Zircon Crystallization Temperatures Research Articles

In situ probing of the present-day zircon-bearing magma chamber at Krafla, Northeastern Iceland

Active felsic magmatism has been rarely probed in situ by drilling but one recent exception is quenched rhyolite sampled during the 2009 Iceland Deep Drilling Project (IDDP). We report finding of rare zircons of up to ∼100 µm in size in rhyolite glasses from the IDDP-1 well products and the host 1724 AD Viti granophyres. The applied SHRIMP U-Th dating for both the IDDP and the Viti granophyre zircons gives zero-age (±2 kyr), and therefore suggests that the IDDP-1 zircons have crystallized from an active magma intrusion rather than due to the 20–80 ka post-caldera magmatic episodes recorded by nearby domes and ridges. Ti-in-zircon geothermometer for Viti granophyre reveals zircon crystallization temperatures ∼800°C–900°C, whereas IDDP-1 rhyolite zircon cores show Ti content higher than 100 ppm, corresponding to temperatures up to ∼1,100°C according to the Ti-in-zircon thermometer. According to our thermochemical model at such elevated temperatures as 1,100°C, rhyolitic magma cannot be saturated with zircon and zircon crystallization is not possible. We explain this controversy by either kinetic effects or non-ideal Ti incorporation into growing zircons at low pressures that start to grow from nucleus at temperatures ∼930°C. High temperatures recorded by IDDP-1 zircon together with an occurrence of baddeleyite require that the rhyolite magma formed by partial melting of the host granophyre due to basaltic magma intrusion. Zr concentration profiles in glass around zircons are flat, suggesting residence in rhyolitic melt for >4 years. In our thermochemical modeling, three scenarios are considered. The host felsite rocks are intruded by: 1) a basaltic sill, 2) rhyolite magma 3) rhyolite sill connected to a deeper magmatic system. Based on the solution of the heat conduction equation accounting for the release of latent heat and effective thermal conductivity, these data confirm that the rhyolite magma could be produced by felsic crust melting as a result of injection of a basaltic or rhyolite sill during the Krafla Fires eruption (1975 AD).

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.

Early cretaceous ridge subduction in the Shandong Peninsula, Eastern China, indicated by Laoshan A-type granite

Early Cretaceous A-type granites are widespread in the Shandong Peninsula, which can be used to elucidate the tectonic evolution of the eastern China and the destruction of the North China Craton. However, their genesis is still controversial. Several competing models, ranging from slab break-off, postorogenic extension, foundering of the lower crust and ridge subduction, were proposed. Here, we report zircon U–Pb ages, whole-rock and apatite geochemical compositions of the Laoshan granite and discuss its tectonic implications. The Laoshan granite has typical characteristics of A-type granite with high FeOT/(FeOT + MgO) ratios (0.90–0.97) and 10000*Ga/Al ratios (2.70–3.36) and high total alkali (Na2O + K2O: 7.95–8.70 wt%) contents and Zr+Nb+Ce+Y (most >350 ppm) concentrations. The Laoshan granite is further classified as A1-type based on the Yb/Ta-Y/Nb and Ce/Nb-Y/Nb diagrams and the Nb-Y-3Ga and Nb-Y-Ce triangular discriminant diagrams. Zircon U–Pb dating of two Laoshan granite samples yielded emplacement ages of 117.8 ± 1.0 Ma and 120.1 ± 1.3 Ma, respectively. The oxygen fugacity of the Laoshan granite magma is low, as indicated by zircon Ce4+/Ce3+ ratios (most <300). The crystallization temperature of zircon varies significantly, ranging from 652 to 830°C. The apatite compositions show that the Laoshan granite has high F (2.09–2.72 wt%) and low Cl (0.01–0.09 wt%) contents, consistent with influence by fluid released from the decomposition of phengite. Apatite rare earth elements show that mantle sources are also involved in Laoshan A-type granite. Combined previous studies of A-type granitic plutons in the Shandong Province and the Lower Yangtze River belt with the drifting history of the Pacific plate, we propose that the flat subduction of the spreading ridge between the Pacific and the Izanagi plates was responsible for the formation of Laoshan A-type granite.

Petrogenesis and geochronology of the bronze fox porphyry Cu-Au deposit: Implications for the geodynamic evolution of the Gurvansaykhan Island arc, southern Mongolia

ABSTRACT A diversity of Devonian-Carboniferous volcanic arcs and granitoid intrusions are exposed in the Gurvansaykhan arc terrane of southern Mongolia. Several giant porphyry Cu-Au-Mo deposits are associated with intrusions within these arcs. The Bronze Fox deposit located in the central part of the Gurvansaykhan arc terrane is an example of a potentially economic deposit with affinity of adakite-like granitoid porphyry (i.e. 428 Mt grading 0.26% to 0.30% Cu for up to 2,437 Mlb of copper and 0.84 Moz gold). The genesis and magmatic evolution of the causative rocks of the Bronze Fox porphyry Cu-Au hosting rocks are poorly understood, and different models are still under debate. In the current study, we present new field observations and petrographic, geochemical, and geochronological data to describe the magmatic evolution and geodynamic settings. In addition, the mineral chemical data are reported from some selected rock-forming minerals to identify crystallization conditions of the studied rock suites by employing geothermobarometric methods. Zircon chronological data reveal the emplaced age at 331 ± 3.4 Ma for the volcanic lava, followed by mineralized granodiorite and monzodiorite with intruded ages of 326.5 ± 3.4 Ma and 326.5 ± 2.7 Ma, respectively. The studied intrusive and extrusive rocks are high-K-alkaline and metaluminous, characterized by high Mg#. They are enriched in large ion lithophile elements (Sr and Ba) and light rare earth elements (LREEs) and depleted in high field strength elements (Nb, Ta, and Ti) and heavy rare earth elements (HREEs). The highly mineralized granodiorite intrusion yields zircon crystallization temperatures ranging from 631.8 to 779.4°C, and the monzodiorite intrusion, which contains relatively low-grade mineralization, has a wider range (599.0 to 803.4°C). The formation pressures of the former range from 0.5 to 0.2 kbar, which is equivalent to a depth of 1.38 km based on amphibole geochemical calculations. The results of mineral chemistry indicate that the unmineralized diorite unit was formed under medium pressure (~50 MPa) and high temperature (~750°C) at depths of 6.8 to 7.5 km. Similarities in geochemistry and petrographic features suggest that the igneous rocks probably originated from a similar magma source (i.e. from the melting of ancient oceanic crust and sediments) but experienced different degrees of partial melting or mixing with lower juvenile crust melts. The estimated fO2 values obtained by amphibole chemistry in the granodiorite and quartz diorite of the Bronze Fox copper-gold mineralization range from −13.0 to −11.8 and −13.2 to −10.8, respectively, confirming that these igneous rocks probably crystallized under relatively oxidized conditions above the nickel-nickel oxide (NNO) buffer. Such results yield an overall range between NNO +2.6 and NNO +1.4, which are characterized by high oxygen fugacities and high Ce4+/Ce3+ ratios that reflect oxidizing magma, thus indicating quite promising potential for porphyry Cu-Au mineralization in the region. Based on the findings of this work, we suggest a new tectonic model of arc magmatism that is a response to the subduction of the Paleo-Asian Ocean in the early Carboniferous period.

Tectono-Thermal History of the Neoarchean Balehonnur Shear Zone, Western Dharwar Craton (Southern India)

Abstract A widely spaced Neoarchean shear zone network traverses the granite-greenstone terrains of the Western Dharwar craton (WDC). The NNW-SSE trending Balehonnur shear zone traverses the largest part of the preserved tilted Archean crustal ensemble in the Western Dharwar craton (WDC) from the amphibolite-granulite transition in the south to greenschist facies in the north and eventually concealed under Deccan lava flows. Published tectonic fabrics data and kinematic analysis, with our data reveal a sinistral sense of shearing that effectuate greenstone sequences, Tonalite-Trondhjemite-Granodiorite Gneisses (TTG), and Koppa granite as reflected in variable deformation and strain localization. A profound increase of strain towards the core of the shear zone in the ca. 2610 Ma Koppa granite is marked by a transition from weak foliation outside the shear zone through the development of C-S structures and C-prime fabrics, mylonite to ultramylonite. The mineral assemblages in the Koppa granite and adjoining greenstone indicate near peak P-T conditions of 1.2 Gpa, 775-800°C following a slow cooling path of 1.0 GPa and 650°C. Field-based tectonic fabrics data together with U-Pb zircon ages reveal that the Koppa granite emplaced along the contact zone of Shimoga-Bababudan basin ca. 2610 Ma, coinciding with the emplacement of ca. 2600 Ma Arsikere-Banavara, Pandavpura, and Chitradurga granites further east which mark the stabilization of WDC. Significant variation in major element oxide (SiO2 = 56-69 wt.%) together with high content of incompatible elements (REE, Nb, Zr, and Y) and high zircon crystallization temperatures (~1000°C) of Koppa granite suggests derivation by partial melting of composite sources involving enriched uppermost mantle and lower crust. The development of widely spaced shear zones is probably linked to the assembly of eastern and western blocks through westward convergence of hot oceanic lithosphere against already cratonized thick colder western block leading to the development of strain heterogeneities between greenstone and TTGs due to their different mineral assemblages leading to rheological contrast in the cratonic lithologies.

Distribution of trace elements controlled by sector and growth zonings in zircon from a miaskite pegmatite of the Vishnegorsky massif, the Southern Urals

Data on the content and distribution of trace and rare-earth elements (SIMS method) in sectors and growth zones of a large zircon crystal from miaskite pegmatites of the Vishnegogorsky massif are presented. The morphology of the zircon crystal is a combination of a dipyramid {111} and prism {010}. It has been established that the growth sector of dipyramid {111} is characterized by almost one order of magnitude higher contents of Y, Nb, REE, Th; higher Th/U and Eu/Eu* values; REE distribution spectra are flatter compared to prism {010} growth sector. A regular decrease in the content of trace and rare-earth elements in the direction from the central zone to the marginal zone of crystal growth was revealed. A smooth regression of zircon crystallization temperature of zircon from 960 °C in the central zone to 740 °C in the marginal zone of the dipyramid sector and 700-650 °C in the prism sector has been revealed, which may be a reflection of thermal evolution of the crystallization process. It is assumed that crystallization of the central zone of zircon occurred at early stages from a relatively trace-еlement-rich melt. The crystallization was completed at lower temperatures, probably, simultaneously with the formation of REE-concentrating minerals, which resulted in natural decrease of content of trace and rare-earth elements in the melt and, consequently, in zircon crystallizing from it.

Permian-Triassic granites of the Schladming complex (Austroalpine basement): Implications for subduction of the Paleo-Tethys Ocean in the Eastern Alps

Permian magmatic activity is widespread in the Austroalpine Unit stretching from Eastern Alps to the Western Carpathians and Pannonian region, the link between magmatism and the subduction of Paleo-Tethys Ocean is not well constrained up to now. Here, we report, for the first time, a Middle Triassic granite in the Austroalpine basement, which is at odds with a common opinion of the passive margin setting of this unit. The Tweng and Schladming Complexes representing a Neoproterozoic to Early Paleozoic magmatic arc were intruded by Permian and Triassic granites, related to back-arc extension triggered by the subduction of Paleo-Tethys Ocean. To reveal the tectonic affinity of these granitic rocks, we systematically analyzed their zircon geochronology and whole-rock geochemistry. The zircon U-Pb data show that the Tweng granitic gneisses and leucogranites were formed at 261 Ma and 244 Ma, respectively. The Schladming granitic gneisses were formed at 244–241 Ma, their εHf(t) values (-0.9 to + 5.6) indicate their derivation from the lower crust and involved continent material. Geochemically, the granitic rocks from the Tweng Complexes have common features with subduction-related continental arc andesitic affinity. The granitic gneisses of Tweng and Schladming Complexes show A-type granite-affinity with high zircon crystallization temperatures of 784–835 ℃ and 781–818 ℃, respectively, whereas the Tweng leucogranites exhibit I-type granite characteristics and low zircon crystallization temperatures of 720–724 ℃. The geochemical features suggest that the Tweng granitic gneisses and leucogranites originated from the lower crust, while the Tweng leucogranites are explained by remelting of Tweng granitic gneisses, and interacted with subduction-related fluids. Therefore, we propose that the Permian-Triassic granitic rocks of Tweng and Schladming Complexes were formed in a back-arc setting during the subduction of the Paleo-Tethys Ocean, which finally resulted in the opening of the Meliata oceanic back-arc basin during Middle Triassic times.

Diffusion anisotropy of Ti in zircon and implications for Ti-in-zircon thermometry

Ti-in-zircon thermometry has become a widely used tool to determine zircon crystallization temperatures, in part due to reports of extremely sluggish Ti diffusion perpendicular to the crystallographic c-axis in this mineral. We have conducted Ti-in-zircon diffusion experiments, focusing on diffusion parallel to the c-axis, at 1 atm pressure between 1100 and 1540°C, with oxygen fugacities equivalent to air and the Ni-NiO buffer. There is no resolvable dependence of Ti diffusion in zircon upon silica or zirconia activity, or upon oxygen fugacity. The diffusion coefficient of Ti in zircon is found to be a weak function of its own concentration, spanning less than 0.5 log units across any profile induced below 1300°C. Ti diffusion in zircon, parallel to the c-axis at 1 atm pressure, is well described using:log10⁡DTi=[1.34(±1.44)−555425(±44820)Jmol−12.303RT(K)]m2s−1 where R is the gas constant in J/(mol⋅K). In conjunction with diffusion coefficients for Ti in zircon perpendicular to the c-axis reported by Cherniak and Watson (2007), strong diffusion anisotropy for Ti in zircon is observed. Diffusion parallel to the c-axis is ∼4-5 orders of magnitude faster than diffusion perpendicular to the c-axis within the experimentally constrained temperature range shared between these two studies (1540-1350°C). This difference increases if the data are extrapolated to lower temperatures and reaches ∼7.5-11 orders of magnitude between 950-600 °C, a typical range for zircon crystallization. Diffusion of Ti in natural zircons will predominantly occur parallel to the c-axis, and the Ti-in-zircon thermometer appears susceptible to diffusive modification under some crustal conditions. Temperatures calculated using this system should therefore be evaluated on a case-by-case basis, particularly when considering high-T, slowly cooled, reheated and/or small zircons.

Stable isotope geochemistry of silicon in granitoid zircon

Zircon is often used to study granites (sensu lato) and continental crust because it is very resistant and can be analyzed for various isotope systems that provide time and source information about their parental melts. Granites with different petrological histories have distinct bulk-rock silicon isotope compositions but it is unclear if these differences are also detectable in zircon because of superimposed fractionation effects (e.g., related to temperature, silica content, magmatic processes). The present study explores the Si isotope signatures of zircon from various granite types to constrain their isotope fractionation behavior and uses them as igneous petrogenetic tools, and possibly as granite source discriminators when zircon is found in detrital sediments. Our results show that although Si isotope compositions in zircon can be modified by secondary (post-crystallization) processes such as alteration/weathering and metamorphism, they are primarily controlled by zircon-melt isotope fractionation, which depends on both zircon crystallization temperature and magma silica content. Once these fractionation effects are understood and filtered out, a pattern emerges between Si isotope signatures of zircons from different granite types that is consistent with theoretical and experimental results as well as with known Si isotope differences at the bulk-rock scale. Silicon isotope ratios in zircon can track magma evolution (e.g., temperature and SiO2 changes) and, hence, reveal complex processes that involved magma mingling, fractional crystallization, and/or multiple sources. This study, therefore, illustrates that Si isotopes in zircon can be used to investigate magma evolution and represents a useful complement to existing techniques in granite studies involving zircon (e.g., U-Th-Pb, Lu-Hf and O isotopes) provided that it is not used as a stand-alone technique.

Cenozoic temporal variation of crustal thickness in the Urumieh-Dokhtar and Alborz magmatic belts, Iran

We present regional variations of whole-rock Sr/Y and (La/Yb)N ratios of magmatic rocks along the Cenozoic Urumieh-Dokhtar and Alborz magmatic belts, Iran. Both the magmatic belts are located at the north of the main Zagros Neo-Tethyan suture. The Urumieh-Dokhtar magmatic belt (UDMB), which trends NW-SE for 1000 km across Iran, was characterized by the intensive volcanism and plutonism, and defined the magmatic front (MF) of the Zagros orogenic belt. The Alborz magmatic belt (AMB) is situated to the north, and characterized by less intense magmatic activity. The Alborz magmatic belt was formed behind it in the rear-arc (RA) domain. A striking feature of the both magmatic belts is the transition from normal calc-alkaline arc magmatism during the Eocene–Oligocene to adakite-like calc-alkaline magmatism during the Middle to Late Miocene–Pliocene. The late-Cenozoic magmatism of the UDMB and AMB shows higher Sr/Y and (La/Yb)N. However, it should be noted that crustal thickening event is intensive in the UDMB than AMB during Late Cenozoic. Using the composition of the Lale-Zar zircons from the SE UDMB we determined the oxygen fugacity (fO2) during zircon crystallization to be between FMQ (fayalite–magnetite–quartz buffer) -0.69 to +2.41, whereas those of the Hashroud-Teckmdash-Gormolla zircons from NW AMB range from −1.22 to +5.99. The fO2 estimates suggest relatively more oxidized conditions for the Late Cenozoic igneous rocks of the AMB. Compiled data from the UDMB and AMB intrusions show an increase in average zircon crystallization temperatures with decreasing age. These outcomes have been interpreted in terms of variation of the crustal thickness, from 30 to 35 km during Eocene-Oligocene to 40–55 km during the middle-late Miocene. We propose the increase in crustal thickness is associated with the collision between the Arabian plate and Iran and subsequent convergence during the middle-late Miocene.

Zircon geochronology suggests a long-living and active magmatic system beneath the Ciomadul volcanic dome field (eastern-central Europe)

Ciomadul is the youngest volcano in eastern-central Europe. Although its last eruption occurred at ca. 30 ka, there are independent indications for a high-crystallinity magma reservoir persisting beneath the volcano until present. In order to further test the hypothesis of long-lived melt presence and to better constrain the nature and timescales associated with the subvolcanic magma storage system, over 500 zircon U-Th and U-Pb spot ages (crystal interiors and outer surfaces) were interpreted from dacitic rocks of the most productive eruptive period (the Young Ciomadul Eruptive Period; YCEP, 160-30 ka). Zircon surface ages from lava dome and pumice samples range from ca. 600 ka up to the youngest eruption event at 30 ka. They form a continuous age distribution and some single crystals reveal significant age zonation (>150 kyr difference from core to rim). The oldest zircon ages of YCEP overlap with the last eruption events of the Old Ciomadul Eruptive Period (1000–330 ka). The zircon age spectra, combined with textural data, point to a prolonged (several 100's kyr) residence in a highly crystalline mush state. The range in zircon crystallization temperature (from ∼750°C to the solidus at ∼680°C) is consistent with the results of thermometry on amphibole and plagioclase from felsic crystal clots, which represent crystal mush fragments. To maintain magma reservoir for such a long time above solidus, continuous magma input by deeper recharge is required. Zircon crystallization model calculations constrained by thermal modelling imply an average rate of magma input of about 1.3 × 10−4 km3/yr over 2 Myr. Such estimate allows us to calculate an extrusive/intrusive ratio of 1:25–1:30. The model calculations suggest that a crystal mush zone of about 35 km3 is still present within the subvolcanic magma reservoir. Importantly, the Ciomadul plumbing system thus remains thermally primed and renewed magma injection could lead to rapid reawakening and eruption of the apparently inactive volcano.

TZT: a windows program for calculating zircon crystallization temperature based on Ti-in-zircon thermometry

A Free Windows software has been programmed by Visual Basic-2012 under the Windows platform. The program allows users to calculate the zircon crystallization temperature. The designed program employs a simple scoring algorithm to measure the zircon crystallization temperature based on the function of titanium-in-zircon thermometry (Watson and Harrison, 2005). During data entry, the input data option includes letters (upper case or lower case), numbers and even symbols. The obtained results are saved into output separate and editable Microsoft Excel sheet. The saved sheet can be used for data editing during opened existing file option. The results accuracy is validated by comparing with analyses taken from the literature and showing no considerable differences between the both results.

Geochemistry, zircon U–Pb dating, and Hf isotopes composition of Carboniferous granitoids in Xing'an Block, NE China: Record of slab break‐off event in eastern Central Asian Orogenic Belt

The Guang‐Tou‐Shan (GTS) granites, located in the Xing'an Block (XB), record Late Palaeozoic magmatism during the stage of convergence between micro‐blocks in the eastern Central Asian Orogenic Belt (CAOB). We performed a detailed geochronological and geochemical study of GTS granites intending to obtain better constraints on the subduction processes of the Palaeo‐Asian Ocean (PAO) in eastern CAOB. LA–ICP–MS U–Pb dating of zircons from GTS granitic rocks in western XB yielded emplacement ages of 332 ± 2 Ma and 301 ± 2 Ma, respectively, indicating a two‐stage emplacement process. Geochemical analyses show that the Early Carboniferous monzogranite (332 ± 2 Ma) have low values of the A/CNK and A/NK, are enriched in LILE (Rb, Sr, Ba) and LREE (LREE/HREE = 5.6–10.8), and depleted in HFSE (Nd, Ta, Zr, Hf) and P, Ti, which belong to I‐type granites. The Late Carboniferous granodiorite (301 ± 2 Ma) also displays LILE, LREE enrichment, and HFSE depletion with negative Nb–Ta anomalies, and contains relatively high Zr + Nb + Ce + Y contents (495–524 ppm) and high zircon crystallization temperature, indicating the Late Carboniferous granodiorite shows geochemical characteristics similar to those of typical A‐type granites. The two‐stage granitic rocks have εHf(t) values of +7.7 to +11.6 and yield a two‐stage depleted mantle Hf model age of 0.66–0.88 Ga, suggesting that the GTS granitic rocks were derived from partial melting of juvenile arc‐related mafic to intermediate rocks. The results, combined with regional magmatic information, indicate that the eastern CAOB experienced a subduction setting in the Early Carboniferous and extension setting in the Late Carboniferous. This change was most likely caused by the subduction slab break‐off of PAO from the Late Carboniferous.

Petrogenesis of Cretaceous granitoids in the Bengbu–Wuhe area, southeastern North China Craton: Implications for gold mineralization

Sinistral slip along the Tancheng–Lujiang Fault Zone (TLFZ) during the Mesozoic resulted in an offset of approximately 400 km between the Dabie and Sulu orogens. The Bengbu–Wuhe gold belt (BWGB) represents an extension of the Zhaoyuan–Yexian gold belt to the west of the TLFZ. Early Cretaceous intrusions are widely distributed throughout the BWGB. Gold deposits in the BWGB are commonly associated with intermediate acid dykes that are roughly contemporaneous with mineralization, and some orebodies are hosted in granitoids. This paper investigates granitoids within the BWGB to constrain their petrogenesis, tectonic setting, and metallogenic significance using systematic whole-rock elemental geochemistry, apatite geochemistry, zircon U–Pb dating, zircon Lu–Hf isotopes, and whole-rock Sr, Nd, and Pb isotopes. The timing of Au mineralization (134–113 Ma) in the area is consistent with the age of intrusive adakite-like rocks (130–115 Ma) and normal calc-alkaline granitoids (114–109 Ma). The adakite-like samples are characterized by pronounced enrichment in light rare earth elements (REE), depletion in heavy REE ((La/Yb)N = 30.5–66.2), an absence of Eu anomalies, high Sr contents and Sr/Y ratios, low Y and Yb contents, and fractionated REE patterns ((Dy/Yb)N = 1.22–1.69). They have relatively low Mg# values (38–42) and variable Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sr(t) = 0.7083–0.7111; εNd(t) = −22.2 to −14.9; 206Pb/204Pb(t) = 17.219–18.010; zircon εHf(t) = −21.0 to −12.3). These features indicate that the parental magmas of the adakites were derived from partial melting of thickened lower crust of the South China Plate and/or the North China Craton. The normal calc-alkaline granitoids have low Sr/Y (6.0–40.5), La/YbN (6.8–24.7), and Dy/YbN (0.90–1.28) ratios, exhibit negative Eu anomalies, and have variable Sr–Nd–Pb–Hf isotopic compositions (87Sr/86Sr(t) = 0.7082–0.7085; εNd(t) = −22.7 to −17.7; 206Pb/204Pb(t) = 16.686–16.713; zircon εHf(t) = −24.9 to −19.0), indicating they were derived from the middle to lower crust of the North China Craton. Compared with the normal calc-alkaline granitoids, the adakite-like rocks have higher zircon Ce4+/Ce3+, higher apatite SO3 and lower apatite MnO contents, lower and more variable zircon crystallization temperatures, and higher apatite molar OH/F and Cl/F ratios, indicating that the adakitic magmas were more enriched in volatiles and more highly oxidized. Taken together, field observations and geochemical data suggest that the adakite-like rocks related to the subducted South China Block are the ore-causative rocks.

Zircon melt inclusions in mafic and felsic rocks of the Bushveld Complex – Constraints for zircon crystallization temperatures and partition coefficients

Melt inclusions in zircon represent time capsules, which provide deep insights into igneous rock formation including timing, physicochemical conditions, and the compositional evolution of cogenetic magmas with respect to major, trace, and volatile elements. However, their full potential as petrogenetic indicators, in particular their usability and consistency as geothermometers, is poorly investigated. Therefore, we present new mineralogical and chemical data for recrystallized and homogenized melt inclusions and host zircon from different mafic and felsic rocks of the Bushveld Complex (BC), South Africa. Samples include rutile-bearing cumulate rocks of the Marginal and Critical zones, as well as rutile-free, magnetite-ilmenite-titanite-bearing diorites and granites of the Upper Zone, Rashoop Granophyre and Lebowa Granite Suite. All melt inclusions, irrespective of rock type, have rhyolitic compositions with SiO2 contents ranging from 65 to 78 wt.%, and H2O from 1.6 to 4.0 wt.%, whereas trace element contents differ systematically between rock types. In rutile-bearing mafic rocks, melt inclusions commonly show higher Ti contents (>800 ppm), higher Th/U ratios (up to 38), and lower REE contents (ƩREE < 150 ppm) compared to those in rutile-free mafic and felsic rocks (Ti < 800 ppm; Th/U < 5; ƩREE > 150 ppm). Liquidus temperatures of melt inclusions obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment mostly at 930–850 °C (at 200 MPa), tailing down in some samples to 700 °C. For rutile-bearing rocks, these temperatures overlap with those obtained by TiO2-in-melt and Ti-in-zircon geothermometry. For all rutile-free mafic and felsic rocks, reduced TiO2 activities of aTiO2 ∼ 0.3 are required for Ti-in-zircon geothermometry, and TiO2rutile ∼ 30 wt.% for TiO2 saturation geothermometry, to match temperatures from other geothermometers. Furthermore, partition coefficients obtained from melt inclusion - host zircon pairs are within error identical for mafic and felsic rocks and also reveal no systematic dependency on melt inclusion size, composition and entrapment temperature. The results of this study demonstrate that melt inclusions in zircon are a powerful tool for geothermometry and to constrain magma compositions, H2O contents, and TiO2 activities, which are critical for the understanding of magmatic processes shaping Earth’s crust.

Geochronological and Geochemical Study of Zircon from Tourmaline-Muscovite Granites of the Archaean Kolmozero–Voronya Greenstone Belt: Insights into Sources of the Rare-Metal Pegmatites

In order to determine the U-Pb crystallization age of zircon from the tourmaline-muscovite granites of the Kolmozero–Voronya greenstone belt located in the northeastern Fennoscandian Shield (Kola Peninsula), an isotope-geochronological study of the zircon grains was performed using a SHRIMP-RG microprobe. The belt is represented by the Archaean volcano-sedimentary rocks (2.9–2.8 Ga). Deposits of rare-metal pegmatites (Li and Cs with associated Nb, Ta, and Be) occur within the belt and on its margins. The age of the pegmatites within the belt was estimated at 2.7–2.6 Ga. Until now, there has been no generally accepted view on the genetic relation of the pegmatites with granite. Various authors have suggested that the pegmatites could potentially be associated with many type of granitoids within the region, i.e., plagiogranites, tonalites, amphibole-biotite granodiorites, microcline granites, alkaline granites, or muscovite-tourmaline granites. Zircon crystals from the muscovite-tourmaline granites are heterogeneous; they have less altered cores and strongly altered rims. The zircon cores are slightly enriched in U at a value of 173–1030 ppm, Th/U = 0.1–0.4. The zircons’ rims are heavily enriched in U at a value of 700–3300 ppm, Th/U = 0.03–0.08, indicating metasomatic processes. Zircon characteristics show that it crystallized from a melt enriched in a fluid phase. Fluid activity lasted after zircon crystallization as reflected in the irregular composition of the mineral and its rare earth element (REE) patterns that are typical of a metasomatic zircon. The computed zircon crystallization temperature in the tourmaline-muscovite granites is in the range of 650–850 °C. The discordant age calculated for five analyzed points of the zircon crystal cores is 2802 ± 13 Ma. The discordant age for four analyzed points of the zircon crystal rims is found to be 2728 ± 14 Ma. On the basis of the obtained isotope-geochronological data, we conclude that the tourmaline-muscovite granites located in the immediate vicinity of rare-metal pegmatite veins are the most probable source of matter for the pegmatites.

Magmatic evolution of ediacaran alkali rhyolites from the Acampamento Velho volcanism in the Tupanci area, southern Brazil: A study based on mineral chemistry, LA-ICP-MS Ti-in-quartz and zircon saturation geothermometry

Post-eruptive, metamorphic, and weathering processes are issues that affect not only the identification of geochemical signature but also mineral paragenesis on ancient volcanic successions. Still, the estimate of pressure, temperature and oxygen fugacity conditions prevalent during magma crystallization can be achieved for volcanic systems of any age by combining experimental, thermodynamic and natural data using geothermobarometers. In this regard, highly resistant minerals such as zircon and quartz can provide key information on the origin and evolution of silicic magmatic chambers. For example, the substitution of Ti4+ for Si4+ in quartz is a function of the activity of TiO2 in the magma and in the crystal, temperature and pressure. Therefore, the aim of this study is to apply Ti-in-quartz (TitaniQ) coupled to Zircon saturation geothermometry to provide a better insight into the igneous processes that originated post-collisional Ediacaran sodic-alkaline rhyolites from the Acampamento Velho in southern Brazil. Zircon saturation geothermometry estimates range from ca. 825 °C–1000 °C (within an error of ~25 °C), in agreement with previously reported zircon crystallization temperatures for the Acampamento Velho volcanism in the Ramada and Taquarembó plateaus. Temperature estimates using the Ti in quartz thermometer range from 700 to 710 °C for the intrusive rocks and from 710 to 810 °C for the lava flows that show quartz zonation. The Ti zonation pattern in quartz was probably generated by partial phenocryst dissolution in response to a magma chamber recharge and increase in the pre-eruption temperature. This type of process could be similar to the injection of mafic melts rich in volatiles that originated giant magma bodies, indicating that the Acampamento Velho volcanism was likely formed in a supereruption event with associated caldera collapse, as previously evidenced by detailed petrography studies in other occurrences in the Sul-Rio-Grandense shield.

Evidence for diverse lunar melt compositions and mixing of the pre-3.9 Ga crust from zircon chemistry

Lunar samples collected during Apollo missions are typically impact-related breccias or regolith that contain amalgamations of rocks and minerals with various origins (e.g., products of igneous differentiation, mantle melting, and/or impact events). The largest intact pre-Nectarian (∼≥3.92 Ga) fragments of igneous rock contained within the breccia and regolith rarely exceed 1 cm in size, and they often show evidence for impact recrystallization. This widespread mixing of disparate materials makes unraveling the magmatic history of pre-Nectarian period fraught with challenges. To address this issue, we combine U-Pb geochronology of Apollo 14 zircons (207Pb–206Pb ages from 3.93 to 4.36 Ga) with zircon trace element chemistry and thermodynamic models. Zircon crystallization temperatures are calculated with Ti-in-zircon thermometry after presenting new titania and silica activity models for lunar melts. We also present rare earth element (REE), P, actinide, and Mg + Fe + Al concentrations. While REE patterns and P yield little information about the parent melt origins of these out-of-context grains, U and Th concentrations are highly variable among pre-4.2 Ga zircons when compared to younger grains. Thus, the distribution of heat-producing radioactive elements in melt sources pervading the early lunar crust was heterogenous. Melt composition variation is confirmed by zircon Al concentrations and thermodynamic modeling that reveal at least two dominant magma signatures in the pre-4.0 Ga zircon population. One inferred magma type has a high alumina activity. This magma likely assimilated anorthosite-rich rocks (e.g., Feldspathic Highlands Terrane; FHT), though impact-generated melts of an alumina-rich target rock is a viable alternative. The other magma signature bears more similarities to KREEP basalts from the Procellarum KREEP Terrane (PKT), reflecting lower apparent alumina activities. Melt diversity seems to disappear after 4.0 Ga, with zircon recording magma compositions that largely fall in-between the two main groups found for pre-4.0 Ga samples. We interpret <4 Ga zircons to have formed from a mixture of KREEP basalts and FHT-like rocks, consistent with the upper ∼15 km of the crust being thoroughly mixed and re-melted by basin-forming impacts during the pre-Nectarian period.

Genesis of Early–Middle Jurassic Intrusive Rocks in the Erguna Block (NE China) in Response to the Late-Stage Southward Subduction of the Mongol–Okhotsk Oceanic Plate: Constraints from Geochemistry and Zircon U–Pb Geochronology and Lu–Hf Isotopes

The subduction processes and geodynamic scenarios of the late-stage southward subduction of the Mongol–Okhotsk oceanic slab since the Early Jurassic are subjects of great debate. This contribution presents new U–Pb zircon dating, trace element geochemistry, Ti-in zircon geothermometry, and Lu–Hf isotopes of zircon, as well as bulk-rock geochemical data for Early–Middle Jurassic intrusive rocks in the Erguna Block, NE China. Approximately 181–198 Ma monzogranites and ca. 162–174 Ma quartz monzonites were identified in the block. The Early Jurassic monzogranites are high-K calc-alkaline I-type granites, which display moderately concave-upward rare earth element (REE) patterns with slightly negative Eu anomalies, and low zircon crystallization temperatures. The Middle Jurassic quartz monzonites have low Yb and Y concentrations, high Sr/Y ratios, and strong high field strength elements (HFSEs) depletions, that are in excellent agreement with adakitic rocks. They exhibit right-sloping REE patterns with negligible Eu anomalies, and a wide range of zircon crystallization temperatures. The intrusions yield εHf(t) values between −4.1 to +4.8 and juvenile two-stage model (TDM2) ages varying from 918–1488 Ma. The geochemical and isotopic signatures suggest that the monzogranites were likely derived by the partial melting of K-rich meta-basalts within the lower part of a juvenile crust that had medium-thickness (≤40 km), with the involvement of minor mantle materials. Whereas, the quartz monzonites were possibly produced by partial melting of a thickened continental lower crust (≥50 km). The Mongol–Okhotsk tectonic regime played a dominant role in accounting for their formation. An Andean-type continental arc setting was developed during the Early–Middle Jurassic, with gradual thickening of the continental crust. The significant crustal thickening may reach its ultimate stage at ca. 162–174 Ma, which marks the tectonic transition from compression to extension. The southward subduction beneath the Erguna Block was continuous and stable during the Early Jurassic. Rollback of the subducted slab occurred at ca. 174–177 Ma, followed by moderate magmatic activities represented by adakitic rocks.

Oceanic Zircon as a Petrogenetic Indicator

Abstract —We present results of a study of the morphology, internal structure, and chemical composition of oceanic zircon, which show that zircon is a sensitive indicator of tectonic and physicochemical processes occurring in the lower oceanic crust. Crystallization of magmatic zircon in gabbroids is not an instantaneous process; it proceeds in the course of differentiation of parental melts in the tectonically active mid-ocean ridge (MOR) setting. The main geochemical indicator of crystallization differentiation during magma cooling is an increase in Hf content toward the zircon grain edge. Zoning is also observed in magmatic zircons from oceanic plagiogranites (OPG), but it is weaker, apparently because of the narrower temperature range of zircon crystallization in these rocks. The OPG zircons are depleted in REE as compared with the least altered magmatic zircons of gabbro, which is probably due to the formation of OPG during the partial melting of gabbro with the participation of concentrated water–salt fluid, a derivate of seawater, and due to the co-crystallization of zircon and apatite. High-temperature hydrothermal processes within slow-spreading MORs lead to a partial or complete recrystallization of zircon as a result of dissolution/redeposition. A significantly reduced cerium anomaly and the presence of microinclusions of xenotime, uranium and thorium oxides or silicates, and, sometimes, baddeleyite in zircon alteration zones indicate a reducing type and high alkalinity of the hydrothermal fluid. The fluid, a derivate of seawater, acquires these features during circulation near the axial zone of ridges as a result of phase separation in the system H2O–NaCl and interaction of the fluid with abyssal peridotites of oceanic core complexes. The estimated solubility of zircon in basic melts indicates that even near-solidus crystallization of zircon is highly unlikely in anhydrous basaltic melts but is possible in differentiates of deep-seated hydrous basic magmas. The Ti-in-Zrn geothermometer must be used with caution, because variations in the Ti content in zircon might be controlled not only by temperature but also by other factors, especially when mineral inclusions in zircon testify to a drastic change in its growth (dissolution) conditions. A geothermometer based on the distribution of Zr and Hf between zircon and the host rock has several advantages over indicators of the crystallization temperature of magmatic zircon that are based on the zircon saturation index and the content of Ti in zircon. It does not depend on the composition of melt and on the correct estimates of the SiO2 and TiO2 activity. In addition, reconstruction of the Zr and Hf fractionation trends during crystallization of zircon from granitoid melts makes it possible to evaluate the temperature of separation of more differentiated melt fractions.