Metamorphic Episode Research Articles

Evidence of back-folding in the Beas Valley puts Himalayan tectonic models on trial

Field structural observations from the Himachal Himalaya, in the northwest of the mountain belt, challenge existing tectonic models and raise questions as to their validity. Microstructures and geochronological data reveal two discrete episodes of Barrovian metamorphism, the earliest during the Eocene-Oligocene transition, before an early period of recumbent folding. This metamorphic event occurred in association with a km-scale extensional ductile shear zone that is itself now recumbently folded on the km-scale, with an axial plane pressure solution cleavage. The Eocene-Oligocene gneiss complex is thus exposed in its core. The second episode of Barrovian metamorphism occurred in association with another regional-scale extensional shear zone during the Oligo-Miocene transition, thus synchronous to the South Tibetan Detachment System. This transects the recumbent fold stack. Microstructures show that the Main Central Thrust was initiated after the second phase of extension, and the associated second episode of Barrovian metamorphism had ceased operating. Further, the previously unrecognized km-scale Phojal Back-fold affects all of the above structures. Confusion caused by the misidentification of this structure led to the tectonic-wedge model, but this hypothesis can be invalidated by the structural evidence presented here. Our data support an alternative hypothesis that requires tectonic mode-switches in association with a succession of accretion events as India indents into Eurasia.

Two episodes of Early Palaeozoic high-pressure metamorphism in North Balkhash ophiolite zone (Central Kazakhstan, western Central Asian Orogenic Belt): Evidence for tectonic evolution of Junggar–Balkhash Ocean

The structure of serpentinite mélange of the North Balkhash ophiolite zone (Central Kazakhstan; west Central Asian Orogenic Belt) is ascertained to be high-grade formations assigned to epidote-glaucophane eclogites and garnet blueschists, and their varieties. The rocks follow a clockwise ‘subduction-type’ evolution with the estimated near–peak metamorphic conditions of 1.6–1.9 GPa and 500–560 °C. 40Ar39Ar phengite ages of ∼491 Ma and ∼ 465 Ma obtained for the eclogites and garnet blueschists, respectively, are interpreted to reflect the near-peak to shortly retrograde stages of rock evolution and to record the late Cambrian and Middle Ordovician episodes of high-pressure re-equilibration in the North Balkhash zone. Protoliths of the eclogites were N-MORB-like mafic rocks, which comprised structurally different (from the lower gabbroic to the upper dolerite/basalt) parts of pre-late Cambrian oceanic crust and were formed in the spreading centre setting at the expense of depleted mantle source. Protoliths of the garnet blueschists and associated rocks were represented by volcanogenic–sedimentary, predominantly mafic, complexes (tuffaceous sandstones, greywackes), an accumulation of which and subsequent involvement into subduction occurred at ∼478–465 Ma in the intra-oceanic (fore-arc) setting. Zircon core ages indicate formation of the source of the protoliths of the garnet blueschists occurred in the 509–478 Ma range. A comprehensive correlation of the metamorphic and igneous formations of the North Balkhash zone with those assigned to the adjacent complexes of the southern part of the West Junggar area (NW China) suggested their mutual Early Palaeozoic tectonic evolution within the Junggar–Balkhash Ocean.

The Kangâmiut dykes in West Greenland: markers of the tectono-metamorphic evolution of the southern Nagssugtoqidian orogen and its foreland

The general extent and structural evolution of the southern Nagssugtoqidian orogen of West Greenland were first described by Hans Ramberg who based much of his paper on the deformation of the regional Kangâmiut dyke swarm. The southern boundary is marked by a transition from undeformed, discordant dykes in the south to highly deformed dykes and host rocks to the north. Our analysis of the southern Nagssugtoqidian orogen and its southern foreland uses a comprehensive compilation of available data and covers the area from Sisimiut in the north to Alanngua, south of Maniitsoq. This represents almost the entire c. 200 km latitudinal extent of the Kangâmiut dyke swarm and encompasses the complete range of Nagssugtoqidian overprint on these dykes and their country rocks. South of Itillip Ilua (Itilleq), the structural and metamorphic overprints on the dykes exhibit a considerable range in both intensity and P–T conditions between and even within outcrops. In contrast, north of Itillip Ilua, the rocks show more systematic gradual increases in the degree of structural overprints and metamorphic grade, culminating in the Ikertooq thrust zone where granulite facies rocks are brought southwards over amphibolite facies rocks. Currently, available age data from the Nagssugtoqidian orogen permits the identification of two metamorphic episodes at c. 1850–1800 Ma and c. 1780–1720 Ma. These groups of metamorphic ages are supported by recent 40Ar–39Ar ages from dykes in the same area, which cluster at c. 1860 Ma and c. 1740 Ma, respectively. Albeit geographically sporadic, both age intervals support a subdivision of the Nagssugtoqidian structural and metamorphic overprints across the southern Nagssugtoqidian orogen and its foreland into two distinguishable temporal phases. Further geochronological investigations may well, however, find these two phases to be part of a tectonic continuum. For now, it is thought that the older event records south-directed thrusting over the foreland and concomitant loading of this crust, at least as far south as Maniitsoq. This c. 1860–1800 Ma crustal shortening and thrusting likely also closed a depositional basin located at the current latitude of Ikertooq, which could have formed during an early-orogenic extensional event that enabled and accompanied the c. 2035 Ma emplacement of Kangâmiut dykes. Up to 50–100 Ma later, a younger (c. 1780–1720 Ma) phase of shearing and thrusting mainly affected the Itillip Ilua – Ikertooq area and likely overprinted elements of the former event. This local younger overprint generated a separate trend of distinctly northward-increasing deformation and metamorphism.

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

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

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

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

Retrograde Assemblages in the Muscovite-Biotite Gneiss of Oluyole Southwestern Nigeria, an Indication of Shear-Zone Environment

Petrographic and whole-rock geochemical study of biotite-muscovite gneiss was determined in order to interpret the metamorphic evolution of the Basement Complex of Southwestern, Nigeria. The gneiss shows a millimetric banding, and in some cases the quartzo-feldspathic bands running up to 10 cm. The gneiss has mineral assemblage biotite + plagioclase + quartz + garnet + K-feldspar + muscovite + chlorite + ilmenite ±titanite. Chlorite occurs along cleavage planes of biotite, and in some cases forms reaction rims around porphyroblasts of garnet. K-feldspar crystals are surrounded by muscovite. Titanite crystals are sub-idioblastic to xenoblastic in form, and have inclusions of ilmenite. Titanite, where present, occurs in close association with biotite and opaque minerals (ilmenite). Also, titanite forms a reaction rim around apatite. Mylonitic texture, fine-grained matrix of mica and quartz ribbons were observed. In addition, there is stretching of the quartz crystals. The SiO2 content is greater than 60 wt %, while CaO ranges from 3.05-6.91 wt %. The M1 foliation comprise of mineral biotite some of which are included in the opaque mineral, M2 represents the metamorphism which gave rise to porphyroblasts of ilmenite, while the M3 gave rise to foliations that forms a wraparound structure on the porphyroblasts of ilmenite. The last metamorphism gave rise to retrograde minerals; chlorite, titanite, and muscovite. The study suggests that this area of the Basement Complex has been subjected to multiple deformations, as well as multiple episodes of metamorphism. The structures observed are similar to those associated with shear zone environment.

Garnet-Staurolite-Mica Schist from Southern Sonbhadra District, U.P.: Constraints from Geothermobarometry and P-T Pseudosection Modelling

Abstract The area around Renukoot town shows the exposure of garnetstaurolite- mica schist belonging to the Chhotanagpur Granite Gneiss Complex (CGGC). The characteristic mineral assemblage present in the rock are garnet–biotite–staurolite–quartz along with minor opaque minerals like rutile, ilmenite and hematite. For the constituent minerals, the calculated XMg value decreases in the order: chlorite>biotite>staurolite>garnet. The metamorphic conditions of the garnet-staurolite-mica schist were determined using the winTWQ and Perple_X programmes, and the peak pressure-temperature conditions for the rock are estimated to be 5.3 kbar and 570°C. The metamorphic episode in the CGGC during Mesoproterozoic time may be attributed to the global level Columbian Super-continental accretionary orogeny.

FIELD GEOLOGY AND PETROGRAPHY AROUND NABARDO-TORO ENVIRONS, NORTH EASTERN, NIGERIA

Background: The mapping exercise within the study ar ea on a scale of 1:25,000 was carried out to study the rocks, determine their distribution, composition, structures and assess the economic potential of the area. Objectives: The geology mapping aim to identify and understand the various geological structures present in the area to gain insights into the tectonic history and deformation processes that have shaped the area and to identify the minerals present in the rock, to understand the rock formation, composition and potential geologic processes involved. Method: Method of mapping involved road, footpath, rivers and stream traverses. Representative rock samples were collected for petrographic studies and geochemical analysis to establish the geology of the study area. Result: The study area which is part of Sheet 148, Toro SE is entirely typical of the Nigerian Basement Complex and is underlain by the following rock units: Migmatitic Gneiss, Granite gneiss, mica schist and porphyritic granite. The Migmatitic Gneiss constitutes the largest group of the rock units covering more than 45% of the mapped area and trending mainly NW/SE with few exceptions in an almost N/S to NE/SW direction. It is pertinent to know that almost E-W structures were also observed which forms relics of the older structure believed to have been affected by the eburnean (possibly) orogenic event. The granite gneiss shares boundary mainly with the migmatitic gneiss showing a weak foliation of light and dark materials. The mica schist is exposed at the western end, highly weathered with a general trend of NW/SE. The porphyritic granite intruded both the migmatite and granite gneiss occurring prominently as hilly and bouldery outcrops. However, flat lying low lying was also observed in some other localities. With respect to field observation and petrographical study the lithologies encountered were grouped into nine (9) categories- homogeneous, inhomogeneous, stromatic, banded orthogneiss, nebulitic, apo granite, pegmatite, intermediate and schist.Conclusion: Field evidence suggests that the study area has undergone an episode of metamorphism and deformations as depicted by the structural features like preferred orientation of the mineralogical and lithological units. It was also observed from the field measurements, that the main structural trend within the study area is from NNE/SSW.

Late Paleozoic magmatism and foreland deformation associated with opening and closing of marginal basins in the North Patagonian Andes

Remnants of coeval Devonian oceanic and continental foreland rocks are preserved in the basement of the North Patagonian Andes. Our previous studies of the igneous rocks here have shown that the primitive oceanic and continental igneous rocks are coeval and belong to a marginal basin that opened and closed over 50 Myr. A structural study and four new U–Pb SHRIMP ages and zircon Hf–O determinations allowed the identification of three metamorphic episodes, the first one of which, M 1 (D 1 -S 1 ), is coeval with Andean-type granite intrusion in the foreland (405–380 Ma). This activity was simultaneous with the development of an oceanic ridge and a marginal basin, at the outer edge of which a primitive granitic oceanic arc formed (380–385 Ma; zircon δ 18 O 3.6–5.2‰). The extinction of this ridge initiated under-thrusting of the oceanic crust below the continent and an important mid- to high-grade metamorphic event, M 2 (D 2 -S 2 ) at 375–360 Ma, its peak dated by metamorphic zircon rims in migmatite at 365 ± 3 Ma. Basin closure occurred after the intrusion of S-type granites (357 ± 2 Ma; zircon δ 18 O 7.4–10.4‰) in the foreland and the accretion of gabbros, cumulate gabbros and trondhjemites at the proto-Pacific margin. Compression prevailed for 20 Myr in the foreland, causing a mylonitic medium-grade M 3 (D 3 -S 3 ) event. Supplementary material: Datasets are available at https://doi.org/10.6084/m9.figshare.c.6989873

Paleoarchean metamorphism in the Acasta Gneiss Complex: Constraints from phase equilibrium modelling and in situ garnet Lu–Hf geochronology

AbstractThe oldest known evolved (felsic) rocks on Earth (c. 4.03 Ga) are found in the Acasta Gneiss Complex (AGC) in north‐western Canada and represent a fundamental keystone in unravelling the geological processes governing crustal growth and differentiation during the Hadean and early Archean. Although the timing of multiple episodes of magmatism, metamorphism and deformation in these tonalitic gneisses has been investigated extensively, the metamorphic pressure–temperature (P–T) conditions recorded by the rocks are poorly constrained. Here, we use phase equilibrium modelling coupled with in situ garnet Lu–Hf geochronology and trace element analysis for two garnet‐bearing tonalitic gneisses to decipher the metamorphic history of the AGC. The observed mineral assemblages are consistent with peak metamorphic conditions of T = 725–780°C and P = 4.5–6.2 kbar and the generation of a small amount of melt (<7 vol.%). Garnet geochronology constrains the age of metamorphism to 3.3–3.2 Ga, consistent with previous evidence for a late Paleoarchean tectono‐metamorphic event in the AGC. Subsequent isotopic disturbance of garnet at c. 1.9 Ga is interpreted to correspond to a modification of the primary Lu–Hf systematics in response to garnet resorption/recrystallization during the Paleoproterozoic Wopmay orogeny, resulting in significant scatter between these two age components. Our study adds to the small number of published P–T data for metamorphic rocks older than 2.8 Ga and shows that tonalitic gneisses in the AGC record a high apparent thermal gradient of ~140°C/kbar in the late Paleoarchean. This thermal gradient is the highest among the limited dataset, but is broadly similar to data from other Paleoarchean‐Mesoarchean crustal rocks in recording high T/P ratios (>77.5°C/kbar).

New U-Pb and Hf data of the contact region between the Oriental and Cabo Frio terranes, Central Ribeira Belt, Brazil: Implications for the closure of the Ediacaran-Cambrian oceanic space

The Ribeira Belt represents a deeply eroded orogen which exposes rocks from the different stages of magmatic arcs and associated metasedimentary basins. This contribution focus on the last collisional episode between the Outer Magmatic Arc System (OMAS) and the Cabo Frio Terrane (Angola craton). The OMAS represents a long-lived magmatic arc that collided against the Inner Magmatic System (IMAS) at ca. 590 Ma (M1 metamorphism), and against the Cabo Frio Terrane at ca. 530 Ma (M2 metamorphism). The nature of the magmatism recorded in the first collisional episode, and the subduction polarity of the second is a current topic of debate. New U-Pb age and Lu-Hf analyses in zircon grains from units close to the contact are presented to propose a tectonic model for the closure of the oceanic space between the OMAS and the Cabo Frio Terrane. The results in combination with published data suggest that the metasedimentary rocks of the São Fidélis Group are in a back-arc position of the OMAS and were affected by both the M1 and M2 metamorphic episodes, reflected by the presence of metamorphic zircons with ages between ca. 592–553 Ma and 543–533 Ma. The orthogneiss of the Tinguí Complex intrusive in the back-arc metasedimentary units yielded a crystallization age of 578 ± 9 Ma, coeval with the syn-collisional stage M1. A younger metamorphic overprint at 529 ± 7 Ma was also detected (syn-M2). The orthogneiss of the Região dos Lagos Complex (Cabo Frio Terrane) rendered a crystallization age of 1968 ± 26 and a M2 metamorphic overprint at 522 Ma. Zircon analyses from folded leucosomes in the orthogneiss also indicated a crystallization age of 523 ± 10 Ma, which constrain the time of folding after thrusting. Undeformed post-collisional granitoids and pegmatites yielded crystallization ages between 496 and 499 Ma, closing the tectonic evolution of the belt with the orogen collapse. Our results corroborate with terrane accretion model for the Central Segment of the Ribeira Belt.

Regulation of energy properties and thermal behavior of bio-coal from lignocellulosic biomass using torrefaction

The energy properties in bio-coal, such as proximate analysis, element composition, high heat value, colour, thermal behavior are detailed elaborated. The results show bio-coal production processes from biomass can be divided into three stages which is close to coalification through the data in energy properties. The first stage called mild torrefaction (≤220 °C) seen as biomass to peat phase during coalification in the result of unstable oxygenated functional groups in hemicellulose are decomposed and Maillard reaction induces the colour modification. Second stage called moderate torrefaction (220–260 °C) seen as lignite formation process during coalification leads to degradation of celluloses. Biomass colour from brown to sepia due to the chromophoric groups. The heating value of maple sawdust (MS) increases to 30.02 MJ/kg of MS-260. Third stage called severe torrefaction (260–300 °C) considered as metamorphic episode of lignite to bituminous coal results of covalent bond cleavage, oxygenation of lignin and condensation of aromatic carbons. As functional groups decomposition during bio-coal production, three characteristic temperatures increase with the torrefied temperature increasing read from TG and DTG curves. The MS with the highest cellulose and aliphatic C–C bonds, is effective raw materials for bio-coal production by torrefaction with the close energy properties, thermal behavior to bituminous coals.

Garnet–Quartz Inclusion Thermobarometry and Lu–Hf Chronology Detail the Pre-Ultra-High Pressure Metamorphic History of the Grapesvare Nappe, Scandinavian Caledonides

Abstract The subduction–exhumation history of the Grapesvare nappe in the northern Seve Nappe Complex (Scandinavian Caledonides) is recorded by late Cambrian/Early Ordovician ultra-high pressure (UHP) and subsequent amphibolite facies metamorphic events. Records of these events obscured earlier metamorphic episodes that are important for understanding the tectonics of the orogen. To extract the pre–UHP metamorphic records, garnet Lu–Hf geochronology, Titanium-in-Quartz thermobarometry, and Quartz-in-Garnet elastic thermobarometry were applied to garnet porphyroblasts in metasedimentary rocks and eclogite. Metasedimentary rocks contain chemically homogeneous garnet (Grt-M1) with shape-matured quartz inclusions. In some rocks, these garnets are overgrown by garnet with bell-shaped Mn-zoning (Grt-M2) containing irregularly-shaped quartz inclusions. This evolution is interpreted as partial dissolution of Grt-M1 and subsequent growth of Grt-M2. Garnet in the eclogite is volumetrically dominated by eclogite-facies garnet (Grt-E1) that envelope remnants of an older, chemically distinct generation (Grt-E0) with highly irregular and diffuse boundaries. Shape-matured quartz inclusions are present within both garnet generations and define a zoning pattern that is not reflective of the chemical zoning. Collectively, these characteristics are interpreted as replacement of Grt-E0 by Grt-E1 via interface-coupled dissolution–reprecipitation, with the latter inheriting the shape-matured quartz inclusions of the former. Pressure–temperature (P–T) conditions extracted from the quartz inclusions in Grt-M1 and Grt-E0/E1 are 1.08 to 1.21 GPa at 645°C to 695°C and 0.94 to 1.03 GPa at 605°C to 640°C, respectively. These conditions are interpreted as cooling of the rocks from a high temperature metamorphic history, altogether preceding subduction of the Grapesvare nappe. The quartz inclusions in Grt-M2 record 1.04 to 1.21 GPa at 620°C to 675°C, interpreted as prograde metamorphic growth of Grt-M2 during subduction at 495.7 ± 3.2 Ma. Subsequent eclogite-facies metamorphism was responsible for the formation of Grt-E1 at the expense of Grt-E0. The collective results indicate a prolonged polymetamorphic history of the Grapesvare nappe prior to UHP metamorphism that has not been recognized previously.

Early Precambrian basement components of the Cathaysia Block in South China exhumed by Phanerozoic orogenic events

Ancient rock assemblages formed during the early history of the Earth are normally regarded as continental nuclei and they occur in old cratons. In recent years, they have been reported from different orogenic belts, but the reasons why they occur in orogenic belts are yet to be constained. The Cathaysia Block in South China is a continental block that witnessed multiple episodes of orogenic events and ancient basement rocks sporadically occur in the Wuyi area, where intense metamorphic alteration can be seen. This area is therefore ideal for addressing the above issue. New data, in combination with a review of previous studies, show that the ancient basement components of the Cathaysia Block formed during > 1.93 – 1.75 Ga and are mostly reworked ancient crustal materials (∼3.8 – 2.5 Ga). The first episode of metamorphism recorded in this area can be traced back to the Paleoproterozoic, but the features of this metamorphism are unclear. The early Paleozoic metamorphic rocks occur across the Wuyi area, and their protolith ages are dominantly Neoproterozoic, with a small amount of Paleoproterozoic rocks. This episode of metamorphism is characterized by high geothermal gradients, possibly corresponding to an arc setting. The early Mesozoic high-grade metamorphism of the Wuyi area is characterized by low geothermal gradients, indicating significant crustal thickening during the orogenic event. The spatial distribution of the early Mesozoic high-grade metamorphic rocks and the early Precambrian basement rocks of the Cathaysia Block are almost the same. Considering that the ancient basement rocks of the Cathaysia Block have been found in the Wuyi area, it is inferred that both the early Paleozoic and the early Mesozoic orogenic events should have facilitated the exhumation of the early Precambrian basement components. It is further suggested that the final exhumation of these ancient basement rocks is dominated by the early Mesozoic orogenic event because compared with other orogenic events of this continental block, this orogeny influenced rocks from deeper crustal levels (with the highest metamorphic pressures). Such a mechanism of ancient basement rocks exhumed by overprinted orogenic events might apply to many other high-grade terranes.

Multistage evolution of the Angara orogenic belt (SW Siberian craton) from granulite to ultrahigh-temperature metamorphism

An exotic block of andalusite-bearing garnet-cordierite-sillimanite gneisses was identified in the area of ultrahigh-temperature metamorphic complex of Angara-Kan block (southwestern Siberian Craton). Study of PT-conditions of these rocks indicate that peak PT-parameters correspond to granulite facies at moderate pressure (T = 820 °C, P = 6 kbar) and was followed by clockwise retrograde PT-path with changing direction from near isothermal decompression to near isobaric cooling at pressure about 3 kbar. The new U-Pb SHRIMP and LA-ICP-MS data on zircons and monazite indicate the time of granulite metamorphism at 1865 ± 5 Ma. The age data, metamorphic mineral assemblages and micro-textures indicate that garnet-cordierite-sillimanite gneisses were not affected by later ultrahigh-temperature metamorphism (1.78–1.76 Ga) widespread in the Angara-Kan block. Thus, the block of garnet-cordierite-sillimanite gneisses can be considered as a fragment of the early granulite metamorphic complex, which was tectonically combined with UHT metamorphic block of the Angara-Kan terrane. The observations prove multistage metamorphic history of the Angara-Kan block and indicate collisional event at ca. 1.86 Ga typical for Siberian craton amalgamation. This metamorphic episode caused melting out of a large volume of post-collisional granitoids and probably played a key role in pre-conditioning of the crust for achieving of UHT conditions during the following metamorphism.

In situ Pb–Pb garnet geochronology as a tool for investigating polymetamorphism: a case for Paleoarchean lateral tectonic thickening

Abstract The Barberton Granite–Greenstone Belt remains a key location in the debate concerning the nature of Archean tectonic processes. Much work has focused on deciphering the tectonic significance of the c. 3.23 Ga metamorphism, as this has been correlated with lower geothermal gradient conditions potentially indicating Archean subduction. However, several studies also found evidence of an earlier, 3.45 Ga metamorphic episode, overprinted by the 3.23 Ga event. Here we apply in situ Pb–Pb dating and P–T modelling to a large (3 cm diameter) garnet crystal, allowing for the direct dating of the metamorphic conditions obtained from the garnet. The garnet core produced an isochron age of 3435 ± 45 Ma, corresponding to an increase in P and T evolution reaching peak conditions of at least 7 kbar and 700°C. Analyses obtained from the garnet rim give an isochron age of 3245 ± 41 Ma, corresponding to P–T conditions reaching 8–9 kbar and 700°C. The preservation of two moderate- to high-pressure events occurring 200 million years apart is consistent with lateral tectonic processes producing crustal thickening at 3.2 Ga and may also be a viable process for the earlier event.

Expanding Azania at the heart of Gondwana: Terrane correlation from Southern India to Sri Lanka

The reconstruction of megacontinent Gondwana was arguably the first crowning achievement of paleogeography, nonetheless the kinematics of its assembly remains controversial. The now-outmoded concept that Gondwana assembly was simply the amalgamation of East and West Gondwana has been replaced by models involving multiple terrane accretions and continental collisions along different orogenic belts. Although the affinities of large cratons are well established, figuring out the relationships between the multiple “Pan-African” orogenic belts has been hindered by poorly defined smaller intervening terranes. At the center of Gondwana, the polymetamorphic terranes in Sri Lanka have been correlated to various crustal blocks without broad consensus, mainly because of a paucity of high-quality geochronological results. Terrane correlations and figuring out the spatial distributions of “Pan-African” orogenic belts in the central part of Gondwana are, therefore, hampered by the controversies about Sri Lanka. We present new data on the high-grade metamorphic rocks occurring in the Highland Complex of Sri Lanka, which reveal the widespread Paleoproterozoic magmatism with U-Pb ages of ≥ 2465 – 1803 Ma and uniform Archean Hf model ages. Our data confirm the pervasive metamorphic ages of ∼ 610 – 510 Ma, and for the first time, we present multiple episodes of metamorphism at Paleoproterozoic (1854 – 1803 Ma) and Neoproterozoic (∼637 – 635 Ma) which are comparable with those reported from the East African Orogen. Based on a comparison with neighboring crustal blocks, we suggest that these early Precambrian basement components as well as the multiple metamorphism of the Highland Complex in Sri Lanka can be correlated with micro-continent Azania. These new data not only confirm the widespread early Precambrian basement components in Sri Lanka, but also open up new ways for terrane correlations from East Africa through Sri Lanka to East Antarctic.

Tectonic evolution of Southeast Asia documented by two periods of high-grade metamorphism in the Kon Tum Massif, Central Vietnam

The Kon Tum Massif in the eastern Indochina Block (ICB) contains extensive Ordo-Silurian and Permo-Triassic metamorphic rocks. These two episodes of metamorphism not only affected the Kon Tum Massif, but were also widely developed in the whole of Southeast and East Asia, including the South China Block (SCB). Comparisons of the metamorphic characteristics among these blocks can reveal the crustal evolution and their affinity in supercontinents. Two gneiss samples in the eastern part of the Kan Nack Complex of the Kon Tum Massif have similar peak metamorphic ages of 488–482 Ma and mineral assemblages of garnet + plagioclase + K-feldspar + biotite + sillimanite + ilmenite + quartz. P-T calculations and thermodynamic modelling constrain a pressure peak (M1) at 770–820 °C and ∼1.1–1.2 GPa, and a temperature peak (M2) at 790–850 °C and 0.85–0.98 GPa, suggesting a near-isothermal decompression. The post-peak M3 assemblage developed at 650–760 °C and 0.48–0.74 GPa. The clockwise P-T-t path with medium and variable geothermal gradients probably reflects an orogeny process from compression to extension, which was triggered by the closure of the Tam Ky-Phuoc Son Ocean in the northern flank of the Kon Tum Massif. The P-T-t path and tectonic setting of this period of metamorphism in the Kon Tum Massif is similar to that in the SCB, but occurred a little earlier than that in the SCB, suggesting that the ICB and the SCB were adjacent on the northern margin of East Gondwana and together experienced a diachronous early Paleozoic orogeny.A granulite sample from the western Kan Nack Complex contains garnet + orthopyroxene + plagioclase + quartz + ilmenite ± K-feldspar. Decomposition of garnet to plagioclase and orthopyroxene suggests a decompression process. Coupled with features of a gneiss sampled at the same spot, P-T calculations and thermodynamic modelling indicate a pressure peak of 885–960 °C and 0.83–0.95 GPa and a temperature peak at 890–970 °C and 0.61–0.73 GPa, corresponding to ultrahigh-temperature (UHT) metamorphism. LA-ICP-MS monazite U-Th-Pb dating constrains a peak metamorphic age of 254 Ma, and Sm-Nd mineral isochrons date the retrograde stage at 243–236 Ma. This UHT metamorphism with a clockwise P-T-t path probably resulted from the excess heat supplied by asthenospheric upwelling following the break-off of the subducting slab of the Paleo-Tethyan Ocean. The Permo-Triassic metamorphism and other geological records in the ICB are significantly different from those in the SCB, implying that the geological processes in the two blocks were controlled by different geodynamic regimes during Late Permian to Early Triassic.

An extended Neoarchaean to Neoproterozoic history of the Sandmata Complex (Aravalli Craton, northwestern India): Insights from metamorphic evolution and zircon-monazite geochronology of high-grade quartzofeldspathic gneisses

The Sandmata Complex in the Aravalli Craton of northwestern India experienced syn- to post-collisional high-grade metamorphism and migmatization related to the 1.9–1.7 Ga Aravalli orogeny. Garnet-bearing quartzofeldspathic gneisses are an important rock type in the Sandmata Complex. Their prograde assemblage is represented by garnet cores and inclusion minerals Bt + Pl + Qz, whereas the assemblage Pl + Kfs + Qz + Btmatrix + Sil + Ilm, along with chemically homogeneous garnet porphyroblasts, represents peak metamorphism. Post-peak retrograde metamorphism is reflected in the growth of muscovite, chlorite and biotite, and diffusion adjustments in garnet rims. Mineral paragenesis, mineral chemistry, conventional thermobarometry, and phase equilibrium modelling indicate P–T conditions of 6.7–10.2 kbar at 725–830 °C for peak metamorphism, and 4.8–6.9 kbar at 450–610 °C for post-peak retrogression. The P–T conditions determined from the quartzofeldspathic gneisses define a near-isobaric cooling P–T path. In contrast, previously studied high-grade pelitic gneisses and metagranitoids from the Sandmata Complex define an isothermal decompression P–T path. U–Pb dating of magmatic zircon cores in the quartzofeldspathic gneisses provides the crystallization age of their felsic protoliths as ∼ 2.52 Ga. Zircon and monazite geochronology show that peak metamorphism occurred at ∼ 1.90–1.78 Ga and thus shortly preceded the emplacement of the Gyangarh-Asind igneous suite and Anjana granite, previously dated at ∼ 1.72–1.64 Ga. A model of oceanic crust subduction followed by continent–continent collision during the Aravalli orogeny can explain granulite facies metamorphism in the Sandmata Complex. The combined results of this and previous studies support the operation of modern-style plate tectonic processes involving subduction and continental collision during Palaeoproterozoic time. We also identify a younger (∼0.90–0.78 Ga) metamorphic episode, preserved mostly in recrystallized monazite grains elongated parallel to the final deformation fabric, likely related to the South Delhi orogeny.

Andrieslombaardite, RhSbS, a new platinum-group mineral from the platiniferous Onverwacht Pipe, Republic of South Africa

Abstract A hundred years after the discovery of the Merensky Reef in 1924, it is appropriate to present the new mineral andrieslombaardite in honour of Andries Frederik Lombaard who was instrumental in its discovery. Andrieslombaardite, RhSbS, was first described as an unknown mineral from placer deposits associated with the Tulameen Alaskan-Uralian type complex, British Colombia, Canada (Raicevic and Cabri, 1976) but has since been reported from several other deposits including the platiniferous Driekop, Mooihoek, and Onverwacht pipes in the eastern Bushveld Complex, South Africa. The mineral and the name were approved by the Commission on New Minerals Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA no. 2022-076) based on data in the co-type samples from Onverwacht and a co-type sample from the Yubdo stream, Birbir River, Ethiopia. Andrieslombaardite in the Onverwacht sample is a single 8 x 20 μm grain attached to laurite in a matrix of altered silicate and Fe-oxyhydroxide minerals. In the Yubdo samples, there are many grains of pale brownish gray andrieslombaardite up to 25 x 55 μm in size, included in Pt-Fe alloys, some associated with erlichmanite, and others attached to bornite and chalcopyrite. The reflectance values (R%) measured in air and in oil at the COM wavelengths are 48.3 and 33.0 (470 nm), 49.3 and 34.0 (546 nm), 51.0 and 35.9 (589 nm), and 51.8 and 36.7 (650 nm). The colour values x, y, Y, λd, and Pe in air are 0.317, 0.322, 50.3, 580, and 3.2, and in oil are 0.319, 0.324, 35.6, 579, and 4.5. The composition of andrieslombaardite is ideally RhSbS, but it contains variable amounts of Fe, Pt, Pd, and Ir that may substitute for Rh. The mineral is cubic with unit-cell dimensions of a = 6.0278(4) Å, V = 219.01(6) Å3 and Z = 4. It was synthesised at 400 and 550°C using stoichiometric elemental amounts. It is a member of the cobaltite group. The mineralisation of the intrusive dunite pipes was probably introduced at high temperatures, under magmatic conditions. The primary assemblages were to a certain degree overprinted and redistributed by low-temperature hydrothermal fluids. The Pt-Fe alloys from Yubdo containing PGM inclusions such as andrieslombaardite in the Yubdo-Alaskan-type complex were formed at some post-magmatic stage owing to PGE remobilisation during hydrothermal or metamorphic episodes.