Mingguo Zhai

SHRIMP U–Pb zircon geochronology of the Fuping Complex: implications for formation and assembly of the North China Craton

Abstract The Fuping Complex, located within the central zone of the North China Craton, is composed of amphibolite to granulite facies orthogneisses, interleaved with minor supracrustal rocks at similar metamorphic grade. The oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a SHRIMP U–Pb zircon age of 2708±8 Ma. We consider these enclaves to represent fragments of ∼2.7 Ga continental materials incorporated in the biotite gneiss. The biotite gneiss has a SHRIMP U–Pb zircon age of 2513±12 Ma, interpreted to be time of magmatic crystallisation of the igneous precursor, based on the igneous characteristics of the zircons. This indicates a major magmatic episode at 2.52 Ga in the Fuping Complex, identical to the age of felsic volcanism within the low-grade Wutai Complex which crops out immediately to the west. A gneissic granite that intrudes the biotite gneisses has a poorly-defined 207Pb/206Pb age of 2045±64 Ma. This is within error of the age of 2097±46 Ma obtained from a fine-grained gneiss interlayered with amphibolite of the Wanzi Supracrustal Suite (WSS), interpreted to be volcanic in origin. Zircons from both these samples have strong oscillatory zoning and provide the first indication of a Palaeoproterozoic magmatic event in the area, again similar in age to magmatic events recently recognised in the adjacent Wutai Complex. These data indicate a comparable history for the Fuping and Wutai Complexes and support geochemical evidence that they had a common origin and formed part of a Late Archaean arc, affected by later Palaeoproterozoic re-activation. Low-uranium zircons without oscillatory zoning, separated from a sample of biotite gneiss, yield data clustered at 1817±26 Ma, which is interpreted to reflect a period of new zircon growth during a major metamorphic event. These data support the recently-proposed tectonic model that amalgamation of the North China Craton occurred due to collision of the Eastern and Western Blocks along the central zone at ∼1.8 Ga ago.

Metamorphic evolution, mineral chemistry and thermobarometry of schists and orthogneisses hosting ultra-high pressure eclogites in the Dabieshan of central China

As is typical of ultra-high pressure (UHP) terrains, the regional extent of the UHP terrain in the Dabieshan of central China is highly speculative, since the volume of eclogites and paragneisses preserving unequivocal evidence of coesite and/or diamond stability is very small. By contrast, the common garnet (XMn=0.18–0.45)–phengite (Si=3.2–3.35)–zoned epidote (Ps38–97)–biotite–titanite–two feldspars–quartz assemblages in the more extensive orthogneisses have been previously thought to have formed under low P–T conditions of ca. 400±50°C at 4 kbar. However, certain orthogneiss samples preserve garnets with XCa up to 0.50, rutile inclusions within titanite or epidote and relict phengite inclusions within epidote with Si contents p.f.u. of up to 3.49 — overlapping with the highest values (3.49–3.62) recorded for phengites in samples of undoubted UHP schists. These and other mineral composition features (such as A-site deficiencies in the highest Si phengites, Na in garnets linked to Y+Yb substitution and Al F Ti−1 O−1 substitution in titanites) are taken to be pointers towards the orthogneisses having experienced a similar metamorphic evolution to the associated UHP schists and eclogites. Re-evaluated garnet–phengite and garnet–biotite Fe/Mg exchange thermometry and calculated 5 rutile+3 grossular+2SiO2+H2O=5 titanite+2 zoisite equilibria indicate that the orthogneisses may indeed have followed a common subduction-related clockwise P–T path with the UHP paragneisses and eclogites through conditions of Pmax at ca. 690°C–715°C and 36 kbar to Tmax at ca. 710°C–755°C and 18 kbar, prior to extensive re-crystallisation and re-equilibration of these ductile orthogneisses at ca. 400°C–450°C and 6 kbar. The consequential conclusion, that it is no longer necessary to resort to models of tectonic juxtapositioning to explain the spatial association of these Dabieshan orthogneisses with undoubted UHP lithologies, has far-reaching implications for the interpretation of controversial gneiss–eclogite relationships in other UHP metamorphic terrains.

First Finding of Eclogite Facies Metamorphic Event in South Korea and Its Correlation with the Dabie‐Sulu Collision Belt in China

Metabasite lenses occur within granitic gneiss at Bibong in the southwestern part of the Gyeonggi massif, South Korea. Bibong metabasites experienced an initial eclogite facies metamorphism (17.0–20.9 kbar, 835°–860°C), which was succeeded by a granulite facies metamorphism (11.5–14.6 kbar, 830°–850°C) and finally overprinted by an amphibolite facies metamorphism (6.7–11.0 kbar, 570°–740°C). The metabasites have Sm‐Nd internal isochron ages at 225–258 Ma, and the major elements of the metabasites display an island arc affinity. The petrochemical and geochronological data suggest the southwestern part of the Gyeonggi massif as an extension of the Triassic Dabie‐Sulu collision belt in China.

Zircon U-Pb SHRIMP dating for the volcanic rocks of the Xiong’er Group: Constraints on the initial formation age of the cover of the North China Craton

The volcanic rocks of the Xiong’er Group occur widely in the southern part of the North China Craton, which mark the beginning of the cover in the southern part of the North China Craton. The age of the volcanic rocks is thus crucial to understand the tectonic regime and evolutionary history of the North China Craton in the Proterozoic age. Zircons from five volcanic rocks and intrusions were dated by U-Pb SHRIMP method. The results indicate that the Xiong’er Group formed in 1.80-1.75 Ga of Paleo-Proterozoic. Since the Xiong’er Group formed earlier than the Changcheng System, the earliest rocks in the Changcheng System is therefore assumed to be formed in 1.75 Ga. A thermal-tectonic event of ca. 1.84 Ga is indicated by new zircon U-Pb SHRIMP ages in the southern part of the North China Craton. The volcanic rocks of the Xiong’er Group thus represent the initial magmatism of the Paleo-Proterozoic breakup of the North China Craton. Numerous inherited zircons in the volcanic rocks mainly formed in ∼2.20 Ga, indicating that the source magma of the volcanic rocks may be derived from the ∼2.20 Ga crust, or from a mantle magma with significant contamination of the ∼2.20 Ga crust.

Sm–Nd geochronology and petrography of garnet pyroxene granulites in the northern Sulu region of China and their geotectonic implication

Abstract Abundant garnet-bearing granulite lenses are widely distributed in the northern part of the Sulu region and adjacent areas. They are possibly re-metamorphosed high-pressure metamorphic rocks. On the basis of detailed petrographic study, samples WD01, WD04 and ML06 from Laixi and Wendeng were identified as high-pressure granulites, and WH1 from Weihai as an original coesite-bearing eclogite. Three high-pressure granulite samples give mineral-WR isochron ages of 1846±76, 1743±79 and 1752±30 Ma. TDM ages are 3.3, 3.0 and 2.8 Ga. The Sm–Nd mineral-WR isochron ages are interpreted to date as the metamorphic resetting within the medium-pressure granulite facies, representing an isotopic re-homogeneity during uplifting of the high-pressure granulites from deep continent crust. It is important that Sm–Nd chronological characteristics are the same as Archaean high-pressure granulites in the North China craton. However, sample WH1 from Weihai demonstrates abnormal Sm–Nd characteristics. Its whole rock eNd (0) value is +129. TDM age is 1.3 Ga, and constrains the minimum age of re-metamorphosed eclogite protolith formation to the mid-Proterozoic. This result is identical to those reported by Jahn (1994) , showing complicated processes of metamorphism and metasomatism. The data in this paper provide further evidence to define the boundary between the North China craton and UHPM belt in eastern Shandong and to understand the geotectonic nature of the boundary.

Precambrian key tectonic events and evolution of the North China craton

Abstract The North China craton (NCC) is one of oldest cratons in the world, with crust up to c. 3.8 Ga old, and has a complicated evolution. The main Early Precambrian geological events and key tectonic issues are as follows. (1) Old continental nuclei have been recognized in the NCC, and the oldest remnants of granitic gneiss and supracrustal rocks are 3.8 Ga old. The main crustal growth in the NCC took place at 2.9–2.7 Ga. The NCC can be divided into several microblocks, which are separated by Archaean greenstone belts that represent continental accretion surrounding the old continental nuclei. (2) By 2.5 Ga, the microblocks amalgamated to form a coherent craton by continent–continent, arc–continent or arc–arc collisions. The tectonic processes in Neoarchaean and modern times appear to differ more in degree than in principle. Extensive intrusion of K-granite sills and mafic dykes and regional upper amphibolite- to granulite-facies metamorphism occurred, and marked the beginning of cratonization in the NCC. Coeval ultramafic–mafic and syenitic dykes of c. 2500 Ma in Eastern Hebei indicate that the NCC became a stable, thick and huge continent at the end of the Archaean, and probably was a part of the Neoarchaean supercontinent that has been suggested by previous studies. (3) In the period between 2500 and 2350 Ma, the NCC was tectonically inactive, but the development of a Palaeoproterozoic volcanic and granitic rocks occurred between 2300 and 1950 Ma. The volcanic–sedimentary rocks are termed Palaeoproterozoic mobile belts; these have a linear distribution, and were affected by strong folding and metamorphism at 1900–1850 Ma, and intruded by granites and pegmatites at 1850–1800 Ma. The Palaeoproterozoic mobile belts formed and evolved within the craton or continental margin (epicontinental geosyncline). Some 2.30–1.95 Ga rift-margin, passive continental margin deposits, analogous arc or back-arc assemblages, as well as HP and HT–UHT metamorphic complexes seem to be comparable with many in the late Phanerozoic orogenic belts. Regarding Palaeoproterozoic orogeny in other cratons, it is possible that a global Palaeoproterozoic orogenic event occurred, existed and resulted in the formation of a pre-Rodinian supercontinent at c. 2.0–1.85 Ga. (4) In contrast, the c. 1800 Ma event is an extension–migmatization event, which includes uplift of the lower crust of the NCC as a whole, the emplacement of mafic dyke swarms, continental rifting, and intrusion of an orogenic magmatic association. This event has been considered to be related to the break-up of the pre-Rodinian supercontinent at 1.8 Ga, attributed to a Palaeoproterozoic plume. (5) As HP and HT–UHT metamorphic rocks occur widely in the NCC, their high pressure of 10–14 kbar has attracted attention from researchers, and several continental collisional models have been proposed. However, it is argued that these rocks have much higher geothermal gradient and much slower uplift rate than those in Phanerozoic orogenic belts. Moreover, HP and HT–UHT rocks commonly occur together and are not distributed in linear zones, suggesting that the geological and tectonic implications of these data should be reassessed.

Occurrence and field relationships of ultrahigh-pressure metagranitoid and coesite eclogite in the Su-Lu terrane, eastern China

Coesite eclogite is associated with metagranitoid in a 50×100 m2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.

Early Permian high-K calc-alkaline volcanic rocks from NW Inner Mongolia, North China: geochemistry, origin and tectonic implications

Abstract: Knowledge of the characteristics of the extensive late Palaeozoic volcanic rocks across the northern China–Mongolia tract is essential for understanding the tectonic evolution and continental crustal growth in the Central Asian Orogenic Belt. This geochronological and geochemical study documents the Early Permian mafic and felsic volcanic rocks from northwestern Inner Mongolia. The mafic rocks form two magma series with distinctive geochemical characteristics; one showing large ion lithophile element (LILE) enrichment relative to high field strength elements (HFSE) and an asthenosphere-like Sr–Nd–Pb isotopic signature, and the other featuring an elevated Nb and lithospheric isotopic signature. This result indicates that two mantle source components are involved in the magma generation: the subduction-related metasomatized asthenosphere and lithospheric mantle. The felsic rocks show strong enrichment of LILE and light REE, depletion in HFSE, and indistinguishable isotopic compositions from mafic ones. Such features are consistent with partial melts of mixed sources composed of predominant juvenile basaltic underplates and minor ancient crustal materials. These mafic and felsic rocks constitute a post-subduction high-potassium calc-alkaline magmatic suite possibly under a geodynamic regime of Palaeo-Asian Ocean slab breakoff. This regime not only provides a feasible trigger for the flipping of subduction polarity in the Solonker suture zone, but also presents a favourable venue for vertical continental crustal growth.

Sr–Nd isotopic characteristics of the Mesozoic magmatism in the Taihang–Yanshan orogen, North China craton, and implications for Archaean lithosphere thinning

Voluminous felsic rocks (mainly monzonitic) and coeval mafic rocks (mainly monzogabbro–diorites) were emplaced in the Taihang–Yanshan orogen of eastern North China craton in Mesozoic time. The monzogabbro–diorites have high Sr (mostly >1300 ppm) and low εNd(t) values (−9.5 to −15), indicating a long-term incompatible element enriched subcontinental lithospheric mantle source for their genesis. The monzonitic rocks show elemental geochemistry (e.g. high Sr, and REE patterns) and isotopic compositions similar to the monzogabbro–diorites, which leaves little doubt that the two rock suites share a similar origin. These mafic and felsic rocks thus represent a significant addition of juvenile continental crust from an enriched lithospheric mantle source in the Mesozoic, and their generation via melting of enriched portions of the subcontinental lithospheric mantle is probably an important mechanism responsible for the lithosphere thinning beneath eastern North China craton.

Sm-Nd age dating of high-pressure granulites and amphi-bolite from Sanggan area, North China craton

The high pressure (HP) metamorphic age has been dated to HP rocks from the Sanggan area, North China craton. We have got garnet+whole rock isochron ages of (1 842±38) Ma for HP granulite, and (1 856 ±26) Ma for HP amphibolite. The Sm-Nd whole rock isochron of HP granulites give out an age of (1870±150) Ma with Nd depleted mantle model age of (2402–2482) Ma. Considering the Nd isotope homogenization during the peak metamorphism of the HP granulite, Sm-Nd closure temperature and the retention of Nd isotopic memory in garnets partially broken down during decompression, all these isochron ages are thought to be HP metamorphic age. Furthermore, we proposed that the HP metamorphism took place at the end of Paleoproterozoic during the large-scale collision and assembly of the North China craton.