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High-grade granulite facies rocks (one of extreme granulites) refer in particular to a part with high metamorphic temperature (HT) and high metamorphic pressure (HP) within granulite facies. A part of basement rocks of the North China Craton (NCC) experiences high-grade metamorphisms up to HP-HT granulite-facies or granuite-eclogite transition facies in Paleoproterozoic, which offers ideal natural laboratories to probe into early Precambrian tectonic evolution of the Earth. The studies of Precambrian high-grade metamorphic rocks have made steady progresses on distributions and occurrences of high-grade granulite facies rocks, their mineral interactions (including micro-inclusion mineral), metamorphic conditions, metamorphic chronologies, and metamorphic processes and P-T paths in recent decades. The concept of Incipient Plate Tectonics was proposed by Zhai (1997, 2004, 2012, 2014) and Zhai and Santosh (2011). However, some intractable and controversial scientific problems still exist, which are challenging and significant issues to understand early tectonic evolution of continent crust and the Earth. This paper reviews study advances of Paleoproterozoic high-grade granulite terranes in the NCC and overviews its prospect. Three fundamental characteristics are distinct: 1) Paleoproterozoic high-grade metamorphic rocks seemingly occur in area or broad pan-belt, which widely spread in the northwest NCC, middle NCC, south NCC, eastern NCC and Korean peninsula, in almost all outcrops of Precambrian basements of the NCC (Sino-Korean Craton) except for some coverage districts of Phanerozoic rocks; Three kinds of high-grade rocks have been distinguished, which are HP mafic granulite, HT pelitic granulite and ultramafic granulite. As their country rocks, some tonalite-trondhjemite-granodiorite gneisses (TTGs) could have undergone metamorphism under the same P-T conditions. 2) The HP mafic granulites and HT pelitic granulites are commonly spatially-associated with concordant occurrences, showing the both underwent contemporaneous deformation and metamorphism since peak metamorphic stage. The HP mafic granulites are suggested to be metamorphosed gabboric dykes by previous studies. Detail district geological mappings in some terranes also demonstrate that the gabbroic dykes intruded into sedimentary rocks and had been in position before peak metamorphic stage. In Jiaobei region of northern East Shandong Province, ultramafic granulites are distributed associated with HP mafic granulites and/or HT pelitic granulites. Metamorphism studies demonstrate that all the three granulite facies rocks record uplift processes and they were uplifted to surface as a whole. 3) All the three types of granulite facies rocks underwent three major metamorphic stages, which are peak HP-HT granulite facies, moderate granulite facies and amphibolites facies. The corresponding metamorphic ages are ∼1980–1900 Ma, ∼1890 ∼ 1820 Ma, and ∼1800 Ma, respectively. Some sedimentary rocks underwent ultra-high temperature (UHT) metamorphism with the indicating mineral assemblages of sappirine + quartz. Eclogite facies minerals were found in a minor amount of HP mafic granulites, which were presented by pseudomorphed omphacite ± rutile ± quartz preserved as inclusion minerals in garnets, or by some matrix clinopyroxenes broken down to wormy symplektite of albite + Na-poor clinopyroxene. Base on P-T conditions of the metamorphism and the corresponding geochronologic data, it is calculated that the Paleoproterozoic metamorphic terranes in the NCC have obviously high geothermal gradient of about 14–28 °C/km (average 21 °C/km), and abnormally slow uplift rate of about 0.18–0.24 mm/yr, both of which are remarkably different from Phanerozoic continent collisional belts. The concept of incipient plate tectonics is established for Paleoproterozoic metamorphism in the NCC. Being clearly different from tectonic style of Archean high-grade region and greenstone belt, Paleoproterozoic terranes are seemingly more similar to tectonic style of Phanerozoic continent-continent collision. Metamorphic pressures of ∼10–14 Kbar in the HT pelitic granulites indicate that the supracrustal rocks had sunk into lower crust with depth of 40–50 km and then were uplifted to the surface again. This implies that the NCC had attained a considerable thickness and an extra drive force could be necessary for so great vertical displacement of supracrustal rocks. In consideration of the broad distributions of these high-grade metamorphic rocks, some “quasi-rigid” blocks were proposed to have preliminarily formed in Paleoproterozoic, which could facilitate sinking and uplifting motion of these rocks by subduction-collision-like processes between micro-blocks. In other words, the principle of plate tectonics might have been operated at that time. However, these blocks are limited in rigidity for their disability to sink into depth of boundary between lithosphere and asthenosphere but activation only in crustal scale. In this regard, a process of “hot” subduction-collision is considered, which comprehensively includes various elements, such as smaller continent blocks, high geothermal gradient, tiny density distinction between subducted slab and wall rock (lower crust), which together define a motion limited in crustal scales with lacking in enough strengths of thrust force and buoyancy. The incipient plate tectonics have displayed lateral compressional structure and kinematic interaction between blocks, but it is much smaller in scale relative to Phanerozoic collisional orogeny, although the tectonic style is more or less similar.

Paleoproterozoic granulite-facies metapelites are widespread in northern part of the North China Craton (NCC), and their metamorphic conditions and P-T paths provide important constraints for tectonic evolution of the NCC. However, it is still controversial on the metamorphic evolution of these rocks due to variations of retrograde modification. In combination with detailed petrological studies, geological mapping, and geochronology, we here present new evidence from Gushan, Datong, conjunction zone of the Ji’ning Complex and Huai’an Complex to constrain P-T evolution of the metapelites. Field investigations reveal that the pelitic granulites are spatially associated with high-pressure (HP) mafic granulites, showing concordant characteristics of deformation and metamorphism. Fine mineralogical and microstructural studies reveal that kyanite-bearing HP granulite-facies indicators preserved in garnet porphyroblasts of the pelitic granulites. It suggests the previously assumed moderate- to low-pressure (MP-LP) and high- to ultrahigh-temperature (HT-UHT) pelitic granulites have experienced HP granulite-facies metamorphism. Four metamorphic generation assemblages (M1–M4) are recognized in the studied samples. The pre-Pmax assemblages (M1) are characterized by mineral inclusions within garnet core, including Bt-Qz-Rt±Ms±Liq, representing the late stage of prograde metamorphism. The Pmax assemblages (M2, HP granulite-facies) are represented by garnet mantle and its inclusion-type minerals, i.e., Grt-Ky-Kfs-Bt-Rt-Liq-Qz±Ms±Zn Spl. It is followed by heating until the maximum of temperature stage (M3, Tmax), featured by disappearance of biotite and transition from kyanite into sillimanite, with mineral assemblages of Grt-Sil-Kfs-Liq-Qz-Ilm±Zn Spl. The cooling stage (M4) is featured by breakdown of garnet to Bt+Qz±Pl±Sil±Ilm. Phase equilibrium modeling and titanium-in-biotite thermometer are combined to constrain the P-T evolution of the studied samples. The P-T conditions for the M1–M4 are respectively estimated around ∼11–11.5 kbar/∼770–790 °C, ∼10.3–13.8 kbar/∼820–835 °C, ∼6.5–9.5 kbar/∼850–925 °C, ∼6 kbar/∼645–761 °C, which defined a clockwise P-T path. Zircon U-Pb ages of two representative samples demonstrated comparable age clusters of ∼1950 Ma and ∼1850 Ma, which are interpreted to represent the timing of HP and MP-LP granulite-facies stages, respectively. Combining with other studies of the NCC, we propose that the pelitic granulites and associated HP mafic granulites have experienced the common metamorphic processes from peak stage (HP-HT granulite-facies) to retrograde metamorphism. Due to intensive and pervasive overprint of sillimanite-type granulite-facies metamorphism, the HP granulite-facies mineral assemblages of the pelitic granulites are partially or completely reworked. The metamorphic peak conditions and relatively high thermal gradients of these granulite-facies rocks suggest that their formation is probably in response to a crustal-scale subduction-collision process during the late Paleoproterozoic.

Geological implications of ∼1.95 Ga and ∼1.85 Ga metamorphic ages for HP granulites in the Huai’an terrane are still controversial, which have affected the crustal evolution models for the North China Craton. This study reports two geological sections in which distinct geological relationships of a quartz-dioritic and a tonalitic dike cutting through mafic HP granulite and granulite facies meta-sedimentary rocks are described. In these two sections, magmatic dikes, mafic HP granulites and meta-sedimentary rocks preserve conformable foliation to indicate their similar deformation history. The U–Pb dating and Lu–Hf isotopic analysis of zircons from metamorphic and meta-igneous rocks provide new constraints on the timing and mechanism of thermal events associated with metamorphic and deformation events. Both magmatic dikes were formed around ∼1.95 Ga and overprinted by later metamorphic event during ∼1.85 Ga. The S-type granite, at geological section (GS) 1, revealed its formation age was formed around 1959 ± 34 Ma (n = 3, MSWD = 1.5), and overprinted by later metamorphic event at 1840 ± 8 Ma (n = 10, MSWD = 1.2). Additionally, zircons from the mafic HP granulite in the GS2 are dominated by metamorphic ages of 1972 ± 22 Ma (n = 6; MSWD = 0.03) with few grains recording an age of 1856 ± 29 Ma (n = 3; MSWD = 0.17). In situ Lu–Hf analyses on zircons from the quartz-dioritic dike, tonalitic dike, mafic HP granulite and S-type granite, reveal that the magmatic dikes were derived from partial melting of ∼2.0–2.2 Ga meta-basic rocks and the S-type granite was generated by melting of meta-sediments beyond 840 ± 50 °C. It suggested that those meta-sediments were taken into the lower crust in a certain way at granulite facies conditions ca. 1.95 Ga ago. A clockwise P-T path acquired from mafic HP granulite revealed peak metamorphic conditions of 880–890 °C and 11–13 kbar. The temperature results recorded in the ∼1.95 Ga zircons of mafic HP granulite, similar to those of high-pressure mineral assemblage in matrix, suggest that the HP granulite metamorphism was approximately ∼1.95 Ga ago. According to the results above, this contribution concludes that mafic and meta-sedimentary rocks were metamorphosed together under high-pressure granulite metamorphic conditions ∼1.95 Ga ago, followed by high-temperature granulite facies melting event that gave rise to granite and S-type granite at ∼1.95 Ga. The granulite facies metamorphic rocks and magmatic dikes in the Huangtuyao graphite mine experienced the same evolution history after ∼1.95 Ga. The mafic and pelitic HP granulites in the Huai’an terrane are interpreted to be collisional products of the Paleoproterozoic cratonization of the NCC.

The first report of Paleoproterozoic mafic high-pressure (HP) granulite terranes in Manjinggou area of the Huai’an Complex, North China Craton (NCC) since 1992 has attracted broad attention from geologists, and accordingly various Early Precambrian continent-continent collision or micro-blocks amalgamation hypotheses were suggested. ​However, one important issue is still controversial whether the pelitic granulites and associated mafic H P granulites underwent similar metamorphic history. Here we report newly recognized pelitic H P granulites in Manjinggou area of the Huai’an Complex to provide direct evidence that both pelitic and mafic H P granulites suffered similar metamorphic history. The metamorphic peak mineral assemblage of the pelitic granulites is characterized by Grt-Ky-Kfs-Bt-Rt-Qz-Liq±Ms, and the subsequent medium-pressure (M P ) granulite facies retrogression is characterized by Grt-Sil-Kfs-Pl-Bt-Rt-Liq-Qz. Pseudosection approaches were undertaken in the Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O (NCKFMASHTO) system to account for textural development, mineral composition and P – T evolution of the pelitic granulites. The estimated peak and retrograde granulite facies conditions are 11.5–15 kbar, 810–860 °C and ∼9.5 kbar, ∼850 °C, respectively, comparable with those of associated mafic H P granulites. Consistently, field geological observations and available geochronological data also indicate their similar deformation and metamorphic history from H P granulite facies stage (∼1.96–1.90 Ga) to retrograde stages (∼1.88–1.80 Ga). This finding changes previous views that the pelitic rocks and associated mafic granulites are different slabs with individual metamorphic history. The khondalite series from the Huai’an Complex and adjacent Ji’ning Complex were probably experienced H P granulite facies metamorphism, but previously derived M P granulite facies conditions might be attributed to mineral re-equilibrium due to long dwell time under M P granulite facies. Additionally, these granulite facies rocks record high apparent geothermal gradients and slow exhumation rates relative to H P –UH P rocks in the Phanerozoic continental collisional orogens. It seems to indicate that the thermal regimes and tectonic mechanisms of Paleoproterozoic probably differ from those of Phanerozoic eons, and these H T –H P granulite terranes from the NCC were likely formed in a hot and slow cooling orogen.

The Taihua Complex is exposed mostly in the central-western part along the southern margin of the North China Craton (NCC). In this study, field geological, geochemical, LA-ICP-MS zircon U-Pb age dating and Lu-Hf isotopic analysis are applied to the orthorgneisses, felsic leucosomes, and granites of the Taihua Complex, in order to provide constraints on its crustal evolution from the Mesoarchean to Paleoproterozoic (ca. 2.9–1.8 Ga). The ca. 2.90–2.73 Ga magmatism was an important episode of contemporaneous crustal growth and crustal reworking. Among them, the ca. 2.90 Ga granodioritic enclaves have features similar to those of the Archean sanukitoids, and were generated by the direct partial melting of peridotitic mantle wedge in the subduction processes, representing the early continental nucleus. The ca. 2.84 Ga high Mg# TTGs were derived from the partial melting of subducted oceanic crust after interaction with mantle wedge, whereas the ca. 2.82–2.77 Ga low Mg# TTGs were originated from the partial melting of underplated basaltic crust, pointing to a series of continuous arc magmatic activities. The formation of the ca. 2.75–2.73 Ga crust-derived granites and felsic leucosomes in association with the contemporaneous metamorphism indicate the presence of a crustal anatexis, which probably was the result of an important magmatic underplating event. The magmatism between 2.9 Ga and 2.73 Ga indicates that the increased crust-mantle interaction and crustal differentiation stimulated the crust in the Taihua Complex to become more matural. The ca. 2.57–2.43 Ga magmatism represents another prominent magmatism in the Taihua Complex. The TTG gneisses and dioritic gneisses aged at ca. 2.57–2.50 Ga were derived from partial melting of the hydrated basalts under eclogite to garnet amphibolite facies conditions with interaction between initial melts and mantle wedge, indicating that incessant subduction led to the amalgamation of micro-blocks. The intrusion of granitic rocks, felsic leucosomes, TTG-like gneisses accompanied by metamorphism during ca. 2.50–2.43 Ga indicate that a dominant crustal anatexis event occurred in association with the cratonic stabilization processes. The ca. 2.36–2.24 Ga magmatism was also characterized by reworking of the basement rocks, which are represented by the intrusion of calc-alkaline monzogneisses, A-type granitic gneisses and mafic dykes. This scenario was coeval with the formation of volcanic-sedimentary sequences, indicating the presence of an extension event in the Taihua Complex. The ca. 1.96–1.85 Ga tectono-thermal event represented the last phase of compressional deformation and consequent metamorphism, leading to reworking of the pre-existed basement rocks. This event formed a WNW-ESE trending metamorphic belt that extends over 1000 km along the southern margin of the NCC and was well concordant with the world-wide orogeny associated with the amalgamation of Nuna supercontinent. Subsequently, widespread A-type granites, mafic dykes and volcanic rocks aged of ca. 1.84–1.74 Ga formed, marking the final consolidation of the crystalline basement at the Taihua Complex. Since then, the southern NCC became stable and entered into the stage of development of platform.