Zircon Saturation Research Articles

Granitik Buzul Çakıllarında Zirkon Tipolojisi Çalışması: Toridler’den Örnek Bir Çalışma (Türkiye)

The Late Ordovician glacial rocks from the northern part of the Gondwana are encountered in Taurides and Southeast Anatolia Autochtone Zone. In this study, zircon typology and zircon saturation temperature invetigations are performed on zircon minerals separated from the granitic glacial pebbles from the Feke-Saimbeyli-Ceralan and Feke-Gokmenler (Adana) areas in the east Taurides and Silifke-Ovacik (Mersin) in the central Taurides. Based on the zircon typology method, among the studied samples, DPK-30 is described as calc-alkaline hybrid granitoids bearing mafic enclaves, DPK-44 is identified as granitoids formed from wholly or partly continental crust with rare mafic microgranular xenoliths and low mantle contribution, and DPK-72 is defined as alkaline series granitoids, which are mantle originated rocks observed in subvolcanic and anorogenic complexes pointing out to hot and dry magma source. These descriptions are generally agreeable with the previous geochemical assessments, including isotopic data, on these rocks. Formation temperatures of studied granitic rocks are suggested as 750°C and 789°C for DPK-30, 700°C and 845°C for DPK-44, and 650°C and 790°C for DPK-72, based on zircon typology method and by zircon saturation temperature calculation, respectively. These temperature ranges show similarities to the present temperature data in literature on the Neoproterozoic granitoids from the Sina Peninsula and the Eastern Egypt Desert. As a result, zircon typology method and zircon saturation temperature calculation studies, combined with the other analytical data, can be beneficial in determining the source regions of the granitic glacial pebbles.

Theoretical versus empirical secular change in zircon composition

We generate theoretical curves for zircon growth during cooling of tonalitic and A-type granitic magmas and compare these with empirical Ti-in-zircon populations from the Paleoarchean Pilbara Craton, Australia, Mesoarchean Akia Terrane, Greenland, and the Mesoproterozoic Musgrave Province, Australia. Our models predict variable zircon growth rates on magma cooling dependant on magma composition, crystallizing assemblage, and zircon growth process. In most modelled magma compositions, higher-temperature grains growing close to the zircon saturation temperature are more abundant, with yields decreasing continuously thereafter. However, there are important dissimilarities in the cumulative zircon growth curve for different magma compositions and whether zircon growth is by equilibrium or disequilibrium processes. For a given starting melt Zr concentration, A-type granite magmas grow zircon at higher temperatures than tonalitic magmas. This compositional distinction is most pronounced at lower starting melt Zr concentrations, and in low Zr tonalite the rate of zircon growth may even increase on cooling. The dependence of zircon growth on magma composition and crystallization process leads to predictive differences in cumulative Ti-in-zircon distributions. Greater disequilibrium growth yields more sigmoidal cumulative growth curves that are dissimilar to predictions from phase equilibrium models. When applied to Mesoarchean-aged zircon grains from the Akia Terrane, calculated Ti-in-zircon temperatures decrease over the 3100–2900 Ma interval. This magmatic episode also reveals a change in cumulative zircon growth curve topology from steeper to shallower, consistent with a reduction in the relative proportion of disequilibrium growth, greater crystal–liquid communication, and enhanced infracrustal reworking. The temporal variability in cumulative zircon growth and its implication for melt interconnectedness are powerful tools in understanding magmatic processes and indicate an important secular change point at c. 3.0 Ga in the Akia Terrane where zircon growth dynamics changed.

Jurassic post‐collisional extension in the Songpan–Ganze Terrane, eastern Tibetan Plateau: Evidence from weakly peraluminous A‐type granites within the Zheduo–Gongga Massif

The Mesozoic–Cenozoic Zheduo–Gongga Massif, eastern Songpan–Ganze Terrane, can provide important clues for the crustal evolution in response to syn‐ and post‐orogeny. Here, we document petrological and geochemical characteristics of Early Jurassic granites (181.0 ± 2.2 Ma) within the massif along the Xianshuihe Fault in the eastern Tibetan Plateau. The granites have high contents of SiO2, Na2O + K2O, FeOT/MgO, 10,000 × Ga/Al, REE (except Eu) and HFSE (e.g., Nb, Ta, Zr, Hf), low CaO, TiO2, P2O5 and negative Eu, Sr, and Ba anomalies. Zircon REE shows a similar pattern with the whole‐rock REE. Our new data demonstrate weakly peraluminous A‐type affinity with A/CNK >1.0 and alkali‐calcic, ferroan features. Zircon saturation temperatures and geochemical features (e.g., high SiO2 and low MgO contents, high Rb/Ba and Rb/Sr ratios) indicate that the granites resulted from high‐temperature (>800°C) partial melting of Triassic flysch rocks in relatively shallow crust (<500 MPa or <20 km). Zircon εHf(t) values range from 0.55 to 2.91 and the corresponding two‐stage TDMC are older than the neighbouring Kangding complex and Yangtze margin, implying possible contribution of mantle materials to the parent magma. Considering the tectonic events and the regional geology, the magma was generated in a post‐collisional setting, which was possibly driven by the upwelling of hot mid–lower crustal materials due to the break‐off of subducted Palaeo‐Tethys Ocean slab. Under this situation, the magma may move upwardly into the pre‐existing Xianshuihe Fault during the post‐collisional extension of the thickened crust. Thus, slab break‐off played a key role in the genesis of the Zheduo–Gongga Massif studied herein.

A Novel Approach to Identifying Mantle‐Equilibrated Zircon by Using Trace Element Chemistry

AbstractOne of the requirements for inferring mantle redox states via zircon Ce/Ce* oxybarometry and Ti‐in‐zircon thermometry is to select mantle‐equilibrated zircon derived from melts that have not interacted with the hydrosphere nor assimilated exogenous materials. Traditional protocols for identifying mantle‐equilibrated zircon require a comprehensive examination of whole rock geochemistry in addition to zircon morphology, texture, and trace element and isotopic measurements. This study proposes a simple filter scheme for selecting mantle‐equilibrated zircon using trace element systematics alone (P, Ti, Y, Nb, rare earth elements [REE], Hf, Th, and U) to provide the geological context of zircon from three perspectives: (1) mineral inclusions; (2) source melt petrogenesis; and (3) zircon saturation context. The filter scheme presented here, comprising of 13 criteria, is established based on detailed classifications of 2,173 zircon analyses from 30 independent references and is shown to distinguish between nonmagmatic zircon, magmatic zircon with significant inclusions and/or sourced from highly enriched source melts, and mantle‐equilibrated zircon. The filter scheme is validated with known, well‐characterized mantle‐equilibrated Jack Hills zircon (n = 53) and derives an average mantle redox state of (sd) circa 4,400 Ma, in agreement with previously published results. Further application of the filter scheme to zircon dataset (n = 76) from three metasedimentary rocks successfully reveals the mantle‐equilibrated zircon in the dataset, and implies a mantle redox state of (sd) circa 2,950 Ma. This filter scheme can be applied to studies of out‐of‐context detrital and/or xenocrystic zircon in the future.

Geochemical discrimination of igneous zircon in the Gawler Craton, South Australia

Zircon from spatially and temporally distinct igneous rock units across the Gawler Craton, Australia, show subtle differences in trace and rare earth element ratios. These igneous suites range in composition from granite, rhyolite to gabbro and originate from the ca 2450 Ma Sleaford Complex, the ca 1850 Ma Donington Suite, the ca 1633–1608 Ma St Peter Suite, the ca 1595–1587 Ma Gawler Range Volcanics and the ca 1595–1575 Ma Hiltaba Suite granites. The geochemical characterisation of zircon is carried out on relatively unaltered samples in order to establish primary signatures and to understand the processes controlling the geochemical signatures of the igneous suites. A range of zircon morphologies and internal textures that are preserved in these igneous suites infer the geological history of the host rock, including metamorphism and multiple episodes of zircon saturation suggesting a complex magma evolution. Despite differences in whole-rock geochemistry between igneous suites, zircon largely overlaps geochemically. However, subtle differences in Th/U, U/Hf and (Nb + Ta)/P ratios are evident between the Donington Suite, Gawler Range Volcanics and Hiltaba Suite. This is likely due to the tectonic setting, composition of the crystallising magma and timing of zircon crystallisation in the magma evolution. The subtle geochemical differences in zircon show the potential of using these ratios to aid in provenance studies with large populations of zircon that are of similar age. However, as the dataset is limited and focused only on unaltered samples, further work needs to be undertaken to look at the altered counterparts of these igneous suites. KEY POINTS Zircon recovered from unaltered samples of spatially and temporally distinct igneous suites in the Gawler Craton is used here. Th/U, U/Hf and (Nb + Ta)/P ratios of zircon can discriminate the Donington Suite, Gawler Range Volcanics and Hiltaba Suite. Zircon morphologies, internal textures and geochemical characteristics are a value aid to provenance studies.

Geochronology and petrogenesis of the composite Zuluhong Granite, North Xinjiang Province of China: Implications for the crust-mantle interaction and continental crustal growth in Western Tianshan Orogen

The mineralogy, whole rock geochemistry, zircon U-Pb-Hf isotopic compositions, and Sr–Nd isotopes were analysed from the composite Zuluhong Granite at the Alataw Mountains in NW China. The aim is to determine the granite's nature and tectonic setting, and gain a better understanding of the continental growth of the Western Tianshan Orogen (WTO), which is commonly regarded as part of the Central Asian Orogen (CAO). The Zuluhong Granite is divided into a monzogranite and porphyritic granodiorite. The monzogranite yields a zircon U–Pb date of 320 ± 2 Ma interpreted as its emplacement age, and the porphyritic granodiorite yields a zircon U–Pb date of 309 ± 1 Ma interpreted as its emplacement age. The monzogranite is characterised by its high SiO2 content (72.7–74.4 wt%), moderate 10,000 × Ga/Al values(2.59–2.81), strongly negative Eu anomaly (0.39–0.43), and relatively low zircon saturation temperatures (Tzr) of 726–807 °C, which are indicative of a highly fractionated monzogranite. The combined arc-like geochemical signatures (e.g. enrichment in LILE and LREE relative to HFSE, negative anomalies in Nb and Ta), and relatively high positive εNd(t) values of +4.4 to +4.8, indicate that the monzogranite originated from the partial melting of arc rocks in the lower crust. In contrast, the granodiorite has higher Al2O3 assays averaging 16 wt% and Sr/Y ratios averaging 40, and lower Yb assays of 1.38–1.49 ppm with a weak Eu anomaly, which are characteristics of adakite-like rocks. The combination of the granodiorite's relatively lower SiO2 content of 63.7–65.3 wt%, higher Mg# values of 53–57, higher εHf(t) values of +10.9 to +13.4, and complex oscillatory zoned plagioclase phenocrysts are indicative of a source derived from material derived from the mantle and lower crust. Our data on the Zuluhong Granite, coupled with the results of previous studies of the Late Carboniferous to Early Permian magmatism in the WTO, indicate that a regional tectonic transition from compression to extension took place in the CAO during the end of the Early Carboniferous (ca. 320 Ma). Furthermore, substantial post-collisional reworking of the Precambrian rocks along the northeastern margin of the Yili Terrane is more pronounced than the West Junggar Terrane to the east. The generation of widespread magmatism and continental growth in the CAO in a post-collisional tectonic setting including the recycling of earlier juvenile-arc complexes, and underplating of mantle-derived magma.

The juxtaposition of Cambrian and early Ordovician magmatism in the Tafí del Valle area. Characteristics and recognition of Pampean and Famatinian magmatic suites in the easternmost Sierras Pampeanas

Along more than 1000 km, the boundary between the Pampean and Famatinian belts separates the Cambrian magmatism to the east (540-515 Ma) from the easternmost granitic rocks ascribed to the Famatinian orogeny to the west (early Ordovician). The geochronological and geochemical study presented in this work has revealed the first Cambrian magmatism in the Sierras de Aconquija and Cumbres Calchaquíes at the northeastern edge of the Famatinian belt. Pabellón (522 ± 2.8 Ma) and Ñuñorco Grande (478.3 ± 2.3 Ma) intrusive units form part of the small plutons exposed together in the Tafí del Valle area, which have been related according to their geochemical characteristics. The good correlation observed between ages and characteristic geochemical signatures have motivated the comparison with other intrusive complexes along the Pampean and eastern Famatinian belts, resulting in regional diagnostic geochemical features and a petrogenetic and evolutive proposal for the Cambrian and early Ordovician magmatism in the eastern Sierras Pampeanas. Easternmost early Ordovician granitic rocks constitute a calcic, weak to moderately peraluminous series. They are Sr-rich tonalitic to monzogranitic magmas (SiO2 >62 %wt.) with steep REE patterns, positive and negative Eu anomalies, high Sr/Y and moderate Ce/Yb ratios. Accordingly, the water-fluxed melting of a mafic source under high to moderate pressures may be involved in the origin of silica- and Ca-rich parental magma. The shallower evolution of these melts consists of Pl-dominated fractionation trends that finally constituted Pl-rich crystal mushes (tonalites) and more felsic residual liquids. Regarding Cambrian granodiorites and monzogranites, any petrogenetic scheme must consider the magnesian, high-K calc-alkalic characteristics for a short-range silica variation, low to moderate peraluminosity and high zircon saturation temperatures (>850 °C). Source conditions are determined by the presence of Pl and Hbl in the solid assemblage based on the REE patterns. These geochemical characteristics point to the extensive HT-LP melting of a dominantly crustal source, probably assisted by the injection of hot H2O-rich mafic melts.

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.

Assessment of magmatic fertility using pXRF on altered rocks from the Ordovician Macquarie Arc, New South Wales

This research presents a new method for assessing magmatic fertility using the portable X-ray fluorescence spectrometer (pXRF) in altered rocks from porphyry terranes. A study of Northparkes and global Cu--Au porphyries highlighted low Zr and Y trends associated with mineralising intrusions. Zircon saturation occurs at low Zr concentrations owing to low-temperature crystallisation depressed by high dissolved H2O in the melt. Yttrium depletion indicates hydrous magmatic conditions that cause hornblende to crystallise early. Previously geochemical differentiation of hydrous, potentially ore-forming (fertile) porphyries has relied on ratios of Sr/Y and Sr/MnO from geochemical and/or pXRF analysis of least altered rocks. Strontium is highly mobile and limits its use in altered terranes. Finding samples in porphyry terranes where Sr can be demonstrated to be retained is difficult given the ubiquity of alteration associated with porphyry emplacement. This precludes the widespread use of pXRF, as quantitative assessment of alteration is beyond the capabilities of pXRF technology. Using immobile elements overcomes issues with alteration and provides more reliable indicators of magmatic fertility. Using a systematic pXRF workflow, plots of Zr vs Y and Zr*Y vs Al2O3/TiO2 were successful in identifying 100% of the mineralising intrusions in the Northparkes intrusive complex and provide an indication of magmatic fertility and a proxy for melt hydration state. The ratios of Zr/Nb vs Y/Nb are insensitive to alteration including weathering and are indicative of evolution and dissolved H2O content of magmatic sources. Moving from bivariate plots to ratio plots shifts the data into a real-number space and provides a linear function to assess against magmatic fertility. Application of the fertility indicators to a regional dataset from New South Wales highlights areas of known porphyry mineralisation indicating that the method has the potential to be a useful indicator of fertility beyond the scope of the pilot study. KEY POINTS The fertility of porphyry intrusions is directly related to their dissolved H2O content and oxidation state. High dissolved H2O content in magmas causes early crystallisation of hornblende, and temperature depression. As a result Zr saturation occurs at low Zr concentrations that results in depletion of Y and the MREE in hydrous rocks compared with normal arc rocks. Magmatic fertility can be rapidly and cheaply assessed in the field using Zr and Y concentrations from pXRF analyses. The use of immobile elements facilitates magmatic fertility assessment in altered rock, which was previously impossible owing to the use of mobile elements Sr and MnO.

Petrogenesis of the Granites in the Yandangshan Area, Southeastern China: Constraints from SHRIMP U-Pb Zircon Age and Trace Elements, and Sr-Nd-Hf Isotopic Data

New geochemical and geochronological data of two types of granites, which are located in Yandangshan area, southeastern Zhejiang Province, were presented to constrain their magma condition, origin and the genetic relationship between them. The SHRIMP zircon U-Pb dating of Dongshan and He-sheng granite in Yandangshan area shows that they were formed at 114±1 and 103±2 Ma, respectively. Samples from the Dongshan granite have high SiO2 (76.4 wt.%–76.9 wt.%) and total alkaline (K2O+Na2O=8.35 wt.%–8.47 wt.%) contents, but low FeOT (0.89 wt.%–1.15 wt.%), MgO (0.21 wt.%–0.22 wt.%), and CaO (0.24 wt.%–0.34 wt.%) contents and high A/CNK (∼1.1) values, belonging to the peraluminous and magnesian granite. The Hesheng granite has high SiO2 (72.2 wt.%–77.5 wt.%), total alkaline (K2O+Na2O=8.05 wt.%–9.41 wt.%) and FeOT contents (1.20–2.06), and high A/CNK values (1.0–1.1), but low in MgO (0.12 wt.%–0.29 wt.%) and CaO (0.24 wt.%–0.34 wt.%) contents. Samples from the Hesheng granite also have high FeOT/MgO (6.9–10.0) and 10 000×Ga/Al (2.6–3.4) ratios similar to the ferroan/A-type granite. All the samples are enriched in LREE but have produced negative Eu anomalies (Eu/Eu*Dongshan=0.45–0.47; Eu/Eu*Hesheng=0.17–0.55), Ba, Nb, and Ta, while the REE contents of the ferroan/A-type granite (Hesheng) are higher than that of the magnesian granite (Dongshan). The (87Sr/86Sr)i value of the magnesian granite is slightly higher than that of the ferroan/A-type granite and its eNd(t) value (−6.8) is lower than the latter (−6.0–−5.9). In addition, the eHf(t) value (−11.8–−4.2) of magnesian granite is also lower than that of the ferroan/A-type granite (−8.3–−2.0), indicating that there may be more mantle-derived components in the source area of the ferroan/A-type granite. Zircon saturation thermometer (TZr) and Ti-in-zircon thermometer (TZircon) are used to estimate the temperature of the magma source, and the results show that the magma temperature of the magnesian granite (average TZr=798 °C; average TZircon=792 °C) is lower than that of the ferroan/A-type granite (average TZr=862 °C; average TZircon=859 °C). And the oxygen fugacity of magnesian granite (ΔFMQ=1.16–3.47) are also higher than those of the ferroan/A-type granite (ΔFMQ= −0.41–1.14). Our new data indicate that both granites in this study are derived from a mixed source that consists of mantle-derived and crust-derived material. Based on the previous studies, both of the granitic plutons were formed under extension setting, and the granites transformed from magnesian to ferroan in the study area may indicate the extension was enhanced, which may be caused by the roll-back or delamination of the Paleo-Pacific oceanic slab.

Inherited or not inherited: Complexities in dating the atypical ‘cold’ Chopok granite (Nízke Tatry Mountains, Slovakia)

Zircon U-Pb SIMS dating combined with in-context (in thin section) monazite and xenotime U + Th-total Pb dating was used to clarify the Palaeozoic evolution of the ‘cold’ Chopok granite (Nízke Tatry Mountains, Slovakia). Four distinct zircon, monazite and xenotime age domains testify to a prolonged evolution from igneous formation to multi-stage metasomatism and hydrothermal overprinting. The geological interpretation of age patterns from ‘cold’ granites, expected to have low zircon saturation temperatures (<800 °C) and relatively high amounts of zircon inheritance, requires special care, especially for what concerns proper attribution of zircon inheritance and igneous growth ages. These issues can be resolved using zircon saturation temperatures (TZrn) as proxy for the amount of zircon inheritance in combination with the temperature differences between TZrn and the granite solidus. In this respect, the Chopok granite is an atypical ‘cold’ granite. Due to TZrn being substantially lower (ca. 80 °C ± 50 °C) than the granite solidus temperature, practically no zircon inheritance was found. The zircon age data indicates that the Chopok granite is a product of an Early Ordovician (475.8 ± 3.3 Ma) magmatic event, corresponding with the widespread Early Palaeozoic magmatism recorded throughout the European Variscan belt. This is further corroborated by phosphate mineral ages. The post-magmatic activity recorded in the U-Pb systematics of zircon and phosphates overgrowths can be related to the different phases of the evolution of the Variscan orogen: Early Carboniferous (ca. 352 Ma) metasomatism documents the main Variscan orogenic event, whereas the Permo-Triassic age (ca. 255 Ma) reflects thermo-tectonic activity associated with large-scale crustal extension, contemporaneous with the initial continental leading to the break-up of Pangea.

Thermotectonic evolution of the Paleozoic granites along the Shangdan suture zone (central China): Crustal growth and differentiation by magma underplating in an orogenic belt

Abstract The nature of source rocks and the pressure-temperature-hydration (P-T-H2O) condition are the two main factors that control the geochemical properties of granites. Therefore, the evolution of P-T-H2O conditions can be used to deduce the tectonic setting of granites. In this paper, we report on three Paleozoic granite plutons along the Shangdan suture that revealed increasing melting temperature and decreasing pressure from 437 to 403 Ma, suggesting a crustal thinning process. The Tieyupu granodiorites (437 ± 4 Ma) display Na-rich adakite affinity, i.e., SiO2 = 69.1–70.1 wt%, Na2O/K2O = 1.9–2.26, positive zircon εHf(t) values (+4.29 to +12.04), and high Sr/Y (137–160) and Y/Yb (9.89–10.25) ratios, implying a garnet-rich residue in their source. In combination with moderate zircon saturation temperatures (814–822 °C), we infer that the Tieyupu granodiorites were formed by melting of Neoproterozoic metabasites under high-pressure (&amp;gt;1.5 GPa) and moderate-temperature (HP-MT) conditions. The Liangchahe granodiorites (415 ± 8 Ma) also display Na-rich adakite affinity, i.e., higher Na2O/K2O (2.16–3.11) and lower Sr/Y (77–88) ratios, and higher zircon saturation temperatures (854–874 °C), and they are interpreted to have been derived from melting of metabasites under moderate-pressure (&amp;gt;1.0 GPa) and high-temperature (MP-HT) conditions. Their variable zircon εHf(t) values (−14.97 to +9.80) and the existence of zircon xenocrysts suggest that the primitive adakitic melts were assimilated by evolved crustal components. The Yaogou monzogranites (403 ± 4 Ma) have the highest K2O/Na2O (0.81–1.00) ratios and total rare earth element (ΣREE; 105–191 ppm) contents, lowest Sr/Y (14–43) ratios, positive zircon εHf(t) values (+6.79 to +12.22), and highest zircon saturation temperatures (891–973 °C), showing they were formed by high-temperature melting of intermediate rocks under low-pressure conditions (&amp;lt;1.0 GPa). The evolution of P-T conditions revealed by these three granites suggests that crustal growth and differentiation were related to gradual extensional and melting of mafic protoliths in the orogenic belt.

The physico-chemical conditions of crystallization of the Grenvillian arfvedsonite granite of Dimra Pahar, Hazaribagh, India: constraints on possible source regions

The Dimra Pahar Pluton is composed of arfvedsonite ± aegirine - alkali-feldspar granites with homophanous and mylonitic features. The pluton was emplaced along a regional shear zone during the post-collisional stage of the orogeny. Arfvedsonite and aegirine crystallized after alkali feldspar and quartz. The rocks are slightly peraluminous (alkali-index – AI: 0.92) to peralkaline (AI: 1.11). The pluton was emplaced at shallow depth at around 2–3 kbar pressure. The zircon saturation temperature varies between 889 and 1071 °C which is consistent with the liquidus temperature of the granitic melt. However, somewhat lower temperatures of crystallization of zircons (822–836 °C) were obtained by Ti-in-zircon thermometer. Two feldspar thermometry yields crystallization temperature ranging from 647 to 705 °C. F-free arfvedsonite crystallized at around 670 °C. Aegirine also crystallized around 670 °C. Arfvedsonite and aegirine crystallization indicates near solidus temperature of the magma. Zr-poor nature of both arfvedsonite and aegirine and presence of discrete zircon crystals suggest high oxygen fugacity of the magma. During the crystallization of zircon (near liquidus condition of the magma), the oxygen fugacity fluctuated around QFM buffer (ΔQFM = −1.22 to +1.29) as indicated by the Ce-anomalies in zircon. Later on, during the crystallization of arfvedsonite (near solidus condition of the magma), the granitic melt achieved oxygen fugacity condition above QFM buffer as indicated by the high Fe3+/(Fe2++Fe3+) ratios (0.2–0.23) of arfvedsonites. High Si/(Na + Fe + Si) ratios (0.52–0.57) of these arfvedsonites also suggest that oxygen fugacity reached above the NNO buffer during the later stage of crystallization. Pseudosection analysis of the studied rocks suggests that the mineral assemblage was equilibrated within the temperature range from 505 to 515 °C and oxygen fugacity range from −19 to −23 log units (bar), i.e. above NNO-buffer. The nearly Ti-free aegirines crystallized in highly oxidized condition near HM buffer. The development of coronal aegirine around arfvedsonite indicates the crystallization following an oxidizing path, towards the HM buffer. Water content of the granitic melt during crystallization of quartz and feldspar is estimated to be around 6 wt%. During the crystallization of arfvedsonite water content of the melt came down to 0.02 wt%. The high oxidation state and water saturated parental melts of the Dimra Pahar granite require a highly oxidized, hydrated source for the melts. A subduction-modified, metasomatically enriched lithospheric mantle or asthenospheric mantle can be considered as the source for the basaltic parental melt of Dimra Pahar granite.

Formation and stabilization of the Dunhuang Block, NW China: Constraints from the Late Paleoproterozoic A-type granites of the Dunhuang Complex

The geochemical composition and temporal evolution of A-type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust. Moreover, these aspects may provide an indication of when orogenic processes ended and craton formation/stabilization began in Early Precambrian blocks. In this study, we report the results of zircon U–Pb ages, Hf isotope data and whole-rock geochemistry for the Proterozoic Huoyanshan potassic granites in the Dunhuang Block, northwest China. Zircon U–Pb dating reveals that the Huoyanshan granites were emplaced at ca. 1791–1786 Ma, representing a Late Paleoproterozoic magmatism event. The granites have high SiO2 (70.71–72.20 wt%), total alkali contents (K2O + Na2O = 8.81–8.96 wt%; K2O/Na2O = 1.57–1.66), TiO2/MgO ratios (2.19–2.57), low CaO (0.77–1.27 wt%), MgO contents (0.12–0.16 wt%), and Mg# values (7.72–8.67), with A/CNK ratios of 0.96–1.04, pointing to alkaline and metaluminous to weakly peraluminous affinities. These granites are characterized by high total rare-earth element concentrations (∑REE = 419–755 ppm) and variable (La/Yb)N values (8–24) with significantly negative Eu anomalies (Eu/Eu* = 0.31–0.42). The primitive mantle-normalized trace element patterns suggest that the Huoyanshan plutons are depleted in Ba, Nb, Ta, Sr, P, and Ti, but enriched in Rb, Th, and Pb. Additionally, the granites have a high zirconium saturation temperature of 855–897 °C, with an average of 878 °C. These geochemical characteristics indicate that Huoyanshan potassic granites are an aluminous A-type granite, and further classified as A2-type. Together with the zircon Hf isotope data (εHf(t) = −11.21 to −2.76), indicates that these granites were generated from a crustal source. The heterogeneous Hf isotope compositions in zircons are principally caused by the disequilibrium melting of Zr-bearing sources. In addition, two-stage Hf model ages of zircons from the granites are ca. 2.6–3.1 Ga and plot on the average crustal evolution trend of Archean tonalite–trondhjemite–granodiorite (TTG) gneiss (~2.5 to 3.0 Ga) in the Dunhuang Block. Thus, we propose that A-type granites in the study area were generated from the disequilibrium partial melting of Archean tonalitic to granodioritic rocks under high-temperature and low-pressure conditions according to their low Al2O3 + CaO + Na2O + K2O (molar) values and high 10000 × Ga/Al values (>2.9). Subsequently, the magma experienced prominent fractional crystallization of K-feldspar, plagioclase, orthopyroxene, and/or magnetite. Our results, withavailabledata, suggest that the Huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting, and serve as a precursor for the formation and stabilization of the Dunhuang Block during the Late Paleoproterozoic. Furthermore, it is probable that the post-collisional magmatism event at ca. 1.79 Ga in the Dunhuang Block was closely related to the assembly of the Columbia supercontinent.

Modulation of zircon solubility by crystal–melt dynamics

Abstract Zircon dating is commonly used to quantify timescales of magmatic processes, but our appreciation of the consequences of internal magma body dynamics lags behind ever-increasing analytical capabilities. In particular, it has been shown that crystal accumulation and melting of cumulates by recharge-delivered heat may affect melt chemistry within magma bodies. We considered the effect of such processes on zircon solubility in highly evolved silicate melts of diverse chemical affinities. Our modeling shows that in most cases cumulate melting perpetuates the zircon saturation behavior of the first melts emplaced at shallow storage levels. Once cumulate melting is established, the ease of saturating in zircon is controlled by cumulate mineralogy, with a particular effect of the amount of cumulate zircon and its availability for resorption. The fidelity of zircon as a recorder of magma system history thus depends on both the system’s chemical affinity and mineralogy, and the history itself.

First Record of Circa 970 Ma Post-Collisional A-Type Magmatism in the Sendra Granitoid Suite, Central Aravalli Orogen, Northwest India

This study provides the first record for the emplacement of post-collisional A-type granites in extensional regime during the late Grenvillian period in northwest India. The ca. 970 Ma granites of the Sendra Granitoid Suite (Chang pluton) intrude calc-silicate rocks of the South Delhi Supergroup in the central Aravalli orogen. The Chang pluton is composed of granite sensu stricto ; the granites are metaluminous, ferroan, calc-alkalic, and are characterized by high Ga/Al (>2.5), Nb + Y (>60 ppm), Ta + Yb (>6 ppm), REE, HFSE and zircon saturation temperatures, typical of A-type granites. The Y/Nb >1.2 further classified the rocks as A2-subtype, signifying their derivation from crustal sources in a post-collisional setting. The crustal source is also supported by their high LILE (Rb, K and Ba), and Pb, Th and REE. The geochronological data and tectonics of the region indicate that the granites were emplaced about 30 Myr after the Grenvillian collisional orogeny. This scenario likely resulted due to delamination of the lower part of the thickened orogenic lithosphere. These results are expected to have significant implications for the assembly tectonics of the Rodinia supercontinent.

Architecture of a Super-sized Magma Chamber and Remobilization of its Basal Cumulate (Peach Spring Tuff, USA)

Abstract Using a combination of petrological and geochemical approaches, we investigate processes prior to and during eruption of the Miocene supereruption of the Peach Spring Tuff (PST; Arizona–California–Nevada), including those leading to assembly and destruction of its reservoir(s). We compare the dominant high-silica rhyolite outflow of the PST with the sparsely exposed but distinctive crystal-rich trachyte capping unit, which matches intracaldera trachyte in composition, texture, and phenocryst content. The details of the diverse glass chemistry in fiamme and pumice in the capping unit, coupled with glass compositions in the rhyolite outflow and phase chemistry in general, illuminate critical aspects of chamber geometry, conditions, and processes at the onset of the supereruption. Our results are consistent with a relatively simple single-chamber reservoir for the PST where the crystal-poor, high-silica rhyolite portion directly overlies a mushy, cumulate base. Rhyolite-MELTS phase-equilibria and amphibole geobarometers indicate that the high-silica rhyolite was extracted from its cumulate mush at a depth of ∼9·5–11 km (∼260–300 MPa) and subsequently stored and crystallized at ∼7·0–8·5 km (190–230 MPa). Three types of glass are distinguishable in PST pumice: trachyte (Trg; ∼68 wt% SiO2), low-silica rhyolite (LSRg; ∼72), and high-silica rhyolite (HSRg; ∼76·5). As many as three discrete, complexly mingled glasses are present in single trachyte fiamme. Trace element concentration profiles in sanidine and plagioclase phenocrysts from both the trachyte and HSR support growth from multiple distinct melts (Trg, LSRg, and HSRg). Glasses in trachyte fiamme have zircon saturation temperatures ≥100 °C higher than HSR glasses (850–920 vs ∼770 °C) and compositions indicating dissolution of cumulate phases: very high Zr and Zr/Hf (zircon), REE (chevkinite and titanite), Ba and Sr (feldspars), and P (apatite). Dominant processes of crystal accumulation in the formation of a mushy base, followed by efficient melt extraction, led to the formation of the voluminous high-silica rhyolite melt-rich body overlying a residual cumulate of trachytic composition. This was followed by heating, partial dissolution, and remobilization of the basal cumulate. This history is reflected in the contrasts that are evident in the PST (elemental compositions of pumice, phenocrysts, and glasses; crystal-fraction; temperatures). Reheating was presumably a result of injection of hot mafic magma, but isotopic uniformity of trachyte and rhyolite indicates minimal chemical interaction with this magma. Variability in dissolution textures in phenocrysts in the trachyte, revealed by resorbed and embayed shapes, and the large range of glass trace element concentrations, together with variable temperatures recorded in glasses by zircon and apatite saturation thermometry, suggest that heat transfer from the hotter rejuvenating magma was unevenly distributed. The late-stage heating event probably contributed to the onset of eruption, providing the thermal energy necessary to reduce the crystal fraction within the cumulate below the mechanical lock point. We estimate ∼50 % of the original cumulate phenocrysts dissolved before eruption, using Rhyolite-MELTS and trace element modeling. Sharp contacts with micron-scale compositional gradients between contrasting glass types in individual trachyte fiamme suggest that juxtaposition of contrasting magmas from different parts of the reservoir occurred during eruption.

Polyphase Zircon Growth during Slow Cooling from Ultrahigh Temperature: an Example from the Archean Pikwitonei Granulite Domain

AbstractQuantifying the timescales of Archean ultrahigh temperature (UHT) metamorphism is essential for constraining the style of plate tectonics on the early Earth. However, such timescales can be difficult to quantify, due to the antiquity of Archean rocks and the extreme thermal conditions of UHT metamorphism. We constrain the timescales of Archean UHT metamorphic processes recorded by a single rock sample from the Pikwitonei granulite domain (northwestern Superior Province), through the integration of two U–Pb zircon petrochronologic techniques. In this study we combine: (1) high-spatial resolution laser ablation split-stream inductively coupled mass spectrometry (LASS) on in situ zircon (in thin section) and hand-picked zircon; and (2) high-precision isotope dilution thermal ionization mass spectrometry (ID-TIMS) analyses on microsampled fragments from the same hand-picked zircon analysed by LASS. Phase equilibria modelling and Zr-in-rutile thermometry suggest the rock followed a P–T path characterized by decompression at &amp;gt; 960 °C, followed by near-isobaric cooling at ∼0·8 GPa. In situ LASS zircon analyses could be interpreted to record zircon growth at broadly ∼2665 Ma, though the large uncertainties on isotopic dates make potentially distinct growth episodes difficult to distinguish. ID-TIMS U–Pb dates of zircon fragments reveal a polyphase zircon growth history over a 24 Ma duration, from 2673 to 2649 Ma. Zircon trace element compositions, textures, and microstructural relationships, as well as evaluation of zircon-garnet equilibrium, suggest zircon grew during melt crystallization, after UHT decompression and garnet resorption. Variable Ti concentrations within zircon domains indicate: (1) zircon crystallized through the temperature interval of ∼875 °C to ∼730 °C, potentially in isolated rock domains with variable zircon saturation temperature; and/or (2) zircon crystallized over a narrower temperature interval in isolated rock domains with variable aTiO2 and/or aSiO2. Collectively, the data suggest the west-central Pikwitonei granulite domain reached peak UHT conditions prior to 2673 Ma, after which suprasolidus conditions in the lower crust persisted for at least 24 Ma. Such an interpretation would be impossible if based on either the LASS or ID-TIMS zircon data alone, which highlights the utility of applying both techniques in tandem to constrain metamorphic timescales in ancient UHT terranes.

Crust–mantle interaction during subduction zone processes: Insight from late Mesozoic I-type granites in eastern Guangdong, SE China

Zircon U–Pb dating and in-situ Hf–O isotopic compositions, combined with whole-rock geochemical and Sr–Nd isotope data, are systematically presented to constrain the timing and petrogenesis of the late Mesozoic granitoids in eastern Guangdong Province, SE China. Zircon U–Pb chronology shows that two intrusions were emplaced from the Middle Jurassic to Early Cretaceous (166–144 Ma). All the granitoids are classified as calc-alkaline I-type granites with highly diverse elemental and isotopic compositions. The systematic variations in zircon Hf–O isotopes show that the granites can be categorized into three groups. The Donggang granitic porphyries (166 Ma) have the highest zircon δ18O values of 6.3–8.2‰ (average of 7.7‰) but the lowest εHf(t) values of −8.0 to −1.4, with the oldest Hf model ages of 1.30–1.70 Ga. The Liuzhoushan monzogranites (144 Ma) have intermediate zircon δ18O values of 6.0–6.9‰ (average of 6.4‰) and εHf(t) values of −5.0 to 0.3, with Hf model ages of 1.17–1.51 Ga. The late Early Cretaceous granitoids have the lowest δ18O values of 4.9–6.6‰ (average of 5.5‰) but the highest εHf(t) values of −3.2 to 3.3, with the youngest Hf model ages of 0.95–1.36 Ga. Zircon saturation temperatures of the Donggang, Liuzhoushan, and Xinwei (Sanrao) intrusions are calculated to be 642 °C, 722 °C, and 748 °C, respectively. Together with the variations in Sr–Nd isotopes, it appears that these granitoids were derived from the mixing of different proportions of mantle-derived components with Paleoproterozoic basement crust-derived magmas. The underplating of mantle-derived magmas into the continental crust, which supplied both heat and material contribution, was likely responsible for the generation of the most late Mesozoic I-type granites in eastern Guangdong. From Middle Jurassic to late Early Cretaceous, the increasing εHf(t) and εNd(t) values as well as the decreasing δ18O values and TDM2 ages indicate a progressive role of the crust–mantle interaction, corresponding to a gradually enhanced extensional tectonic environment. The northeast (NE)-ward younging trend and the enhanced crust–mantle interaction along coastal SE China were probably induced by the changing direction of the subducted plate from oblique to parallel to the continental margin.

Coexisting A1 and A2 granites of Kudaru Complex: implications for genetic and tectonic diversity of A-type granite in the Younger Granite province, north-central Nigeria

The Nigerian alkaline ring complexes have long been regarded as typical anorogenic A-type granite. However, uncertainty still surrounds many aspects of their petrogenesis and tectonic implication. In this paper, a combined study of whole rock geochemistry, Nd-Pb isotope and zircon U-Pb dating was carried out on the Kudaru ring complex from north-central Nigeria. The complex consists of mildly alkaline fayalite granite porphyry, peralkaline arfvedsonite granite and metaluminous biotite granite, which were emplaced between 176.9 ± 2.5 and 180.55 ± 0.6 Ma. These rocks are characterized by variably high alkalis, HFSEs, Ga/Al ratios, and zircon saturation temperature, typically of A-type granites. Notably, the popular subdivision scheme of A-type granite permits the discrimination of the alkaline-peralkaline granites as typical “A1-type” granites and the metaluminous granite as “A2-type”. These co-existing A1 and A2 granites show distinct isotopic compositions with less negative eNd (t) values and higher 206Pb/204Pb ratios for the former, indicating that the magma which gave rise to the A1 and A2 suites was derived from an enriched mantle source but modified to different extent during ascent. Notably, the coexistence of A1-A2 granites do not indicate discrete tectonic regimes (intraplate setting for A1 and post-orogenic setting for A2), but rather point to a significant role of crustal contamination, as seen also from the differences in Y/Nb ratios.