High-grade Metamorphism Research Articles

Rare-metal Be–Nb–Ta mineralisation by pegmatite remelting: Insights from Dakalasu deposit in the Chinese Altai orogenic belt

The rare metals Li, Be, Cs, Nb and Ta are mainly extracted from pegmatite deposits that may have formed through extensive fractional crystallisation or local partial melting of crustal rocks. There are a large number of rare-metal pegmatites that formed through crustal melting in the Chinese Altai, but what controls rare-metal enrichment remains enigmatic. Here, a new columbite UPb age of pegmatite mineralisation (247 ± 1 Ma) that is distinct from the ages of surrounding biotite granites (290–270 Ma) in the Dakalasu Be–Nb–Ta deposit in the Altai is reported. Zircon from the border zone with a medium to coarse-grained graphic texture of pegmatite yields two age groups: 290–270 Ma and 249 ± 7 Ma, corresponding to the age of biotite granite and the mineralisation age of pegmatite, respectively. The consistent age of the older group and biotite granite (290–270 Ma) and the elevated εHf(t) ratios and lower Nb/Ta ratios of pegmatite suggest that pegmatite is the end member of biotite granite after fractional crystallisation. The similar age of the younger group and mineralisation, the myrmekitic texture and albite trace elements of pegmatite imply pegmatite remelting and mineralisation occurred at ∼250 Ma. Our modelling agrees with geochronology, petrology and geochemistry that Be–Nb–Ta pegmatite mineralisation requires two melting events. The first step with anatexis of metasedimentary rocks generated biotite granitic melts whose extensive fractional crystallisation generated pegmatite with preliminary Be–Nb–Ta enrichment. The second step with pegmatite remelting (<5% degree) generated the significant enrichments of Be, Nb and Ta, which are sufficient for beryl and columbite crystallisation. The Be–Nb–Ta mineralisation mechanism through pegmatite remelting is likely common in orogenic belts with high-grade metamorphism and, therefore, should be considered in future explorations.

Detrital zircon geochronology of quartzites from the Central Zone of the Limpopo Complex in southern Africa: Implications for different models and the timing of Zimbabwe Craton–Kaapvaal Craton amalgamation

U–Pb geochronological analyses have been conducted on 765 detrital zircon grains from 11 quartzite samples in the Central Zone of the Limpopo Complex, southern Africa, for constraining the depositional age and provenance of the protolith quartz-sandstone and evaluating the effect of high-grade metamorphism on zircon age data in polymetamorphic terranes. The age data have also been used to evaluate two contrasting tectonic models for the evolution of the complex; collision of the Zimbabwe and Kaapvaal Cratons and diapir-driven intracratonic orogeny, both during the Neoarchean. Magmatic cores of the zircons from the Beit Bridge, Phikwe, and Motloutse Complexes in the Central Zone show a wide age range from 3828 Ma to 1995 Ma. Ages from the Beit Bridge Complex in Zimbabwe (ca. 3.6–3.1 Ga) and South Africa (ca. 3.5–2.4 Ga) are dominantly Paleoarchean to Neoarchean with minor Paleoproterozoic ages. Similar Eoarchean to Neoarchean ages were also obtained from the Phikwe Complex (ca. 3.8–2.6 Ga), whereas ages from the Motloutse Complex are slightly younger (early Mesoarchean to Paleoproterozoic; ca. 3.0–2.4 Ga). Although the age patterns from the three complexes are different, the distribution patterns of discordant and concordant data suggest Eoarchean to Mesoarchean (ca. 3.8–3.1 Ga) ages for magmatic cores of detrital zircon grains. Homogeneous zircon rims around magmatic cores gave metamorphic ages of ca. 2.61 Ga and ca. 2.27–2.02 Ga, probably formed by Neoarchean and Paleoproterozoic high-grade metamorphisms. Similar Neoarchean (ca. 2.7–2.6 Ga) and Paleoproterozoic concordant ages obtained from magmatic zircon cores are interpreted as reset ages due to Pb loss during the metamorphic events. The smaller peak of the Paleoproterozoic age population compared to that of the Neoarchean age data suggests the dominant high-grade metamorphism of the Central Zone took place at ca. 2.7–2.6 Ga, either by collision of the Zimbabwe and Kaapvaal Cratons or by diapir-driven intracratonic orogeny. Therefore, cratonization of the Azanian (Kalahari) Craton probably occurred before ca. 2.6 Ga. Based on the age patterns, the depositional age of the protolith quartz-rich sandstone is constrained as 3.1–2.7 Ga. The provenance of the Paleoarchean to Mesoarchean (3.6–3.1 Ga) detrital zircons has been inferred as orthogneisses and granitoids of the southern Zimbabwe Craton (e.g., Tokwe Segment). The oldest Eoarchean detrital zircon (3828 Ma) obtained from the Phikwe Complex suggests the presence of unknown older crust in this provenance.

Neoproterozoic amphibolite-facies metamorphism of the Douling complex in the northern Yangtze Craton and its tectonic implications: Constraints from petrology and zircon U-Pb-Hf-O isotopes

Neoproterozoic evolution of the Yangtze Craton has long been debated. The Douling complex, representing one of the oldest crystalline basements, records Neoproterozoic high-grade metamorphism and may provide pivotal clues into a reliable Neoproterozoic configuration of the Yangtze Craton. However, its metamorphic evolution and tectonic significance have not been well constrained. In this contribution, we conduct an integrated study of petrography, zircon U-Pb age and Hf-O isotopic compositions for two garnet amphibolites and one gneiss from the Douling complex. Phase equilibrium modeling and conventional geothermobarometry calculation define peak P-T conditions of 9–10 kbar and 670-690 °C, and shape a clockwise P-T path. Zircon grains from the gneiss mostly reveal core-rim structure. The cores display oscillatory zoning, high Th/U ratios and trace element contents, as well as HREE-enriched patterns, high 176Lu/177Hf ratios and δ18O values, consistent with a magmatic origin. They yield a weighted mean 206Pb/238U age of 868 ± 9 Ma, which is taken as the protolith age. The cores have strongly positive ƐHf(t) values of 7.0–15.6 and young two-stage Hf model ages of ca. 1310–760 Ma, indicating that the protolith was dominantly sourced from prompt reworking of Meso-Neoproterozoic juvenile crust. On the other hand, the zircon overgrowth rims in the gneiss exhibit no zoning patterns, low Th/U ratios and trace element contents, flat HREE patterns and pronouncedly negative Eu anomalies, variable 176Lu/177Hf ratios and comparable ƐHf(t) values to the magmatic cores, indicating their coeval formation with garnet and plagioclase under amphibolite-facies metamorphism and might be generated by dissolution-reprecipitation. Zircon crystals from the two amphibolites perform weak zoning patterns, low Th/U ratios and trace element contents, high formation temperatures, flat to weakly enriched HREE patterns, insignificantly and/or clear negative Eu anomalies, and low 176Lu/177Hf ratios and ƐHf(t) values, suggesting that they may form near the peak stage of amphibolite-facies metamorphism. Metamorphic zircon in the three samples yield homogeneously high δ18O values (10.81 ± 0.17‰, 11.45 ± 0.40‰, 10.92 ± 0.08‰), indicating the participation of external fluid originated from surrounding metasedimentary rocks. These metamorphic zircon grains together yield a weighted mean age of 817 ± 4 Ma (MSWD = 0.63), which is taken as the best estimated age for the upper amphibolite-facies metamorphism in the Douling complex. Considering the widespread contemporaneous magmatism in the northern Yangtze Craton, we propose that the Douling complex recorded a tectonic regime switch from compression to extension at ca. 830–820 Ma, in response to outboard oceanic subduction on the periphery of the Yangtze Craton and then tectonic extension and collapse of the thickened lithosphere.

Gahnite, garnet and magnetite compositions of metamorphosed sediment-hosted Zn-Pb-(Cu[sbnd]Ag) deposits of the Mesoproterozoic Nova Brasilândia Group: Vectors for SEDEX deposits with Broken Hill-type affinities in the western Amazonian Craton, Brazil

Stratiform and stratabound Zn-Pb-Ag sulfide deposits hosted in siliciclastic sedimentary rocks constitute some of the world's largest accumulations of base metals. This research paper is the first to describe the DM-1 Zn-(PbAg) and PQ Zn-(CuAu) sulfide deposits, located about 20 km apart in the Mesoproterozoic Nova Brasilândia belt at the southwest margin of the Amazonian Craton. These deposits occur in a NW-SE trending sequence of metamorphosed siliciclastic rocks. The Zn-rich mineralization in both deposits occurs as massive, semi-massive and banded sulfide orebodies concordant with the layering of the country rocks. The DM-1 deposit (1.2 Mt at 12 wt% Zn) has up to 30 m thick zones of sulfide ore consisting predominantly of sphalerite, galena, pyrrhotite and pyrite while the PQ deposit (5 Mt at 5.6 wt% Zn) contains up to 20 m thick zones of sulfide ore consisting mainly of pyrite, pyrrhotite, and sphalerite along with minor chalcopyrite. The host rocks of both deposits consist of schist, gneiss and quartzite with variable modal proportions of garnet, sillimanite, gahnite, magnetite and biotite. These host rocks are up to 50 m thick and commonly show gradational contacts with country rocks. Country rocks, Zn-rich orebodies and their host rocks were overprinted by a post-ore upper amphibolite facies regional metamorphic event.Gahnite compositions of the PQ and DM-1 deposits are characterized by high contents of Zn (68 to 81 % of the gahnite end member) and moderate contents of Fe+2 and Mg. Their major elements compositions plot within the field for metamorphosed massive sulfide deposits hosted by hydrothermally altered Fe-Al-rich metasedimentary and metavolcanic rocks. Moreover, they display compositional trends like those reported for Pb-Zn-Ag deposits in the Broken Hill Domain, Australia. Gahnite from the DM-1 and PQ deposits shows unusually low Cd concentrations (<0.04 ppm), which are 1 to 2 orders of magnitude lower than values reported for gahnite in other Zn-rich sulfide deposits. Very high (from 400 to 900 ppm) Cr concentrations were obtained in gahnite from the PQ deposit. Garnet from the DM-1 and PQ deposits varies from Mn-rich (30 to 62 % of the spessartine end member) in samples from the ore zone or closely associated sulfide-rich host rocks to Mn-poor in sulfide-poor host rocks (<5.5 % of the spessartine end member). Magnetite crystals from ore samples from both deposits have distinctively higher MnO contents than those from host rocks, which follows the Mn-rich and Mn-poor garnets distribution. Our results support the concept that the DM-1 and PQ deposits share common features with BHT deposits as well as some SEDEX deposits affected by high-grade metamorphism, suggesting that they may be classified as clastic-dominated SEDEX deposits with BHT affinities. In addition, gahnite in heavy mineral concentrates is the best pathfinder for regional exploration, and the identification of Mn-rich garnet and/or Mn-rich magnetite in heavy mineral concentrates or preliminary drilling may be used as a proximal pathfinder for Zn-rich sulfide ore.

The tectonic evolution of Thelon tectonic zone, Canada: a new model based on petrological modeling linked with Lu–Hf garnet and U–Pb accessory mineral geochronology

Petrological modeling and geochronology (Lu–Hf garnet, U–Pb monazite, zircon, and titanite) for seven mid-amphibolite to granulite-facies metamorphic rocks in the Thelon tectonic zone (Ttz) provide new insights into the evolution of this orogen. A Grt-Sp-Sil diatexite records 2.01–2.00 Ga garnet and monazite growth during &gt;830 °C contact metamorphism associated with voluminous convergent margin plutonism. The most widespread, 1.96–1.90 Ga metamorphism is associated with clockwise pressure ( P)–temperature ( T) paths, indicating it was driven by crustal thickening. Earlier (1.96–1.92 Ga) and lower temperature (630 °C–730 °C; 6–8 kbar) metamorphism in the eastern Slave craton contrast with ∼860 °C (7.5 kbar), 1.91–1.90 Ga diatexites in the central, Paleoproterozoic plutonic rock-dominated, core of the Ttz. These differences are interpreted to reflect lower vs. upper plate settings, respectively. A contribution of mantle heat is suggested by the counter-clockwise P–T path and 885 °C (5.7 kbar) conditions recorded by diatexite associated with c. 1.9 Ga peraluminous leucogranite that dominates a corridor along the Ttz–Rae craton boundary. A tectonic model is proposed wherein the Ttz evolved at an accretionary margin after rifting of Ttz microcontinent (mTtz) off the Rae craton at c. 2.2–2.1 Ga. Voluminous plutonism at 2.01–1.98 Ga formed via east-dipping subduction under mTtz. Following 1.97 Ga collision of the Slave craton and mTtz, west-dipping subduction generated 1.96–1.95 Ga plutonism in the composite mTtz-Slave upper plate. Collision of Rae craton at 1.95–1.94 Ga led to crustal thickening, widespread 1.91–1.90 Ga high-grade metamorphism and extensive crustal melting in the central Ttz.

Multiple timings of garnet-forming high-grade metamorphism in the Neoproterozoic continental collision zone revealed by petrochronology in the Sør Rondane Mountains, East Antarctica

The Sør Rondane Mountains (SRM), East Antarctica is located in a key area of Gondwana formation where the East African-Antarctic Orogen and the Kuunga Orogen cross. Timing of peak metamorphism has been considered at ca. 650–600 Ma in the SRM, on which previous tectonic models were built. In this study, timing of garnet-forming metamorphisms is determined using in situ U-Pb dating of zircon and petrochronological approach utilizing distribution coefficient of rare earth elements (DREE) between zircon and garnet and REE patterns of zircon. As a result, we obtained several different periods of garnet-forming metamorphism at ≥600 Ma and ≤580 Ma both from the NE- and SW-terranes. This is the first report of multiple timings of garnet formation detected within single samples from the SRM. The latest garnet formation in each sample is probably caused by regional metamorphism, because the garnets accompany pressure shadows and are wrapped by the penetrative foliation defined mainly by the arrangement of biotite. Therefore, multiple timings of garnet formation during a single metamorphism or polymetamorphism are both likely. In the former case, the SRM may be affected by a single long-lived regional metamorphism. This implies that the regional metamorphism belongs only to the East African-Antarctic Orogeny or that the East African Orogeny continuously transformed into the Kuunga Orogeny in the deep crust. In the latter case, the former regional metamorphism corresponds to the East African(-Antarctic) Orogeny, whereas the latest metamorphism may correspond to the final stage of East African-Antarctic Orogeny or the Kuunga Orogeny. Any temperature drops during these two garnet-forming timings support the polymetamorphism, which needs to be cleared in the future works.

Exotic mineral products from the Chupa gneisses of the Belomorides

Exotic mineral products (EМP) in the Chupa paragneisses of the Belomorian rock complex occur as an association of submicroscopic “spherules”, “pipes”, “chips” and their combined varieties. EМPs were derived by multi-stage allochemical stress-metamorphism of host gneisses at high pressure in temperature regimes typical of amphibolite (in the Late Archean) and epidote-amphibolite (in the Early Proterozoic) facies. One distinctive feature of EMPs is the presence of carbon in all of their mineral phases. The metal-like substance of “chips” and the cores of “spherules” and “pipes” is comparable in chemical composition and the Ме/С index (Ме = Fe + impurity elements) with known iron carbides, and is similar in composition to terrestrial minerals, such as chalypite, yarlongite, cohenite, and haxonite. The average bulk carbon content of core-free EMPs and core margin substance (at.%) is 7 for Archean and 7-11 for Proterozoic rocks. The average carbon content of iozite, a mineral phase prevalent in “spherules” and “pipes”, is 11 at.% for Archean and 14 at.% for Proterozoic units. The carbon content of the mineral fragments of host rocks (quartz, plagioclase, garnet, and kyanite), which form inclusions in EMPs, is 7-20 to 46 at. %. The distinctive characteristics of the EMPs, publications on the gas composition and soluble carbonaceous endogenic material of gas-liquid inclusions in the rock-forming minerals of Chupa gneisses, and the presence of graphite therein provide evidence for the significant, yet poorly understood contribution of carbon and its chemical compounds as fluid constituents to the highgrade metamorphism of the Belomorides.

Sulfide anatexis during high-grade metamorphism: a case study from the Bodenmais SEDEX deposit, Germany

Many sedimentary-exhalative (SEDEX) sulfide deposits have been subject to regional metamorphism and, if the metamorphic grade was high enough, this could have resulted in sulfide anatexis. Although experiments and textures indeed showed that some deposits were partially molten, there is an ongoing debate as to the extent to which metamorphosed ore deposits were molten. Since some SEDEX deposits underwent amphibolite to granulite facies metamorphism, not only sulfides but also the host silicate rocks should have reached anatectic conditions. Due to the two immiscible silicate and sulfide melts, the formation of typical mingling and emulsion textures, as already known from magmatic sulfide deposits, should form. To test this hypothesis, we investigate sulfide-silicate textures from the granulite-facies Bodenmais SEDEX deposit (Germany). Textures from Bodenmais are similar to magmatic sulfide deposits including sulfide-matrix breccia, emulsion textures, pegmatitic leucosomes, and massive sulfides overlain by net-textured intergrowths of refractory quartz, which is interpreted to be a relic of silicate anatexis. Minerals crystallized during the interaction of both immiscible melts differ in their chemistry compared to the same minerals found in the adjacent migmatitic host rocks: for example, garnet in sulfides is Mn-rich (spessartine), but Fe-rich (almandine) in the migmatites and sulfide-enclosed cordierite is more enriched in Mg (Mg/(Mg + Fe): 0.84) than migmatitic cordierite (Mg/(Mg + Fe): 0.54). The textures themselves, their spatial arrangement within the deposit, the differences in mineral chemistry, and the observed crystallization sequence provide unequivocal evidence that the sulfides at Bodenmais were molten to a large extent under granulite facies conditions.

The polyphase evolution of the mafic rocks of the Juiz de Fora Complex: The record of two supercontinent cycles

The Juiz de Fora Complex (JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, is a basement unit displaying a variety of rock types that is tectonically imbricated with the supracrustal rocks of the Raposos Group in the Upper Thrust System of the Occidental Terrane, Ribeira Belt. Here we report new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data on mafic garnet-granulites from the JFC with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. This work presents the most comprehensive metamorphic study on the Juiz de Fora Complex rocks to date. Metamorphic P-T estimates for the metamorphic peak conditions of 800–900 °C and 8–9 kbar in basic garnet-bearing granulites were obtained for the JFC, indicating that metamorphism took place at deeper crustal levels. Geochemical, zircon U–Pb geochronology, whole-rock Sm–Nd, Rb–Sr isotopes, and Lu–Hf isotopic data of mafic granulites yielded U–Pb ages of ca. 2201 Ma, and moderately juvenile εNd(t) = −0.5 and were obtained from the JFC marking the timing of volcanic arc magmatism during the Rhyacian period. This arc-related stage was followed by the post-collisional period during the Early Paleoproterozoic with ca. 1.94 Ga E-MORB signature granulites that shows evidence of magma sourced from Neoarchean (∼2.8 Ga) crust with addition of juvenile components. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which placed the Neoproterozoic metasediments and the JFC rocks at the same level. The integrated information sheds light on two collisional episodes where the oldest is represented by arc-related granulites during the Rhyacian Orogeny and the youngest is the higher-pressure Neoproterozoic overprint during the Brasiliano Orogeny.

Crustal evolution of the Laurentian continental margin from the Paleo- through Mesoproterozic: A zircon U–Pb and Hf transect through the western Grenville Province, Ontario, Canada

The Great Proterozoic Accretionary Orogeny along the southern and southeastern margin of Laurentia is arguably the first long-lived, modern-style active plate margin on Earth, representing significant crustal growth and reworking. Here, we present new detrital zircon U–Pb and Hf data, along with zircon and monazite metamorphic ages, from tectonic domains in the southwestern Grenville Province that represent depositional basins formed and reworked during a series of events between 2.2 and 1.2 Ga. As such, the data allow a glimpse into the evolution and assembly of this extensive margin. The Nepewassi domain contains Archean basement overlain by mature sandstone characterized by near-unimodal, ca. 2.7–2.6 Ga detrital zircon peaks and likely correlative with Superior Province basement and its Huronian cover. The Nepewassi domain underwent both Archean (ca. 2.65 Ga) and Killarnean (ca. 1.76 Ga) metamorphism. In addition, all units preserve evidence of Grenvillian high-grade metamorphism between ca. 1.05 and 0.99 Ga. The structurally highest parts of the Nepewassi domain also contain younger sedimentary rocks that received detritus from Killarnean sources, with evolved Hf isotopic compositions indicative of reworked Archean crust. Sedimentary rocks in the Tomiko domain preserve detrital zircon of Penokean and Killarnean age (1.9–1.7 Ga) with evolved Hf compositions, along with Labradorian (1.7–1.6 Ga) grains of more juvenile composition. Detrital zircon from the Kiosk domain yield Penokean through Labradorian ages, with Hf compositions ranging from evolved to juvenile. In contrast to the sedimentary rocks found in these Laurentia-derived parautochthonous domains, similar rocks from younger, more allochthonous units, including the Shawanaga and basal Parry Sound domain, dominantly contain zircon ranging from Pinwarian (ca. 1.45 Ga) through ca. 1.20 Ga that show exclusively juvenile isotopic compositions. The data show that sedimentary basins received detritus from nearby active sources as well as from the nearby Superior continent, and that successively younger sources and basins became increasingly less influenced by continental input as the margin grew wider. The data add to a growing database of similar data from other parts of this several thousand-kilometer-long margin that seemingly displayed little along-strike variation in overall evolution, implying the existence of a vast, Pacific-scale ocean.

Zircon geochronology of high-grade metamorphic rocks from outcrops along the Prince Olav Coast, East Antarctica: Implications for multi-thermal events and regional correlations

This paper reports laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb ages of a suite of high-grade metamorphic rocks collected from Sinnan Rocks, Akebono Rock, Niban Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock along the Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The dating results indicate that a newly detected ~ 990 Ma metamorphism of garnet–sillimanite–biotite gneiss from Niban Rock. A thermal event at 931.7 ± 9.8 Ma is recorded in zircon from staurolite-bearing garnet–gedrite–biotite–chlorite gneiss in Akebono Rock. The metamorphic zircon grains in other analyzed samples provide Ediacaran to Cambrian ages. Their multi-growth textures and age populations are possibly interpreted to exhibit three metamorphic stages during >600–580 Ma, 580–550 Ma, and 550–500 Ma. Combined with previous reports, the metamorphic rocks in Cape Hinode, Niban-nishi Rock of Niban Rock, and Akebono Rock might have experienced earlier high-temperature metamorphism at ~ 990–930 Ma without younger overprinting. Extensive high-grade metamorphism during ~ 650–500 Ma is recorded from not only the granulite-facies zone in the west of the LHC but also the amphibolite-facies zone in its east. The main metamorphic episode in the LHC is likely to be subdivided into a preceding thermal event (either independent single metamorphic event or prograde stage) at pre-580 Ma, near-peak condition stage during 580–560 Ma, and subsequent retrograde stage after 550 Ma. In regional context this indicates that the assembly at the central Gondwana started with the collision of early and late Neoproterozoic terranes prior to 580 Ma, as a part of the East Africa-Antarctic Orogeny. Subsequent collisions took place among late Neoproterozoic igneous terrane, above terranes collided at pre-580 Ma, and Neoarchean terrane, which were probably driven by the Kuunga Orogeny.

Carboniferous mafic-ultramafic intrusions in the Eastern Pontides (Pulur Complex): Implications for the source of coeval voluminous granites

This study deals with the age and petrogenesis of mafic-ultramafic intrusions ranging in size from a few meters to 10 km within the Early Carboniferous high-grade gneisses of the Pulur Complex in the Eastern Pontides. The intrusions comprise dunite, wehrlite, gabbronorite, leucogabbro, anorthosite and ilmenite-bearing gabbronorite of cumulus origin, and are crosscut by dikes of ilmenite-bearing gabbronorite, leucogranite and microdiorite. U Pb dating on zircons from gabbronorite, anorthosite and leucogranite yielded igneous crystallization ages of 322–326 Ma, indicating that the intrusions were emplaced ca. 5–7 Ma after the peak of high-grade metamorphism, and form part of the Late Carboniferous high-volume magmatism in the region. In most cumulate rocks, Cr Al spinel, olivine and plagioclase were early crystallizing phases, followed by orthopyroxene, clinopyroxene and hornblende. Whole rock geochemical data suggest that wehrlite, gabbronorite, leucogabbro and anorthosite stem from a common magma, and ilmenite-bearing gabbronorite and dikes of leucogranite and microdiorite from different magmas. Application of mineral/melt partition coefficients to trace element compositions of clinopyroxene and hornblende in cumulate rocks suggests that the main cumulate body was derived from middle- to high-K calc-alkaline basic melts, and relatively late ilmenite-bearing gabbronorites from hypersthene-normative Ca-rich melts. All the rock types display radiogenic Sr and Pb isotopic signatures, and unradiogenic Nd isotopic ratios, which are indistinguishable from those of the coeval voluminous high-K calc-alkaline I-type granites in the region; the isotopic ratios are probably related to the metasomatism of the lithospheric mantle by sediment-derived melts. We suggest that the parental melts of the mafic-ultramafic intrusions and those of the high-K calc-alkaline granites were genetically related, and melts of the high-K calc-alkaline granites were probably derived from the melting of newly underplated calc-alkaline basic material at lower crustal depths, that were compositionally comparable to the parental magmas of the mafic-ultramafic intrusions. • The Pulur mafic-ultramafic intrusions (NE Turkey) formed at 326–322 Ma. • They are part of the Late Carboniferous high-volume magmatism. • Parental melts of the mafic-ultramafic intrusions were middle- to high-K calc-alkaline basic melts. • The high-volume magmatism probably resulted from the convective thinning of the lithosphere.

Exceptional preservation of organic matter and iron-organic colloidal mineralization in hydrothermal black smoker-type sulfide mineralization from the Paleoarchean seafloor

Fossil organic matter (OM) in Paleoarchean rocks is an invaluable tracer of ancient life, yet it is often contentious due to low preservation potentials of its original organic molecular characteristics under generally high metamorphic grades. This study reports on exceptionally preserved OM within black smoker-type sulfide mineralizations from the 3.24 billion-years-old Sulphur Springs volcanic-hosted massive sulfide deposit, East Pilbara Terrane, northwestern Australia. Fine scale mineralogical and organic molecular variations – documented by SEM and TEM techniques, Raman and FTIR Spectroscopy, as well as ToF-SIMS analysis – trace the formation of millimetre-scale pyritic hydrothermal orifices. Overmature OM (≳200–250 °C) enriched in aromatic hydrocarbons occurs within earliest formed colloform-banded pyrite that could have precipitated at high temperature upon mixing of hot hydrothermal fluids with cold seawater. In contrast, mature OM (within the catagenesis window; ∼100–150 °C) with lower proportions of aromatic hydrocarbons occasionally occurs alongside pyrite within barite near the inner paleofluid conduits of the hydrothermal orifices. A deposition of this OM generation from cooler fluids enriched in sulfate during waning hydrothermal activity is corroborated by its association with original mineralogy, particularly the occurrence of isolated nanoinclusions of OM within hydrothermal barite away from grain boundaries or fissures and cracks, which renders impossible an origin from post-depositional hydrocarbon migration. When compared to the overmature OM within colloform-banded pyrite, the better-preserved inner OM not only contains less abundant, smaller, and less ordered aromatic hydrocarbons, but also higher amounts of aliphatic molecules with longer chain lengths and higher proportions of carbonyl and amide functional groups. Although the latter characteristics are consistent with contributions by microbial biomass, abiotic origins are equally plausible because hydrothermal processes can produce OM with similar organic molecular attributes. Irrespective of these uncertainties on the ultimate sourcing of OM, common intergrowths of the cooler OM with nanoscopic iron oxides – hematite and lesser magnetite – hint at its hydrothermal deposition from colloidal particles that have been stabilized by iron-organic complexation. Further research is required to ascertain the significance of this process for the hydrothermal cycling and release of organic carbons and bound iron into the Paleoarchean oceans. Collectively, our results show that ancient marine hydrothermal systems can preserve OM with differing degrees of thermal degradation, which allows for insights into its sourcing, cycling, and deposition.

Arc-like magmatism in syn- to post-collisional setting: The Ediacaran Angra Fria Magmatic Complex (NW Namibia) and its cross-Atlantic correlatives in the south Brazilian Florianópolis Batholith

Ediacaran syn-tectonic plutonic rocks (amphibole gabbros, quartz diorites/tonalites to biotite- and muscovite-bearing granites) of the Angra Fria Magmatic Complex (Kaoko Belt, north-western Namibia) belong to two compositionally similar, magnesian, transitional tholeiitic–calc-alkaline suites, the Older (∼625–620 Ma) and the Younger (∼585–575 Ma). Both have counterparts in the broadly contemporaneous Florianópolis Batholith (southern Brazil), from which they were separated during the Cretaceous opening of the southern Atlantic. In the Angra Fria Magmatic Complex, the only unequivocal mantle contributions are identified in mingling zones of the Younger Suite and hybrid mafic–intermediate dykes of uncertain age. Previously published Hf-in-zircon isotopic data, together with new whole-rock geochemical and Sr–Nd isotopic signatures, underline an important role of crustal anatexis of a material with late Palaeoproterozoic to early Mesoproterozoic mean crustal residence (1.9–1.5 Ga). This interval resembles some of the published Nd model ages for Tonian ‘Adamastor Rift’-related felsic magmatic rocks in the Namibian Coastal and Uruguayan Punta del Este terranes. In detail, the Older Suite probably originated mainly by fluid-present melting of metabasalts and metatonalites, followed by (near) closed-system fractional crystallization (with or without accumulation) of amphibole ± plagioclase. For the Younger Suite, the principal process was the dehydration melting of relatively felsic lower crustal protoliths (metagreywackes or intermediate–acid orthogneisses >> metapelites), leaving garnet in the residue. Based on the geological context, the conspicuous enrichment of hydrous-fluid-mobile large ion lithophile over the conservative high field strength elements is not interpreted through a classic model of oceanic plate subduction, devolatilization, and fluxed-melting of the overriding mantle wedge. Instead, it is thought to reflect high-grade metamorphism of deeply buried continental crust and attendant water-fluxed melting of the overlying crustal lithologies, connected with inversion of the Tonian ‘Adamastor Rift’.

Anticlockwise metamorphic paths at ca. 890–790 Ma from the NE Baidrag block, Mongolia, indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neoproterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic–early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ∼720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ∼750 °C and ∼9 kbar and decompression to ∼650 °C and ∼8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ∼620 °C and 6 kbar, followed by further burial to ∼590 °C and 8.5 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926–768 Ma in the Grt-Sil-Ky gneiss, and ca. 937–754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27–32 °C/km is estimated at ca. 890–853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18–22 °C/km dated at ca. 835–815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568–515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ∼ 0.01–0.06 in Grt-Sil-Ky gneiss with 877 ± 7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice – probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri-Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal.

Zircon inheritance, sources of Devonian granitic magmas and crustal structure in central Victoria

In central Victoria, inherited zircon in Devonian igneous rocks and detrital zircon in metasedimentary country rocks and an amphibolite-facies xenolith show that Mesoproterozoic parts of the underlying Selwyn Block cannot be the source for all the silicic magmas. Zircon inheritance in S-type samples reveals significant thermal events at 525–425 Ma and 1200–1100 Ma. Both S- and I-type samples have prominent zircon age peaks at 420–410 Ma, which record high-grade metamorphism of the deep crust during the terminal phases of the Benambran and Bindian orogenies. All I-type rocks have 650–500 Ma peaks, suggesting derivation from an arc-related metavolcanic source in the upper Selwyn Block. Protoliths of the greenschist-facies Ordovician metasediments and the amphibolite-facies Cambrian metasedimentary xenolith were deposited in distal backarc settings. Most inherited zircon cores are metamorphic, and the strongest zircon inheritance occurs in hornblende-bearing I-type rocks, highlighting their largely crustal origin. Zircon populations at ca 1400 Ma, thought to signal sediment derivation from East Antarctica and Rodinia-Nuna, are mostly absent in I-type samples and some S-types. The ca 1400 Ma signal probably applies to the upper, metasedimentary Selwyn Block, so Devonian S-type magmas were sourced mainly in the deeper sections. Zircon inheritance in the Devonian igneous rocks was not influenced by the exposed metasedimentary country rocks. Two samples from one of the smaller plutons have contrasting patterns of zircon inheritance, suggesting relatively small-scale source heterogeneity. Many rounded and corroded cores in zircon crystals yield the same ages as the crystallisation dates for the rocks, and thus are antecrysts. Higher whole-rock Zr contents generally correlate with higher proportions of inherited zircon, and differentiation does not affect this relationship. The degree of partial melting of a magma source and the efficiency of crystal entrainment are critical in governing zircon inheritance.

Rhyacian evolution of high-grade metamorphic rocks of the Porto Nacional Granulite Complex, based on geocronological data U-Pb-Hf in zircon and U- Pb in monazite

The Porto Nacional Complex in the northern portion of the Goiás Massif, in north-central Brazil, represents a Paleoproterozoic/Archean microcontinent reworked in the Neoproterozoic during the Brasiliano Cycle. This NE-SW-trending belt of high-grade metamorphic rocks consists, predominantly, of orthogranulites in addition to aluminous paragneisses that reached high-grade metamorphic conditions in the granulite facies (∼860 °C and 9 kbar). The zircon U–Pb geochronological results obtained on orthogranulite (enderbite) crystal cores yielded an age of 2.16 Ga, however, the crystal borders present lower values (2.09 Ga). These results represent, respectively, the age of igneous crystals of tonalitic protoliths, and high-grade metamorphism. The zircon crystals of another enderbite sample yielded lower values of about 2.09 Ga, reinforcing the interpretation of metamorphic zircons. The U–Pb geochronological results between 2.09 Ga and 2.10 Ga obtained on monazite from the aluminous paragneisses, coincide with the values obtained for enderbite metamorphic zircon crystals, allowing us to define the high-grade metamorphism age and the formation of igneous protoliths in the Rhyacian. This small age difference demonstrates the metamorphism role contemporary to the magmatism of the tonalitic protoliths of the Porto Nacional Complex, related to the Late-Transamazonian orogeny. The Hf-TDMC model ages suggest two crust formation episodes that gave rise to the following rocks: one from the Siderian (2.40–2.48 Ga) and another from the Neo-Mesoarchean (2.52–3.01 Ga). The positive petrogenetic parameters ƐHf(t) (+3.9 to +5.2) evidence a mantle source derivation for the Siderian material, while the negative and positive petrogenetic parameters ƐHf(t) (−4.6 to +3.3) indicate a mixture between crustal and juvenile material for the Neo-Mesoarchean material. These results allow us to disregard the proposed correlation between this high-grade metamorphic terrain with the Porangatu-Talismã Granulitic Belt, which occurs to the south of this region, whose thermo-tectonic evolution is related to the late Neoproterozoic. On the other hand, there are records of several other Rhyacian similar granulitic terrains, such as the granulites of the Bacajá and Tartarugal Grande in the Amazonian Craton, the Itabuna-Salvador-Curaçá Granulitic Belt in the São Francisco Craton, as well as in the Bakhuis Belt, in Suriname, and Limpopo Belt in the West African Craton.

Genetic linkage between carbonaceous materials and Triassic gold deposits in the Liaodong Peninsula, northeastern North China Craton: Insights from Raman spectroscopy and C[sbnd]O isotope analysis

The Qingchengzi gold ore-concentration district, in the Liaodong Peninsula, northeastern North China Craton, is dominated by altered rock-type gold deposits and hosted within metamorphic rocks of the Paleoproterozoic Liaohe Group. The deposits are mainly controlled by a low-angle, north-dipping thrust structural system, containing numerous carbonaceous materials (CMs) and syn-deformational ore-bearing quartz veins and adjacent clastic rocks. Raman spectroscopy and CO isotope analysis were performed on CM samples from the fault zone to decipher their possible origins and potential genetic links with gold mineralization. Raman spectra analysis distinguishes three types of CM with low (CML), medium (CMM) and high (CMH) crystallinity and maturity. The estimated Raman-derived temperatures of CML range from 308 °C to 401 °C corresponding to greenschist- to blueschist-facies metamorphism. Large temperature variations of 363–678 °C for CMM and 442–664 °C for CMH suggest higher metamorphic grades from low blueschist to amphibolite-facies. The organic carbon isotopic valuess (δ13Corg = -20.9 ‰ – –23.1 ‰) suggest an organic carbon source, mixing with carbonate wall rocks characterized by the δ13Ccarb of −1.92 ‰ to −6.22 ‰. Combined with the Raman spectrum, the CMs are formed by the demethanation of the organic matter during the greenschist- to amphibolite-facies metamorphism. The disseminated CML coexists with Au-bearing quartz veins and is inferred to have facilitated the migration of Au in the fault zone as a solid lubricant and the precipitation of Au from hydrothermal fluids as a reductant. CMM and CMH are recognized in medium- to high-grade metamorphism with higher temperatures and occur with fewer Au-bearing pyrite or quartz veins, and thus they are unrelated to gold mineralization. In summary, the disseminated CML with low crystallinity contributes to the formation of the Triassic gold deposits and serves as a potential indicator for further prospecting.

Hot metamorphic complex in the Foreland Zone of the Variscan chain: insights from the Monte Filau orthogneiss (SW Sardinia), Italy

The Mt. Filau Orthogneiss belongs to the low-pressure (LP) - high-temperature (HT) Sulcis Metamorphic Complex located in the foreland zone of the Variscan belt (southern Sardinia). It consists of a core of orthogneiss and high metamorphic grade host rock tectonically juxtaposed below weakly- to non-metamorphic tectonic units, by the Tuerredda ductile shear zone. Our new petrographic and microstructural data on the fine–grained leucocratic facies of the Mt. Filau Orthogneiss are consistent with metamorphic peak under conditions of incipient partial melting in the sillimanite stability field at 680–700°C and 0.32–0.46 GPa. The retrograde P–T path occurred in the andalusite stability field, according to the growth of andalusite after sillimanite in leucocratic facies. The P-T conditions of metamorphic peak and incipient melting are both consistent with development, in the foreland zone, of a thermal anomaly accompanied by the upward rise of deep hot crust in the footwall of the Tueredda ductile shear zone. This transtensive shear zone acted as preferential channel for upward advection of deep hot melts during tectonically-driven isothermal decompression of middle crust. Therefore, we interpret the Sulcis Metamorphic Complex as the result of a tectonically controlled thermal perturbation, and underline the role of this late Variscan crustal scale tectonic structure in the development of hot metamorphic complex in an anomalous position within the foreland zone of the Variscan collisional belt.

The Fate of Pyroxenes in Mafic Xenoliths from the Kinnaur Kailash Granite, Sutlej Valley, NW Himalaya: Effect of Retrograde Hydration and Insights on the Rare Occurrence of High-Grade Metamorphic Rocks in the Himalayan Orogen

Abstract Pseudosection modelling of post-peak mineral assemblages in metamorphosed mafic xenoliths from the Kinnaur Kailash Granite, Northwest Himalaya, has provided new insights into pre-Himalayan metamorphism and constrained the P–T conditions of hydration. P–T and P–MH2O pseudosections constructed in the NCFMASH system show that retrograde hydration occurred during destabilisation of orthopyroxene and clinopyroxene in mafic granulites after decompression to 0.3 GPa pressure, and as a result, water-containing phases such as cummingtonite and hornblende formed. Results from two of the studied xenoliths indicate that the secondary mineral assemblage is controlled not only by P and T, but also by the amount of water available for hydration. In contrast, another example indicates the development of cummingtonite from hornblende via dehydration reaction after decompression to conditions appropriate for cummingtonite stability. The study shows that the Higher Himalayan Crystalline Sequence (HHCS), from which the xenoliths have been derived, must have been subjected to pre-Himalayan high-grade metamorphism, although it is not evident because of pervasive overprinting by Himalayan metamorphism. The present-day metamorphic pattern of the HHCS is expectedly the result of both pre-Himalayan and Himalayan metamorphism, and careful petrological investigation is required to distinguish their signatures so that a better understanding of the tectonothermal evolution of the Himalayas can come into picture. Isotopic dating of the xenoliths is required to establish the timings of the pre-Himalayan metamorphism and decompression.