Search Results (20)

The Xing’an–Mongolia Orogenic Belt (XMOB) is located in the eastern part of the Central Asian Orogenic Belt (CAOB). The region’s notable tectonic complexity and extensive tungsten mineralization offer a unique opportunity to explore metallogeny mechanisms in orogenic areas. This study focuses on the Shamai tungsten deposit as a case study, presenting results from LA–ICP–MS U–Pb dating of fine-grained, medium-grained, and porphyritic biotite monzogranite samples from the deposit, along with in situ zircon Hf isotopic and plagioclase Pb isotopic analyses. The fine-grained, medium-grained, and porphyritic biotite monzogranite were emplaced at 142.5, 141.9, and 140.2 Ma, respectively. These samples contain zircons with εHf(t) values ranging from 3.2 to 7.9 and 4.2 to 7.6, respectively, yielding T_DM2 model ages from 996 to 692 Ma and 923 to 708 Ma. These findings suggest that the magmas in the Shamai deposit were produced by partial melting of juvenile crustal material mixed with mantle-derived components. The tungsten mineralization periods in the Eastern XMOB region can be divided into three stages: Early Paleozoic (ca. 520–475 Ma), Triassic (ca. 250–200 Ma), and Jurassic to Early Cretaceous (ca. 190–130 Ma). The highest concentration of tungsten mineralization in the XMOB occurs within the Xing’an Block during the Jurassic to Early Cretaceous period. Yanshanian magmatism and the most significant tungsten metallogenic events are likely influenced by an extensional setting and oceanic slab rollback, shaped by the tectonic evolution of the Mongol-Okhotsk Ocean and the Paleo-Pacific Ocean. Full article

Himalayan leucogranite is an excellent target for understanding the orogenic process of the India–Asia collision, but its origin and tectonic significance are still under debate. An integrated study of geochronology, geochemistry, and in situ Sr-Nd-Hf isotopes was conducted for a tourmaline-bearing leucogranite in the eastern Tethyan Himalaya using LA-ICP-MS, X-ray fluorescence spectroscopy, and ICP-MS and LA-MC-ICP-MS, respectively. LA-ICP-MS U-Pb dating of zircon and monazite showed that it was emplaced at ~19 Ma. The leucogranite had high SiO₂ and Al₂O₃ contents ranging from 73.16 to 73.99 wt.% and 15.05 to 15.24 wt.%, respectively. It was characterized by a high aluminum saturation index (1.14–1.19) and Rb/Sr ratio (3.58–6.35), which is characteristic of S-type granite. The leucogranite was enriched in light rare-earth elements (LREEs; e.g., La and Ce) and large ion lithophile elements (LILEs; e.g., Rb, K, and Pb) and depleted in heavy rare-earth elements (e.g., Tm, Yb, and Lu) and high field strength elements (HFSEs; e.g., Nb, Zr, and Ti). It was characterized by high I Sr (t) (0.7268–0.7281) and low ε Nd (t) (−14.6 to −13.2) and ε Hf (t) (−12.6 to −9.47), which was consistent with the isotopic characteristics of the Higher Himalayan Sequence. Petrogenetically, the origin of the leucogranite is best explained by the decompression-induced muscovite dehydration melting of an ancient metapelitic source within the Higher Himalayan Sequence during regional extension due to the movement of the South Tibetan Detachment System (STDS). The significantly high lithium and beryllium contents of the leucogranite and associated pegmatite suggest that Himalayan leucogranites possess huge potential for lithium and beryllium exploration. Full article

In the process of in situ zircon U–Pb dating, it is an effective means to overcome the matrix effect by using a matrix-matched external reference material. However, the limited number of available zircon reference materials still makes it difficult to meet the research needs. In this paper, we performed a preliminary analysis of the gemological characteristics, trace elements and U–Pb ages of natural zircon CAM18 to assess its suitability as a reference material for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating. This tawny, gem-quality zircon has no visible inclusions and weighs approximately 0.55 g. Its density, full width at half maximum (FWHM) of the Raman peak and alpha flux (Dα) indicate that the sample has suffered mild-to-moderate radiation damage without any thermal treatment. The LA-ICP-MS U–Pb dating results reveal that the trace elements content and U–Pb ages of the sample are fairly homogeneous at the 50 μm scale, and there is no obvious loss of radiogenic Pb. The ²⁰⁶Pb/²³⁸U age (571.0 ± 3.0 Ma, 2s) and ²⁰⁷Pb/²³⁵U age (573.4 ± 6.0 Ma, 2s) are consistent within the analytical uncertainty, and the calculated concordia age is 571.4 ± 1.4 Ma (2s, n = 20). The variation in the ²⁰⁶Pb/²³⁸U ages is small, with a measurement repeatability of 0.46% (RSD), which is within the uncertainty of the age accuracy obtained by LA-ICP-MS. The oscillatory zoning, Th/U ratio (0.2) and chondrite-normalized rare-earth element (REE) pattern imply a magmatic origin of zircon CAM18. The Ti-in-zircon temperature ranges from 714 to 742 °C, and the oxygen fugacity ranges from ΔFMQ−2.87 to ΔFMQ−3.17, suggesting that it is crystallized in a reducing environment. All the results show that zircon CAM18 may has great potential in LA-ICP-MS U–Pb dating. Full article

The Jizerka Quaternary alluvial placer in the Czech Republic has been a well-known source of gemstones since the 16th century, and the only one in Europe that has yielded a significant amount of jewel-quality sapphire. Besides Mg-rich ilmenite (“iserine”), which is the most common heavy mineral at the locality, some other minerals have been mined for jewellery purposes. These are corundum (sapphire and ruby varieties), zircon (“hyacinth” gemstone variety) and spinel. Here, we present a detailed petrological and geochronological investigation of the enigmatic relationship between the sapphires and their supposed host rocks, supporting their xenogenetic link. Our hypothesis is based on thermal resetting of the U–Pb isotopic age of the zircon inclusion found inside Jizerka blue sapphire to the estimated time of the anticipated host alkaline basalt intrusion. The host rocks of the gemstones (sapphire and zircon) and Mg-rich ilmenite are not yet known, but could be related to the Cenozoic volcanism located near the Jizerka gem placer (Bukovec diatreme volcano, Pytlácká jáma Pit diatreme and Hruškovy skály basalt pipe). The transport of sapphire, zircon and Mg-rich ilmenite to the surface was connected with serial volcanic events, likely the fast ascent of alkali basalts and formation of multi-explosive diatreme maar structures with later deposition of volcanoclastic material in eluvial and alluvial sediments in nearby areas. All mineral xenocrysts usually show traces of magmatic corrosion textures, indicating disequilibrium with the transporting alkali basalt magma. In order to constrain the provenance and age of the Jizerka placer heavy mineral assemblage, zircon inclusion and associated phases (niobian rutile, baddeleyite and silicate melts) in the blue sapphire have been studied using LA–ICP–MS (laser ablation–inductively coupled plasma–mass spectrometry) geochemistry and U–Pb in situ dating. Modification of the zircon inclusion into baddeleyite by exposure to temperature above 1400 °C in a basaltic melt is accompanied by zircon U–Pb age resetting. A zircon inclusion in a Jizerka sapphire was dated at 31.2 ± 0.4 Ma, and its baddeleyite rim at 31 ± 16 Ma. The composition of the melt inclusions in sapphire and incorporated niobian rutile suggests that the parental rock of the sapphire was alkali syenite. The Eocene to late Miocene (Messinian) ages of Jizerka zircon are new findings within the Eger Graben structure, as well as among the other sapphire–zircon occurrences within the European Variscides. Jizerka blue sapphire mineral inclusions indicate a provenience of this gemstone mineral assemblage from different parental rocks of unknown age and unknown levels of the upper crust or lithospheric mantle. Full article

The Yangshan gold belt is renowned for its igneous rock formations, particularly dykes that form in tectonically weak zones. Some of these rock formations exhibit a close spatial relationship with gold mineralization, and a tiny portion of the granitic dykes serve as gold ore bodies by themselves. In order to investigate the nature of granitic dykes and their association with gold mineralization, we conducted a comprehensive study consisting of zircon U-Pb chronology, petrography, and in situ Hf isotope analysis of 25 granitic dyke samples collected from east to west across the belt. According to LA-ICP-MS zircon U-Pb dating results, the granitic dykes inherited zircon ages that are concentrated between 745.0 and 802.0 Ma, and magmatic intrusion ages that mainly fall between 201.0 and 213 Ma. Moreover, the granitic dykes display a calc-alkaline to high-K calc-alkaline peraluminous series, which is relatively enriched in light over heavy REE, with moderate Eu anomalies. These dykes are rich in large-ion lithophile elements and poor in high-field-strength elements. The zircon Lu-Hf isotope data range from εHf(t) values of −1.5 to 0.1, mantle model (T_DM¹) ages range from 859 to 937 Ma, and crustal model (T_DM²) ages range from 1111 to 1218 Ma. The granitic dykes found in the Yangshan gold belt were formed between 200 and 213 Ma ago, during a period of intracontinental extension following the late collision between the Yangtze plate and Qinling microplates. These dykes originated from the volcanic basement of the Mesoproterozoic Bikou Group, which was formed by the melting of the upper crust under the crustal thickening caused by the subduction and collision of the Qinling microplate. Subsequently, the dykes were transported along a tectonically weak zone, assimilating surrounding rocks and undergoing a transformation from “I”-type to “S”-type granite before finally evolving into granite with specific “A”-type characteristics. Our study provides new insights into the petrogenesis of granitic dykes in the Yangshan gold belt, as well as the relationship between gold mineralization and magmatic activity, which has significant implications for mineral exploration and the geological understanding of gold mineralization in this region. Full article

The NE-trending multiphase Late Devonian Eagle Lake granite (ELG) in southwestern New Brunswick is mineralized, consisting of hypabyssal porphyritic stocks and dikes that intruded Silurian metabasic volcanic rocks; however, its various phases, ages, and associations with notable stockwork Cu–Mo–Au mineralization and alteration have yet to have been studied. The ELG suite is predominantly composed of phenocrysts and a microcrystalline groundmass of quartz, K-feldspar, and plagioclase, with minor biotite and accessory minerals. In situ LA ICP-MS U–Pb zircon dating of this pluton yielded 360 ± 5 Ma (Late Devonian), so this pluton is considered part of the Late Devonian granitic series in southwestern New Brunswick. The isotopic analysis of two granitic samples yielded an initial ¹⁴³Nd/¹⁴⁴Nd of 0.512164 and 0.512184, initial ⁸⁷Sr/⁸⁶Sr of 0.70168 and 0. 70675, and initial ¹⁷⁶Hf/¹⁷⁷Hf of 0.282619 and 0.282631. The εNd (360 Ma) is −0.37 to +0.03, whereas the εHf (360 Ma) values are +2.1 and +2.5. Pb isotopic analysis yielded a ²⁰⁶Pb/²⁰⁴Pb of 18.49 and 18.72, ²⁰⁷Pb/²⁰⁴Pb of 15.62 and 15.63, and ²⁰⁸Pb/²⁰⁴Pb of 38.26 and 38.37, indicative of a relatively radiogenic source contaminating a primitive mantle melt. Potassic alteration and pyrite-quartz stockwork Cu–Mo–Au veining is evident in some parts of these porphyries. Petrographic and geochemical evidence indicates that this composite pluton is a low-T, I-type granite with zircon saturation temperatures between 720° and 825 °C, with emplacement depths of 10.3 to 4.4 km. ELG was emplaced along a major structural trend manifested by contemporaneous faults and shear zones, i.e., the Belleisle Fault Zone in southern New Brunswick. Full article

The Jinzhuping and Changkeng granites are related to the Helong W–Sn ore field in southern Jiangxi Province, China. Three different phases can be found in the Jinzhuping pluton, and their LA-ICP-MS zircon U–Pb ages are 155.2 ± 0.68 Ma, 154.0 ± 0.56 Ma, and 153.4 ± 0.99 Ma, respectively, indicating two types of granitic rocks. All granites in the Helong ore field have similar geochemical characteristics, they have high contents of SiO₂ (73.99 wt.%–77.68 wt.%), and total alkali (7.56 wt.%–8.76 wt.%) and are weakly to strongly peraluminous. They are slightly enriched in HREE and depleted in Eu, Ba, Sr, P, and Ti. Zircon εHf(t) values of the Jinzhuping three granites are from −14.4 to −10.4, from −15.3 to −11.4, and from −18.1 to −10.5, and the Hf TDM model ages range from 1.83 to 2.06 Ga, from 1.89 to 2.14 Ga, and from 1.83 to 2.31 Ga, respectively. Whole-rock geochemistry and Hf isotope analysis indicate that the Helong granites experienced a high degree of differentiation and evolution derived by partial melting of the Late Paleoproterozoic crustal materials, and they formed in a backarc caused by low-angle subduction of the Paleopacific plate. Full article

The Chang’an Chong Cu-Mo deposit is located in the Chang’an Cu-Mo-Au ore cluster in the southern Ailaoshan tectonic belt in southwestern China. There are six intrusive bodies in the mining area, among which the No.Ⅱ intrusive body is the largest and most closely related to Cu-Mo mineralization of skarn. The No. 1 main orebody is composed of the No. 1 copper orebody and No. 1 molybdenum orebody, which are distributed in parallel with similar shapes. In this paper, the age of skarn is determined by the LA-SF-ICP-MS U-Pb dating of garnet, and it is nearly consistent with the age of alkaline porphyry in this region (41–32 Ma). Compared with the U-Pb age of zircon from the ore-bearing porphyry and the Re-Os age of molybdenite, the U-Pb age of garnet was consistent with them within error, indicating that they were the same mineralization event, which further proves that the porphyry-skarn Cu-Mo-Au mineralization event along the Ailaoshan-Red River fault zone mainly occurred at 38~32 Ma. In-situ S isotope results show that the δ³⁴S mean values of disseminated pyrite (PyI), pyrite of sulfide veins (PyⅡ) and chalcopyrite (Ccp) in the main mineralization period are 2.35‰, 3.60‰ and 0.55‰, respectively. These δ³⁴S values are similar to those of magma and slightly enriched in δ³⁴S, and the δ³⁴S value of chalcopyrite is mainly concentrated near 0‰, so it can be considered that the S of the ore-forming fluid came from magmatic-hydrothermal fluids. Based on the comprehensive analysis of the regional metallogenic background, deposit chronology and isotope geochemistry, it is concluded that the Chang’an Chong Cu-Mo deposit was formed in an intra-plate post-collision strike-slip environment. Full article

The South China Craton comprises the Yangtze and Cathaysia blocks and is one of the largest Precambrian continental blocks in East Asia. However, the early geological and geographical evolution of the Cathaysia block is relatively poorly understood, due to the sparse exposure of pre-Neoproterozoic rocks and reworking during Phanerozoic polyphase magmatism and metamorphism. In this contribution, we carried out detrital zircon U-Pb geochronology and sedimentary analyses on five Proterozoic meta-sedimentary rocks collected from the northeastern Cathaysia block, which belong to the previously defined Chencai, Mayuan, and Mamianshan Groups (strata). LA-ICP-MS U-Pb dating results of the detrital zircons show various ~1.85–1.35 Ga maximum depositional ages. They are significantly older than the previously constrained Neoproterozoic formation ages of these Proterozoic strata of northeastern Cathaysia, suggesting that their deposition and formation were probably initiated as early as the late Paleoproterozoic. Provenance analyses reveal that the late Paleoproterozoic to early Mesoproterozoic detrital zircons with igneous-origin were derived from in situ contemporary crystalline basements in eastern Cathaysia. In addition, by implication, the easternmost part of Cathaysia was probably an emerged area (i.e., the “proto-Cathaysia Land”) under active erosion. It had a ~NWW orientation and provided detrital sediments to the neighboring marine basin (i.e., the Cathaysia Sea) during the late Paleoproterozoic to early Mesoproterozoic. Finally, the Paleoproterozoic evolution of Cathaysia was involved in the assembly of the Nuna supercontinent. Our results, together with the published data, reveal a distinct late Paleoproterozoic (~1.8 Ga) detrital zircon age peak, which seems to support the view that eastern Cathaysia had close tectonic affinities with terranes such as the Precambrian terranes of current northern India, in the framework of the Nuna supercontinent reconstruction. Full article

Paleoproterozoic tectonic evolution of the northern North China Craton has been a hot research topic. We firstly identified a 1.85 Ga hornblendite from the Gaositai mafic–ultramafic complex, in northern Hebei. Systematic studies of petrology, zircon U-Pb geochronology, and in situ mineral major and trace elements of hornblendite are the key to revealing the petrogenesis of the Paleoproterozoic ultramafic rock and the tectonic evolution of northern North China Craton. LA-ICP-MS zircon U-Pb dating suggests the Gaositai hornblendite formed at 1851 ± 44 Ma. The late Paleoproterozoic ultramafic rocks, together with coeval post-collisional granites, formed a bimodal igneous assemblage. Both hornblende and its equilibrium melt compositions show strongly fractionated HREE patterns, relative enrichments in LREEs and LILEs, and depletions in HREEs and HFSEs. The phlogopite-bearing hornblendite magma could have originated from a hydrous garnet-facies mantle source metasomatized by slab-derived silicate melt. Furthermore, the variations of major and trace elements in hornblende from core to rim also reveal the mineral fractional crystallization and magma recharge. Zircon trace elements, melt composition equilibrium with hornblendes, and the bimodal igneous assemblage suggest that the generation of the Gaositai Paleoproterozoic hornblendite was likely the product of post-collisional extension related to the collision between eastern and western North China blocks. Full article

As a major constituent in magmatic–hydrothermal ore deposits, cassiterites, with moderate amounts of U and low Pb, can be dated with U–Pb geochronology. The tetragonal lattice structure makes cassiterites capable of incorporating dozens of elements within its crystal lattice (e.g., Fe, Ti, W, Zr, Hf, Ta, Nb, Mn, Sc, V, and Sb). Variations of these elements record information of potential elemental substitution mechanisms and precipitation environments of cassiterites. In this study, we collected cassiterite grains from four different ore styles of the Gejiu tin polymetallic deposit to perform LA–ICP–MS U–Pb dating, multiple element mapping, and in situ trace element analysis on these cassiterites. Systematic U–Pb dating yielded Tera–Wasserburg lower intercepted ages at around 85 Ma, coinciding with zircon U–Pb ages of regional Late Yanshanian granitoids, within their respective analytical uncertainties. Such age coincidence, combined with the spatial association, suggests that tin mineralization may be genetically related to the Late Cretaceous granitic magmatism. Multielemental mapping shows that the distribution of Nb, Ta, and Ti in the cassiterite grains correlates well with the regular oscillatory zoning patterns in cathodoluminescence (CL) images. The relatively high Sb, Fe, W, Ga, and U concentrations control the dark luminescing domains in these cassiterite grains. The systematic variations in chemical compositions suggest that trace elements such as Sc, V, Fe, and Ga incorporate in cassiterites via coupled substitutions of Sc³⁺ + V⁵⁺ ↔ 2 (Sn, Ti)⁴⁺, Fe³⁺ + Ga⁵⁺ ↔ 2 (Sn, Ti)⁴⁺ and Fe³⁺ + OH^– ↔ Sn⁴⁺ + O^2– or Fe³⁺ + H⁺ ↔ Sn⁴⁺. The covariation of redox sensitive elements such as W, U, Fe, and Sb indicates that the ”tin-granite” type of cassiterites were formed under an oxidized state whereas cassiterites from skarn, massive sulfide, and oxidized ore styles were precipitated in a reducing environment. Full article

The Lake George antimony mine was at one time North America’s largest producer of antimony. Despite being widely known for the antimony mineralization, the deposit also hosts a range of styles of mineralization such as multiple generations of W-Mo bearing quartz veins as well as a system of As-Au bearing quartz–carbonate veins. In situ U-Pb zircon geochronology, using LA ICP-MS, of the Lake George granodiorite yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 419.6 ± 3.0 Ma. Step heating of phlogopite separated from the lamprophyre dykes produced a ⁴⁰Ar/³⁹Ar plateau segment date of 419.4 ± 1.4 Ma. Single molybdenite crystal analysis for Re-Os geochronology was conducted on two W-Mo-bearing quartz veins, which cross-cut altered granodiorite and altered metasedimentary rocks and yielded two dates of 415.7 ± 1.7 Ma and 416.1 ± 1.7 Ma respectively. ⁴⁰Ar/³⁹Ar geochronology of muscovite from alteration associated with Au-bearing quartz–carbonate veins yielded one representative plateau segment date of 414.1 ± 1.3 Ma. The dates produced in this study revealed that the different magmatic–hydrothermal events at the Lake George mine occurred over approximately a 10-million-year period at the end of the Silurian and the start of the Devonian following the termination of the Acadian orogeny. Full article

The paper provides new U–Pb, Sm–Nd, and Nd–Sr isotope-geochronological data on rocks of the Paleoproterozoic Kandalaksha-Kolvitsa gabbro-anorthosite complex. Rare earth element (REE) contents in zircons from basic rock varieties of the Kandalaksha-Kolvitsa area were analyzed in situ using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Plots of REE distribution were constructed, confirming the magmatic origin of zircon. Temperatures of zircon crystallization were estimated using a Ti-in-zircon geochronometer. The U–Pb method with a ²⁰⁵Pb artificial tracer was first applied to date single zircon grains (2448 ± 5 Ma) from metagabbro of the Kolvitsa massif. The U–Pb analysis of zircon from anorthosites of the Kandalaksha massif dated the early stage of the granulite metamorphism at 2230 ± 10 Ma. The Sm–Nd isotope age was estimated on metamorphic minerals (apatite, garnet, sulfides) and whole rock at 1985 ± 17 Ma (granulite metamorphism) for the Kolvitsa massif and at 1887 ± 37 Ma (high-temperature metasomatic transformations) and 1692 ± 71 Ma (regional fluid reworking) for the Kandalaksha massif. The Sm–Nd model age of metagabbro was 3.3 Ga with a negative value of εNd = 4.6, which corresponds with either processes of crustal contamination or primary enriched mantle reservoir of primary magmas. Full article

The Mogok area in Myanmar (Burma) is known since historic times as a source for some of the finest rubies and spinels in the world. In this study, we focus on in-situ U–Pb geochronological analyses of zircon and zirconolite, either present as inclusions in gem-quality ruby and spinel or as accessory minerals in ruby- and spinel-bearing marble and adjacent granulite facies gneisses. The age determination was carried out using both laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS) and sector-field mass spectrometry (LA-ICP-SF-MS). In addition, we present multi-element data (REE) of zircon and zirconolite collected with LA-ICP-TOF-MS to further characterize these inclusions. Most of the studied zircon grains display growth zoning (core/rim) regardless if as inclusion in gemstones, or as accessory mineral in host rock samples. U–Pb dating was conducted on both core and rim of zircon grains and revealed most ages ranging from ~200 Ma in the core to ~17 Ma in the rim. The youngest U–Pb ages determined from the rim of zircon inclusions in gem-quality ruby and spinel are 22.26 ± 0.36 Ma and 22.88 ± 0.72 Ma, respectively. This agreement in U–Pb ages is interpreted to indicate a simultaneous formation of ruby and spinel in the Mogok area. In ruby- and spinel-bearing marble from Bawlongyi, the youngest zircon age was determined as 17.11 ± 0.22 Ma. Furthermore, U–Pb age measured on the rim of zircon grains in a biotite-garnet gneiss reveals a Late Oligocene age (26.13 ± 1.24 Ma), however older ages up to Precambrian age were also recorded in the cores of zircon as accessory minerals from this gneiss. These old ages point to a detrital origin of the analysed zircon cores. Although non-matrix matched standard was applied, zirconolite U–Pb age results are narrower in distribution from ~35 Ma to ~17 Ma, falling within the range of zircon ages. Based on results which are well in accordance with previous geochronological data from the Mogok Metamorphic Belt (MMB), we deduce that gem-quality ruby and spinel from Mogok probably formed during a granulite-facies regional metamorphic event in Oligocene to Early Miocene, related to post collision tectonics of the Eurasian and Indian plates. Our data not only provide key information to understand the formation of gem-quality ruby and spinel in the so-called Mogok Stone Tract, but also provide assisting evidence when determining the country of origin of gemstones in gemmological laboratories. Full article

A metasomatic zone formed between the contact of a 2940 ± 5 Ma intrusive trondhjemite sheet in the Archean dunite of the Seqi Ultramafic Complex, SW Greenland, consists of three distinct mineral zones dominated by (1) talc, (2) anthophyllite, and (3) phlogopite. These zones supposedly resulted from a process of dissolution of olivine by silica rich fluid residual from the trondhjemite magma, with crystallization of secondary minerals along a compositional gradient in the fluid phase. A zircon crystal inclusion in a large (4 cm) olivine porphyroblast was dated in situ via LA-ICP-MS U–Pb isotope analysis, yielding a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2963 ± 1 Ma, which coincides with granulite facies metamorphism and potential dehydration. Considering phase relations appropriate for the dunite composition, we deduced the talc forming conditions to be at temperatures of 600–650 °C and at a pressure below 1 GPa. This is supported by oxygen isotope data for talc, anthophyllite and phlogopite in the metasomatic zone, which suggests formation in the temperature range of 600–700 °C from fluids that had a δ¹⁸O of ~8‰ and a Δ’¹⁷O_0.528 of about −40 ppm, i.e., from fluids that could have been derived from the late stage trondhjemite sheet. Full article