Lu-Hf Isotope Systematics Research Articles

Experiments on two techniques for the removal of barite from detrital zircon

Abstract Barite (BaSO4) is a common mineral in sandstone that must be removed during separation of detrital zircon (ZrSiO4). One widespread technique for the removal of barite exploits its lesser tenacity by milling the barite and zircon mixture in a ball mill. Here we test the extent to which such milling affects zircon and thus whether the milling could introduce bias into the detrital zircon sample. We then describe a new chemical technique for the removal of barite from detrital zircon. We find that milling a mixture of barite and zircon both breaks and causes loss of zircon grains, potentially introducing bias into a detrital zircon sample. Boiling barite in a 0.94 molar aqueous solution of sodium carbonate for 4 h converts most grains to barium carbonate. The barium carbonate grains are opaque white and thus visually distinguishable from zircon, allowing separation by hand under a stereoscopic microscope. Alternatively, the barium carbonate grains can be dissolved by boiling in 16 wt% nitric acid for 30 min. In our experiments, boiling zircon in sodium carbonate solution and/or concentrated (65 wt%) nitric acid cleaned the surfaces of and the cracks in the grains but did not visibly change the zircon surfaces in other ways. Boiling only in concentrated nitric acid did not measurably affect the U-Pb and Lu-Hf isotopic systems in zircon interiors, and boiling in a sodium carbonate solution followed by concentrated nitric acid did not detectably alter the Lu-Hf isotopic system. However, boiling in a concentrated sodium carbonate solution followed by concentrated nitric acid did disturb the U-Pb isotopic system in zircon interiors. We recognize this disturbance due to a ~0.5% reduction in 206Pb/238U dates of treated zircon. Our results highlight the importance of proper technique during zircon isolation to minimize the introduction of bias into the sample.

The effects of Antarctic alteration and sample heterogeneity on Sm-Nd and Lu-Hf systematics in H chondrites

Long-lived radioactive isotope systematics, such as Sm-Nd and Lu-Hf, are useful tools as important chronometers and tracers for chemical differentiation processes. Even though Antarctic meteorites include rare meteorites such as ungrouped meteorites, the effects of Antarctic alteration on the Sm-Nd and Lu-Hf systems in chondrites have not yet been evaluated in detail. Moreover, the heterogeneity of Sm-Nd and Lu-Hf data in bulk chondrites prevents the determination of precise average Sm-Nd and Lu-Hf values (e.g., for individual chondrite groups). To examine the effects of Antarctic alteration and sample heterogeneity on the Sm-Nd and Lu-Hf isotope systematics, ten Antarctic H chondrites (HCs) and three HCs from hot deserts were characterized for their modal abundances, elemental abundances, and Sm-Nd and Lu-Hf isotopic compositions. Regardless of the classical weathering index for Antarctic meteorites and the normalized Rb abundance used as a chemical alteration indicator in this study, the modal and elemental abundances in Antarctic HCs appear to be in good agreement with those in non-Antarctic HCs. The Sm-Nd and Lu-Hf isotopic compositions of the characterized H chondrites fall within the range measured for both HC falls and for falls of other chondrite groups, except in the case of the most heavily altered sample. Consequently, the effects of Antarctic alteration processes on the Sm-Nd and Lu-Hf systematics in HCs appear to be limited, except in the case of Asuka 09516. The latter meteorite exhibits severe mineralogical and chemical alteration, with considerable losses of even the rare earth elements (REEs), which are considered relatively immobile. The 147Sm/144Nd, 143Nd/144Nd, 176Lu/177Hf, and 176Hf/177Hf of bulk HCs correlate with their P/Mg and Y/Mg. Furthermore, the Lu-Hf ratios correlate strongly with their P/Ca and Y/Ca as well as their P/Mg and Y/Mg. Thus, the distribution of the elements between constituent minerals in ordinary chondrites (OCs) may control the heterogeneity observed for the bulk Sm-Nd and Lu-Hf data. In this context, the weight ratio of Ca-phosphates to Ca-pyroxene, or at least that of Ca-phosphates to silicates, may be a key factor leading to the observed elemental and isotopic variations. This observation indicates that the nugget effect of Ca-phosphates in OCs as the result of insufficient homogenization or terrestrial alteration leads to the heterogeneities displayed by the Sm-Nd and Lu-Hf data. Moreover, it also indicates that the use of equilibrated OCs for the determination of Sm-Nd and Lu-Hf data is affected more by sample heterogeneity, especially with respect to Ca-phosphates, than is the case for unequilibrated OCs, based on the re-distribution of REEs during thermal metamorphism on their parent bodies. This study demonstrates that Antarctic meteorites commonly preserve their original Sm-Nd and Lu-Hf isotopic compositions as much as chondrite falls, although exceptions are possible in the case of severe alteration. Similar to previous studies, we recommend the use of unequilibrated chondrites, for which the re-distribution of REEs is less extensive, for the determination of well-constrained average Sm-Nd and Lu-Hf isotopic compositions for individual chondrite groups as well as their robust Chondritic Uniform Reservoir values.

Variation in the analytical figure of merit for in situ Hf isotopic measurements by LA-MC-ICPMS in zircon standards at variable spatial resolutions.

Lu-Hf isotopic systematics is a powerful tool and gained importance as a geochemical tracer in geosciences. A multi-collector inductively coupled plasma mass spectrometer coupled with a laser ablation system has been used to perform zircon hafnium (Hf) isotopic analysis, and protocol is established for measurement of Hf isotopes. Toward this, two zircon standards, Z91500 and Plesovice, have been used for Hf isotopic measurements at four different spot sizes (50, 40, 25, and 20 μm). The isotopic data for ≥25 μm have an excellent agreement with published data, but for 20 μm, the results are erroneous and irrelevant to geological application. It has been observed that Hf beam intensity has a linear relation to the volume of the target (RM) ablated. 178 Hf/177 Hf ratio and ɛHf for both standards are comparable with the published recommended values. The present study indicates that 176 Lu/177 Hf ratio for Z91500 is more homogeneous than Plesovice zircon. The 176 Hf/177 Hf isotopic ratio for 50 and 25 μm shows an excellent agreement with the previously reported data. However, for 40-μm spot size, a slightly higher but negative offset of -123 and -105 ppm in 176 Hf/177 Hf ratio have been observed for Plesovice and 91500, respectively.

Zircon U-Pb geochronology and Lu-Hf isotope systematics of the Araguaia Belt basement Rocks: Evidence of links with the southeastern Amazonian Craton, Brazil

Archean and Paleoproterozoic basement rocks are exposed in the Araguaia Belt, a Neoproterozoic orogen bordering the eastern portion of the Amazonian Craton. Zircon LA-MC-ICPMS U-Pb and Lu-Hf isotopic systematics allowed a more accurate dating of the basement rocks and investigating the evolution of this large Archean/Paleoproterozoic crustal segment and discuss the relationship with the southeastern Amazonian Craton. Archean TTG orthogneisses (2.93 to 2.87 Ga) prevail in the northern belt while distinct magmatic events generated their protoliths in approximately 60 Ma. Zircon Hf model ages (~3.0 to 3.4 Ga) and ƐHf(2.87-2.93) from −4.3 to +5.3 suggest crustal reworking of Mesoarchean juvenile sources, with the subordinated contribution of older crust. A Mesoarchean source is also indicated by the Hf model ages (2.93–3.25 Ga) and ƐHf(−1.86 Ga) from −4.8 to −1.6 of a 1.86 Ga granitic gneiss intrusive in the Archean TTG. An orthogneiss and an intrusive granitoid from southern segment of the belt were dated 2.06 Ga and 1.87 Ga, respectively. The Hf model ages (2.65–2.86 Ga) determined for these rocks, the ƐHf(-2.06 Ga) (−4.8 to −1.6) for the orthogneiss and the ƐHf(1.87 Ga) (−15.8 to −9.7) found for the granitoid reveal a dominant crustal source for the southern basement rocks. The Archean and Paleoproterozoic basement constituted a single crustal segment by the Orosirian, since both segments present intrusive granites of this geological period. The Archean orthogneisses of the Araguaia Belt and the granitoids of the Carajás Province Mesoarchean domains in southeastern Amazonian Craton share some similarities, such as the TTG affinity magmatism formed in a relatively similar time interval, the Orosirian intrusive granites, and the Mesoarchean juvenile source. Therefore, the basement rocks cropping out in the northern segment of the Araguaia Belt may represent portions of the adjacent Amazonian Craton.

The U-Pb Zircon Geochronology (LA-ICP-MS) of Geological Processes in Granulites of Middle Bouh Area. Article 3. Rock association in the lower reaches of the Yatran river

LA-ICP-MS method was applied to investigate U-Pb and Lu-Hf isotope systematics of zircon crystals from charnockitic gneiss and biotite-garnet-hypersthene enderbite that occur in the lower reaches of the Yatran river (Yatran block of the Bouh river area). According to the obtained isotope data, charnockitic gneiss hosts three zircon populations. The oldest one is represented by three crystals that have isotope age between 3125 and 3300 Ma, and εHf values between –2.3 and –7.5. The next population is well-defined, it has an age of 2038±25 Ma and large variations of Hf isotope composition: 176Hf/177Hf — from 0.28122 to 0.28261, εHf — from –9.3 до 4.6. However, the ages of most of the analyzed zircons spread along the concordia between 2300 and 2800 Ma. All zircons in this population have a similar Hf isotope composition 176Hf/177Hf = 0.28072 to 0.28092, which does not depend on the age. It is characteristic that the oldest (with preserved U-Pb isotope systematics) crystals have positive or slightly negative εHf values. Most of the U-Pb isotope analyses of zircons from enderbite fall on the discordia line that has an upper interception age of 2029 ± 18 Ma. A small number of discordant grains have 207Pb/206Pb ages up to 2500 Ma. Hafnium isotope composition in zircons from enderbite varies widely: 176Hf/177Hf = 0.28131 to 0.28151, and εHf from –6.2 to 1.8.

Anatexis of the deeply subducted continental crust during exhumation: A case study of Rongcheng migmatite in the Sulu orogen

Migmatite in the Sulu ultra‐highpressure (UHP) terrane is an ideal sample to provide information on anatexis during the exhumation of the deeply subducted continental crust. The studied migmatite is composed of the leucosome and melanosome with strongly ductile shear deformation. Petrographic observations clearly show an anatexis process, for example, cuspate and triangular K‐feldspars in triple junction and along grain boundaries, felsic veinlets and pockets mainly consisting of K‐feldspar + quartz + plagioclase ± biotite. The zircon grains of the migmatite have a core‐rim structure: the inherited magmatic core and the anatectic overgrowth rim. The inherited cores exhibit oscillatory zonings in the cathodoluminescence (CL) images and display strong heavy rare earth element (HREE) enrichment with clearly positive Ce and negative Eu anomalies. The inherited cores have an upper intercept U‐Pb age of 814 ± 45 Ma with εHf(t) values of −5.6 to ‐10.6 (mean = −8.5 ± 0.8, n = 16). Thus, protolith of the migmatite is a mid‐Neoproterozoic granitic rock. The overgrowth rims exhibit a typical anatexis genesis, that is, a bright homogeneous luminescence in the CL images, very low Th/U ratios (<0.02) with low U (<100 ppm) and Th (<1 ppm) contents, and lower total REE and Y contents with lower (Gd/Lu)N ratios and higher Hf/Y ratios than the inherited cores. They are Late Triassic with ages ranging from 211 to 234 Ma (mean = 223 ± 8 Ma), which, combined with previous studies on the Sulu UHP terrane, suggests that the anatexis occurred during the early exhumation stage. They have higher Ti‐in‐zircon temperatures ranging from 760 to 813°C (mean = 791 ± 16°C) than the pre‐anatexis zircon (244 ± 5 Ma and 739°C), suggesting a ‘hot’ exhumation. They have εHf(t) values of −17.6 to −14.5 (mean = −15.5 ± 0.6) which are lower than the estimated value (~12.0) on the crustal evolution array, suggesting an addition of crustal fluid during the anatexis. The anatexis also dramatically affected the zircon Lu–Hf isotopic system, that is, 176Yb/177Hf and 176Lu/177Hf isotopic ratios are markedly decreased but initial 176Hf/177Hf ratios are increased from the inherited magmatic cores to the anatectic rims. Thus, we propose that the anatexis plays a critical role in the exhumation of the UHP metamorphic rocks. The anatexis could lubricate the subducted slices and further facilitate their buoyant rise from the mantle depth to the crustal level.

Neoarchean–Paleoproterozoic crustal growth and tectonic evolution of the Trans-North China Orogen, North China Craton: evidence from granite–greenstone successions in the Dengfeng Complex

Granite–greenstone successions and associated tonalite–trondhjemite–granodiorite (TTG) suites constitute dominant components of Precambrian cratons. Here, we present petrology, geochemistry, zircon U–Pb geochronology, and Lu–Hf isotopes on monzonite, granodiorite, granite, and amphibolite from the Dengfeng Complex in the southern part of the North China Craton (NCC). Zircon U–Pb data yield weighted mean 207Pb/206Pb ages of 2498 ± 6 Ma for monzonite, 2481 ± 16 Ma for granodiorite, and 2475 ± 21 Ma for granite. Lu–Hf isotope systematics show eHf(t) values ranging from 1.3 to 6.7 and crustal model ages ( $$T_{{{\text{DM}}}}^{{\text{C}}}$$ ) in the range of 2584–2934 Ma for the rocks. The monzonite, granodiorite, and granite show moderate total rare-earth element (REE) contents, enrichment in light REEs (LREE) and depletion in heavy REEs (HREE), negative Nb, Ta, and Ti anomalies, high Al2O3, Sr/Y, and (La/Yb)N ratios, with low Cr and Ni, indicating that the magma was possibly sourced from partial melting of the lower crust. The geochemical features of amphibolites are characterized by low SiO2, Na2O, K2O, Sr/Y, (La/Yb)N and high FeO, low total REEs values, limited LREE/HREE fractionation, and absence of obvious Ce anomaly, showing similarity to island arc magmatic rocks. Our results are consistent with the model of subduction and arc–continent collision in the Trans-North China Orogen during the Neoarchean to Paleoproterozoic.

Carbonates at the supergiant Olypmic Dam Cu-U-Au-Ag deposit, South Australia part 2: Sm-Nd, Lu-Hf and Sr-Pb isotope constraints on the chronology of carbonate deposition

Mineralization at the supergiant Cu-U-Au-Ag Olympic Dam deposit (South Australia), the ‘uranium endmember’ of the iron-oxide copper–gold (IOCG) spectrum of ore deposits, is hosted in a breccia complex developed entirely within granite of the 1.59 Ga Hiltaba Suite (Gawler Craton). Earlier studies suggested brecciation and mineralization occurred within a magmatically-driven hydrothermal system at 1.59 Ga, with a critical role for mafic–ultramafic intrusions. In contrast, recent radiometric dating of the breccia complex indicates a prolonged, multi-stage history of brecciation and mineralization from 1.59 to 0.5–0.4 Ga. Ca-Fe-Mg-Mn-carbonate gangue minerals are associated with ore minerals at virtually every stage of mineralization. In a companion study (Apukhtina et al., 2020), this mineralogically, texturally and compositionally diverse carbonate mineral suite was assigned to seven associations defined on the basis of host lithology and texture. Here we report Sm-Nd, Pb-Pb and Lu-Hf isotope ages for these carbonates, which are used to examine the chronology of carbonate deposition. Initial Sr-Nd isotopic compositions are used to place constraints on fluid sources. Sm-Nd and Pb-Pb isotope systematics of calcite veins in ~1.59 Ga IOCG ore indicate 1.59–1.55 Ga deposition ages. Likewise, locally abundant laminated siderites have Sm-Nd ages in this age interval. A world-first attempt to apply Lu-Hf dating to carbonate gangue in an ore deposit yields ages that are 70–100 Ma younger than corresponding Sm-Nd ages, presumably reflecting isotopic exchange of carbonate Lu-Hf isotope systems with host rocks. Sm-Nd ages for carbonates assigned to other carbonate associations (hosted in highly altered inferred 1.59 Ga basalt and picrite; diverse settings within granite-dominated breccia; locally abundant megaclasts of green and red bedded sandstone/mudstone sequences; ~0.82 Ga doleritic dykes) are more diverse and range from ~1.59 to 0.5 Ga. The structurally youngest carbonates (unbrecciated fluorite-barite veins; carbonate matrix in polymict conglomerate above the breccia complex) yield ~0.50 Ga Sm-Nd ages. Inferred carbonate ages are broadly consistent with radiometric dates for other hydrothermal minerals (e.g., hematite, uraninite, apatite, fluorite). They suggest that mineralization initiated at 1.59 Ga was reworked and possibly increased in size in response to large-scale tectonic, magmatic, sedimentary and hydrothermal events. Initial 87Sr/86Sr in the carbonates is higher and more variable (0.710–0.752, average ~0.721) than could be explained by ore and gangue mineral formation from magmatic-hydrothermal fluids during a single event at 1.59 Ga, a model favored in several earlier studies. By contrast, carbonate formation over a long period, as inferred from the Sm-Nd chronology presented here, would allow ingrowth of 87Sr in the granitic host rocks to develop the heterogeneous initial 87Sr/86Sr recorded in the carbonates. Carbonate-bearing fluids appear to have sourced Nd (and most likely also Sr) locally, within the host granite and breccia, with contributions from mafic rocks. The emerging evidence for protracted, multi-stage mineralization implies that single-stage models for Olympic Dam need to be revisited and that all studies of sulfide and gangue minerals in this deposit require careful radiometric dating. We speculate that the polymetallic nature and unusually large metal reserves of the Olympic Dam mineralization are related to its multi-stage formation history.

Discovery of Neoproterozoic highly fractionated syenogranite in the southwestern part of the Erguna Massif in NE China and its geological implication

ABSTRACT Earlier studies of the Neoproterozoic magmatism and tectonic evolution in Erguna Massif of NE China are spatially confined to the central and northern parts of the area, but the southwestern part of the massif is poorly studied. This contribution documents new zircon U-Pb geochronology, Lu-Hf isotopic systematics, and whole-rock geochemistry from Neoproterozoic granites in the southwestern part of the area. The aim of this study is to determine the petrogenesis of the granites and further elucidate on the geodynamic setting of Neoproterozoic magmatism in the massif and their geological implications. Syenogranite samples from the region yield LA-ICP-MS U-Pb zircon weighted mean ages of 853 ± 5 Ma and 824 ± 3 Ma, and both dates are evidence for Neoproterozoic magmatism in the southwestern part of the massif. Both syenogranite samples are peraluminous (A/CNK = 1.06–1.43), calc-alkaline with high-K values, and assay 76.06–76.52 wt% SiO2, 1.98–3.20 wt% Na2O, and 4.63–4.77% wt% K2O. The syenogranites’ Chondrite-normalized REE pattern display a ‘gull-wing’ shape, with low (La/Yb)N ratios of 2.28–4.29 and significant negative Eu anomalies of δEu = 0.09–0.19. The syenogranites are characterized by depletion in Ba, Nb, Ta, Sr, P, and Ti and enrichment in Rb, Th, U and K compared to the N-MORB and non-fractionated granites elsewhere in the massif. These geochemical characteristics show that the Neoproterozoic syenogranite plutons in SW Erguna Massif are highly fractionated. Zircons from the granites also have εHf(t) values of +7.68 to −1.48 with corresponding two-stage model ages of ca. 1828 to 1258 Ma, indicating that they formed from the partial melting of a Palaeo- and Mesoproterozoic igneous source in the middle crust. In combination with previously published geological data, it is proposed that the Neoproterozoic granites in the southwestern study area were emplaced in an extensional environment before or during the initial stages of the break-up of the Rodinia.

Petrogenesis and Lu–Hf Dating of (Ultra)Mafic Rocks from the Kutná Hora Crystalline Complex: Implications for the Devonian Evolution of the Bohemian Massif

Abstract The Lu–Hf isotope system and Sr–Nd–Hf–Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm–Nd and U–Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian–Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutná Hora Crystalline Complex (KHCC: Úhrov, Bečváry, Doubrava, and Spačice localities). Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Úhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Bečváry and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively. Multiphase solid inclusions enclosed in garnet from Úhrov and Bečváry peridotites represent trapped H2O ± CO2-bearing metasomatizing agents and Fe–Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Úhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spačice) and crustal-derived material (Doubrava) both metamorphosed at high P–T conditions. The Lu–Hf age of 395 ± 23 Ma obtained for the Úhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spačice and Doubrava eclogites yield younger Lu–Hf ages of ∼350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains. We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Úhrov) beneath the Teplá–Barrandian at ≥380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Bečváry, Doubrava). This was followed by continental subduction (∼370–360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at ∼350–330 Ma.

Zircon U–Pb and Lu–Hf isotopic systems in ediacaran to Fortunian “Taourirt” granitic ring complexes (Silet and In Tedeini terranes, Tuareg shield, Algeria)

The Tuareg shield in North-West Africa is composed of an assembly of Neoproterozoic continental and oceanic terranes, as well as Archean and Paleoproterozoic terranes separated by major continental shear zones. In the Silet terrane, the igneous suite of Silet-Taourirts is composed of post-collisional ring-complexes of ferroan alkaline and alkali-calcic granites. Six ring-complexes have been dated using the U–Pb SHRIMP technique on zircon. The alkaline Tin Erit (584.8 ± 2.0 Ma), Tihoiiarene (569.8 ± 4.8 Ma) and Tioueine (561 ± 6 Ma) complexes are older than the alkali-calcic Issedienne (538.7 ± 2.9 Ma), Tesnou (536.5 ± 6.7 Ma) and Ait Oklan (529.3 ± 3.1 Ma) complexes. Sharing an enriched mantle source, the complexes display a secular variation of crustal contamination marked by decreasing εHf(t). The oldest alkaline complexes are related to the tectonic escape of the Tuareg terranes to the north after the climax of Pan-African orogeny, while the youngest alkali-calcic complexes were emplaced after the 575-555 Ma intra-continental Murzukian event.

Uranium-Lead and Lu–Hf Isotope Systematics of Zircon and Sm–Nd Isotopes in Rocks of the Dyumtaley Ore-Bearing Intrusion (Taimyr, Russia): New Evidence for the Role of the Depleted Mantle in Its Genesis

This study utilized SHRIMP, LA MC–ICP–MS and ID-TIMS analytical techniques and presents for the first time U–Pb age, Lu-Hf-isotope features of zircon and the Nd-isotope composition of variously mineralized rocks from the ore-bearing Dyumtaley intrusion, one of the most promising targets for platinum-group element (PGE)-Cu–Ni ores within the Taimyr metallogenic province. 206Pb/238U ages of zircon from monzodiorite and gabbro of the Dyumtaley intrusion (250.9 ± 3.1 to 253 ± 1 Ma, respectively) indicate their close temporal relationship to a Permo-Triassic boundary and synchronism with a tholeiite-basaltic volcanism of the Siberian platform. Hafnium-neodymium isotope signatures (eHf(t) 6.9–8.5, eNd(t) 2.9–4.2) imply the significant role of the depleted mantle material in magma generation of the Dyumtaley intrusion.

A clearer view of crustal evolution: U-Pb, Sm-Nd, and Lu-Hf isotope systematics in five detrital minerals unravel the tectonothermal history of northern China

AbstractMuch of the global picture of crustal evolution has been constructed using zircon. While this has revealed a rich and complex history, this view is necessarily incomplete because of the lithology-specific affinity of zircon and the high temperatures needed to reset the U-Pb and Lu-Hf systems inherent within it. Here we use a five mineral, multi-isotope system approach to compare the record of crustal evolution recorded by zircon versus the picture provided by monazite, titanite, apatite, and rutile from the Yong-Ding and Luan rivers, northern China. These other minerals sample more diverse lithologies and temperature-pressure conditions that reflect additional tectonothermal events to those recorded solely by zircon. Zircon from both studied rivers predominantly reflects magmatic features, yielding age peaks at 2.6–2.3, 2.0–1.8, and 0.38–0.13 Ga, corresponding to the major magmatic events in their catchments. However, the detrital zircon record from both catchments fails to record and detail several important tectonothermal events. Specifically, the detrital monazite U-Pb ages cluster into two Paleoproterozoic peaks of ca. 1.95 and 1.85 Ga, while detrital apatite and rutile ages document unimodal and protracted U-Pb age peaks at 1.9–1.6 Ga. The different U-Pb closure temperatures of monazite, apatite, and rutile likely record two metamorphic events and the subsequent cooling history—key details that are absent from or obscured in the zircon record. The Phanerozoic mineral U-Th-Pb ages correspond to multiple magmatic events between 0.40 and 0.24 Ga and subsequent 0.24–0.20 Ga metamorphism. The 0.60–0.25 Ga rutile U-Pb ages along with 0.33–0.24 Ga U-Pb ages in some zircon grains with radiogenic Hf isotope compositions from the Luan River do not match the geological records in the North China Craton, but instead reflect the protracted subduction-accretionary history of the Central Asian Orogenic Belt. In addition to their U-Th-Pb ages, Nd model ages of monazite, titanite, and apatite, plus zircon Hf model ages provide additional constraints on regional crustal evolution. The Nd model age information is blurred by the fact that the relationship between the Sm/Nd of these minerals and their former host rocks is not precisely known. Taken at face value, the monazite Nd model ages have two Neoarchean peaks at 2.9–2.7 and ca. 2.5 Ga, that may correspond to two crustal growth episodes, while the titanite Nd model ages with predominant peaks at 2.2–1.8 and 1.5–1.3 Ga broadly correspond with those derived from the whole-rock analyses of the wide spread Phanerozoic granitoids, and hence record extensive crustal reworking. In contrast, the zircon Hf model ages are strongly skewed to a 2.9–2.7 Ga period and fail to record the post-Archean evolution of this region. These data highlight the power of integrating the U-Th-Pb age and Lu-Hf/Sm-Nd isotope compositions of multiple detrital minerals, with a broad range in geochemical behavior and closure temperatures, to gain a more complete understanding of tectonothermal history and crustal evolution than zircon alone.

Two metamorphic gold mineralization events confirmed by Lu-Hf isotope dating of garnet in the Late Archean Storø Au deposit, Nuuk region of SW Greenland

We report Lu-Hf isotope ages for garnet from the Late Archean Storø Supracrustal Belt (SSB) ranging from 2697 ± 8 Ma to 2646 ± 13 Ma. These ages essentially overlap with two previously reported Re-Os isotope ages for arsenopyrite associated with Au mineralization within the SSB, which implies that both Au-forming events took place while garnet was stable and thus occurred during amphibolite-facies metamorphic conditions.Furthermore, we report bulk-rock Lu-Hf isotope data for amphibolite and aluminous gneiss from the SSB. The latter rock type has previously been suggested to represent the pre-metamorphic hydrothermal alteration product associated with low temperature epithermal Au formation. However, the initial Hf isotope compositions of these two rock types are significantly different, and therefore demonstrate that they do not share a common origin, because the Lu-Hf isotope system is essentially fluid-immobile. Instead, the aluminous gneiss represents a pelitic rock with mixed mafic and felsic sources, which is in agreement with the formation of both Au mineralization events during orogenic amphibolite-facies metamorphic processes, which were therefore structurally controlled.A too young bulk-rock errorchron age of 2553 ± 72 Ma for the amphibolite unit (>2720 Ma) is interpreted to reflect resetting of the system during the intrusion and melt infiltration of the ca. 2560 Ma Qôrqut Granite Complex.

The U-Pb Zircon Geochronology (LA-ICP-MS) of Geological Processes in Granulites of Middle Bouh Area. Article 2. Rock association in the Chausove open pits

The LA-ICP-MS method was applied to investigate U-Pb and Lu-Hf isotope systematics of zircon crystals from charnockitoids that crop out in the open pits near Chausove village, Pervomaisk district. The association of enderbite gneisses, mafic and ultramafic (pyroxenite) granulites and vein bodies of pink biotite granites occur in the open pits. Enderbites are rather heterogeneous, and their composition varies from leucocratic quartzite-like rocks to enderbite-diorite. The Pervomaisk open pit is located within charnockite massif. Our data indicates that the association of enderbites and charnockites crystallised at 2850-2760 Ma from juvenile material (Hf isotope composition in zircons from enderbite-diorite is 176Hf/177Hf = 0.280915 ± 12, εHf2850 = +2; from leucocratic charnockite 176Hf/177Hf = 0.280875 ± 12, εHf2760 = –1; and from charnockite 176Hf/177Hf = 0.280875 ± 12, εHf2760 = –1). These rocks represented the newly formed Meso-Neoarchean crust, in contrast to the reworked older continental crust. This association has experienced a significant transformation in Paleoproterozoic. The last stage of transformation was accompanied by crystallisation of a new generation of zircon and formation of monazite at 2.04-2.06 Ga.

Decoupling of Lu-Hf and Sm-Nd Isotopic System in Deep-Seated Xenoliths from the Xuzhou-Suzhou Area, China: Differences in Element Mobility during Metamorphism

This paper presents whole-rock Hf isotopic data for a suite of eclogite and garnet clinopyroxenite xenoliths hosted in the Early Cretaceous dioritic intrusions from the Xuzhou-Suzhou area along the southeastern margin of the Eastern Block of the North China Craton (NCC). Six of the eight studied xenolith samples plot significantly above the terrestrial Hf-Nd isotopic array and have eHf(0) value up to +60. All the samples define a well correlated 147Sm/144Nd-143Nd/144Nd age of 2 081 Ma, which is considered to record the granulite-facies metamorphism. In contrast, the Lu-Hf isotope system faithfully records the protolith information. The mineralogical assemblage, especially garnet and/or zircon (rutile to some extent) mainly controlled the decoupling of Hf-Nd isotope. The metamorphic modification on protolith characteristics and the differences in element mobility during metamorphism may also reinforce the observed decoupling between the Sm-Nd and Lu-Hf isotope systems; i.e., the absence of the correlations in eNd-eHf and also 87Sr/86Sr-143Nd/144Nd diagram. The Lu/Hf isochron age of 2 424 Ma is similar to the zircon age peak of the studied xenoliths and the dominant age of NCC basement, indicating that the igneous protolith has an affinity to the Archean basement of the NCC. Furthermore, the positive eHf(t) values at 2 500 Ma indicate a crustal growth event of 2 500 Ma in the NCC.

Constraints on Archean crust formation from open system models of Earth evolution

Establishing the mode and rate of formation of the continental crust is crucial for quantifying mass exchange between Earth’s crust and mantle. The limited crustal rock record, particularly of early Archean rocks, has led to a variety of different models of continental growth. Here, we present an open-system model of silicate Earth evolution incorporating the Sm-Nd and Lu-Hf isotope systematics with the aim to constrain crustal growth during the Archean and its effect on the chemical and isotopic evolution of Earth’s crust-mantle system. Our model comprises four reservoirs: the bulk continental crust (CC), depleted upper mantle (UM), lower mantle (LM), and an isolated reservoir (IR) where recycled crust is stored transiently before being mixed with the LM. The changing abundance of isotope species in each reservoir is quantified using a series of first order linear differential equations that are solved numerically using the fourth order Runge–Kutta method at 1 Myr time steps for 4.56 Gyr (the age of the Earth). The model results show that only continuous and exponential crustal growth reproduces the present-day abundances and isotope ratios in the terrestrial reservoirs. Our preferred crustal growth model suggests that the mass of the CC by the end of Hadean (4.0 Ga) and end of Archean (2.5 Ga) was ∼30% and ∼75% of the present-day mass of the CC, respectively. Models proposing formation of most (∼90%) of the present-day CC during the initial 1 Gyr or nearly 50–60% during the last 1 Gyr are least favorable. Significant mass exchange between crust and mantle, that is, both the formation and recycling of crust, started in the Hadean with Sm-Nd and Lu-Hf isotope evolution typical for mafic rocks. Depletion of the UM (in incompatible elements) during the early Archean is mitigated by the input of recycled crust, so that the UM maintained a near-primitive Hf-Nd isotope composition. The LM also retained a near-primitive Hf-Nd isotope composition during the Archean, but for different reasons. In contrast to the UM, the crustal return flux into the LM is transiently stored (∼ 1 Gyr) in an isolated reservoir (IR), which limits the mass flux into and out of the LM. The IR in our model is distinct from other mantle reservoirs and possibly related to stable crustal blocks or, alternatively, to recycled crust in the mantle that remains temporarily isolated, perhaps at the core-mantle boundary (LLSVPs).

Dating post-Archean lithospheric mantle: Insights from Re-Os and Lu-Hf isotopic systematics of the Cameroon Volcanic Line peridotites

Highly depleted Archean peridotites have proven very amenable to Re-Os model age dating. In contrast, due to the increasing heterogeneity of mantle Os isotope compositions with time, the Re-Os system has not been as effective in dating post-Archean peridotites. The timing of depletion and accretion of post-Archean lithospheric mantle around cratons is important to understand within the context of the evolution of the continents. In an attempt to precisely date post-Archean peridotite xenoliths, we present a study of the petrology, mineralogy and geochemistry, including whole-rock Re-Os isotopes, highly siderophile elements and clinopyroxene-orthopyroxene Sr-Nd-Hf isotopes of peridotite xenoliths from Lake Nyos in the Cameroon Volcanic Line (CVL). Eight Nyos peridotite xenoliths, all fresh spinel lherzolites, are characterized by low to moderate olivine Fo contents (88.9–91.2) and low spinel Cr# (8.4–19.3), together with moderate to high whole-rock Al2O3 contents (2.0–3.7%). These chemical characteristics indicate that they are mantle residues of a few percent to <20% partial melting. However, trace element patterns of both clinopyroxene and orthopyroxene are not a pristine reflection of melt depletion but instead show various extents of evidence of metasomatic enrichment. Some of the samples contain orthopyroxene with 143Nd/144Nd lower than its coexisting clinopyroxene, which is best explained by recent short-timescale alteration, most likely by infiltration of the host basalt. Because of these metasomatic effects, the Sr-Nd isotope systematics in pyroxenes cannot sufficiently reflect melt depletion signatures. Unlike Sr-Nd isotopes, the Lu-Hf isotope system is less sensitive to recent metasomatic overprinting. Given that orthopyroxene hosts up to 33% of the Lu and 14% of the Hf in the whole rock budget of these rocks and has 176Hf/177Hf similar to, or higher than, coexisting clinopyroxene, it is necessary to reconstruct a whole-rock Lu-Hf isochron in order to constrain the melt depletion age of peridotites. The reconstructed Nyos Lu-Hf isochron from ortho- and clinopyroxenes gives an age of 2.01 ± 0.18 Ga (1σ), and when olivine and spinel are considered, is 1.82 ± 0.14 Ga (1σ). Both ages are identical within error, and they are within error of the alumina-187Os/188Os pseudo-isochron ages (1.2–2.4 Ga) produced on the peridotites from Lake Nyos, consistent with their oldest rhenium depletion Os model ages (2.0 Ga). We conclude that the Nyos peridotites, and the lithospheric mantle that they represent, were formed at ∼2.0 Ga, indicating that the reconstructed whole-rock Lu-Hf isotope system can be a powerful radiometric dating tool that is complementary to and in some instances, more precise than the Re-Os isotope system in dating well-preserved post-Archean peridotites. The recognition of ∼2.0 Ga subcontinental lithospheric mantle (SCLM) in the Nyos area suggests that the Nyos region was assembled as a Paleoproterozoic block, or that it represents fragments of the SCLM from the nearby Paleoproterozoic domain juxtaposed through collisional emplacement during the Pan African Orogeny. With regards to the origin of the CVL, our data reveal that the Hf isotopic compositions of the Nyos peridotites are too radiogenic to be the main source of the CVL basalts.

New insights into Neoarchean–Paleoproterozoic crustal evolution in the North China Craton: Evidence from zircon U–Pb geochronology, Lu–Hf isotopes and geochemistry of TTGs and greenstones from the Luxi Terrane

Tonalite–trondhjemite–granodiorite (TTG) suites constitute dominant components of Precambrian continental crust and provide important insights into the crustal evolution history of the early Earth. Here we investigate a suite of TTGs and associated greenstones from the Luxi Terrane in the south–eastern part of the North China Craton (NCC). We present data from petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes to characterize these rocks and understand their genesis. Mineral assemblage in the greenstones and TTGs suggests that the rocks have undergone amphibolite–facies metamorphism with regression under greenschist–facies. Zircon U–Pb data from the magmatic grains yield upper intercept ages of 2753 ± 37 and 2582 ± 40 Ma for greenstone, 2552 ± 13 Ma for TTG gneiss and 2476 ± 18 Ma for tonalite, and weighted mean 207Pb/206Pb age of 2507 ± 22 Ma for granodiorite. Lu–Hf isotope systematics show εHf(t) values ranging from −2.13 to 7.29 and crustal model ages (TCDM) in the range of 2603–3125 Ma for the TTGs, whereas the εHf(t) values are up to 8.53 with TCDM ages of 2644–3006 Ma for the greenstone. The results suggest that the protolith for the greenstone was derived from a Meso- to Neoarchean mantle contaminated by the crust source, whereas the TTGs were mostly sourced from juvenile crustal sources with reworking of older basement. The geochemical features of greenstone are similar to island arc basalt, with low Na2O, K2O and TiO2, low total rare earth elements (REEs) values, relative enrichment in light REEs and depletion in heavy REEs absence of obvious Ce anomaly, and negative Nb, Ta and Ti anomalies. The TTGs show similar features, except for the high K2O content and moderate total REEs values. We propose that the greenstone likely represent a part of the early and initial crustal growth at ca. 2750–2600 Ma, and the TTGs were generated subsequently at ca. 2550–2450 Ma. Our results are consistent with the previous models on the crustal growth models in the NCC and provide further insights into the Neoarchean to Paleoproterozoic tectonic history of the craton.

Archean crustal evolution of the Aravalli Banded Gneissic Complex, NW India: Constraints from zircon U-Pb ages, Lu-Hf isotope systematics, and whole-rock geochemistry of granitoids

The Aravalli Banded Gneissic Complex forms a block of Archean gneisses and granitoids in the southern domain of the Aravalli orogen in NW India. Results of U-Pb-Hf isotope analyses on zircon grains combined with whole-rock geochemical data reveal that the gneissic complex is made up of TTGs, which intruded during the Paleoarchean (3310 Ma) and Neoarchean (2563–2548 Ma), and granite-granodiorite-quartz diorite emplaced between 2545 and 2485 Ma. Zircon xenocrysts additionally point to Meso- and Neoarchean magmatic episodes at 3100–3000, 2880–2800, 2690 and 2660–2630 Ma. Ages from zircon rims in conjunction with previously published Sm-Nd isotope data suggest metamorphic events at ca. 2450 and 520 Ma. Medium-HREE characteristics and near chondritic εHf3.31 Ga of −0.4 suggest formation of the Paleoarchean TTGs by partial melting of a deep-seated mafic crust, derived from CHUR-like mantle immediately prior to the TTG formation. In contrast, the high-HREE characteristics and subchondritic εHf2.56-2.55 Ga from −3 to −8 of the Neoarchean TTGs suggest partial melting at relatively shallow depth of a younger oceanic crust, which was derived from a depleted mantle source at <3.1–3.4 Ga. The Neoarchean granitoids (2540–2485 Ma) with εHft from −8 to +2 in magmatic and xenocrystic zircon were likely originated by internal reworking of a heterogeneous felsic crust, consisting of Paleo-, Meso- and Neoarchean rocks. The Neoarchean quartz diorite (ca. 2545 Ma) represents a hybrid rock, which resulted from mixing of enriched mantle-derived mafic melts (εHf2.55Ga = −3.4) with felsic melts formed by anatexis of Paleoarchean crust (εHf2.55Ga = −11.7). Comparison of our new data from the Aravalli Banded Gneissic Complex with those of the adjacent Bundelkhand Craton indicate that both cratonic nuclei underwent different evolutionary histories during the Archean.