Residual Zircon Research Articles

Preparation of zirconium oxide nanoparticles from rosette concentrate using two distinct and sequential techniques: hydrothermal and fusion digestion

The preparation of zirconium dioxide nanoparticles (ZrO2-NPs) as hard ceramics was accomplished from rosette zircon concentrate through two consecutive alkaline digestion reactions. The rosette zircon concentration in the Abu Khashaba area consists mainly of zircon and monazite minerals. Using different operating conditions, the hydrothermal digestion by autoclave and the conventional alkaline fusion methods was performed upon the non-magnetic concentrate of rosette in order to complete the removal of monazite firstly and to complete the purification of zircon metal secondly. All monazite content and undesirable impurities were removed by the hydrothermal method using optimal digestion conditions such as 4 mol/L NaOH solutions, 1/6 solid to liquid, 2 h dissolving time, and a temperature of 423 K. The residual zircon (84% Zr) was subjected to complete digestion using NaOH with a zircon-to-alkali ratio of 1/1.5 and a fusion temperature of 923 K. ZrO2-NPs were synthesized using the hydrothermal technique at 473 K for 7 h. The calcined ZrO2-NPs were characterized by X-ray diffraction, scan electron microscope, and transmittance electron microscope. Purified silica was also obtained as a by-product from washing solutions of fused zircon.

Zircon evolution from migmatite to crustally-derived granite: A case study of late Neoarchean migmatite in the Yishan area, western Shandong, North China Craton

Sequential changes in the texture, composition and U-Pb age of zircon from the late Neoarchean migmatites and related rocks in the Yishan area of western Shandong, eastern North China Craton, record the evolution of melt and magma from paleosome to crustally-derived granite. Zircon grains from the paleosome and metatexite have features of recrystallization, with narrow rims; those from leucosome have inherited cores and wide homogenous rims; those from diatexite preserve evidence of a complex evolution involving dissolution and growth, with the euhedral morphology of the rims, and the cross-cutting relationship between cores and rims, consistent with a magmatic origin. With the increase in melt content toward granite, the newly-grown zircon has clearer magmatic zoning and higher Th/U ratios. The low whole-rock Zr saturation temperatures (684–728 ℃) indicate that the diatexites, with abundant residual zircon, might be the product of low-temperature melting. The contemporaneous, crustally-derived granites, however, contain no or much less xenocrystic zircon and have high zircon saturation temperatures (740–868 ℃). Only at the higher temperature was sufficient partial melt produced to start the migration of magma out of the source region to form the granites. Anatectic (magmatic) zircons in diatexite commonly contain xenocrystic (residual) zircons, no matter how the magmatic system evolves further, the xenocrystic zircons in the anatectic (magmatic) zircons will still be retained. On the other hand, the crustlly-derived granites widely distributed in the area contain few residual zircons. Therefore, the crustally-derived granites widely distributed in western Shandong cannot form by further evolution of the diatexites, but require a higher degree of partial melting under higher temperature conditions. SHRIMP U-Pb zircon dating, combined with CL imaging, reveals that zircon crystals from the diatexites mostly are characterized by inherited cores with thick magmatic overgrowths, recording a regional anatectic event at ∼2.51 Ga. Residual source rock enclaves and xenocrystic zircons in the diatexites mostly have ages of ∼2.70 Ga and ∼2.53 Ga, suggesting that the diatexites were derived from Neoarchean continental crust.

Zircon and quartz inclusions in garnet used for complementary Raman thermobarometry: application to the Holsnøy eclogite, Bergen Arcs, Western Norway

Mineral inclusions are common and have been widely used to investigate complex geological history. When a rock undergoes cooling and decompression after the entrapment of an inclusion into a host mineral, residual pressure may develop within the inclusion because of the differences in thermal expansivity and compressibility between the inclusion and host. By combining laser Raman spectroscopy and experimental data relating hydrostatic pressure and Raman shift, it is possible to estimate the entrapment pressure–temperature (P–T) conditions using an isotropic elastic model. In this study, we report Raman spectroscopic data on both zircon and quartz inclusions in garnet host from the Holsnoy eclogite, Bergen Arcs, Norway. Averaged residual pressures based on different Raman peaks for zircon and quartz inclusions are obtained to be ca. 0.6 GPa and ca. 0.65 GPa respectively. Using the equation of state for zircon and quartz, the entrapment P–T conditions are constrained to be 1.7–1.9 GPa, 680–760 °C, consistent with previous estimates based on phase equilibria. Heating/cooling experiments are performed on an entrapped zircon inclusion. A clear trend is found between the residual zircon inclusion pressure and the externally controlled temperature. We show that the residual zircon inclusion pressure sealed in garnet host is very sensitive to the entrapment temperature, and can be used as a Raman thermometer. The effects of laser heating and the thermo-elastic anisotropy of zircon inclusion are quantified and discussed.

Nd-O-Hf isotopic decoupling in S-type granites: Implications for ridge subduction

The generation of S-type granites in the continental crust generally requires partial melting of metasedimentary rocks at relatively high temperatures, and is typically associated with syn- or post-collisional tectonics. These melts are generally expected to inherit the geochemical characteristics of their metasedimentary protoliths, and their Nd, Hf, and O isotopic compositions have been widely used to trace the origin of the magmas. Here we show that these isotopic systems are decoupled in S-type granites from the Chinese Altai, displaying a wide range of zircon ɛHf(t) values (−0.7 to +6.2), and a limited range of whole rock ɛNd(t) values (−6.1 to −2.2) and zircon δ18O values (+8.49‰ to +12.91‰). Zircon xenocrysts are common in these plutons, and show markedly negative ɛHf(t) values (from −30.7 to −18) and highly positive δ18O values (from +9.61‰ to +13.03‰). In addition, these granites exhibit high Lu/Hf (0.15 to 1.38) ratios, and substantial depletions in Zr and Hf. We develop a new model to explain the observed relationship between residual zircon content and Nd–Hf–O isotope decoupling. We propose that these S-type granites formed from partial melting of metasedimentary crustal rocks, associated with a ridge subduction-related heat source. Limited crustal heating, due to mantle upwelling through the slab window on a small scale, may have resulted in significant disequilibrium melting of the metasedimentary crust. Our findings provide new insights into melting processes in the deep continental crust.

Tracking the timing and nature of protolith, metamorphism, and partial melting of tourmaline‐bearing migmatites by zircon U–Pb and Hf isotopic compositions in the Yuka terrane, North Qaidam UHP metamorphic belt

An integrated study using cathodoluminescence (CL) images, U–Pb geochronology, and Hf isotopic characterization of zircons was performed on migmatites from the Yuka terrane to provide insights into the timing and nature of the protoliths, metamorphism, and partial melting of these rocks. Zircon grains separated from the migmatites have three distinct domains. Residual zircon cores in samples YKT01 and YKT02 exhibit oscillatory zoning, high Th/U ratios (0.10–0.79), steep heavy rare earth element (HREE) patterns, and mean ages of 936 ± 5 Ma and 927 ± 11 Ma, respectively, indicating that they are igneous in origin and record Neoproterozoic magmatism during the Grenvillian Orogeny. CL‐grey cores record an ultrahigh‐pressure (UHP) peak metamorphism age of 435 ± 5 Ma and have low Th/U and 176Lu/177Hf ratios, weak negative Eu anomalies, and relatively flat HREE patterns. The CL‐bright rims have steep HREE patterns with lower middle rare earth element (MREE) concentrations but higher HREE concentrations compared with the metamorphic cores and a weighted average age of 422 ± 4 Ma, which records the timing of melt crystallization. The migmatites incorporate garnet relicts coexisting with fluid inclusions and hydrous minerals like peritectic phengite and peritectic large anhedral poikilitic tourmaline in the leucosomes. Peak temperatures are less than 750°C, suggesting that the anatectic melts may be derived from melting of felsic gneiss in the presence of water. The timing of melt crystallization is ca. 13 Ma later than UHP metamorphism, indicating that the partial melting took place during the exhumation stage. The metamorphic zircon cores have higher 176Hf/177Hf(t) ratios than the residual magmatic cores, which provides powerful evidence for the generation of the metamorphic zircons through the incorporation of internal high Hf minerals other than zircon in a closed system. Anatectic rims with lower 176Hf/177Hf(t) ratios but higher 176Lu/177Hf ratios than metamorphic cores suggest that the main formation mechanism is dissolution‐reprecipitation of pre‐existing zircons. In conclusion, the elevated 176Hf/177Hf(t), TDM2 age and decreasing 176Lu/177Hf(t) ratios in the metamorphic and anatectic domains imply that the new zircons in some migmatites and granites may not reflect the Hf isotope compositions of their source rocks, accounting for the variable Hf isotope compositions in S‐type granites.

Hydrothermal Metasomatism and Gold Mineralization of Porphyritic Granite in the Dongping Deposit, North Hebei, China: Evidence from Zircon Dating

A porphyritic granite intrusion was recently discovered in the Zhuanzhilian section of the Dongping gold deposit. There is as many as one tonnage of Au in the fractured shear zone within the porphyritic granite intrusion, but no relevant reports concerning the origin and age of the intrusion has been published as yet. In this paper, zircon U-Pb dating is used to study the geochronology of porphyritic granite, in order to find out the evidence of age and the relationship with gold mineralization. There are two groups of zircon 207Pb/235U-206Pb/238U concordant ages of porphyritic granites: The concordant age of 373.0 ± 3.5 Ma, with the weighted mean age of 373.0 ± 6.4 Ma; and the concordant age of 142.02 ± 1.2 Ma with the weighted mean age of 142.06 ± 0.84 Ma. We believe that the first group might represent the age of residual zircon of alkaline complex, while the second group might be related with main gold mineralization. The obtained results of the petrography and electron probe analysis indicate that the porphyritic quartz and porphyritic granite, as well as gold mineralization, might be products of a late replacement of tectonic-hydrothermal fluid, which was rich in Si, Na and K originally and later yielded gold-forming fluids.

Impact of residual zircon on Nd-Hf isotope decoupling during sediment recycling in subduction zone

The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate these processes. Here we present a case where these isotopic systems are decoupled in a suite of granites from the Chinese Altai, showing a wide range of εHf(t) values (from −4.7 to +10.8) in contrast to a limited range of εNd(t) values (from −5.8 to −1.9). The zircon xenocrysts occurring frequently in these rocks show markedly negative εHf(t) values (from −34.3 to −6.5) and positive δ7Li values (from +12.5 to +18.2). We propose a model to explain the observed relationship between residual zircon and Nd–Hf isotope decoupling. We suggest that the Altai granites originated from partial melting of subducted slab components under relatively low temperature conditions which aided the residual zircon from oceanic sediments to inherit and retain a significant amount of 177Hf in the source, thereby elevating the 176Hf/177Hf ratio of the melt, and decoupling from the 143Nd/144Nd ratio during the subsequent magmatic processes. Our study illustrates a case where sediment recycling in subduction zone contributes to decoupling of Nd and Hf isotopic systems, with former providing a more reliable estimate of the source characteristics of granitic magmas.

Breakdown of residual zircon in the Izu arc subducting slab during backarc rifting

Breakdown of residual zircon in the Izu arc subducting slab during backarc rifting

Sediment recycling and crustal growth in the Central Asian Orogenic Belt: Evidence from Sr–Nd–Hf isotopes and trace elements in granitoids of the Chinese Altay

Felsic igneous rocks are common constituents of volcanic arcs, and contain valuable information about subduction-related magmatism. In this study we investigate nine granitoids with S-type volcanic arc affinity from the Chinese Altay, emplaced from 507 to 391Ma in an active subduction zone during the early–middle Paleozoic. These granitoids are characterized by moderate to high SiO2 contents (61.01–75.30wt.%), moderate total alkalis (Na2O+K2O, 3.43–7.64wt.%), and high Al2O3 contents (13.29–17.18wt.%). Negative εNd(t) values (−6.1 to −1.0), the wide range of εHf(t) values (−7.0 to +9.0), and enrichment of LILEs such as Pb, Th and U, all suggest that the granitoids were probably derived from the partial melting of subducting oceanic sediments and the associated mantle wedge. This inference is further supported by the Nd-isotope data. The high initial 87Sr/86Sr ratios (0.703963–0.719428), low Ba/Th ratios (7.00–118.93), and uniformly negative εNd(t) values (−6.1 to −1.0) indicate that slab-derived aqueous fluids were vital in generating the initial magma of these granitoids, and assimilation played only a minor role. Our data demonstrate that residual zircon retains a substantial amount of Hf during the partial melting of oceanic sediments, therefore, Hf may not be an effective tracer for the input of recycled sediments. We conclude that sediment recycling played an important role in the generation of arc magmatism and the growth of the Central Asian Orogenic Belt (CAOB).

Subduction zone mantle enrichment by fluids and Zr–Hf-depleted crustal melts as indicated by backarc basalts of the Southern Volcanic Zone, Argentina

We aim to identify the components metasomatizing the mantle above the subducting Nazca plate under part of the Andean Southern Volcanic Zone (SVZ). We present new major and ICP-MS trace element and Sr, Nd and high-precision Pb isotope analyses of primitive olivine-phyric alkali basalts from the Northern Segment Volcanic Field, part of the Payenia province in the backarc of the Transitional SVZ. One new 40Ar–39Ar age determination confirms the Late Pleistocene age of this most northerly part of the province. All analysed rocks have typical subduction zone type incompatible element enrichment, and the rocks of the Northern Segment, together with the neighbouring Nevado Volcanic Field, have isotopic compositions intermediate between adjacent Transitional SVZ arc rocks and southern Payenia OIB-type basaltic rocks. Modelling the Ba–Th–Sm variation we demonstrate that fluids as well as 1–2% melts of upper continental crust (UCC) enriched their mantle sources, and La–Nb–Sm variations additionally indicate that the pre-metasomatic sources ranged from strongly depleted to undepleted mantle. Low Eu/Eu* and Sr/Nd also show evidence for a UCC component in the source. The contribution of Chile Trench sediments to the magmas seems insignificant. The Zr/Sm and Hf/Sm ratios are relatively low in many of the Northern Segment rocks, ranging down to 17 and 0.45, respectively, which, together with relatively high Th/U, is argued to indicate that the metasomatizing crustal melts were derived by partial melting of subducted UCC that had residual zircon, in contrast to the UCC melts added to Transitional SVZ arc magmas. Mixing between depleted and undepleted mantle, enriched by UCC and fluids, is suggested by Sr, Nd and Pb isotopes of the Northern Segment and Nevado magmas. The metasomatized undepleted mantle south of the Northern Segment is suggested to be part of upwelling OIB-type mantle, whereas the pre-metasomatically depleted mantle also can be found as a component in some arc rocks. The fluid-borne enrichment seems to have been derived from South Atlantic wedge mantle with no significant transfer of solubles in the slab fluids from the subducting altered Pacific oceanic crust to the wedge. The Northern Segment magmatism is proposed to be related to the steepening of Nazca plate subduction in the Pleistocene after a shallow slab period, where melts of subducted UCC plus slab fluids metasomatized the overlying depleted wedge mantle. During this steepening, the enriched depleted and undepleted mantle mixed or interacted, and yielded the Northern Segment and Nevado magmas.

Factors controlling the geochemical variations of lateritic bauxites formed upon different parent rocks in Peninsular Malaysia

Three lateritic bauxites representing felsic (Rompin area), intermediate (Johor area), and mafic (Kuantan area) parent rocks from Malaysia were subjected to detailed mineralogical and geochemical investigations to examine the factors controlling their geochemical variations. The bauxitic mineral gibbsite occurs as in all areas. Non-bauxitic minerals are represented by kaolinite, quartz, and goethite in bauxites from granites and intermediate tuffs and by goethite only in bauxite from basalts. Bauxites from basalts and intermediate tuffs have higher TiO2, Fe2O3, Ba, V, Cu, Pb, and Zn but lower SiO2, Th, Rb, and rare earth elements (REE) when compared with bauxite from granites. Bauxite from basalts has higher concentrations of Cr, Nb, Cu, and Zn and lower Ba, As, and REE when compared with bauxite from intermediate tuffs. Chondrite-normalized REE patterns of bauxite from granites showed heavy rare earth elements (HREE) enrichments over light rare earth elements (LREE), pronounced negative EuN/EuN* anomalies and strong positive CeN/CeN* anomalies. In contrast, bauxites from intermediate tuffs and basalts exhibited LREE enrichment relative to HREE, slight to strong negative CeN/CeN* anomalies, and absence or slight negative EuN/EuN* anomalies. Compositional variations among the studied bauxites are controlled not only by the parent rock compositions but also by the degree of lateritization and mineralogy of its products. The effect of each of these factors can be traced by a number of compositional parameters in these bauxites. The influence of parent rock compositions is indicated by absence of kaolinite and quartz, high TiO2 and Fe2O3 and low SiO2 in bauxites from basalts and intermediate tuffs as well as negative EuN/EuN* anomalies in bauxite from granites and absence of EuN/EuN* anomalies in bauxites from basalts and intermediate tuffs. The impact of lateritization degree was evident by abundance of quartz and kaolinite in bauxite from granites as well as relative LREE enrichments over HREE in bauxites from basalts and intermediate tuffs. The effect of lateritic products mineralogy was observed in the variations of REE patterns between bauxite from granites that are characterized by HREE enrichment and Ce anomaly due to occurrence of REE as residual zircon and bauxites from intermediate tuffs and basalts that are characterized by low HREE and high LREE due to occurrence of REE as authigenic florencite and/or cerianite. The distinct geochemical composition of Fe concretions such as lower TiO2 and most of the trace elements, enrichment of HREE relative to LREE and negative EuN/EuN* anomaly compared to the host bauxite at Johor area also indicates the effect of mineralogy of lateritic products on their geochemistry.

Hf–Nd–O isotopic evidence for melting of recycled sediments beneath the Sulu Orogen, North China

The mantle source of late Mesozoic mafic magmas in the southeastern North China Craton is generally considered to have been enriched by deeply subducted continental lithosphere of the Yangtze Block. The mechanisms through which enrichment occurs are still unclear. Here we document an Early Cretaceous (zircon U–Pb age at 115±2Ma) dioritic intrusion with some geochemical features similar to sanukitoids (i.e., Al2O3>17.5%, Ba=1692–1819ppm, Sr=1042–1111ppm and δ18OZircon=6.2±0.2‰) from the Sulu Orogen in North China. Compared with the regional older (120–130Ma) mafic magmas (dykes), the diorite has less radiogenic Sr (87Sr/86Sr(i)~0.7075), higher εNd(t) (−9.0 to −8.6) with less radiogenic Hf than Nd (ΔεHf(t)=−6.1 to −5.2; ΔεHf(t)=εHf(t)−1.36×εNd(t)−2.89), signatures typically observed from continental zircon-bearing sediments. The primary magma of the dioritic intrusion was probably derived from a mantle enriched by the addition of a significant volume of a restitic component of recycled sediment that had been previously melted to leave residual zircon. Combined Hf–Nd–O isotopic data for the Rushan dioritic intrusion and the earlier mafic rocks requires the involvement of recycled sediments or their melts, incorporated into the lithospheric mantle as lenses or metasomatic veins, to produce a pudding-cake structure beneath the Sulu Orogen.

The nature of orogenic lithospheric mantle: Geochemical constraints from postcollisional mafic–ultramafic rocks in the Dabie orogen

Postcollisional mafic–ultramafic rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr–Nd–Pb isotopes in addition to zircon U–Pb ages and Lu–Hf isotopes. The results provide insights into the nature of orogenic lithospheric mantle in the continental collision zone. The zircon U–Pb dating gave consistent ages of 125±3 to 129±1Ma for magma crystallization. A few residual zircon cores exhibit U–Pb ages of 234±5Ma and 739±9Ma, respectively, in agreement with ages of tectonothermal events for ultrahigh-pressure metamorphism and protolith formation. The mafic–ultramafic rocks have high contents of MgO (up to 18.0wt.%), Cr (up to 1546ppm) and Ni (up to 349ppm), but low contents of SiO2 (41.0–51.9wt.%), and exhibit arc-like trace element distribution patterns and enriched Sr–Nd–Pb isotope compositions. These geochemical features indicate their derivation from partial melting of special mantle sources that are fertile in lithochemistry and enriched not only in LILE and LREE but also in Sr–Nd–Pb isotopes. The mantle sources are suggested to be part of the orogenic lithospheric mantle and they would be generated by melt–peridotite reaction in the continental subduction channel. The enriched signatures of incompatible trace elements and radiogenic isotopes in the mantle sources would be caused by metasomatic reaction of the overlying subcontinental lithospheric mantle (SCLM) peridotite with different compositions of felsic melts derived from the subducted continental crust during the Triassic continental collision. In this regard, slab–mantle interaction is evident during the continental deep subduction, with the postcollisional mafic–ultramafic rocks as its derivatives. Significant differences in element and isotope compositions between different mafic–ultramafic intrusions suggest that the orogenic lithospheric mantle is geochemically heterogeneous, with possible existence of hornblende-rich and pyroxene-rich lithologies in the mantle sources. The heterogeneity is primarily attributed to differences in the compositions of felsic melts derived from the subducted crust with a tectonic affinity to the South China Block, whereas the same SCLM of the North China Block was involved in the crustal metasomatism in the subduction channel. Therefore, the postcollisional mafic–ultramafic rocks provide a snapshot of the compositional and lithological variations in the orogenic lithospheric mantle in the continental collision zone.

Geochronology of multi-stage metamorphic events: Constraints on episodic zircon growth from the UHP eclogite in the South Altyn, NW China

Petrography, mineral chemistry and pressure–temperature (P–T) estimates were carried out for the eclogite from the South Altyn in NW China. The results suggest three stages of metamorphism: an ultra-high pressure (UHP) eclogite-facies metamorphism at 717–871°C and ≥2.8GPa, a high pressure (HP) granulite-facies metamorphism at 624–789°C and 1.42–1.52GPa, and an amphibolite-facies metamorphism at 597–728°C and 0.99–1.17GPa. Cathodoluminescence investigation revealed that zircons from the retrograde eclogite display a distinct core-rim structure. Cores are grey-white luminescent and contain mineral inclusions of Garnet+Omphacite+Rutile+Quartz, suggesting eclogite-facies metamorphic origin. The rims are dark grey luminescent and contain Garnet+Clinopyroxene+Pagioclase inclusions, forming at HP granulite-facies conditions. A few residual zircon grains with mottled internal structure also occur as the metamorphic cores. LA-ICPMS zircon U–Pb dating yielded three discrete age groups: (1) a Neoproterozoic protolith age of 752±7Ma for the residual grains, (2) an eclogite-facies metamorphic age of 500±7Ma for the metamorphic cores, and (3) a HP granulite-facies retrograde age of 455±2Ma for the rims. These ages indicate that the protolith of the Altyn eclogite probably formed in response to breakup of the Rodinia supercontinent during the Neoproterozoic; it was subjected to continental deep subduction and UHP metamorphism during early Paleozoic (~500Ma) and subsequently underwent two stages of retrograde metamorphism during exhumation. The petrological and geochronological data suggest a clockwise P–T–t path for the UHP eclogite. According to pressures and ages for the peak UHP eclogite-facies and the retrograde HP granulite-facies metamorphism, an exhumation rate of 1.2mm/yr was estimated for the eclogite, which is considerably slower than that of some UHP rocks from other UHP terranes (>5mm/yr). While the peak metamorphic age of 500Ma is consistent with previous dates of 480–504Ma, it is 40–60Myr older than the HP/UHP metamorphic ages of 420–461Ma for UHP eclogites in North Qaidam. The retrograde metamorphic age is 455±2Ma for the Altyn eclogite, which is 30–55Myr older than ~400–425Ma for the North Qaidam eclogites. These age differences suggest that the South Altyn and North Qaidam eclogites do not belong to the same HP/UHP metamorphic zone.

Zircon Hf–O isotope evidence for crust–mantle interaction during continental deep subduction

In-situ SIMS zircon U–Pb dating and O isotope analysis as well as LA-(MC)-ICPMS zircon U–Pb dating and Lu–Hf isotope analysis were carried out for postcollisional mafic–ultramafic rocks in the Dabie orogen, China. The zircon U–Pb dating gave consistent ages of 126 ± 1 to 131 ± 1 Ma for magma crystallization. Survival of residual zircon cores is identified by CL imaging and U–Pb dating, yielding ages of 697 ± 10 and 770 ± 11 Ma that agree with protolith U–Pb ages of UHP metaigneous rocks in the Dabie orogen. The zircon Hf–O isotope compositions show systematic variations that can be categorized into three groups. Group I has the lowest δ 18O values of 2.0 to 2.9‰ but the highest ε Hf(t) values of − 3.5 to − 1.0 with the youngest Hf model ages of 1.2 to 1.4 Ga. Group II displays intermediate δ 18O values of 4.0 to 5.1‰ and ε Hf(t) values of − 22.5 to − 13.2 with Hf model ages of 2.0 to 2.6 Ga. Group III exhibits the highest δ 18O values of 5.2 to 7.3‰ but the lowest ε Hf(t) values of − 29.1 to − 18.6 with the oldest Hf model ages of 2.4 to 3.0 Ga. The three groups of Hf–O isotope compositions correspond to a three-layer Hf–O isotope structure in the subducted continental crust, suggesting their involvement in the mantle source. Along with existing data for whole-rock Sr–Nd isotopes and trace elements, it appears that the mantle source for the postcollisional mafic–ultramafic rocks is characterized by fertile lithochemistry, the arc-like signature of trace elements, the heterogeneous enrichment of radiogenic isotopes, the differential incorporation of supracrustal materials, and the variable concentrations of water. Clearly, such a source is neither the asthenospheric mantle nor the refractory subcontinental lithospheric mantle (SCLM). It is a kind of the orogenic SCLM that would be generated by reaction of the overlying SCLM-wedge peridotite with hydrous silicate melts derived from different layers of the subducted continental crust. Therefore, the postcollisional mafic–ultramafic rocks provide a petrological record of crust–mantle interaction during the continental deep subduction.

Hf–Nd isotope and trace element constraints on subduction inputs at island arcs: Limitations of Hf anomalies as sediment input indicators

New Nd–Hf isotope and trace element data for Javanese volcanoes are combined with recently published data to place constraints on subduction inputs at the Sunda arc in Indonesia and assess the value of Hf anomalies (expressed as Hf/Hf* and Sm/Hf ratios) as tracers of such inputs. Hf anomaly does not correlate with Hf isotope ratio in Javanese lavas, however, Hf/Hf* and Sm/Hf ratios do correlate with SiO 2. Contrary to previous work, we show that Hf anomaly variation may be controlled by fractionation of clinopyroxene and/or amphibole during magmatic differentiation and does not represent the magnitude or type of subduction input in some arcs. Correlation of Sm/Hf with indices of differentiation for other arcs (e.g., Vanuatu, New Britain, and Mariana) suggests that differentiation control on Sm/Hf ratios in volcanic arc rocks may be a relatively common phenomenon. This study corroborates the use of Nd–Hf isotope co-variations in arc volcanic rocks to ascertain subduction input characteristics. The trajectories of regional volcano groups (East, Central and West Java) in Nd–Hf isotope space reveal heterogeneity in the subducted sediment input along Java, which reflects present-day spatial variations in sediment compositions on the down-going plate in the Java Trench. The high Sm/Hf ratio required in the sediment end-member for some Javanese basalts suggests that partial melting of subducted sediment occurs in the presence of residual zircon, and is inconsistent with residual monazite or allanite.

Geochronology and geochemistry of Late Paleozoic magmatic rocks in the Lamasu–Dabate area, northwestern Tianshan (west China): Evidence for a tectonic transition from arc to post-collisional setting

Voluminous Late Paleozoic igneous rocks and associated Cu–Au–Mo deposits occur in the northwestern Tianshan district, Xinjiang, west China. However, the tectonic setting and petrogenesis of these rocks remain controversial. This paper reports zircon U–Pb and Hf CUU isotopic data, major and trace elements, and Sr–Nd–Pb isotopic data for the intrusive rocks and minor dacites in the Lamasu–Dabate area of northwestern Tianshan adjacent to the Cu–Au–Mo deposits. LA-ICPMS U–Pb zircon analyses suggest that the Lamasu porphyries were formed at 366 ± 3 Ma and contain 907–738 Ma inherited zircons, the Dabate dacites were formed at 316 ± 4 Ma, and granite porphyries were formed at 289 ± 3 Ma with ~ 319 Ma inherited zircons. The Lamasu porphyries consist of plagioclase granite and granodiorite, and are geochemically similar to adakites, e.g., having high Al 2O 3 (14.54–19.75 wt.%) and Sr (308–641 ppm) and low Y (7.84–16.9 ppm) contents, with fractionated rare earth element (REE) patterns and slightly positive Sr anomalies. However, they have variable initial ratios of 87Sr/ 86Sr (0.7072–0.7076) and 206Pb/ 204Pb (18.139–18.450), and variable ε Nd(t) (− 5.6 to − 0.8) and positive ε Hf(t) (+ 1.4 to + 10.6) values. They also have variable Mg # (100 × Mg 2+/(Mg 2++ Fe 2+)) (41–73) and low Th (3.13–8.09) and Th/Ce (0.14–0.28) values. We suggest that the Lamasu adakitic magmas were generated through partial melting of southward subducted Junggar oceanic crust, with subsequent melt-mantle interaction and assimilation of basement rocks. The Dabate dacites show typical arc-like geochemical characteristics (e.g., enrichment of large ion lithophile elements (LILE) and strong negative anomalies of Ta, Nb, P and Ti), with variable ε Nd(t) (+ 0.1 to + 3.3). They were probably generated by melting of juvenile basaltic lower crust as a result of magma underplating. The Dabate granite porphyries are geochemically similar to A 2-type granites, e.g., high SiO 2 (75.6–77.6 wt.%) and alkalis (Na 2O + K 2O = 8.27–8.70 wt.%), low CaO (0.28–0.34 wt.%) and Mg # (2–10), and obvious negative Eu, Ba and Sr anomalies. They have variable values of ε Nd(t) (− 1.7 to 0) and 206Pb/ 204Pb i (18.567–18.721) and zircon ε Hf(t) (+ 0.5 to + 11.1). The Dabate A-type granite porphyries contain Late Carboniferous residual zircon cores, suggesting that their source rock possibly contained Carboniferous arc igneous rocks. Taking into account all available data from Late Paleozoic magmatic rocks and Cu–Mo–Au mineralization in the northern Tianshan district, we suggest that the Dabate–Lamasu area was a continental arc during the Late Devonian–Carboniferous but had entered a post-collisional stage by the Early-Permian (~ 290 Ma).

Late Cretaceous alkaline sills of the south Tethyan suture zone, Pakistan: Initial melts of the Réunion hotspot?

During the closure of the Tethyan Ocean, Triassic to early Jurassic marine sediments containing alkali dolerite and ultramafic lamprophyre sills were accreted to the margin of Greater India. The 69.7 ± 0.2 Ma weighted mean 40Ar- 39Ar plateau age of the sills, their location, and their incompatible element and Nd–Pb–Sr isotopic composition, which is similar to that of recent Réunion shield lavas, are consistent with their derivation from the Réunion hotspot. Mantle melt modelling suggests that the alkali dolerites are the result of < 3% melting of a moderately incompatible element-depleted, garnet-bearing source. However, some of the ultramafic lamprophyres have lower heavy rare-earth element, Nb and Zr contents that cannot be explained by melting of the same mantle source as the alkali dolerites. This unusual feature can be explained by small-degree melting of a metasomatised mantle source containing small amounts (< 0.02 vol.%) of residual zircon. Isotopic signatures of the ultramafic lamprophyres are similar to those of the alkali dolerites, implying that metasomatism was not significantly older than the melting event that formed the sills. The metasomatism may have been due to small-degree melting and re-solidification at the leading edge of the starting head of the Réunion plume. This alkaline magmatism may represent the arrival of the Réunion plume at the base of the Indian lithosphere several million years before the onset of the main phase of Deccan flood basalt volcanism.

New insights into the origin of O–Hf–Os isotope signatures in arc lavas from Tonga–Kermadec

O, Hf and Os isotope data are presented for lavas from the highly depleted Tonga–Kermadec arc. O isotope values overlap with those of MORB limiting the amount of interaction with the arc crust. δ 18O does not increase northwards as would be expected from the ~ 4 fold increase in subduction rate if slab-derived fluids had high 18O/ 16O ratios. Thus, the overall northward decrease in HFSE concentrations likely reflects depletion due to prior melt extraction, not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes consistent with mixing of subducted pelagic sediment into the mantle wedge and do not require Hf to be fluid mobile. With the exception of a boninite from the north Tongan trench, the northern Tonga lavas do not overlap the Hf isotope composition of either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the Pb isotope signatures in these lavas must have been added by fluids and sediment melts derived from the Louisville volcaniclastics with minimal mobilisation of Hf. This suggests conservative behaviour for this element due to the formation of residual zircon during partial melting of the subducted sediments. 187Os/ 188Os ranges from 0.1275 to 0.4731 and the higher Os isotope ratios reflect the sensitivity of this system to even minor interaction with altered arc crust. Conversely, the lowest Os ratios are subchondritic and indicate that transfer of radiogenic Os from the slab is not all pervasive and provide an important constraint on the composition of the mantle wedge. Remarkably, the least radiogenic sample is a dacite demonstrating that evolved magmas can develop by fractionation from mantle-derived magmas with minimal interaction with the arc crust.

Geochemistry and sedimentary petrology of Archean clastic sedimentary rocks at Mt. Goldsworthy, Pilbara Craton, Western Australia: Evidence for the early evolution of continental crust and hydrothermal alteration

Geochemical and petrological analyses of Archean clastic metasedimentary rocks at Mt. Goldsworthy in the Pilbara Craton, Western Australia reveal the early evolution of continental crust and the Earth's surface environment. The succession correlative to the ca. 3.4 Ga Strelley Pool Chert is composed dominantly of silicified medium-grained to very coarse-grained sandstone, siltstone and shale, with minor chert and precipitative beds of nahcolite and barite. The sandstone is generally rich in detrital quartz, with the coarse-grained sandstone containing abundant, very well rounded mono-crystalline quartz that is assumed to be derived from older sedimentary rocks. The recycled detritus is mixed with first-cycle detrital material such as feldspar and barite that is sourced from reworked precipitative beds. The sandstone also contains zircon, Fe–Ti oxides, sulfides and lithic fragments of various origins including mafic-ultramafic volcanic rocks, sedimentary and possibly metamorphic rocks. Al–Ti–Zr systematics indicates that the source area for the Mt. Goldsworthy clastic metasedimentary rocks was locally subjected to hydrothermal alteration. Reaction with the acid solution resulted in Al-dissolution and the formation of Zr- and Ti-enriched residual detrital material. The enrichment in heavy rare earth elements and low Al 2O 3/TiO 2 values of Mt. Goldsworthy samples reflects the inflow of the residual zircon- and Ti-oxides-enriched detritus. Th/Sc, La/Sc and Eu/Eu * ratios that are unaffected by contamination by the residual detritus indicate a mixed provenance with a significant contribution from granitoid rocks. The presence of detrital zircon older than 3.5 Ga was also demonstrated in this study.