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Garnet amphibolite and semi-pelitic gneiss preserving three stages of metamorphic mineral assemblages, i.e., the prograde metamorphic inclusion assemblage (M1) preserved in the garnet porphyroblast, metamorphic peak assemblage (M2) and retrograde symplectic assemblage (M3), have been recognized in the Aketashitage area, northwestern China. Geothermobarometric computation shows that the M1, M2 and M3 assemblages record the metamorphic P-T conditions of 600–700 °C/4.2–5.3 kbar, 730–790 °C/6.3–7.1 kbar, and 770 °C/5.6 kbar, respectively. These three metamorphic stages define clockwise P-T paths including nearly isothermal decompression (ITD) process, indicating typical orogenic metamorphism. The peak metamorphic P-T conditions reach the transition zone between the high amphibolite facies and granulite facies, and belong to intermediate P/T facies series. The SHRIMP and LA-ICP-MS U-Pb dating of metamorphic zircon implies that the subduction might occur as early as ∼1.97 Ga and last as late as 1.84 Ga. Combined with literature data, it is concluded that the Aketashitage area was a Paleoproterozoic orogenic belt, which is possibly related to the assembly of the basements of the Columbia supercontinent.

Garnet-bearing mafic granulites occur as lenses within TTG gneiss or metapelite in the Fuping Complex, middle Palaeoproterozoic Trans-North China Orogen. They are composed mainly of garnet, clinopyroxene, orthopyroxene, hornblende, plagioclase, quartz and accessary zircon and titanite. Four stages of metamorphic mineral assemblages have been found in these mafic granulites: (a) the prograde assemblages (M1) are inclusions preserved in the garnet porphyroblasts, including clinopyroxene, hornblende, plagioclase and quartz; (b) the metamorphic peak assemblages (M2) consist of garnet porphyroblasts plus matrix minerals clinopyroxene + orthopyroxene + hornblende + plagioclase + quartz + zircon + titanite; (c) the first retrograde assemblages (M3) are represented by clinopyroxene + orthopyroxene + plagioclase + quartz symplectites rimming the garnets; and (4) the final retrograde assemblages (M4) are hornblende + plagioclase + quartz symplectites rimming or adjacent to the garnets. The retrieved metamorphic P-T paths pass from 530–600 °C/6.5–7.0 kbar (M1) through 680–820 °C/7.2–11.7 kbar (M2) to 730–760 °C/3.2–3.9 kbar (M3) and finally to 600–640 °C/3.2–3.7 kbar (M4). All the P-T paths of the mafic granulites are clockwise, two of these even contain near isothermal decompression (ITD) and subsequent near isobaric cooling (IBC) segments. SHRIMP U-Pb dating of metamorphic zircons of the mafic granulites yields a mean weighted age of ∼1849 ± 9Ma, which is consistent with the metamorphic time of surrounding TTG gneisses. Therefore, the Fuping Complex records the collision event between the Eastern and Western Blocks of the North China Craton that led to the final assembly of this craton during the late Paleoproterozoic.

Mafic and felsic granulite from the Miyun metamorphic complex, northeast Beijing, North China Craton, occur as either interlayers or lenses within the Late Archean TTG gneiss. In the mafic granulite, the metamorphic peak assemblages (M1) are represented by porphyroblast minerals plus matrix minerals (orthopyroxene + clinopyroxene + plagioclase + quartz ± K-feldspar ± hornblende ± biotite) as well as accessory minerals of zircon + apatite + ilmenite ± magnetite, the post-peak assemblages (M2) are characterized by “red-eye socket” symplectites (garnet + clinopyroxene + plagioclase + quartz) rimming the orthopyroxene or clinopyroxene porphyroblast as well as accessory minerals of rutile, and the final retrograde assemblages (M3) are marked by the hornblende-biotite intergrowth. As for the felsic granulite, the peak assemblages (M1) are represented by the porphyroblast and matrix minerals (garnet + orthopyroxene + biotite + plagioclase + quartz ± K-feldspar) as well as accessory minerals in the matrix (zircon + monazite + apatite + pyrite), the post-peak assemblages (M2) are characterized by “red-eye socket” symplectites (garnet + plagioclase + quartz) rimming the orthopyroxene porphyroblast, and the final retrograde assemblages (M3) are marked by the biotite-calcite intergrowth. Geothermobarometric computation suggests that the M1, M2 and M3 assemblages of the mafic granulite were formed under P-T conditions of ∼800 to 860 °C/∼6 to 10 kbar, ∼620 to 700 °C/11 to 12 kbar and ∼590 to 660 °C/4 to 6 kbar, respectively, suggesting that these granulites record anticlockwise P-T paths including final nearly isothermal decompression (ITD) segments, indicative of orogenic process. For the felsic granulite, metamorphic P-T conditions of the M1 and M3 assemblages were estimated to be ∼680 to 750 °C/6–8 kbar and ∼670 to 680 °C/5 kbar, respectively, also including ITD process. The widely penetrative gneissosity of the granulite and these P-T paths, suggest a collisional event of cold nappes overthrusting on hot ones or hot nappes being buried/subducted into cold ones and finally experienced fast tectonic exhumation. High-precision SIMS and LA-ICP-MS U-Pb dating of metamorphic zircon of the granulite suggest that the metamorphism occurred at ∼2.5 Ga. These data confirm that the Miyun metamorphic complex experienced a Neoarchean collisional event within the Eastern Block of the North China Craton.

The discrete mafic granulite and amphibolite enclaves within the Pingyang trondhjemitic pluton, Fuping metamorphic terrane, middle Trans-North China Orogen, ubiquitously record three stages of metamorphic assemblages. For the mafic granulite enclaves, the prograde metamorphic assemblage (M1) is represented by mineral inclusions (clinopyroxene + hornblende + plagioclase + quartz ± apatite) preserved in the garnet porphyroblast, the metamorphic peak (M2) assemblage consists of garnet and matrix minerals of clinopyroxene + orthopyroxene + hornblende + plagioclase + quartz ± magnetite ± ilmenite ± zircon, and the retrograde (M3) stage is represented by symplectite assemblage of orthopyroxene + plagioclase ± clinopyroxene ± hornblende ± quartz rimming the garnet porphyroblast. The metamorphic P-T paths of the mafic granulite enclaves are defined to pass from 560–750 °C/6–9 kbar (M1) through 740–880 °C/11–14 kbar (M2) to 720–800 °C/6–8 (M3), indicative of a collisional orogenic process. The metamorphic P-T path of the amphibolite enclave experienced from 670–730 °C/9.3–11.8 kbar to 710–730 °C/3.9–5.5 kbar, further certify such metamorphic feature of an orogenic belt. SIMS U-Pb dating of zircons of the trondhjemitic gneiss yields the magmatic crystallization age of ∼2.52 Ga, and both the trondhjemitic gneiss and the mafic granulite enclaves yield the metamorphic age of ∼1.89–1.86 Ga. Therefore, it can be concluded that the Fuping terrane was involved in the late Palaeoproterozoic subduction-collision-uplifting tectonic process caused by amalgamation of the Eastern and Western Blocks of the North China Craton along the Trans-North China Orogen.

The Trans-North China Orogen has witnessed the assembly of the Western and Eastern blocks of the North China Craton during Late Paleoproterozoic. Numerous studies have focused on the subduction/collision timing and metamorphic evolution of the Trans-North China Orogen, whereas the thermal evolution and exhumation history are still poorly understood. The Fuping and Wutai Complexes are widely exposed of middle- to high-grade metamorphic rocks, which are excellent regions to investigate the thermal evolution and exhumation history of the Trans-North China Orogen. Previous U-Pb geochronological data of metamorphic zircon and monazite suggest that the peak granulite facies metamorphism in the Fuping Complex occurred during 1.92–1.80 Ga, and peak amphibolite facies metamorphism in the northeastern part of the Wutai Complex happened at 1.85–1.82 Ga. Here we report newly obtained hornblende 40Ar/39Ar age of 1813–1806 Ma in the Fuping Complex, and biotite 40Ar/39Ar age of 1802–1786 Ma in the northeastern part of the Wutai Complex, which representing the cooling ages. Combined with previous thermochronological data, the cooling rate of 2.0–8.5 °C/Ma was obtained for the middle to southern segments of the Trans-North China Orogen, possibly suggesting a moderate to fast exhumation relative to other Proterozoic orogens.

The Paleoproterozoic Aketashitage orogenic belt lies in between the two large accretionary orogenic systems, i.e., Paleo-Asian ocean and Paleo-Tethys ocean systems, in northwest China. Previous geochronologic studies reveal widespread Paleoproterozoic orogeny-related magmatism and high-grade metamorphism in this area, determined by the U-Pb dating of zircon and monazite. In this paper, 40Ar/39Ar dating of hornblende of three representative amphibolite samples yielded cooling ages of ∼1.88–1.85 Ga. Comparison of the zircon/monazite U-Pb and hornblende 40Ar/39Ar ages suggests that these amphibolite samples record different exhumation and cooling processes. These new geochronologic data further confirm that the Aketashitage orogenic belt only records Paleoproterozoic metamorphism but no overprint of Paleozoic thermal event. It is thus inferred that the Aketashitage area might act as a continental fragment/terrane which was passively collaged onto the ambilateral Paleozoic orogens but was not involved in the Paleozoic subduction.

The Wutai Complex lies in the middle part of the Trans-North China Orogen, mainly consisting of metamorphosed volcanic-sedimentary rocks (named as “Wutai Group”) and Late Archean TTG gneisses. Garnet biotite schist from the northeastern part of the Wutai Group (lower Shizui Subgroup) mainly consists of garnet porphyroblast plus biotite, staurolite, kyanite, sillimanite/fibrolite, plagioclase, quartz as well as accessory minerals including ilmenite, magnetite, hematite, apatite, monazite and zircon. Petrographic observation and mineral chemistry indicate that these metapelites underwent the sequential prograde (M1), peak (M2) and retrograde (M3) metamorphic stages. Geothermobarometry computation and phase equilibrium modelling show that the representative samples record clockwise P-T paths passing from 3–7 kbar/570–630 °C (M1) through 9–10 kbar/615–660 °C (M2P) to 6–9 kbar/655–680 °C (M2T) and finally to a speculated exhumation stage (M3), indicative of orogenic metamorphism. SRHIMP and LA-ICP-MS U-Pb dating on inherited detrital zircons yielded three age peaks of ~2.7 Ga, ~2.5 Ga and ~2.1 Ga. The youngest 207Pb/206Pb age of 2064 ± 21 Ma of detrital zircon possibly constrains the maximum depositional age of the Shizui Subgroup. SIMS and SHRIMP U-Pb dating of metamorphic monazite and zircon constrain the metamorphic age of the garnet-bearing mica schist to be 1853–1823 Ma. Combined with previous data, the Wutai Complex was demonstrated to have been involved in a prolonged subduction-collision process (1.97–1.82 Ga) between the Western and Eastern Blocks in the Paleoproterozoic.

The Miyun area is located in the western margin of the Eastern Block of the North China Craton (NCC), and granulite- to amphibolite facies tectono-metamorphic events occurred in the Precambrian. In this contribution, high-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon from garnet-pyroxene amphibolite yielded the metamorphic age of 2507 ± 4 Ma. SIMS U-Pb dating of monazite from garnet-biotite gneiss yielded the metamorphic ages of 2496 ± 16 Ma and 1834 ± 21 Ma, respectively. Furthermore, 40Ar/39Ar dating of hornblende from amphibolite yielded plateau ages of 1817 ± 41–1795 ± 19 Ma, and the biotite yielded a plateau age of 1744 ± 12 Ma. Combined with previous studies and the closure temperatures of the dating minerals, it is inferred that the Miyun area experienced a late Neoarchean granulite-facies metamorphic event occurred at ~2.50 Ga, followed by a late Paleoproterozoic amphibolite-facies metamorphic event occurred at ~1.83–1.74 Ga. These two tectono-metamorphic events are ascribed to the Precambrian orogenic-related environments.

High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly represented by the symplectic minerals surrounding the embayed garnet and the retrograded hornblende rimming matrix-type clinopyroxene. Metamorphic pressure and temperature (P-T) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by thermobarometry are all clockwise, passing from 640 to 720 °C/6.2–12.6 kbar (M1) through 840–920 °C/14.6–16.2 kbar (M2) to 750–815 °C/5.5–7.9 kbar (M3) for high-pressure mafic granulite, from ~650 °C/5.7 kbar (M1) through ~750 °C/9.2 kbar (M2) to ~780 °C/8.1 kbar (M3) for amphibolite, and from ~615 °C/7.9 kbar (M1) through 730–820 °C/8.6–11.7 kbar (M2) to 675–740 °C/5.4–8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore, pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits prograde metamorphism in a P-T range of 510–800 °C/8.5–13 kbar and demonstrates peak metamorphic P-T conditions of ~850 °C/16 kbar, which are consistent with the thermobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U Pb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430–420 Ma (M2) and ca. 400–390 Ma (M3), respectively. Metamorphism was followed by the intrusion of granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subduction-collision complexes in this orogenic belt.

As a key accessory mineral in metapelite, monazite forms in the metamorphic event and therefore, places important direct constraint on the timing of the tectono-metamorphic event. In this study, we investigate four representative metapelites from different metamorphic terranes in the central-south Trans-North China Orogen (TNCO), applying secondary ion mass spectrometry (SIMS) U–Th–Pb dating on monazite. The dating results reveal that monazite collected from the Huashan, Luoning, Zanhuang and Wutai metamorphic complexes record the metamorphic ages of ~1.83 Ga, ~1.95 Ga, ~1.85 Ga and ~1.91 to 1.83 Ga, respectively. Combined with the literature data of metamorphic geochronology on the discrete terranes within the TNCO, it is suggested that the amalgamation between the Western and Eastern Blocks of the North China Craton along the nearly NS-trending TNCO stared as earlier as ~1.97 Ga and continued to as late as ~1.80 Ga, which was in fact a long-lived tectono-metamorphic event.