Search Results (42)

In the absence of appropriate tools and a knowledge base for exploring high-grade metamorphic terrains, felsic gneiss complexes at granulite facies have long been considered barren and have remained undermapped and understudied. This was the case of the Bondy gneiss complex in the southwestern Grenville Province of Canada which consists of 1.39–1.35 Ga volcanic and plutonic rocks metamorphosed under granulite facies conditions at 1.19 Ga. Iron oxide–apatite and Cu-Ag-Au mineral occurrences occur among gneisses rich in biotite, cordierite, garnet, K-feldspar, orthopyroxene and/or sillimanite-rich gneisses, plagioclase-cordierite-orthopyroxene white gneisses, magnetite-garnet-rich gneisses, garnetites, hyperaluminous sillimanite-pyrite-quartz gneisses, phlogopite-sillimanite gneisses, and tourmalinites. Petrological and geochemical studies indicate that the precursors of these gneisses are altered volcanic and volcaniclastic rocks with attributes of pre-metamorphic Na, Ca-Fe, K-Fe, K, chloritic, argillic, phyllic, advanced argillic and skarn alteration. The nature of these hydrothermal rocks and the ore deposit model that best represents them are further investigated herein through lithogeochemistry. The lithofacies mineralized in Cu (±Au, Ag, Zn) are distinguished by the presence of garnet, magnetite and zircon, and exhibit pronounced enrichment in Fe, Mg, HREE and Zr relative to the least-altered rocks. In discrimination diagrams, the metamorphosed mineral system is demonstrated to exhibit the diagnostic attributes of, and is interpreted as, a metasomatic iron and alkali-calcic (MIAC) mineral system with iron oxide–apatite (IOA) and iron oxide copper–gold (IOCG) mineralization that evolves toward an epithermal cap. This contribution demonstrates that alteration facies diagnostic of MIAC systems and their IOCG and IOA mineralization remain diagnostic even after high-grade metamorphism. Exploration strategies can thus use the lithogeochemical footprint and the distribution and types of alteration facies observed as pathfinders for the facies-specific deposit types of MIAC systems. Full article

New U-Pb zircon ages are reported for granulite facies crustal xenoliths brought to the surface by mafic lavas in the Snake River Plain. All samples yield Meso-to-Neoarchean ages (2.4–3.6 Ga) that significantly expand the known extent of the Archean Wyoming Craton at least as far west as the west-central Snake River Plain. Most zircon populations indicate multiple growth episodes with complexity increasing eastward, but they bear no record of major Phanerozoic magmatic episodes in the region. To extrapolate this work further west to the inferred craton boundary, zircons from southwestern Idaho batholith granodiorites were also analyzed. Although most batholith zircons record Cretaceous formation ages, all samples have zircons with inherited cores—with some recording Proterozoic ages (approaching 2 Ga). These data enhance our perspectives regarding lithosphere architecture beneath southern Idaho and adjacent areas and its possible influence on Cenozoic magmatism associated with the Snake River Plain–Yellowstone “melting anomaly”. Full article

The Gangdese metallogenic belt in Tibet is an important polymetallic metallogenic belt formed during the subduction of the Neo-Tethys Ocean and subsequent India–Asia collision. Adakitic rocks are widely distributed in this belt and are considered to be closely related to porphyry–skarn Cu-Mo polymetallic mineralization. However, the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt remain controversial. In this study, we focus on the quartz diorite in the Nuri Cu-W-Mo deposit along the southern margin of the eastern Gangdese belt. LA-ICP-MS zircon U-Pb dating yields a Late Cretaceous age of 93.6 ± 0.4 Ma for the quartz diorite. Whole-rock geochemistry shows that the quartz diorite possesses typical adakitic signatures, with high SiO₂, Al₂O_3, and Sr contents, but low Y and Yb contents. The relatively low K₂O content and high MgO, Cr, and Ni contents, as well as the positive zircon εHf(t) values (+6.58 to +14.52), suggest that the adakites were derived from the partial melting of the subducted Neo-Tethys oceanic slab, with subsequent interaction with the overlying mantle wedge. The Late Cretaceous magmatic flare-up and coeval high-temperature granulite-facies metamorphism in the Gangdese belt were likely triggered by Neo-Tethys mid-ocean ridge subduction. The widespread occurrence of Late Cretaceous adakitic intrusions and associated Cu mineralization in the Nuri ore district indicate a strong tectono-magmatic-metallogenic event related to the Neo-Tethys subduction during this period. This study provides new insights into the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt, and has important implications for Cu polymetallic deposit exploration in this region. Full article

Pressure–temperature estimates of a xenolith found within a post-obduction granodiorite in southern New Caledonia provide evidence for subcrustal, granulite facies, peak crystallisation conditions (ca. 850 °C—8.5 ± 1.0 kbar), followed by isobaric cooling to 700 °C, and final decompression with partial rehydration at ca. 650 °C—3.5 kbar. The xenolith, dated at 24.7 Ma (U-Pb zircon), i.e., the same age as the granodiorite host rock, has low SiO₂ (35.5 wt%) and high Al₂O₃ (33.2 wt%) contents, suggesting that it is the restite of a previous melting episode, while the elevated Ca (Ba and Sr) contents suggest mantle metasomatism. Although the concentrations of Rb, K, Ca, Ba, and Sr have been strongly modified, some geochemical (REE patterns and some “immobile” trace element ratios) and isotopic (Sr and Nd isotopic ratios, U-Pb zircon age) characteristics of the granulite facies xenolith are similar to those of the xenoliths found in other Late Oligocene intrusions in southern New Caledonia; therefore, this rock is interpreted to be related to an early magmatic episode. The rock protolith was emplaced and equilibrated at the base of the crust where it underwent ductile deformation. Younger ascending magma picked it up and they eventually crystallised together at a shallow crustal level, near the tectonic sole of the ophiolite. The recrystallisation and ductile deformation at ~8.5 kbar suggest that a rheological discontinuity existed at about 25–28 km, probably representing the Moho. It is concluded that a continental crust of normal thickness must have existed beneath New Caledonia at about 24 Ma, i.e., 10 Ma after obduction. Full article

Granulite facies rocks provide important keys to evaluating collisional metamorphism in orogenic belts. The mafic granulites of Baoding in the Fuping Complex of the North China Craton occur within the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional orogen. Here, we present results from detailed investigations on newly discovered garnet pyroxenite, garnet two-pyroxene granulite, and garnet-bearing-plagioclase amphibolite using petrographic, mineralogical, geochemical, and zircon U-Pb dating methods. Our results show that the Fuping Complex metamorphic evolution in this study evolved in four stages: prograde (M₁), high-pressure granulite facies (M₂), granulite facies (M₃), and retrograde (M₄) stages. The mineral assemblage of the prograde stage (M₁) consists of Amp + Pl + Q within garnet cores. The mineral assemblage of high-pressure granulite facies at the peak stage (M₂) consists of Gt + Cpx + Pl + Q ± Amp, forming the garnet pyroxenite. The granulite facies stage M₃ is characterized by the occurrence of orthopyroxene, with a mineral assemblage of Gt + Cpx + Opx + Amp+ Pl + Q. The early retrograde stage M_4-1 includes clinopyroxenes scattered inside amphiboles, following the breakdown of garnet and clinopyroxene. The mineral assemblage of this stage comprises Amp + Pl + Q + Ilm ± Cpx. Later, in the late retrograde stage M_4-2, the composition of amphiboles changed to actinolite, and epidote and chlorite started to appear in the matrix. Traditional geothermobarometry yielded P-T conditions of 700~706 °C and 6.0~6.2 kbar for prograde stage M₁, 854~920 °C and 13.0~13.8 kbar for high-pressure granulite facies stage M₂, 912~939 °C and 8.1~9.9 kbar for M₃, 661~784 °C and 3.1~4.4 kbar for M_4-1, and 637~638 °C, 1.1~1.3 kbar for M_4-2, along a clockwise P-T path with a nearly isothermal decompression (ITD) and slight heating. Zircon LA-ICP-MS U-Pb dating constrains the timing of the high-pressure granulite facies metamorphic event to be between 1.83 and 1.86 Ga. Geochemical features suggest that the protoliths of the high-pressure granulites may have formed in an island arc environment within a convergent margin setting. Together with results from previous studies, our data suggest that the ~1.85 Ga metamorphic age recorded in the Fuping Complex represents a regional metamorphism in the TNCO, associated with the subduction–collision and assembly of the Eastern and Western Blocks of the NCC. Full article

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase ± K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825–910 °C/7.2–10.8 kbar, which is similar to that (800–870 °C/7.0–10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl–pl–qz thermobarometry yielded the metamorphic PT conditions of 732–810 °C/3.0–6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 °C, 768 °C and 780 °C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800–910 °C/7.0–10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl–pl–qz thermobarometer. In addition, LA-ICP-MS U–Pb analysis on metamorphic zircons yielded weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 1901 ± 22–1817 ± 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex. Full article

The Paleoproterozoic gneiss domes are important structures of the Khondalite Belt in the northwestern North China Craton. However, less attention has been paid to their formation and evolution, and it thus hampers a better understanding of the deformation history of the Khondalite Belt. In this paper, we conducted structural and geochronological studies on the Qianlishan gneiss domes of the Khondalite Belt. The field observations and zircon U–Pb dating results show that the Qianlishan gneiss domes consist of 2.06–2.01 Ga granitoid plutons in the core, rimmed by granulite facies metasedimentary rocks (khondalites) of the Qianlishan Group. Both of them were subjected to two major phases of deformation (D1–D2) in the late Paleoproterozoic. Of these, D1 deformation mainly generated overturned to recumbent isoclinal folds F1 and penetrative transposed foliations/gneissosities S1 at ~1.95 Ga. Subsequently, D2 deformation produced the NW(W)–SE(E)-trending doubly plunging upright folds F2 at 1.93–1.90 Ga, and they have strongly re-oriented S1 gneissosities, giving rise to the Qianlishan gneiss domes. Combined with previous studies, we argue that the Qianlishan gneiss domes were the products of the Paleoproterozoic collisional orogenesis between the Yinshan and Ordos Blocks. Additionally, the development of doubly plunging antiforms is considered an important dome-forming mechanism in the Khondalite Belt. Full article

Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamorphic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Permian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies conditions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corresponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with subduction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization. Full article

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (<modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En_69–74) and/or clinopyroxene (En_33–53Fs_1–14Wo_44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ¹³C_graphite in the graphite schist is −38 to −17‰; δ¹³C_calcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ¹³C_calcite = −8.7‰ to −9.5‰ and heavier values for δ¹³C_graphite = −11.5‰ to −8.9‰. δ¹⁸O_calcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration. Full article

Accretionary and collisional orogeny are often accompanied by the disturbance of the geothermal gradient, leading to high-temperature metamorphism. High-temperature metamorphic rocks are significant in their ability to help the reconstruction of the thermal histories of orogenic belts. The Tianshan Orogenic Belt, at the southwest margin of the Central Asian Orogenic Belt, is a record of the long-term subduction–collision–post-collision orogenic process that has taken place in the Phanerozoic Eon. Here, we report the discovery of mafic granulites in the Muzhaerte area, SW Tianshan. Petrographic observation reveals that the mafic granulites underwent two metamorphic stages. The peak mineral assemblage of the first stage is dominated by clinopyroxene + orthopyroxene + plagioclase + quartz + hornblende (hb¹) ± biotite, and the post-peak mineral assemblage of the second stage is dominated by clinopyroxene + plagioclase + quartz + hornblende (hb²) + biotite. The calculated results obtained from the two-pyroxene thermobarometers and the Al-in-hornblende barometer for the mafic granulites indicate that the metamorphic conditions of mafic granulites are 760–860 °C, <0.39–0.41 Gpa. The mafic granulites recorded a high-grade granulite facies thermal metamorphic event with the highest temperature limit currently recorded in the Central Tianshan Block. Full article

The Qinling Complex is located in the core of the northern Qinling Orogen and plays a key role in understanding the tectonic evolution of the Qinling Orogen, but its metamorphic evolution remains controversial. The combined investigation of petrographic observation, zircon U-Pb dating, and phase equilibria modeling for garnet amphibolites from the Tianshui area in the West Qinling Orogen is reported in this study. The results show that the garnet amphibolites record a clockwise P-T path characterized by a pre-T_Max decompression heating stage, a temperature peak at P-T conditions of 0.84–0.99 GPa and 869–886 °C, followed by a decompression cooling stage. Zircon U-Pb dating yields four age populations of ~479 ± 4 Ma, ~451 ± 8 Ma, ~411 ± 4 Ma, and ~377 ± 6 Ma. The 479–450 Ma reflects the timing of the pre-T_Max high–medium pressure upper amphibolite-facies metamorphism. The metamorphism at peak temperature condition occurred at c.411 Ma and was followed by decompression cooling to c.377 Ma. The Ordovician high–medium pressure metamorphism is related to the continental collision, which is slightly later than the HP–UHP eclogite-facies metamorphism in the East Qinling Orogen. The HT granulite-facies metamorphism at peak temperature condition took place at reduced pressures, suggesting thinning of the collision-thickened orogenic crust. Therefore, the northern West Qinling Orogen experienced a tectonothermal evolution from initial crust thickening to thinning during the Paleozoic collisional orogeny. Full article

The Jiao-Liao-Ji Belt (JLJB) is the most representative Paleoproterozoic orogenic belt in the North China Craton (NCC). The sedimentation, metamorphism and magmatism of the Ji’an Group and associated granites provide significant insights into the tectonic evolution of the JLJB. In this study, we have synthesized published geochemistry and geochronology data on metasedimentary, metavolcanic and igneous rocks. According to the available data, the protoliths of the metasedimentary rocks are sets of shale, wacke, arkose, quartz sandstone and carbonate, while the protoliths of the metavolcanic rocks are calc-alkaline basalt, basaltic andesite, andesite, dacite and rhyolite. The rock assemblages indicate a transformation of the tectonic environment from a passive margin to an active continental margin following the onset of plate convergence and subduction. The A2-type gneissic granite (Qianzhuogou pluton) is formed in a subsequent back-arc basin extension setting at 2.20–2.14 Ga. The Ji’an Group was finally deposited in an active continental margin during the closure of a back-arc basin at 2.14–2.0 Ga. Then, the sediments were involved in a continent–arc–continent collision between the Longgang and Nangrim blocks at ~1.95 Ga. This process was accompanied by HP granulite-facies metamorphism at ~1.90 Ga. The subsequent exhumation and regional extension resulted in decompression melting during 1.90–1.86 Ga, producing metamorphism with an isothermal decompression clockwise P–T path. The resulting metapelites are characterized by perthite + sillimanite, and mafic granulites are characterized by orthopyroxene + clinopyroxene. The S-type porphyritic granite (Shuangcha pluton) is formed during the crustal anatexis. Meanwhile, extensive anatexis produced significant heating and triggered prograde to peak metamorphism with an anticlockwise P–T path. Cordierite-bearing symplectites around the garnet in the metapelites indicate a superposed isobaric cooling metamorphism. The ages of monazites and anatectic zircons suggest that the post-exhumation cooling occurred at 1.86–1.80 Ga. The Paleoproterozoic magmatism, sedimentation and metamorphism suggest a process of subduction back-arc basin extension and closure, collision and exhumation for the tectonic evolution of the JLJB. Full article

The present study aims to investigate the textural, mineralogical and geochemical characteristics of lean grade placer monazite from the Bramhagiri beach sand deposit to assess the possibility for its use in industrial applications. The bulk back dune sand deposit with 18 samples showed the elements uranium and thorium in traces, phosphorus and calcium in minor amounts, and alumina, silica and titanium in major amounts. Since apatite was absent in this placer deposit, P and Ca were attributed to monazite only. Based on the chemical analysis, it was established that the monazite mineral exists in this deposit. The monazite is generally below the −150- to +90-micron size range, and the concentration of the monazite mineral in the bulk back dune sand is around 0.01% by weight. The structural data and complete chemical analysis established that the monazite is Ce-monazite. The monazites with other heavy mineral sands of the Bramhagiri beach placer deposits were derived from the Eastern Ghats, which closely resembles the mineralogical composition of khondalite, charnockite, leptynite and pegmatite groups of rocks. The Eastern Ghats’ provenance appears to be the primary source for the heavy mineral assemblages of the Bramhagiri placer deposit. Thus, these monazite sands are derived from the granulite facies of metamorphic rocks such as khondalites and charnockites from the Eastern Ghats group of rocks. Garnet is the major mineral, following ilmenite and sillimanite. Zircon, rutile and monazite are minor minerals in the deposit. All these minerals are well liberated and have uniform shapes with variable densities and size ranges, with different magnetic, electrical and surface properties. Hence, the occurrences of these heavy minerals are of economic importance. Further, these minerals can be recovered individually for industrial applications. Full article

Archean supracrustal rocks from the Qingyuan area of the northern Liaoning terrane, the North China Craton, occur as enclaves or rafts of various scales within tonalite–trondhjemite–granodiorite (TTG) domes. They were normally subjected to metamorphism at amphibolite facies with locally granulite facies. We collected biotite two-feldspar gneiss from the Hongtoushan of the Qingyuan area and conducted petrography, mineral chemistry, phase equilibrium modeling and monazite dating to reveal its metamorphic evolution. The peak condition was constrained to be 750–775 °C at ~7 kbar based on the stability of the inferred peak mineral assemblage and mineral compositions including the pyrite and grossular contents in the garnet core, and X_Mg in biotite. The final condition was constrained to be ~700 °C at ~6 kbar on the solidus based on the presence of muscovite in the final assemblage. The post-peak near-isobaric cooling process was consistent with the core→rim decreasing pyrite content in garnet. Monazite dating yielded a metamorphic age of ~2.50 Ga for the sample, coeval with the final magmatism of TTGs in the terrane. By combining other geological features, we suggest a vertical sagduction process to be responsible for the metamorphic evolution of the Qingyuan area. This process may be correlated with Archean mantle plume. Full article

Fluid and solid inclusions in magmatic garnet from Permian pegmatites of the Koralpe Mountains were investigated. On the basis of MnO/(MnO + FeO) ratios, different degrees of melt fractionation during garnet growth were linked with fluid inclusion densities and chemistries. It is shown that garnet indicating low-melt fractionation trends contained primary CO₂ ± N₂-rich fluid inclusions of the highest densities, up to 1.15 g/cm³, compared to garnet samples of increased fractionation trends comprising CO₂-N₂-rich fluid inclusions with lower densities up to 0.85 g/cm³. This fluid composition is interpreted as a part of an unmixed CO₂ ± N₂-H₂O-rich fluid that was present during garnet crystallization. Variabilities in the nitrogen composition up to 40.83 mol% resulted from different degrees of partial melting of mica and plagioclase from the metapelitic host rock. Densities, fluid chemistries, and mineral chemical data enabled a continuous upward trend for garnet crystallization during anatexis from lower (ca. 25 km) up to middle crustal levels (12–15 km). Resulting amphibolite/granulite facies conditions of 7.6 kbar/700 °C for garnet crystallization in spodumene-free pegmatites were significantly higher than previously suggested for pegmatite formation in the Koralpe Mountains. Full article