High-pressure Pelitic Granulites Research Articles

Geochemistry, geochronology and metamorphism of high-pressure mafic granulites in the Huai'an Complex, North China Craton: Implications for the tectonic evolution of the Paleoproterozoic orogeny

The Paleoproterozoic was a critical tectonic transition stage for the North China Craton (NCC), with several collisional orogenic belts identified. However, the tectonic boundaries, temporal-spatial evolution, and geodynamic mechanisms of these orogens remain controversial. In this contribution, we document a newly identified largest high-pressure (HP) mafic granulite outcrop at Xizhaojiayao in the Huai'an Complex, which is composed of metamorphosed supracrustal (volcano-sedimentary) sequences, including HP mafic granulites (metabasalts), marbles (metalimestones), quartzites (metasilicalites), and pelitic granulites (metapelites). They occur as tectonic slices within Neoarchean migmatized tonalitic gneisses. The Xizhaojiayao HP mafic granulites show different field occurrences and rock assemblages from the dike-type HP mafic granulites that are widely documented in the Huai'an and Hengshan Complexes. Microstructural studies, geothermobarometer calculations, and phase equilibrium modeling of the HP mafic granulites defined a clockwise pressure–temperature (P–T) path with prograde evolution from amphibolite-facies (M1, 5.2–5.5 kbar/538–668 °C) to HP granulite-facies (M2, 11.2–12.2 kbar/850–880 °C), followed by a significant near-isothermal decompression to medium-low-pressure granulite-facies (M3-1, 8.4–9.1 kbar/800–900 °C; M3-2, 4.9–5.6 kbar/779–805 °C). Zircon/titanite U–Pb dating constrains peak HP metamorphism, granulite-facies decompression, and amphibolite-facies cooling retrograde at 1902–1909 Ma, ∼1830 Ma, and < 1818 Ma, respectively. Whole-rock major and trace-element compositions, as well as positive εNd(t) (+2.0 to + 4.9) and zircon εHf(t) values (+2.9 to + 8.8) of these HP mafic granulites, indicate a mid-ocean ridge basalt geochemical affinity, whereas an island-arc signature is common for the dike-type HP mafic granulites. The Xizhaojiayao supracrustal sequences, which represent dismembered and metamorphosed upper oceanic crustal fragments, were subducted from the upper crust to depths of 35–40 km, followed by rapid exhumation to the middle-shallow crust and were tectonically involved in the melt-weakened tonalitic crust. These tectono-metamorphic scenarios illustrate the Paleoproterozoic orogenic evolution of a consecutive subduction-collision-exhumation process from 1.95 to 1.80 Ga, which led to the final assembly of the NCC.

Clockwise P−T−t paths of late Neoarchean high-pressure pelitic granulites from the Qingyuan terrane, eastern North China Craton

Precambrian high-pressure (HP) granulites provide record for the geodynamic process of ancient continental nuclei. Here we report the newly discovered late Neoarchean HP pelitic granulites in the Qingyuan terrane, eastern North China Craton (NCC). The Neoarchean basement of the Qingyuan terrane comprises meta-supracrustal rocks, tonalite–trondhjemite–granodiorite (TTG) gneisses, and a small amount of charnockites. The HP pelitic granulites were recognized from the meta-supracrustal rocks as fragments within the TTG gneisses. Detailed petrographic observations, mineral chemistry and phase equilibrium modelling results show that the prograde stage (M1) of HP pelitic granulites can be presented by the inclusions of garnet porphyroblasts including kyanite, plagioclase, quartz, biotite and ilmenite. The prograde P−T conditions were constructed via melt-reintegration approach and Ti-in-quartz thermometer (710–745 °C at 8–9.3 kbar). The peak stage (M2) can be represented by the core of garnet porphyroblasts and matrix minerals containing kyanite, plagioclase, quartz, biotite, K-feldspar and rutile with the P−T conditions of 12.2–13.6 kbar and 800–825 °C retrieved by Ti-in-quartz thermometer, garnet-Al2SiO5-plagioclase-quartz (GASP) barometer, garnet barometer and phase equilibria modelling. The peak metamorphic P−T conditions suggest that surficial sediments have been brought to a depth over 40 km. The post-peak stage (M3) is presented by the inner rim of garnet porphyroblasts (recorded in QY01) or the core of garnet (recorded in QY02) with some matrix minerals containing sillimanite, plagioclase, quartz, biotite, K-feldspar and ilmenite, indicating the P−T conditions of 7.8–9.0 kbar and 810 °C combined with Ti-in-quartz thermometer, GASP barometer, garnet barometer and phase equilibria modelling. The final retrograde stage (M4) is presented by the minor matrix sillimanite, plagioclase, quartz, K-feldspar, ilmenite and biotite with the absence of melt and the P−T conditions of 668–733 °C at 5–8 kbar using Ti-in-biotite thermometer. Monazite LA-ICP-MS U-Pb dating reveals the peak or near peak metamorphic ages ∼2.49 Ga of the HP pelitic granulites. Therefore, the HP pelitic granulites of the Qingyuan terrane record the clockwise P−T−t paths with sequential isothermal decompression (ITD) and isobaric cooling (IBC) segments, suggesting a subduction−collision environment at the late Neoarchean (∼2.49 Ga) in the eastern NCC.

Metamorphic PT path, U-Pb zircon dating and tectonic implications of High-pressure Pelitic Granulites from the Kharta region, Southern Tibet, China

HP pelitic granulites associated with eclogites from the Kharta Gneiss are found in the Thongmön area, central Himalaya, which provides significant constraints for the tectonic evolution of the Himalayan orogeny. Three generations of garnets are identified in pelitic granulites (DR15P15). The first generation of garnet is core sections of euhedral to subhedral porphyroblasts (Grt-I core) followed by growth of Grt-I mantle during the heating stage. The third generation of garnet is represented by thin rims of Grt-I and small grains (Grt-II) in the matrix, which likely nucleated and grew in response to peritectic reaction during exhumation. Petrographic evidence and P–T pseudosection calculation reveal a four-stages metamorphic evolution: 1) a prograde metamorphic stage (M1) represented by Grt-I core and its mineral inclusions rutile, ilmenite and quartz with metamorphic conditions of 740–760 °C and 13–16 kbar; 2) a near isobaric heating stage with slightly decompression to HP granulite-facies (M2, 820–840 °C, 13–14 kbar) indicated by the composition of Grt-I mantle and the increase of Zr concentrations of rutile in it; 3) a heating decompression stage (M3) supported by a decrease in grossular content in the Grt-I rim and higher XAn in plagioclase, yielding T > 860 °C and P = 10–11 kbar in the sillimanite stability field; 4) a final decompression stage followed by cooling (M4, T ~ 800 °C and P = 6–8 kbar) indicated by biotite replacing sillimanite and lower Ti content in biotite in the matrix. Based on the zircon trace element geochemistry, zircon inclusions, and the REE partitioning between garnet and zircon, the metamorphic zircon rims with middle Miocene age (16–13 Ma) probably grew during Grt-I rim/Grt-II formation at granulite facies stage (M3). The P–T history of the pelitic granulites of the Kharta Gneiss indicates that they were subducted beneath the Asian plate and exhumed to middle-lower crustal depths prior to 14–13 Ma due to the activity of STDS.

Discovery of kyanite in typically cordierite/sillimanite-bearing low- to medium-pressure pelitic granulites from the Jiaobei terrain, North China Craton

The pelitic granulites along the Paleoproterozoic Jiao-Liao-Ji belt (JLJB) are dominated by cordierite/sillimanite-bearing low- to medium-pressure (LMP) conditions. Recent studies have recognized a few typical high-pressure (HP) pelitic granulites without retrograde cordierite from the Jiaobei terrane, the southern part of the JLJB. The relationship between the dominant LMP and rare HP pelitic granulites is not well constrained, and whether the LMP types are retrograde products of the HP ones remains uncertain. This uncertainty precludes a full understanding of the original spatial distribution of HP pelitic granulites and the tectonic affinity of the JLJB. We identify in this study for the first time, relict kyanite and the mineral assemblage of kyanite + K-feldspar + rutile + garnet from typically cordierite/sillimanite-bearing LMP pelitic granulites within the Jiaobei terrane. Integrated results from petrology, pseudosection modeling, geothermobarometry, and zircon and monazite U-Pb geochronology suggest that these LMP pelitic granulites experienced an earlier HP granulite-facies metamorphism possibly at ~1.95 Ga, with minimum P-T conditions of 13–14 kbar and 860–870 °C. Subsequently, these rocks underwent near isothermal decompression to 4–7 kbar and >840 °C, followed by isobaric cooling to ~600 °C and 4–6 kbar. The decompression and cooling triggered LMP granulite-facies retrogression at ~1.85 Ga that eclipsed the earlier HP granulite-facies records. The clockwise P-T-t path defined is consistent with those reported from typical HP pelitic and mafic granulites elsewhere in the Jiaobei terrane. The results confirm that at least some portions of the typically cordierite/sillimanite-bearing LMP pelitic granulites are retrograded from the HP types. This greatly expands the spatial distribution of HP pelitic granulites and suggests a linear HP granulite-facies metamorphic belt within the Jiaobei terrane. The linear distribution of HP pelitic granulites whose protoliths are sedimentary rocks from Earth’s surface and the consistent clockwise P-T-t paths require Paleoproterozoic subduction, collision, and exhumation processes similar to those operating in modern plate tectonics. Our results from the JLJB thus provide new metamorphic constraints on the nature of Paleoproterozoic plate tectonics, and have implications for other Precambrian high-grade terranes worldwide where pelitic granulites are dominated by LMP types.

Ultra-high temperature overprinting of high pressure pelitic granulites in the Huai'an complex, North China Craton: Evidence from thermodynamic modeling and isotope geochronology

Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton (NCC). The Huai'an terrane, located within the northern segment of the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional belt in the central NCC expose mafic and pelitic granulites as well as TTG (tonalite-trondhjemite-granodiorite) gneisses. Here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M1) is recorded by relict biotite and the compositional profile of Xca (grt) isopleths. The Pmax (M2) is distinguished by the Xca (grt) isopleths, which corresponds to the kyanite stable area with an inclusion mineral assemblage of Grt-c–(Ky)-Qz-Rt-Kfs-liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 °C. However, kyanite is absent in thin sections suggesting its consumption during later stages. The Tmax metamorphism (M3) is characterized by the assemblage: Grt-m-Qz-Pl-Rt-Kfs-Sil-liq in the garnet mantle and also reflected in the compositional profile. Two-feldspar geothermometry yields a P-T range of 940 °C–950 °C and 9.5–10.5 kbar, indicating ultra-high temperature (UHT) metamorphic overprinting. The subsequent retrograde metamorphic stage (M4) is characterized by Grt-r-Bt-Sil-Kfs-Pl-Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770 °C–840 °C and 6.5–8 kbar. The pelitic granulites show a clockwise path, with P-T estimates higher than those in estimated in previous studies using conventional techniques.LA-ICP-MS U–Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1 Ma and 1873.3 ± 9.9 Ma. We suggest that the protoliths of the Manjinggou HP-UHT granulites were deep subducted where they experienced HP metamorphism associated with the collision of the Ordos and Yinshan blocks at ca. 1.97 Ga. Subsequently, the UHT metamorphic overprint occurred during the assembly of the unified Western and Eastern Blocks of the NCC along the TNCO at ca. 1.87 Ga.

Relics of a Paleoproterozoic orogen: New petrological, phase equilibria and geochronological studies on high-pressure pelitic granulites from the Pingdu-Laiyang areas, southwest of the Jiaobei terrane, North China Craton

High-pressure (HP) pelitic granulites have been used as important targets in evaluating ancient geodynamic processes similar to subduction, collision and exhumation operating in modern plate tectonics. In this contribution, relic HP pelitic granulites were identified from the Pingdu-Laiyang areas of the Jiaobei terrane that are dominated by low- to medium-pressure (LMP) pelitic granulites, suggesting their linear distribution and that at least some of LMP types should be retrograde products of the HP pelitic granulites. Their metamorphic P-T-t paths were traced by integrated methods of petrographic observation, mineral chemistry, P-T-X pseudosection modeling, Zr-in-rutile thermometry, and zircon and monazite geochronology. These HP pelitic granulites record consistent clockwise P-T-t paths that are manifested by similar peak metamorphism of high-pressure granulite-facies, with P-T conditions of P > 13 kbar, T > 850 °C possibly during 1950–1900 Ma, followed by near isothermal decompression (ITD) to mid-crustal level at 1900–1850 Ma and subsequent near isobaric cooling (IBC) to low temperature during 1850–1800 Ma. The consistent clockwise P-T-t paths and linear distribution of HP pelitic granulites explicitly indicate a sequence of geodynamic processes similar to modern-style plate tectonics must have operated during formation of the Paleoproterozoic Jiao-Liao-Ji belt (JLJB). These include a process similar to subduction that brought sedimentary precursors of HP pelitic granulites down, which was followed by continental orogeny during 1950–1900 Ma, leading to crustal thickening (40–50 km) and high-pressure granulite-facies metamorphism, and a post-orogenic collapse during 1900–1800 Ma that resulted in exhumation and cooling of HP granulites along ITD and IBC paths. Therefore, it is argued that ancient HP pelitic granulites can be regarded as relics of orogen and ascertain tectonic interpretation of the dominated LMP pelitic granulites in the same Precambrian metamorphic terranes. Notably, the apparent thermal gradients estimated from peak metamorphism are 15–20 °C/km, much higher than those of typical products in Phanerozoic plate tectonics, and the HP pelitic granulites that contain well-preserved kyanite probably require relatively high exhumation rates during ITD to middle crustal level. These seem to mark the differences in thermal regimes and tectonic mechanisms between Paleoproterozoic and Phanerozoic collisional orogenic systems.

Phase equilibrium modelling and SHRIMP zircon U–Pb dating of medium-pressure pelitic granulites in the Helanshan complex of the Khondalite Belt, North China Craton, and their tectonic implications

The Helanshan complex in the Khondalite Belt, North China Craton (NCC) is characterized by the occurrence of both medium-pressure (MP) and high-pressure (HP) pelitic granulites. This paper presents new results from phase equilibrium modelling and SHRIMP zircon U–Pb dating on the MP garnet-sillimanite granulites, in order to better understand the relationship between the MP and HP pelitic granulites, the tectonic implications of the MP pelitic granulites for the formation of the Khondalite Belt, and their geodynamic implications for the Paleoproterozoic orogenesis. Based on petrographic observations, the metamorphic evolution can be divided into four stages (M1–M4). The M1–M3 stages are suprasolidus as reflected by the migmatitic structure on the outcrop. The peak metamorphic stage (M1) is represented by the rim of garnet porphyroblasts along with inclusions of sillimanite + quartz ± biotite, and rutile relicts + plagioclase + K-feldspar in the matrix; the M2 is defined by the ilmenite that have partially replaced rutile; and the M3 is characterized by the replacement of garnet by cordierite. The subsolidus mineral assemblage is garnet + biotite + plagioclase + K-feldspar + sillimanite + cordierite + ilmenite + quartz (M4). Combined with the phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 chemical system, the MP pelitic granulites record a clockwise P–T path, which is characterized by near-isothermal decompression (ITD) from ∼10.1 kbar at ∼830 °C to ∼5.8 kbar at ∼800 °C and subsequent close-to-isobaric cooling (IBC). SHRIMP zircon U–Pb dating yielded a metamorphic age of 1931 ± 21 Ma, interpreted as the time of the retrograde cooling. Integrated with previously reported results, we propose that slab break-off subsequent to Yinshan-Ordos collision at ca. 1.95 Ga led to uplift of the MP and HP granulites in the Khondalite to the middle crustal level in an extensional regime as indicated by their ITD evolution, which was followed by slow cooling at ca. 1.93 Ga as evidenced by their nearly IBC evolution.

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.

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.

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.

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.

Long-lived high-temperature granulite-facies metamorphism in the Eastern Himalayan orogen, south Tibet

Abstract The Namche Barwa Complex exposed in the Eastern Himalayan Syntaxis, south Tibet, underwent high-pressure (HP) and high-temperature (HT) granulite-facies metamorphism and associated anatexis. The HP pelitic granulites contain garnet, kyanite, sillimanite, cordierite, biotite, quartz, plagioclase, K-feldspar, spinel, ilmenite and graphite. These minerals show composite reaction texture and varying chemical compositions and form four successive mineral assemblages. Phase equilibrium modeling constrains the P–T conditions of 10–12 kbar and 550–700 °C for the prograde stage, 13–16 kbar and 840–880 °C for the peak-metamorphic stage, and 5–6 kbar and 830–870 °C for the late retrograde stage, indicating that the HP granulites recorded a clockwise P–T path involving the early heating burial and anatexis through dehydration melting of both muscovite and biotite, and the late isothermal decompression and gradual melt crystallization under HT granulite-facies conditions. The zircon U–Pb dating reveals that the HT granulite-facies metamorphism probably initiated at ca. 40 Ma, and lasted to ca. 8 Ma. Therefore, the present study provides robust evidence for a long-lived HT metamorphism and associated anatexis in the deeply buried Indian continent and important constraints on the leucogranite generation and tectonic evolution of the Himalayan orogen.

U–Pb dating of zircons from granitic leucosomes in migmatites of the Jiaobei Terrane, southwestern Jiao–Liao–Ji Belt, North China Craton: Constraints on the timing and nature of partial melting

In the Jiaobei Terrane of the Jiao–Liao–Ji Belt, eastern China, layers and irregular lenses of granitic leucosomes are widely distributed within high-pressure (HP) mafic granulites, medium-pressure (MP) granulites, amphibolites, TTG gneisses, and HP and MP pelitic granulites. The layers and lenses may be parallel to or cross-cut the foliation in the host rocks on different scales, and they show evidence of coalescence and migration in the formation of centimeter- to decimeter-scale segregations. The migmatized rocks extend over an area of at least 120km×75km from Laixi, through Laiyang and Qixia, to SW Yantai. A combined study of mineral inclusions, cathodoluminescence (CL) images, and zircon U–Pb LA–ICP–MS dates provides clear evidence on the nature and timing of the partial melting in these granulite facies metamorphic rocks. Most zircons from the granitic leucosomes are either inherited crystals (igneous or detrital) with distinct overgrowths, or are simply new euhedral crystals. Both the overgrowths and the new crystals commonly exhibit perfect euhedral shapes, pronounced oscillatory zoning, and contain mineral inclusions of Qtz+Kfs+Pl+Ap. In contrast, the inherited cores are rounded or irregular in shape, and exhibit signs of dissolution. These observations suggest that the new zircons are anatectic in origin, and that they crystallized from melts derived from dehydration melting reactions and partial melting of the host granulite-facies rocks. Numerous U–Pb spot analyses of anatectic zircons reveal that the Jiaobei Terrane experienced a major episode of partial melting in the Paleoproterozoic, with crystallization of the melt occurring in two distinct periods from 1860.9±2.2 to 1852.1±2.8Ma, and 1839.9±2.9 to 1833.3±5.3Ma, as represented, respectively, by the magmatic cores and rims of the anatectic zircons. The first age group probably represents the initial crystallization of melts derived from partial melting within the Jiaobei Terrane, while the second age group may represent the completion of migmatization and crystallization of granitic leucosomes. This significant partial melting event was genetically associated with the Paleoproterozoic MP granulite-facies ‘hot’ stage of exhumation of the southwestern part of the Jiao–Liao–Ji Belt, and it induced extensive retrogression of the TTG gneisses and mafic and pelitic granulites, as well as the formation of regional migmatites.

Metamorphism and partial melting of high-pressure pelitic granulites from the Qianlishan Complex: Constraints on the tectonic evolution of the Khondalite Belt in the North China Craton

High-pressure pelitic granulites with a peak assemblage of garnet+kyanite+K-feldspar+plagioclase+biotite+quartz have been found from the Qianlishan Complex in the westernmost part of the Khondalite Belt, an east–west-trending Paleoproterozoic orogen between the Ordos and Yinshan Blocks in the North China Craton. Petrographic evidence indicates that high-pressure pelitic granulites underwent the prograde, peak and post-peak decompression stages. The early prograde metamorphic stage (M1) is represented by an inclusion-type mineral assemblage of quartz+plagioclase+muscovite+biotite+sillimanite, which occurs within the core of garnet porphyroblasts. The peak metamorphic stage (M2) is marked by the mantle growth of garnet porphyroblasts and matrix minerals kyanite, K-feldspar, biotite, plagioclase and quartz. The decompression stage (M3) is indicated by sillimanite replacing kyanite, the formation of the cordierite+sillimanite symplectite in the matrix (M3–1), and the cordierite corona replacing garnet (M3–2). These mineral assemblages and their P–T estimates based on the assemblage stability fields of P–T pseudosection constructed in NCKFMASHTO define a clockwise P–T path involving near-isothermal decompression. Combined with available metamorphic age data, the clockwise P–T paths reconstructed for high-pressure pelitic granulites from the Qianlishan Complex and medium-pressure pelitic granulites from other complexes in the Khondalite Belt are in accord with collision between the Yinshan and Ordos Blocks to form the Western Block at ∼1.95Ga, followed by the exhumation and associated decompressional partial melting at ∼1.88Ga.

Metamorphic P–T path and tectonic implications of medium-pressure pelitic granulites from the Jiaobei massif in the Jiao-Liao-Ji Belt, North China Craton

Both medium- and high-pressure pelitic granulites were found in the Jiaobei massif in the southern segment of the Jiao-Liao-Ji Belt, which is a Paleoproterozoic tectonic belt in the Eastern Block of the North China Craton. Studies on mineral assemblages and metamorphic reaction textures reveal that the medium-pressure pelitic granulites from the Jiaobei massif underwent four metamorphic stages: (1) the prograde stage (M1) represented by mineral inclusions within the core of garnet grains; (2) the peak stage (M2) indicated by the formation of sillimanite+K-feldspar and the growth of inclusion-free garnet mantle, (3) the decompression stage (M3) characterized by the cordierite+sillimanite symplectite, and (4) the decompression-cooling stage (M4) forming cordierite coronas. An integrated study of mineral reaction textures, mineral chemistry and pseudosection modelling in the NCFKMASHTO system with the THERMOCALC technique constrains the P–T conditions of 9.4–10kbar and 870–900°C for the M2 stage, 6.3–6.6kbar and 840–900°C for the M3 stage, and 4–5.2kbar and 815–830°C for the M4 stage. The P–T conditions of the early prograde (M1) assemblage cannot be estimated due to reworking at later metamorphic stages (M2–M4). The mineral assemblages of the M2–M4 stages and their P–T conditions define a clockwise P–T path involving isothermal decompression (ITD) and subsequent decompression-cooling for the medium-pressure pelitic granulites, which are consistent with a continent–continent collision environment. This suggests that the Jiao-Liao-Ji Belt was not formed simply by the closure of a Paleoproterozoic rift basin as conventionally considered, but the tectonic evolution of its southern segment must have been involved in subduction and/or continent–continent collisional processes. Based on this study and previous data, we propose that the Jiao-Liao-Ji Belt represented a Paleoproterozoic rifting- and collision-belt along which the Langgang and Langrim Blocks amalgamated to form the Eastern Block of the North China Craton.

Metamorphic P–T path and implications of high-pressure pelitic granulites from the Jiaobei massif in the Jiao-Liao-Ji Belt, North China Craton

High-pressure pelitic granulites have recently been found from the Jiaobei massif in the southern segment of the Jiao-Liao-Ji Belt (JLJB), which separates the Eastern Block of the North China Craton into the Longgang and Langrim Blocks. Petrological evidence from these high-pressure pelitic granulites indicates three distinct metamorphic stages (M1–M3). The pre-peak (M1) stage is represented by a mineral assemblage of garnet (core)+inclusion-type minerals within the garnet (biotite+kyanite+muscovite+plagioclase+quartz+ilmenite). The peak (M2) stage is marked by a mineral assemblage of garnet (mantle)+K-feldspar+kyanite+plagioclase+biotite+rutile+ilmenite+quartz. The post-peak (M3) stage is featured by a mineral assemblage of garnet (rim)+sillimanite+plagioclase+biotite+ilmenite+quartz. Using the THERMOCALAC technique, the NCKFMASHTO system has been applied to construct pseudosections for a representative high-pressure pelitic granulite sample. P–T conditions for the M1, M2 and M3 stages are constrained at 9.3–10.7kbar/645–670°C, 14.8–16.2kbar/860–890°C, and 6.3–8.5kbar/710–740°C, respectively. The mineral assemblages and P–T conditions of the high-pressure pelitic granulites define a clockwise P–T path involving decompression and cooling following the peak high-pressure granulite facies metamorphism. This suggests that the high-pressure pelitic granulites experienced the initial crustal thickening (M1 and M2), followed by exhumation and cooling (M3), which implies that the southern segment of the JLJB must have been involved in subduction- or collision-related tectonic processes. Therefore, the JLJB may represent another Paleoproterozoic collisional belt along which the Longgang and Langrim Blocks amalgamated to form the Eastern Block.

Timing of metamorphism in the Paleoproterozoic Jiao-Liao-Ji Belt: New SHRIMP U–Pb zircon dating of granulites, gneisses and marbles of the Jiaobei massif in the North China Craton

The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U–Th–Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded 207Pb/ 206Pb ages of 1956 ± 41 Ma and 1884 ± 24 Ma. One metamorphic zircon from a garnet–sillimanite gneiss also gave an apparent 207Pb/ 206Pb age of 1939 ± 15 Ma. These results are consistent with interval of ages of c. 1.93–1.90 Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt. Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207Pb/ 206Pb ages of 1837 ± 8 Ma, 1821 ± 8 Ma and 1836 ± 8 Ma respectively. Two diopside–olivine–phlogopite marbles yielded weighted mean 207Pb/ 206Pb ages of 1817 ± 9 Ma and 1790 ± 6 Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85 Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86–1.80 Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event.

EPMA U-Th-Pb monazite and SHRIMP U-Pb zircon geochronology of high-pressure pelitic granulites in the Jiaobei massif of the North China Craton

The Jiaobei massif constitutes the southern segment of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton, and contains high-pressure (HP) mafic and pelitic granulites. The HP pelitic granulites contain four distinct mineral assemblages: garnet + kyanite +biotite + muscovite + rutile (M₁), garnet + kyanite + ternary-feldspar + biotite + muscovite + rutile (M₂), garnet + sillimanite + biotite + perthite + anti-perthite (M₃), and biotite + muscovite + sillimanite + plagioclase (M₄), of which M₂ is a typical HP pelitic assemblage. The origin of these HP granulites is important for understanding the tectonic evolution of the Jiao-Liao-Ji Belt, but the timing of the HP event still remains unknown. This study applied the EPMA U-Th-Pb monazite and SHRIMP U-Pb zircon dating techniques to determine the metamorphic ages of the HP pelitic granulites in the Jiaobei massif. CL images and Th/U ratios show that all monazite and zircon grains in the analyzed samples are of metamorphic origin. Eighty-two EPMA analyses on 16 monazite grains from two samples yielded three age groups at 1895 to 1882 Ma, 1814 to 1813 Ma and 1706 to 1692 Ma, and the ages of 16 SHRIMP analyses on 14 zircon grains from one sample range from 1864 to 1803 Ma. The oldest ages of 1895 to 1882 Ma were yielded from high-Y monazite grains enclosed in garnet and sillimanite pseudomorph after kyanite, and thus interpreted as the time of the HP (M₂) granulite-facies metamorphism. The EPMA monazite ages of 1814 to 1813 Ma and SHRIMP zircon ages of 1864 to 1803 Ma are interpreted as the approximate ages of the post-HP granulite-facies metamorphism (M₃), most probably resulting from the exhumation of the HP granulites to the medium-pressure granulite-facies levels. The youngest age group of 1706 to 1692 Ma obtained from low-Y monazite grains in the matrix can be interpreted as the age of late cooling and retrograde metamorphism (M₄) that occurred when the HP pelitic granulites were exhumed to the upper crust. The presence of Paleoproterozoic HP mafic and pelitic granulites in the southern segment of the Jiao-Liao-Ji Belt suggests that the evolution of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton must have been involved in subduction- or collision-related tectonic processes.