High-temperature Metamorphism Research Articles

Effects of high temperature annealing and low temperature metamictization on Archean zircon: constraints from U–Pb isotopes, trace elements and Raman dating

Abstract Zircon is a common accessory mineral that can survive extreme temperature conditions in the crust, although U-Th series radioactive decay is responsible for its self-destruction over long time scales. Upon exhumation at lower temperatures, metamict zircon grains are then more susceptible to isotopic and trace element disturbance. In this work, we investigated the isotopic and trace element behavior of zircon through LA-ICP-MS, coupled with Raman spectroscopy, from two Archean mylonitic samples within the São José do Campestre Massif, northeastern Brazil. U–Pb in zircon dating indicates Paleoarchean (3.3 Ga) sedimentation followed by Mesoarchean (3.0–2.9 Ga) high temperature metamorphism in the mylonitic paragneiss sample, while the mylonitic amphibolite was emplaced during the late Neoarchean (2521 ± 22 Ma) and reached high-temperature conditions during shear zone development and deformation during the Paleoproterozoic (2.0 Ga). Inherited zircon grains in the amphibolite are consistent in morphology and ages with zircon grains from the host paragneiss. However, high temperature annealing and structure recovery upon entrainment of inherited zircon grains in the hot mafic magma led to homogenization of trace elements and preservation of internal textures. On the other hand, zircon grains from the host paragneiss accumulated radiation damage for a longer time, leading to self-destruction of the crystalline structure. Upon later low temperature fluid influx, non-formula elements were incorporated in metamict grains, as evidenced by the positive correlation between trace elements and radiation damage. Raman zircon dating indicates Cambrian to Late Paleozoic exhumation for these rocks, which agrees with U–Pb in zircon lower intercept ages. Although often avoided in geochronological studies, metamict zircon can provide important constraints on low temperature events. Collectively, these observations show a complex trans-crustal history of zircon recycling and exhumation from the Paleoarchean to the Paleozoic and the rock-dependent behavior of zircon upon fluid flow and deformation. The results have implications for the interpretation of trace 36 elements in naturally annealed zircon grains, especially those inherited in mafic rocks.

Crustal thickening, exhumation and metamorphic cooling of Neoproterozoic eclogites in NE Brazil: Timescale for the assembly of West Gondwana

The high-pressure followed by high-temperature metamorphism related to the evolution of collisional orogens is the expected Pressure-Temperature-time (P-T-t) path for modern-style plate tectonics. However, these P-T-t paths are rarely preserved even in Phanerozoic orogens. In this matter, the Campo Grande migmatite-gneiss area presents an exhumed Archean crust with heterogeneous retrograde eclogites overprinted by amphibolite-facies in NE Brazil. Retrogressed eclogites in the area core are formed by Mn-rich garnet + clinopyroxene porphyroblasts layer with a minor amphibole + quartz + titanite assemblage, whereas the margins display dismembered lenses of Mg-rich garnet porphyroblasts with plagioclase + amphibole coronae and clinopyroxene + plagioclase + ortopyroxene sympleticte mainly within shear zones. Phase equilibria modelling reveals that these distinct retro-eclogites record similar eclogite conditions (up to 18 kbar and 660 ± 20 °C). Mn-rich eclogites display widespread magmatic zoned zircon grains from Mesoarchean to Paleoproterozoic and recrystallized metamorphic grains of 607.7 ± 5.4 Ma. Zircon from Mg-rich eclogites crystallized at 2.67 Ga and overgrowth rims formed at 2.0 Ga and 590.1 ± 1.8 Ma. REE in zircon lack Eu anomaly, displaying flat HREE patterns in 590.1 ± 1.8 Ma Neoproterozoic grains indicate recrystallization under eclogite-facies conditions. In situ U-Pb titanite dating yielded two younger age sets at 574.7 ± 0.5 and 562.8 ± 2.1 Ma. Lastly, in situ U-Pb dating of rutile inclusions in garnet cores present ages of 606.1 ± 4.0 Ma, whereas rutile included in retrograde coronae and sympleticte yielded concordant ages from 559.0 ± 2.3 to 523.2 ± 1.9 Ma. Thus, combined phase equilibria modelling and U-Pb ages reveal a progressive change in metamorphic conditions from eclogite conditions (at least 607.7 ± 5.4 to 590 ± 1.8 Ma; zircon dating and rutile ages included in eclogite garnet) to nearly isothermal decompression (574.7 ± 0.5 Ma; titanite age) of ancient Archean deep crust, which was followed by suggested final exhumation and metamorphic cooling at shallower crustal levels between 559.0 ± 2.3 and 523.2 ± 1.9 Ma (rutile age in retrogressed assemblage). Therefore, the high-pressure conditions followed by decompression and metamorphic cooling during sin- to post-collisional settings were associated with the final assembly of West Gondwana.

The “greywacke problem” explored in the Neoproterozoic of Saxo-Thuringia: new insights into sediment composition and metamorphic overprint

AbstractGreywackes make up a substantial part of the Cadomian basement of Saxo-Thuringia. Here, their classification as greywackes and the timing of metamorphic overprint are re-evaluated using a multi-method approach. Immature monotonous greywacke sequences from the Lausitz (Lausitz Block) and Leipzig groups (North Saxon Anticline), as well as from the eastern Thuringian Basin and parts of the Weesenstein Group (Elbe Zone) probably belong to a coherent unit, based on microscopic investigations supported by SEM Automated Mineralogy analyses and point counting data. However, due to the low matrix content (< 15%), the sedimentary rocks are more likely classified as lithic sandstones. The heterogeneous composition and in particular the highly mature character of the Clanzschwitz Group (North Saxon Anticline) and parts of the Weesenstein Group (Seidewitz Formation) suggest a younger, Late Cambrian to Early Ordovician sedimentation age. Typically, the metamorphic overprint of the “greywacke units” is very weak. Previous assumptions of Cadomian contact metamorphism triggered by Early Cambrian intrusions (ca. 540 Ma) could not be confirmed due to the local differences in the determined metamorphic ages. Late Cambrian to Early Ordovician (521–461 Ma) Th–U–Pb monazite ages are likely related to the tectonic transition from the collisional regime of the Cadomian orogeny to extensional processes in the course of the opening of the Rheic Ocean. Sporadic Late Ordovician (458–445 Ma) Th–U–Pb monazite and K–Ar fine-fraction ages were also obtained but the specific thermal trigger is still subject of debate. The Permo-Carboniferous metamorphic ages (314–286 Ma) indicate high-temperature metamorphism related to the post-Variscan extensional processes of Central Europe during this period. The youngest dated monazites are Jurassic in age and may have grown in association with the hydrothermal activity known from Central Europe at that time. Graphical abstract

The monazite record of (ultra)high-temperature metamorphism in southern Madagascar: Gondwana terrane correlations and structural trapping of hot middle crust

Ancient orogens eroded to midcrustal levels provide insight about strain accommodation, metamorphism, and melting in Himalaya-type continent-continent collisions. This study focuses on the Neoproterozoic–Cambrian Eastern-Africa / Kuunga orogen exposed in Madagascar, where uncertainty about the terrane correlations, and therefore structural framework, of the orogen persists. We present a comprehensive dataset of monazite petrochronology and thermobarometry across the southern Madagascar basement to quantify the regional and temporal variability of metamorphism. We argue that the ultrahigh-temperature Anosyen domain and associated Androyen domain have a shared geological history, recording two successive tectonic events at 630–600 Ma and 580–500 Ma. Other Madagascar domains record primarily the former (Vohibory domain to the west) or latter (all other domains to the northeast) event. From this inference, we discuss terrane correlations with Africa and India, then present a structural framework for the orogen in which the Anosyen–Androyen domain was structurally confined in a central, lithosphere-scale transpressional shear system between divergent, diachronous thrust belts. By limiting exhumation, extrusion, and collapse, the structural trapping of the Androyen–Anosyen domain facilitated longer-lasting, higher- T metamorphism than associated rocks in the adjacent nappe systems. Such structural trapping may be an important control on high- T metamorphism in the cores of Himalaya-type orogens in general.

Paleo-to-Mesoarchean crustal evolution in the Uauá Block, Northeastern São Francisco Craton, Brazil: New insights from petrography, U–Pb geochronology, Lu–Hf isotopes and trace elements in zircon

The timing of Earth's initial crustal growth and reworking through collisional settings remains an issue of extensive debate. Here, we report a Paleo-to Mesoarchean migmatite-gneiss, temporally- and spatially-associated with a convergent plate boundary in the Uauá Block, Brazil. This rock records an amphibolite-bearing tonalitic residue and pyroxene-bearing granitic leucosome. Orthopyroxene indicates high-temperature metamorphism during leucosome generation, while the widespread retrogressed hornblende in the residue indicates conditions corresponding to amphibolite facies. Zircon grains display LREE-depleted magmatic cores characterized by distinct zoning, crystallized at 3325 ± 70 and 3245 ± 8 Ma. They are enclosed by LREE-enriched metamorphic overgrowth zones and rims formed at 3120 ± 7 and 3069 ± 9 Ma, accompanied by an increase in Ti content. The εHf(t) values decrease with time from +1.55 to −7.06 and Lu/Hf ratios (0.001–0.011) are consistent with the evolution of the Paleo-to Eoarchean crustal sources with TDM Hf model ages between 3.56 and 3.73 Ga. Our findings provide direct evidence of crustal reworking, implicating a Paleoarchean protolith submitted, during early Mesoarchean, to a high-temperature metamorphism spatially-associated with a regional collisional setting in the São Francisco Craton. Further, we surmise that convergent tectonic processes may have played an important role in crustal reworking during the Paleo-to Mesoarchean transition. These observations allowed a comparison between the crustal evolution of the Uauá and Gavião Blocks in the northern São Francisco Craton, as well as with other Archean cratons.

Monazite and zircon U–(Th–)Pb dating reveals multiple episodes of HT metamorphism in the Cima Lunga unit (Central Alps): implications for the exhumation of high‐pressure rocks

High- to ultrahigh-pressure (HP–UHP) rocks recording high-temperature (HT) > 700 °C are well exposed in the Central Alps, making it an ideal region to study the timing of metamorphic stages and the mechanisms of deep-seated rocks exhumation. Here, we report an integrated dataset of petrological and U–(Th–)Pb dating of metapelites surrounding ultramafic lenses from the Cima Lunga unit. At the interface with ultramafics preserving (U)HP–HT assemblages (1.5–3.1 GPa, 650–850 °C), metapelites record higher P‒T values (1.3–2.7 GPa, 700–850 °C) and traces of partial melting, whereas the rest of the unit is dominated by amphibolite-facies conditions. U–Th–Pb dating on zircon and monazite from migmatites indicates that partial melting was episodic involving at least two stages at ~38 to 35 Ma and 33–30 Ma, respectively. While the 38–35 Ma stage matches the HP conditions (> 1.5 GPa) and it is recorded around only one lens with scarce volumes of melt, partial melting at 33–30 Ma is witnessed at lower pressure (~1 GPa) and more widely distributed around the lenses, as within the major shear zones. Far from the ultramafics, zircon from the amphibolite-facies metasedimentary rocks record inherited pre-Variscan ages, while monazite ages at ~22 Ma document mineral growth during the Barrovian cooling. Field and petro-chronological evidence highlight that multiple episodes of partial melting locally developed at the rheological interface promoted by the interplay of fluids extracted from the ultramafic lenses associated with shear heating. New evidence suggests that local variation of P‒T equilibria play a significant role during the exhumation history.Graphical

Paleoarchean sedimentation and Mesoarchean high-temperature metamorphism in northeastern Brazil: Tracing sources and depositional ages in polymetamorphic rocks

Supracrustal rocks offer a window into tectonic processes of the early Earth, since they are common in the Archean lithosphere. However, these rocks are usually affected by several episodes of metamorphism that can compromise their UPb systematics, leading to equivocal interpretations of depositional ages and sources. In northeast Brazil, supracrustal rocks are frequent within the Archean basement of the São José do Campestre Massif. These metapelitic migmatites show a high-temperature mineral assemblage, with garnet + sillimanite ± spinel and retrograde cordierite, with abundant anatectic melt migration at conditions of upper amphibolite to granulite facies. Zircon UPb dating coupled to trace elements analysis through LA-ICP-MS, as well as zircon internal zoning patterns suggest a maximum depositional age of 3305 ± 16 Ma followed by high-temperature metamorphism in the Mesoarchean, Paleoproterozoic and Neoproterozoic. Mesoarchean high temperature metamorphism occurred between 3084 ± 4 and 3006 ± 6 Ma and generated a wide range of textures that could be grouped in, at least, three stages of zircon growth. The first, during prograde heating, led to dissolution of detrital cores through the process of Ostwald Ripening and reprecipitation in oscillatory zoned rims. The second, probably at peak conditions, occurred above the stability of monazite, as evidenced by high Th/U ratios within zircon grains, rounded shape and sector-zoned cores. The third, during retrograde cooling, is mostly driven by garnet breakdown, and resulted in the crystallization of convoluted rims. Ti-in-zircon temperatures indicate minimum temperatures of 712 ± 21 °C for prograde/retrograde stages and 881 ± 50 °C for peak conditions. The Paleoarchean sedimentation and Mesoarchean metamorphism are coeval with similar events in Kaapvaal and Dharwar Cratons (South Africa and South India, respectively), but show no correlation to any Archean domains in South America to date. Solid-state recrystallization during the Paleoproterozoic (ca. 2.0 Ga) and the Neoproterozoic (ca. 0.6 Ga) correlates with orogenic events in both the Borborema Province and São Francisco Craton, suggesting a common evolution since the Rhyacian.

Discovery of the Naturally Occurring Pure CaTiO3 Cubic Perovskite from Tazheran Massif

As the parent compound of the perovskite-structured family and an analogue of davemaoite CaSiO3 in Earth’s lower mantle, CaTiO3 perovskite is widely used in electronic ceramic materials, immobilizing radioactive waste, and geosciences. Here we report the discovery of a natural pure CaTiO3 perovskite-C in calc-silicate veins in brucite marble in the Tazheran Massif, Russia. The mean composition of perovskite-C is 97 wt% total CaO and TiO2, and minor impurities of Na, Al, and Zr, yielding the chemical formula CaTi0.97Al0.01Na0.01Zr0.01O3. The cubic crystal structure—Pm3m, Z = 1: a = 3.8301 Å, V = 56.19 (2) Å3 (ρ = 4.019 g/cm3) has been refined to R1 = 0.0451. Compared with the cubic perovskite discovered in Israel, the cubic perovskite discovered in this study is very pure in Ca and Ti. The naturally occurring perovskite-C from Tazheran implies that the host skarn might have experienced high-temperature (> 1247–1374 °C) metamorphism and/or metasomatism at an early stage. Moreover, the discovery of natural perovskite-C also draws attention to considering crystal-structure-related matrix effects in the future of in situ U-Pb geochronology, especially using Tazh international perovskite standards.

Inside the Ryoke Magmatic Arc: Crustal Deformation, High-T Metamorphism, and Magmatic Pulses

The Ryoke belt represents the root of a volcanic arc exposed across SW Japan. It records successive deformation phases, high-temperature metamorphism, and several magmatic pulses that occurred during the Late Cretaceous. Successive magma intrusions at different crustal levels raised the overall geothermal gradient of the arc crust, and their thermal influence was contrastingly recorded in metamorphic zircon and monazite. Despite a broadly similar duration of magmatic activity (20–30 My) along the belt, the timing and periodicity of magma pulses varied. An along-arc variation in lower crustal magma generation together with a fluctuating crustal stress regime likely controlled the formation and evolution of this magmatic arc section.

Dravite of Terny Meteoritic Crater (Kryvyi Rih District)

During mineralogical investigations of breccia and volcanic rocks of the Terny structure (Veseli Terny, Kryvyi Rih), so-called Terny meteoritic crater, the mineral tourmaline, previously unknown here, was found. Tourmaline crystals, with a length from 0.05 to 1.0 mm and more and up to 0.1—0.2 mm wide, are associated with quartz, potassium feldspar, graphite and apatite. The morphology and chemical composition of tourmaline were examined using optical microscope and EPMA in transparent thin sections. The tourmaline crystals are unusually parted along two transverse systems of microcracks oriented obliquely to the prism faces. EPMA reveals that tourmaline is dravite. The specific deformations of dravite and observed mineral associations probably indicate the rock formation under conditions of high pressures and temperatures of impact metamorphism. It is also confirmed by the presence of planar fractures in quartz.

The Alpine Geological History of the Hellenides from the Triassic to the Present—Compression vs. Extension, a Dynamic Pair for Orogen Structural Configuration: A Synthesis

In this paper, the Hellenic orogenic belt’s main geological structure and architecture of deformation are presented in an attempt to achive a better interpretation of its geotectonic evolution during Alpine orogeny. This study was based not only on recent research that I and my collaborators conducted on the deformational history of the Hellenides but also on more modern views published by other colleagues concerning the Alpine geotectonic reconstruction of the Hellenides. The structural evolution started during the Permo–Triassic time with the continental breaking of the supercontinent Pangea and the birth of the Neotethyan ocean realm. Bimodal magmatism and A-type granitoid intrusions accompanied the initial stages of continental rifting, followed by Triassic–Jurassic multiphase shallow- and deep-water sediment deposition on both formed continental margins. These margins were the Apulian margin, containing Pelagonia in the western part of the Neotethyan Ocean, and the European margin, containing continental parts of the Serbo-Macedonian and Rhodope massifs in the eastern part of the Neotethyan ocean. Deformation and metamorphism are recorded in six main deformational stages from the Early–Middle Jurassic to the present day, beginning with Early–Middle Jurassic Neotethyan intra-oceanic subduction and ensimatic island arc magmatism, as well as the formation of a suprasubduction oceanic lithosphere. Compression, nappe stacking, calc-alkaline magmatism, and high-pressure metamorphic events related to subduction processes alternated successively over time with extension, orogenic collapse, medium- to high-temperature metamorphism, adakitic and calc-alkaline magmatism, and partial migmatization related to the uplift and exhumation of deep crustal levels as tectonic windows or metamorphic core complexes. A S- to SW-ward migration of dynamic peer compression vs. extension is recognized during the Tertiary Alpine orogenic stages in the Hellenides. It is suggested that all ophiolite belts in the Hellenides originated from a single source, and this was the Neotethyan Meliata/Maliac-Axios/Vardar ocean basin, parts of which obducted during the Mid–Late Jurassic on both continental margins, Apulian (containing Pelagonia) and European (containing units of the Serbo-Macedonian/Rhodope nappe stack), W-SW-ward and E-NE-ward, respectively. In this case, the ophiolite nappes should be considered far-traveled nappes on the continental parts of the Hellenides associated with the deposition of Middle–Late Jurassic ophiolitic mélanges in basins at the front of the adjacent ophiolite thrust sheets. The upper limit of the ophiolite emplacement are the Mid–Upper Jurassic time(Callovian–Oxfordian), as shown by the deposition of the Kimmeridgian–Tithonian Upper Jurassic sedimentary carbonate series on the top of the obducted ophiolite nappes. The lowermost Rhodope Pangaion unit is regarded as a continuation of the marginal part of the Apulian Plate (External Hellenides) which was underthrust during the Paleocene–Eocene time below the unified Sidironero–Kerdylia unit and the Pelagonian nappe, following the Paleocene–Eocene subduction and closure of a small ocean basin in the west of Pelagonia (the Pindos–Cyclades ocean basin). It preceded the Late Cretaceous subduction of the Axios/Vardar ocean remnants below the European continental margin and the final closure of the Axios/Vardar ocean during the Paleocene–Eocene time, which was associated with the overthrusting of the European origins Vertiskos–Kimi nappe on the Sidironero–Kerdylia nappe and, subsequently, the final collision of the European margin and the Pelagonian fragment. Subsequently, during a synorogenic Oligocene–Miocene extension associated with compression and new subduction processes at the more external orogenic parts, the Olympos–Ossa widow and the Cyclades, together with the lower-most Rhodope Pangaion unit, were exhumed as metamorphic core complexes.

The inhibited response of accessory minerals during high‐temperature reworking

AbstractU–Pb zircon and monazite geochronology are considered to be among the most efficient and reliable methods for constraining the timing of high‐temperature (HT) metamorphic events. However, the reliability of these chronometers is coupled to their ability to participate in reactions. A case study examining the responsiveness of zircon and monazite has been conducted using granulite facies metapelitic and metamafic lithologies in the Warumpi Province, central Australia. In some instances, metapelitic granulites from this locality are polymetamorphic, with an early M1 assemblage containing orthopyroxene, cordierite, biotite, quartz, ilmenite and magnetite, and an M2 assemblage represented by garnet, sillimanite, orthopyroxene, cordierite, biotite, sapphirine, ilmenite and magnetite. M2 metamorphism is linked to HT peak conditions of 8–10 kbar and 850–915°C. Detrital and metamorphic zircon and monazite from these rocks dominantly record U–Pb dates of 1670–1610 Ma and have trace element compositions suggesting they grew prior to peak M2 garnet in the rock. Lu–Hf geochronology from M2 garnet gives ages of c. 1150 Ma. Zircon and monazite are therefore suggested to have remained largely inert during HT metamorphism. We attribute the relatively minor response of zircon and monazite during high‐temperature Mesoproterozoic metamorphism to the localized development of refractory bulk compositions at c. 1630 Ma during M1 metamorphism. This created refractory Mg–Al‐rich bulk compositions that were unable to undergo significant partial melting, despite experiencing subsequent temperatures of ~900°C at c. 1150 Ma. In contrast, metapelitic and metamafic rocks in the area that did not develop refractory bulk compositions during M1 metamorphism were able to partially melt and record c. 1150 Ma accessory mineral U–Pb ages. These results contribute to a small, but growing number of case studies investigating the systematics of the U–Pb system in zircon and monazite in polymetamorphic HT terranes and their apparent resistance to isotopic resetting. Where disequilibrium is apparent, garnet Lu–Hf geochronology can form an important tool to interrogate the significance of accessory U–Pb ages. In the Warumpi Province in central Australia, c. 1640 Ma zircon U–Pb ages had previously been interpreted to reflect the formation of HT garnet‐bearing granulites during a collisional event. Instead, the garnet‐bearing assemblages formed at c. 1150 Ma during the Mesoproterozoic, calling into question the existence of a late Palaeoproterozoic collisional system in central Australia.

Origin of Archean Pb isotope variability through open-system Paleoarchean crustal anatexis

Abstract Lead isotopic data imply that thorium and uranium were fractionated from one another in Earth’s early history; however, the origin of this fractionation is poorly understood. We report new in situ Pb isotope data from orthoclase in 144 granites sampled across the Archean Yilgarn craton (Western Australia) to characterize its Pb isotope variability and evolution. Granite Pb isotope compositions reveal three Pb sources, a mantle-derived Pb reservoir and two crustal Pb reservoirs, distinguished by their implied source 232Th/238U (κPb). High-κPb granites reflect sources with high 232Th/238U (~4.7) and are largely co-located with Eoarchean–Paleoarchean crust. The Pb isotope compositions of most granites, and those of volcanic-hosted massive sulfide (VHMS) and gold ores, define a mixing array between a mantle Pb source and a Th-rich Eoarchean–Paleoarchean source. Pb isotope modeling indicates that the high-κPb source rocks experienced Th/U fractionation at ca. 3.3 Ga. As Th/U fractionation in the Yilgarn craton must have occurred before Earth’s atmosphere was oxygenated, subaerial weathering cannot explain the apparent differences in their geochemical behavior. Instead, the high Th/U source reflects Eoarchean–Paleoarchean rocks that experienced prior high-temperature metamorphism, partial melting, and melt loss in the presence of a Th-sequestering mineral like monazite. Archean Pb isotope variability thus has its origins in open-system high-temperature metamorphic processes responsible for the differentiation and stabilization of Earth’s continental crust.

Differences in Geochemical Signatures and Petrogenesis between the Van Canh and Ben Giang-Que Son Granitic Rocks in the Southern Kontum Massif, Vietnam

Permian Ben Giang-Que Son and Triassic Van Canh granitic rocks are widely distributed across the southern Kontum Massif, the basement of which consists mainly of metasedimentary rocks. The Ben Giang-Que Son granitic rocks are classified as I- to S-type and ilmenite-series granitic rocks, while the Van Canh granitic rocks are classified as I-type and magnetite-series granitic rocks. Both granitic rock suites exhibit more or less adakitic properties, suggesting that the subduction of the high-temperature Song Ma Ocean crust, part of the Paleo-Tethys Ocean, beneath the Indochina Block produced adakitic magma. It is hypothesized that the differences between the two granitic rock suites were caused by differences in the quantities of incorporated continental crustal materials and carbon or graphite in clastic sedimentary rocks when their adakitic magma intruded into the continental crust. Based on their high initial Sr isotope ratios, the Ben Giang-Que Son granitic rocks evidently incorporated a higher quantity of continental crustal materials compared to the Van Canh granitic rocks, resulting in the former showing the signatures of ilmenite-series and I- to S-type granitic rocks. Consequently, the Ben Giang-Que Son granitic rocks have relatively high A/CNK ratios and high total Al contents in their biotite, whereas the Van Canh granitic rocks have low A/CNK ratios and low total Al contents in their biotite. The intrusion of the Ben Giang-Que Son granitic rocks caused high-temperature metamorphism, which decomposed some of the carbon or graphite in the surrounding continental crustal materials, such as clastic sedimentary rocks. Meanwhile, the Van Canh granitic rocks, which intruded later than the Ben Giang-Que Son granitic rocks, incorporated smaller quantities of carbon or graphite in continental crustal materials, resulting in them retaining the chemical characteristics of adakitic, magnetite-series, and I-type granitic rocks, different from the Ben Giang-Que Son granitic rocks.

THE Fe-Cu DISCONNECT: UNRAVELING A COMPOSITE IRON OXIDE COPPER-GOLD DEPOSIT IN THE OLYMPIC Fe-Cu-Au PROVINCE, GAWLER CRATON

Abstract The northern Olympic Cu-Au province, Gawler craton, Australia, includes a series of magnetite-dominated deposits/prospects associated with minor Cu-Au mineralization such as the 8.37 million tonne Cairn Hill deposit. Cairn Hill has long been considered a deep, magnetite end member of the iron oxide copper-gold (IOCG) family that is largely represented in the southern Olympic province by the 1590 Ma hematite-dominated Olympic Dam, Carrapeteena, and Prominent Hill deposits. In contrast to the southern district, the deposits in the northern Olympic Cu-Au province are hosted in rocks that experienced multiple phases of high-temperature metamorphism and deformation. New U-Pb zircon geochronology shows the magnetite-hornblende lodes at Cairn Hill were formed at ca. 1580 Ma at amphibolite facies conditions. The magnetite lodes are crosscut by ca. 1515 Ma granitic dikes. A second high-temperature event is recorded by U-Pb monazite geochronology at ca. 1490 Ma and involved deformation and metamorphism along the Cairn Hill shear zone at conditions of 4.6 to 5.3 kbar and 740° to 770°C. The 1490 Ma event reworked the iron lodes and 1515 Ma granitic dikes. However, Cu mineralization at Cairn Hill occurs in brittle fractures and quartz-biotite veins, overprinting the 1490 Ma deformation and metamorphism. Despite a spatial association between magnetite and Cu, the long thermal history that affected magnetite mineralization and the clear petrographic links between magnetite and high-temperature granulite facies minerals contrast with the late, low-temperature hydrothermal Cu mineralization and indicate the two are not paragenetically related. Therefore, the spatial but not temporal association between magnetite and Cu has effectively overlain two distinct episodes of mineralization to create the Fe-Cu deposit observed today. Although this fits within the broad IOCG deposit family, exploration strategies for Cairn Hill-style composite deposits should be distinct from IOCG deposits with cogenetic Fe and Cu.

Fluid-fluxed partial melting of the Buncheon granitic gneiss in the Yeongnam Massif, Korea: Protracted (ca. 1.86–1.84 Ga) reworking of the Paleoproterozoic Korean arc

The fluid-fluxed melting of igneous protoliths provides an opportunity to assess the role of fluid for the formation and modification of continental crust during high-temperature metamorphism. In order to investigate such an anatexis in the Buncheon granitic gneiss, Yeongnam Massif, we combined field, microstructural, and geochemical studies with the U-Pb and oxygen isotopic analyses of zircon and monazite. The granitic gneisses comprise metatexitic migmatites containing leucosomes, although neither melanosomes nor peritectic phases are associated with neosomes. These migmatites underwent syn-kinematic wet melting in the presence of ca. 0.9–1.4 wt% H2O (estimated from the pseudosection modeling) via the reaction, plagioclase + K-feldspar + quartz + H2O = melt, that took place at ∼650–750 °C and 4–6 kbar. The leucosomes differ in the degree of Eu anomalies and have variable amounts of rare earth elements, Zr, Y, and Th. Such variations are attributed to the segregation and migration of melts as well as the entrainment of residual phases. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of zircon and monazite were determined from two granitic gneisses and three leucosomes together with an adjacent, undeformed Hongjesa granite. Zircon grains are oscillatory-zoned, except for homogeneous overgrowth rims rarely present in two leucosomes. Both zircon domains were dated at ca. 1.98–1.97 Ga and ca. 1.86 Ga, respectively, suggesting that igneous protoliths were partly recrystallized during the late Orosirian. Monazites from two gneiss samples contain submicroscopic polycrystalline inclusions of K-feldspar + quartz + apatite, reflecting the presence of former melts, and yielded a pooled 207Pb/206Pb age of 1859 ± 6 Ma (n = 30). In contrast, monazites from a leucosome are inclusion-free, and yielded the youngest age of 1840 ± 16 Ma (n = 6). This result is interpreted to represent the melt crystallization at ca. 1.84 Ga following the ca. 1.86 Ga anatexis. On the other hand, monazite from the Hongjesa granite, yielded a weighted mean 207Pb/206Pb age of 1878 ± 5 Ma (n = 11), most likely reflecting a prograde metamorphic growth. The δ18O values of ca. 1.98–1.97 Ga igneous zircon in granitic gneisses are 7.1–7.2 ‰, but those from the leucosome are reduced at 5.9–6.7 ‰. The latter is largely identical to 5.7–6.7 ‰ estimated from ca. 1.86 Ga domains of zircon in the leucosome and monazite in the granitic gneiss; in contrast, a slightly lower δ18O value of 5.46 ± 0.61 ‰ is recorded in the ca. 1.84 Ga monazite of leucosome. The relatively low δ18O values are most likely accounted for by an influx of fluid that has apparently triggered partial melting in the granitic gneiss, and the fluid itself is possibly linked to the crystallization of mafic magmas in an arc complex. Taken together, in contrast to the majority of zircons preserving the crystallization age of igneous protoliths, monazites are more susceptible to recrystallization during the fluid-present melting and permit us to decode a protracted (∼20 m.yr.) episode from partial melting to melt crystallization in the Yeongnam Massif. Such a longevity of melt-present system typifies the prolonged crustal reworking of Paleoproterozoic Korean arc situated at the eastern margin of the North China Craton.

Corundum-bearing and spinel-bearing symplectites in ultrahigh-pressure eclogites record high-temperature overprint and partial melting during slab exhumation

Abstract. Corundum- and spinel-bearing symplectites after muscovite were found in ultrahigh-pressure (UHP) eclogites from the Dabie terrane, China. Three types of symplectites were recognized based on their mineral assemblages: (1) symplectitic intergrowths of corundum + plagioclase + biotite after phengite (CPB), (2) symplectitic intergrowths of spinel + plagioclase + biotite after phengite (SPB), and (3) symplectitic intergrowths of spinel + plagioclase after paragonite (SP). The microtextures and mineral assemblages of the symplectites, in combination with the results of thermodynamic modeling on local regions, indicate that these symplectites formed by the breakdown of phengite and paragonite during the granulite-facies metamorphic overprint (770–850 ∘C) of the eclogite at pressures of 0.8–0.9 GPa. Dehydration partial melting reactions occurred during the breakdown of muscovite, which leads to the formation of thin plagioclase films (silicate melts) along grain (garnet, rutile, quartz) boundaries. Mass balance calculations indicate that the development of CPB and SPB symplectites after phengite requires the introduction of Al, Ca, Na, and Fe and loss of Si, Mg, and K. However, the formation of SP symplectites after paragonite requires the input of Mg, Ca, and Fe and removal of Si, Al, and Na. By summarizing the occurrence and growth mechanism of corundum- and spinel-bearing symplectites in global UHP terranes, we find that such symplectites can form by both the subsolidus replacement of an Al-rich anhydrous mineral (kyanite) and the dehydration melting of an Al-rich hydrous phase during high-temperature metamorphism. This study reveals that muscovite-bearing eclogites may experience multiple episodes of partial melting during the slab exhumation, not only at the high-pressure (HP) exhumation stage but also at the lower-pressure metamorphic overprinting stage. Kyanite is a reaction product during the HP partial melting, whereas the low-pressure (LP) melting will consume kyanite. We propose that the occurrence of corundum- and spinel-bearing symplectites after muscovite in eclogites is a potential mineralogical indicator of LP melting in exhumed slabs.

Paleoproterozoic alkaline-carbonatite magmatism in the convergent tectonic setting: Evidences from 2.07 Ga Dubravinsky complex in the Eastern Sarmatia

The Dubravinsky alkaline-carbonatite complex (ACC) include two relatively large plutons: the arcuate shape Dubravinka pluton and linear Chernyanka pluton intruding the Paleoarchean TTG of the Kursk block of Sarmatia and consisting of three main lithologies: alkaline pyroxenites, carbonatites (together with silicocarbonatites and phoscorites) and syenites (including alkaline granites). The carbonatites and silicocarbonatites are enriched in trace and rare earth elements, which contents vary significantly.The source for the alkaline pyroxenites and carbonatites could be enriched protoliths from the subcontinental lithospheric mantle, produced by melting and release of fluids from the subducted oceanic slab at c. 2.1 Ga. The primary igneous C and O isotope composition is preserved in the carbonatite: δ13C (‰ VPDB) = (−4.9) - (−6.4), δ18O (‰ VSMOW) = (+8.0) – (+9.8). The alkaline granites display well preserved isotope markers of a long-lasting crustal prehistory indicating Paleoarchean source. The syenites have a lesser contribution of Paleoarchean crustal material.The alkaline pyroxenites and carbonatites are undoubtedly initially igneous rocks that have undergone c. 2.07 Ga-old high-temperature metamorphism. The Dubravinsky ACC is most likely 2.07–2.08 Ga-old, not much older than the metamorphic event. The Dubravinsky ACC has been formed in a suprasubduction setting from both mantle and crustal sources. It is the earliest known carbonatite complex in the World resulted from subduction-collision processes, and the first sign of transition to deep subduction of the modern style.

REGIONAL METAMORPHISM AND STRATIGRAPHY OF THE BASEMENT OF THE UKRAINIAN SHIELD

The stratigraphic dismembrement of the Lower Precambrian has always been inextricably linked with the study of metamorphism. The degree of metamorphism of the complexes was even used for some time as an indication of their relative age. This sign was not confirmed by isotope dating and was no longer taken into account in the stratigraphic dismembrement of the basement of the shields, including the preparation of official stratigraphic schemes of the Precambrian of the Ukrainian Shield. The authors of the article believe that the degree of metamorphism can still be used in developing the stratigraphy of the Ukrainian Shield. The possibility of such use of metamorphism is considered in a series of publications. The first article describes the stratigraphy and metamorphism of all megablocks of the Ukrainian Shield. It is shown that older stratigenic complexes in each of the megablocks are characterized by higher temperature metamorphism. At the same time, the distinctive features of the composition and metamorphism of the stratigenic complexes, according to the authors, are a reflection of large successive stages of the geological development of the Ukrainian Shield in the Early Precambrian and can serve as the basis for compiling a regional stratigraphic scheme on a historical-geological basis. A version of the regional stratigraphic scheme of the Ukrainian Shield on a historical-geological basis, compiled at the level of complexes, proposed in the second article of the series. In this final article of the cycle, the conditions and duration of lithogenesis and accompanying metamorphism of various sequentially formed stratigenic complexes of the Lower Precambrian are considered. The idea is put forward that the initial pre-metamorphic composition of the complexes and their metamorphism are determined by the temperature state of the upper shells of the Earth and their directed thermal evolution at the early stages of geological history. The existence of hydrospheric, early thermohydrospheric, late thermohydrospheric and normohydrospheric global stages of lithogenesis, lasting from 300 to 700 Ma, and continuous metamorphism from 3.8 to 2.0–1.9 Ga are assumed. During this time, a single metamorphic (paleotemperature) zoning of the basement of the Ukrainian Shield was formed, in which all stratigenic complexes of different ages participate.

Comparative characteristics of diamonds from areas with a high density of kimberlite bodies

Background. This paper presents a comparative study into diamonds from kimberlite bodies of the Verkhnemunsky field (Zapolyarnaya, Novinka, Komsomolskaya-Magnitnaya and Poiskovaya), the results of which allow these minerals to be distinguished from those found in other fields of the Siberian platform. Diamonds from the Zapolyarnaya kimberlite pipe are characterized by an increased prevalence of dodecahedroids with shagreen and plastic deformation bands, mainly of a violet-brown color. A significant part of these diamonds contains cavities. In this respect, diamonds from the Zapolyarnaya kimberlite pipe are similar to other bodies of the Verkhnemunsky kimberlite field (Komsomolskaya-Magnitnaya, Novinka and Poiskovaya) and different from diamonds mined at the Malo-Botuobinsky, Daldyn-Alakitsky and Sredne-Markhinsky diamondiferous areas.Aim. To analyze the characteristics of diamonds from the Verkhnemunsky field with a high density of kimberlite bodies.Materials and methods. The research was based on a large amount of data and materials collected over long-term prospecting and exploration works carried out by industrial and research organizations in Western Yakutia. A comprehensive study of diamonds was conducted under the guidance and participation of the authors.Results. Kimberlite ore bodies in the region under study were found to differ from similar ore bodies of the central fields (Mirninsky, Daldynsky and Alakit-Markhinsky) by a low content of xenoliths of sedimentary rocks, many of which underwent high-temperature metamorphism. In general, the kimberlite ore bodies of the field are characterized by relatively weak secondary changes. Therefore, the largest part of the kimberlites of individual diatremes (Zymnyaya, Komsomolskaya-Magnitnaya, Novinka and Legkaya) preserve fresh olivine of the second generation, which is usually replaced by later monticellite and periclase.Conclusion. Diamonds from the Verkhnemunsky field are characterized by a set of typomorphic characteristics, which can be used to differentiate them from minerals in other Yakutia deposits. The main typomorphic characteristics of Zapolyarnaya diamonds include the pronounced predominance of dodecahedroids with shagreen and plastic deformation bands, mainly of a violet-brown color and numerous cavities. In terms of this feature, these diamonds are close to other kimberlite bodies of the Verkhnemunsky field (Komsomolskaya-Magnitnaya, Novinka and Poiskovaya) but different from those in the Daldyn-Alakitsky, Malobotuobinsky and Sredne-Markhinsky diamondiferous areas.