Pelitic Granulites Research Articles

Early Paleozoic granulite-facies metamorphism and anatexis in the northern West Qinling orogen: Monazite and zircon U-Pb geochronological constraints

In the northern West Qinling orogen (WQO), granulite-facies metamorphic rocks are recognized within the Qinling Complex. These rocks are composed of amphibole-bearing two-pyroxene granulite and garnet-sillimanite gneiss with widespread migmatitization. We investigate three granulite-facies samples and one leucosome sample from the Qinling Complex, which are suitable for U-Pb analyses of zircon and monazite. SHRIMP and LA-ICPMS U-Pb age dating of zircon and monazite from two pelitic granulites provides weighted mean ages of 430±4 Ma (MSWD=0.88) and 433±4 Ma (MSWD=0.27), respectively. Based on the petrographic characteristics and zircon CL imagery, we postulated a ca. 430 Ma metamorphic timing for the pelitic granulites. LA-ICPMS zircon U-Pb data from an amphibole two-pyroxene granulite sample reports two weighted mean age groups: 424±3 Ma (MSWD=0.45) and 402±3 Ma (MSWD=1.4), which were interpreted as granulite-facies metamorphic and retrograde ages, respectively. LA-ICPMS U-Pb dating of zircons from the leucosome sample yields a weighted mean age of 426±2 Ma (MSWD=0.3), which is interpreted as the crystallization age of the leucosome. These data indicate that the West QOB experienced early Paleozoic granulite-facies metamorphism and anatexis similar to the East QOB. However, it remains unclear whether the early Paleozoic granulite facies metamorphism resulted from an arc setting created by the northward subduction of the Shangdan ocean or from a continental collisional orogenic event.

Late Archean high-pressure pelitic granulites in the Yinshan Block, North China Craton

Ancient high-pressure metamorphism provides constraints for the understanding of Archean tectonic processes. Archean high-pressure metamorphic rocks, however, are rarely preserved in old cratons. Here, we report the newly discovered Archean high-pressure metapelites in the Yinshan Block of the North China Craton. The peak metamorphic stage (M2) is represented by kyanite, garnet, plagioclase, K-feldspar, quartz, rutile and melt. Kyanite was replaced by sillimanite during post-peak (M3) decompression process. Phase equilibria modelling in the MnNCKFMASHT system indicates nearly isothermal decompression with peak conditions of 840–870°C and >14–11kbar. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating of leucosome (13XWL35) and monazite U–Pb dating of kyanite-garnet-bearing metapelite (13XWL22) yield similar ages of ∼2520Ma, which represent the crystallization age of granulite-facies melt. The high-pressure metapelites are enriched in Na2O (1.8–3.0wt.%), MgO (3.4–5.7wt.%), Cr (245–421ppm) and Ni (21.0–91.9ppm) contents, suggesting that volcanic rocks are probably the main sedimentary provenance. High δ18O(WR) values (up to 12.4‰) and zircon oxygen isotopes (av. 11.4‰) of the metasediment source indicate they have undergone low-temperature alteration. The pressure-temperature-time constraints of this study provide a rare record indicating that surficial sediments have been buried to a depth of >40km at ca. 2520Ma in the Yinshan Block. Such crustal thickening, combined with available synchronous magmatism suggests that high-pressure metamorphism was triggered in a late Archean deep arc setting in the Yinshan Block of the North China Craton.

Metamorphic evolution and zircon ages of pelitic granulites in eastern Hebei, North China Craton: Insights into the regional Archean P–T–t history

Archean supracrustal granulites occur as localized slivers or rafts in granitoids, and this particular occurrence of supracrustal granulites has not been much concerned from the perspective of metamorphic evolution. The pelitic granulites in eastern Hebei occur as slivers in TTG gneisses and charnockite. Three pelitic granulite samples involving orthopyroxene–garnet gneiss (JD1334), biotite–garnet gneiss (JD1453) and sillimanite–garnet gneiss (JD15126) were collected from Taipingzhai and Laolijia areas. Based on textural observations and mineral compositions, four generations of assemblages involving final, peak, inclusion and overprinting stages are identified. The observed final assemblage yield P–T conditions of 8.0–9.5kbar/870–890°C. The observed peak assemblage characteristic of the absence of the matrix biotite record condition of T>950°C with fixed P=9kbar. An isobaric cooling path from peak to final assemblage is constrained by mineral isopleths. The inclusion assemblages characterized by plagioclase inclusions with high XAn contents are predicted to be stable at low pressure and/or high temperature field. They form an anticlockwise P–T path. The overprinting assemblage characteristic of coronary garnet (g2) coexisting with fine-grained mineral aggregates defines P–T condition of 10–12kbar/830–880°C. The deposition age of the sedimentary protolith of pelitic granulites was ∼2.5Ga, coeval with the crystallization age of TTG gneisses. Both of them were later subjected to granulite facies metamorphism at ∼2.48Ga. The Archean anticlockwise P–T paths suggest a tectonic scenario to explain the particular occurrence of the Archean supracrustal granulites. Firstly, supracrustal sediments experienced thermal event under low pressures attributed to the intrusion of high-temperature TTGs, resulting in dehydrations of hydrous phases. Then, the dried metasediments interlayered with iron-bearing formations potentially denser than the surrounding lithologies tend to drop into the TTG magma ocean, causing the pressure-increase in the metasediments. Thirdly, the dropped metasediments experienced cooling process under lower crust in coupled with the crystallization of the surrounding TTG magma to form the observed final assemblages. Finally, they uplifted as a consequence of the doming processes in Archean terranes.

Zircon U–Pb SHRIMP and monazite EPMA U–Th–total Pb geochronology of granulites of the western boundary, Eastern Ghats Belt, India: a new possibility for Neoproterozoic exhumation history

Abstract We present detailed and high-precision geochronological data on granulites occurring along the western boundary of the Eastern Ghats Belt, India. Age data on systematically sampled rocks coupled with geochemical observation have a potential to unravel the overprinted tectonothermal events operated during Precambrian time. Zircon U–Pb SHRIMP and monazite electron probe microanalysis (EPMA) U–Th–total Pb analyses, chemical zoning, microtextural investigation, and pressure–temperature calculations were carried out on samples of four different rock types. Inherited zircons from migmatitic quartzofeldspathic gneiss and mafic granulite yielded ages of approximately 2900–2350 Ma, representing an older crustal component. The age of granulite metamorphism recorded from charnockite and pelitic granulite ranges between approximately 950 and 930 Ma (from zircon and monazite). A possible decompression event from this area that occurred during Rodinia break-up is recorded from the Y-rich zones of monazite closely associated with porphyroblastic garnet in pelitic granulite and dates from approximately 800 to 750 Ma. Zircon grains of charnockite also yield a similar age. The youngest age of approximately 525–510 Ma documented from the monazite grains of migmatitic quartzofeldspathic gneiss and pelitic granulite, along with a spot age from zircon of migmatitic quartzofeldspathic gneiss, testifies to the final assembly of East Antarctica with cratonic India as a part of East Gondwana.

Metamorphic P–T–t paths of pelitic granulites of the Taihua metamorphic complex in the Mts. Huashan area and tectonothermal implications for the Palaeoproterozoic Trans-North China Orogen

Metamorphic evolution and geochronology of pelitic granulites were reported for the first time in the Taihua metamorphic complex, Mts. Huashan area, southern segment of the trans-North China Orogen (TNCO). Three generations of metamorphic mineral assemblages are recognized: (1) the prograde metamorphic mineral assemblages (M1) are represented by mineral inclusions within the garnet porphyroblasts; (2) the metamorphic peak assemblage (M2) are the garnet porphyroblasts and minerals in the matrix (biotite+orthopyroxene+plagioclase+quartz+K-feldspar+ilmenite+zircon+magnetite); and (3) the retrograde metamorphic mineral assemblages (M3) are represented by the symplectic assemblages around the garnet porphyroblasts, resulted from decomposition reactions between garnet rims and neighboring minerals in the matrix. Calculated by both conventional thermobarometry and pseudosection modeling in the NCKFMASHT system using the Perple_X technique, the P–T conditions of these three metamorphic stages are constrained to be of 4–5kbar/520–530°C for the M1 stage, 6.8–8.6kbar/730–810°C for the M2 stage and 4.1–6.4kbar/570–740°C for the M3 stage, respectively. The derived clockwise P–T paths imply that the Mts. Huashan terrane involved in the subduction and subsequent collision between the Eastern and Western Blocks of the North China Craton (NCC) along the Palaeoproterozoic Trans-North China Orogen (TNCO). High resolution SIMS U–Pb dating of metamorphic zircons reveals the metamorphic ages of 1.85–1.82Ga. Combined with geochronological data from the literature, it is concluded that the tectonothermal evolution between the Eastern and Western Blocks started as early as ∼1.97Ga and lasted as late as 1.80Ga. Furthermore, the eastern Taihua complex records older metamorphic ages and higher peak metamorphic pressures than those of the western Taihua complex, possibly suggesting an eastward subduction model for the TNCO.

P-T-t evolution of Neoarchaean to Paleoproterozoic pelitic granulites from the Jidong terrane, eastern North China Craton

Four metamorphic mineral assemblages, formed at different metamorphic stages, are identified in pelitic granulites exposed in the Jidong terrane, east Hebei Province, North China Craton (NCC). Prograde mineral assemblages (M1) are represented by biotite, plagioclase, quartz, and ilmenite, preserved as mineral inclusions within garnet porphyroblasts. Peak mineral assemblages (M2), which are represented by the garnet cores, rutile, pseudomorphs of sillimanite after kyanite, and some matrix minerals, formed at P–T conditions of ∼12–13kbar/820–850°C. Post-peak mineral assemblages (M3), which consist of the inner rims of garnets and some matrix minerals, formed at P–T conditions of ∼11kbar/850°C. The final retrograde mineral assemblages (M4) are mainly represented by matrix biotite without rutile exsolution, and minor matrix plagioclase with relatively high anorthite content. High-resolution secondary ion mass spectrometry (SIMS) U−Pb dating of metamorphic zircons demonstrates that the pelitic granulites possibly reached the peak metamorphic stage at ∼2.5Ga. Younger metamorphic ages (2.41–2.45Ga) possibly represent the cooling ages of the granulites. Accordingly, the pelitic granulites exposed in the Jidong terrane record clockwise P–T–t paths with sequential isothermal decompression (ITD) and isobaric cooling (IBC) segments, likely caused by a subduction–collision event during the late Neoarchean to early Palaeoproterozoic in the eastern NCC.

High-pressure pelitic granulites from the Jiao-Liao-Ji Belt, North China Craton: A complete P-T path and its tectonic implications

The Paleoproterozoic khondalite series rocks were widely distributed along the Jiao-Liao-Ji Belt (JLJB), North China Craton (NCC). Metamorphic investigations on these rocks can definitely shed light on the tectonic evolution of the JLJB and provide some clues for Paleoproterozoic history of the NCC. Here we present a comprehensive metamorphic study of high-pressure (HP) pelitic granulites from the Jiaobei terrane in the southern part of the JLJB and combine microstructure, mineral chemistry, Raman spectroscopy, thermobarometry, and P-T-X pseudosections in NCKFMASHTO to trace the metamorphic evolution of these rocks. Staurolite inclusion in kyanite was firstly reported using Raman spectroscopy and four successive mineral assemblages (M1–M4) were identified. Integrated method of thermobarometry and pseudosection modeling constrains P-T conditions of 6–8kbar at below 660°C, 15–16.6kbar at 850–860°C, 8.8–10.5kbar at 830–870°C, 5.8–7.8kbar at 660–700°C, for M1–M4, respectively. By employing the melt-reintegrated method, a prograde P-T trajectory is evaluated and hence a complete P-T path is defined that involves a prograde trajectory with marked increase both in pressure and temperature to peak metamorphism, followed by isothermal decompression and subsequent near-isobaric cooling. Such a P-T path implies that a subduction process before collision followed by subsequent extension and cooling processes, was involved in the formation the JLJB, consistent with an orogenic event. Available geochronological data suggests that HP granulites-facies metamorphism probably initiated at 1.90–1.95Ga and that exhumation and cooling of the metamorphic terrane occurred at 1.80–1.85Ga, indicating a long-lived hot orogeny during Paleoproterozoic. This contribution provides mineralogical evidence for low-pressure prograde metamorphism in HP pelitic granulites and hence gives a complete P-T path, placing a more rigorous metamorphic constraint on evolution of the JLJB.

The Garnet Exsolution Texture and Petrological Investigations on a Typical Pelitic Granulite from Eastern Himalaya Syntaxis

The Garnet Exsolution Texture and Petrological Investigations on a Typical Pelitic Granulite from Eastern Himalaya Syntaxis

Neoarchean tectonothermal imprints in the Rengali Province, eastern India and their implication on the growth of Singhbhum Craton: evidence from zircon U–Pb SHRIMP data

AbstractA detailed zircon U–Pb (SHRIMP) geochronological study of the amphibolite to granulite‐grade rocks of the Rengali Province of eastern India records the growth history of the southern margin of the Singhbhum Craton. Pelitic and mafic granulites from the gneissic belt exhibit contrasting styles of metamorphism. Zircon of the pelitic granulites from the eastern segment yields c. 3528–3064 Ma detrital ages. Charnockitic gneiss from the eastern segment has protolith age of 3058 ± 15 Ma while that from the central segment has protolith age of 2861 ± 30 Ma. The latter rock records high‐grade metamorphism at 2818 ± 15 Ma. Hornblende gneiss from the central sector has a protolith age of 2828 ± 9 Ma. Deformed leucogranite in the central and undeformed granitoid in the eastern segment were emplaced at 2807 ± 13 and 2809 ± 13 Ma respectively. The protolith of felsic gneiss from the central sector was emplaced at 2776 ± 24 Ma. Most of the zircon samples contain overgrowths of c. 2500 Ma, inferred to be the age of reworking of the gneissic belt. Our data suggest that the Rengali Province evolved as an orogenic belt in the Neoarchean time (c. 2800–2500 Ma) during southward growth of the Singhbhum Craton. These tectonothermal imprints at the margin of the Singhbhum Craton are possibly related to its inclusion within the supercontinent Ur.

Phase equilibria modelling and zircon age dating of pelitic granulites in Zhaojiayao, from the Jining Group of the Khondalite Belt, North China Craton

AbstractThe Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton and is dominated by a series of granulite facies rocks involving ‘normal’ pelitic granulites recording peak temperatures of ~850 °C and ultrahigh‐temperature (UHT) pelitic granulites recording peak temperatures of 950–1100 °C. The P–T paths and ages of these two types of granulites are controversial. Three pelitic granulite samples in the Jining Group comprising two sillimanite–garnet gneiss samples (J1208 and J1210) and one spinel–garnet gneiss sample (J1303) were collected from Zhaojiayao, where ‘normal’ pelitic granulites occur, for determination of their metamorphic evolution and ages. Samples J1208 and J1210 are interpreted to record cooling paths from the Tmax stages with P–T conditions respectively of ~870–890 °C/7–8 kbar and >840 °C/>7.5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase, garnet and biotite compositions in pseudosections. Sample J1303 is interpreted to record pre‐Tmax decompression from the kyanite‐stability fields to the Tmax stage of 950–1020 °C/8–9 kbar and a post‐Tmax cooling path revealed mainly from the stability field of the observed mineral assemblage, the plagioclase zoning and the biotite composition isopleth in pseudosections. The post‐Tmax cooling stage can be divided into suprasolidus and subsolidus stages. The suprasolidus cooling may not result in an equilibrium state at the solidus in a rock. Therefore, different minerals may record different P–T conditions along the cooling path; the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss. Plagioclase compositions, especially its zoning in plagioclase‐rich granulites, are predicted to be useful for recording the higher temperature conditions of a granulite's thermal history. The three samples studied seem to record the temperature range covering those of the ‘normal’ and UHT pelitic granulites in the Jining Group, suggesting that UHT conditions may be reached in ‘normal’ granulites without diagnostic UHT indicators. LA‐ICP‐MS zircon U–Pb data provide a continuous trend of concordant 207Pb/206Pb ages from 1.89 to 1.79 Ga for sample J1210, and from 1.94 to 1.80 Ga for sample J1303. These continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post‐Tmax cooling P–T paths recorded by the pelitic granulites, which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c. 1.95 Ga as suggested by the pre‐Tmax decompression P–T path.

Discovery of pelitic high-pressure granulite from Manjinggou of the Huai’an Complex, North China Craton: Metamorphic P–T evolution and geological implications

The first report of Paleoproterozoic mafic high-pressure (HP) granulite terranes in Manjinggou area of the Huai’an Complex, North China Craton (NCC) since 1992 has attracted broad attention from geologists, and accordingly various Early Precambrian continent-continent collision or micro-blocks amalgamation hypotheses were suggested. ​However, one important issue is still controversial whether the pelitic granulites and associated mafic HP granulites underwent similar metamorphic history. Here we report newly recognized pelitic HP granulites in Manjinggou area of the Huai’an Complex to provide direct evidence that both pelitic and mafic HP granulites suffered similar metamorphic history. The metamorphic peak mineral assemblage of the pelitic granulites is characterized by Grt-Ky-Kfs-Bt-Rt-Qz-Liq±Ms, and the subsequent medium-pressure (MP) granulite facies retrogression is characterized by Grt-Sil-Kfs-Pl-Bt-Rt-Liq-Qz. Pseudosection approaches were undertaken in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O (NCKFMASHTO) system to account for textural development, mineral composition and P–T evolution of the pelitic granulites. The estimated peak and retrograde granulite facies conditions are 11.5–15kbar, 810–860°C and ∼9.5kbar, ∼850°C, respectively, comparable with those of associated mafic HP granulites. Consistently, field geological observations and available geochronological data also indicate their similar deformation and metamorphic history from HP granulite facies stage (∼1.96–1.90Ga) to retrograde stages (∼1.88–1.80Ga). This finding changes previous views that the pelitic rocks and associated mafic granulites are different slabs with individual metamorphic history. The khondalite series from the Huai’an Complex and adjacent Ji’ning Complex were probably experienced HP granulite facies metamorphism, but previously derived MP granulite facies conditions might be attributed to mineral re-equilibrium due to long dwell time under MP granulite facies. Additionally, these granulite facies rocks record high apparent geothermal gradients and slow exhumation rates relative to HP–UHP rocks in the Phanerozoic continental collisional orogens. It seems to indicate that the thermal regimes and tectonic mechanisms of Paleoproterozoic probably differ from those of Phanerozoic eons, and these HT–HP granulite terranes from the NCC were likely formed in a hot and slow cooling orogen.

Application of the revised Ti-in-zircon thermometer and SIMS zircon U-Pb dating of high-pressure pelitic granulites from the Qianlishan-Helanshan Complex of the Khondalite Belt, North China Craton

The Ti-in-zircon thermometer is a newly established, single-mineral trace element thermometer, which has been successfully used to estimate the peak and post-peak temperatures of high-grade metamorphic rocks. Here the revised Ti-in-zircon thermometer and in situ SIMS U-Pb dating are applied to detrital and metamorphic zircons of pelitic granulites from the Qianlishan-Helanshan Complex. The SIMS U-Pb results show that detrital zircons yield 207Pb/206Pb ages scattering from 2021 to 2449Ma with a peak age of 2030Ma, possibly sourced from felsic to intermediate igneous rocks at a relatively low temperature magmatic environment. Most metamorphic zircons yield ages of 1953±15Ma, 1954±20Ma and 1948±18Ma with Ti contents of 3.9–82.6ppm, corresponding to metamorphic temperatures of 664–992°C. Minor metamorphic zircons give 207Pb/206Pb ages of 1923±9Ma and Ti contents of 5.8–16.3ppm with temperatures of 697–794°C. These define metamorphic ages and temperature conditions at peak and post-peak metamorphic stages of pelitic granulites, respectively. Combined with previous studies, our new data establish a linking between zircon U-Pb ages and metamorphic thermal evolution. This demonstrates a ∼1.95Ga peak temperature granulite-facies metamorphism and subsequent near-isothermal decompression process at ∼1.92Ga, which is considered to record a ∼1.95Ga continent-continent collisional event between the Yinshan and Ordos blocks, followed by post-orogenic exhumation at ∼1.92Ga.

UHT granulites of the Highland Complex, Sri Lanka II: Geochronological constraints and implications for Gondwana correlation

The regional ultrahigh-temperature (UHT) metamorphism of the Highland Complex, Sri Lanka is well established and has an important role in our understanding of the tectonic history of the Gondwana supercontinent. U-Pb zircon dating of sapphirine-bearing Mg-Al granulites yielded two major metamorphic age populations at approximately 620-590 and 563-525 Ma with no evidence of older zircon cores. Pelitic granulite samples with a Grt-Sil-Spl-Crd assemblage have similar metamorphic ages with concordant data clusters at similar to 602, 563, and 526 Ma and inherited zircon cores aged from 2040 to 1600 Ma. The pelitic granulites also underwent two stages of metamorphism (565-520 and 622-580 Ma). Some of these pelitic granulite samples have inherited zircon cores ranging from 3060 to 760 Ma. Zircons in mafic granulite samples have age ranges of 566-533 and 620578 Ma. A calc-silicate granulite sample also has similar age populations at 591, 541, and 524 Ma. Combining these new results with previously published ages from Sri Lanka and formerly adjacent continental fragments of Gondwana, we propose that the terranes in southern Madagascar (south of Ranotsara Shear Zone), Northern and Southern Madurai and the Trivandrum Blocks of southern India, the Highland Complex of Sri Lanka, and the Skallen Group in the Lutzow-Holm Complex of East Antarctica represent a unique metamorphic belt that regionally experienced the Ediacaran-Cambrian UHT event during the amalgamation of the Gondwana super-continent.

High-pressure pelitic granulites from the Helanshan Complex in the Khondalite Belt, North China Craton: Metamorphic P-t path and tectonic implications

This paper presents petrographic and mineral chemistry data and an inferred P-T path for the high-pressure pelitic granulites from the Helanshan Complex in the Khondalite Belt, a Palaeoproterozoic collisional belt along which the Yinshan and Ordos blocks joined to form the Western Block in the North China Craton. Petrographic observation indicates that these high-pressure pelitic granulites experienced four metamorphic stages: the early prograde (M1), peak high-pressure (M2), post-peak decompression (M3) and late retrograde (M4) stages. The early prograde assemblage (M1) is preserved as mineral inclusions within the cores of garnet, represented by plagioclase + biotite + quartz + muscovite + kyanite + ilmenite. The peak high-pressure assemblage (M2) is garnet + kyanite + biotite + K-feldspar + plagioclase + quartz + ilmenite in the matrix. The post-peak decompressional stage (M3) is represented by sillimanite replacing kyanite and formation of cordierite + sillimanite symplectites (M3-1) and cordierite ± spinel coronas replacing garnet porphyroblasts (M3-2). The late retrograde stage (M4) is indicated by the formation of staurolite + chlorite in the matrix. Pseudosection modeling for a representative sample in the NCKFMASHTO system constrains the P-T conditions of M1, M2, M3-1, M3-2 and M4 stages at 9.3-9.7 kbar/656-674 °C, 10.2-11.2 kbar/792-805 °C, 5.5-5.7 kbar/810-820 °C, 4.5-5.0 kbar/780-785 °C and 3.4-4.4 kbar/580-610 °C, respectively. These mineral assemblages and their P-T conditions define a clockwise P-T path involving isothermal decompression and isobaric cooling following the peak high-pressure metamorphism, suggesting that the high-pressure pelitic granulites underwent initial crustal thickening (M1-M2), followed by isothermal exhumation (M3) and final retrogression and cooling (M4). Such a clockwise P-T path involving near-isothermal decompression is in accord with the P-T paths reconstructed for other complexes in the Khondalite Belt, and, combined with available geochronological data, is considered to record a continent-continent collisional event that led to the amalgamation of the Yinshan and Ordos Blocks to form the Western Block of the North China Craton at ∼1.95 Ga. This is coincident with these global-scale collisional events (2.1–1.8 Ga) leading to the assembly of the Palaeo-Mesoproterozoic supercontinent, further supporting an early proposal that the North China Craton was one of the components of the Palaeo-Mesoproterozoic Columbia (or Nuna) supercontinent.

Late-Neoproterozoic ultrahigh-temperature metamorphism in the Highland Complex, Sri Lanka

The Sri Lankan crustal blocks preserve the records of Neoproterozoic tectonothermal events associated with the final assembly of the Gondwana supercontinent. Here we present petrological and U–Pb zircon geochronological data on a suite of ultrahigh-temperature (UHT) metapelites and mafic granulite from the Highland Complex in central Sri Lanka which provide convincing evidence for Late Neoproterozoic UHT metamorphism. Three sapphirine granulites, two corundum bearing metapelites and one mafic granulite were studied for petrology and zircon U–Pb geochronology. The petrographic observations coupled with conventional thermobarometric calculations indicate that the pelitic granulites attained P=11–12kbar and T=850°C during prograde evolution. Subsequently the rocks have undergone a prograde decompression stage with increasing temperature up to UHT conditions. The peak metamorphism was attained at P–T conditions around 900–975°C and 9–10kbar, followed by near isobaric cooling and rapid decompression.LA-ICPMS zircon U–Pb analyses of magmatic cores in detrital zircons from the sapphirine granulites show 206Pb/238U ages ranging from 834±12 to 722±14Ma, reflecting the timing of crystallization of these grains in their protoliths. Metamorphic zircons in these samples yield ages ranging 535±5 and 567±4Ma representing the age of the Neoproterozoic UHT metamorphism. The metamorphic zircons from corundum-bearing metapelites yield ages from 530±4 to 578±4Ma. An older age population between 1676Ma and 2365Ma from these metasediments suggests the timing of emplacement of the Paleoproterozoic sources. Our data suggest variable degree of input from multiple sources into the protolith sediments of the Highland Complex prior to the latest Neoproterozoic UHT metamorphism at ∼530–580Ma which is broadly coeval with the collisional assembly of the Gondwana supercontinent.

Petrological evidence for isobaric cooling of ultrahigh-temperature pelitic granulites from the Khondalite Belt, North China Craton

The ultrahigh-temperature (UHT) pelitic granulites from the Khondalite Belt, North China Craton (NCC), contain ilmenite in the matrix, which has been partially replaced by rutile. Based on this observation and the growth of biotite by garnet-consuming reaction, the UHT rocks are inferred to have recorded three metamorphic stages: the peak metamorphic stage (M1) and two retrograde metamorphic stages (M2 and M3). The M1 stage is represented by the assemblage of perthite + sillimanite + ilmenite in the matrix, and quartz inclusions bearing (in the cores) garnet porphyroblasts. The M2 stage is defined by rutile-replacing ilmenite and growth of garnet mantles and rims containing acicular sillimanite inclusions, with the garnet + perthite + sillimanite + rutile + ilmenite + quartz assemblage. The M3 stage is recorded by the growth of biotite in the matrix, with the garnet + biotite + perthite + sillimanite + rutile + ilmenite + quartz assemblage. Based on phase equilibrium modeling, an isobaric cooling path is reconstructed, which is consistent with the idea that mantle-derived magma provided the heat for the UHT metamorphism in the Khondalite Belt, NCC.

Petrology and geochemistry of ultrahigh‐temperature granulites from the South Altay orogenic belt, northwestern China: Implications for metamorphic evolution and protolith composition

AbstractUltrahigh‐temperature (UHT) granulites in the South Altay orogenic belt of Northwestern China provide important clues for the lower crustal components and tectonic evolution of the Central Asian Orogenic Belt during the Paleozoic. In this paper, we studied whole‐rock geochemistry and mineral characteristics to understand the protolith and metamorphic evolution of the Altay UHT granulite. The Altay granulite shows negative discriminant function values (DF) of −9.27 to −3.95, indicating a sedimentary origin, probably an argillaceous rock. The peak metamorphic temperature–pressure conditions of 920–1010 °C and > 9 kbar were estimated from the geothermobarometry, together with the stability of spinel (low ZnO) + quartz and orthopyroxene (Al2O3 up to 9.2 wt.%) + sillimanite + quartz in the Altay UHT rock, indicate a UHT metamorphic condition has been achieved. Two stages of retrograde conditions are recognized in these rocks; the first is an isothermal decompression to approx. 750 °C at 5.2–5.8 kbar at the early stage, and the second is the cooling down to 520–550 °C at 4.8–5.2 kbar. Combined with previous study, the formation of the Altay UHT pelitic granulite with a clockwise retrograde P–T path is inferred to be related with collisional and accretional orogenic process between the Siberian and Kazakhstan–Junggar plates.

Late Neoproterozoic granulite facies metamorphism in the Menderes Massif, Western Anatolia/Turkey: implication for the assembly of Gondwana

The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamorphism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana.

Rare evidence for formation of garnet + corundum during isobaric cooling of ultrahigh temperature metapelites: New insights for retrograde P–T trajectory of the Highland Complex, Sri Lanka

We report the occurrence of coexisting garnet+corundum in spinel- and corundum-bearing, garnet–sillimanite–biotite–graphite gneiss (pelitic granulites) from the Highland Complex (HC), Sri Lanka. In the investigated pelitic granulites, two domains such as quartz-saturated and quartz-undersaturated are distinguishable. The quartz-saturated domains consist of porphyroblastic garnet, quartz, plagioclase, alkali-feldspar and biotite flakes rimming garnet. The quartz-undersaturated domains are constituted of two generations of garnet (Grt1 and Grt2), sillimanite, plagioclase, alkali-feldspar, corundum, spinel and biotite. Grt1 encloses rare Ti-rich biotite and numerous rutile needles and apatite rods. Grt2 contains rare sillimanite and/or spinel inclusions. Corundum occurs in mutual contact with Grt2, partially embedded at the rim or as an inclusions in Grt2.Thermobarometry on inclusion phases in Grt1 indicates that during the prograde history pelitic granulites attained a P of 10.5–11kbar at T of ~850°C. Textural observations coupled with both pseudosections calculated in the NCKFMASHTMnO system and Ti-in-Garnet geothermobarometry suggest that peak metamorphism occurred at ultrahigh temperature (UHT) conditions of 950–975°C and pressures of 9–9.5kbar. Peak T was followed by a period of isobaric cooling that formed corundum and Grt2 at approx. 930°C along with exsolution of rutile needles and apatite rods in Grt1. Thermodynamic modelling confirms that corundum appears along an isobaric cooling path at about 920–930°C and 9–9.5kbar. Therefore, the investigated granulites provide a rare example of post-peak crystallization of garnet+corundum along a retrograde metamorphic trajectory under UHT conditions. Thus, isobaric cooling at the base of the crust could be regarded as an alternative process to form coexisting garnet+corundum.

Anatomy of zircon growth in high pressure granulites: SIMS U–Pb geochronology and Lu–Hf isotopes from the Jiaobei Terrane, eastern North China Craton

Granulite facies rocks that witnessed high pressure and high temperature metamorphism preserve important records of the growth and recrystallizaton of zircons during the multiple stages of tectonic evolution of orogenic belts developed in complex subduction–collision settings. Here we investigate zircons in the Paleoproterozoic high pressure (HP) granulites from Jiaobei Terrane in the eastern North China Craton that experienced a multi-stage metamorphic evolution. We present SIMS zircon U–Pb ages for zircons from garnet amphibolites, mafic granulites and pelitic granulites. Our data show four age peaks at 2.5Ga, 1.92Ga, 1.87–1.85Ga and 1.83Ga. Zircons belonging to the latter three age groups show similar characteristics of low Th and U contents, low Th/U ratios (<0.1), sector zoning or unzoned internal structures, corresponding to the features of zircon growth during metamorphism. Using zircon Lu–Hf isotopic compositions combined with a careful analysis of the morphology of the zircons, we evaluate the processes of solid-state recrystallization, intergranular melt growth, coupled dissolution–reprecipitation and subsolidus growth. Zircons formed through solid-state recrystallization during peak metamorphic stage inherited the relatively high 176Lu/177Hf ratios and low 176Hf/177Hf ratios of zircons from the protoliths. The youngest U–Pb age of ca. 1899Ma is interpreted as the maximum estimate of the time of peak metamorphism. Zircons formed through intergranular melt growth during low- to medium-pressure retrogression display homogeneous Lu–Hf isotope compositions, low 176Lu/177Hf and high 176Hf/177Hf ratios. The U–Pb age of 1870–1850Ma yielded by zircons crystallized from intergranular melt is interpreted to represent the time of low- to medium-pressure granulite facies retrogression. Some zircon grains with prismatic morphology and low 176Lu/177Hf and 176Hf/177Hf ratios display heterogeneous Lu–Hf isotopic compositions, possibly related to coupled dissolution–reprecipitation. The U–Pb ages of these zircons are identical to those formed from intergranular melt and these two groups of zircons formed during the same metamorphic stage. Zircons formed through subsolidus growth show large variations in Lu–Hf isotopic compositions and their ca.1833Ma age represents the timing of amphibolite facies retrogression. The HP granulite facies metamorphism in Jiaobei is thus constrained between 1870 and 1899Ma, with low- to medium-pressure granulite facies retrogression at ca. 1850–1870Ma followed by amphibolite facies retrogression at ca. 1833Ma.Our data show that several zircon cores and recrystallized grains were entrained from basement rocks during the emplacement of the mafic magmatic protoliths in the case of mafic granulites. For the pelitic granulites, the older zircons are detrital grains of magmatic origin. The Hf crustal model ages of these zircons that show peaks at ca. 2.7Ga and 2.9Ga are consistent with the timing of mafic magmatism in the region as identified in previous studies. The Lu–Hf isotopes of newly grown metamorphic zircons which have equilibrated chemical compositions trace the nature of the protolith. The zircon Hf crustal model age of ca. 2.5–2.7Ga from the pelitic granulites is consistent with that of the basement rocks in the Jiaobei Terrane and the zircon Hf model ages (TDM) of ca.2.0Ga and 2.2–2.3Ga from the mafic granulites might represent the time of magma generation from the mantle with crustal contamination. The geochronology of the basement rocks as revealed by the old zircon cores and the metamorphic evolution of the Jiaobei Terrane are comparable with the tectonic history of the central and western North China Craton and provide insights into the complex tectonothermal events associated with the Neoarchean–Paleoproterozoic crustal evolution history of the craton. Also, the pervasive ~2.5Ga zircons in a variety of rocks seem to suggest that the Jiaobei Terrane was already part of a coherent tectonic unit at such time.