Garnet Staurolite Schist Research Articles

Metamorphic P–T Evolution and In Situ Biotite Rb–Sr Geochronology of Garnet–Staurolite Schist From the Ramba Gneiss Dome in the Northern Himalaya

The North Himalayan gneiss domes provide a window for looking into the deeper crust and record abundant clues of continent collisional orogenesis. This study carried out detailed petrology, in situ LA–ICP–MS biotite Rb–Sr dating, and phase equilibrium modeling on garnet–staurolite–two-mica schist in the Ramba gneiss dome in order to constrain metamorphic P–T evolution and the timing of metamorphism. A clock-wise P–T path, involving an early prograde process that evolves from ∼540°C at ∼4.4 kbar to ∼630°C at ∼6.0 kbar, was constructed for garnet–staurolite–two-mica schist in the Ramba gneiss dome. In situ LA–ICP–MS biotite Rb–Sr analysis yielded two metamorphic ages of 37.17 ± 5.66 and 5.27 ± 3.10 Ma, corresponding to the timing of retrograde cooling and the cooling age of the dome following the thermal resetting by the emplacement of ca. 8 Ma leucogranite pluton in the core of the dome, respectively. The peak metamorphism is inferred to be older than ca. 37 Ma. Based on these results and the data previously published, the garnet–staurolite–two-mica schist recorded the Eocene crustal thickening, following the India–Asia collision and later the exhumation process.

Proterozoic High-Temperature–Low-Pressure Metamorphism in the Mahakoshal Belt, Central Indian Tectonic Zone (India): Structure, Metamorphism, U-Th-Pb Monazite Geochronology, and Tectonic Implications

To understand the protracted accretionary evolution along the Central Indian Tectonic Zone (CITZ), we investigated garnet-staurolite schists and associated lithologies from the central Mahakoshal Belt (MB). Mesoscale structures, porphyroblast growth, garnet zoning, and pseudosection modeling were coupled with U-Th-Pb monazite dating to reconstruct a clockwise pressure-temperature-time (P-T-t) path for the garnet- and staurolite-bearing schists. The prograde path is characterized by near isobaric heating conditions, which initiates at pressure-temperature (P-T) conditions of 4.5–5.0 kbar and 550°–560°C and attains peak metamorphism at 5.5–6.0 kbar and 610°–620°C. The peak metamorphism was contemporaneous with the emplacement of southern-margin granitoids at ~1.70 Ga, resulting in the perturbation of geotherms in the collision setting. The retrograde arm (MR) of the P-T path passes through isobaric cooling at ~5.0 kbar and ~500°C. This late Paleoproterozoic P-T-t path is overprinted by hitherto uncharacterized tectonism that coincides with the Sausar orogeny and provides evidence for the northward extension of 0.95–0.85 Ga orogenic activity within the CITZ. The 1.80–1.55 and 0.95–0.85 Ga tectonothermal events identified in this study support that crustal evolution in the CITZ involved a mosaic of domains that were accreted together in the Neoproterozoic time.

Automated petrography analysis by QEMSCAN® of a garnet-staurolite schist of the San Lorenzo Formation, Sierra Nevada de Santa Marta massif

El análisis petrográfico automatizado integra hardware de microscopía electrónica de barrido y espectroscopía de rayos X con software experto para generar mapas de composición de rocas a escala de micras. Si bien las soluciones de petrografía automatizada, como QEMSCAN®, se usan ampliamente en las industrias de minería, procesamiento de minerales y petróleo para caracterizar los depósitos minerales y las formaciones rocosas subsuperficiales, no se ha utilizado en petrología metamórfica. Este estudio aplica el análisis petrográfico automatizado utilizando QEMSCAN® a un esquisto con granate y estaurolita de la Formación San Lorenzo, provincia geológica de Sevilla (macizo de la Sierra Nevada de Santa Marta), y demuestra que esta técnica analítica tiene una clara aplicación potencial en estudios petrológicos.

Geochemistry, mineral chemistry and P-T evaluation of metasediments of Bahram-Gur complex, ES Sanandaj-Sirjan zone, Iran

Abstract The Bahram-Gur area in the southeastern part of the Sanandaj – Sirjan metamorphic zone, contains metabasites and metasediments. The metasedimentary rocks are mainly garnet schists and garnet-staurolite schists that were metamorphosed under amphibolite facies conditions. The rocks consist of garnet ± staurolite, biotite, muscovite, chlorite and quartz. The geochemistry of the Bahram-Gur metasediments classifies them as quartziferous sedimentary rocks. The protoliths of the metasedimentary rocks were close to greywackes from an ensialic arc basin depositional setting, with a source comprising mostly mixture of acid and intermediate magmatic rocks in the upper continental crust. The metamorphic conditions of formation of the Bahram-Gur metasedimentary are investigated by geothermobarometric methods. The results show that the metasedimentary rocks formed at temperatures of 600-750°C and pressures of 5-7.5 kbar.

Mineral chemistry and thermobarometery of garnet staurolite schists from the Hamedan area

Mineral chemistry and thermobarometery of garnet staurolite schists from the Hamedan area

Pressure–temperature–time (P–T–t) evolution of fore-arc and foreland schist in the Qinling Orogenic Belt, China: Implications for Late Paleozoic and Triassic subduction termination

The Qinling Orogenic Belt (QOB) is a segment of the larger Central China Orogen and marks the amalgamation of the South and North China Cratons during a protracted period spanning the Neoproterozoic through to Triassic. The complex evolution of the QOB has been extensively studied through U–Pb zircon geochronology, but lacks fundamental characterisation of its time-integrated thermal history. Moreover, the comparatively few metamorphic studies that exist focus exclusively on high-pressure rocks at the margins of major tectonic divisions within the QOB. Samples for this study are metapelitic schists sourced from the Qinling Mountains, North Qinling Belt (NQB), deep in the QOB interior. Cordierite schist has metamorphic monazite age affinities to Late Triassic magmatism and metamorphism in the South Qinling Belt (SQB) occurring at 230–220 Ma. Calculated phase equilibria modelling constrains metamorphism to 1.0–2.0 kbar and 530–545 °C, corresponding to a steep apparent thermal gradient between ~275–530 °C/kbar. This probably represents contact metamorphism of the Liuling Group turbidite sequence by intruding magma. Garnet–staurolite schist has metamorphic monazite age data that overlaps with Late Paleozoic events occurring between 420 and 400 Ma. Calculated phase equilibria modelling constrains peak metamorphism to ~7.1 kbar and 615 °C, corresponding to a shallower thermal gradient of ~87 °C/kbar. This represents Barrovian-style metamorphism of fore-arc sedimentary units during arc–continent collision marking the closure of the Shangdan Ocean. Metamorphism of these fore-arc sequences has a comparable thermal gradient to Guishan Complex equivalents in the Tongbai Orogen, which is a continuation of the Qinling Orogen to the east. This study establishes previously undocumented contact metamorphism in the northern SQB and Barrovian-style metamorphism in the NQB, providing key constraints for tectonic models that explain the evolution of the QOB and broader Central China Orogen.

Reconstruction of the metamorphic P–T path from the garnet zoning in aluminous schists from the Tsogt Block, Mongolian Altai

The paper presents original authors’ data on aluminous schists in the Tsogt tectonic plate in the Southern Altai Metamorphic Belt. The nappe includes a medium-temperature/medium-pressure zonal metamorphic complex, whose metamorphic grade varies from the greenschist to epidote-amphibolite facies. The garnet and garnet–staurolite schists contain three garnet generations of different composition and morphology. The P–T metamorphic parameters estimated by mineralogical geothermometers and geobarometers and by numerical modeling with the PERPLEX 668 software provide evidence of two successive metamorphic episodes: high-gradient (of the andalusite–sillimanite type, geothermal gradient approximately 40–50°/km) and low-gradient (kyanite–sillimanite type, geothermal gradient approximately 27°/km). The P-T parameters of the older episode are T = 545–575°C and P = 3.1–3.7 kbar. Metamorphism during the younger episode was zonal, and its peak parameters were T = 560–565°C, P = 6.4–7.2 kbar for the garnet zone and T = 585–615°C, P = 7.1–7.8 kbar for the staurolite zone. The metamorphism evolved according to a clockwise P–T path: the pressure increased during the first episode at a practically constant temperature, and then during the second episode, the temperature increased at a nearly constant pressure. Such trends are typical of metamorphism related to collisional tectonic settings and may be explained by crustal thickening due to overthrusting. The regional crustal thickening reached at least 15–18 km.

Textural relations, P-T path, polymetamorphism and also geodynamic significance of metamorphic rocks of the Aligudarz-Khonsar region, Sanandaj-Sirjan zone, Iran

The metamorphic rocks of the Aligudarz-Khonsar region can be divided into nine groups: slate, phyllite, sericite schist, biotite-muscovite schist, garnet schist, garnet-staurolite schist, staurolite schist, mylonitic granite, and marble. In this metamorphic region, four phases of metamorphism can be identified (dynamothermal, thermal, dynamic and retrograde metamorphism) and there are three deformation phases (D1, D2 and D3). Paleozoic pelagic shales experienced prograde metamorphism and polymetamorphism from the greenschist to amphibolite facies along the kyanite geotherm. The metapelites show prograde dynamothermal metamorphism from the greenschist to amphibolite facies. Maximum degree of dynamothermal metamorphism is seen in the Nughan bridge area. Also development of the mylonitic granites in the Nughan bridge area shows that dynamic metamorphism in this area was more intense than in other parts of the AligudarzKhonsar metapelitic zone. The chemical zoning of garnets shows three stages of growth and syn-tectonic formation. With ongoing metamorphism, staurolite appeared, and the rocks reached amphibolite facies, but the degree of metamorphism did not increase past the kyanite zone. Thus, metamorphism of the pelitic sediments occurred at the greenschist to amphibolite facies (kyanite zone). Thermodynamic studies of these rocks indicate that the metapelites in the Aligudarz-Khonsar region formed at 490–550°C and 0.47–5.6 kbar.

The U–Pb age and Lu–Hf isotope composition of detrital zircon from metasedimentary rocks of the Onot greenstone belt (Sharyzhalgay uplift, southern Siberian craton)

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7Ga. Older source rocks (2.80–3.35Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources.

Metamorphism and deformation of golpayegan metapelitic rocks, Sanandaj-Sirjan Zone, Iran

The metapelitic schists of the Golpayegan region can be divided into four groups based on their mineral assemblages: (1) garnet-chloritoid schists, (2) garnet schists, (3) garnet-staurolite schists, and (4) staurolite-kyanite schists. Paleozoic pelagic shales experienced progressive metamorphism and polymetamorphism from greenschist to amphibolite facies along the kyanite geotherm. Mylonitic granites are concentrated in the central part of the region more than in other areas, and formed during the dynamic metamorphic phase by activity on the NW-SE striking Varzaneh and Sfajerd faults. The presence of chloritoid in the metapelites demonstrates low-grade metamorphism in the greenschist facies. The textural and chemical zoning of garnets shows three stages of growth and syntectonic formation. With ongoing metamorphism, staurolite appeared, and the rocks reached amphibolite facies, but the degree of metamorphism did not increase past the kyanite zone. Thus, metamorphism of the pelitic sediments occurred at greenschist to lower amphibolite facies. Thermodynamic studies of these rocks indicate that the metapelites in the north Golpayegan region formed at 511–618°C and 0.24–4.1 kbar.

Insights into the metamorphic evolution of the Belt–Purcell basin; evidence from Lu–Hf garnet geochronology

We have determined Lu–Hf garnet ages from spatially separated garnet bearing localities in northern Idaho. The Lu–Hf ages are diverse and reflect a progression of Mesoproterozoic metamorphic events. The oldest Lu–Hf garnet age determined in this study is 1463 ± 24 Ma for garnet within a kyanite schist exposed in the core-zone of the Boehls Butte metamorphic complex. A garnet schist from the Priest River complex yields a well-defined age of 1379 ± 8 Ma. A garnet–staurolite schist, a garnet–mica schist, and a gem-grade Idaho star garnet sample all from the general vicinity of Clarkia, Idaho, yield ages of 1064 ± 10, 1081 ± 20, and 1078 ± 17 Ma, respectively. A garnet amphibolite, also collected from the Boehls Butte metamorphic complex, yields an age of 1151 ± 41 Ma for garnet porphyroblasts and 137 ± 25 Ma for diffuse overgrowths. The Hf in all of the Proterozoic garnets is extraordinarily radiogenic with εHf values ranging from +1210 to +10546, which attest to both their high Lu/Hf ratios and their great antiquity. All of the samples analyzed in this study are from the rocks of the Mesoproterozoic Belt–Purcell Supergroup or its basement. These data provide evidence of polymetamorphism in northwestern Laurentia during the Mesoproterozoic.

PTt path in metamorphic rocks of the Khoy region (northwest Iran) and their tectonic significance for Cretaceous–Tertiary continental collision

Metamorphic rocks in the Khoy region are exposed between obducted ophiolites to the southwest and sedimentary rocks of Precambrian–Paleozoic age to the northeast. The Qom formation (Oligocene–Miocene) with a basal conglomerate transgressively overlies all of these rocks. The metamorphic rocks consist of both metasediments and metabasites. The metasediments are micaschist, garnet–staurolite schist and garnet–staurolite sillimanite schist with some meta-arkose, marble and quartzite. The metabasites are metamorphosed to greenschist and amphibolite facies from a basaltic and gabbroic protolith of tholeiitic and calc-alkaline rocks. Geothermobarometry based on the equivalence of minerals stability and their paragenesis in these rocks and microprobe analyses by several different methods indicate that metamorphism occurred in a temperature range between 450 and 680 °C at 5.5 and 7.5 kb pressure. Rims of minerals reveal a considerable decrease of pressure (<2 kb) and insignificant decrease of temperature. The PTt path of this metamorphism is normal. The MFG line passes above the triple junction of Al 2SiO 5 polymorphs, and the average geothermal gradient during metamorphism was from 27 to 37 °C/km, which is more concordant with the temperature regime of collision zones. We infer that crustal thickening during post-Cretaceous (possibly Eocene) collision of the Arabian plate and the Azerbaijan–Albourz block was the main factor that caused the metamorphism in the studied area.

Stratigraphy and structural development of the Precambrian rocks in the area north-east of Disko Bugt, West Greenland

The Precambrian rocks in the area north-east of Disko Bugt can be divided into a lower gneiss group and an upper supracrustal group. The lower part of the supracrustal group consists of quartzites, amphibolites and garnet-staurolite schists, while the upper part is mainly semipelitic schist. The gneisses were affected by three successive phases of deformation, while in the supracrustals only the two latest phases of folding can be recognized. Evidence is given in support of the idea that the supracrustals were deposited on a gneiss basement after the first phase of folding, the basement being later reactivated mainly during the third and last deformation. Two main tectonic compLexes can be distinguished in the area mapped: a southern complex characterized by a predominarit ENE direction of the fold axes and a northern complex in which the main fold axes are strongly curved around a central gneiss dorne. The transition zone between the southern and northern complex is marked by the presence of severaL important faults and mylonites. The gneiss dorne is flanked in its northern part by a thick granite sheet. The dorne structure appears to have been forrned by a combination of diapiric movements and the interference between two successive deformations. The gneisses and lower supracrustals recrystallized under amphibolite facies conditions, while greenschist facies conditions prevaiLed during the recrystallisation of the upper supracrustals.