High-pressure Granulite-facies Metamorphism Research Articles

Considerable heating during clockwise decompression of a long-lived Paleoproterozoic hot orogen: Evidence from the Xuanhua Complex, North China Craton

Granulites comprise the root of collisional orogens, and decoding their evolution may provide key information on orogenic processes from collisional thickening to post-collision extensional thinning. However, the records of collisional orogeny in most cases have been obscured or even erased by later high-/ultrahigh-temperature (HT/UHT) overprinting. Here, we report new petrological, geochemical and geochronological data for granulite-facies rocks in the Xuanhua Complex of the northern Trans-North China Orogen (TNCO). We reveal the complex metamorphic features and tectonic evolution of a long-lived Paleoproterozoic hot orogen as well as their link with Phanerozoic orogens. The rocks are composed mainly of garnet + clinopyroxene + orthopyroxene ± amphibole + plagioclase + quartz + rutile + ilmenite with symplectitic intergrowths of clinopyroxene + orthopyroxene ± amphibole + plagioclase ± quartz. Pseudosection modeling and thermobarometric calculations suggest that the high-pressure (HP) mafic granulites record clockwise pressure–temperature (P-T) paths, from the prograde stage (M1) to the pressure-peak stage (M2; 13.5–15.0 kbar and 850–875 °C), followed by considerable heating during decompression to the temperature-peak stage (M3a; 8.0–9.6 kbar and 910–950 °C) and final cooling to the M3b stage. Magmatic zircon U-Pb dating reveals c. 2.1–2.0 Ga protolith ages for the granulites. Metamorphic zircon and titanite U-Pb results, and trace elements in zircons indicate that the age of c. 1.92 Ga is possibly close to the lower limit of HP granulite-facies pressure-peak, and c. 1.84–1.81 Ga ages are related to the timing of the final cooling stage. Hence, by comparison with existing geological data, the synthesized metamorphic P-T-t data of the Xuanhua granulites indicate two stages of tectonic evolution, including: (1) crustal thickening by continental collision between the Western and Eastern Blocks at c. 1.95–1.92 Ga, resulting in an early HP granulite-facies metamorphism at relatively low thermal gradients of 16–19 °C/km; and (2) the collision-thickened orogenic lithosphere was thinned controlled by continental extension and gravitational instability, which might provide additional heat in the interval c. 1.90–1.84 Ga, resulting in a later UHT overprinting at reduced pressures and higher gradients of 30–36 °C/km. In addition, the post-collisional extension accompanying exhumation decompression shows a long-lived timescale of > 80 Myr, corresponding roughly to a slow exhumation rate of < 0.3 mm/yr. To sum up, these features of long-lived tectono-thermal evolution (tectonic transition from crustal thickening to lithospheric thinning, and prolonged orogeny and exhumation), slow exhumation rates, and the occurrence of UHT overprinting, are becoming more commonly identified in Precambrian hot orogens like the TNCO.

Late Neoarchean and Paleoproterozoic tectonothermal and metamorphic evolution recorded in high-pressure granulites: Evidence from the Xuanhua Complex, northern North China Craton

Archean cratons usually underwent reworked processes during the late Paleoproterozoic orogeny, which may result in multiple tectonothermal records preserved in some high-grade metamorphic rocks; thus, these rocks provide a key opportunity on understanding the tectonic styles during different periods. In this study, we report new petrological, mineral chemical and geochronological data of a suite of newly discovered mafic and felsic metamorphic rocks in the Xuanhua Complex from the northern North China Craton (NCC), revealing the complex metamorphic thermal history and the transition of tectonic regime from the late Neoarchean to the late Paleoproterozoic. The mafic granulite preserves the prograde quartz and clinopyroxene inclusions within garnets (M1), and the peak stage (M2) is recorded by garnet, clinopyroxene, hornblende, plagioclase, quartz and ilmenite. The first retrograde (M3a) and final cooling stages (M3b) are defined by the appearance of orthopyroxene, and the amphibolite-facies overprinting, respectively. The felsic granulite records the similar peak mineral assemblage (M2), but has higher volumes of feldspar and quartz than the mafic granulite. The retrograde stage (M3) is represented by amphibolite-facies assemblage (hornblende ± biotite ± clinopyroxene) and fine-grained intergrowth of hornblende + plagioclase around garnet. Pseudosection modeling and thermobarometry results suggest that both high-pressure (HP) mafic and felsic granulites record clockwise P–T paths, including isothermal decompression and cooling from the peak stage (M2: 10.4–10.8 kbar/830–860 °C and 9.6–11.2 kbar/850–930 °C, respectively) to the retrograde stages (M3a and M3b: 7.5–8.0 kbar/840–870 °C and 7.8–8.2 kbar/650–700 °C, respectively; M3: 7.2–8.6 kbar/670–800 °C). Zircon U–Pb dating shows that the magmatic precursors of these rocks were emplaced at c. 2.5–2.47 Ga. The metamorphic zircons reveal two groups metamorphic ages of c. 2.51–2.45 Ga and c. 1.90–1.85 Ga. The c. 2.51–2.45 Ga ages indicate a late Neoarchean tectonothermal event, and the c. 1.95–1.85 Ga ages are interpreted to represent the Paleoproterozoic HP granulite-facies metamorphism overprinting. The highest positive εHf(t) value of 7.64 and 2.8–2.6 Ga Hf model ages of c. 2.5 Ga magmatic zircons indicate that the c. 2.5 Ga mantle-derived mafic magma and the c. 2.5 Ga partial melting of 2.8–2.6 Ga basaltic juvenile crust generated the protoliths of mafic and felsic granulites, respectively, revealing 2.8–2.6 Ga and 2.5 Ga crustal growth and c. 2.5 Ga crustal reworking in the northern NCC.To sum up, we consider that the late Neoarchean magmatism and coeval metamorphism recorded in our samples are probably resulted from the amalgamation of micro-blocks. The Paleoproterozoic clockwise P–T–t paths with near-isothermal decompression segments and low geothermal gradients (c. 21–26 °C/km) of HP mafic and felsic granulites indicate a rapid post-collisional exhumation process after the significantly crustal thickening, and this long-term orogeny along the TNCO (c. 1.95–1.85 Ga; > 100 Myr) is related to the incorporation of the NCC into the Columbia supercontinent.

Paleoproterozoic high-pressure granulite facies metamorphism in the Yinshan Block, North China craton

The Yinshan Block located in the northern margin of the North China Craton (NCC) has been regarded as a typical Archean cratonic block, but it is argued whether the block had been affected by Paleoproterozoic tectonic and metamorphic events. This paper reports the late Paleoproterozoic high-pressure (HP) granulites for the first time from the Xiwulanbulang area in the Yinshan Block, and presents a systematic study on their petrography, mineral chemistry, phase equilibria modelling and zircon dating to confirm their metamorphic evolution and tectonic implications. Two representative samples contain the typical HP basic granulite assemblages involving garnet, clinopyroxene, plagioclase, amphibole and quartz, with garnet commonly occurring as coronae around clinopyroxene and plagioclase. Both samples show clockwise P–T paths with their peak conditions of ∼930 °C/∼15kbar (20NM01) and 970–980 °C/∼13.5 kbar (21NM05) respectively, constrained using the maximum Ti contents in amphibole and minimum anorthite (XAn) in the mantle of plagioclase on P–T pseudosections. Zircon dating yields a magmatic age of ∼ 2.3 Ga and metamorphic ages of ∼2.5 Ga and ∼1.8 Ga. The ∼2.3 Ga magmatic age represents the time of protolith gabbro crystallization, which can be comparative to the extension-related bimodal magmatism in the Khondalite Belt. The ∼ 1.8 Ga age was determined from metamorphic zircons that have flat HREE patterns with the presence of garnet, interpreted as the time of the post-peak cooling and uplifting of the HP granulite facies metamorphism. The ∼2.5 Ga is defined from the captured zircon grains by gabbroic intrusions from the country Neoarchean rocks. The HP granulite facies metamorphism registers a geothermal gradient of 19–20 °C/km, indicating a collision-related orogenic event that may have occurred at ∼1.85 Ga along the northern margin of the NCC. Therefore, the Yinshan Block may represent a weak deformation domain in the orogenic belt on the northern margin of the NCC.

Paleoproterozoic (2.01 Ga) high-pressure granulite facies metamorphism and isothermal decompression of garnet–orthopyroxene-cordierite–orthoamphibole gneisses from the Central Zone of the Limpopo Belt in eastern Botswana

Paleoproterozoic (2.01 Ga) high-pressure granulite facies metamorphism and isothermal decompression of garnet–orthopyroxene-cordierite–orthoamphibole gneisses from the Central Zone of the Limpopo Belt in eastern Botswana

Tracking the multi-stage metamorphism and exhumation history of felsic gneisses in the South Altyn ultra-high pressure metamorphic belt, Western China

Felsic gneisses are usually the dominant rock types in most high-pressure (HP) and ultrahigh-pressure (UHP) terranes, although their peak metamorphic conditions and complete P–T paths are rarely known. This lack of knowledge is attributed to the high variance of mineral assemblages at HP/UHP conditions of the felsic gneisses, or to the effects of retrograde-related overprinting. Thus, it is difficult to distinguish whether felsic gneisses underwent HP/UHP metamorphism. In this paper, we focus on the felsic gneiss that has been previously documented as having formed under HP granulite-facies metamorphism with peak P–T of > 1.15 GPa and > 860 °C from the Danshuiquan locality in the South Altyn HP–UHP belt. The Danshuiquan felsic gneiss is strongly deformed and is spatially associated with weakly deformed garnet-rich felsic gneiss. On the basis of detailed microstructure observations on mineral assemblages and phase equilibria modelling, three stages of metamorphic evolution for the weakly deformed garnet-rich felsic gneiss are identified. The first stage is represented by garnet cores, revealing a prograde P–T path from 1.5 GPa/750 °C to 2.1 GPa/900 °C. The second stage is characterized by garnet mantles with multiphase inclusions, defining HP eclogite-facies metamorphic conditions of 2.2–2.6 GPa/950–1100 °C. The third stage is represented by the composition of garnet rims, biotite, and plagioclase, suggesting an HP granulite to amphibolite facies decompression cooling process to 1.1–1.3 GPa/710–770 °C. In contrast, the strongly deformed felsic gneiss just records a deformation-related decompression process from 1.1 to 1.2 GPa/750–760 °C to 0.87–0.98 GPa/760–770 °C, and finally to 0.83 GPa/740 °C. Zircon U–Pb dating yields c. 920 Ma protolith ages, c. 500 Ma eclogite-facies ages, and c. 480 Ma granulite to amphibolite facies retrograde ages for the Danshuiquan felsic gneisses. Our results show that the Danshuiquan felsic gneisses underwent a similar metamorphic evolution process to that of Yinggelisayi HP–UHP rocks in the eastern segment of the South Altyn Tagh. Most HP mineralogical records tend to be eliminated by the strong deformation at crustal depths (∼30 km), meaning that the peak metamorphic conditions of the studied felsic rocks in UHP metamorphic terranes are previously underestimated.

Neoproterozoic HP granulite and its tectonic implication for the East Kunlun Orogen, northern Tibetan Plateau

The P–T path of high-pressure (HP) granulite is a sensitive indicator providing metamorphic evidence on the convergence process and on the evolutionary history of orogenic belts during the supercontinent circles. The Wulonggou mafic granulites in the East Kunlun Orogen are intercalated as lenses/boudins within the dominant amphibolite facies basement rocks. Based on microstructures and mineralogical observations, three mineral assemblage generations can be recognized in the granulite samples: (1) inclusions within garnet and amphibole comprising cpx + pl + amp + qtz + rt; (2) the matrix assemblage grt + cpx + pl + amp + qtz + rt ± bt ± ilm; and (3) a retrogressive assemblage consisting of kfs + pl + qtz + amp coronas around garnet. Phase equilibrium modeling and conventional thermobarometric calculations reveal that the Wulonggou granulite has experienced (1) first HP granulite facies metamorphism at > 12 kbar and 810–840 °C, (2) dominant HP granulite facies stage of 800–860 °C at 10.7–12.3 kbar, as well as (3) a retrograde stage of 620–650 °C and 6.7–8.1 kbar, implying a clockwise P–T path. Zircon U–Pb geochronological results indicate that the mafic granulites and the gneissic country rocks yield similar Neoproterozoic metamorphic ages of 939–941 Ma. The granulite also yields a zircon U–Pb age of 444 ± 7 Ma and amphibole 40Ar/39Ar plateau age of 433 ± 1 Ma, suggesting an Early Paleozoic metamorphic overprint at ca. 440 Ma. Together with our previous studies on the contemporaneous Neoproterozoic magmatism and metamorphism, all the above lines of evidence constrain that the Wulonggou mafic granulite has experienced high pressure granulite facies metamorphism at ca. 940 Ma, suggesting a Neoproterozoic collisional orogeny in the East Kunlun Orogen, which has been overprinted by the Early Palaeozoic orogenic processes.

High-pressure granulite-facies metamorphism in the junction between the Siberian and Kazakhstan-Junggar continents and implications for the assembly of Pangea

The assembly of Pangea is usually thought to be finally completed by the accretion of the Siberian continent to Laurussia, but the exact timing of the accretion is poorly constrained in global paleogeographic reconstructions, which lagged behind the Kazakhstan-Laurussian amalgamation in the latest Mississippian and was possibly coeval with, or after, the amalgamation of the Tarim and Kazakhstan-Junggar continents in the Pennsylvanian. In this respect, the geological features, such as the medium-pressure facies series metamorphism in the Irtysh Complex, NW China, from the junction between the Siberian and Kazakhstan-Junggar continents are expected to provide crucial constraints on the timing of the accretion. The metamorphic rocks in the Irtysh Complex record the high-pressure granulite-facies metamorphism with the assemblage of kyanite + K-feldspar + garnet and show a complete clockwise metamorphic P-T-t path, involving rapid burial to the peak P-T conditions of ∼11 Kbar and 800 °C at ∼300 Ma and complicated exhumation to the surface during the Permian. The Pennsylvanian (∼322–300 Ma) rapid burial was coeval with the collision of Siberian and Kazakhstan-Junggar continents, whereas the Permian exhumation was triggered by post-collisional extension and sinistral shearing and can be divided into three substages: isothermal decompression at ∼300 Ma, near-isobaric cooling (∼300–268 Ma), and decompression-cooling to the surface prior to ∼253 Ma. The high-pressure granulite-facies metamorphism suggests that the Siberian continent was amalgamated with the Kazakhstan-Junggar continent in the Pennsylvanian, almost coeval with the amalgamation of the Tarim and Kazakhstan-Junggar continents. Therefore, the assembly of Pangea was completed in the latest Carboniferous at least.

A refined study of Paleoproterozoic high-pressure granulite-facies metamorphism in the Kongling complex of northern Yangtze block

The Paleoproterozoic era is a critical tuning period for the transitions in global geodynamic regimes and plate tectonic processes. However, detailed pressure–temperature–time (P–T–t) paths of Paleoproterozoic metamorphic rocks worldwide are scarce, which hinders our understanding of deep thermal state and relevant tectonic processes at that time. Here, we identified a prolonged Paleoproterozoic (from ca. 2.05 Ga to ca. 2.0 Ga) metamorphism by zircon U–Pb geochronology of mafic granulite in the Kongling Complex of the northern Yangtze Block. The metamorphic zircon grains/domains show distinct weakly to strongly fractionated heavy rare earth elements (HREE), corresponding to competitive growth with or without garnet during the metamorphism. Mineral assemblages of orthopyroxene + plagioclase are distinctly found both within and around (as corona) garnet, indicating high-temperature decompression. P–T estimations from mineral compositions, chemical potential modeling and phase equilibrium modeling together yield a clockwise path with peak metamorphism at 10.0 ± 0.3 kbar and 860 ± 30 °C followed by decompression to 6.0 ± 0.5 kbar and 840 ± 40 °C in a period of about 50 Myr. This contradicts a Paleoproterozoic ultrahigh-pressure collision in northern Yangtze Block and may exemplify the assembly of micro-continents during formation of the Paleoproterozoic Columbia (Nuna) supercontinent. According to the available P-T paths of worldwide Paleoproterozoic metamorphism, we propose a typical warm subduction/collision in Paleoproterozoic plate tectonic regime.

Metamorphic evolution of the Sittampundi Layered Complex, India, during the Archaean–Proterozoic boundary: insight from pseudosection modelling and zircon U–Pb SHRIMP geochronology

AbstractIn the Palghat–Cauvery Shear/Suture Zone of the Southern Granulite Terrane, the Sittampundi Layered Complex occurs as a mappable unit. The Sittampundi Layered Complex consists of mafic, ultramafic and anorthositic rocks with chromitite layers and has been interpreted as an arc/ophiolite complex that formed in an Archaean suprasubduction zone arc setting. In the Sittampundi Layered Complex, reddish-black metabasites occur as layers or boudins with a rim of amphibolite within anorthosite. The peak metamorphic assemblage of the metabasites is garnet + clinopyroxene + quartz + rutile ± plagioclase ± orthopyroxene, and symplectite consisting of amphibole and plagioclase formed around the garnet during retrograde metamorphism. The protolith of metabasite may have intruded in a suprasubduction zone arc setting during the late Neoarchaean (c. 2540–2520 Ma), and then underwent high-pressure granulite-facies peak metamorphism (900–800 °C and 11–14 kbar) in the early Palaeoproterozoic (c. 2460–2440 Ma), followed by amphibolite-facies metamorphism (550–480 °C and 5.5–4.5 kbar) in the middle Palaeoproterozoic (c. 1900–1850 Ma). The results obtained in this study, together with previous studies, indicate that: (1) the high-pressure granulite-facies metamorphism in the study area indicates that subduction occurred during the Archaean–Proterozoic boundary with a higher apparent average geothermal gradient (∼20–16 °C km−1) than the modern-day Earth, and (2) the apparent average geothermal gradient of the subduction zone was ∼29–14 °C km−1 during the Archaean–Proterozoic boundary, which was still too high to enter the realm of eclogite-facies metamorphism.

Reply to ‘Comment on “Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implication for the western extension of the Limpopo Complex”’ by Mfa et al.

Reply to ‘Comment on “Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implication for the western extension of the Limpopo Complex”’ by Mfa et al.

Comment on “Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex”

Comment on “Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex”

Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen

Abstract Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.

Tectonic burial of sedimentary rocks drives the building of juvenile crust of magmatic arc

Abstract Continental arcs grow primarily by addition of mantle-derived magmas, thus forming juvenile crust, although geophysical evidence, alongside field investigation of exhumed terranes, show that supracrustal rocks are common components of the lower portions of continental arcs. The mechanisms by which metasedimentary rocks are transported to the deep arc crusts and their contributions to the juvenile arc crusts are ambiguous. Here, we conduct a systematic petrological, geochronological, and geochemical study of pelitic migmatites within Late Cretaceous meta-gabbros from the lower crust of the eastern Gangdese arc, southern Tibet. Our results show that the pelitic migmatites were derived from the Late Carboniferous sedimentary rocks of the upper arc crust, have significantly enriched Sr-Nd-Hf-O isotopic compositions, and underwent Late Cretaceous (95–80 Ma) high-pressure granulite-facies metamorphism and partial melting at ~850 °C and 15 kbar to generate voluminous granitic melts. This indicates that the eastern Gangdese arc underwent significant crustal thickening and chemical differentiation during the final stages of subduction of the Neo-Tethys. We suggest that the metasedimentary rocks were transported into the lower crust of the Gangdese arc by underthrusting of arc crust and accretion of mantle-derived magma, and the deeply buried supracrustal rocks altered the petrological constitution and chemical compositions of juvenile lower arc crust, and assimilated the mantle- and juvenile crust-derived melts. We propose that tectonic burial of sedimentary rocks is a key mechanism driving the building and reworking of juvenile crust of magmatic arcs throughout most of geological time.

Late Cretaceous Metamorphism and Anatexis of the Gangdese Magmatic Arc, South Tibet: Implications for Thickening and Differentiation of Juvenile Crust

AbstractMagmatic arcs are the primary sites of growth of post-Archean continental crust; however, the mechanisms and processes for transforming primary arc crust into mature continental crust are subject to disagreement. We conducted a detailed petrologic and geochronological study on mafic and felsic migmatites from the eastern Gangdese magmatic arc, which is typical of continental arcs worldwide. The studied mafic migmatites contain amphibole, garnet, plagioclase, epidote, white mica, quartz, rutile and ilmenite in melanosomes, and plagioclase, garnet, epidote, amphibole, white mica, and quartz in leucosomes. The leucosomes occur as diffuse patches, concordant bands, or concordant and discordant networks and veins in the melanosomes. The migmatites have protolith ages between ~157 and ~86–87 Ma, and metamorphic ages of ~83–87 Ma and underwent high-pressure granulite-facies metamorphism at peak P–T conditions of ~850–880°C and 15–17 kbar. Heating, burial, and associated partial melting preceded near-isobaric cooling with residual melt crystallization. Significant melt (&amp;gt;16 wt.%) generated during heating and loading had a granitic composition. Compositional comparison to low-grade meta-gabbros implies that any extracted melt had adakitic affinities (high Sr/Y and highly fractionated REE patterns). The eastern Gangdese magmatic arc experienced crustal thickening during Late Cretaceous late-stage evolution of the arc due to magma loading and tectonic shortening and thrusting of the arc crust. Crustal thickening and chemical differentiation of the Gangdese arc occurred during late subduction of the Neo-Tethys, prior to the India–Asia collision. Metamorphism nearly completely erased all prior igneous mineralogy and mineral chemistry, and consequent partial melting represents a potential source for Late Cretaceous granitoids of the upper arc crust. Although prior studies demonstrate the significance of fractional crystallization, deep-seated metamorphic processes largely drove chemical differentiation to produce mature continental crust in the Gangdese arc during the late Cretaceous.

Comment on Volodichev et al. Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding. Minerals 2021, 11, 1029

Volodichev et al. (Volodichev et al., 2021) reported on the first finding of omphacite (23%–25% Jd) inclusions in 2.68 Ga metamorphic zircons from Gridino eclogites and presented it as evidence for Archean eclogite-facies metamorphism in the Belomorian Mobile Belt. We believe that the Archean age of the garnets referred to by the above authors was estimated incorrectly. Our interpretation is that omphacite origin is related to Archean high-pressure granulite-facies metamorphism.

Neoarchean arc magmatism and Paleoproterozoic high-pressure granulite-facies metamorphism in the southern Motloutse Complex, eastern Botswana: Implications for the western extension of the Limpopo Complex

We provide the first record of the occurrence of high-pressure garnet-bearing amphibolite and mafic granulite hosted by amphibolites from the Maope area of the southern Motloutse Complex in eastern Botswana. The mineral assemblages of the rocks are garnet + edenite + plagioclase + ilmenite + rutile + quartz (garnet-bearing amphibolite), magnesio-hornblende + orthopyroxene + clinopyroxene + plagioclase + quartz (mafic granulite), and magnesio-hornblende + plagioclase + quartz + ilmenite + epidote + titanite (host amphibolite). The poikiloblastic garnet in the garnet-bearing amphibolite contains inclusions of edenite, ilmenite, quartz, and plagioclase as prograde minerals, while it is mantled by ferro-edenite + plagioclase coronae formed during retrograde metamorphism. The geochemical signatures of the mafic granulite and amphibolite, combined with the enrichment of large-ion lithophile elements and depletion of high-field strength elements (Nb and Ta), indicate subduction-related volcanic arc affinity, whereas the garnet-bearing amphibolite indicates subduction-related volcanic arc or within-plate basalt affinity. The peak metamorphic condition of the garnet-bearing amphibolite was constrained as 850–910 °C and 10.0–10.5 kbar based on phase equilibrium modeling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCFMASHTO) system and geothermobarometry. Slightly lower conditions were obtained from the mafic granulite (745–825 °C) and amphibolite (610–750 °C). Zircon grains from the garnet-bearing amphibolite with lower Th/U ratios of 0.02–0.23 yielded a weighted-mean 207Pb/206Pb age of 2020.2 ± 4.9 Ma as the timing of high-grade metamorphism. In contrast, oscillatory-zoned zircons from the mafic granulite with higher Th/U ratios of 0.44–0.86 recorded a magmatic age of 2648.8 ± 9.8 Ma. As the age is consistent with previously reported age of a tonalite gneiss (2645 Ma) from the same region, we infer bimodal arc magmatism during Neoarchean. The results of this study indicate that the southern Motloutse Complex underwent high-pressure granulite-facies metamorphism with post-peak decompression along a single clockwise P–T evolution during Paleoproterozoic Era (∼2.02 Ga). Our integrated petrological and geochronological studies suggest that the southern Motloutse Complex is nearly equivalent to the Beit Bridge Complex as a possible western extension of a collisional suture (the Central Zone of the Limpopo Complex) between the Archean Kaapvaal and Zimbabwe Cratons. The occurrence of ca. 2.0 Ga high-pressure granulite-facies rocks from the southern Motloutse Complex might suggest multiple collisions of discrete crustal units during Neoarchean to Paleoproterozoic.

High-pressure granulite-facies metamorphism and anatexis of deep continental crust: New insights from the Cenozoic Ailao Shan–Red River shear zone, Southeast Asia

Studies of crustal anatexis have provided valuable insights into the evolution of metamorphism, deformation, and tectonic processes at convergent plate margins during the orogeny. The transition of metatexite to diatexite migmatites records crucial information about the tectonothermal evolution and rheology of the deep crust. Along the Ailao Shan–Red River shear zone, metatexite migmatites, diatexite migmatites, and leucogranites are widely distributed within the upper amphibolite and granulite facies zones of the Diancang Shan metamorphic complex. The high-pressure granulite-facies metamorphism, with the mineral assemblage comprising garnet + kyanite + K–feldspar + plagioclase + biotite + quartz + melt, is first recognized from the patch of metatexite migmatites in the complex. Detailed petrographic evidence and thermodynamic modeling revealed that the migmatite underwent nearly isothermal decompression metamorphism, presenting a clockwise P–T path. The peak metamorphic P–T conditions are constrained at approximately 11 kbar and 810 °C using thermodynamic modeling, and the amount of melt generated during heating is up to 18 mol%. The extraction and segregation of the melts are evidenced by the presence of leucosomes within the migmatites and leucogranite dikes, which record the melt flow network through the crust. Zircon and monazite geochronology of migmatites record the timing of the melting episode that began at approximately 36 Ma and lasted until 20 Ma. All these results are consistent with orogenic crust thickening accompanied by pervasive anatexis during the Late Eocene to the Early Oligocene in the Ailao Shan–Red River shear zone. Combined with the available data related to the other continental–exhumed shear zone, we propose that the crustal anatexis had a significant effect on the thermal state of deep–seated shear zones, thus controlling the rheological behavior of the lithosphere and playing an essential role in the initial localization of shearing in the lower crust.

High pressure granulite facies metamorphism at the interface of the Archean Bastar craton and the Proterozoic Eastern Ghats Belt, India

Nd model ages show that the suture between the Archean Bastar craton and Proterozoic Eastern Ghats Province (a part of the Eastern Ghats Belt), India is marked by a broad shear zone that contains a mixture of rocks from both the blocks. In this suture zone, amphibolite facies rocks of the Bastar craton were subjected to high pressure granulite facies metamorphism possibly at ~500 Ma (published isotopic data). New petrological, geothermobarometric and P-T pseudosection studies show that dehydration-melting of amphibole (in mafic protolith) and of biotite (in quartzofeldspathic protolith) along a clockwise P-T path led to peak metamorphic conditions of 9.75 ± 0.5 kbar; 875 ± 30 °C. Mafic granulites from the Eastern Ghats Province had an earlier metamorphic history of isobaric cooling from unknown peak metamorphic conditions, but currently record P-T maximum values of 9.25 ± 0.25 kbar; 825 ± 25 °C. Followed by cooling through ~100 °C from the peak metamorphic conditions, mafic granulites from the Bastar craton were exhumed to mid-crustal levels by decompression-cooling, probably as a tectonic wedge. This decompression-cooling is also shared by charnockitic rocks of the Bastar craton and mafic granulites of the Eastern Ghats Province. We interpret further burial of the Bastar craton rocks as a result of underthrusting beneath the Eastern Ghats Province, consistent with available seismic data. Total absence of any evidence of ocean opening or closure between the Bastar craton and the Eastern Ghats Province at around 500 Ma, coupled with interpretation of deduced P-T paths, suggest the shear zone resulted from far-field stress related to the Kuunga orogeny in an intraplate setting. This would support a model of Tonian age amalgamation of the Bastar craton and the Eastern Ghats Province as a part of the Greater Indian Landmass.

On the origin of high-pressure mafic granulite in the Eastern Himalayan Syntaxis: Implications for the tectonic evolution of the Himalayan orogen

The Himalayan orogen, resulting from the Early Cenozoic collision of the Indian and Asian plates, is an ideal vehicle to study active orogenic processes and test geodynamic models of how the crust responds to collisional orogeny. This paper focused on migmatitic high-pressure (HP) mafic granulite and associated leucosome from the Greater Himalayan Crystallines (GHC) in the Eastern Himalayan Syntaxis (EHS) in order to understand the conditions and timescales over which high-grade rocks and partial melts were produced during the Himalayan orogeny. Combining with previous study results from the Western and Central Himalayas and Trans-Himalayan magmatic arc, we obtained the following conclusions: (1) The mafic granulites from the EHS underwent HP and high-temperature (HT) granulite facies metamorphism and partial melting, with peak metamorphic conditions of 15–17 kbar and 820–880 °C. The GHC, at least its western part of the EHS, underwent coherent HP granulite-facies metamorphism. (2) The HP mafic granulites experienced long-lived dehydration melting of amphibole from ~40 Ma to ~20 Ma during prograde metamorphism and generated up to ~16 vol% partial melt. The variable degrees of dehydration melting of the HP mafic, pelitic and felsic granulites in the EHS generated voluminous granitic melts with distinct compositions, and provided the source for the Himalayan granites. (3) Peak metamorphic pressure of the GHC gradually decreases, whereas the metamorphic temperature progressively increases from the Western to Eastern Himalayas. This indicates that the Indian continental crust deeply subducted into the mantle in the Western Himalaya after the Indo-Asia collision, whereas the Indian crust underthrusted or relaminated beneath the Asian continental crust, and formed the thickened lower crust in the Central and Eastern Himalayas and Gangdese arc. (4) The melts derived from the underthrusted Indian crust probably resulted in isotopic compositional enrichment of the Early Cenozoic mantle- and juvenile crust-derived magmatic rocks of the Gangdese arc.

Chicheng high-pressure granulites record the paleoproterozoic tectonic evolution in the northern North China Craton

High-pressure (HP) mafic granulites are widely distributed along the Trans-North China Orogen (TNCO), and they provide important insights into the timing and tectonic processes involved in the formation of the TNCO. We discovered three types of HP granulite, including mafic, intermediate, and felsic granulites, exposed in Chicheng area, in the northern segment of the TNCO. Their mineral assemblages of prograde, peak, and post-peak metamorphic stages were identified based on petrographic investigations. SIMS U-Pb zircon analyses yielded an upper intercept age of 2547 ± 38 Ma and a lower intercept age of 1916 ± 100 Ma for one HP mafic granulite and a discordant 207Pb/206Pb age of 2479 Ma with a weighted mean 207Pb/206Pb age of 1905 ± 9 Ma for the other HP mafic granulite. For the HP intermediate granulite, an upper intercept age of 2544 ± 20 Ma was obtained. The HP felsic granulite yielded an upper intercept age of 2524 ± 23 Ma and a concordant 207Pb/206Pb age of 1906–1907 Ma. These results reveal that the protoliths of HP granulites were formed at ~ 2.5–2.55 Ga and underwent high pressure granulite facies metamorphism at ~ 1.9 Ga. The elevated Hf isotopic compositions of the metamorphic zircons from two granulite samples indicate that metamorphic fluids that have high 176Hf/ 177Hf ratios but low 176Lu/177Hf ratios were involved in their metamorphism. The low 18O isotopic features of the zircons from the intermediate granulite suggest alteration by δ18O-depleted seawater or meteoric fluids at high temperatures. By integrating geochronological data from previous studies, we infer that the formation of the TNCO may have begun as early as ~ 1.9 Ga and lasted until ~ 1.8 Ga, supporting the hypothesis that it was formed by a long-lived tectono-metamorphic event