Simon A. Wilde

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.

Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.

No crustal rocks are known to have survived since the time of the intense meteor bombardment that affected Earth between its formation about 4,550 Myr ago and 4,030 Myr, the age of the oldest known components in the Acasta Gneiss of northwestern Canada. But evidence of an even older crust is provided by detrital zircons in metamorphosed sediments at Mt Narryer and Jack Hills in the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia, where grains as old as ∼4,276 Myr have been found. Here we report, based on a detailed micro-analytical study of Jack Hills zircons, the discovery of a detrital zircon with an age as old as 4,404 ± 8 Myr—about 130 million years older than any previously identified on Earth. We found that the zircon is zoned with respect to rare earth elements and oxygen isotope ratios (δ18O values from 7.4 to 5.0‰), indicating that it formed from an evolving magmatic source. The evolved chemistry, high δ18O value and micro-inclusions of SiO2 are consistent with growth from a granitic melt with a δ18O value from 8.5 to 9.5‰. Magmatic oxygen isotope ratios in this range point toward the involvement of supracrustal material that has undergone low-temperature interaction with a liquid hydrosphere. This zircon thus represents the earliest evidence for continental crust and oceans on the Earth.

Review of global 2.1-1.8 Ga orogens: implications for a pre-Rodinia supercontinent

Abstract Available lithostratigraphic, tectonothermal, geochronological and paleomagnetic data from 2.1–1.8 Ga collisional orogens and related cratonic blocks around the world have established connections between South America and West Africa; Western Australia and South Africa; Laurentia and Baltica; Siberia and Laurentia; Laurentia and Central Australia; East Antarctica and Laurentia, and North China and India. These links are interpreted to indicate the presence of a supercontinent existing before Rodinia, referred to herein as Columbia, a name recently proposed by Rogers and Santosh [Gondwana Res. 5 (2002) 5] for a Paleo-Mesoproterozoic supercontinent. In this supercontinent, the Archean to Paleoproterozoic cratonic blocks were welded by the global 2.1–1.8 Ga collisional belts. The cratonic blocks in South America and West Africa were welded by the 2.1–2.0 Ga Transamazonian and Eburnean Orogens; the Kaapvaal and Zimbabwe Cratons in southern Africa were collided along the ∼2.0 Ga Limpopo Belt; the cratonic blocks of Laurentia were sutured along the 1.9–1.8 Ga Trans-Hudson, Penokean, Taltson–Thelon, Wopmay, Ungava, Torngat and Nagssugtoqidian Orogens; the Kola, Karelia, Volgo–Uralia and Sarmatia (Ukrainian) Cratons in Baltica (Eastern Europe) were joined by the 1.9–1.8 Ga Kola–Karelia, Svecofennian, Volhyn–Central Russian and Pachelma Orogens; the Anabar and Aldan Cratons in Siberia were connected by the 1.9–1.8 Ga Akitkan and Central Aldan Orogens; the East Antarctica and an unknown continental block were joined by the Transantarctic Mountains Orogen; the South and North Indian Blocks were amalgamated along the Central Indian Tectonic Zone; and the Eastern and Western Blocks of the North China Craton were welded together by the ∼1.85 Ga Trans-North China Orogen. The existence of Columbia is consistent with late Paleoproterozoic to Mesoproterozoic sedimentary and magmatic records. The ∼2.0 Ga fluvio-deltaic deposits have been found in all cratonic blocks in South America and West Africa, and they are interpreted to have formed within foreland basins during the latest stage of the 2.1–2.0 Ga Transamazonian–Eburnean collisional event that resulted in the assembly of South America and West Africa. In Laurentia and Baltica, a 1.8–1.30 Ga subduction-related magmatic belt extends from Arizona through Colorado, Michigan, South Greenland, Sweden and Finland to western Russia. The occurrence of temporally and petrologically similar rocks across a distance of thousands of kilometers between these continents supports the existence of a Paleo-Mesoproterozoic supercontinent. Accretion, attenuation and final breakup of this supercontinent were associated with the emplacement of 1.6–1.2 Ga anorogenic anorthosite-mangerite-charnockite-rapakivi (AMCR) suites, 1.4–1.2 Ga mafic dyke swarms and the intrusion of kimberlite–lamproite–carbonatite suites throughout much of the supercontinent.

A-type granites in northeastern China: age and geochemical constraints on their petrogenesis

Abstract A-type granites are widely distributed in northeastern China (NE China). They were emplaced during three major episodes (the Permian, late Triassic to early Jurassic, and early Cretaceous) and evolved in different tectonic regimes. According to their mineralogical and geochemical characteristics, two subgroups of A-type granites (aluminous and peralkaline) can be recognized. The peralkaline subgroup contains alkali mafic minerals, such as riebeckite, arfvedsonite and sodic pyroxene, while the aluminous subgroup contains annite and Fe-rich calcic- or sodic-calcic amphibole. With respect to the aluminous subgroup, the peralkaline granites contain higher Rb, Ga and total rare earth elements (REE), but lower MgO, CaO, Al 2 O 3 , Ba and Sr. Based on the discrimination criteria of Eby [Geology 20 (1992) 641], the Permian and late Triassic to early Jurassic A-type granites belong to the A 2 (post-orogenic) type, whereas the early Cretaceous granites are of the A 1 (anorogenic) type. Nd isotopic compositions of these A-type granites indicate their derivation from a dominantly juvenile crustal source. Their origin is thought to have involved partial melting of an underplated lower crustal source. Because the generation of A-type granites requires high melting temperature, we propose models involving slab break-off, lithospheric delamination and extension. The Permian A-type granites have the same age range as those in eastern Junggar, southern Mongolia and central Inner Mongolia. They occur along a major suture and form a narrow belt between the north China and Siberian Cratons. We suggest that their formation was associated with post-collisional slab break-off. The late Triassic to early Jurassic A-type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late Paleozoic to early Triassic. The early Cretaceous A-type granites have an anorogenic affinity and were possibly associated with rifting in eastern China at this time, associated with the onset of paleo-Pacific subduction. Consequently, we conclude that the A-type granites in NE China were generated at three different times, involving multiple processes operative in different tectonic environments.

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, ...

A cool early Earth

No known rocks have survived from the first 500 m.y. of Earth history, but studies of single zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. Surface temperatures are inferred from high d 18 O values of zircons. The range of d 18 O values is constant throughout the Archean (4.4‐2.6 Ga), suggesting uniformity of processes and conditions. The hypothesis of a cool early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquidwater oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought.

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.

A review of the geodynamic setting of large-scale Late Mesozoic gold mineralization in the North China Craton: an association with lithospheric thinning

Abstract Abundant gold deposits are distributed along the margins of the North China Craton (NCC). Occurring throughout the Precambrian basement and located in or proximal to Mesozoic granitoids, these deposits show a consistent spatial–temporal association with Late Jurassic–Early Cretaceous magmatism and are characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock. Their ages are mainly Early Cretaceous (130–110 Ma) and constrain a very short period of metallogenesis. Sr–Nd–Pb isotopic tracers of ores, minerals and associated rocks indicate that gold and associated metals mainly were derived from multi-sources, i.e., the wall rocks (Precambrian basement and Mesozoic granites) and associated mafic rocks. Previous studies, including high surface heat flow, uplift and later basin development, slow seismic wave speeds in the upper mantle, and a change in the character of mantle xenoliths sampled by Paleozoic to Cenozoic magmas, have been used to suggest that ancient, cratonic mantle lithosphere was removed from the base of the NCC some time after the Ordovician, and replaced by younger, less refractory lithospheric mantle. The geochemistry and isotopic compositions of the mafic rocks associated with gold mineralization (130–110 Ma) indicate that they were derived from an ancient enriched lithospheric mantle source; whereas, the mafic dikes and volcanic rocks younger than 110 Ma were derived from a relatively depleted mantle source, i.e., asthenospheric mantle. According to their age and sources, relation to magmatism and geodynamic framework, the gold deposits were formed during lithospheric thinning. The removal of lithospheric mantle and the upwelling of new asthenospheric mantle induced partial melting and dehydration of the lithospheric mantle and lower crust due to an increase of temperature. The fluids derived from the lower crust were mixed with magmatic and meteoric waters, and resulted in the deposition of gold and associated metals.

Mesozoic crust-mantle interaction beneath the North China craton : A consequence of the dispersal of Gondwanaland and accretion of Asia

We present evidence from zircons entrained within lower-crustal xenoliths in the Cenozoic Hannuoba Basalt of multiple melting events beneath the North China craton in the late Mesozoic. Peak activity was between 180 and 80 Ma, the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks, emplacement of dioritic and lamprophyric dikes, and widespread gold mineralization. The process involved partial loss of mantle lithosphere, accompanied by wholesale rising of asthenospheric mantle beneath eastern China. We correlate these events with lithospheric thinning resulting from the breakup and dispersal of Gondwanaland, accompanied by a major mantle overturn, fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of Asia.