Granulite-facies Conditions Research Articles

Timescales of magmatism and metamorphism in the Connemara Caledonides: insights from the thermal aureole of the Dawros–Currywongaun–Doughruagh Complex, western Ireland

AbstractZircon separates from the contact aureole of the syn-tectonic Dawros–Currywongaun–Doughruagh Complex, western Ireland, are studied to constrain the nature and timing of magmatism associated with the early stages of the Grampian Orogeny. The samples analysed come from the uppermost part of the Dalradian Supergroup in northern Connemara (the Ben Levy Grit Formation), where a laterally extensive (>10 km) package of metamorphosed siliciclastic sedimentary rocks containing heavy mineral seams crops out. The seams mainly comprise magnetite, but zircon is also present in greater than accessory quantities. The seams have been locally reworked at granulite-facies metamorphic conditions during intrusion of the Dawros–Currywongaun–Doughruagh Complex magmas. Here we combine in situ mineral chemical and U–Pb geochronological analyses of zircons from samples of these heavy mineral seams collected at different locations in the Dawros–Currywongaun–Doughruagh Complex thermal aureole. An important finding is that the zircons studied have magmatic trace-element compositions, interpreted here as a function of their growth during contact metamorphic-induced partial melting. The zircons yield a range of U–Pb spot ages whose uncertainties suggest a maximum duration of zircon growth of ˜11 Ma, between 477.1 and 466.1 Ma, though it is likely that zircon growth occurred much more quickly than this. The age constraints revealed here match well with the range of 475 to 463 Ma previously proposed for the Grampian Orogeny overall in Connemara and lend useful support to models that argue for high-intensity, relatively short-lived Grampian orogenesis in the Connemara Caledonides.

U-Pb zircon age, geochemistry and petrogenesis of Mesoarchean anorthositic rocks from the Holenarsipur Greenstone Belt, Western Dharwar Craton: Implications for accretionary tectonics in southern India

The Doddakadanur Anorthosite Complex is a well-preserved sequence of anorthosites, gabbroic anorthosites, and anorthositic gabbros intrusive into amphibolites of the Mesoarchean Holenarsipur Greenstone Belt in the Western Dharwar Craton of southern India. Although the whole sequence is metamorphosed at upper amphibolite to lower granulite facies conditions, igneous cumulus texture is preserved in many of these rocks. The peak P-T conditions corresponding to metamorphic resetting, synchronous with 2.5 Ga regional metamorphism, is estimated to be ~650–700 °C and 7–11 kbar. The major and trace elemental systematics suggest a moderately fractionated parental magma (La/Yb)n = 3.3) with an overall tholeiitic affinity. The trace elemental systematics of these rocks indicates enrichment of the magma source regions by subduction-derived fluids. Uranium-Pb zircon ages yield an emplacement age of 3242 ± 17 Ma and also help to constrain two metamorphic events at 3159 ± 25 Ma and 2538 ± 11 Ma. Considering the intrusive contact relationship between the anorthositic rocks of the Doddakadanur Anorthositic Complex and ultramafics of the Holenarsipur Greenstone Belt, the new U-Pb ages suggest that the mafic-ultramafic rocks of the Holenarsipur Greenstone Belt are at least 3.24 Ga old. Integrating the new age data with available ages for the different magmatic rocks of the region, a spatial and temporal link between anorthosite emplacement and crustal accretion processes in the Western Dharwar Craton can be established. On the basis of whole-rock geochemistry and mineral chemistry, we propose a convergent margin tectonic setting for the rocks of the Doddakadanur Anorthosite Complex.

Early Palaeoproterozoic granulite-facies metamorphism and partial melting of eclogite-facies rocks in the Salma association, eastern Fennoscandian Shield, Russia

The Salma-type Archaean eclogites exposed along the northwestern boundary of the Belomorian Eclogite Province in the eastern Fennoscandian Shield formed as a result of the Mesoarchaean–Neoarchaean subduction and collision. The common protoliths of the Salma-type subduction-related eclogites were oceanic layered gabbro and volcanic-sedimentary assemblage. The eclogite-facies pillow lavas and associated alumina-siliceous sediments that fill interpillow space and intercalate with lava flows are the main objects of our work. The kyanite-garnet–phengite–quartz rocks formed after alumina-siliceous sediments contain fluid inclusions trapped in large relic quartz grains. The fluid inclusions yielded an isochore that corresponds to PT-conditions of a beginning of the Salma oceanic rock subduction from the seafloor level that generally confirms the sedimentary provenance of these rocks. The alumina-siliceous sediments underwent the eclogite-facies metamorphism at pressure no lower than 21 kbar and temperatures of 650–750 °C and transformed into kyanite-garnet–phengite–quartz rocks. During exhumation under granulite-facies conditions at temperatures up to 900 °C and pressure down to 9 kbar, eclogite facies metasediments underwent partial melting accompanied by disequilibrium breakdown of phengite + quartz association with formation complex polymineralic pseudomorphs consisting of feldspars, biotite, muscovite, kyanite, corundum, and dumortierite. U-Pb dating of Th-rich igneous zircon from melted metasedimentary and mafic rocks using the LA-ICP-MS and TIMS methods yielded the time of granulite facies event accompanied by partial melting processes at ~2.45 Ga. After this, zircon underwent fluid-induced alteration, causing partial dissolution followed by precipitation of new Th–poor zircon and zircon rims around ancient grains at ~1.9 Ga ago

Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

Abstract —The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Recovering P–T–t paths from ultra-high temperature (UHT) felsic orthogneiss: An example from the Southern Brasília Orogen, Brazil

Mineral assemblages that are diagnostic of ultrahigh-temperature (>900 °C) metamorphism are generally restricted to Mg- and Al-rich rocks. However, in many metamorphic terrains such compositions do not occur, and the rocks are dominated by orthogneisses that typically contain high-variance mineral assemblages that are not diagnostic of particular pressure–temperature (P–T) conditions. Here we use whole-rock and mineral chemistry, thermodynamic phase equilibrium modelling, and zircon U–Pb–Hf isotopic data to constrain the P–T–time history of granulites from the Socorro-Guaxupé Nappe of the Southern Brasília Orogen. These rocks occur as part of a stack of Neoproterozoic nappes that were assembled during the formation of West Gondwana. Based on phase equilibrium modelling, the peak assemblage in a garnet-bearing felsic granulite (ortho- and clinopyroxene, garnet, K-feldspar, plagioclase, quartz, ilmenite, and silicate melt) records pressures of around 11.5 kbar and temperatures of ~1000 °C, consistent with the high-Al content of orthopyroxene. The retrograde path records a small decrease in pressure to 9 kbar marked by orthopyroxene replacing garnet and the exsolution of magnetite from ilmenite followed by near-isobaric cooling that led to the development of coronas of hornblende around pyroxene due to crystallization of the last vestiges of melt. Zircon cores yielding U–Pb ages of ca. 625 Ma are interpreted as dating close-to-peak conditions during high-temperature metamorphism. In one sample, zircon rims and ‘soccer ball’ zircon neoblasts both have high U contents and yield ages of ca. 615 Ma, interpreted to mark the onset of melt crystallization. In two other samples, low-U rims and ‘soccer ball’ zircons grains yield ages of ca. 600–595 Ma, interpreted to date the end of melt crystallization. These data reveal a protracted regional metamorphic event in which the lower crust remained at granulite-facies conditions for at least 30 Myr during collision and assembly of West Gondwana.

Monazite recorded Paleoproterozoic granulite-facies metamorphism and Triassic fluid modification of the Weihai pelitic granulite, Sulu orogen

Monazite is an important accessory mineral in the pelitic granulites (Weihai area, Sulu orogen), and is also a powerful monitor for understanding the metamorphic evolution of the granulites. The pelitic granulites incongruously occur as lenses in granitic gneisses. The lithologies of the lenses gradually change from core to margin: The undeformed coarse-grained granulite, the foliated fine-grained granulite, the garnet-biotite-gneiss, and the migmatized granulite. The lens cores mostly preserve a peak granulite-facies metamorphic mineral assemblage of garnet + plagioclase (antiperthite) + quartz + sillimanite + biotite with accessory minerals of rutile, zircon, and monazite. The lens margins display a fluid-induced retrogression. In order to decipher the metamorphic processes of the pelitic granulites, a combined study of BSE imaging, U-Pb dating, and trace element composition for the monazites from the metapelitic lens were conducted. Monazites from the undeformed coarse-grained granulite only record a Paleoproterozoic age (1832±7 Ma, n=40). Monazites from the other lithologies yield the inherited Paleoproterozoic age and Triassic overgrowth age. For example, monazites from the migmatite yield intercept ages of 1818±10 and 211±22 Ma (n=56) with Triassic concordant age of 223.8±2.9 Ma. The Paleoproterozoic monazites are characterized by remarkable depletion in HREE and Y with obviously negative Eu anomalies, indicating their formation equilibrated with garnet and feldspar under granulite-facies conditions. During Triassic fluid modification, the monazite bright rims assimilated Th and Si but released U, HREE, Y, and P. This process resulted in the Triassic overgrowth monazites have higher HREE and Y contents, and lower Th and U contents with relatively lower Th/U ratios. Thus, the monazites in the pelitic granulites recorded a Paleoproterozoic metamorphic event and Triassic fluid modification. The Weihai pelitic granulites might have a tectonic affinity with the North China Craton. Therefore, the Paleoproterozoic pelitic granulites were mechanically drawn into the orogen during the Triassic continental collision, and subsequently were remoulded by the fluids during its exhumation.

Provenance and tectonic evolution of the Andrelândia Group in the region between the Socorro and Guaxupé nappes, Southern Brasília and Ribeira orogens, Brazil

Two metasedimentary successions, the Paleoproterozoic Itapira Group and the Neoproterozoic Andrelândia Group, are present in the southwestern extreme of the Southern Brasília Orogen, also affected by superposition of Ribeira Orogen tectonics. The first orogen resulted from a collision between the São Francisco paleocontinent, lower plate, and the Paranapanema paleocontinent, upper plate. The Andrelândia and Itapira groups, composed of paragneisses, quartzites, schists, and mafic/ultramafic rocks, were deformed and metamorphosed, in the studied region, under amphibolite to granulite facies conditions between 630 and 600 Ma. These metasedimentary rocks show an unconformable and tectonic contact with orthoderived basement rocks of the Amparo and Pouso Alegre complexes, considered part of the São Francisco paleocontinent. Here we present a new 1:50.000 scale geological map of the southwestern part of the Southern Brasília Orogen and six new geochronological data (LA-ICP-MS). The ages obtained in two basement samples are 2783 ± 12 Ma and 2143 ± 6 Ma. Detrital zircon analyzes in three paragneiss samples provided maximum depositional ages of ca. 1572 Ma, 1070 Ma, and 815 Ma and subordinate Archean and Paleoproterozoic ages. The maximum sedimentation ages of 1070 and 815 Ma indicate that part of the studied metasedimentary rocks is Neoproterozoic. The unconformable contact of these metasedimentary rocks with Archean and Paleoproterozoic basement rocks of the São Francisco paleocontinent justifies the interpretation that they belong to the Andrelândia Group. The crystallization age of 1716 ± 15 Ma obtained in an orthogneiss intrusive in metasedimentary rocks proves the presence of the Paleoproterozoic Itapira Complex.

Neoproterozoic crustal growth in southern Malawi: New insights from petrology, geochemistry, and U–Pb zircon geochronology, and implications for the Kalahari Craton–Congo Craton amalgamation

The southern part of the Malawi Basement Complex is regarded as being part of the Mozambique Belt, which was formed by a series of collisions between Archean cratons and/or Proterozoic magmatic arcs during Neoproterozoic to Cambrian time. The region is particularly important because of its location near the junction between the East-African (ca. 600–550 Ma) and Kuunga (ca. 560–530 Ma) orogens, which means that complex magmatic or metamorphic episodes may have been recorded in high-grade metamorphic rocks in the region. Here we report on new petrological, geochemical, and zircon U–Pb age data on felsic to mafic orthogneisses as well as on a syn-tectonic syenite from the Lilongwe–Zomba–Blantyre area in southern Malawi, with a view to evaluating the timing and pressure–temperature (P–T) conditions of the collision events as well as the protolith formation. The geochemical data on the felsic orthogneisses suggested magmatic-arc signatures, whereas the mafic orthogneisses had both magmatic-arc and non-arc signatures. Metamorphic P–T estimates for the orthogneisses based on conventional geothermometry and phase-equilibria modeling indicated high-pressure granulite-facies conditions of 9.1–11.6 kbar and 840 °C–930 °C. Zircons obtained from charnockite, biotite gneiss, and garnet-bearing mafic granulite yielded magmatic ages of 1011–808 Ma, 1013 Ma, and 1076 Ma, respectively, suggesting the presence of Stenian–Tonian crustal growth related to arc magmatism. Ediacaran (573 Ma) intrusion of granitic rock was also inferred from magmatic zircons found within a granitoid gneiss. The timing of the high-grade metamorphism was constrained by the ages of the zircon rims and found to be 569–557 Ma, thus coinciding with the intrusion of syn-tectonic syenite at 562 Ma. The results of this study suggest that the southern part of Malawi is composed of remnants of the Stenian–Tonian magmatic arc, which were metamorphosed together with surrounding metasediments around 560 Ma. The Ediacaran high-grade metamorphism is consistent with the age of the Zambezi Belt located immediately southwest of the Malawi Basement Complex. Both the Lilongwe–Zomba–Blantyre area and the Zambezi Belt may correspond to a single suture zone formed by the latest Neoproterozoic collision between the Kalahari and Congo Cratons.

Newly discovered MORB-Type HP garnet amphibolites from the Indus-Yarlung Tsangpo suture zone: Implications for the Cenozoic India–Asia collision

High pressure (HP) and ultrahigh pressure (UHP) oceanic rocks remain very scarce along the Indus-Yarlung Tsangpo suture zone in the Tibetan Plateau. Such rock types, however, could provide new and important insight into the understanding of the India-Asia collision tectonics and regional metamorphic processes. This contribution focuses on newly discovered HP garnet amphibolites that occur in the North Ayilari range along the Indus suture zone in the Western Himalaya. Petrographic and mineralogical studies as well as pressure–temperature (P–T) calculations show that the garnet amphibolites underwent at least three metamorphic stages. A prograde amphibolite-facies stage at 0.9–1.2 GPa and 605–680°C (M1) was followed by HP granulite-facies metamorphism at 680–750°C and 1.1–1.5 GPa (M2) and cooling to amphibolite-facies metamorphism at 628–650°C, 0.6–0.8 GPa (M3). Zircon and apatite U–Pb dating shows that the garnet amphibolites experienced granulite-facies conditions at 33–32 Ma and a cooling age of 12.5 ± 2.6 Ma at temperatures of 375–550 °C. The garnet amphibolites have light rare earth element (LREE)-depleted distribution patterns, low (87Sr/86Sr)i = 0.70727–0.70833, but high εNd(t) (+8.9 – +9.8) in whole-rock, and high εHf(t) (+13.9 – +16.0) values of zircon, which are similar to those of normal mid-ocean ridge basalt. This confirms that the protolith represents an oceanic slice that dominantly sourced from depleted mantle material. Geochemical, geochronological and geothermobarometrical data demonstrate that the garnet amphibolites represented a part of the Neo-Tethyan ocean, that experienced a profound amphibolite facies metamorphism before reaching HP granulite-facies conditions at ca. 32 Ma as a consequence of crustal thickening during the India–Asian collision. Thus, the studied HP oceanic-type slice represents an important puzzle piece that, together with other continental portions that experienced different P-T conditions such as HP and UHP eclogite and different metapelites and quartzo-feldspathic gneisses, helps to decipher the tectonic evolution of the Western Himalaya orogen, from oceanic subduction to continental collision and subsequent exhumation.

Eclogite resembling metamorphic disequilibrium assemblage formed through fluid-induced metasomatic reactions

Equilibrium omphacite-garnet-bearing mafic rocks have been classified as eclogites, either pristine or retrogressed, that were formed at great depths in the lithosphere. Here we report a unique natural example of eclogite resembling assemblage in disequilibrium formed through fluid-induced metasomatic reactions under the amphibolite to granulite facies. Primarily, the amphibolized protolith experienced a garnet-amphibolite facies metamorphism at ~ 500–700 °C and ~ 0.8–1 GPa. Subsequently, CO2 fluid induced fracturing and dissolution-reprecipitation reactions occurred at peak metamorphic conditions of ~ 700 °C and ~ 1 GPa. Occasional omphacite-albite assemblage, which gradually replace diopside-oligoclase symplectite adjacent to albite veins along fractures, indicates fluid-induced coupled dissolution-reprecipitation disequilibrium reactions. Here the albite-omphacite assemblage is in local equilibrium at least on 1 mm length scale, during cooling, below ~ 600 ºC and ~ 1 GPa, within the amphibolite facies conditions. The results from this study clearly suggest that disequilibrium garnet-omphacite assemblage in mafic rocks could be formed by crustal reworking processes below granulite facies conditions, and their textural equilibrium is an important criterion while defining eclogite facies.

The formation of TTGs by hydrous partial melting and anatexis from a gabbroic mantle source in Viti Levu, Fiji Islands

Tonalite, trondhjemite, granodiorite (TTG) rocks in Viti Levu, Fiji Islands formed through hydrous partial melting of gabbroic oceanic crust at low-pressure amphibolite-facies conditions caused by flat subduction of an oceanic plateau from Yavuna creek during late-Eocene time. This was followed by the formation of a quartz diorite unit by anatexis of the gabbro in the mid-Miocene. The TTGs occur as migmatitic discordant stocks and dykes within the older gabbroic unit of the Yavuna Pluton. Zircon ages show the parental gabbro to be ∼ 47.5 Ma in age, whereas the TTGs, consisting of a tonalite and a quartz diorite suite, are ∼ 37.1 Ma and ∼ 16.5 Ma old, respectively. The average δ18O value of 4.8‰ in zircons from the gabbro and the tonalite suggest a homogenous mantle source. However, about 50% of the analyzed zircons from the gabbroic and tonalitic rock samples show lower δ18O values than this average, and are interpreted as reflecting interaction of hydrothermally altered seafloor with the deep mantle source. The zircon εHf of 13 in the analyzed TTGs is interpreted as reflecting typical juvenile continental crust. PerpleX whole-rock calculations supports that the tonalite formed by hydrous partial melting of the gabbro through decompression under water-saturated amphibolite-facies conditions at a temperature of ∼ 770 °C at ∼ 3.8 kbar, whereas the quartz diorite formed by anatexis under granulite-facies conditions at ∼ 800 °C at ∼ 1.7 kbar at very shallow depth. Our investigation provides new evidence that the tonalite formed by hydrous partial melting and the quartz diorite by metasomatism and anatexis of an extant gabbroic rock/crust at < 0.1 Ga in the South Pacific region.

Active crustal differentiation beneath the Rio Grande Rift

Silicon-rich continental crust is unique to Earth. Partial melting during high- to ultrahigh-temperature metamorphism (700 °C to >900 °C) promotes the long-term stability of this crust because it redistributes key elements between the crust and mantle and ultimately produces cooler, more-differentiated continents. Granulites—rocks formerly at high- to ultrahigh-temperature conditions—preserve a record of the stabilization of Earth’s continents, but the tectonic mechanisms that drive granulite formation are enigmatic. Here we present an analysis of lower-crustal xenoliths from the Rio Grande Rift—a nascent zone of extension in the southwestern United States. Uranium–lead geo- and thermochronology combined with thermobarometric modelling show that the lower 10 km of the crust currently resides at granulite-facies conditions, with the lowermost 2 km at ultrahigh-temperature conditions. Crust and mantle xenoliths define a continuous pressure-and-temperature array, indicating that a thin lithospheric mantle lid mediates elevated conductive heat transfer into the crust. These findings establish a direct link among ultrahigh-temperature metamorphism, collapse of the Laramide orogen and lithospheric mantle attenuation. Other indicators of modern ultrahigh-temperature metamorphism are consistent with these conditions prevailing over thousands of square kilometres across the US–Mexico Basin and Range province. Similarities between the pressure-and-temperature path from the Rio Grande lower crust and those from exhumed granulite terranes imply that post-thickening lithospheric extension is a primary mechanism to differentiate Earth’s continental crust. A link between post-thickening lithospheric extension and the differentiation of continental crust is implied by granulite conditions beneath the Rio Grande Rift, inferred from analysis of lower-crustal xenoliths and thermobarometric modelling.

Petrology and U–Pb geochronology of high-grade metavolcano-sedimentary rocks from central Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is a high-grade metamorphic belt exposed along the southern margin of the North American plate in Mexico. Its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels, which culminated in the Paleocene. On the basis of field and petrographic work, this study integrates a petrological modelling approach with LA-ICP-MS U–Pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt. The study area in central XC includes a suite of alternating migmatitic paragneiss and garnet-bearing mafic schist. Petrological evidence and phase equilibria calculations demonstrate that a staurolite–kyanite grade metamorphism occurred at ~640–670 °C and 8–9 kbar before extensive migmatization took place. Differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration-melting reactions, and continued during peak metamorphism at granulite-facies conditions of ~800–820 °C and ~5–7 kbar. According to Ti-in-zircon thermometry, melt generation/mobilization progressed during cooling until crystallize at ~700 °C. The age of prograde metamorphism was likely recorded by monazite cores that give mid- to Late Cretaceous ages. Cooling and melt crystallization could be constrained by leucosome zircon dated at 61.8±0.6 Ma. Both zircon and low-Y monazite from whole rock and melanosome portions, respectively, are consistent with an episode of growth around 60.9±0.5 Ma. Monazite, however, would have continued its crystallization process for a period of ca. 10 Ma. Thin zircon and high-Y monazite rims show ages between ~34–25 Ma and are interpreted to reflect a stage of reheating by magmatic advection related to large Oligocene plutonism in the region. Age and nature of central XC basement show good correlation with other Early-Cretaceous volcano-sedimentary basins of central and southern Mexico. A period of crustal thickening would have buried the sequence at depths of ~30 km, causing Barrovian-type metamorphism during mid- to Late Cretaceous time and eventually, anatexis that evolved at granulite-facies conditions during orogenic collapse in the Paleocene (≥62 Ma). The time span from leucosome crystallization to late-stage reheating in central XC would imply a protracted high-temperature evolution indicating a middle- to upper-crustal residence time of at least ca. 30 Ma.

Pb and Nd Isotopic Data on Granitoids from the Lake Zone, Mongolian and Gobi Altai with Implications for the Crustal Growth of the Central Asian Orogenic Belt

Pb and Nd isotopic composition of granitoids from the Lake Zone, Mongolian and Gobi Altai was analyzed to decipher the continental crustal growth of heterogeneous terranes of the Central Asian orogenic belt. The granitoids were generated in the Late Neoproterozoic–Early Paleozoic juvenile crust of the Lake Zone during island-arc, accretion, and post-accretionary stages at 535–440 Ma. Their Pb and Nd isotopic characteristics are similar and illustrate the dominance of juvenile material in the source of granitoids of all three stages. An insignificant terrigenous contribution in their source provides dispersion of the Pb isotopic composition at a weak effect on the Nd isotopic composition. The Pb and Nd isotopic study of Middle Paleozoic (∼380–355 Ma) synkinematic and Late Paleozoic (∼350–270 Ma) post-kinematic granitoids in the Mongolian and Gobi Altai revealed a significant heterogeneity of their source, which is mainly inherited from terrigenous rocks of the Altai accretionary wedge variably metamorphosed under greenschist to granulite facies conditions. The main source of Altai terrigenous sediments was volcanic rocks from the Lake Zone juvenile crust with subordinate contribution of more mature rocks from the Tuva-Mongolian and Dzabkhan microcontinents, which supplied sediments with highly radiogenic Pb and Nd isotopic composition. The Pb isotopic compositions of the granitoids from the Lake Zone, Mongolian and Gobi Altai, and also the Trans-Altai Gobi (Savatenkov et al., 2016) show that their mantle source has an elevated Th/U ratio compared to the depleted mantle (Kramers and Tolstikhin, 1997). This is a hallmark of the depleted mantle source of the Paleo-Asian province. Juvenile continental crust terrains of the Central Asian orogenic belt (Lake Zone and Trans-Altai Gobi terranes) were formed in various tectonics settings. The island-arc complexes of the Lake Zone developed near the Precambrian Tuva-Mongolian and Dzabkhan microcontinents, which supplied terrigenous material with highly radiogenic Pb to granitoid sources. The Trans-Altai Gobi was a system of ensimatic island arcs separated from the Siberian paleocontinent by a continent-dipping subduction zone, which prevented the influx of ancient terrigenous material from the continent.

Tectonic setting of the Bipindi greenstone belt, northwest Congo craton, Cameroon: Implications on BIF deposition

The banded iron formations (BIFs) in the Bipindi greenstone belt (GB) are hosted within metavolcano-sedimentary sequence of the Nyong Group at the northwestern edge of the Congo Craton (Ntem Complex) in southern Cameroon. New data on the BIFs and its associated rocks allow us to discuss the tectonic setting of this greenstone belt in a regional perspective. Metavolcanic rocks consist of epidosites, mafic granulites, and garnet-pyroxene gneisses, whereas the metasedimentary rocks comprise chlorite-epidote schists. These rocks have been metamorphosed under greenschist-to granulite-facies conditions. The geochemical features of the metavolcanic rocks such as the trace element ratios and rare earth elements (REE) patterns suggest that their protolith corresponded to tholeiitic to transitional and calc-alkaline basalts. The associated metasedimentary rocks are characterized by low La/Sc and Th/Sc discrimination ratios, and REE abundances, indicating that they were derived from mafic and intermediate igneous rocks. Similar protoliths were recently described for metamorphic rocks interbedded with BIFs at the Nyong Group, which were formed in a shallow basin near the continent. The Bipindi BIFs are further characterized by both weak (1.31–1.34) and slightly high (2.00–2.51) positive (Eu/Eu)SN ratio, that are typical of Superior- and Algoma-type BIFs respectively. However, the geochemical features of the host rocks (e.g., REE patterns, Th/Nb, La/Yb, and La/Sc ratios) suggest that the Bipindi BIFs were deposited at an extensional geodynamic setting likely related to a back-arc, similar to the depositional environment for many Algoma-type BIFs. Archean-Paleoproterozoic arc setting is widely distributed across the Congo Caton and the West African Craton, suggesting integration of the tectonic evolution of the Bipindi GB into a continental-scale paleogeography.

Contribution of magmatism, partial melting buffering and localized crustal thinning on the late Variscan thermal structure of the Agly massif (French Pyrenees)

AbstractLow pressure‐high temperature (LPHT) metamorphism, with geothermal gradients in the order of 50–100°C/km, is a common feature of the late evolution of collisional orogens. These abnormal thermal conditions may be the results of complex interactions between magmatism, metamorphism and deformation. The Agly massif, in the French Pyrenees, preserves the metamorphic footprints of the late Variscan thermal structure of an almost continuous section from the upper and middle continental crust. The upper crust is characterized by a very high geothermal gradient of ~55°C/km, evolving from greenschist to amphibolite facies, while the middle crust, exposed in a gneissic core, exhibits granulite facies conditions with a near isothermal geothermal gradient (&lt;8°C/km) between 740 and 790°C. The abnormal and discontinuous crustal geothermal gradient, dated at c. 305 Ma on syn‐granulitic monazite by LA‐ICP‐MS, is interpreted to be the result of magmatic intrusions at different structural levels in the crust: the Ansignan charnockite (c. 305 Ma) in the deepest part of the gneissic core, the Tournefort granodiorite (c. 308 Ma) at the interface between the gneissic core and the upper crust and the Saint‐Arnac granite (c. 304 Ma) in the upper section of the massif. The heat input from these magmas combined with the thermal buffering effect of the biotite dehydration‐melting reaction resulted in the near isothermal geothermal gradient in the gneissic core (melt‐enhanced geotherm). The higher geothermal gradient (&gt;50°C/km) in the upper crust is only due to conduction between the hot middle crust and the Earth's surface. The estimated maximum finite pressure range suggests that ~10 to 12 km of crust are exposed in the Agly massif while the present‐day thickness does not exceed 5–6 km. This pressure/depth gap is consistent with the presence of several normal mylonitic shear zones that could have contributed to the subtraction of ~5 km of the rock pile. Monazite U–Th–Pb ages carried out on monazite overgrowths from a highly mylonitized sample suggest that this vertical thinning of the massif occurred at c. 296–300 Ma. This later Variscan extension might have slightly perturbed the 305 Ma geothermal gradient, resulting in an apparent higher conductive geothermal gradient in the upper crust. Although the Agly massif has been affected by Cretaceous extension and Eocene Alpine compression, we suggest that most of the present‐day thickness of the column rock was acquired by the end of the Palaeozoic.

Zircon U-Pb Dating and Petrogenesis of Multiple Episodes of Anatexis in the North Dabie Complex Zone, Central China

The North Dabie complex zone (NDZ), central China, is a high-T ultrahigh-pressure (UHP) metamorphic terrane. It underwent a complex evolution comprising of multistage metamorphism and multiple anatectic events during the Mesozoic continental collision, characterized by granulite-facies overprinting and a variety of migmatites with different generations of leucosomes. In this contribution, we carried out an integrated study including field investigation, petrographic observations, zircon U-Pb dating, and whole-rock element and Sr-Nd-Pb isotope analysis for the migmatites in the NDZ and their leucosomes and melanosomes. As a result, four groups of leucosomes have been recognized: Group 1 (garnet-bearing leucosome), strongly deformed leucosomes with coarse-grained peritectic garnet; Group 2 (amphibole-rich leucosome), weakly deformed to undeformed amphibole-rich leucosomes with coarse-grained peritectic amphibole and no garnet; Group 3 (amphibole-poor leucosome), weakly deformed to undeformed amphibole-poor leucosomes with minor fine-grained amphibole; Group 4 (K-feldspar-rich leucosome), K-feldspar-rich leucosomes mainly composed of coarse-grained quartz, plagioclase and K-feldspar. Zircon SHRIMP and LA-ICPMS U-Pb dating suggest that the Group 1 leucosomes formed at 209 ± 2 Ma whereas the rest of the leucosome groups (Groups 2–4) occurred between 145–110 Ma, in response to decompression under granulite-facies conditions during the early stage of exhumation, and to heating during post-orogenic collapse, respectively. Furthermore, the garnet-bearing leucosomes were resulted from fluid-absent anatexis related to biotite dehydration melting, while the other three groups of leucosomes were formed during large-scale fluid-present partial melting and coeval migmatization. This migmatization comes from heating from the mountain-root removal and asthenosphere upwelling, together with the influx of fluids derived from country rocks at mid-upper crustal levels. However, all the leucosomes and melanosomes display Pb-isotopic compositions similar to those observed for the NDZ UHP rocks (eclogites and granitic gneisses), suggesting a common source from the Triassic subducted Neoproterozoic lower-crustal rocks. In addition, the Cretaceous partial melting and migmatization began at 143 ± 2 Ma with three age-peaks at 133 ± 3 Ma, 124 ± 3 Ma and 114 ± 7 Ma, respectively.

Sm-Nd geochronology and petrologic investigation of sub-ophiolite metamorphic sole from the Dinarides (Krivaja-Konjuh, Ophiolite Complex, Bosnia and Herzegovina)

The Dinaridic segment of Neotethys was affected by a widespread shortening and related subduction-accretion-obduction processes that commenced in the middle Jurassic. In the Dinarides, the Krivaja-Konjuh Ophiolite Complex (KKOC) stands as the largest ophiolite complex with a well-exposed metamorphic sole which is the key to understanding the dynamics of intraoceanic subduction initiation in this part of Neotethys. In this contribution we present Sm-Nd geochronology on a granulite facies amphibolite from the KKOC, as well as a detailed petrological description. A five-point isochrone age calculated from clinopyroxene, plagioclase, garnet, amphibole and whole rock is 162 ± 14 Ma (MSWD = 6.2), whereas garnet and whole rock yield 160 ± 7 Ma. Ages calculated from all data points except clinopyroxene are 162 ± 5 Ma (MSWD = 1.09). Petrographic investigations suggest that these ages date granulite facies metamorphic conditions (i.e. peak metamorphism of Grt-Cpx amphibolite) rather than post-peak exhumation or obduction processes. Phase textural relationships are in line with previous research, which indicated a peak metamorphism equilibration pressure and temperature of ~1 GPa and ~800 °C, respectively. Granulite facies conditions are elucidated for an igneous precursor, which underwent a multi-stage metamorphism that gave rise to recrystallization of igneous clinopyroxene and plagioclase, epitaxial growth of amphibole, and garnet blastosis. Taking into account the age of gabbronorite from the youngest segment of the KKOC oceanic crust (Taorcian to Bathonian) and ages of radiolarian assemblages from the KKOC melange (Bajocian to Bathonian), it may be inferred that within ~25 Ma the Dinaridic segment of Neotethys evolved rapidly from active ridge spreading through a stage of intraoceanic subduction and arc magmatism toward sub-ophioliteexhumation and further obduction along the Adria passive margin at the end of the Jurassic era.

High Temperature Reduced Granulite-Facies Nature of Garnetites in the Khabarny Mafic–Ultramafic Massif, Southern Urals: Evidence from Fluid and Mineral Analyses

Abstract High-grade metamorphic rocks underlying the intrusive layered dunite–pyroxenite–gabbronorite East-Khabarny Complex (EKC) are integrated in the complex Khabarny mafic–ultramafic Massif in the Sakmara Allochthon zone in the Southern Urals. These rocks are associated with high-temperature shear zones. Garnetites from the upper part of the metamorphic unit close to the contact with EKC gabbronorite are chemically and texturally analysed to estimate their formation conditions and fluid regime. Fluids provide crucial information of formation conditions and evolution of these garnetites during high-grade metamorphism, and are preserved in channel positions within Si6O1812- rings of cordierite, and in fluid inclusions in quartz and garnet. Minerals and fluid inclusions of the garnetites are studied with X-ray fluorescence spectrometry, electron microprobe analyses, Raman spectroscopy, and microthermometry. The garnetites mainly consist of garnet (up to 80 vol. %), cordierite and quartz. Accessory minerals are rutile, ilmenite, graphite, magnetite and cristobalite. Granulite-facies metamorphic conditions of the garnetites are estimated with the garnet–cordierite–sillimanite–quartz geothermobarometer: temperatures of 740 to 830 ˚C and pressures of 770–845 MPa. The average garnet composition in end-member concentrations is 48·5 mole % almandine (±3·9), 34·7 mole % pyrope (±3·3), 10·3 mole % spessartine (±1·1), 1·8 mole % grossular (±1·5), and 1·5 mole % andradite (±1·5). The cordierite electron microprobe analyses reveal an average Mg2+ fraction of 0·79 ± 0·01 in the octahedral site. Relicts of a strong positive temperature anomaly (up to 1000 ˚C) are evidenced by the preservation of cristobalite crystals in garnet and the high titanium content of quartz (0·031 ± 0·008 mass % TiO2) and garnet (0·31 ± 0·16 mole % end-member Schorlomite-Al). The fluid components H2O, CO2, N2 and H2S are detected in cordierite, which correspond to a relatively oxidized fluid environment that is common in granulites. In contrast, a highly reduced fluid environment is preserved in fluid inclusions in quartz nodules, which are mono-fluid phase at room temperature and composed of CH4 (&amp;gt;96 mole %) with locally minor amounts of C2H6, N2, H2S and graphite. The fluid inclusions occur in homogeneous assemblages with a density of 0·349 to 0·367 g·cm-3. The CH4-rich fluid may represent peak-temperature metamorphic conditions, and is consistent with temperature estimation (∼1000 ˚C) from Ti-in-garnet and Ti-in-quartz geothermometry. Tiny CH4-rich fluid inclusions (diameter 0·5 to 2 µm) are also detected by careful optical analyses in garnet and at the surface of quartz crystals that are included in garnet grains. Graphite in fluid inclusions precipitated at retrograde metamorphic conditions around 300–310 ± 27 ˚C. Aragonite was trapped simultaneously with CH4-rich fluids and is assumed to have crystallized at metastable conditions. The initial granulite facies conditions that led to the formation of a cordierite and garnet mineral assemblage must have occurred in a relative oxidized environment (QFM-buffered) with H2O–CO2-rich fluids. Abundant intrusions or tectonic emplacement of mafic to ultramafic melts from the upper mantle that were internally buffered at a WI-buffered (wüstite–iron) level must have released abundant hot CH4-rich fluids that flooded and subsequently dominated the system. The origin of the granulite-facies conditions is similar to peak-metamorphic conditions in the Salda complex (Central Urals) and the Ivrea–Verbano zone (Italian Alps) as a result of magmatic underplating that provided an appearance of a positive thermal anomaly, and further joint emplacement (magmatic and metamorphic rocks together) into upper crustal level as a high temperature plastic body (diapir).

Sulfide melts in ore deposits from low-grade metamorphic settings: Insights from fluid and Tl-rich sulfosalt microinclusions from the Monte Arsiccio mine (Apuan Alps, Tuscany, Italy)

Sulfide melting is increasingly recognized as an efficient ore remobilization process in several ore bodies deformed and metamorphosed under amphibolite to granulite facies conditions. Actually, sulfide melts may also occur at lower metamorphic conditions, provided the abundance of the so-called Low-Melting point Chalcophile Elements (LMCE). The Monte Arsiccio ore body (Apuan Alps, northern Tuscany, Italy) is strongly enriched in several LMCE, i.e., Ag, As, Hg, Pb, Sb, and Tl, and experienced greenschist facies metamorphism (T = 350–450 °C; P = 0.3–0.4 GPa) during the Alpine orogeny. Trails of polyphase sulfosalt inclusions (up to five phases), coexisting with fluid inclusions, were observed within quartz and baryte crystals found in late-stage veins. Sulfosalt assemblages are formed by LMCE-rich phases, the main being ferdowsiite, Ag8As3Sb5S16, chabournéite, Tl2Pb(Sb,As)10S17, and intermediate members of the arsiccioite/routhierite isotypic pair, (Ag1−xCux)Hg2TlAs2S6, along with minor realgar. Sulfosalt inclusions range between 200 nm and more than 100 µm in size, whereas larger inclusions, likely formed through the coalescence of smaller melt droplets, are up to 500 µm. Sulfosalt inclusions are associated with primary two-phase liquid-rich fluid inclusions (Type 1a), containing CO2 and N2, as revealed by microthermometric investigations and micro-Raman spectroscopy. Secondary two-phase liquid-rich fluid inclusions (Type 1b) were also identified. Microthermometric measurements carried out on Type 1a fluid inclusions gave homogenization temperatures (Th) in the range 265–296 °C, whereas Type 1b fluid inclusions showed lower Th, between 222 and 246 °C. As microthermometric heating experiments on sulfosalt inclusions revealed homogenization temperatures around 270 °C, it is very likely that sulfosalt inclusions were in the liquid state during their entrapment, in agreement with many Tl-bearing systems. These results are fully consistent with the occurrence of LMCE-rich sulfide melt during the formation of the sulfosalt assemblages observed at the Monte Arsiccio mine. In addition, the polyphase nature of most of the observed sulfosalt inclusions may be related to the unquenchable nature of sulfide melts. The formation of LMCE-rich sulfosalt assemblages could be related to the release of these elements from pyrite during its metamorphic recrystallization. Consequently, pyrite ores may produce sulfide melts, with potential implications in the change of speciation of LMCE, from trace to major elements.