Search

The ca. 2.5 Ga as the time boundary between the Archean and the Proterozoic eons is a landmark, indicating the most important continental crust evolving stage of the Earth, that is, the global cratonization or the formation of supercraton(s) that was unseen before and is unrepeated in the following history of the Earth9s formation and evolution. The North China Craton (NCC) is one of the best recorders of the ca. 2.5 Ga event, and therefore studies in the thorough understanding of early Precambrian continental evolution are continuous. The period from 2.8 to 2.6 Ga is the major crustal growth period of the NCC and formed seven micro-blocks. All the micro-blocks in the NCC were surrounded by 2.6 to 2.54 Ga greenstone belts. The clear geological presentations are as follows: (1) Archaic basement rocks in North China (various micro-blocks) experienced strong partial melting and migmatization. The granitoid rocks derived from crustal partial melting include potassium, TTG and monzonitic granitoids, which come, respectively, from continental crust (sedimentary rocks with TTG gneisses), juvenile crust (mafic rocks with TTG gneisses) or mixed crust; (2) the BIF-bearing supracrustal rocks are mainly distribute in greenstone belts. The lithologic associations in the greenstone belts within the NCC are broadly similar, belonging to volcano-sedimentary sequences, with common bimodal volcanic rocks (basalt and dacite) interlayered with minor amounts of komatiites in the lower part, and calc-alkalic volcanic rocks (basalt, andesite and felsic rocks) in the upper part; (3) nearly all old rocks of >2.5 Ga underwent ∼2.52 to 2.5 Ga metamorphism of amphibolite–granulite facies. Most metamorphosed rocks show high-temperature-ultra-high-temperature (HT–UHT) characteristics and record anticlockwise P–T paths, albeit a small number of granulites seemingly underwent high-pressure granulite facies metamorphism and record clockwise P–T paths; (4) ∼2.5 Ga mafic dikes (amphibolites), granitic dikes (veins) and syenitic–ultramafic dikes developed across these archaic basements and were strongly deformed or un-deformed; (5) the extensive 2.52 to 2.48 Ga low-grade metamorphic supracrustal covers has been recognized in eastern, northern and central parts of the NCC, which are commonly composed of bi-modal volcanic rocks and sedimentary rocks. The above mentioned ∼2.5 Ga geological rocks and their characters imply that the seven micro-blocks have been united through amalgamation to form the NCC. The metamorphosed late Neoarchean greenstone belts, as syn-formed mobile belts, welded the micro-blocks at the end of the Neoarchean. However, the metamorphic thermal grades of the greenstone belts are lower than those of the high-grade terranes within the micro-blocks, suggesting that the latter might have developed under a higher geothermal gradient than the former. Besides, the greenstone belts surround the various micro-blocks in the late Neoarchean when both the old continental crust and the oceanic crust were hotter. The subduction during the amalgamation, if it happened, must have been much smaller in scale as compared to those in the Phanerozoic plate tectonic regime, and all stages occurred at crust-scale instead of lithosphere-scale or mantle-scale. This is why most rocks record HT-UHT and anti-clockwise metamorphism, while only a few samples record high-pressure granulite facies metamorphism with clockwise P–T paths. The micro-block amalgamation was accompanied by extensive crust partial melting and granitization, which finally gave rise to the stabilization of the NCC. Except for the vast granitoid intrusions, mafic-syenitic dike swarms and sedimentary covers are also landmarks of cratonization. The ca. 2.5 Ga cratonization is a global epoch-making geological event, although the accomplishment of cratonization in various cratons is somewhat different in time. Cratonization declared the formation and stabilization of global-scale supercratons or cratonic groups coupling with lithosphere, which was followed by a “silent period” with rare tectonic-thermal action lasting 150 to 200 Ma (from 2.5 Ga – 2.3 or 2.35 Ga), and then followed by the Great Oxidation Event (GOE).

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.

Ultra-high pressure (UHP) metamorphism is recorded by garnet clinopyroxenite enclaves enclosed in an undeformed, unmetamorphosed granitic pluton, northeastern Paleozoic Dunhuang orogenic belt, northwestern China. The protoliths of the garnet clinopyroxenite might be basic or ultrabasic volcanic rocks. Three to four stages of metamorphic mineral assemblages have been found in the garnet clinopyroxenite, and clockwise metamorphic pressure&ndash, temperature (P-T) paths were retrieved, indicative of metamorphism in a subduction environment. Peak metamorphic P-T conditions (790&ndash, 920 &deg, C/28&ndash, 41 kbar) of garnet clinopyroxenite suggest they experienced UHP metamorphism in the coesite- or diamond-stability field. The UHP metamorphic event is also confirmed by the occurrence of high-Al titanite enclosed in the garnet, along with at least three groups of aligned rutile lamellae exsolved from the garnet. Secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic titanite indicates that the post-peak, subsequent tectonic exhumation of the UHP rocks occurred in the Devonian period (~389&ndash, 370 Ma). These data suggest that part of the Paleozoic Dunhuang orogenic belt experienced UHP metamorphism, and diverse metamorphic facies series prevailed in this Paleozoic orogen. It can be further inferred that most of the UHP rocks in this orogen remain buried.

The Paleoproterozoic metapelites within the major collisional orogenic belts of the North China Craton are generally characterized by clockwise metamorphic P-T-t paths. Here we report the case where an isobaric cooling followed by a final decompression is defined from high grade metapelites in the Jingshan Group at the southern part of the Jiao-Liao-Ji belt. The earlier prograde trajectory is constrained to be considerable heating with slight pressure increase to the peak metamorphism of 7.9–9.2 kbar at 763–770 °C by integrated methods of petrology, geothermobarometry and P-T-X pseudosection, suggesting an anticlockwise P-T path. Zircon and monazite geochronology combined with previous data suggests that the prograde metamorphism probably commenced at 1.90–1.85 Ga, followed by a slow isobaric cooling at mid-crustal level during 1.85–1.80 Ga and a final exhumation to upper crustal level presumable at 1.60–1.70 Ga. The defined P-T-t path seems to contrast with previously reported clockwise P-T-t paths. In the light of available geological evidence and recent numerical modelling, these contrasting P-T-t paths are reconciled by a tentative geodynamic model in which orogenic collapse and hot exhumation of the orogenic root during 1.90–1.85 Ga resulted in isothermal decompression of the granulites with clockwise P-T paths and triggered the prograde metamorphism for those regions displaying anticlockwise P-T paths. The pressure increase in the prograde trajectory of anticlockwise P-T path may be related to the sinking of upper-middle crustal materials to compensate for the contemporaneous ascend of the hot orogenic root. Subsequently, rocks of these contrasting P-T paths experienced a common slow isobaric cooling during 1.85–1.80 Ga. The 1.95–1.90 Ga metamorphic ages that are common in the Paleoproterozoic khondalitic rocks elsewhere in the NCC are absent in our samples can be ascribed to the high-temperature metamorphism related to this orogenic collapse and subsequent slow cooling. Therefore, our new data further supports that the JLJB is a long-lived collisional orogenic belt with post-orogenic extension at 1.90–1.85 Ga and subsequent slow cooling during 1.85–1.80 Ga. Hence, this study provides new metamorphic constraint on the post-orogenic evolution of the JLJB and insights into the mechanism of generating contrasting P-T-t paths in regional metamorphic orogens.

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.

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.

Four metamorphic mineral assemblages, formed at different metamorphic stages, are identified in pelitic granulites exposed in the Jidong terrane, east Hebei Province, North China Craton (NCC). Prograde mineral assemblages (M 1 ) are represented by biotite, plagioclase, quartz, and ilmenite, preserved as mineral inclusions within garnet porphyroblasts. Peak mineral assemblages (M 2 ), which are represented by the garnet cores, rutile, pseudomorphs of sillimanite after kyanite, and some matrix minerals, formed at P–T conditions of ∼12–13 kbar/820–850 °C. Post-peak mineral assemblages (M 3 ), which consist of the inner rims of garnets and some matrix minerals, formed at P–T conditions of ∼11 kbar/850 °C. The final retrograde mineral assemblages (M 4 ) are mainly represented by matrix biotite without rutile exsolution, and minor matrix plagioclase with relatively high anorthite content. High-resolution secondary ion mass spectrometry (SIMS) U−Pb dating of metamorphic zircons demonstrates that the pelitic granulites possibly reached the peak metamorphic stage at ∼2.5 Ga. Younger metamorphic ages (2.41–2.45 Ga) possibly represent the cooling ages of the granulites. Accordingly, the pelitic granulites exposed in the Jidong terrane record clockwise P–T–t paths with sequential isothermal decompression (ITD) and isobaric cooling (IBC) segments, likely caused by a subduction–collision event during the late Neoarchean to early Palaeoproterozoic in the eastern NCC.

Garnet-bearing mafic granulites and amphibolites from the Hongliuxia area of the southern Dunhuang orogenic belt, northwestern China, commonly occur as lenses or boudinages enclosed within metapelite or marble, which represent the block-in-matrix feature typical of orogenic melange. Three to four generations of metamorphic mineral assemblages are preserved in these rocks. In the high-pressure amphibolites, prograde mineral assemblages (M1) occur as inclusions (hornblende + plagioclase + quartz ± chlorite ± epidote ± ilmenite) preserved within garnet porphyroblasts, and formed at 550–590 °C and 7.7–9.2 kbar based on geothermobarometry. The metamorphic peak mineral assemblages (M2) are composed of garnet + hornblende + plagioclase + quartz + clinopyroxene, as well as titanite + zircon + rutile + apatite as accessory minerals in the matrix, and are estimated to have formed at 640–720 °C and 14.1–16.0 kbar. The first retrograde assemblages (M3) are characterized by “white-eye socket” symplectites (hornblende + plagioclase + quartz ± biotite ± epidote ± magnetite) rimming garnet porphyroblasts, which formed at the expense of the garnet rims and adjacent matrix minerals during the decompression stage under P – T conditions of 610–630 °C and 5.6–11.8 kbar. The second retrograde assemblages (M4) are intergrowths of actinolite and worm-like quartz produced by the breakdown of the matrix hornblendes, and formed under P – T conditions of ∼490 °C and ∼2.8 kbar. For the high-pressure mafic granulites, the prograde assemblages (M1) are represented by plagioclase + quartz preserved within the garnet porphyroblasts. The metamorphic peak assemblages (M2) are garnet + matrix minerals (clinopyroxene + plagioclase + quartz + hornblende + rutile + zircon) and were estimated to have formed at ∼680 °C and ∼15.4 kbar. The retrograde assemblages (M3) are characterized by fine-grained patches of hornblende + plagioclase + quartz rimming the garnet porphyroblasts, as well as hornblende rimming clinopyroxene in the matrix, and were inferred to have formed at ∼620 °C and ∼4.2 kbar. For the metapelitic gneiss, the metamorphic peak assemblages are the garnet porphyroblasts plus the matrix minerals (biotite + plagioclase + quartz + ilmenite + zircon), which were estimated to have formed at ∼630 °C and ∼8.9 kbar. The mafic granulites and amphibolites record fairly similar clockwise P–T paths that include nearly isothermal decompression processes, which suggest that they experienced subduction and subsequent rapid tectonic exhumation. SIMS and LA-ICP-MS U–Pb dating of zircons and 40Ar/39Ar dating of hornblende suggest that the metamorphism occurred at ∼430–390 Ma. Field occurrences, different protolith ages of the mafic granulites and amphibolites, and the considerable gap in peak P – T conditions between the amphibolite and mafic granulite boudinages and their country rock may suggest a melange accumulation process during the Paleozoic caused by the Silurian–Devonian orogeny, which is possibly associated with the closure of the Liuyuan ocean, a branch of the Paleo-Asian ocean near the southern Central Asian Orogenic Belt.

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

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 6–8 kbar at below 660 °C, 15–16.6 kbar at 850–860 °C, 8.8–10.5 kbar at 830–870 °C, 5.8–7.8 kbar 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 1.90–1.95 Ga and that exhumation and cooling of the metamorphic terrane occurred at 1.80–1.85 Ga, indicating a long-lived hot orogeny during Paleoproterozoic. This contribution provides mineralogical evidence for low-pressure prograde metamorphism in HP pelitic granulites and hence gives a complete P-T path, placing a more rigorous metamorphic constraint on evolution of the JLJB.

The Taihua metamorphic complex in the southern part of the North China Craton is composed of tonalite-trondhjemite-granodiorite (TTG) gneisses, amphibolites, metapelitic gneisses, marbles, quartzites, and banded iron formations (BIFs). The protoliths of the complex have ages ranging from ∼2.1 to ∼2.9 Ga and was metamorphosed under the upper amphibolite to granulite facies conditions with NWW-SEE-striking gneissosity. Metapelitites from the Wugang area have three stages of metamorphic mineral assemblages. The prograde metamorphic mineral assemblage (M1) includes biotite + plagioclase + quartz + ilmenite preserved as inclusions in garnet porphyroblasts. The peak mineral assemblage (M2) consists of garnet porphyroblasts and matrix minerals of sillimanite + biotite + plagioclase + quartz + K-feldspar + ilmenite + rutile + pyrite. The retrograde mineral assemblage (M3), biotite + plagioclase + quartz, occurs as symplectic assemblages surrounding embayed garnet porphyroblasts. Garnet porphyroblasts are chemically zoned. Pseudosection calculated in the NCKFMASHTO model system suggests that mantles of garnet porphyroblasts define high-pressure granulites facies P–T conditions of 12.2 kbar and 830 °C, whereas garnet rims record P–T conditions of 10.2 kbar and 840 °C. Integrating the prograde mineral assemblages, zoning of garnet porphyroblasts with symplectic assemblages, a clockwise metamorphic P–T path can be retrieved. High resolution SIMS U−Pb dating and LA−ICP−MS trace element measurements of the metamorphic zircons demonstrate that metapelites in Wugang possibly record the peak or near peak metamorphic ages of ∼1.92 Ga. Furthermore, 40Ar/39Ar dating of biotite in metapelites suggests that the cooling of the Taihua complex may have lasted until ∼1.83 Ga. Therefore, a long-lived Palaeoproterozoic metamorphic event may define a slow exhumation process. Field relationship and new metamorphic data for the Taihua metamorphic complex does not support the previous model in which the Tran-North China Craton (TNCO) was formed through the collision between the East and West blocks.

The Kadavur massif-type anorthosite, which intruded the basement of the Madurai Block in southern India, comprises anorthosite, leucogabbro, and (noritic) gabbro with minor Fe-Ti oxide ores. The U-Pb zircon SIMS dating indicates that the magmatism occurred during the Neoproterozoic period (Tonian), i.e., noritic gabbro (790.4 ± 5.1 Ma), gabbro (793.0 ± 4.1 Ma), leucogabbro (784.9 ± 4.1 Ma), and anorthosite (790.7 ± 4.3 Ma). Combined with the published age data from the coeval charnockite and A-type granite, we conclude that the anorthosite-charnockite-rapakivi granite suite was emplaced during ca. 785–805 Ma. In-situ zircon Hf-O isotopic data of the Kadavur gabbros and anorthosite provide insights on the source and evolution of the parental magmas of the massif-type anorthosites. Zircons from the anorthosite are characterized by ancient crustal eHf(T) values (−11.4 to −6.5) and higher δ18O values (5.92 to 6.4‰). In contrast, zircons from the coarse-grained gabbro have relatively primitive eHf(T) values (−6.3 to −2.3) and mantle-like δ18O values (4.86–5.73‰). Moreover, zircons from the noritic gabbro and leucogabbro have eHf(T) values (−11.1 to −3.7) and δ18O values (5.35 to 6.77‰) distributed between those of the anorthosite and coarse-grained gabbro. The new zircon Hf-O data demonstrate that the parental melt of the anorthosite was subjected to crustal contamination during the early evolution stages, producing an evolved magma with crustal isotope and trace element signatures. In contrast, the gabbros show less crustal influence and like to represent original magma source of the Kadavur Complex. The zircon Hf-O isotope compositional array from the primitive gabbros to the more-evolved anorthosite demonstrates that the parental magmas were derived from partial melting of the upper mantle with varying crustal input, which can be up to 30–40% for the anorthosite formation. Contamination of the ponded basaltic magmas by the felsic crust can effectively increase the SiO2, Al2O3, Na2O, and Sr contents in the magmas, which was likely essential for enormous plagioclase fractionation in the massif-type anorthosites.

Garnet-bearing mafic granulite and amphibolite exposed as lenses, boudins, or interlayers within metasediments in the Qingshigou area, Dunhuang block, southernmost Central Asian orogenic belt, record important information for understanding the tectono-metamorphic evolution of subduction and collision zones in the southern Central Asian orogenic belt during the mid-Paleozoic. Three stages of metamorphic assemblages (M1, M2, M3) are recognized in the high- and medium-pressure mafic granulite and amphibolite. In the high-pressure mafic granulite, the prograde assemblage (M1) is represented by inclusion minerals (hornblende + plagioclase + quartz) preserved in garnet porphyroblasts; the metamorphic peak assemblage (M2) is characterized by garnet porphyroblasts and matrix minerals (garnet + clinopyroxene + plagioclase + quartz ± zircon ± titanite); and the retrograde assemblage (M3) is marked by coronitic symplectite (hornblende + plagioclase + quartz ± magnetite) rimming the garnet porphyroblasts. In the medium-pressure mafic granulite, the prograde assemblage (M1) of hornblende + plagioclase + quartz is included in the garnet porphyroblasts; the peak assemblage (M2) consists of garnet + orthopyroxene + clinopyroxene + plagioclase + quartz ± zircon ± titanite (M2) in the matrix; and the retrograde assemblage (M3) of hornblende + orthopyroxene + plagioclase + quartz (M3) surrounds the garnet porphyroblasts. In the amphibolite, the prograde assemblage (hornblende + plagioclase + quartz + ilmenite) is preserved as inclusions in garnet (M1); the peak assemblage (M2) is composed of garnet + hornblende + plagioclase + quartz ± zircon ± titanite; and the retrograde assemblage (M3), consisting of hornblende + biotite + plagioclase + quartz + epidote + magnetite, rings the garnet porphyroblasts. Geothermobarometric calculations suggest that the metamorphic pressure-temperature paths pass from 568 °C and 8.8 kbar through 607 °C and 10.6 kbar and 861 °C and 16.9 kbar and finally to 598 °C and 4.4 kbar for the high-pressure mafic granulite; from 756 °C and 9.0 kbar through 750–874 °C and 9.3–11.6 kbar to 675 °C and 4.7 kbar for the medium-pressure mafic granulite; and from 686 °C and 7.6 kbar through 715–766 °C and 10.6–11.2 kbar to 671 °C and 5.6 kbar for the amphibolite, and the paths show clockwise pressure-temperature loops typical of an orogenic process. The metamorphic peak of the high-pressure mafic granulite lies in the eclogite facies, which is indicative of a subduction zone environment. High-resolution secondary ion mass spectrometry (SIMS) U-Pb dating of metamorphic zircon indicates that the metamorphism occurred in the Early Silurian (ca. 430 Ma) and lasted for at least 65 m.y. This study reveals a possible southward subduction history of a branch of the Paleo–Asian Ocean, the Liuyuan Ocean, from the Silurian to Late Devonian, which may be an important event in the accretionary history of the Central Asian orogenic belt.

The cuprous oxide-reduced graphene oxide (Cu2O/rGO-x) composites were prepared via a simple wet-chemical method by using CuSO4·5H2O and graphene oxide as precursors and ascorbic acid as a reducing agent, respectively. These Cu2O/rGO-x were employed as photocatalysts for degrading emerging contaminants and organic dye pollutants (i.e., sulfamethoxazole (SMX) and methylene blue (MB)) under visible light. A variety of different spectroscopic and analytical techniques, such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman scattering spectroscopy and UV-Visible spectroscopy were used to characterize the physical properties of photocatalysts. In the photodegrading experiments, it can be found that the Cu2O/rGO-80 photocatalyst has the superior visible-light response of ca. 50% removal efficiency of SMX within 120 min and 100% removal efficiency of MB within 40 min. These observations may be attributed the well-dispersed and visible-light-responsive Cu2O nanoparticles are supported on the surface of rGO sheets that can enhance absorption of visible light during photocatalysis.

The Taihua metamorphic complex along the southern margin of the North China Craton (NCC) mainly consists of tonalite–trondhjemite–granodiorite (TTG) gneisses, amphibolites, metapelites, marbles, quartzites, and banded iron formations (BIFs). The region has been subjected to upper amphibolite–granulite facies metamorphism and intense deformation, with NWW–SEE striking gneissosity. Garnet porphyroblasts from metapelites of the Lushan area are chemically zoned. Based on petrological and geochemical analyses, quantitative phase equilibria modeling, and traditional geothermobarometry, a clockwise pressure–temperature (P–T) path with a nearly isothermal decompression (ITD) segment was retrieved from the Lushan metapelites. The peak metamorphic conditions of ~770 °C and 11.7 kbar were recorded in sample L93, which reached to the upper amphibolite–granulite facies. High-resolution secondary ion mass spectrometry (SIMS) zircon U–Pb dating demonstrated that the metapelites record a peak or near-peak metamorphic age of ~1.93 Ga. Hornblende 40Ar/39Ar dates from amphibolite illustrated that the Taihua metamorphic complex have cooled to 500–600 °C by ~1.84 Ga. Accordingly, a prolonged Paleoproterozoic continental collision event is supported.

Three morphologies (octahedral, hierarchical and rhombic dodecahedral) of crystal Cu2O with different facets ({111}, {111}/{110}, and {110}) incorporating graphene sheets (denoted as o-Cu2O-G, h-Cu2O-G and r-Cu2O-G, respectively) have been fabricated by using simple solution-phase techniques. Among these photocatalysts, the r-Cu2O-G possesses the best photocatalytic performance of 98% removal efficiency of methyl orange (MO) with outstanding kinetics for 120 min of visible light irradiation. This enhancement is mainly due to the dangling &ldquo, Cu&rdquo, atoms in the highly active {110} facets, resulting in the increased adsorption of negatively charged MO. More importantly, the unique interfacial structures of Cu2O rhombic dodecahedra connected to graphene nanosheets can not only decrease the recombination of electron-hole pairs but also stabilize the crystal structure of Cu2O, as verified by a series of spectroscopic analyses (e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM)). The effective photocatalysts developed in this work could be applied to the efficient decolorization of negatively charged organic dyes by employing solar energy.

To further understand the metamorphic evolution of the Trans-North China Orogen (TNCO), a combined study of petrography, pseudosection modeling and conventional thermobarometry was completed on garnet-sillimanite-bearing metapelitic rocks of the Jiehekou Group from the southern Luliang metamorphic complex, in the central segment of the TNCO. The Luliang metapelitic rocks preserve peak assemblages of garnet, biotite, sillimanite, plagioclase and quartz, with or without K-feldspar, that reflect c. 750 °C and 7.0 kbar. Garnet has mostly homogeneous grain cores with sparse mineral inclusions, enclosed successively by: (1) rims of higher spessartine and lower pyrope content and (2) symplectitic intergrowths of cordierite, biotite and plagioclase that present a spectacular “white-eye socket’ reaction microstructure. Quantitative phase equilibria modeling and traditional thermobarometric calculation results are consistent with this retrograde microstructure having developed during decompression to T ≈ 650 °C and P

A simple solution-chemistry method has been investigated to prepare crystal cuprous oxide (Cu2O) incorporated with reduced graphene oxide (designated as Cu2O-rGO-x, where x represents the contents of rGO = 1%, 5% and 10%) in this work. These Cu2O-rGO-x composites combine the prospective advantages of rhombic dodecahedra Cu2O together with rGO nanosheets which have been studied as visible-light-sensitive catalysts for the photocatalytic production of methanol from CO2. Among the Cu2O-rGO-x photocatalysts, the methanol yield photocatalyzed by Cu2O-rGO-5% can be observed to be 355.26 &mu, mol g&minus, 1cat, which is ca. 36 times higher than that of pristine Cu2O nanocrystal in the 20th hour under visible light irradiation. The improved activity may be attributed to the enhanced absorption ability of visible light, the superior separation of electron&ndash, hole pairs, well-dispersed Cu2O nanocrystals and the increased photostability of Cu2O, which are evidenced by employing UV-vis diffuse reflection spectroscopy, photoluminescence, scanning electron microscopy/transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. This work demonstrates an easy and cost-effective route to prepare non-noble photocatalysts for efficient CO2 recovery in artificial photosynthesis.

The rhombic dodecahedral cuprous oxide-reduced graphene oxide/core-shell Fe3O4@SiO2 composites (denoted as rCu2O-rGO/Fe3O4@SiO2) are successfully synthesized facilely via a wet-chemical route. The resulting rCu2O-rGO/Fe3O4@SiO2 combines the unique structure of Cu2O, electronic characteristics of reduced graphene oxide (rGO) and magnetic property of Fe3O4@SiO2 to be an effective and recoverable photocatalyst for the degradation of methyl orange (MO). The obtained results show that rCu2O-rGO/Fe3O4@SiO2 is capable of completely degrading MO in the presence of a very low catalyst concentration (0.125 g l-1) within a short time (60 min) under visible light compared to the reported catalysts. The observations may be due to the distinctive interfacial structures of rhombic dodecahedral Cu2O nanoparticles connected to rGO sheets that can enhance the separation of photogenerated electron-hole pairs, stabilize the Cu2O and increase MO adsorption, as evidenced by a variety of spectroscopic analyses (transmission electron microscopy, x-ray photoelectron spectroscopy and photoluminescence). More importantly, these efficient photocatalysts can easily be recovered under a magnetic field and remain highly photoactive towards the degradation of MO after cyclic tests, and may be promising photocatalysts for practical applications in the solar-energy purification of wastewater.