Sillimanite Gneiss Research Articles

Metamorphic and tectonic evolution at 31°36′S across a deep crustal zone from the Sierra Chica of Córdoba, Sierras Pampeanas, Argentina

A geological section at 31°36′S cut the main structures of a representative area of the deep crust in the Sierra Chica (Sierras Pampeanas of Córdoba, Argentina), where associations of high-grade gneisses, migmatites, marbles, amphibolites and ultramafic rocks are recognized. A group of ultramafic bodies composed of serpentinized spinel harzburgites are associated with a basement affected by a M2–D2 regional metamorphism that developed the main S2 metamorphic foliation. Applying different thermobarometers to the varieties of garnet sillimanite gneisses and leucocratic garnet gneisses, a range of P–T conditions of 768–840 °C and 7.3–8.6 kb were obtained for M2. A strong D3 ductile deformation structured the ultramafic bodies, more viscous than the associated metamorphic rocks, generating large west-vergent sheath folds with tubular geometry. This constrictional deformation was developed in a simple shear regime, under high-temperature conditions, with a tectonic transport direction toward the northwest (∼N 300°). Later, decompression and cooling down to amphibolite facies conditions occurred in all the rocks of the area during a retrograde M3 event. A range of P–T conditions of 588–710 °C and 5.2–7.3 kb were calculated for M3 in the gneisses of the area. The Sierra Chica ultramafic rocks outcrop as folded bodies, isolated between them and locally parallel to the S2 metamorphic foliation, therefore they cannot be correlated as a regional belt that could be interpreted as an ancient suture. Our hypothesis is that the ultramafic bodies are upper mantle relics of earlier emplacements into an accretionary prism that was entirely reworked during the Pampean orogeny. This same outcrop-pattern can also be recognized in the other ultramafic bodies in the Sierras de Córdoba. Hence, we suggest to avoid the use of the terms Eastern and Western Belts for the treatment of the ultramafic rocks in the Sierras de Córdoba. Una sección geológica a los 31°36’ de latitud sur atraviesa perpendicularmente las principales estructuras de un sector representativo de la corteza profunda en la Sierra Chica (Sierras Pampeanas de Córdoba, Argentina), donde se reconocen asociaciones de gneises, migmatitas, mármoles, anfibolitas y rocas ultramáficas. Un grupo de cuerpos ultramáficos compuestos por harzburgitas espinélicas serpentinizadas se encuentra asociado a un basamento afectado por un metamorfismo regional M2–D2 de alto grado, que desarrolló la foliación metamórfica principal S2. Aplicando diferentes termobarómetros a las variedades de gneises biotíticos sillimaníticos y gneises granatíferos leucocráticos, se obtuvieron rangos de temperaturas de 768–840 °C y de presiones de 7.3–8.6 kb para el evento M2. Se determinó que una fuerte deformación dúctil D3 estructuró los cuerpos ultramáficos, de mayor viscosidad que las rocas metamórficas asociadas, generando grandes estructuras plegadas en vaina con geometría tubular, con vergencia al oeste. Esta deformación constriccional se habría desarrollado en un régimen de cizalla simple, en condiciones de alta temperatura, con una dirección de transporte tectónico hacia el noroeste (∼N 300°). Posteriormente, durante un evento retrógrado M3, se habrían producido la descompresión y enfriamiento de todas las rocas del área hasta condiciones de la facies de anfibolitas. Se calcularon rangos de temperaturas de 588–710 °C y de presiones de 5.2–7.3 kb para el evento M3 en los gneises del área. Las rocas ultramáficas de la Sierra Chica se encuentran como cuerpos plegados aislados entre sí y localmente concordantes con la foliación metamórfica, por lo que no se pueden correlacionar como una única faja regional que pueda interpretarse como una antigua sutura. Nuestra hipótesis es que los cuerpos ultramáficos serían escamas del manto superior emplazadas tectónicamente en un prisma de acreción. Este mismo patrón de afloramiento puede reconocerse en los otros cuerpos ultramáficos de las Sierras de Córdoba. De acuerdo con esto, en este trabajo se sugiere evitar el uso de los términos Fajas Oriental y Occidental para el tratamiento de las rocas ultramáficas en las Sierras de Córdoba.

Osumilite and a spinel + quartz association in garnet–sillimanite gneiss from Rundvågshetta, Lützow-Holm Complex, East Antarctica

This is the first report of osumilite occurring as fine isolated inclusions within garnet porphyroblasts, as observed in garnet–sillimanite gneiss from Rundvågshetta, Lützow-Holm Complex, East Antarctica. The osumilite is characterized by high Si content (10.60 and 10.95 atoms based on 30 oxygens per formula unit), low Al content (2.99 and 3.82), a high content of M site-occupying cations (2.51 and 3.03), and high X Mg values (about 0.81). We also report a spinel + quartz association found as inclusions within garnet porphyroblasts. Spinel grains, which are in direct contact with quartz and are spatially associated with sillimanite, show extremely high Zn contents ( X Zn Spl = 0.33 − 0.46) and high X Mg values (0.45–0.54). The garnet is rimmed by sillimanite, K-feldspar, plagioclase, and quartz. Biotite and cordierite are found only as inclusions within garnet porphyroblasts, where biotite coexists with spinel–quartz or with rutile. Porphyroblastic garnet contains rutile needles and has low X Mg values (about 0.36). The sillimanite contains a high Fe content (about 1.2 wt.% Fe 2O 3). The occurrence of osumilite and spinel + quartz indicates a clockwise pressure–temperature path of ultrahigh-temperature metamorphism, involving the following events: (1) the Rundvågshetta granulites suffered prograde metamorphism within the kyanite and sapphirine + quartz fields; (2) subsequent retrograde metamorphism, involving near-isothermal decompression, occurred in the orthopyroxene + sillimanite + quartz field; (3) the granulites passed through the garnet + cordierite + sillimanite + quartz field during decreasing temperature; (4) the granulites entered the osumilite stability field at around 8 kbar and 950 °C; and (5) the granulites retain a final record of retrograde metamorphism within the biotite + sillimanite + K-feldspar and quartz field at 6.1 kbar and about 830 °C.

Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt: New SHRIMP U–Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton

The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U–Th–Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/ 206Pb ages of 1956 ± 41 Ma and 1884 ± 24 Ma. One metamorphic zircon from a garnet–sillimanite gneiss also gave an apparent 207Pb/ 206Pb age of 1939 ± 15 Ma. These results are consistent with interval of ages of c. 1.93–1.90 Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/ 206Pb ages of 1837 ± 8 Ma, 1821 ± 8 Ma and 1836 ± 8 Ma respectively. Two diopside–olivine–phlogopite marbles yielded weighted mean 207Pb/ 206Pb ages of 1817 ± 9 Ma and 1790 ± 6 Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85 Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86–1.80 Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event.

Pan-African metamorphic and magmatic rocks of the Khanka Massif, NE China: further evidence regarding their affinity

AbstractThe Khanka Massif is a crustal block located along the eastern margin of the Central Asian Orogenic Belt (CAOB) and bordered to the east by Late Jurassic–Early Cretaceous circum-Pacific accretionary complexes of the Eastern Asian continental margin. It consists of graphite-, sillimanite- and cordierite-bearing gneisses, carbonates and felsic paragneisses, in association with various orthogneisses. Metamorphic zircons from a sillimanite gneiss from the Hutou complex yield a weighted mean206Pb/238U age of 490 ± 4 Ma, whereas detrital zircons from the same sample give ages from 934–610 Ma. Magmatic zircon cores in two garnet-bearing granite gneiss samples, also collected from the Hutou complex, yield weighted mean206Pb/238U ages of 522 ± 5 Ma and 515 ± 8 Ma, whereas their metamorphic rims record206Pb/238U ages of 510–500 Ma. These data indicate that the Hutou complex in the Khanka Massif records early Palaeozoic magmatic and metamorphic events, identical in age to those in the Mashan Complex of the Jiamusi Massif to the west. The older zircon populations in the sillimanite gneiss indicate derivation from Neoproterozoic sources, as do similar rocks in the Jiamusi Massif. These data confirm that the Khanka Massif has a close affinity with other major components of the CAOB to the west of the Dun-Mi Fault. Based on these results and previously published data, the Khanka Massif is therefore confirmed as having formed a single crustal entity with the Jiamusi (and possibly the Bureya) massif since Neoproterozoic time.

Single zircon grains record two Paleoproterozoic collisional events in the North China Craton

The Western Block of the North China Craton consists of the Yinshan Block in the north and the Ordos Block in the south which were amalgamated along the east-west trending Khondalite Belt at ∼1.95 Ga. The Western Block then collided with the Eastern Block to form the coherent basement of the North China Craton along the north-south trending Trans-North China Orogen at ∼1.85 Ga. The Huaian Complex, a high-grade terrrane located at the conjunction of the Khondalite Belt and Trans-North China Orogen, records metamorphic events associated with both collisions. The complex consists of lithologies from both the Khondalite Belt and Trans-North China Orogen, of which the former consist of graphite–garnet–sillimanite gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble, together called the Khondalite series. Zircons in the graphite–garnet–sillimanite gneiss can be divided into three types: (1) spherical grains without internal structures, (2) grains with a core-and-rim structure; and (3) grains with a dark core surrounded by double rims. Except for the dark core in type 3, all other types of zircon domains are structureless and highly luminescent, with very low Th/U ratios, typical of a metamorphic origin. Analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 ± 26 and 1947 ± 22 Ma, similar to previously determined metamorphic ages from the Khondalite Belt and thus interpreted as the time of collision between the Yinshan and Ordos Blocks. Analyses on type 1 zircons, rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 ± 15 and 1857 ± 16 Ma, interpreted as the time of collision between the Eastern and Western Blocks. Thus, zircons in the graphite–garnet–sillimanite gneiss of the Huaian Complex record both of the Paleoproterozoic collisional events in the North China Craton.

Crustal melt granites and migmatites along the Himalaya: melt source, segregation, transport and granite emplacement mechanisms

ABSTRACTIndia–Asia collision resulted in crustal thickening and shortening, metamorphism and partial melting along the 2200 km-long Himalayan range. In the core of the Greater Himalaya, widespread in situ partial melting in sillimanite+K-feldspar gneisses resulted in formation of migmatites and Ms+Bt+Grt+Tur±Crd±Sil leucogranites, mainly by muscovite dehydration melting. Melting occurred at shallow depths (4–6 kbar; 15–20 km depth) in the middle crust, but not in the lower crust. 87Sr/86Sr ratios of leucogranites are very high (0·74–0·79) and heterogeneous, indicating a 100 crustal protolith. Melts were sourced from fertile muscovite-bearing pelites and quartzo-feldspathic gneisses of the Neo-Proterozoic Haimanta–Cheka Formations. Melting was induced through a combination of thermal relaxation due to crustal thickening and from high internal heat production rates within the Proterozoic source rocks in the middle crust. Himalayan granites have highly radiogenic Pb isotopes and extremely high uranium concentrations. Little or no heat was derived either from the mantle or from shear heating along thrust faults. Mid-crustal melting triggered southward ductile extrusion (channel flow) of a mid-crustal layer bounded by a crustal-scale thrust fault and shear zone (Main Central Thrust; MCT) along the base, and a low-angle ductile shear zone and normal fault (South Tibetan Detachment; STD) along the top. Multi-system thermochronology (U–Pb, Sm–Nd, 40Ar–39Ar and fission track dating) show that partial melting spanned ̃24–15 Ma and triggered mid-crustal flow between the simultaneously active shear zones of the MCT and STD. Granite melting was restricted in both time (Early Miocene) and space (middle crust) along the entire length of the Himalaya. Melts were channelled up via hydraulic fracturing into sheeted sill complexes from the underthrust Indian plate source beneath southern Tibet, and intruded for up to 100 km parallel to the foliation in the host sillimanite gneisses. Crystallisation of the leucogranites was immediately followed by rapid exhumation, cooling and enhanced erosion during the Early–Middle Miocene.

Presence of high-grade rocks in NW Venezuela of possible Grenvillian affinity

Abstract High-grade metamorphic rocks – marble, charnockite, meta-anorthosite, metapelite, clinopyroxenite and garnet amphibolite – have been found in northwestern Venezuela. They occur as: (a) xenoliths in the Oligo-Miocene lavas of Cerro Atravesado, Central Falcón; (b) possibly olistoliths in Nuezalito Formation, NW Portuguesa; (c) in Cerro El Guayabo, an elongated east–west oriented hill in the Nirgua Complex, Yaracuy; (d) rounded clasts of marble in the basal conglomerate of Casupal Formation, Falcón; (e) rounded clasts of anorthosite and sillimanite gneiss in a conglomerate of Matatere Formation, Lara; and (f) basement cores extracted from La Vela Gulf, Falcón. These high-grade rocks probably suffered a retrograde metamorphism to amphibolite facies of Palaeozoic age, and an even more retrograde event to the greenschist facies during Early Cenozoic, together with strong shearing and hydrothermal alteration. They indicate the possible existence of an extensive high-grade basement, or a mosaic of such blocks, under northwestern Venezuela, especially below the Falcón petroleum basin. Similar rocks crop out extensively in northern and central Colombia, Mexico, Ecuador and Peru. This is the first time they are described from Venezuela. Their high-grade lithology, pre-Mesozoic positions, and the tectonic evolution of Northern South America allow interpretation of a possible Grenvillian affinity, related to the supercontinents of Rodinia and Pangaea.

Metamorphism, graphite crystallinity, and sulfide anatexis of the Rampura–Agucha massive sulfide deposit, northwestern India

Located adjacent to the Banded Gneissic Complex, Rampura–Agucha is the only sulfide ore deposit discovered to date within the Precambrian basement gneisses of Rajasthan. The massive Zn–(Pb) sulfide orebody occurs within graphite–biotite–sillimanite schist along with garnet–biotite–sillimanite gneiss, calc–silicate gneisses, amphibolites, and garnet-bearing leucosomes. Plagioclase–hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720–780°C, whereas temperatures obtained from Fe–Mg exchange between garnet and biotite (580–610°C) in the pelites correspond to postpeak resetting. Thermodynamic considerations of pertinent silicate equilibria, coupled with sphalerite geobarometry, furnished part of a clockwise P–T–t path with peak P–T of ∼6.2 kbar and 780°C, attained during granulite grade metamorphism of the major Zn-rich stratiform sedimentary exhalative deposits orebody and its host rocks. Arsenopyrite composition in the metamorphosed ore yielded a temperature [and log f(S 2)] range of 352°C (−8.2) to 490°C (−4.64), thus indicating its retrograde nature. Contrary to earlier research on the retrogressed nature of graphite, Raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals (490 ± 43°C with a maximum at 593°C). The main orebody is mineralogically simple (sphalerite, pyrite, pyrrhotite, arsenopyrite, galena), deformed and metamorphosed while the Pb–Ag-rich sulfosalt-bearing veins and pods that are irregularly distributed within the hanging wall calc–silicate gneisses show no evidence of deformation and metamorphism. The sulfosalt minerals identified include freibergite, boulangerite, pyrargyrite, stephanite, diaphorite, Mn–jamesonite, Cu-free meneghinite, and semseyite; the last three are reported from Agucha for the first time. Stability relations of Cu-free meneghinite and semseyite in the Pb–Ag-rich ores constrain temperatures at >550°C and <300°C, respectively. Features such as (1) low galena–sphalerite interfacial angles, (2) presence of multiphase sulfide–sulfosalt inclusions, (3) microcracks filled with galena (±pyrargyrite) without any hydrothermal alteration, and (4) high contents of Zn, Ag (and Sb) in galena, indicate partial melting in the PbS–Fe0.96S–ZnS–(1% Ag2S ± CuFeS2) system, which was critical for metamorphic remobilization of the Rampura–Agucha deposit.

An overview of geological studies of JARE in the Napier Complex, Enderby Land, East Antarctica

Abstract Subsequent to the reconnaissance fieldwork in 1982, the Japanese Antarctic Research Expedition (JARE) carried out extensive geological studies that focused on structural and tectonic aspects, petrology, geochemistry and geochronology of the Napier Complex in Enderby Land, East Antarctica. Detailed field investigations in several key areas, including geological mapping of the Mt. Riiser-Larsen area and Tonagh Island, revealed that the Napier Complex comprises layered and massive gneiss units, of which the layered unit is composed of garnet felsic gneiss, orthopyroxene felsic gneiss, pelitic and basic gneisses, impure quartzite, and minor metamorphosed banded iron formation, whereas the massive unit consists mainly of orthopyroxene felsic gneiss. The boundary between the units is transitional in the Mt. Riiser-Larsen area, in which metamorphosed anorthosite and ultramafic rocks occur as thin layers, or blocks or pods, but on Tonagh Island the boundary is closely associated with the shear zone. Nine deformation episodes (D 1 –D 9 ) were suggested for Tonagh Island. These results of fieldwork were presented in detail in two geological maps. Geochemical studies showed that (1) garnet–sillimanite gneisses and garnet-rich felsic gneisses were derived from mudstone and sandstone, respectively, both enriched in MgO, Cr and Ni; (2) orthopyroxene felsic gneisses have a close REE affinity with Archaean tonalite–trondhjemite–granodiorite (TTG); (3) basic gneisses were derived from light rare earth element (LREE)-enriched or -depleted basalts; (4) meta-ultramafic rocks are comparable with komatiite and related depleted mantle peridotite. This suite of protoliths is reminiscent of Archaean greenstone–granite belts. Precise analyses of physical conditions of metamorphism were carried out by using reliable approaches such as feldspar thermometry, alumina content of orthopyroxene, inverted pigeonite and bulk-rock compositions, and clino- and orthopyroxene compositions with different textures (porphyroblastic and neoblastic), and the results suggested that the maximum metamorphic temperature might have reached 1130 °C (i.e. ultrahigh-temperature (UHT) metamorphism). P – T evolution of the Napier UHT metamorphism was examined by analyses of reaction textures combined with fluid inclusion studies, suggesting both clockwise (Bunt Island) and counterclockwise (Mt. Riiser-Larsen and Tonagh Island) P – T – t paths. U–Pb sensitive high-resolution ion microprobe and secondary ionization mass spectrometry zircon ages from the Mt. Riiser-Larsen area and Tonagh Island indicate three stages of protolith formation at around 3.28–3.23, 3.07 and 2.68–2.63 Ga, and two contrasting ages for the timing of peak UHT metamorphism at either c. 2.55 or c. 2.51–2.45 Ga. On the basis of these results, more comprehensive studies on the Napier Complex are essential in the future for understanding (1) the role and age of TTG protolith and (2) the origin and timing of UHT metamorphism.

Geochronological and Petrological Constraints for Tectonic Evolution of the Central Greater Himalayan Sequence in the Kharta Area, Southern Tibet

Abstract Within the Kharta area, east of Mount Qomolangma (Everest), garnet sillimanite gneisses and granites including mafic lenses that form the Greater Himalayan Sequence (GHS) were displaced beneath the North Col Formation by the lower South Tibetan Fault (LSTF) and above the Lesser Himalayan Sequence by the Main Central Thrust (MCT). Zircons from mafic lenses, probably former dikes, were dated by thermal ionization mass spectrometry to yield discordant U‐Pb results. One sample gave a discordia line with an upper intercept age of 971 Ma, suggesting a late Proterozoic age for emplacement of the dike. Zircon SHRIMP analyses show that the garnet sillimanite gneisses and granites were derived mainly from early Paleozoic rocks produced by high‐temperature metamorphism and/or partial melting of Neoproterozoic (meta)sedimentary rocks. These crystalline rocks were buried beneath southern Tibet and experienced metamorphic conditions of 750°–800°C and 14 kbar at \documentclass{aastex} \usepackage{amsbsy} \usepa...

Petrogenesis of the Puerto Edén Igneous and Metamorphic Complex, Magallanes, Chile: Late Jurassic syn-deformational anatexis of metapelites and granitoid magma genesis

A suite of schists, gneisses, migmatites, and biotite granitoids from the Puerto Edén Igneous and Metamorphic Complex (PEIMC) and biotite–hornblende granitoids of the South Patagonian batholith (southern Chile) has been studied. For that purpose, the chemistry of minerals and the bulk rock composition of major and trace elements including Rb–Sr and Sm–Nd isotopes were determined. Mineralogical observations and geothermobarometric calculations indicate high-temperature and low-pressure conditions (ca. 600–700 °C and 3 to 4.5 kbar) for an event of metamorphism and partial melting of metapelites in Late Jurassic times (previously determined by SHRIMP U–Pb zircon ages). Structures in schists, gneisses, migmatites and mylonites indicate non-coaxial deformation flow during and after peak metamorphic and anatectic conditions. Andalusite schists and sillimanite gneisses yield initial 87Sr/ 86Sr ratios of up to 0.7134 and εNd 150 values as low as − 7.6. Contemporaneous biotite granitoids and a coarse-grained orthogneiss have initial 87Sr/ 86Sr ratios between 0.7073 and 0.7089, and εNd 150 values in the range − 7.6 to − 4.4. This indicates that metamorphic rocks do not represent the natural isotopic variation in the migmatite source. Thus, a heterogeneous source with a least radiogenic component was involved in the production of the biotite granitoids. The PEIMC is considered as a segment of an evolving kilometre-sized and deep crustal shear zone in which partial melts were generated and segregated into a large reservoir of magmas forming composite plutons in Late Jurassic times. A biotite–hornblende granodiorite and a muscovite–garnet leucogranite show initial 87Sr/ 86Sr ratios of 0.7048 and 0.7061, and εNd 100 values of − 2.6 and − 1.8, respectively, and are thus probably related to Early Cretaceous magmas not involved in the anatexis of the metasedimentary rocks.

Further evidence for ∼1.85 Ga metamorphism in the Central Zone of the North China Craton: SHRIMP U–Pb dating of zircon from metamorphic rocks in the Lushan area, Henan Province

This paper reports SHRIMP zircon U–Pb dating of Precambrian supracrustal and granitic rocks from the Lushan area, Henan Province, in the southern portion of the Central Zone (also referred to as the Trans-North China Orogen) of the North China Craton. A graphite–garnet–sillimanite gneiss (Sample TW0006/1) of the Shangtaihua ‘Group’ gives a range of inherited zircon ages from 2.73 to 2.26 Ga and a metamorphic zircon age of 1.84 ± 0.07 Ga. A garnet-bearing gneissic granitoid (Sample TWJ358/1), which is considered to intrude the Shangtaihua ‘Group’, gives a magmatic zircon age of 2.14 ± 0.02 Ga and a metamorphic zircon age of ∼1.87 Ga. The metamorphic zircon ages of 1.87–1.84 Ga obtained in this study indicate that an important tectonothermal event occurred at the end of the Paleoproterozoic in the Lushan area. This supports the southern continuation of a Central Zone in the North China Craton that workers have recently considered to result from continent–continent collision. It is also evident that the Shangtaihua ‘Group’ was formed during the Paleoproterozoic (between 2.26 and 2.14 Ga), and not during the Archean, as previously considered.

Chemical imprint of highly metamorphosed volcanic-hosted hydrothermal alterations in the La Romaine Supracrustal Belt, eastern Grenville Province, Quebec

The La Romaine Supracrustal Belt and the southeastern end of the Wakeham Group in the eastern Grenville Province, Canada, host a series of Pinwarian, 1.50 Ga felsic-dominated volcanic centres metamorphosed at amphibolite to granulite facies during the Grenville orogeny. The centres are interpreted as being related to the emergence of rhyolitic domes in shallow-marine intra-arc basins within the active Pinwarian continental magmatic arc. High-grade metamorphosed hydrothermal alteration zones are intimately associated with pyroclastic deposits composing these volcanic centres and an overlying composite amphibolite unit. They comprise layers of rhyolitic metatuff bearing networks of aluminous nodules and veins, migmatized garnet–biotite–sillimanite gneiss with well-preserved volcanic fragments, and mottled quartz–cordierite gneiss with textures similar to those of vuggy silica facies. Alteration zones of ironstone, carbonate and calc-silicate rocks, garnetite, diopsidite, epidosite, and sulphide mineralization collectively cut across the internal contacts of a composite amphibolite unit inferred to be a mafic lava and sill complex. Lithogeochemical analysis of inferred metamorphosed altered rocks and precursors highlights chemical changes typical of metamorphosed sericitic zones, advanced argillic and silicic zones, and discharge zones characterized by calcic alterations and copper mineralizations. Such zones involve the interaction of hot, very acidic to neutral fluids. Medium to heavy rare-earth elements (REE) and Zr behave as mobile elements in the hydrothermal system as a result of the presence of F-rich fluids. The chemical changes recorded by the various alteration zones share similarities with those observed in high-sulphidation, volcanic-hosted massive sulphide deposits occurring in proximal, shallow-marine, volcanic sequences.

Late-Svecofennian transpressive deformation in SW Finland—evidence from late-stage D3 structures

Late-Svecofennian deformation was investigated in the Bjensböle area of SW Finland. The deformation history was revealed by following marker horizons of amphibolite, marble and black schist, in addition to microscopic studies and U-Pb monazite geochronology. The D1 structures are transposed and overprinted by D2 structures. The mesoscopic F2 folds are tight to isoclinal with fold axes plunging 20-40[ddot] NE-ENE. Regional large-scale open F3 folds formed during the main D3 phase, and the macroscopic structure in Bjensböle is an F2-F3 interference pattern formed by N-S compression. The area consist of low P/high T amphibolite facies sillimanite gneiss and low P/low T amphibolite facies quartz schist, separated by a low P/low T amphibolite horizon in the central part of the area. The sillimanite gneiss yielded a U-Pb monazite age of 1824±5 Ma. This age gives a timing for the peak metamorphism and the D3 folding on the Kemiö Island. After the peak metamorphism some deformational structures developed. A NE-SW directed crenulation cleavage in the quartz schist and the amphibolite, small open folds in the marble, a foliation in the sillimanite gneiss and a muscovite orientation in the quartz schist all have the same direction. These structures are interpreted as late-stage D3 (D3L). The angle between the NE-SW directed D3L structures and the locally predominant S0 + S1 + S2 schistosity is ca. 25[ddot]. The structural evidence related to the D3L point to a dextral sense of shearing along roughly E-W directed zones with a NW-SE compression. This is related to a regional ductile shearing signifying the vaning stages of the late-Svecofennian transpressive deformation in SW Finland.

Timing of high-grade metamorphism in central Turkey and the assembly of Anatolia

The timing of metamorphism of the crystalline massifs in western and central Turkey (Menderes, Kırșehir, Nigde, Akdag) is important information for understanding the assembly of Anatolia and the thermal–tectonic evolution of the Turkish segment of the Alpine–Himalayan belt. The high-grade basement rocks of the massifs are polymetamorphic, and the complexity of interpreting ages obtained from zircon and other minerals has led to a longstanding debate about the significance and regional extent of Neoproterozoic v. younger (Mesozoic, Cenozoic) metamorphism. New U–Pb SHRIMP (sensitive high-resolution ion microprobe) analyses of monazite from a garnet–sillimanite gneiss in the Kırșehir Massif, central Anatolia, document Late Cretaceous (84.1 ± 0.8 Ma) high-temperature metamorphism. This Late Cretaceous age for central Anatolian metamorphism is at least 50 Ma older than Alpine metamorphism in the Menderes Massif in western Turkey. Models that propose a continuous Anatolide–Tauride tectonic zone, with central Anatolia as a promontory, cannot explain such a large time difference within a small region. Unless the Menderes ages have been reset by Aegean thermal–tectonic events, Anatolia probably formed from a collage of continental fragments rather than by simple collision of an irregular Tethyan platform with Eurasia.

The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet

This paper presents a new geological map together with cross-sections and lateral sections of the Everest massif. We combine field relations, structural geology, petrology, thermobarometry and geochronology to interpret the tectonic evolution of the Everest Himalaya. Lithospheric convergence of India and Asia since collision at c. 50 Ma. resulted in horizontal shortening, crustal thickening and regional metamorphism in the Himalaya and beneath southern Tibet. High temperatures (&gt;620 °C) during sillimanite grade metamorphism were maintained for 15 million years from 32 to 16.9 ± 0.5 Ma along the top of the Greater Himalayan slab. This implies that crustal thickening must also have been active during this time, which in turn suggests high topography during the Oligocene–early Miocene. Two low-angle normal faults cut the Everest massif at the top of the Greater Himalayan slab. The earlier, lower Lhotse detachment bounds the upper limit of massive leucogranite sills and sillimanite–cordierite gneisses, and has been locally folded. Ductile motion along the top of the Greater Himalayan slab was active from 18 to 16.9 Ma. The upper Qomolangma detachment is exposed in the summit pyramid of Everest and dips north at angles of less than 15°. Brittle faulting along the Qomolangma detachment, which cuts all leucogranites in the footwall, was post-16 Ma. Footwall sillimanite gneisses and leucogranites are exposed along the Kharta valley up to 57 km north of the Qomolangma detachment exposure near the summit of Everest. The amount of extrusion of footwall gneisses and leucogranites must have been around 200 km southwards, from an origin at shallow levels (12–18 km depth) beneath Tibet, supporting models of ductile extrusion of the Greater Himalayan slab. The Everest–Lhotse–Nuptse massif contains a massive ballooning sill of garnet + muscovite + tourmaline leucogranite up to 3000 m thick, which reaches 7800 m on the Kangshung face of Everest and on the south face of Nuptse, and is mainly responsible for the extreme altitude of both mountains. The middle crust beneath southern Tibet is inferred to be a weak, ductile-deforming zone of high heat and low friction separating a brittle deforming upper crust above from a strong (?granulite facies) lower crust with a rheologically strong upper mantle. Field evidence, thermobarometry and U–Pb geochronological data from the Everest Himalaya support the general shear extrusive flow of a mid-crustal channel from beneath the Tibetan plateau. The ending of high temperature metamorphism in the Himalaya and of ductile shearing along both the Main Central Thrust and the South Tibetan Detachment normal faults roughly coincides with initiation of strike-slip faulting and east–west extension in south Tibet (&lt;18 Ma).

Geology and tectonic history of the Lohit Valley, Eastern Arunachal Pradesh, India

The Lohit River section of eastern Arunachal Pradesh comprises four tectonic units. From SW to NE these are: the Lesser Himalayan rocks, the Mishmi Crystallines, the Tidding Suture Zone and the Lohit Plutonic Complex. The Mishmi Thrust underlies the basal Lesser Himalayan unit, while the Mishmi Crystallines are thrust over the Lesser Himalayan unit along the Main Central Thrust. The grade of metamorphism in the Mishmi Crystallines increases up the structural section from chlorite to staurolite–kyanite zones, exhibiting inverted metamorphism. The relationship between deformation and metamorphism shows that the metamorphic peak was syn- to post-tectonic in relation to the main ductile shearing event. Continued deformation, after the metamorphic peak, was accommodated along millimetre scale shear zones, developed throughout the sequence, parallel to the regional schistosity. Movement along these shear zones has resulted in inversion of the metamorphic zones. The rocks of the Tidding Suture represent an ophiolitic mélange, thrust over the Mishmi Crystallines, which in turn are overthrust by the Lohit Plutonic Complex along the Lohit Thrust. The Lohit Plutonic Complex is subdivided into western and eastern belts separated by the Walong Thrust. The western belt consists of deformed quartz-diorite, diorite, gabbro and trondhjemite, intruded by basic and acid dykes. The eastern belt comprises garnet–sillimanite gneiss, intercalated with crystalline marble bands, followed by a complex zone of leucogranites, aplites and pegmatites, which intrude the early foliated quartz-diorite, soda-rich granite and microdiorite. The rocks of the eastern belt are the northward continuation of the Mogok Gneissic Belt of central Burma. The occurrence of intrusive rocks in the eastern belt suggests that the magmatism related to subduction extended to the east, far from the subduction zone. The peraluminous leucogranites, aplites and pegmatites are the products of crustal melting, induced by crustal thickening related to the intracontinental Walong Thrust. Subsequent to metamorphism and shearing, the whole sequence was folded into an antiform, forming the Eastern Syntaxis, and this deformation steeply tilted the earlier low angle thrusts and foliations. Later compression partitioned into right-lateral strike-slip motion, producing a superimposed sub-horizontal lineation observed mostly in the Lohit Plutonic Complex.

Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains

Research Article| November 01, 2001 Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains James E. Fraser; James E. Fraser 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Michael P. Searle; Michael P. Searle 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Randall R. Parrish; Randall R. Parrish 2Department of Geology, University of Leicester, and Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar Stephen R. Noble Stephen R. Noble 3Natural Environment Research Council Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar GSA Bulletin (2001) 113 (11): 1443–1455. https://doi.org/10.1130/0016-7606(2001)113<1443:CODMAM>2.0.CO;2 Article history received: 03 Dec 1999 rev-recd: 09 Aug 2000 accepted: 16 May 2001 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation James E. Fraser, Michael P. Searle, Randall R. Parrish, Stephen R. Noble; Chronology of deformation, metamorphism, and magmatism in the southern Karakoram Mountains. GSA Bulletin 2001;; 113 (11): 1443–1455. doi: https://doi.org/10.1130/0016-7606(2001)113<1443:CODMAM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb dating of metamorphic and igneous rocks from the Hunza Valley and Baltoro regions of the Karakoram Mountains in northern Pakistan addresses the thermal and magmatic evolution of the thickened Asian plate crust before, during, and after the collision of the Kohistan arc and the Indian plate. Crustal thickening and high- temperature, sillimanite-grade metamorphism in the southern Karakoram Mountains followed the collision and accretion of the Kohistan arc during the Late Cretaceous. U-Pb ages of metamorphic monazites from sillimanite gneisses in the Hunza Valley are 63.3 ± 0.4 Ma, ca. 50–52 Ma, and 44.0 ± 2.0 Ma, and monazites from a kyanite-grade schist from the Baltoro region are 28.0 ± 0.5 Ma. Metamorphic monazites from a highly graphitic garnet + staurolite schist from the Hunza Valley yield a crystallization age of 16.0 ± 1.0 Ma. Sillimanite gneisses from the Dassu dome have magmatic zircons of 1855 ± 11 Ma, reflecting a Proterozoic continental crustal source, and metamorphic monazites of 5.4 ± 0.2 Ma. Magmatism was also sporadic; early granodiorite, monzogranite, and leucogranite dikes yield zircon, monazite, and uraninite ages of 50–52 Ma and 35.0 ± 1.0 Ma. Widespread lower crustal melting during the latest Oligocene–early Miocene culminated with emplacement of the Baltoro Plutonic Unit in the Karakoram batholith that cuts deformation fabrics in the high-grade gneisses to the south. The youngest magmatic phase dated is the 9.3 ± 0.2 Ma Sumayar leucogranite pluton. On the basis of detailed structural field studies combined with U-Pb geochronology, sillimanite-grade metamorphism was either a protracted event lasting as long as 20 m.y. (64–44 Ma) or peaked at different times within the lower crust following collision of first, the Kohistan arc, and later, the Indian plate. We also present evidence for southward propagation of peak metamorphism and postmetamorphic thrusting and folding of isograds within the past 5 m.y. Detailed geochronology shows that deformation, metamorphism, and magmatism in the middle and lower crust of the south Asian margin has been occurring within the Karakoram metamorphic complex for more than 60 m.y. Similar processes may also have affected the unplumbed depths of the south Tibetan crust. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tracking fluid flow during deep crustal anatexis: metasomatism of peridotites (Naxos, Greece)

Horizons of ultramafic lenses were metamorphosed with host felsic gneisses at upper amphibolite facies conditions during the M2 event on the island of Naxos, Greece. The synkinematic peak M2 Ol–Opx–Hbl–Chl–Spl assemblage of the Main, migmatite-associated, Ultramafic Horizon (MUH) retains mantle-like chemical and oxygen isotope compositions and thus shows no evidence of infiltration of fluids from the host rocks. A bimodal distribution of temperatures, grouped at 700 and 1,200 °C, is given by oxygen isotope Opx–Ol thermometry in the MUH meta-peridotites and indicates partial oxygen exchange during M2 superposed over previous mantle fractionation. The Agia Ultramafic Horizon (AUH), a coarser-grained and unfoliated peridotite, occurs within sillimanite gneisses in northwest Naxos and contains talc–enstatite and olivine domains. Recrystallization of the AUH peridotite during post-peak M2 infiltration of silica-rich, high δ18O fluids is indicated by lack of deformation, increased activity of silica required to stabilize the talc–enstatite assemblage, extremely high δ18O values of Ol and Opx and Δ18O(Opx–Ol) temperatures of 520–650 °C. The source for these fluids is inferred to be aplitic and pegmatitic dikes emanating from the migmatitic core of Naxos and intruding the AUH. At peak M2 temperatures and during anatexis of gneisses, volumes of fluid were small and fluid composition was locally buffered in the deeper part of the Naxos section. Crystallization of melts within the migmatitic core released siliceous fluids and initiated an episode of retrograde hydrous metamorphism in the overlying sequences, as observed in the AUH.

Extension of a newly identified 500 Ma metamorphic terrane in North East China: further U–Pb SHRIMP dating of the Mashan Complex, Heilongjiang Province, China

New single zircon SHRIMP U–Pb dating of paragneiss and garnet-bearing granite from the Mashan Complex in the Jiamusi Massif at Sandaogou and Xi Mashan, Heilongjiang Province, China, indicates that these rocks underwent granulite facies metamorphism ∼500 Ma ago. Sillimanite gneiss from Sandaogou contains a single population of metamorphic zircons which have a 206Pb/ 238U age of 496±8 Ma. Sillimanite gneiss from Xi Mashan, approximately 16 km west of Sandaogou, has a 206Pb/ 238U metamorphic zircon age of ∼500 Ma, but also includes several populations of detrital zircon grains that extend back to ∼1700 Ma. A garnetiferous granite from Xi Mashan has a 206Pb/ 238U igneous zircon age of 507±12 Ma. It contains an enclave of garnet paragneiss and this has a metamorphic zircon age of ∼500 Ma. The enclave also contains zircons that extend back to ∼1900 Ma, with a concordant population at ∼1100 Ma. These data establish that the southern outcrop of the Mashan Complex, extending west from Liu Mao to Xi Mashan, a total distance of 22 km, records the same early Palaeozoic metamorphic event. The older zircon populations in the sillimanite gneiss and garnet gneiss enclave from Xi Mashan indicate derivation from both Palaeo- and Mesoproterozoic sources. These data, together with the ∼500 Ma age for the granulite facies metamorphic event, suggest that the Mashan Complex formed part of a Late Neoproterozoic–Early Palaeozoic orogenic terrane. High-grade metamorphic rocks of slightly younger age (∼480 Ma) are recorded from southern Siberia to the north of the study area, whereas slightly older ages (550–515 Ma) are recorded from various Pan-African terranes formerly associated with Gondwanaland. However, the closest analogue in the literature is with the Delamerian Orogen in southern Australia, where high-grade metamorphism has been dated at 503±7 Ma. Although speculative, it is argued that the Mashan Complex may possibly once have been located in Gondwanaland, drifting northward along with the North China, South China and Tarim blocks to dock with the Siberian Craton sometime between the late Permian and late Jurassic.