Geochronological and geochemical evidence for multi-stage apatite in the Bafq iron metallogenic belt (Central Iran), with implications for the Chadormalu iron-apatite deposit
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Detrital zircon grains preserved within clasts and the matrix of a basal diamictite sequence directly overlying the Carrapateena IOCG deposit in the Gawler Craton, South Australia are shown here to preserve U–Pb ages and geochemical signatures that can be related to underlying mineralisation. The zircon geochemical signature is characterised by elevated heavy rare-earth element fractionation values (GdN/YbN ≥ 0.15) and high Eu ratios (Eu/Eu* ≥ 0.6). This geochemical signature has previously been recognised within zircon derived from within the Carrapateena orebody and can be used to distinguish zircon associated with IOCG mineralisation from background zircon preserved within stratigraphically equivalent regionally unaltered and altered samples. The results demonstrate that zircon chemistry is preserved through processes of weathering, erosion, transport, and incorporation into cover sequence materials and, therefore, may be dispersed within the cover sequence, effectively increasing the geochemical footprint of the IOCG mineralisation. The zircon geochemical criteria have potential to be applied to whole-rock geochemical data for the cover sequence diamictite in the Carrapateena area; however, this requires understanding of the presence of minerals that may influence the HREE fractionation (GdN/YbN) and/or Eu/Eu* results (e.g., xenotime, feldspar).
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Abstract The Epembe Complex is one of the Mesoproterozoic (~1200 Ma) carbonatite alkaline complexes situated along the southern margin of the Congo Craton in northwestern Namibia. Nepheline syenites and minor syenites constitute the main lithologies, cross-cut by a calcite-carbonatite dyke. In order to constrain zircon forming-processes and magma sources, cathodoluminescence (CL) imaging combined with trace elements (including REE) as well as Hf isotope compositions of zircon grains extracted from one syenite, five nepheline syenite samples and one carbonatite sample are presented. Syenite zircons are generally unaltered and are characterised by positively sloping REE patterns in a chondrite-normalised diagram, with positive Ce anomalies. Syenite zircon further displays significant negative Eu anomalies attributed to earlier plagioclase formation and fractionation. These features are consistent with zircon formation in a magmatic environment. In the nepheline syenite samples, two zircon types are recognised. Type 1 zircon is magmatic, with homogeneous-grey, unzoned and oscillatory-zoned domains in CL, while type 2 zircon underwent low temperature fluid alteration and displays a cloudy appearance. Type 2 zircon is characterised by enrichment in LREE, Nb and Ti when compared to magmatic type 1 zircon. Carbonatite zircon displays a variety of textures and variable chemical compositions suggestive of the presence of both xenocrystal, altered and magmatic zircon. The Hf concentration and Hf isotope composition of type 1 and type 2 zircon are similar suggesting that zircon alteration did not affect the Hf isotope systematics. The similarity of ƐHf(t) values in zircon from syenite (+0.5 ± 0.4 to +1.5 ± 0.4), nepheline syenite (+1.6 ± 0.3 to +2.7 ± 0.5) and carbonatite (+1.5 ± 0.2 to +1.9 ± 0.1) is consistent with the melts having been derived from a moderately Depleted Mantle.
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Granitoids play a key role in the geological structure of the Ros-Tikych megablock. Supercrustal rocks of the Ros-Tikych series have been preserved in the granitoids only in the form of isolated fragments such as elongated remains, small skialites and even smaller "melted" xenoliths. In particular, in the Ostrivsky quarry, located on the right bank of the Ros River east of Bila Tserkva, granitoids are found (even-grained, porphyry-like granites) among which, as a rule, small bodies of granodiorites, plagiogranites and amphibolites occur. In order to determine the source of the parent magmas of rocks the properties of zircon crystals and the isotopic composition (87Sr/86Sr ratio) of apatite were studied. An analysis of the zircon crystals of the crystalline rocks exposed at the Ostrivsky quarry allows us to propose that the and plagio- and difeldspar granites were formed from one protolith. This is because they contain similar virtually identical zircon relics as nucleus. In addition, none of the granitoids contain zircon crystals whose internal structure is similar to zircon crystals found in amphibolite. This suggests that the granitoids were not derived by melting of amphibolites. Most likely, amphibolites are relicts of the protolith that were not assimilated during granite formation. The occurrence of heterogeneous zircon crystals (relic zircon cores of the protolith) in the protolith of the various studied granitoids indicates that they formed from volcanic-sedimentary rocks. Apatites in plagiogranitoids and porphyry granite contain strontium of similar isotopic composition. Their 87Sr/86Sr isotopic ratio is 0.70680 in apatite granodiorite and 0.70822 in granite. A high ratio of 87Sr/86Sr = 0.77940 was measured for apatite from monazite-bearing granite, thus indicating a different source for its parent magma.
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The petrogenesis of the Pridoli to Early Lochkovian granites in the Miramichi Highlands of New Brunswick, Canada, is controversial. This study focuses on the Pridoli Nashwaak Granite (biotite granite and two-mica granite). In situ trace elements and O and Hf isotopes in zircon, coupled with O isotopes in quartz, are used to reveal its magmatic sources and evolution processes. In the biotite granite, inherited zircon cores have broadly homogenous δ18OZrc ranging from +6.7‰ to 7.4‰, whereas magmatic zircon rims have δ18OZrc of +6.3‰ to 7.2‰ and εHf(t) of −0.39 to −5.10. The Hf and Yb/Gd increase with decreasing Th/U. Quartz is isotopically equilibrated with magmatic zircon rims. The biotite granite is interpreted to be solely derived by partial melting of old basement rocks of Ganderia and fractionally crystallized at the fO2 of 10−21 to 10−10 bars. The two-mica granite has heterogeneous inherited zircon cores (δ18OZrc of +5.2‰ to 9.9‰) and rims (δ18OZrc of +6.2‰ to 8.7‰), and εHf(t) of −11.7 to −1.01. The two-mica granite was derived from the same basement, but with supracrustal contamination. This open-system process is also recorded by Yb/Gd and Th/U ratios in zircon and isotopic disequilibrium between magmatic zircon rims and quartz (+10.3 ± 0.2‰).
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Abstract Zircon geochronology has contributed to our understanding of the longevity of transcrustal magmatic systems; however, most studies focus on zircon records from felsic rocks due to the restricted occurrence of zircon in mafic-ultramafic rocks. We present U–Pb age, geochemical, and Hf–O isotope data for zircons from a hornblendite peridotite in the Hida Belt, Japan, that offers a unique opportunity to investigate the lifetime of a long-lived mafic plumbing system in an arc setting. We found two zircon U–Pb age clusters: an incompatible element-rich cluster at 196 Ma and an incompatible element-poor cluster at 186 Ma. Their homogeneous isotopic signatures (δ18O = 7.7‰ ± 0.8‰, εHf = 10.3‰ ± 1.7‰) indicate the same magma source despite the 10 m.y. age gap. These two clusters are explained by different zircon formation mechanisms that differ depending on whether or not zircon saturation requires differentiated melt with high SiO2. The enriched older zircons formed by local zircon saturation at the mafic melt-olivine interface, whereas the younger depleted zircons precipitated from the last drop of interstitial felsic melt co-existing with hornblendes. Our finding substantiates the longevity of mafic systems at lower crusts, which sustain transcrustal magma systems and crustal evolution.
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Zircon megacrysts occur in association with mafic alkaline volcanic fields worldwide and have been used as indicators for the chemical characteristics of their mantle sources. However, their origins from magmas that are strongly undersaturated in zircon remain enigmatic. To resolve this conundrum, better constraints on the temporal and chemical relations between zircon megacrysts and associated mafic alkaline magmas are required. For six volcanoes from the West and East Eifel Volcanic Fields (WEVF, EEVF), Germany, we report concordant middle to late Pleistocene zircon megacryst crystallization ages from (230Th)/(238U) disequilibrium and disequilibrium-corrected 206Pb/238U geochronology, which generally agree with independently constrained eruption ages. Trace elements in Eifel zircon megacrysts indicate crystallization from highly fractionated melt pockets in which zircon competed with other accessory minerals (e.g. apatite, titanite, pyrochlore) for incompatible elements enriched in residual melts, such as the rare earth elements, Th, and U. Eifel zircon megacrysts display systematic covariation between indices of differentiation (Eu/Eu*, Zr/Hf) and isotopic signatures of continental crustal contamination, revealing magmatic differentiation of parental mafic melts via coupled assimilation and fractional crystallization (AFC). Isotopic compositions of εHf and δ18O in Eifel zircon megacrysts are consistent with mid- to upper-crustal AFC end-members, which are represented by xenolithic ejecta in WEFV and EEVF volcanic deposits, although not necessarily the same ones that yielded zircon megacrysts. Lower-crustal mafic granulites, by contrast, are a poor match for the isotopic trends displayed by the Eifel zircon megacrysts. These lines of evidence support that the zircon megacrysts in the Eifel originated from mantle melts that differentiated in the mid- to upper crust where they fractionated and partially solidified as syenitic intrusive bodies. Mafic magma recharge en route to the surface then scavenged and disintegrated syenitic rock fragments, in some cases liberating zircon crystals as the only recognizable survivors of their plutonic hosts. Zircon megacrysts in mafic alkaline magmas thus should be treated cautiously as tracers for mantle isotopic compositions. Mixing between mafic magmas and accessory-mineral rich syenites can selectively enrich incompatible trace elements, and potentially compromise the genetic interpretation of trace element patterns in mafic rocks.
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Eastern Tianshan hosts a number of porphyry Cu deposits. However, these mainly formed in the Jueluotage Belt, in the middle part of Eastern Tianshan. The Tonggou porphyry Cu mineralization is an exception to this, since it is located in the Bogda Orogenic Belt, north of Eastern Tianshan. We obtained new zircon U-Pb ages, whole-rock geochemical data, zircon Hf isotope data, and zircon trace element compositions. LA-ICP-MS zircon U-Pb dating indicates a crystallization age of 302.2–303.0 Ma for the Tonggou mineralized granodiorite (TMG), which suggests that the Tonggou porphyry Cu mineralization formed in the Late Carboniferous period. εHf (t) data (1.8–14.1) for TMG suggests it was sourced from juvenile crustal melts, mixed with some mantle materials. TMG displays low ΣREE, compatible elements (Ba, Sr, Zr, and Hf), Zr/Hf and Nb/Ta ratios, as well as clearly negative Eu anomalies in whole rocks analyses. In addition, TMG is enriched in P, Hf and Th/U ratios in zircon, and has lower crystallization temperatures (734 to 735 °C) than the Daheyan barren granodiorite (DBG) (753 to 802 °C). Whole rock and zircon geochemical analyses show that the TMG was formed by fractional crystallization to a greater extent than the DBG in the Bogda Orogenic Belt. Moreover, zircon grains of the TMG show high Ce4+/Ce3+ ratios (159–286), which are consistent with related values from large porphyry deposits of the Central Asian Orogenic Belt (CAOB). High Ce4+/Ce3+ ratios reflect oxidizing magmas as a result of fractional crystallization, which indicates that the Tonggou deposit has potential to host a large porphyry Cu deposit.
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