► China contains three major Precambrian blocks, the North China, South China and Tarim cratons, sutured by Phanerozoic orogenic belts. ► The North China Craton (NCC) is divided into the Yinshan, Ordos, Longgang and Langrim blocks, which amalgamated in the period 1.95–1.85 Ga ► South China formed by collision between the Yangtze and Cathsysia blocks along the Jiangnan Belt most probably in the period 825–815 Ma ► The Tarim Craton consists of minor Neoarchean and Paleoproterozoic rocks and abundant early-middle Neoproterozoic metamorphosed rocks. China contains three major Precambrian blocks, the North China, South China and Tarim cratons, separated and sutured by Phanerozoic orogenic belts. The North China Craton (NCC) contains rocks as old as 3.8 Ga, but is dominated by Neoarchean igneous rocks that were formed during two magmatic events at 2.8–2.7 Ga and 2.55–2.50 Ga. The 2.8–2.7 Ga magmatic pulse is considered as a major phase of juvenile crustal growth in the craton, though exposure of these rocks is limited. The 2.55–2.50 Ga rocks make up ∼80% of Archean basement in the NCC, but their rock associations, structural patterns, and metamorphic age and P–T paths in the eastern and western parts (Eastern and Western blocks) are different from those in the central part (Trans-North China Orogen). In the Eastern and Western blocks, the end-Neoarchean rocks are exposed as gneissic domes and dominated by tonalitic–trondhjemitic–granodioritic (TTG) gneisses and mafic to komatiitic rocks that were metamorphosed at ∼2.5 Ga, along anticlockwise P–T paths involving isobaric cooling, that is interpreted to reflect underplating of mantle-derived magmas. In the Trans-North China Orogen, the end-Neoarchean rocks occur as linear structural belts and are composed of arc-related granitoids and volcanic rocks that formed in a supra-subduction zone setting and were metamorphosed at ∼1.85 Ga along a clockwise P–T paths involving isothermal decompression in a continent–continent collisional setting. Although magmatic arc models can explain the origin of the 2.55–2.50 Ga TTG rocks in the Eastern and Western blocks, a mantle plume model is favored because it best explains many other features, including the formation of komatiitic rocks. The 2.55–2.50 Ga rocks and associated Paleoproterozoic rocks in the Trans-North China Orogen exhibit the same structural and metamorphic characteristics that typify continental margin arcs and collisional belts. Spatially, Paleoproterozoic rocks in the NCC are related to collisional assembly of the disparate parts of the Eastern and Western blocks including a ∼1.95 Ga collisional event that led to amalgamation of the Yinshan and Ordos blocks to form the Western Block, which then collided with the Eastern Block to form the Trans-North China Orogen at ∼1.85 Ga. Following the final assembly at ∼1.85 Ga, the interior of the NCC underwent on-going extension, leading to widespread emplacement of 1.80–1.75 Ga mafic dyke swarms, 1.75–1.68 Ga anorthosite–mangerite–granite–rapakivi suites, and deposition of Mesoproterozoic and Neoproterozoic strata. Meanwhile, a large Paleo-Mesoproterozoic volcanic belt (Xiong’er Group) developed on the southern margin of the NCC, and is variously interpreted as an intracontinental rift zone or an Andean-type continental margin arc. In the late Mesoproterozoic, the northern margin of the NCC underwent rifting (Zhaertai-Bayan Obo rift zone), coincident with breakup of the Columbia (Nuna) supercontinent. The South China Craton consists of the Yangtze and Cathaysia blocks, which are considered to have collided along the Jiangnan Belt in the Neoproterozoic. Archean and Paleoproterozoic basement rocks in the Yangtze Block are only locally exposed. The late Mesoproterozoic to early Neoproterozoic folded belts in the Yangtze Block are divided into the Jiangnan Belt in the southeast and the Panxi-Hannan Belt in the west and north. The former is dominated by early Neoproterozoic metamorphosed volcanic-sedimentary strata intruded by middle Neoproterozoic peraluminous (S-type) granites and unconformably overlain by the middle Neoproterozoic Banxi Group and its equivalents. The Panxi-Hannan Belt consists of late Mesoproterozoic to early Neoproterozoic metamorphosed volcanic-sedimentary units and plutonic complexes. A number of mutually exclusive models (e.g. plume-rift, slab-arc, plate-rift, etc.) have been proposed for these belts. The Cathaysia Block is composed predominantly of Neoproterozoic basement rocks with Paleoproterozoic rocks only exposed in southwest Zhejiang and north Fujian, and Mesoproterozoic rocks limited to Hainan Island. The Paleoproterozoic rocks consist of 1890–1830 Ma granitoids and 1850–1815 Ma supracrustal rocks, which were metamorphosed at 1.89–1.88 Ga and locally reworked at 250–230 Ma. Neoproterozoic rocks make up ∼90% of the Precambrian basement in the Cathaysia Block and mainly consist of volcanic-sedimentary strata metamorphosed from greenschist to granulite facies. Some of the volcanic rocks have arc affinities, suggesting the existence of a Neoproterozoic magmatic arc in the Cathaysia Block. The Neoproterozoic rocks were metamorphosed at 460–420 Ma. The tectonic setting of this event has been related to both intracontinental orogeny and continental margin subduction and collision. The Precambrian basement of the Tarim Craton consists of Neoarchean and Paleoproterozoic rocks and late Mesoproterozoic to early-middle Neoproterozoic sedimentary and volcanic strata metamorphosed at greenschist and blueschist facies, which are unconformably overlain by unmetamorphosed late Neoproterozoic (Sinian) cover. The Neoarchean and Paleoproterozoic rocks are exposed in the Kulukatage and Dunhuang complexes on the northern and northeastern margins of the craton, respectively. The Neoarchean rocks consist of granitoid rocks and minor supracrustal rocks, including TTG gneisses, calc-alkaline granites and Kf-granites, most of which were emplaced at 2.6–2.50 Ga. The Paleoproterozoic basement rocks are also composed of granitoid and supracrustals rocks (Xingditage and Dunhuang groups), of which the granitoids were emplaced in two stages at 2.45–2.35 Ga and ∼1.9 Ga. In the Kulukatage Complex, the Neoarchean and Paleoproterozoic rocks underwent metamorphic events at 1.9–1.8 Ga and 1.1–1.0 Ga that are related to assembly of the Columbia (Nuna) and Rodinia supercontinents, respectively. In the Dunhuang Complex, Archean rocks underwent metamorphic events at ∼2.5 Ga and 1.9–1.8 Ga, similar to two major metamorphic events occurring in the Western Block of the NCC, leading to speculation that the complex is the western extension of the Alax Complex of the NCC. Late Mesoproterozoic to early-middle Neoproterozoic metamorphosed strata are exposed on the peripheral margins of the Tarim Craton and are considered to have formed in Andean-type continental margins that were deformed and metamorphosed between 1.0 Ga and 0.9 Ga, probably related to the assembly of Rodinia. During middle Neoproterozoic to Cambrian time, the Tarim Craton became a stable platform overlain by middle-late Neoproterozoic to Cambrian unmetamorphosed cover, of which the middle-late Neoproterozoic units contain four sequences of tillite correlated with the global snowball Earth events. Widespread middle to late Neoproterozoic ultramafic–mafic complexes and mafic dyke swarms with the Tarim Craton are related to a mantle plume event that led to the final breakup of Rodinia.
► Metamorphic ages suggest that the amalgamation of the Eastern and Western Blocks along the Trans-North China Orogen occurred at ∼1.85 Ga. ►The polarity of subduction in the Trans-North China Orogen cannot be determined simply based on the senses of thrusting. ► Available data are not consistent with the existence of the Paleoproterozoic Inner Mongolia-North Hebei Orogen. ►High- and medium-pressure pelitic granulites in the Khondalite Belt resulted from collision between the Yinshan and Ordos blocks at ∼1.95 Ga. ►The Jiao-Liao-Ji Belt can be regarded as a Paleoproterozoic rift-and-collision belt that formed at ∼1.9 Ga. Geological and geophysical data indicate that the Precambrian basement of the North China Craton (NCC) formed by amalgamation of a number of micro-continental blocks. The number of blocks, when they existed and how they came together are controversial, and in particular the following issues are disputed: (1) the timing of collisional event(s) leading to the amalgamation of the Eastern and Western blocks along the Trans-North China Orogen (TNCO); (2) the polarity of the subduction between the Eastern and Western blocks; (3) the validity of an old continental block (Fuping Block) that collided with the Eastern Block at ∼2.1 Ga; (4) the tectonic setting of the northern margin of the NCC in the Paleoproterozoic; (5) the tectonic nature of high-pressure (HP) and ultrahigh temperature (UHT) granulite-facies events in the Khondalite Belt of the Western Block; and (6) the tectonic setting of the Paleoproterozoic Jiao-Liao-Ji Belt in the Eastern Block. Analysis and integration of available stratigraphic, structural, geochemical, metamorphic and geochronologic data enable the development of an internally consistent and coherent model for assembly and stabilization of the various Archean blocks of the NCC in the Paleoproterozoic. All metamorphic ages obtained for the TNCO are around 1.85 Ga, which establishes that the final amalgamation of the Western and Eastern blocks of the craton occurred at ∼1.85 Ga. The TNCO is characterized by a fan-shaped pattern of structural features, with the top-to-the-NW and top-to-the-SE thrusting in the northwest and southeast, respectively. This pattern does not constrain subduction polarity for the collisional assembly of the Eastern and Western blocks. Structures in lithospheric mantle and asthenosphere in the TNCO have been significantly modified/replaced in the Mesozoic and Cenozoic, and hence the present-day orientation of these structures, even if they relate to Paleoproterozoic assembly of the craton cannot be used to infer associated subduction polarity. There are no unique structural data or available metamorphic data to supporting the existence of an old continental block that intervened between the Eastern and Western Blocks, which collided with the Eastern Block at ∼2.1 Ga. Available data are also inconsistent with the existence of the Paleoproterozoic Inner Mongolia-North Hebei Orogen along the northern margin of the NCC that formed through accretion of an exotic arc at ∼2.3 Ga and incorporated into the Paleoproterozoic Columbia (Nuna) Supercontinent at 1.92–1.85 Ga. We interpret the north Hebei portion of this inferred orogen as part of the TNCO, and the Inner Mongolian portion as an independent continental block (Yinshan Block). This block is separated from the Ordos Block by the Paleoproterozoic Khondalite Belt. The high-/medium-pressure granulite facies metamorphic event in the Khondalite Belt is considered to have resulted from collision between the Yinshan and Ordos blocks to form the Western Block at ∼1.95 Ga, whereas the ∼1.92 Ga UHT metamorphism within the belt was related to the underplating or intrusion of mantle-derived magmas during the post-collisional extension. The Jiao-Liao-Ji Belt in the Eastern Block likely formed through Paleoproterozoic rifting to form the Longgang and Langrim blocks, and subsequent basin closure and collision in the period 2.2–1.9 Ga.
An evaluation of recent S-wave receiver functions, S-wave velocities and two versions of P-wave tomographic images along various transects in the North China Craton provides some clues on the subduction-collision history of the different crustal blocks and their final amalgamation within the Paleoproterozoic Columbia supercontinent. Interpretation of a N–S seismic section of the craton suggests thick slab debris sinking to various depths in the mantle. The W–E seismic corridors show the preservation of a thick (>200 km) lithospheric root (tectosphere) beneath the Ordos Block and its variable and extensive erosion towards the Yanliao Block (Eastern Block). This zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted Paleoproterozoic oceanic lithosphere. The present day lithosphere-asthenosphere boundary beneath this region probably marks the ‘erosional plane’ along which decratonization occurred through subduction-erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east. Within the asthenosphere below the Yanliao Block, younger and thinner slabs predominate, in the absence of any prominent thick high velocity layers. These younger slabs define a westward polarity and constitute a mega-scale duplex formed by underplating through Phanerozoic subduction process, particularly the Pacific plate subduction from the east. The lithologic associations within the Inner Mongolia Suture Zone dividing the Yinshan Block to the north and Ordos Block to the south correspond to an accreted ocean plate stratigraphic sequence, with the tonalite-trondhjemite-granodiorite (TTG) gneisses, charnockites and calc-alkaline granites representing a continental arc built up through subduction from the north. The seismic transects bring out a contrasting polarity in the subduction regime with an oblique east- to southward subduction of the Yinshan Block and a westward subduction of the Yanliao Block. Here I propose a double-sided subduction history for the NCC, similar to the ongoing subduction process in the Western Pacific. Such double-sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of NCC are therefore considered to be critical in evaluating the final assembly of the Paleoproterozoic supercontinent Columbia.
This paper presents a brief synthesis of the current state of knowledge on the formation and break-up of the early-Neoproterozoic supercontinent Rodinia, and the subsequent assembly of Gondwanaland. Our discussions are based on both palaeomagnetic constraints and on geological correlations of basement provinces, orogenic histories, sedimentary provenance, the development of continental rifts and passive margins, and the record of mantle plume events. Rodinia assembled through worldwide orogenic events between 1300 Ma and 900 Ma, with all, or virtually all, continental blocks known to exist at that time likely being involved. In our preferred Rodinia model, the assembly process features the accretion or collision of continental blocks around the margin of Laurentia. Like the supercontinent Pangaea, Rodinia lasted about 150 million years after complete assembly. Mantle avalanches, caused by the sinking of stagnated slabs accumulated at the mantle transition zone surrounding the supercontinent, plus thermal insulation by the supercontinent, led to the formation of a mantle superswell (or superplume) beneath Rodinia 40–60 million years after the completion of its assembly. As a result, widespread continental rifting occurred between ca. 825 Ma and 740 Ma, with episodic plume events at ca. 825 Ma, ca. 780 Ma and ca. 750 Ma. Like its assembly, the break-up of Rodinia occurred diachronously. The first major break-up event occurred along the western margin of Laurentia (present coordinates), possibly as early as 750 Ma. Rifting between the Amazonia craton and the southeastern margin of Laurentia started at approximately the same time, but only led to break-up after ca. 600 Ma. By this time most of the western Gondwanan continents had joined together, although the formation of Gondwanaland was not complete until ca. 530 Ma.
A set of global paleogeographic reconstructions for the 1770–1270 Ma time interval is presented here through a compilation of reliable paleomagnetic data (at the 2009 Nordic Paleomagnetic Workshop in Luleå, Sweden) and geological constraints. Although currently available paleomagnetic results do not rule out the possibility of the formation of a supercontinent as early as ca. 1750 Ma, our synthesis suggests that the supercontinent Nuna/Columbia was assembled by at least ca. 1650–1580 Ma through joining at least two stable continental landmasses formed by ca. 1.7 Ga: West Nuna (Laurentia, Baltica and possibly India) and East Nuna (North, West and South Australia, Mawson craton of Antarctica and North China). It is possible, but not convincingly proven, that Siberia and Congo/São Francisco were combined as a third rigid continental entity and collided with Nuna at ca.1500 Ma. Nuna is suggested to have broken up at ca. 1450–1380 Ma. West Nuna, Siberia and possibly Congo/São Francisco were rigidly connected until after 1270 Ma. East Nuna was deformed during the breakup, and North China separated from it. There is currently no strong evidence indicating that Amazonia, West Africa and Kalahari were parts of Nuna.
South China was formed through the amalgamation of the Yangtze Block with the Cathaysia Block, but the timing of this amalgamation is controversial, ranging from Mesoproterozoic to Mesozoic. We report here SHRIMP U–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu Group volcanic rocks from the southeastern Yangtze Block. These rocks were strongly deformed, metamorphosed to greenschist-facies, intruded by 849 ± 7 Ma dolerites, and unconformably overlain by Neoproterozoic rift successions of no older than ca. 820 Ma. The Beiwu and Zhangcun volcanic rocks from the middle and uppermost Shuangxiwu Group were dated at 926 ± 15 Ma and 891 ± 12 Ma, respectively. All the studied rocks are characterized by highly positive ɛNd(T) (5.4–8.7) and ɛHf(T) (11.0–15.3) values. The Pingshui basaltic and andesitic rocks from the lower Shuangxiwu Group, which were previously dated at ca. 970 Ma, are high in Al O (15–20%) but low in MgO (69%), low in MgO (0.35–1.2%), and have nearly constant Al O contents of 14–15% and relatively uniform trace element concentrations. They were generated by remelting of juvenile mafic to intermediate arc rocks. Overall, the Shuangxiwu Group volcanic rocks and associated intrusive tonalites and granodiorites constitute a typical calc-alkaline magmatic assemblage of a 970–890 Ma active continental margin. These results and the 849 ± 7 Ma zircon U–Pb age for the undeformed doleritic dikes intruding the Shuangxiwu Group suggest that the tectonic regime of the study region transformed from plate convergence to intracontinental rifting in the time period between ca. 890 Ma and ca. 850 Ma. Previously reported 1.04–0.94 Ga metamorphic and deformation ages from the nearby Tianli Schists and evidence for the final closure of the back-arc basin at ca. 880 Ma (ophilitic obduction at Xiwan), further suggest that the amalgamation between the Yangtze and Cathaysia Blocks, likely through “soft docking” at the eastern segment of the Sibao orogen, was completed at ca. 880 Ma or soon after.
. ► Ultrahigh-temperature (UHT) granulites within the Inner Mongolia Suture Zone in the North Chin Craton (NCC) record robust evidence for extreme crustal metamorphism in a subduction-collision setting. ► The Paleoproterozoic UHT event broadly coincided with the timing of scissor-like closure of oblique collision during 1.95–1.92 Ga between the Yinshan and the Ordos Blocks. ► Plate tectonic models relating asthenospheric upwelling and input of heat and volatiles best account for the formation of the UHT rocks in the NCC. Ultrahigh-temperature (UHT) metamorphic rocks associated with the ‘Khondalite Belt’ within the Inner Mongolia Suture Zone (IMSZ) provide robust evidence for extreme thermal metamorphism in the North China Craton (NCC). The IMSZ marks the collisional suture between the Yinshan Block to the north and the Ordos Block to the South as the NCC was incorporated within the Columbia supercontinent amalgam during Paleoproterozoic. Here we present a synthesis of the salient features of the UHT rocks from the NCC including petrologic indicators, fluid characteristics, and monazite and zircon chronometry on the extreme crustal metamorphism. The granulites carry diagnostic UHT mineral assemblages including sapphirine + quartz, low Zn/Fe spinel + quartz, high alumina orthopyroxene + sillimanite + quartz and high temperature mesoperthite. The stability fields of the typical mineral assemblages, conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these UHT rocks experienced metamorphic temperatures up to or in excess of 1000 °C at ca. 10 kbar, followed by an isobaric cooling segment. The rocks were exhumed along a near-isothermal decompression path. Microstructures, mineral reactions and phase equilibria modeling suggest an anti-clockwise – path, similar to those displayed by metamorphic orogens developed in subduction-collision settings. The dominant category of fossil fluids preserved within the major UHT minerals is CO , consistent with the stability of the broadly anhydrous mineral assemblage in these rocks. Both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the UHT event as around 1.92 Ga. The Paleoproterozoic high grade metamorphism younging from 1.95 Ga in the western domain to 1.92 Ga in the eastern domain of the Khondalite Belt might suggest a scissor-like closure of oblique collision between the Yinshan and the Ordos Blocks. The salient features of the UHT metamorphism in the NCC include: (1) extreme metamorphic temperatures at moderate pressures, (2) dominantly anhydrous nature of the mineral assemblages, typically the stability of orthopyroxene, (3) common presence of CO -rich fluid inclusions as the trace of the ambient fluid, (4) regional extent of the UHT granulites, and (5) the association of the UHT orogen with an accretionary belt in a continental collisional suture. We evaluate the diverse models on the generation of UHT orogens including their formation in thickened and inverted back-arcs, orogen self-heating through heat producing elements, heat and CO input by plume impingement below a carbonated tectosphere, and asthenospheric upwelling through ridge subduction and slab-window process or during post-collisional slab break-off. The ultra-hot and dry UHT rocks in the NCC provide one of the well preserved examples from the Paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm.
. ► A new Pt UHT granulite from Dongpo, the North China Craton, is reported. ► UHT metamorphism was caused by ∼1.93 Ga mantle-derived magma injection. ► Sapphirine granulite shows a decompressional – path following magma heating. Sapphirine granulites occur in the Daqingshan and Jining areas in the Palaeoproterozoic Khondalite belt, which divides the Western Block of the North China Craton into the Yinshan block to the north and the Ordos block to the south. The sapphirine granulites in the Daqingshan area are always in contact with meta-gabbronorite dykes, implying a causal relationship. The sapphirine-bearing rocks are divided into spinel–garnet–sillimanite–biotite–plagioclase–sapphirine gneiss, UHT sapphirine granulite, and spinel–garnet granulite. The sapphirine granulite contains up to 30% sapphirine, garnet (30–50%), spinel (5–15%), sillimanite (5–15%), biotite (10–20%) and plagioclase (10–20%) with minor cordierite, rutile and ilmenite, but without quartz and orthopyroxene. Bulk chemical compositions show that the sapphirine granulites have very low SiO contents (39 wt.%), high Al contents, and low . Biotite contains very high TiO contents up to 7.6 wt.%. Detailed petrographic examination of the sapphirine granulites reveals five mineral assemblages (M –M ): (1) an assemblage (M ) of mineral inclusions within garnet cores, (2) a matrix (peak) assemblage (M ) represented by coarse-grained garnet, sapphirine, spinel, sillimanite, biotite and plagioclase, (3) sapphirine + plagioclase symplectite (M ), (4) spinel + plagioclase symplectite (M ), and (5) retrogressive biotite (M ). The – stability field in the pseudosection of the NCKFMASH system indicates that the temperature of the peak UHT metamorphism of the Daqingshan sapphirine granulites is in the range 910–980 °C (this compares with the peak regional metamorphic temperature of the khondalites of 700–820 °C). The – path inferred from the – stability fields of the mineral assemblages (M –M ) suggests that the peak UHT metamorphism (M ) was followed by nearly isothermal decompression (M and M ) and later cooling (M ). Field relations and geochronological data suggest that the high-heat flow necessary for the UHT metamorphism of the sapphirine granulites from the Daqingshan area was provided by coeval ∼1.93–1.92 Ga gabbronorite intrusions that were most probably generated by ridge subduction, which was also responsible for abundant garnet-rich granites by crust melting the area.
► Early to end Paleoproterozoic subduction-accretion history along two major zones of ocean closure in the North China Craton. ► Long-lived convergent margins through westward subduction of the Eastern Block and southward subduction of the Yinshan Block in a double-sided subduction realm. ► Pacific-type orogeny and accretionary growth with progressive collision marked by exhumation of HP-UHT belts. The Inner Mongolia Suture Zone (IMSZ) and the Trans-North China Orogen (TNCO) incorporate the major Paleoproterozoic accretionary orogens in the North China Craton (NCC), with the Jiao-Liao-Ji Belt (JLJB) representing the third one. Here we investigate the Paleoproterozoic tectonics of the IMSZ and TNCO through zircon SHRIMP geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks. SHRIMP analysis of zircons with textures indicating extreme recrystallization under ultrahigh-temperature (UHT) conditions from the metapelites at Heling’er in the southern domain of the IMSZ reveals a single population with a weighted mean Pb/ Pb age of 1913 ± 17 Ma. The zircons in another UHT granulite from this locality yield a weighted mean Pb/ Pb age of 1910 ± 18 Ma. These data correlate with the ca. 1.92 Ga age reported from zircons in sapphirine-bearing UHT granulites further north and confirm the regional extent of the Paleoproterozoic UHT metamorphism within the IMSZ. Zircons in a charnockite from the southern margin of the Khondalite Belt fringing the UHT granulites in the IMSZ show two distinct age groups: an older population with a magmatic paragenesis and a weighted mean Pb/ Pb age of 1932 ± 24 Ma, and a younger group of metamorphic zircons with an age of 1858 ± 26 Ma. We also report zircon ages from charnockites in two localities around Xing’he in the Huangtuyao belt belonging to the Huai’an Complex within the westernmost domain of the TNCO at the junction with the IMSZ. The charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean Pb/ Pb age of 2477 ± 2 Ma and the younger population showing an age of 1807 ± 38 Ma. The internal structure as revealed from CL images and the overall high Th/U values (up to 2.42) of the older zircons suggest their magmatic affinity, whereas the younger group with extremely low Th/U (0.02–0.09) is of metamorphic origin. Zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older (magmatic) group having a weighted mean Pb/ Pb age of 2147 ± 11 Ma and a minor younger group with an age of 1958 ± 25 Ma. The range of ages from 2477 to 2147 Ma from magmatic zircons in the charnockites from the eastern periphery of the IMSZ, within the western margin of the TNCO, in combination with similar ages reported in recent studies from zircons in magmatic complexes within the IMSZ suggest a prolonged history of subduction-related arc magmatism and accretionary tectonics analogous to those in some of the Phanerozoic belts such as the Central Asian Orogenic Belt and the Western Pacific. Subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high-pressure (HP) and UHT metamorphic rocks. The available data from the IMSZ and TNCO suggest long-lived convergent margins associated with the southward subduction of the Yinshan Block and westward subduction of the Eastern Block in a double-sided subduction realm prior to the final amalgamation of the NCC and its incorporation within the Columbia supercontinent in the late Paleoproterozoic.
Sapphirine granulites occur in the Daqingshan and Jining areas in the Palaeoproterozoic Khondalite belt, which divides the Western Block of the North China Craton into the Yinshan block to the north and the Ordos block to the south. The sapphirine granulites in the Daqingshan area are always in contact with meta-gabbronorite dykes, implying a causal relationship. The sapphirine-bearing rocks are divided into spinel-garnet-sillimanite-biotite-plagioclase-sapphirine gneiss, UHT sapphirine granulite, and spinel-garnet granulite. The sapphirine granulite contains up to 30% sapphirine, garnet (30-50%), spinel (5-15%), sillimanite (5-15%), biotite (10-20%) and plagioclase (10-20%) with minor cordierite, rutile and ilmenite, but without quartz and orthopyroxene. Bulk chemical compositions show that the sapphirine granulites have very low SiO2 contents (39 wt.%), high Al contents, and low X-Mg. Biotite contains very high TiO2 contents up to 7.6 wt.%. Detailed petrographic examination of the sapphirine granulites reveals five mineral assemblages (M-0-M-4): (1) an assemblage (M-0) of mineral inclusions within garnet cores, (2) a matrix (peak) assemblage (M-1) represented by coarse-grained garnet, sapphirine, spinel, sillimanite, biotite and plagioclase, (3) sapphirine + plagioclase symplectite (M-2), (4) spinel + plagioclase symplectite (M-3), and (5) retrogressive biotite (M-4). The P-T stability field in the pseudosection of the NCKFMASH system indicates that the temperature of the peak UHT metamorphism of the Daqingshan sapphirine granulites is in the range 910-980 degrees C (this compares with the peak regional metamorphic temperature of the khondalites of 700-820 degrees C). The P-T path inferred from the P-T stability fields of the mineral assemblages (M-1-M-4) suggests that the peak UHT metamorphism (M-1) was followed by nearly isothermal decompression (M-2 and M-3) and later cooling (M-4). Field relations and geochronological data suggest that the high-heat flow necessary for the UHT metamorphism of the sapphirine granulites from the Daqingshan area was provided by coeval similar to 1.93-1.92 Ga gabbronorite intrusions that were most probably generated by ridge subduction, which was also responsible for abundant garnet-rich granites by crust melting the area. (c) 2011 Elsevier B.V. All rights reserved.
► Two Pre-Devonian litho-tectonic units in the Cathaysia block were divided. ► Four groups of SHRIMP zircon U–Pb age at 860–835 Ma are obtained from mafic rocks. ► A rifting environment at 800–860 Ma and an arc setting at 970 Ma were recognized. ► All mafic samples from Cathaysia show non-ophiolitic geochemical features. The Cathaysia block is an important element for the reconstruction of the Proterozoic tectonic evolution of South China within the Rodinia supercontinent. The Pre-Devonian Cathaysia comprises two litho-tectonic units: a low-grade metamorphic unit and a basement unit; the former was a late Neoproterozoic–Ordovician sandy and muddy sedimentary sequence, the latter consists essentially of metamorphosed Neoproterozoic marine facies sedimentary and basaltic rocks, and a subordinate amount of Paleoproterozoic granites and amphibolites. This block has undergone several tectono-magmatic events. The first event occurred in the late Paleoproterozoic, at ca. 1.9–1.8 Ga, and the tectonic–magmatic event dated at 0.45–0.40 Ga was resulted from the early Paleozoic orogeny that made the Pre-Devonian rocks to undergo a regional lower greenschist to amphibolite facies metamorphism. The Neoproterozoic geodynamic event is poorly understood. In this paper, new U–Pb zircon age, whole-rock chemical and zircon Hf isotopic data for mafic and felsic igneous rocks are used to constrain the tectonic evolution of Cathaysia. Zircon SHRIMP U–Pb analyses on four mafic samples yielded rather similar Neoprotorozoic ages of 836 ± 7 Ma (gabbro), 841 ± 12 Ma (gabbro), 847 ± 8 Ma (gabbro) and 857 ± 7 Ma (basalt). Combined with the published isotopic age data, most of the mafic samples dated at 800–860 Ma show geochemical characteristics of continental rift basalt. By contrast, rhyolitic samples with an age of 970 Ma have a volcanic arc affinity. All mafic samples have LREE-enriched REE patterns, and non-ophiolitic trace element characteristics. However, the zircon Hf isotopic data of mafic samples show positive epsilon ( ) values (+4.1 to +10.5), suggesting that they were originated from a long-term depleted mantle source. All the available ages indicate that the Cathaysia block has registered two stages of Neoproterozoic magmatism. The younger stage corresponds to a continental rifting phase with emplacement of mafic rocks during the period of 860–800 Ma, whereas the older stage represents an eruption of volcanic arc rocks at about 970 Ma. These two magmatic stages correspond to two distinct tectonic settings within the framework of the geodynamic evolution of Cathaysia. Such a similar Neoproterozoic stratigraphy and magmatism between the Cathaysia, Yangtze and Australian blocks provide a significant line of evidence for placing the Cathaysia block within the Rodinia supercontinent.
► There are a mass of metamorphic volcanic rocks in the Lüliang Complex, NCC. ► LA-ICP-MASS zircon U–Pb isotopic dating reveals they erupted at ∼2.2 Ga. ► Their parental magma came from the 5 to 30% melting of the mantle lherzolites. ► These metavolcanic rocks were developed under a continental marginal arc setting. The Paleoproterozoic Lüliang Complex is situated in the central part of the western margin of the Trans-North China Orogen and consists of volcanic rocks, sedimentary rocks and Paleoproterozoic granitoid intrusions. The volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist- to amphibolite-facies. These metamorphosed volcanic rocks are dominated by basalts to basaltic andesites. The parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5% to 30% partial melting of spinel lherzolites to spinel-garnet lherzolites which had been enriched by the subduction melts. Mafic magma experienced fractional crystallization and crustal assimilation. U–Pb zircon dating on two metamorphosed volcanic rocks from the Yejishan and Lüliang groups reveals that they formed at 2210 ± 13 Ma and 2213 ± 47 Ma, respectively, and were metamorphosed at ∼1832 Ma. This suggests that the metamorphosed volcanic rocks in the Yejishan and Lüliang groups formed synchronously in the Paleoproterozoic. These new ages, integrated with recently reported U–Pb zircon ages for the Jiehekou Group and Paleoproterozoic granitoids, suggest that all of the lithological assemblages of the Lüliang Complex formed and were metamorphosed in the Paleoproterozoic, not in the Neoarchean. Petrological, geochronological and geochemical data suggest that the geodynamic evolution of the Paleoproterozoic Lüliang Complex was involved in the development of a magmatic arc system at an active continental margin, generating widespread arc-related magmatism at ∼2.2 Ga. The Lüliang Complex then underwent intense deformation and metamorphism, and was incorporated into the Trans-North China Orogen during the 1.88–1.83 Ga collisional event which was followed by post-collision extension at ∼1.80 Ga.
An outstanding feature of the Archaean Eon is that it was a time of major production and preservation of continental lithosphere. Here I review the geological, geochemical and basic geophysical data that hold key information regarding Archaean crust formation and preservation. This insight is then contrasted with the data for the preceding Hadean and following Palaeoproterozoic, both often portrayed as geological times of apparently much poorer crust preservation. It is concluded that two different paths led to formation of preserved Archaean continental crust. The first was the stabilisation of lithosphere at the Hadean–Archaean boundary, eventually giving rise to the long-lived, relatively slowly maturing, thick cratonic nuclei found today in several Precambrian shields. The second type of Archaean continental lithosphere formed much more rapidly, over periods of 150–300 Ma, at several times during the Archaean. Magmas were initially erupted as stacks of mafic–ultramafic volcanic plateaux that became so thick that they underwent internal differentiation. No Archaean oceanic lithosphere containing evidence for spreading is preserved, but the existence of oceanic basins underlain by relatively thin lithosphere is inferred from a number of observations. An important aspect of studying Archaean lithosphere is to differentiate between the effect of higher radioactive heat production on the crust as opposed to the mantle. In the crust, the strong distillation of K, U and Th into the upper layer, via repeated re-melting, caused non-uniformitarian geological phenomena. Once heat production was concentrated into the top layer, the crust became mechanically strong and its geology more familiar. The effect of higher heat production on the mantle is much less well understood. My hypothesis is that the temperature of the Archaean convecting asthenosphere was not uniformly hotter than at present. Rather, very hot (ca. 1750 °C) upwellings from the lower mantle passed through much cooler (ca. 1400 °C) upper mantle, or the asthenospheric mantle temperature swung rapidly between the two thermal states. The defining feature of the Archaean mantle could have been a barrier layer in the transition zone. Hot Archaean mantle upwellings originated at the base of the barrier, which acted as a thermal boundary layer. The disappearance of this mechanical barrier in the mantle transition zone could have marked the end of the Archaean. The ensuing pulse of re-fertilisation of the highly depleted asthenosphere and the more stable thermal state of the Palaeoproterozoic mantle caused the disappearance of previously widespread highly magnesian lavas, with wide-reaching consequences for petrology of terrestrial magmas, the structure of the continental crust, and the state of the hydrosphere and atmosphere. If the upper mantle was refertilised between 2.5 and 2.6 Ga, the geochemical and isotopic evidence for muted continental growth in the Palaeoproterozoic is misleading because almost all crustal growth models assume a constant mass of the depleted mantle. Until the driver for the Archaean–Proterozoic boundary is better understood, care should be taken with the interpretation of extent of depletion of Proterozoic magma sources.
U–Pb geochronology along with elemental and Nd–Hf–Os isotopic data from the earliest Neoproterozoic metabasic rocks within the Cathaysia Block of the South China Block (SCB) constrain the tectonic setting and paleogeography of the block within the Rodinia supercontinent. The metabasic rocks give zircon U–Pb ages of 969–984 Ma, ( ) values of +1.8 to +15.3 and Hf model ages of 0.92–1.44 Ga. They are subalkaline basalts that can be geochemically classified into four groups. Group 1 has low Nb contents (1.24–4.33 ppm), highly positive ( ) values (+4.3 to +5.2), and REE and multi-elemental patterns similar to fore-arc MORB-type basalt. Group 2 has Nb contents ranging from 3.13 ppm to 6.48 ppm, ( ) of +3.1 to +6.2, low Re and Os contents and high initial Os isotopic ratios, and displays an E-MORB geochemical signature. Group 3 has Nb = 7.18–29.87 ppm, Nb/La = 0.60–1.40, Nb/U = 5.0–37, Ce/Pb = 1.1–6.6, ( ) = +2.9 to +7.0, Re/ Os = 5.87–8.87 and γOs ( ) = 178–772, geochemically resembling to the Pickle Nb-enriched basalt. Group 4 has strong LREE/HREE and HREE fractionation and high ( ) values (+2.3 to +5.6), and is characterized by similar element patterns to arc volcanic rocks. Serpentinites coeval to Group 4 show Os/ Os of 0.1143–0.1442 and γOs ( ) of −7.8 to +0.1. Groups 1 and 2 are interpreted to originate from the N-MORB and E-MORB-like sources with the addition of an arc-like component, genetically linked to fore- and back-arc settings, respectively. Groups 3 and 4 show inputs of newly subduction-derived melt and fluid in the wedge source. These geochronological and geochemical signatures fingerprint the development of an earliest Neoproterozoic (∼970 Ma) arc–back-arc system along the Wuyi-Yunkai domain of the Cathaysia Block. Regional relationships indicate that the Wuyi-Yunkai arc–back-arc system was one of a series of separate convergent margin settings, which included the Shuangxiwu (∼970–880 Ma) and Jiangnan (∼870–820 Ma) systems that developed in the SCB. The formation and closure of these arc–back-arc systems resulted in the northwestwardly episodic amalgamation of various pieces of the Yangtze and Cathaysia to finally form the SCB. These signatures require the SCB to occupy an exterior accretionary orogen along the periphery of Rodinia during 990–820 Ma, rather than to have formed through Mesoproterozoic Sibao orogenesis within the interior of Rodinia.
The Western Block of the North China Craton consists of the Yinshan Block in the north and the Ordos Block in the south which were amalgamated along the east-west trending Khondalite Belt at ∼1.95 Ga. The Western Block then collided with the Eastern Block to form the coherent basement of the North China Craton along the north-south trending Trans-North China Orogen at ∼1.85 Ga. The Huaian Complex, a high-grade terrrane located at the conjunction of the Khondalite Belt and Trans-North China Orogen, records metamorphic events associated with both collisions. The complex consists of lithologies from both the Khondalite Belt and Trans-North China Orogen, of which the former consist of graphite–garnet–sillimanite gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble, together called the Khondalite series. Zircons in the graphite–garnet–sillimanite gneiss can be divided into three types: (1) spherical grains without internal structures, (2) grains with a core-and-rim structure; and (3) grains with a dark core surrounded by double rims. Except for the dark core in type 3, all other types of zircon domains are structureless and highly luminescent, with very low Th/U ratios, typical of a metamorphic origin. Analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 ± 26 and 1947 ± 22 Ma, similar to previously determined metamorphic ages from the Khondalite Belt and thus interpreted as the time of collision between the Yinshan and Ordos Blocks. Analyses on type 1 zircons, rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 ± 15 and 1857 ± 16 Ma, interpreted as the time of collision between the Eastern and Western Blocks. Thus, zircons in the graphite–garnet–sillimanite gneiss of the Huaian Complex record both of the Paleoproterozoic collisional events in the North China Craton.
► The southern segment of the Paleoproterozoic Jiao-Liao-Ji Belt underwent three distinct episodes of folding and two stage of ductile thrust shearing. ► The deformation developed at a period of about 1956 Ma to 1875 Ma. ► The syn-collisional extrusion and thrusting were possibly responsible for fast exhumation of the high pressure granulites. ► A southeastward-directed oblique subduction beneath the Rangrim Block led to collision between the two blocks. The Paleoproterozoic Jiao-Liao-Ji Belt separates the Eastern Block of the North China Craton into two small sub-blocks: the northern Longgang and the southern Rangrim blocks. However, it still remains unknown or controversial about the subduction polarity, collisional deformation and kinematics between two sub-blocks. The southern segment of the belt consists of the Paleoproterozoic Fenzishan and Jingshan groups, and Paleoproterozoic high pressure mafic granulites and serpentinites blocks which are located in the Jiaodong Complex. All of which are separated from the Jiaodong Complex of Neoarchean TTG gneisses by STZ1 ductile shear zones. Structural analysis in this study indicates that most of the rocks in all the units of the southern segment of the Jiao-Liao-Ji Belt underwent three distinct episodes of folding (D to D ) and two stage of ductile thrust shearing (STZ coeval to D and D , STZ between D and D ). The D deformation formed penetrative axial planar foliations (S ), bedding-parallel ductile shear zone, mineral stretching lineations (L ), and rarely preserved small isoclinal D folds in the Jingshan and Fenzishan groups. In the Jingshan Group, however, penetrative deformational transposition resulted in stacking of sedimentary compositional layers which are separated by bedding-parallel ductile shear zones (STZ ) at a period of about 1956 Ma to 1914 Ma. The kinematic indicators of STZ in the Jingshan Group with resultant prograde peak metamorphism up to granulite facies grade and the Fenzishan Group with peak metamorphism up to amphibolite facies grade indicate NW-directed compression. D resulted in crustal thickening with retrograded medium pressure granulite facies grade at about 1914–1893 Ma. The D deformation produced NW-verging asymmetric and recumbent folds, interpreted to have resulted from basement-involved thicken-skin structures. The Jiaodong Complex was also involved into the development of WNW-verging asymmetric tight folds associated with D in the Jingshan and the Fenzishan groups. Ongoing collision led to the development of transpressional ductile shearing (STZ ), forming the transpressional Taipingzhuang dextral ductile shear zone between the Jingshan Group and the southern Archean Complex and the transpressional Tading-Xiadian sinistral ductile shear zone between the Jingshan Group and the northern Archean Complex. All three lithotectonic units were superposed during the late D deformation with amphibolite facies metamorphism. The D deformation developed WNW-trending open to tight upright folds at about 1893–1875 Ma. The structural pattern resulting from superimposition of D and D is a composite synform in the Fenzishan and Jingshan groups. The structural events of D and STZ , and D and STZ deformation were possibly responsible for fast syn-collisional exhumation of the high pressure mafic granulites. The structural patterns and deformational history of the Fenzishan and Jingshan groups suggest a southeastward-directed oblique subduction beneath the northwestern margin of the Rangrim Block, and that the final scissor-shaped closure of the rift led to collision between the two blocks to form the coherent basement of the Eastern Block of the North China Craton.
An early extensive Neoproterozoic (ca. 900 Ma) continental magmatic arc system covering hundreds of kilometers has been reported to occur in the South Beishan Orogenic Belt (SBOB) and the Central Tianshan (CTA) in the southern Central Asian Orogenic Belt (CAOB). However, evidence for coeval high-grade metamorphism and thus the formation of an accretionary orogen in the framework of Rodinia is ambiguous or absent. This study provides new petrological, geochemical and geochronological data for garnet-bearing schists (quartz + garnet + biotite + plagioclase ± muscovite) from the SBOB in order to constrain its Neoproterozoic metamorphic history. The metamorphic zircon rims are either unzoned or display sector zoning in CL-images and reveal REE patterns with flat HREE patterns and negative Eu anomalies, which are interpreted to be in chemical equilibrium with garnet and plagioclase. The zircon U-Pb dating yields concordant U-Pb ages of 900 ± 3 Ma, 897 ± 2 Ma and 898 ± 4 Ma for the metamorphic zircon rims. The inherited detrital zircon cores of one sample display a concordant U-Pb age of 1397 ± 5 Ma that is consistent with the timing of formation for the extensive Mesoproterozoic continental arc in the SBOB and CTA. Based on phase equilibrium geothermobarometry and average P-T thermobarometric calculations, minimum amphibolite-facies P-T conditions are estimated to be >600 °C at pressure >0.6 GPa, which is thought to have been overprinted by subsequent Paleozoic metamorphism. However, the Ti-in-zircon thermometer still reveals temperatures of up to 840 °C using the composition of metamorphic zircon rims, suggesting former ca. 900 Ma granulite-facies peak metamorphic temperatures. The combined petrological and geochronological evidence in conjunction with the continental affinity of the regional metamorphic rocks suggests that the SBOB and the eastern CTA experienced an early Neoproterozoic accretionary orogenesis during the final assembly stage of Rodinia.
► Early Neoarchean TTG and supracrustal rocks occur widely in Western Shandong, China. ► This paper reports SHRIMP U-Pb ages and Hf compositions of zircons from these rocks. ► Large-scale juvenile crustal additions happened at the Early Neoarchean in the area. The evolution of the North China Craton (NCC) is well known for a marked 2.55–2.50 Ga tectonothermal event. However, supracrustal and intrusive rocks of 2.75–2.70 Ga are in fact widely distributed in the western Shandong Province, the most important area of Archaean basement in the eastern part of the NCC. This paper reports SHRIMP U–Pb dating and LA-ICPMS Hf isotopic composition of zircons from 2.75–2.70 Ga supracrustal and trondhjemite–tonalite–granodiorite (TTG) rocks in that area. Three fine-grained (hornblende) biotite gneiss samples (known locally as leptynite, with meta-volcanic or volcanosedimentary rock protoliths) and five TTG samples have SHRIMP zircon U–Pb ages varying from 2.75 to 2.70 Ga and 2.74 to 2.71 Ga, respectively. Zircons from most of the samples have high positive ( ) values (+4.7 to +10.0) and (Hf) ages (2.85–2.60 Ga) similar to their zircon U–Pb ages. This indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before. However, some granitoids show (t) zircon values of −13.6 to +5.1 and (Hf) of 3.51–2.80 Ga. Therefore, the strong 2.75–2.70 Ga tectonothermal event in the western Shandong Province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components. Combined with craton-wide data, it is shown that the NCC is similar to many other cratons around the world where tectonothermal events of ∼2.7 Ga are well developed. However, the main difference is that in the NCC, superimposed ∼2.5 Ga tectonothermal events were much stronger than in most other cratons.
. ► Late Neoarchean syenogranites are widely distributed in the North China Craton. ► Two phases (2.53–2.52 Ga and 2.52–2.50 Ga) of syenogranite magmatism are recognized. ► There are three types of syenogranites in terms of element and isotope compositions. ► They mark a tectono-magmatic event resulting in stabilization of the craton. At the end of the Neoarchean continental evolution, voluminous syenogranites were emplaced in the North China Craton, together with other magmatic rocks (trondhjemite–tonalite–granodiorite (TTG), monzogranite, diorite, gabbro). Syenogranites are widely distributed in Anshan-Benxi, Qinhuangdao and western Shandong, and also occur in southern Jilin, northern Liaoning, northwestern Hebei and central Henan. Based on geological relationships, degree of metamorphism, deformation and magmatic zircon ages, two phases of syenogranite magmatism are recognized. Rocks produced during the first phase show a gneissic texture and were formed between 2.53 and 2.52 Ga and locally comprise abundant TTG. Rocks of the second phase cut late Neoarchean TTG and supracrustal rocks, display a massive structure, and mainly formed between 2.52 and 2.50 Ga. All syenogranites share the same features in major element compositions, being high in SiO and low in CaO, total FeO, MgO, TiO and P O . However, they are different in trace and REE compositions and can be subdivided into three types. (1) Type 1 shows a large variation in total REE contents, low (La/Yb) ratios, strong negative Eu*/Eu anomalies and Ba depletion; (2) Type 2 is similar to Type 1 but has higher (La/Yb) ratios. (3) Type 3 shows a large variation in total REE and (La/Yb) ratios and significantly do not show strongly negative Eu*/Eu anomalies and Ba depletion. Whole-rock Sm–Nd isotopic compositions show large variations in ( ) values and (Nd) modal ages, ranging from −9.49 to −4.72 and 3.70 to 3.25 Ga (Type 1), 0.55–1.03 and 2.77–2.71 Ga (Type 2) and −2.35 to 1.23 and 2.93–2.66 Ga (Type 3), respectively. Hf isotopic compositions of zircons from three samples have ( ) values and (Hf) ages of 0.7–7.2 and 2.84–2.56 Ga (Type 1), 2.6–7.4 and 2.74–2.56 Ga (Type 2) and 2.1–6.3 and 2.76–2.60 Ga (Type 3). It is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages, and most of them were emplaced during the second phase (2.52–2.50 Ga) in an extensional tectonic regime. The formation of these voluminous syenogranites marks a tectono-magmatic event resulting in stabilization of the North China Craton at the end of the Neoarchean.
► 800–760 Ma bimodal volcanic rocks in the eastern Jiangnan orogen (JO). ► Felsic rocks from the partial melting of juvenile arc crustal materials. ► Diversity of magma sources of the mafic rocks. ► Dominant melting of newly-metasomatized lithospheric mantle implies the early significant subduction. ► Passive rifting resulted from post-orogenic extension in the JO has no direct relationship with rifting of Rodinia. We present a systematic geochronological and geochemical study on 800–760 Ma volcanic rocks in the eastern part of the Jiangnan orogen. The Xucun composite dykes are dated at 805 Ma; the mafic components have OIB-like trace-element patterns and positive anomalies in Zr and Hf. The least-contaminated sample has relatively depleted Nd isotopic features, suggesting the Xucun mafic dykes may have been generated from the partial melting of OIB-like asthenosphere with later crustal contamination. The Xucun felsic dykes have decoupled Nd–Hf isotopes, and the Hf-isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early Neoproterozoic juvenile crustal materials, with minor incorporation of Paleoproterozoic crustal components. The 800–790 Ma Shangshu volcanics include two compositional series: calc-alkaline and tholeiitic. The Shangshu calc-alkaline volcanics in the Minjiawu area have low abundances of LILE, HFSE and high Na O contents and Sr/Y ratios, similar to adakitic rocks. The evident arc-like geochemical features and radiogenic Nd isotopes ( ( ) values of +3.7 to +4.8) suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by Na-rich melts released from the subducted slab. The tholeiitic mafic rocks from the Shangshu bimodal volcanics represent two different magma sources. The partial melting of metasomatized lithospheric mantle led to the formation of arc-like basalts with low TiO contents, negative anomalies in Zr and Hf, and high values of Mg and ( ) (+6.2), whereas the partial melting of asthenospheric mantle generated volcanic rocks with high TiO contents and low positive ( ) (+1.4 to +2.7), without negative anomalies of Nb, Ta, Zr and Hf. The Shangshu felsic rocks were formed by the reworking of early Neoproterozoic juvenile arc crustal materials. The 760 Ma mafic rocks from the Puling bimodal volcanics generally have low TiO contents (<0.9 wt%), nearly flat REE distributions and arc-like trace-element patterns. They may have been generated from the high-degree partial melting of metasomatized lithospheric mantle. One sample has a high TiO content (2.41 wt%) and high ( ) (+6.2), with overall OIB-like trace-element patterns, implying the local partial melting of asthenospheric mantle. The occurrence of significant volumes of bimodal volcanics in the eastern part of the Jiangnan orogen suggests an extensional setting in the period 800–760 Ma. The evident partial melting of newly-metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post-orogenic extension shortly after the Neoproterozoic orogenesis may be a better explanation for the genesis of the mid-Neoproterozoic magmatic rocks in the eastern part of the Jiangnan orogen. Post-orogenic extension may be diachronous along the whole orogenic belt, and probably has no direct relationship with the Rodinia rifting event. A more detailed model is presented to illustrate the evolution of the eastern part of the Jiangnan orogen.