust and consequently, the primary sources for these zircons presumably had a chemical composition characteristic of continental crust. Reworked Hadean crust appears to have evolved with a similar low Lu/Hf, such that the early crust was probably evolved with respect to Lu–Hf distributions. The co-variation of Hf isotopes vs. age in zircon grains from Mt. Narryer and Jack Hills zircon grains implies a similar crustal source for both sediments in a single, major crustal domain. Age spectra and associated Hf isotopes in the zircon grains strongly argue for ongoing magmatic reworking over hundreds of millions of years of the felsic crustal domain in which the zircon grains formed. Late-stage metamorphic zircon grains from the Meeberrie Gneiss unit yield a mean U–Pb age of 3294.5 ± 3.2 Ma with initial Hf isotopes that correspond to the evolutionary trend defined by older NGC zircon grains and overlap with other detrital zircon grains, proving their genetic relationship. This ‘Meeberrie event’ is interpret here as the last reworking event in the precursor domain before final deposition. The continuous magmatic activity in one crustal domain during the Archean is recorded by the U–Pb ages and Hf isotope systematics of zircon grains and implies reworking of existing crust. We suspect that the most likely driving force for such reworking of crustal material is ongoing crustal collision and subduction. A comparison of Hf isotope signatures of zircon grains from other Archean terranes shows that similar trends are recognised within all sampled Archean domains. This implies either a global trend in crustal growth and reworking, or a genetic connection of Archean terranes in close paleo-proximity to each other. Notably, the Archean Acasta gneiss (Canada) shows a similar reworking patterns to the Yilgarn Craton of Hadean samples implying either a common Hadean source or amalgamation at the Hadean–Archean transition.