Medium-grade Metasedimentary Rocks Research Articles

Meso-Neoproterozoic mid-crustal metamorphic record from the Ajmer–Shrinagar section, Rajasthan, India and its implication to the assembly of the Greater Indian Landmass during the Grenvillian-age orogenesis

Abstract The Shrinagar–Ajmer section of the South Delhi Fold Belt exposes a package of medium-grade metasedimentary rocks intruded by synkinematic granite, and the entire package was thrust on top of the basement gneisses occurring further east. The metamorphic history is best developed in the staurolite schist that shows an overall increase in modal abundance of staurolite towards the east. Textural analyses, garnet zoning profiles, thermobarometric data and phase equilibria analyses show an increase in metamorphic pressure and temperature, reaching peak conditions of 592±12°C and 7.7±0.11 kbar. In situ monazite dating of a staurolite schist sample yields a pooled age of 980±22 Ma, which is assumed to be close to the age of the peak metamorphism. The Shrinagar granite was possibly emplaced close to the orogeny occurring at approximately 980 Ma and deformed by later events. The style and timing of metamorphism in the Shrinagar–Ajmer section match with the granulite-facies reworking of the basement rocks of the Aravalli–Delhi Mobile Belt. We envisage that the Grenvillian-age orogeny with its characteristic collisional style involved deep- to mid-crustal sections of the Aravalli–Delhi Mobile Belt. Our results further indicate that the Greater Indian Landmass was assembled during the formation of the supercontinent Rodinia. Supplementary material: Electron microprobe data of the garnet used for chemical zoning in Figure 5 are available at https://doi.org/10.6084/m9.figshare.c.3738335

Transient allanite replacement and prograde to retrograde monazite (re)crystallization in medium-grade metasedimentary rocks from the Orlica-Śnieżnik Dome (Czech Republic/Poland): Textural and geochronological arguments

We use textural and geochronological arguments to constrain monazite petrogenesis and the age of metamorphic events in medium-grade metasedimentary rocks from the European Variscan Belt (Orlica-Śnieżnik Dome, Czech Republic/Poland). The relationships between monazite, allanite and other major and accessory phases are characterized for staurolite-, sillimanite- and kyanite-grade, garnet-bearing mica schist samples. Garnet porphyroblasts with prograde compositional zoning include allanite in the core and monazite at the rim. Allanite is virtually absent from the matrix; it shows replacement textures involving a cluster of Th±U±Y-rich silicates or phosphates. Monazite, with frequent micro-cracks or trimmed grain boundaries, abounds in the mica-rich matrix; it occurs as lone grains close to partially resorbed garnet, apatite or staurolite, and as subhedral and compositionally zoned aggregates associated with allanite and xenotime. Electron microprobe analyses of monazite define Pb vs. Th* isochrons (ages=363–343Ma) with positive intercepts (Pb0=40–120ppm) which suggest the presence of initial Pb and age overestimation. Isotopic analyses obtained by laser ablation coupled with mass spectrometry confirm the minor presence of initial Pb which leads to variable discordance in the 206Pb/238U–207Pb/235U system. Conversely, the 206Pb/238U–208Pb/232Th system is not visibly affected; analyses are concordant and 208Pb/232Th ages range from 358 to 299Ma (±7–8Ma) regardless of monazite texture or composition. They define a main group at 330–310Ma, with few older outliers. All observations suggest that a first monazite generation (MnzI) formed via allanite breakdown at prograde to peak P–T conditions (5–7kbar/575–640°C). A comparison with previous geochronological results reveals that the age of this prograde reaction is, however, preserved by only few domains yielding 360–340Ma. Instead, our results emphasize the importance of retrograde processes which built the dominant age population of 330–310Ma. The latter ages are ascribed to a combination of (1) transient monazite growth after allanite and controlled by P availability, (2) variable recrystallization/replacement of older monazite grains, and (3) minor monazite neoformation due to the resorption of garnet or apatite. The young monazite grains or domains (MnzII) mostly formed during fluid influx and chloritization; they constrain the end of the orogenic history when mid-crustal rocks were exhumed and re-hydrated at temperatures below ~300°C.

Fluid‐induced breakdown of monazite in medium‐grade metasedimentary rocks of the Pontremoli basement (Northern Apennines, Italy)

AbstractThe last (decompression) stages of the metamorphic evolution can modify monazite microstructure and composition, making it difficult to link monazite dates with pressure and temperature conditions. Monazite and its breakdown products under fluid‐present conditions were studied in micaschist recovered from the cuttings of the Pontremoli1 well, Tuscany. Coronitic microstructures around monazite consist of concentric zones of apatite + Th‐silicate, allanite and epidote. The chemistry and microstructure of the monazite grains, which preserve a wide range of chemical dates ranging from Upper Carboniferous to Tertiary times, suggest that this mineral underwent a fluid‐mediated coupled dissolution–reprecipitation and crystallization processes. Consideration of the chemical zoning (major and selected trace elements) in garnet, its inclusion mineralogy (including xenotime), monazite breakdown products and phase diagram modelling allow the reaction history among accessory minerals to be linked with the reconstructed P–T evolution. The partial dissolution and replacement by rare earth element‐accessory minerals (apatite–allanite–epidote) occurred during a fluid‐present decompression at 510 ± 35 °C. These conditions represent the last stage of a metamorphic history consisting of a thermal metamorphic peak at 575 °C and 7 kbar, followed by the peak pressure stage occurring at 520 °C and 8 kbar. An anticlockwise P–T path or two clockwise P–T loops can fit the above P–T constraints. The former path may be related to a context of late Variscan strike‐slip‐dominated exhumation with minor Tertiary (Alpine‐related) reworking and fluid infiltration, while the latter requires an Oligocene–Miocene fluid‐present tectono‐metamorphic overprint on the Variscan paragenesis.

K-Ar Dating of White Mica from the Lesser Himalaya, Tansen-Pokhara Section, Central Nepal: Implications for the Timing of Metamorphism

The Lesser Himalaya in central Nepal comprises low- to medium-grade metasedimentary rocks. Metamorphic studies show that they have experienced at least two metamorphic events (M1 and M2). However, exact timing of metamorphism is still controversial. In the present study K-Ar dating of white micas in shales, slates, phyllites, schists and gneisses from the Tansen-Pokhara section was carried out to understand the timing of metamorphism. The muscovite in gneiss from the MCT zone shows an age of about 1255 Ma representing the age of crystallization of parent granite. Detrital mica from the Bhainskati Formation gives an age of about 2441 Ma. Probably this is the age of crystallization of muscovite in its provenance. The recrystallized white micas from slate and phyllite show older ages (Early Paleozoic) in the southern part (279 to 458 Ma). Most probably this represents the timing of M1. Age become gradually younger towards the north due to the mixing of older (M1 related) and younger (M2 related) white micas. Youngest age (10 Ma) was measured from the sample just below the Upper MCT. This age may be related to the M2 which was due to the Late Miocene-Pleistocene reactivation of the Upper MCT.DOI: http://dx.doi.org/10.3126/njst.v12i0.6509 Nepal Journal of Science and Technology 12 (2011) 242-251

Rutile occurrence and trace element behavior in medium-grade metasedimentary rocks: example from the Erzgebirge, Germany

Metamorphic textures in medium-grade (~500–550°C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (~480°C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from ~500 to 630°C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (~530°C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure.

The Neoproterozoic Quatipuru ophiolite and the Araguaia fold belt, central-northern Brazil, compared with correlatives in NW Africa

Abstract The Araguaia Belt, the northern branch of Neoproterozoic Tocantins tectonic province, developed during West Gondwana amalgamation as a result of collision between the Amazon and West African and/or São Francisco/Congo cratons. The external zone of the belt consists of ophiolitic slices and fragments, sedimentary rocks derived from magmatic arc sources, volcanic rocks, and part of a passive continental margin with low-grade metamorphic rocks, while the internal zone corresponds to a pile of low- to medium-grade metasedimentary rocks. The largest and best preserved ophiolitic bodies occur in the southern part of the belt, where the Quatipuru and Morro do Agostinho ophiolites are composed predominantly of mantle peridotites (mainly residual harzburgite) representing the base of the Moho transition zone. They contain chromitite pods, dunitic lensoid bodies and a suite of mafic–ultramafic dykes and/or sills resulting from partial melting, magma impregnation and diapiric up-rise. A Sm–Nd isochron age of 757±49 Ma indicates oceanic crust formation during the Early Neoproterozoic. NW African correlatives of the Araguaia Belt, the Mauritanide–Bassaride–Rokelide belt, show similarities with respect to lithostratigraphic units, the ages of basement and supracrustal rocks, the presence of Neoproterozoic ophiolitic slices and fragments, suture zones characterized by high gravity anomalies and centrifugal tectonic vergence. We conclude that these belts were probably formed around the same Neoproterozoic ocean or several small coeval oceans.

Deciphering polymetamorphic episodes in high-grade metamorphic orogens: Constraints from PbSL, Sm/Nd and Lu/Hf garnet dating of low- to high-grade metasedimentary rocks from the Kaoko belt (Namibia)

Three dating techniques for metamorphic minerals using the Sm–Nd, Lu–Hf and Pb isotope systems are combined and interpreted in context with detailed petrologic data from crustal segments in NW Namibia. The combination of isochron ages using these different approaches is a valuable tool to testify for the validity of metamorphic mineral dating. Here, PbSL, Lu–Hf and Sm–Nd garnet ages obtained on low- to medium-grade metasedimentary rocks from the Central Kaoko Zone of the Neoproterozoic Kaoko belt (NW Namibia) indicate that these samples were metamorphosed at around 550–560 Ma. On the other hand, granulite facies metasedimentary rocks from the Western Kaoko Zone underwent two phases of high-grade metamorphism, one at ca. 660–625 Ma and another at ca. 550 Ma providing substantial evidence that the 660–625 Ma-event was indeed a major tectonothermal episode in the Kaoko belt. Our age data suggest that interpreting metamorphic ages by applying a single dating method only is not reliable enough when studying complex metamorphic systems. However, a combination of all three dating techniques used here provides a reliable basis for geochronological age interpretation.

Deformation history inferred from magnetic fabric in the southwestern Okcheon metamorphic belt, Korea

Magnetic fabric and rock-magnetic studies have been carried out for the non-fossiliferous, low- to medium-grade metasedimentary rocks in the southwestern part of the Okcheon metamorphic belt (OMB). Two major metamorphic events in the study area were previously recognized: regional metamorphism ( M 1) in the late Carboniferous to early Permian and contact metamorphism ( M 2) due to the intrusion of granite in the middle Jurassic. The metamorphic grade of the study area increases from the biotite zone in southeast through the garnet zone to the staurolite zone towards the northwest. Magnetic fabrics of the study area are generally well defined and can be characterized according to the metamorphic zones. Magnetic foliation is the dominant magnetic fabric in the biotite zone, while magnetic lineation prevails in both garnet and staurolite zones. We interpret the metamorphism-related deformation history of the study area based on magnetic fabrics, magnetic mineralogy and previously reported metamorphic evolution as follows. Penetrative NW-dipping cleavage, represented by magnetic foliation, was formed in the study area by prevailing NW–SE shortening event during the M 1 regional metamorphism in the late Carboniferous–early Permian. This shortening event is interpreted to be associated with the collisional event between the North and South China blocks. Cleavages dipping steeply to the southeast in the staurolite zone are attributed to the pressure exerted from the intrusion of Jurassic granite in the northwestern area.

Trace element and isotopic (Sr, Nd, Pb, O) arguments for a mid-crustal origin of Pan-African garnet-bearing S-type granites from the Damara orogen (Namibia)

Geochronological data, major and trace element abundances, Nd and Sr isotope ratios, δ 18O whole rock values and Pb isotope ratios from leached feldspars are presented for garnet-bearing granites (locality at Oetmoed and outcrop 10 km north of Omaruru) from the Damara Belt (Namibia). For the granites from outcrop 10 km N′ Omaruru, reversely discordant U–Pb monazite data give 207Pb/ 235U ages of 511±2 Ma and 517±2 Ma, similar to previously published estimates for the time of regional high grade metamorphism in the Central Zone. Based on textural and compositional variations, garnets from these granites are inferred to be refractory residues from partial melting in the deep crust. Because P– T estimates from these xenocrystic garnets are significantly higher (800°C/9–10 kbar) than regional estimates (700°C/5 kbar), the monazite ages are interpreted to date the peak of regional metamorphism in the source of the granites. Sm–Nd garnet–whole rock ages are between ∼500 and ∼490 Ma indicating the age of extraction of the granites from their deep crustal sources. For the granites from Oetmoed, both Sm–Nd and Pb–Pb ages obtained on igneous garnets range from ∼500 to ∼490 Ma. These ages are interpreted as emplacement ages and are significantly younger than the previously proposed age of ∼520 Ma for these granites based on Rb/Sr whole rock age determinations. Major and trace element compositions indicate that the granites are moderately to strongly peraluminous S-type granites. High initial 87Sr/ 86Sr ratios (>0.716), high δ 18O values of >13.8‰, negative initial ε Nd values between −4 and −7 and evolved Pb isotope ratios indicate formation of the granites by anatexis of mid-crustal rocks similar to the exposed metapelites into which they intruded. The large range of Pb isotope ratios and the lack of correlation between Pb isotope ratios and Nd and Sr isotope ratios indicate heterogeneity of the involved crustal rocks. Evidence for the involvement of isotopically highly evolved lower crust is scarce and the influence of a depleted mantle component is unlikely. The crustal heating events that produced these granites might have been caused by crustal thickening and thrusting of crustal sheets enriched in heat-producing elements. Very limited fluxing of volatiles from underthrust low- to medium-grade metasedimentary rocks may have also been a factor in promoting partial melting. Furthermore, delamination of the lithospheric mantle and uprise of hot mantle could have caused localized high-T regions. The presence of coeval A-type granites at Oetmoed that have been derived at least in part from a mantle source supports this model.