Guochun Zhao

Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P–T path constraints and tectonic evolution

An examination of lithological, geochemical, geochronological, structural and metamorphic P–T path data suggests that the basement of the North China Craton can be divided into Eastern and Western Blocks, separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone, called the Trans-North China Orogen. The Eastern Block consists predominantly of Late Archean domiform tonalitic–trondhjemitic–granodioritic (TTG) batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at ∼2.5 Ga, with anticlockwise P–T paths. Some Early to Middle Archean rocks are locally present in the Eastern Block, but their tectonic history is unclear due to reworking by the 2.5 Ga tectonothermal event. The Western Block has a Late Archean assemblage, structural style and metamorphic history similar to that of the Eastern Block, but it differs in the absence of early to middle Archean assemblages and in being overlain by and interleaved with Paleoproterozoic khondalites, which were affected by a ∼1.8 Ga metamorphic event involving clockwise P–T paths. A mantle plume model is proposed for the formation and evolution of Late Archean basement rocks in the Eastern and Western Blocks based on a combination of extensive exposure of TTG gneisses, affinities of mafic rocks to continental tholeiitic basalts, presence of voluminous komatiitic rocks, dominant diaprism-related domiform structures, anticlockwise P–T paths, and a short time span from the primary emplacement of TTG and ultramafic to mafic rocks until the onset of regional metamorphism. Between the two blocks is the Trans-North China Orogen which is bounded by two major fault systems and is composed of Late Archean to Paleoproterozoic TTG gneisses and granitoids, interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra-arc basin environments. These rocks underwent multiple phases of compressional deformation and peak high-pressure metamorphism followed by rapid exhumation during the Late Paleoproterozoic at ∼1.8 Ga as a result of collision between the Eastern and Western Blocks, resulting in the amalgamation of the North China Craton.

Metamorphism of basement rocks in the Central Zone of the North China Craton: implications for Paleoproterozoic tectonic evolution

Abstract Lithological, structural, metamorphic and geochronological data for the North China Craton enable its division into the Western and Eastern Blocks of Archean to Paleoproterozoic age separated by a north–south trending Paleoproterozoic orogenic belt: the Central Zone. The Central Zone is divisible into a series of low- to medium-grade granite–greenstone belts and high-grade metamorphic terrains containing reworked Archean material and late Archean to Paleoproterozoic juvenile igneous and sedimentary rocks which developed in intra-continental magmatic arc and intra-arc basin environments bordering the western margin of the Eastern Block. The basement rocks from the Central Zone, regardless of their protolith age, composition and metamorphic grade, record a metamorphic history characterized by nearly isothermal decompression (M2) and then retrogressive cooling (M3) following peak metamorphism (M1). The decompression textures are represented by worm-like hypersthene+plagioclase symplectites or clinopyroxene+orthopyroxene+plagioclase coronas in mafic granulites, hornblende/cummingtonite+plagioclase symplectites in amphibolites, and cordierite coronas and cordierite+orthopyroxene or cordierite+spinel symplectites in pelitic rocks. The cooling textures are shown by hornblende+plagioclase symplectites in mafic granulites, chlorite+epidote+mica retrogressive rims around garnet or hornblende grains in amphibolites, and biotite+K-feldspar±muscovite±magnetite replacing garnet, cordierite and sillimanite in pelitic gneisses. These textural relations and their P–T estimates define near-isothermal decompressional clockwise P–T paths, which, in combination with lithological, structural and geochronological constraints, are in accord with collision between the Eastern and Western Blocks of the North China Craton at ∼1.8 Ga.

Thermal Evolution of Archean Basement Rocks from the Eastern Part of the North China Craton and Its Bearing on Tectonic Setting

The basement rocks in the eastern zone of the North China craton are composed predominantly of pretectonic tonalitic-trondhjemitic-granodioritic gneisses and syntectonic granitoids, with rafts of supracrustal rocks consisting of ultramafic to felsic volcanic and sedimentary rocks, metamorphosed over a range of conditions from greenschist to granulite facies. Most mafic granulites, amphibolites, and some pelitic gneisses and schists preserve the prograde, peak, and post-peak textures. The prograde metamorphic stage is indicated by mineral inclusions within minerals of the peak stage, represented by the assemblages of hornblende + plagioclase + quartz ± biotite in mafic granulites, chlorite + actinolite + epidote + plagioclase + quartz in amphibolites, and biotite + plagioclase + quartz in pelitic gneisses. The peak stage is shown by assemblages of orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the mafic granulites, hornblende + plagioclase + quartz + garnet in garnetiferous amphibolites, ...

Assembly, Accretion and Breakup of the Paleo-Mesoproterozoic Columbia Supercontinent: Records in the North China Craton

Abstract Established pre-Rodinian connections between cratonic blocks around the world lead to the proposal of a Paleo-Mesoproterozoic supercontinent, named Columbia, which may have contained nearly all of the earths continental blocks at some time between 1.8 Ga and 1.5 Ga. The assembly of Columbia was completed during the global-scale 2.1-1.8 Ga collisional event, forming the 2.1-2.0 Ga Transamazonian Orogen in South America, 2.1-2.0 Ga Eburnean Orogen in West Africa, ∼2.0 Ga Limpopo Belt in Southern Africa, 1.9-1.8 Ga Trans-Hudsonian and Nagssugtoqidain Orogens in Laurentia, 1.9-1.8 Ga Kola-Karelia Orogen in Baltica, 1.9-1.8 Ga Akitkan Orogen in Siberia, ∼1.8 Ga Central Indian Tectonic Zone in India, and ∼1.85 Ga Trans-North China Orogen in North China. The outward accretion of Columbia is evidenced by the presence of 1.8-1.3 Ga magmatic zones bordering the present southern margins of North America, Greenland, Baltica, North China and North Australia, which consist of juvenile volcanogenic sequences and granitoid suites resembling those of present-day island arcs and active continental margins, representing subduction-related episodic outbuilding on the continental margins of Columbia. The breakup of Columbia commenced at ∼1.6 Ga and continued until ∼1.2 Ga, as indicated by widespread 1.6-1.2 Ga continental rifting, anorogenic magmatism and emplacement of mafic dyke swarms in all cratonic blocks of Columbia. Like most other cratonic blocks, the North China Craton records the history of the assembly, accretion and breakup of Columbia. New data indicate that the evolution of the North China Craton involved discrete Eastern and Western Blocks that developed independently during the Archean and collided to form a coherent craton during a global Paleoproterozoic orogenic event. Following the final amalgamation at ∼1.85 Ga, the North China Craton underwent a long-lived (1.8-1.4 Ga), subduction-related, outgrowth along its southern margin, forming the Xionger volcanic belt. The most robust evidence in the North China Craton for the Mesoproterozoic fragmentation of Columbia comes from the 1.6-1.2 Ga Zhaertai-Bayan Obo-Huade-Weichange rift zone along the northern margin of the craton. The development of this rift zone may have been associated with the separation of the North China Craton from India, whose western margin is considered to have connected to the northern margin of the North China Craton until the start of the Mesoproterozoic when the dispersion of Columbia commenced.

Tectonothermal history of the basement rocks in the western zone of the North China Craton and its tectonic implications

The basement of the North China Craton can be divided into the eastern, central and western zones, based on lithological, structural, metamorphic and geochronological data. The western zone comprises two different petrotectonic units: Archaean tonalitic–trondhjemitic–granodioritic (TTG) grey gneisses and metamorphic mafic rocks, and Palaeoproterozoic khondalite series. The former is characterized by isobaric cooling (IBC)-type anticlockwise P–T paths in the north-northwestern part of the zone and near-isothermal decompression (ITD)-type clockwise P–T paths in the eastern part, adjacent to the central zone. On the other hand, the tectonothermal evolution of Palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling, defining clockwise P–T paths. The Archaean TTG gneisses and associated mafic rocks with anticlockwise metamorphic P–T paths reflects an origin related to underplating and intrusion of mantle-derived magmas which may be derived from mantle plumes. They represent a late Archaean continental block in the western part of the North China Craton. The Palaeoproterozoic khondalite series rocks represent passive continental margin deposits. They were metamorphosed and deformed in the late Palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the North China Craton. The ITD-type clockwise P–T–t paths of the Palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the North China Craton at c. 1800 Ma.

Palaeoproterozoic assembly of the North China Craton

The basement of the North China Craton consists of the Eastern and Western blocks, separated by the Central Zone. Both the Eastern and Western blocks are dominated by late Archaean tonalitic–trondhjemitic–granodioritic gneiss complexes interdigitated with minor supracrustal rocks metamorphosed at ~2.5 Ga, with anticlockwise P–T paths. The Central Zone is composed of reworked late Archaean components and Palaeoproterozoic juvenile crustal materials that underwent regional metamorphism at ~1.85 Ga, with clockwise P–T paths involving isothermal decompression as a result of collision between the Eastern and Western blocks, which resulted in the final assembly of the North China Craton.

Thermal evolution of two textural types of mafic granulites in the North China craton: Evidence for both mantle plume and collisional tectonics

Mafic granulites from the North China craton can be divided into two textural types, referred to as A- and B-types. A-type mafic granulites display garnet + quartz symplectic coronas, and outcrop in the eastern and western zones of the craton, whereas B-type mafic granulites exhibit orthopyroxene + plagioclase ± clinopyroxene symplectites or coronas, and are mainly exposed in the central zone of the craton. Most A-type mafic granulites preserve the prograde (M 1 ), peak (M 2 ) and post-peak near-isobaric cooling (M 3 ) assemblages, which are represented respectively by inclusions of hornblende + plagioclase + quartz, a peak mineralogy of orthopyroxene + clinopyroxene + plagio- clase + quartz + garnet, and overprinted by garnet + quartz symplectic coronas. These mineral assem- blages and their P-T (pressure-temperature) estimates define anticlockwise P-T evolutionary paths. The B-type mafic granulites preserve the peak (M 1 ), post-peak near-isothermal decompression (M 2 ) and cooling (M 3 ) assemblages, which are represented by the peak assemblage of orthopyroxene + clinopyroxene + plagioclase + quartz + garnet ± hornblende, post-peak orthopyroxene + plagioclase ± clinopyroxene symplectites or coronas, and later hornblende + plagioclase + magnetite symplectites, respectively. These mineral assemblages and their P-T estimates define clockwise P-T paths. The anticlockwise P-T paths of the A-type mafic granulites in the eastern and western zones of the North China craton are consistent with a model of underplating and intrusion of mantle-derived magmas. In combination with lithological, structural and geochronological data, the eastern and west- ern zones of the North China craton are considered to represent two continental blocks that devel- oped through the interaction of mantle plumes with the lithosphere from the Palaeoarchaean to the Neoarchaean era. The B-type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and west- ern continental blocks in the late Palaeoproterozoic era. The mineral reaction relations and clockwise P-T paths of the B-type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the North China craton at c. 1800 Ma.

Mesozoic, Not Paleoproterozoic SHRIMP U-Pb Zircon Ages of Two Liaoji Granites, Eastern Block, North China Craton

The Paleoproterozoic Jiao-Liao-Ji orogen in the North China craton is composed mainly of the Liaohe Group (and its equivalents) and the Liaoji Granites. The latter have long been considered to be Paleoproterozoic in age, but have not been precisely constrained by reliable isotopic ages. CL and BSE images reveal the existence of both magmatic and xenocrystal/inherited zircons in one rock sample each from the Dandong and Gaoliduntai granites, two major Liaoji Granite intrusions. The magmatic zircons occur as either single grains or rims surrounding or truncating xenocrystal/inherited zircon cores, and are characterized by euhedral, prismatic grains with well-developed oscillatory zoning, whereas the xenocrystal/inherited zircons occur either as cores surrounded by the igneous zircon rims, or as single grains exhibiting weakly zoned bands. The magmatic zircons yielded ages of 157.4 ± 5.7 Ma and 156.3 ± 4.8 Ma for the Dandong and Gaoliduntai granites, respectively, indicating that these two plutons were emplaced in the Mesozoic, not in the Paleoproterozoic as previously considered. The xenocrystal/inherited zircons in these samples yield ages ranging from 2343 to 1971 Ma, much older than that of the magmatic zircons. Although the precise ages of other Liaoji Granite bodies remain unknown, SHRIMP U-Pb zircon data for one sample of each of the Dandong and Gaoliduntai granites require a re-evaluation of previous tectonic models for evolution of the Paleoproterozoic Jiao-Liao-Ji orogen.

Petrology and P-T history of the Wutai amphibolites: Implications for tectonic evolution of the Wutai Complex, China

Abstract The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M 1 assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M 2 stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P – T conditions for this assemblage have been estimated using the program T weequ at 10–12xa0kbar and 600–650°C. The M 3 assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields P – T conditions of 6.0–7.0xa0kbar and 600–650°C. M 4 is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P – T estimates from M 1 and M 4 and the quantitative P – T estimates from M 2 and M 3 define a clockwise P – T path for the Wutai amphibolites. The estimated P – T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M 1 +M 2 ), subsequent isothermal exhumation (M 3 ), and finally cooling and retrogression (M 4 ). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.

Did South America and West Africa Marry and Divorce or Was it a Long-lasting Relationship?

Abstract The 2.1-2.0 Ga Transamazonian orogen in South America can be well correlated with the 2.1-2.0 Ga Eburnean orogen along the western margin of West Africa. Both the orogens are characterized by early tangential tectonics, marked by large-scale thrusts and sinistral strike-slip faults, followed by later transcurrent tectonics, marked by dextral shear zones. Metamorphic evolution of the two orogens involved an initial phase of crustal thrusting and thickening, followed by exhumation and final cooling. These suggest that South America and West Africa may have been jointed along the Transamazonian-Eburnean orogen at 2.1-2.0 Ga. This is supported by the presence of ∼2.0 Ga fluvio-deltaic formations in nearly every craton in South America and West Africa. The available Paleomagnetic data also indicate that between 2.1-1.5 Ga, coeval rocks in West Africa and South America recorded similar polar wander paths. If South America and West Africa remained coherent from their amalgamation at 2.1-2.0 Ga until their incorporation into Gondwana at 0.6-0.5 Ga, the positions of South America and West Africa in the present configurations models of Rodinia need to be re-evaluated, since these configurations did not bring the two blocks into close proximity.