Zircon Trace Element Compositions Research Articles

Optical-spectroscopic and isotope-geochemical characteristics of zircons from diamond-bearing placers in Yakutia as indicators of diamond ore bodies

Research subject. Zircon grains from diamond placers of the Molodo and Ebelyakh rivers of the Yakut Arctic. Aim. To study the indicator characteristics of zircon to determine sources of diamonds and ways of their transfer to placers; to analyze optical-spectroscopic and isotope-geochemical parameters of zircon grains, their microelemental, U-Pb, and Lu-Hf isotope composition. Materials and methods. A LA-ICP-MS analysis of the U-Pb isotopic and trace element composition of zircon was carried out using a NexION 300S mass spectrometer with an NWR 213 attachment. An analysis of the Lu-Hf isotope system was carried out using a Neptune Plus mass spectrometer with an NWR 213 attachment, located in a room of cleanliness class 7 ISO at the Geoanalitik Center for Collective Use (IGG Ural Branch of the Russian Academy of Sciences, Ekaterinburg). Raman spectra were obtained using a LabRAM HR800 Evolution confocal spectrometer. CL spectra were obtained using a Jeol JSM6390LV scanning electron microscope equipped with a Horiba H-CLUE iHR500 attachment. Results. The local optical spectroscopic characteristics (Raman spectra and catholuminescence) of zircon grains were established, which made it possible to substantiate the conclusion about their high crystallinity and homogeneity (monochrony), as well as to select international zircon standards with similar characteristics for use in LA-ICP-MS analysis to ensure similar conditions for evaporation of the substance and fractionation parameters of the U and Pb elements. LA-ICPMS data on the microimpurity, U-Pb and Lu-Hf isotopic composition of zircon grains from diamond placers, as well as the dose values of their self-irradiation, are presented. Conclusions. The obtained U-Pb dating contributes to reconstructing a more precise history, character, and sequence of manifestation of kimberlite magmatism, tectonic processes, and migration routes of kimberlite material and diamonds across the Siberian craton, within the Yakut kimberlite province.

Regional trends and petrologic factors inhibit global interpretations of zircon trace element compositions

The trace element composition of zircon reveals information about the melt that they are derived from, as such, detrital zircon trace element compositions can be used to interrogate melt compositions, and thus the evolution of the continental crust in time and space. Here, we present a global database of detrital zircon compositions and use it to test whether average global trends for five common petrogenetic proxies truly represent secular changes in continental evolution. We demonstrate that the secular trend is broadly comparable across continental regions for Ti-in-zircon temperatures, but for other trace element ratios interrogated, secular trends are highly variable between continental regions. Because trace element ratios result from multiple petrologic variables, we argue that these petrogenetic proxies can be overinterpreted if projected to global geologic processes. In particular, we caution against the interpretation of crustal thickness from trace elements in zircon, and we argue that our results negate current hypotheses concerning secular changes in crustal thickness.

First identification of the Early Cretaceous Suolong Cu-rich porphyry in the Geji area, central Tibet: A new clue for regional porphyry Cu exploration

The Tibetan Plateau hosts the majority of porphyry Cu deposits in China. There are two large metallogenic belts, the Gangdese and Bangong-Nujiang belts, in the Lhasa Terrane, Central Tibet. The Lhasa Terrane experienced multistage magmatic activity from the Mesozoic to Cenozoic, and the subaerial volcanic areas of the western part of the terrane are regarded as favorable targets of porphyry Cu deposits. In this paper, we describe the geology of our newly identified Early Cretaceous Suolong Cu-rich porphyry in the western Lhasa Terrane and present new U–Pb ages, trace element and Hf isotopic compositions of zircon and whole-rock elemental and Sr–Nd isotopic compositions. The Suolong porphyry contains zircon grains with ages ranging from 136 to 135 Ma, εHf(t) values ranging from −1.5 ∼ +2.7 and logfO2 values ranging from −16.27 ∼ −9.92. Geochemically, the porphyry rocks are peraluminous in nature, with whole-rock εNd(t) values of −5.7 ∼ −5.4. We infer that the Suolong porphyry originated from a mixture of ∼53 % enriched lithospheric mantle and ∼47 % ancient lower crustal melt, triggered by the roll-back of the northward-subducting Neo-Tethyan Ocean. The conditions of high oxygen fugacity and water content and tectonic transition promoted the formation of the Suolong Cu-rich porphyry, but the condition of a thinned crust may have constrained the mineralization scale of the Cu deposits. This new identification of Early Cretaceous mineralization event provides important clues for identifying new potential exploration targets for porphyry Cu deposits in the western segment of the Lhasa Terrane.

Zircon in tin granite as tracer for fluid metasomatism and Sn mineralization

Most tin deposits in the world are genetically related to tin granite and form during complex magmatic-hydrothermal processes. Zircon is a common accessory mineral in granite and related ore systems and can host a number of ore metals, such as Sn, W, Nb, Ta, U and Th, in its crystal lattice. However, whether metal enrichment/depletion can trace ore-forming processes is still unclear. Here, we report that the metal concentrations in various types of zircons from the Mopanshan tin granites in the southern Great Xing'an Range (Northern China) can be used as good indicators of fluid metasomatism and Sn mineralization. Two lithological zones are developed in the Mopanshan pluton, including porphyritic syenogranite (PG) in the center and fine-grained syenogranite (FG) at the margin. Zircons in the PG (PGZ-1, PGZ-2, and PGZ-3) are all magmatic in origin, while zircons in the FG can be categorized into magmatic zircons (FGZ-1 and FGZ-2) and metasomatic zircons (FGZ-3). The PGZ-1 and FGZ-1 grains are transparent prismatic crystals with bright oscillatory zonation, whereas the PGZ-2 grains are murky crystals with dark oscillatory zonation. PGZ-3 and FGZ-2 grains occur as overgrowths of previously formed zircon (PGZ-1, PGZ-2, and FGZ-1) or as brown individual crystals, showing dark and homogeneous cathodoluminescence (CL) textures. The metasomatic FGZ-3 grains are translucent-opaque porous crystals with vermicular CL zonation and commonly replace FGZ-2. The trace element compositions of magmatic zircons are completely melt controlled, providing a record of magmatic evolution as a constant decrease in Zr/Hf ratios and a gradual increase in Th, U, Nb, and Ta contents. Moreover, the structure of magmatic zircons transforms from a crystalline state to an amorphous state as a consequence of radioactive decay of U and Th. A coupled dissolution-reprecipitation process is proposed for the formation of metasomatic FGZ-3. The reactive fluid is the magmatic fluid that exsolved from the melt in the late magmatic stage. The magmatic fluid replaced biotite and rare earth phosphates (mainly monazite and apatite) enclosed within biotite, resulting in significant amounts of Nb, Ta, Sn, P, Al, Ca, Fe, and REEs, which subsequently reacted with the FGZ-2 zircons to leach Th, U, Y, and HREEs. Eventually, REEs, Y, Th, and U in the fluid combined with P to form monazite and xenotime, while the other elements partially precipitated with the crystallization of the FGZ-3 zircons. Although the alteration of biotite only released approximately 190 ppm of Sn into the fluid, this is still a significant Sn source for the Sn deposits surrounding the Mopanshan pluton, taking the granite size (∼50 km2) and the volume proportion of biotite (∼5%) into account. Furthermore, based on previous studies on tourmaline from the Mopanshan granite and regional geochemistry, it may be inferred that the addition of wall rock components may also play an important role in Sn mineralization.

Petrogenesis and REE mineralogical characteristics of Shitouping granites in southern Jiangxi Province: Implication for HREE mineralization in South China

The enrichment of heavy rare earth elements (HREE) in the granitic host-rocks is crucial for generating ion-adsorption HREE deposits in South China. The Shitouping pluton, located in the southern part of Jiangxi Province, South China, develops large-scale HREE deposits in its weathering profile identified by recent geological surveys. These granites can be divided into medium to coarse-grained biotite monzogranite (MGBG), medium to coarse-grained syenogranite (MGSG) and fine-grained alkali feldspar granite (FGAG) based on field observations and mineral assemblages. New geochronologic results show that this composite pluton intruded during the Early Cretaceous (ca. 140 Ma). Three granitic units displaying comparable whole-rock Nd [εNd(t) = −4.5 to −3.4] and zircon Hf compositions [εHf(t) = −4.4 to +1.2], combined with mineral association, hydrothermally altered zircon and whole-rock geochemistry, indicate they were derived from partial melting of hybridized crustal source, followed by intense fractional crystallization and fluid metasomatism in the highly evolved granitic system. The MGSG and FGAG have higher Rb, Nb, HREE contents and Rb/Sr ratios, as well as lower Nb/Ta, K/Rb ratios in comparison with the MGBG, implying they are more evolved units. The overlapping magmatic zircon trace element compositions between MGBG and MGSG, coupled with the subvertical contact boundary between the FGAG and the MGBG, suggest that the generation of Shitouping pluton should be attributed to the periodic magma replenishment, remobilization, and differentiation process in the long-lived magmatic system rather than simple fractional crystallization from a single parental magma. Hydrothermally altered zircon with elevated concentration of HREE and the paragenetic relationship between LREE-phosphate and HREE minerals in the MGSG suggest that the fluid metasomatism promotes the migration and accumulation of HREE. This is consistent with the high HREE concentration recorded in the MGSG, which contains abundant HREE minerals associated with fluorites. Thus, this study highlights that magma differentiation and fluid metasomatism during the late magmatic stage could achieve the relatively high HREE contents in granites to favor the formation of ion-adsorption HREE deposits.

Zircon and cassiterite U-Pb geochronology and petrochemical characteristics of early Tertiary tin mineralization at Mayo Darlé, Cameroon Volcanic Line

Tin mineralization at Mayo-Darlé in the Cameroon Volcanic Line is associated with highly evolved anorogenic ferroan granite magmatism with a pervasive hydrothermal overprint. Mineralization occurs as cassiterite-quartz-muscovite stockwork and disseminated in quartz-mica and quartz-topaz greisen in low-Ca alkali feldspar granite with a composition near the 1 kbar haplogranitic thermal minimum. The trace element composition of zircon indicates elevated temperature (880 ± 55 °C) and very low oxidation state below the quartz-fayalite-magnetite buffer. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) identifies a partly disturbed U-Pb system for igneous zircon (U and Th contents >1000 ppm) but defines Tera-Wasserburg concordia ages and common-lead-corrected weighted 206Pb/238U ages of about 54 Ma, similar to a previous 87Rb/86Sr vs 87Sr/86Sr regression with a date of 56.1 ± 1.7 Ma (MSWD = 58; n = 9) and an initial 87Sr/86Sr ratio of 0.7085 ± 0.0035 (2 s). Hydrothermal cassiterite gives a pooled LA-ICPMS Tera-Wasserburg concordia age of 56.7 ± 0.6 Ma on two samples (each about 20 points, MSWD = 0.54 and 1.3). These new age data define a period of early Cenozoic intraplate calc-alkalic felsic magmatism as a precursor to abundant Oligocene to Recent alkali basalt volcanism along the Cameroon Volcanic Line. Geochemical and Sr–Nd–Hf isotopic data suggest that the granitic rocks from Mayo Darlé were largely derived by partial melting of Mesoproterozoic basement and modified by strong differentiation, all likely powered by heat (and possibly fluids) from the mantle. The Tertiary age of the rare-metal magmatism sets this system apart from the widespread Late Neoproterozoic to Early Paleozoic rare-metal pegmatites and granites in the Pan-African collisional belts of Africa, and also from the anorogenic Mesozoic ring complexes in Central Nigeria.

Accretionary and collisional processes of the Brasiliano Orogeny recorded by the Tonian La Tuna ophiolite, Dom Feliciano Belt, Uruguay

Accretionary to collisional orogenies such as the Brasiliano Orogeny of eastern South America present the opportunity to distinguish between oceanic-continental (accretionary) and continental (collisional) processes. However, the multitude of geotectonic processes involved and the reduced degree of preservation of the accretionary stage restrain the interpretation of evidence. This study focuses on a metasomatite associated with the La Tuna Ophiolite in the southern Dom Feliciano Belt of the Uruguayan Shield, as these can provide crucial insights into the accretionary stage of the Brasiliano Orogeny. The metasomatite occurs as a meter-sized massive tourmalinite body surrounded by a meter-wide chloritite lens, both enclosed by the La Tuna serpentinite. Our SEM-EDS and EPMA data revealed that the serpentines from the La Tuna ophiolite are antigorite and lizardite, while chlorite was identified as clinochlore. Abundant inclusions of sheridanite chlorite in dravite tourmaline suggests that tourmaline overgrew chlorite. Dravite crystals revealed the presence of three diffuse zones with minor compositional variations, mainly enrichments in Ca and Fe towards the rims. The U-Pb LA-ICP-MS dating of metasomatic zircon yielded ages of 743.2 ± 3.4 Ma (n = 22) for the chloritite and 743.1 ± 2.4 Ma (n = 31) for the tourmalinite. High δ11B values (+12.1‰ to + 18.1‰) of tourmaline (LA-ICP-MS) suggest that the metasomatic event was caused by seawater-derived fluids in a fluid-dominated environment. The single metasomatic U-Pb age combined with the high δ11B values suggest that the metasomatites underwent more intense metasomatic events than those associated with the Bossoroca (−8.5‰ to +1.8‰) and the Ibaré ophiolites (+3.2‰ to +5.2‰) further north in the São Gabriel Terrane of the Dom Feliciano Belt. Nonetheless, a few spots in tourmaline located within fracture zones and in contact with chlorite displayed negative (c. −5‰) and low positive δ11B values (c. 2‰). These are interpreted as overprinting by crustal fluids, either during the tectonic emplacement of the La Tuna Ophiolite or during exhumation. The calculated TDM Hf model ages for zircon in tourmalinite and chloritite range from 0.74 to 1.09 Ga and their ɛHf(T) values range from +4.9 to +11.2, indicating derivation from depleted mantle sources. The data corroborate earlier results indicating that the associated La Tuna amphibolites have composition similar to N-MORB. Furthermore, the trace element composition of zircon in the metasomatites is consistent with an oceanic environment (U/Yb < 0.1). Geotectonically, the La Tuna metasomatites formed at 743 ± 2 Ma in an oceanic setting, most likely in a juvenile Adamastor Ocean basin. During the collisional stage of the Brasiliano Orogeny, the La Tuna Ophiolite was tectonically emplaced within the <666 Ma La Micaela Schist of the Paso del Drágon Complex. As such, this study on the La Tuna Ophiolite contributes direct evidence on the oceanic evolution leading up to the accretionary stage of the Brasiliano Orogeny and provides new constraints on the transition towards the collisional stage.

Trace element composition of zircon from granitoids of the Neoarchean alkaline province of the Kola Peninsula

Research subject. Zircon from granitoids of the A- and I-types of the Neorchean Kola alkaline province of the Baltic (Fennoscandian) shield. Aim. To determine the trace element composition of zircon in order to characterize the conditions of its formation in different types of granitoids. To use these data, along with the available results of U-Pb dating and Lu-Hf isotope composition of the studied zircon crystals, to assess the amount of mantle and core matter in the protolith composition and the effect of crystallization conditions on the rare element composition of zircon. Materials and Methods. 50 zircon crystals from 5 samples of the main types of granitoids of the province were studied. Concentrations of trace elements were determined using a Cameca IMS-4f ion microprobe (NF FTIAN, Yaroslavl). Points for analysis were selected using optical and cathodoluminescent images. The size of the crater did not exceed 20 µm, the relative measurement error for the predominant part of the elements was set at 10–15%, the threshold for detecting elements was 10 ppb. Results. There are two main types of zircon of Neoarchean age – zircon-1 and zircon-2, formed, respectively, at the magmatic and autometasomatic stages of crystallization, zircon-4 in inclusions in zircon-1, and Paleoproterozoic zircon-3 of metamorphic genesis. The average isotopic composition of Hf in zircon-1 and zircon-2 of different arrays varies slightly, with the amount of mantle component (Xm) ranging from 23 to 30%, which may indicate a relatively constant and significantly core composition of their protoliths. Conclusions. The change in the trace element composition of Neoarchean zircons was controlled mainly by the composition of protoliths and redox crystallization conditions. Under reducing conditions, the greatest chemical activity was acquired by light (La-Nd) lanthanides, for which a direct dependence on the Ce/Ce* value was established. For heavy lanthanides with similar ion sizes relative to Zr+4, crystallochemical factors were more important and a decrease in the dependence of heavy lanthanide concentrations in zircon on Ce/ Ce* was determined.

Identifying crustal contributions in the Patagonian Chon Aike Silicic Large Igneous Province

The volcanic rocks of the Chon Aike Silicic Large Igneous Province (CASP) are recognized as magmas dominantly produced by crustal anatexis. Investigating the zircon of the CASP provides an opportunity to gain further insight into geochemical and isotopic differences of the potential magmatic sources (i.e., crust versus mantle), to identify crustal reservoirs that contributed to the felsic magmas during anatexis, and to quantify the contributions of the respective sources. We present a combined zircon oxygen and hafnium isotope and trace element dataset for 16 volcanic units of the two youngest volcanic phases in Patagonia, dated here with LA-ICP-MS U–Pb geochronology at ca. 148–153 Ma (El Quemado Complex, EQC) and ca. 159 Ma (western Chon Aike Formation, WCA). The EQC zircon have 18O-enriched values (δ18O from 7 to 9.5‰) with correspondingly negative initial εHf values (− 2.0 to − 8.0). The WCA zircon have δ18O values between 6 and 7‰ and εHf values ranging between − 4.0 and + 1.5. Binary δ18O-εHf mixing models require an average of 70 and 60% melt derived from partial melting of isotopically distinct metasedimentary basements for the EQC and WCA, respectively. Zircon trace element compositions are consistent with anatexis of sedimentary protoliths derived from LIL-depleted upper continental crustal sources. The overlap between a high heat flux environment (i.e., widespread extension and lithospheric thinning) during supercontinental breakup and a fertile metasedimentary crust was key in producing voluminous felsic volcanism via anatexis following the injection and emplacement of basaltic magmas into the lower crust.

Zircon fertility indicators compromised by mineral inclusion contamination: A case study from the Taoxikeng W deposit, South China

Zircon trace element compositions have been widely used as a tool to assess the ore-forming potential of magmatic-hydrothermal systems. However, many zircons contain accessory mineral inclusions which, if accidentally analyzed, could compromise reported zircon trace element signatures, especially when the mineral inclusions are sub-micrometer and thus hard to monitor using conventional microscopes. However, how mineral inclusion contamination influences the robustness of zircon fertility indicators (e.g., Ce and Eu anomalies) is still not well illustrated. In this study, we report U-Pb ages and trace element compositions of zircons from ore-forming muscovite granite plutons of the Taoxikeng W deposit, South China. The studied zircons are characterized by abundant small apatite inclusions. Although these had little influence on zircon Eu/Eu* (and thus probably the interpreted level of magmatic fractionation and melt water concentration), they could have resulted in a misestimation of the magmatic oxidation state either via the zircon Ce/Ce* or Loucks et al. (2020) methods. This is especially important for granite-related W-Sn systems since their magmatic zircons generally contain abundant mineral inclusions, which may be accidentally sampled during zircon trace element analysis. In practice, we can use La as a monitor of REE-enriched mineral (i.e., apatite, titanite) inclusions, although the criterion for identifying pure zircon trace element data is still controversial and awaits further work.

A shifty Toba magma reservoir: Improved eruption chronology and petrochronological evidence for lateral growth of a giant magma body

Polycyclic caldera complexes hold clues to understanding why some magmatic systems develop into supersized magma bodies and how they can recover to produce several caldera-forming eruptions. However, the geologic records of the transitions between successive caldera events are very often inaccessible due to limited preservation of eruptive products of inter-caldera activity, prompting the search for alternative archives of magma evolution such as accessory minerals. Here we applied multiple geochemical tools to study one of the most active caldera centres of the Quaternary, the Toba caldera complex in Sumatra (Indonesia), which produced at least four caldera-forming eruptions in the last 1.6 My, including the iconic Youngest Toba Tuff at 74 ka. We combined feldspar 40Ar/39Ar and zircon U–Pb geochronology of proximal pyroclastic deposits with glass and mineral chemistry of both the tuffs and distal marine tephra to revise the eruption chronology of Toba, obtaining new eruption ages of 1417 -31/+14 ka (zircon) or 1339 ± 39/39 ka (plagioclase, internal/full external 2σ uncertainty) for the Haranggaol Dacite Tuff, 783.81 ± 0.85/1.32 ka (sanidine) for Oldest Toba Tuff, and 503.61 ± 1.36/1.50 ka (sanidine) for Middle Toba Tuff. Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb crystallisation ages, trace element contents and Hf isotopic ratios of zircons illuminate changes in the shallow magma reservoir which saw near-continuous zircon crystallisation over 1.6 My. Prolonged build-ups to each eruption with highly scattered zircon trace element compositions reflect a complex, heterogeneous character of the shallow reservoir, without a clear temporal trend or indications of the eruption trigger. In contrast, hafnium isotopes in zircon display a pronounced shift towards unradiogenic values immediately after the OTT caldera collapse, followed by a gradual recovery to a baseline value of εHf = -7 at the time of YTT eruption, interpreted as a reflection of the shift in magma reservoir position corresponding to change in the character of assimilated crust. We can show in unprecedented detail how a large caldera collapse affects magma geochemistry; however, identification of patterns in the behaviour of the Toba system and making geochemistry-based predictions about its future development remain a challenge.

Zircon double-dating, trace element and O isotope analysis to decipher late Pleistocene explosive-effusive eruptions from a zoned ocean-island magma system, Ascension Island

In this first detailed study of zircon from Ascension Island, South Atlantic, we take a novel approach combining trace element and O isotope compositional data with double-dating (disequilibrium 238U–230Th and (U–Th)/He) to decipher timescales and dynamics of magmatic processes. The Echo Canyon (EC) sequence comprises small-volume explosive-effusive eruptions of trachyte that tapped a compositionally zoned magma system. Associated volcanic hazards may be constrained from the age of volcanism, duration of magma storage, and magma source and plumbing system character. Zircon U–Th–Pb dating of lithic lava clasts has revealed recurrent evolved volcanism at 1.34 and 0.6 Ma, and 95 ka. The (U–Th)/He zircon cooling ages indicate that most of the EC explosive-effusive sequence erupted in a brief episode at ca. 95 ka. Additionally, uniform 238U–230Th zircon crystallisation ages suggest moderately protracted magma storage with melt present at depth for at most 103–104 years before eruption. The enriched character of zircon trace element compositions, relative to MORB, in the absence of a continental crustal signature in the oxygen isotope values (δ18O range 2.67–5.63‰), suggests the presence of an enriched component in the EC magma source. Furthermore, low δ18O zircon compositions imply assimilation of high temperature hydrothermally altered country rock by the source magma. The mineral assemblage in crystal-poor pumices indicates equilibrium storage conditions: zircon saturation and Ti-in-zircon crystallisation temperatures are consistent with alkali feldspar-melt temperatures. Significantly, zircon crystals were preserved both as macrocryst inclusions and in the groundmass of EC explosive and effusive deposits. These rocks preserve evidence of magma evolution by fractional crystallisation. This process led to pre-eruptive compositional stratification, which is evidenced in the range of whole-rock major and trace element compositions and zircon Zr/Hf values. Notably, zircon crystallisation and cooling ages derived from pumice, lava, and accidental lithic lava clasts in highly explosive pyroclastic deposits, have revealed episodes of evolved magmatism that would otherwise have gone undetected. In addition, the zircon trace element and isotope compositions, in combination with the range of crystallisation ages, evidence progressively deeper tapping of less evolved magma stored in discrete lenses. Thus, a combined zircon geochronological-geochemical approach can place constraints on the ca. 0.6 Ma recurrence of past explosive-effusive pulses of millennial to decamillennial duration and their enriched magma sources. This information is relevant for assessing hazards and informing monitoring and forecasting efforts to assist in managing associated risks for small ocean island volcanoes with particularly vulnerable populations and infrastructure.

Variations of Sr–Nd–Hf–O isotopes and redox conditions across a Neoproterozoic arc magmatic belt in the western margin of the Yangtze craton

We use zircon trace elements and HfO isotopes, plus whole-rock chemical and SrNd isotope compositions to investigate the controls on the chemistry of Neoproterozoic arc magmas. The samples are from the gabbrodiorite intrusion of the Gaojiacun mafic-ultramafic complex and from the Tongde diorite batholith of the Panxi Neoproterozoic magmatic belt in the western margin of the Yangtze craton, western China. Zircon UPb ages reveal that both intrusions are coeval, emplaced at ∼820 Ma. Major element compositions of whole rocks and major silicate minerals indicate that the Tongde batholith formed from more evolved magma than the parental magma for the Gaojiacun intrusion. The mantle-normalized trace element patterns of whole rocks from both intrusions are similar, showing light REE enrichments and pronounced negative NbTa anomalies. The SrNd isotopic compositions of the whole rocks are also similar, plotting within or very close to the mantle array, indicating no to minor contamination with the upper crust. Zircon HfO isotopes indicate that the Gaojiacun magma has much higher ƐHf (6.26 ± 0.52) and δ18O (7.0 ± 0.3 ‰) than the Tongde magma (ƐHf = 4.81 ± 0.39; δ18O = 5.7 ± 0.2 ‰). The δ18O values of the Gaojiacun zircons are also higher than the normal mantle (δ18O = 5.3 ± 0.6 ‰). Zircon trace element compositions show that, despite their close spatial association (<10 km), the coeval intrusions have contrasting redox states, with the estimated fO2 values of ΔFMQ+1.0 and ΔFMQ-0.7 for Tongde and Gaojiacun, respectively. The former is within the range of modern arc basalts, whereas the latter is lower than the average value of MORBs. The very low fO2 of the Gaojiacun magma could be due to the presence of subducted oceanic sediments enriched in organic matter (OM) in the source or due to contamination with OM-rich sedimentary rocks in the upper crust. Since such crustal rocks have not been reported for the region, we prefer the former explanation. The preferred model is also supported by Sr–Nd–Hf–O isotopes. The results from this study reveal that both reduced and oxidized magma could be produced simultaneously in a subduction zone.

Diverse magma evolution recorded in trace element composition of zircon from Permo-Carboniferous rhyolites (NE German Basin, NW Polish Basin)

Permo-Carboniferous rhyolitic rocks are widespread in the NE German Basin and NW Polish Basin. Hafnium (Hf) and oxygen (O) isotopes analysed in zircon from these rocks suggest diverse sources and processes involved in the formation of rhyolitic magmas. In this study, detailed core-to-rim trace element compositions were analyzed in zircon from four localities that were previously analyzed for Hf and O isotopes. The trace element analyses, in particular Hf concentrations as well as Eu/Eu*, Ce/U, Yb/Gd, and Th/U ratios, are consistent with prolonged magma evolution in three localities from the NE German Basin (Fehmarn, Slazwedel and Penkun). The fourth locality within the NW Polish Basin (Wysoka Kamieńska) is consistent with a shorter period of magma evolution. Similar stages were distinguished in zircon from the three NE German Basin localities that include: early crystallization followed by rejuvenation with more primitive magma (stage A), subsequent fractional crystallization (stage B) and finally late crystallization in a saturated system or alternatively late rejuvenation with a more primitive magma (stage C). Interestingly magmatic rims on inherited zircon grains have compositions typical for late stage B and stage C, which is consistent with their late addition to evolving rhyolitic magma, most probably during assimilation and not during source melting. The zircon from the fourth, NW Polish Basin locality shows limited compositional variability consistent with the eruption of hot magma not long after the zircon started crystallizing. Thus trace element analyses in zircon provide a record of magmatic processes complementary to that of Hf and O isotope analysis, in that, a detailed analyses of core-to-rim compositional variations are particularly useful in distinguishing respective stages of magma evolution and can pinpoint the relative timing of inherited grains being incorporated into magma.

Paleo–Mesoarchean sedimentary record in the Dharwar Craton, India: Implications for Archean ocean oxygenation

The early Archean oceans were marked by significant redox changes which have subsequently shaped the Earth’s biosphere. Archean chemical sediments of banded Iron and Manganese formations provide important geochemical proxies for these historical shifts in the redox conditions and to trace the ancient sedimentation patterns and protoliths. In this study, we investigate the proto-ore of the Archean Mn-formations of the Sandur, Chitradurga and Shimoga greenstone belts of Dharwar Craton of southern Peninsular India, which is geochemically characterised as quartz arenites, Mn-arenites, Fe-arenites, Mn-argillites and Fe-argillites. The geochemical systematics suggest their deposition in shallow to deeper shelf in the Archean proto-ocean. The detrital zircon U-Pb systematics of Mn arenites and argillites indicate their maximum depositional age of 3230 ± 52 Ma representing the oldest onset of sedimentation during the Paleo–Mesoarchean timeframe in the Chitradurga Group of Dharwar Supergroup. The detrital influx proxies suggest variations in sedimentation rates associated with the Archean transgressive–regressive cycles and fluctuating hydrodynamic conditions, together reflecting an increasing trend in the contributions of recycled sediment from Sandur to Chitradurga and Shimoga greenstone belts. The available detrital zircon ages of the Mn arenites and argillites from these greenstone belts indicate a ∼ 600 Ma prolonged period of Mn deposition for which high-T hydrothermal fluids from Archean mid-oceanic ridges supplied the manganese. The trace element compositions of the concordant detrital zircons suggest 3.3–3.1 Ga Dharwar basement TTG/granitoid source which is corroborated by the zircon crystallization temperatures of 690–820 °C. The source-normalised α-dose rates of the detrital zircons signify greater degrees of sediment transport and multi-cycle nature which correspond to the earliest episode of crustal growth in the Indian sub-continent associated with the Mesoarchean Ur supercontinent. The clastic-chemogenic sedimentation attained through concomitant detrital sediment–seawater-metalliferous hydrothermal fluid mixing at an epicontinental passive margin resulted in the deposition of Mn-arenites and argillites closer to the higher Eh shore, while the Fe-rich sediments formed at a relatively deeper shelf characterised by comparatively lower Eh and more alkaline conditions. The comprehensive geochemical and geochronological data of the Archean Mn arenite-argillite sequences reveal the significance of regional episodes of ocean oxygenation at the shallow shelves of Archean oceans prior to great oxygenation event (GOE) that was mediated by the prolific growth of ancient microbiota which transformed the Earth to a more habitable planet.

Formation of Archean greenstone belts: Insights from an assemblage-scale study in the western Superior craton

Archean greenstone belts contain the oldest volcanic rocks on Earth and can provide critical insights into Earth’s early evolution. This study chooses the Sturgeon Lake greenstone belt in the easternmost Western Wabigoon terrane (WWT) of the Superior craton to illustrate an option for better understanding the formation of Archean greenstone belts. Our dataset includes outcrop observations, bulk compositions of 77 mafic to felsic volcanic samples, and in-situ U-Pb, Lu-Hf, and trace elements of zircon from 14 intermediate to felsic volcanic samples. The results revealed that this greenstone belt had broadly continuous volcanism from ca. 2780 to 2725 Ma, followed by a tectonic quiescence prior to a ca. 2707–2703 Ma volcanism at the terrane boundary. The intermediate volcanic rocks are characterized by variable Th/Nb and highly depleted Hf (zircon ƐHf(i) = +3 to + 6), interpreted to be derived from mantle sources ranging from those of normal mid-ocean ridge basalts (N-MORB) to those of enriched MORB (E-MORB), which commonly assimilated various felsic crustal components. The mafic volcanic rocks are dominated by low La/Sm basalts with negligible Nb, Ta anomalies and limited range of Th/Nb, forming a mantle array on the Th/Yb-Nb/Yb plot and consistent with oceanic crust of N-MORB to enriched N-MORB affinities. Some basalts from the ca. 2738–2725 Ma assemblages, however, gave high La/Sm, negative Nb and Ta anomalies, and limited range of Th/Nb plotting above and parallel to the mantle array. These basalts might be associated with passive subcretion and rollback of oceanic crust of the stagnant-lids tectonics or subduction of oceanic lithosphere as in modern-style plate tectonics, both of which are supported by trace element compositions of zircon that indicate hydrous, oxidized, large-ion lithophile element-enriched, and medium- to high-pressure magmas. Identification of three magma-series would not be possible if the data were lumped together, which is commonly seen in terrane- or craton-scale studies that sometimes propose contrasting interpretations of rock origins and Archean geodynamic processes. This detailed approach of distinguishing rock types and zooming into the subunits of a greenstone belt offers a practical solution to more accurately characterize Archean volcanic rocks and to better understand the early Earth.

Deciphering the parent-daughter relationship between Ediacaran high-silica ignimbrites and their complementary silicic cumulates: Insights from zircon trace element composition

The Ediacaran Campo Alegre-Corupá Basin (CACB) comprises a rare and excellent exposure of caldera-forming pyroclastic rocks and their cogenetic underlaying plutons. The CACB experienced at least two major episodes of bimodal volcanic activity. The Basin Stage (∼605–590 Ma), characterized by transitional OIB-like basaltic lava flows with minor trachydacites and rhyolites, interbedded with surge-like, silicic pyroclastic deposits, and the Caldera Stage (∼583–577 Ma), marked by at least one cycle of silicic alkaline, caldera-forming eruption, dominated by rhyolitic to trachytic ignimbrites and lava flows/domes with subordinate IAB-like basalts. During the Caldera Stage, granitoid plutons were emplaced at shallow crustal levels (∼5–8 km deep), correlating with the depth of the magma chamber responsible for the contemporaneous large silicic eruption(s). Previous studies have suggested a genetic connection between the post-collisional volcanic and plutonic rocks based on their similar ages, composition, and Sr-Nd-Hf isotopic signatures. In this study, we analyze zircon trace elements from the volcanic rocks of both stages in the CACB to constrain their evolutive compositional patterns, redox conditions, and Ti-in-zircon saturation temperatures. We compare our results with zircon compositions of the cogenetic granitoids, focusing on the Corupá Pluton, which likely supplied the magma for the caldera-forming eruption(s). By examining whole-rock and zircon chemical compositions, we establish a petrogenetic link between the volcanic rocks and their potential remnant magma reservoir. Our findings indicate that the silicic volcanic rocks from the Caldera Stage represent highly evolved compositions extracted from a crystal mush that solidified as a syenitic intrusion. During the evolution of the volcanic-plutonic system, zircon crystals tracked compositional changes associated with the extraction of residual silicic liquids from crystal mushes and the formation of intermediate-silicic cumulates. Unique variations in zircon compositions of volcanic and plutonic rocks, such as Eu/Eu*, Zr/Hf, and Th/U ratios support such a connection and suggest crystal-melt segregation processes at relatively low crystallinities (< 30 vol%). Additionally, our trace element data supports the occurrence of intense hydrothermal-induced recrystallization of zircon within the caldera structure, as suggested by the morphologic aspects of crystals previously analyzed for UPb and LuHf isotopes.

Petrogenesis and Geodynamic Evolution in the Northern Westfjords, Iceland, Elucidated by Iceland's Oldest Silicic Rocks

AbstractIceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ18O∼+2 to +4‰ and indicate involvement of a low‐18O component in their source magmas. Hrafnsfjörður zircon Hf (mean sample εHf ∼ +15.3–16.0) and whole rock Hf and Nd (εHf = +14.5 to +15; εNd = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sample εHf ∼ +11.8–13; whole rock εHf = +12.8 to +15.1; εNd = +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (208/204Pb = 37.95–37.96; 206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (208/204Pb = 38.34–38.48; 206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions of εHf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle.

Contrasting magmatic controls on the genesis of Fe-Ti-V oxide deposits in the Emeishan large igneous province using apatite Sr-Nd isotopes and apatite-zircon trace elements

We use Sr-Nd isotopes of apatite and trace element compositions of apatite-zircon pairs from a major ore layer in each of three selected magmatic Fe-Ti-V oxide ore deposits (Hongge, Panzhihua, and Taihe) in the Emeishan large igneous province (LIP) in southwestern China to contrast the magmatic controls on ore formation. The average contents of REEs in apatite from a representative ore zone/layer in each of these deposits are higher than the predicted values of apatite crystallizing from a melt with REE contents assumed to be the same as the average values of high-Ti basalts in the Emeishan LIP, confirming that the ore-forming magmas all experienced higher degrees of fractional crystallization by major silicate minerals than the average basalt. The apatites Sm/Yb and Sr/Y indicate that the Panzhihua magma was generated at a shallower depth and experienced higher degree of plagioclase fractional crystallization than Taihe and Hongge. The apatite Sr-Nd isotopes can be reproduced by ~ 8 wt% contamination with Precambrian gneiss-schist in the parental magma for Hongge and by ~ 10 wt% and ~ 25 wt% contamination with marbles in the parental magmas for Panzhihua and Taihe, respectively. A recycled, high-T altered oceanic gabbroic component in the mantle source is a viable alternative to the marble contamination model for Taihe. Coexisting zircon trace element compositions reveal that the parental magma for Taihe (ΔFMQ+4.0) is more oxidized than those for Hongge (ΔFMQ−0.4) and Panzhihua (ΔFMQ−0.7), questioning that magma oxidation played a critical role in the genesis of Fe-Ti-V oxide ore deposits in the Emeishan LIP.

Insights into the palaeoenvironments, structure and stratigraphy of the lower Miocene of the Eastern Carpathians Bend Zone, Romania

The ages of several Oligocene to Miocene sedimentary formations from the Eastern Carpathians Bend Zone are poorly constrained due to palaeoenvironmental factors, reworking of fossils, structural complexity and limited exposure. To help overcome these issues, this study integrates calcareous nannoplankton and foraminifera biostratigraphy with isotopic age dating (U-Pb) of volcaniclastic zircons, and sedimentological and structural observations/interpretations. Our study was carried out along an ~6-km-long section made from a series of outcrops along the Bizdidel River which exposes several formations such as the Pucioasa, Fusaru, Vinețișu, Starchiojd and Slon. We show that the Fusaru Formation consists of coarse-grained rocks deposited as confined longitudinal channel successions that migrated laterally. It is bounded by the mud-rich Pucioasa and Vinețișu formations which are lateral equivalents of the Fusaru confined channels deposited as levee/overbank units. These genetically related formations appear to reach younger ages – of the lower to middle Burdigalian based on calcareous nannoplankton and foraminifera biostratigraphy – than previously thought (upper Oligocene to lower Burdigalian). The dominant organic-rich mudstones of the Starchiojd Formation represent pelagites/hemipelagites deposited in anoxic conditions. Their middle Burdigalian age is established by a 17.41 ±0.27 Ma zircon U-Pb age of zircons from the Bătrâni Tuff in the Starchiojd Formation. Based on the similar phenocryst content, zircon U-Pb age and zircon trace element composition, the source of the tuff is suggested to be the 17.3 Ma Eger ignimbrite-forming eruption, which has proximal, near-caldera deposits in the Bükkalja Volcanic Field, Hungary. The mud-rich Slon Formation seems to be related to shelf edge/upslope failure that formed cohesive debrite avalanches resulting from foreland propagation of compression. The Slon Formation extends in this area to at least the upper part of the lower Miocene to middle Miocene. These results highlight the need to revise ages of those parts of the sequence which are poorly constrained or different in other parts of the Carpathian Basin. Such revised ages help to better constrain the understanding of the deformation history of the Carpathians.