The crustal growth and stabilization of the North China Craton (NCC) relate to three major geological events in the Precambrian: (1) a major phase of continental growth at ca. 2.7 Ga; (2) the amalgamation of micro-blocks and cratonization at ca. 2.5 Ga; and (3) Paleoproterozoic rifting-subduction-accretion-collision tectonics and subsequent high-grade granulite fades metamorphism-granitoid magmatism during ca. 2.0-1.82. The major period of continental growth during 2.9-2.7 Gain the NCC correlates with the global growth of Earth's crust recognized from other regions. The enormous volume of tonalite-trondhjemite-granodiorite (TTG) rocks and associated komatiite-bearing magmatic suites developed during this period possibly suggest the manifestation of plume tectonics. The cratonization of the NCC at the end of Neoarchean at ca. 2.5 Ga (Archean-Proterozoic boundary) through the amalgamation of micro-blocks was accompanied by granulite facies metamorphism and voluminous intrusion of crustally-derived granitic melts leading to the construction of the basic tectonic framework of the NCC. Several Neoarchean greenstone belts surround the micro-blocks and represent the vestiges of older arc-continent collision. The next major imprint in the NCC is the Paleoproterozoic orogenic events during 2.35 -1.82 Ga which involved rifting followed by subduction accretion -collision processes, followed by plume-triggered extension and rifting, offering important insights into modern-style plate tectonics operating in the Paleoproterozoic. Extreme crustal metamorphism and formation of high pressure (HP) and ultra-high temperature (UHT) orogens during 1950-1820 Ma accompanied the subduction-collision process and the suturing of continental blocks within the Paleoproterozoic supercontinent Columbia. Multiple subduction zones with opposing subduction polarity promoted the rapid assembly of crustal fragments of the NCC and their incorporation into the Columbia supercontinent. The HP and HT-UHT granulites demonstrate two main stages of metamorphism at ca. 1.95-1.89 Ga and at ca. 1.85-1.82 Ga, exhuming the basement rocks from lowermost crust level to the lower-middle crust level. With the emplacement of extensive mafic dyke swarms associated with continental rifting, and the intrusion of anorogenic magmatic suites, the evolution of the NCC into a stable continental platform was finally accomplished. (C) 2011 International Association for Gondwana Research. Published by Elsevier BM. All rights reserved.
Sundaland comprises a heterogeneous collage of continental blocks derived from the India-Australian margin of eastern Gondwana and assembled by the closure of multiple Tethyan and back-arc ocean basins now represented by suture zones. The continental core of Sundaland comprises a western Sibumasu block and an eastern Indochina-East Malaya block with an island arc terrane, the Sukhothai Island Arc System, comprising the Linchang, Sukhothai and Chanthaburi blocks sandwiched between. This island arc formed on the margin of Indochina-East Malaya, and then separated by back-arc spreading in the Permian. The Jinghong, Nan-Uttaradit and Sra Kaeo Sutures represent this closed back-arc basin. The Palaeo-Tethys is represented to the west by the Changning-Menglian, Chiang Mai/Inthanon and Bentong-Raub Suture Zones. The West Sumatra block, and possibly the West Burma block, rifted and separated from Gondwana, along with Indochina and East Malaya in the Devonian and were accreted to the Sundaland core in the Triassic. West Burma is now considered to be probably Cathaysian in nature and similar to West Sumatra, from which it was separated by opening of the Andaman Sea basin. South West Borneo and/or East Java-West Sulawesi are now tentatively identified as the missing "Argoland" which must have separated from NW Australia in the Jurassic and these were accreted to SE Sundaland in the Cretaceous. Revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of Sundaland and adjacent regions are presented. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Central Asian Orogenic Belt contains many Precambrian crustal fragments whose origin is unknown, and previous speculations suggested these to be derived from either Siberia, Tarim or northern Gondwana. We present an age pattern for detrital and xenocrystic zircons from Neoproterozoic to Palaeozoic arc and microcontinental terranes in Mongolia and compare this with patterns for Precambrian rocks in southern Siberia, the North China craton, the Tarim craton and northeastern Gondwana in order to define the most likely source region for the Mongolian zircons. Our data were obtained by SHRIMP II, LA-ICP-MS and single zircon evaporation and predominantly represent arc-related low-grade volcanic rocks and clastic sediments but also accretionary wedges and ophiolitic environments. The Mongolian pattern is dominated by zircons in the age range ca. 350-600 and 700-1020 Ma as well as minor peaks between ca. 1240 and 2570 Ma. The youngest group reflects cannibalistic reworking of the Palaeozoic arc terranes, whereas the Neoproterozoic to late Mesoproterozoic peak reflects both reworking of the arc terranes as well as Neoproterozoic rifting and a Grenville-age crust-formation event. The 700-1020 Ma peak does not exist in the age spectra of the Siberian and North China cratons and thus effectively rules out these basement blocks as potential source areas for the Mongolian zircons. The best agreement is with the Tarim craton where a major Grenville-age orogenic event and early Neoproterozoic rifting have been identified. The age spectra also do not entirely exclude northeastern Gondwana as a source for the Mongolian zircons, but here the Neoproterozoic age peak is related to the Pan-African orogeny, and a minor Grenville-age peak may reflect a controversial orogenic event in NW India. Our Mongolian detrital and xenocrystic age spectrum suggests that the Tarim craton was the main source, and we favour a tectonic scenario similar to the present southwestern Pacific where fragments of Australia are rifted off and become incorporated into the Indonesian arc and microcontinent amalgamation that will evolve into a future orogenic belt. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The geometry and timing of amalgamation of the North China Craton have been controversial, with three main models offering significantly different interpretations of regional structure, geochronology, and geological relationships. One model suggests that the Eastern and Western Blocks of the NCC formed separately in the Archean, and an active margin was developed on the Eastern Block between 2.5 and 1.85 Ga, when the two blocks collided above an east-dipping subduction zone. A second presumes the Eastern Block rifted from an unknown larger continent at circa 2.7 Ga, and experienced a collision with an arc (perhaps attached to the western block) above a west-dipping subduction zone at 2.5 Ga, and the 1.85 Ga metamorphism is related to a collision along the northern margin of the craton when the NCC joined the Columbia supercontinent. A third model suggests two collisions in the Central Orogenic Belt, at 2.1 and 1.88 Ga, but recognizes an early undated deformation event. Recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models, showing that subduction beneath the Central Orogenic Belt was west-directed, and that there is a second, west-dipping paleosubduction zone located to the east of the COB dipping beneath the Western Block (Ordos Craton). The boundaries identified through geophysics do not correlate with the boundaries of the Trans-North China Orogen suggested in the first model, and the subduction polarity is opposite that predicted by that model. High-pressure granulite facies metamorphism at 1.85 Ga is not restricted to the "TNCO" as suggested by the first model, but is documented across the NCC, as predicted by the second model, suggesting a major continent-continent collision along the north margin of the craton at 1.85 Ga. Further, it has recently been shown that in the southern "TNCO", there is no record of metamorphism at circa 1.85 Ga, but only at 2.7-2.5 Ga, showing that the "TNCO", as defined as a circa 1.85 Ga orogen, does not exist. This is further confirmed by recent Re-Os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern COB is late Archean in age, and that a province in the northern NCC is circa 1.8 Ga, correlating with the proposed collision belt of the NCC with the Columbia supercontinent across the entire NCC. The COB is an Archean convergent belt, re-worked in the Paleoproterozoic, and the Paleoproterozoic tectonism is widespread across the NCC, as predicted by the model whereby the previously amalgamated Eastern and Western Blocks experienced a continental collision with Columbia at circa 1.85 Ga, but uplift/exhumation rates are slow, necessitating a re-evaluation of the tectonic models of the NCC. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Paleoproterozoic Jiao-Liao-Ji Belt lies in the Eastern Block of the North China Craton, with its southern segment extending across the Bohai Sea into the Jiaobei massif. High-pressure pelitic and mafic granulites have been recently recognized in the Paleoproterozoic Jingshan Group (Jiaobei massif). New SHRIMP U-Th-Pb geochronology combined with cathodoluminescence (CL) imaging of zircon has been applied to the determination of the timing of the metamorphism of the high-temperature and high-pressure granulites and associated gneisses and marbles. Metamorphic zircons in these high-pressure granulites, gneisses and marbles occur as either single grains or overgrowth (or recrystallization) rims surrounding and truncating oscillatory-zoned magmatic zircon cores. Metamorphic zircons are all characterized by nebulous zoning or being structureless, with high luminescence and relatively low Th/U values. Metamorphic zircons from two high-pressure mafic granulites yielded Pb-207/Pb-206 ages of 1956 +/- 41 Ma and 1884 +/- 24 Ma. One metamorphic zircon from a garnet-sillimanite gneiss also gave an apparent Pb-207/Pb-206 age of 1939 +/- 15 Ma. These results are consistent with interval of ages of c. 1.93-1.90 Ga already obtained by previous studies for the North and South Liaohe Groups and the Laoling Group in the northern segment of the Jiao-Liao-Ji Belt Metamorphic zircons from a high-pressure pelitic granulite and two pelitic gneisses yielded weighted mean Pb-207/Pb-206 ages of 1837 +/- 8 Ma, 1821 +/- 8 Ma and 1836 +/- 8 Ma respectively. Two diopside-olivine-phlogopite marbles yielded weighted mean Pb-207/Pb-206 ages of 1817 +/- 9 Ma and 1790 +/- 6 Ma. These Paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c. 1.85 Ga produced from the Ji'an Group in the northern segment of the Jiao-Liao-Ji Belt and c. 1.86-1.80 Ga obtained for the high-pressure pelitic granulites from the Jingshan Group in the southern segment. As this metamorphic event was coeval with the emplacement of A-type granites in the Jiao-Liao-Ji Belt and its adjacent areas, it is interpreted as having resulted from a post-orogenic or anorogenic extensional event. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The origin of the Antarctic continent can be traced to a relatively small late Archaean cratonic nucleus centred on the Terre Adelie regions of East Antarctica and the Gawler Craton region of South Australia. From the late Archaean to the present, the evolution of the proto-Antarctic continent was remarkably dynamic with quasi-continuous growth driven by accretionary or collisional events, episodically punctuated by periods of crustal extension and rifting. The evolution of the continent can be broken into seven main steps: (1) late Palaeoproterozoic to middle Mesoproterozoic accretion and collision added crust first to the Antarctic nucleus's eastern margin, then to its western margin. These events resulted in the incorporation of the Antarctic nucleus within a single large continent that included all of Proterozoic Australia, a more cryptic Curnamona-Beardsmore Craton and most probably Laurentia (2) Rifting in the middle to late Mesoproterozoic separated a block of continental crust of unknown dimensions to form an ocean-facing margin, the western edge of which was defined by the ancestral Darling Fault in Western Australia and its unnamed continuation in Antarctica. (3) Inversion of this margin followed shortly and led to the Grenville aged collision and juxtaposition of proto-Antarctica with the Crohn Craton, a continental block of inferred Archaean and Palaeoproterozoic age that now underlies much of central East Antarctica. The Pinjarra Orogen, exposed along the coast of Western Australia, defines the orogenic belt marking this collision. In Antarctica the continuation of this belt has been imaged in sub-ice geophysical datasets and can be inferred from sparse outcrop data and via the widespread dispersal of syn-tectonic zircons. (4) Tectonic quiescence from the latest Mesoproterozoic to the Cryogenian was the forerunner to Ediacaran rifting that separated Laurentia and the majority of the Curnamona-Beardsmore craton from the amalgam of East Antarctica and Australia. The result was the formation of the ancestral Pacific Ocean. (5) The rifting of Laurentia was mirrored by convergence along the opposing margin of the continent. Convergence ultimately sutured material with Indian and African affinities during a series of Ediacaran and Cambrian events related to the formation of Gondwana. These events added much of the crust that today defines the East Antarctic coastline between longitudes 30 degrees W and 100 degrees E. (6) The amalgamation of Gondwana marked a shift in the locus of subduction from between the pre-Gondwana cratons to Gondwana's previously passive Pacific margin. The result was the establishment of the accretionary Terra Australis and Gondwanide orogenies. These were to last from the late Cambrian to the Cretaceous. and together accreted vast sequences of Gondwana derived sediment as well as fragments of older and allochthonous or para-allochthonous continental crust to Gondwana's Pacific margin. (7) The final phases of accretion overlapped with the initiation of extension and somewhat later rifting within Gondwana. Extension started in the late Carboniferous, although continental separation did not begin until the middle Jurassic. Gondwana then fragmented sequentially with Africa-South America, India, Australia and the finally the blocks of New Zealand separating between the middle Jurassic and the late Cretaceous. The late Cretaceous separation of Antarctica and Australia split the original Antarctic nucleus, terminating more than 2.4 billion years of shared evolution. The slightly younger separation of New Zealand forme the modern Antarctic continent. Crown Copyright (C) 2010 Published by Elsevier B.V, on behalf of International Association for Gondwana Research. All rights reserved.
The Ediacaran Doushantuo Formation (ca. 635-551 Ma) in South China contains exceptionally well-preserved fossils of multicellular eukaryotes including early animals, and it is one of the most intensively investigated Ediacaran units in the world. Various stratigraphic methods including litho-, chemo-, bio-, and sequence-stratigraphy have been applied to establish a stratigraphic framework for the Doushantuo Formation, but so far regional correlation across the basin relies heavily on two distinctive marker beds, the cap carbonate at the base and the organic-rich black shale at the top of the Doushantuo Formation. The majority of the Doushantuo Formation in the Yangtze platform was deposited on a rimmed carbonate shelf, with a shelf margin shoal complex that restricted the shelf lagoon from the open ocean. Large facies variations are observed in the shallow margins of the shelf lagoon and in the shelf margin-to-slope transition, where depositional environments were near the chemocline of the stratified, anoxic/euxinic shelf lagoon and of the broader Nanhua basin, respectively. Chemocline instability in the shelf lagoon and in the Nanhua basin caused local geochemical cycling, resulting in significant variations in carbon and sulfur isotopes and in redox-sensitive elemental concentrations. Most benthic eukaryotic fossils (including animal fossils) of the Doushantuo Formation have been found from the shallow margins of the shelf lagoon and from the shelf margin-slope transition, but rarely from deep-water environments that may have been below the chemocline for most of the Doushantuo time, implying the sensitivity of eukaryotes to paleogeographically controlled chemocline fluctuations. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Subduction erosion occurs at all convergent plate boundaries, even if they are also accretionary margins. Frontal subduction erosion results from a combination of erosion and structural collapse of the forearc wedge into the trench, and basal subduction erosion by abrasion and hydrofracturing above the subduction channel. High rates of subduction erosion are associated with relatively high convergence rates (> 60 mm/yr) and low rates of sediment supply to the trench ( 440 km(3)/km/my, vary temporally as a function of these same factors, as well as the subduction of buoyant features such as seamount chains, submarine volcanic plateaus, island arcs and oceanic spreading ridge, due to weakening of the forearc wedge. Revised estimates of long-term rates of subduction erosion appropriate for selected margins, including SW Japan (>= 30 km(3)/km/my since 400 Ma), SW USA (30 km(3)/km/my since 150 Ma), Peru and northern Chile (50-70 km(3)/km/my since > 150 Ma), and central (115 km(3)/km/my since 30 Ma) and southernmost Chile (30-35 km(3)/km/my since 15 Ma), are higher than in previous compilations. Globally, subduction erosion is responsible for > 1.7 Armstrong Units (1 AU=1 km(3)/yr) of crustal loss, 33% of the similar to 5.25 AU of yearly total crustal loss, and more than any one other of sediment subduction (1.65 AU), continental lower crustal delamination (>= 1.1 AU), crustal subduction during continental collision (0.4 AU), and/or subduction of rock-weathering generated chemical solute that is dissolved in oceanic crust (0.4 AU). The paucity of pre-Neoproterozoic blueschists suggests that global rates of subduction erosion were probably greater in the remote past, perhaps due to higher plate convergence rates. Subducted sediments and crust removed from the over-riding forearc wedge by subduction erosion may remain in the crust by being underplated below the wedge, or these crustal debris may be carried deeper into the source region of arc magmatism and incorporated into arc magmas by either dehydration of the subducted slab and the transport of their soluble components into the overlying mantle wedge source of arc basalts, and/or bulk melting of the subducted crust to produce adakites. In selected locations such as in Chile, Costa Rica, Japan and SW USA, strong cases can be made for the temporal and spatial correlations of distinctive crustal isotopic characteristics of arc magmas and episodes or areas of enhanced subduction erosion. Nevertheless, overall most subducted crust and sediment, > 90% (> 3.0 AU), is transported deeper into the mantle and neither underplated below the forearc wedge nor incorporated in arc magmas. The total current rate of return of continental crust into the deeper mantle, the most important process for which is subduction erosion, is equal to or greater than the estimates of the rate at which the crust is being replaced by arc and plume magmatic activity, indicating that currently the continental crust is probably slowly shrinking. However, rates of crustal growth may have been episodically more rapid in the past, most likely at times of supercontinent breakup, and conversely, rates of crustal destruction may have also been higher during times of superontinent amalgamation. Thus the supercontinent cycle controls the relative rates of growth and/or destruction of the continental crust. Subduction erosion plays an important role in producing and maintaining this cycle by transporting radioactive elements from the crust into the mantle, perhaps as deep as the 670 km upper-to-lower mantle transition, or even deeper down to the core-mantle boundary, where heating of this subducted crustal material initiates plumes and superplumes. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
U-Pb age and Hf-O isotope data of zircons from the mafic-ultramafic complexes and their related rhyolites and granites in the Eastern Tianshan and Beishan regions were obtained by SIMS and LA-ICPMS. The Hf-O isotopic composition and Hf model age of the zircons are closely related to their tectonic setting and formation age. The zircons from the mafic-ultramafic complexes in the Bogeda-Haerlike and Jueluotage belts have higher epsilon(Hf)(t) ( +8- +17) and lower delta O-18 (+4 parts per thousand-+6 parts per thousand.), whereas most of the zircons in the gabbros, granites and rhyolite from the Middle Tianshan Massif and Beishan Rift display lower epsilon(Hf)(t) (0- + 8) and higher delta O-18 (+5 parts per thousand- + 8 parts per thousand). The positive epsilon(Hf)(t) and relatively lower delta O-18 values suggest that these mafic-ultramafic complexes were derived from depleted mantle which was subjected to subduction-related modification processes. The Hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the Central Asian Orogenic Belt during the Paleozoic. The Hf model ages of the analyzed zircons, together with the regional geology suggest that the Beishan area had a northward subduction, possibly from ca. 900 Ma to ca. 400 Ma, whereas the Eastern Tianshan had a south-directed subduction most likely from ca. 600 Ma to ca. 310 Ma. Additionally, zircons with ca. 280 Ma U-Pb ages display wider and more scattered epsilon(Hf)(t) and delta O-18 variations than the relatively older and younger ones, which further support the Early Permian mantle plume model. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
This paper reports some new results from U-Pb geochronological, Hf isotopic and REE geochemical studies of detrital zircons in the Ordovician sandstones from South Jiangxi within Cathaysia. 426 groups of U-Pb age determinations define five major age populations: 2560-2380 Ma (a peak of 2460 Ma), 1930-1520 Ma (a peak of 1700 Ma), 1300-900 Ma (a major peak at 970 Ma and two subordinate peaks at 1250 Ma and 1130 Ma), 850-730 Ma (a prominent peak of 780 Ma) and 670-530 Ma (a major peak at 540 Ma and a subordinate peak at 650 Ma). We also report zircon U-Pb concordia age of 3.96 Ga, which is the oldest age so far obtained from Cathaysia. The age peak at 2460 Ma correlates with similar ages reported for Neoarchean global continental growth. The 1930-1520 Ma population broadly overlaps with the time of amalgamation and disruption of the Columbia supercontinent. The major age peak at 970 Ma and two secondary peaks at 1250 Ma and 1130 Ma reflect multiple tectonothermal events associated with the assembly of Rodinia. Similar ages are widely reported from the South China Craton (SCC). Our study reveals that the 850-730 Ma population is consistent with the breakup period of Rodinia, suggesting that the SCC within Rodinia began to break up since 850 Ma. Geologically, the evidence for this breakup event is widespread and presented by Neoproterozoic granites, bimodal igneous rocks, basic dyke swarms and formation of continental rift type basins. Our study also reveals a 670-530 Ma population that correlates well with the assembly of Gondwana during end Neoproterozoic. However, direct geological evidence for this event has not yet been found within the studied area. Furthermore, the Hf isotopic model age data suggest two major stages of crustal evolution within Cathaysia. The first is the event dated at 1.6-2.8 Ga and the second one at 3.5-3.9 Ga. The zircons show a large range of epsilon Hf(t) values from +8.64 to -30.54, suggesting that they have different origins with a similar age of crystallization. The fact that most detrital zircons show negative epsilon Hf(t) values suggests the ancient provenances of Cathaysia were dominated by reworked crustal materials. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Neoarchean to Paleoproterozoic Jianping complex in the western Liaoning province is an important component of the Precambrian basement of the North China Craton. This region connects the Neoarchean Fengning-Chengde complex in the west, the Neoarchean to Paleoproterozoic North Chaoyang complex in the northeast and the Yixian-Fuxin Archean greenstone belt in the east. The Precambrian Jianping complex is dominantly composed of metamorphosed supracrustal sequence and dioritic to granitoid gneisses (tonalite-trondhjemite-granodiorite, TTG). Here we present results from LA-ICPMS zircon U-Pb isotope dating from the various lithological units in this complex, which reveal that the magmatic precursors of the metavolcanic rocks associated with the supracrustal sequence were generated during 2555-2550 Ma, and up to 2615 Ma. A major magmatic pulse of dioritic to granitic suite occurred during 2538 to 2495 Ma. This was followed by ca. 2485 Ma granulite facies metamorphism and a retrograde event at ca. 2450-2401 Ma. Our data also reveal a major charnockite emplacement event in this region at ca. 1694 Ma. Integrating our new results with the available geologic and previous geochronological data, we identify three major growth stages in the crustal evolution history of the northern margin of the Eastern Continental Block of the North China Craton at ca. 2550-2495 Ma, ca. 2403-2394 Ma, and ca.1700 Ma. The LA-ICP-MS zircon data provide new insights on the Neoarchean to Paleoproterozoic tectonothermal evolution history in the northern margin of the Eastern Continental Block of the North China Craton. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Dengfeng complex is located on the southern margin of the North China Craton (NCC) and belongs to the southern portion of the Trans-North China Orogen. This terrane is important to understand the formation and evolution of NCC during late Neoarchean (similar to 2.5 Ga). The Dengfeng complex is well exposed in the Junzhao region and comprises two distinct lithologic units: supracrustal assemblage and plutonic rocks. LA-ICPMS magmatic zircon U-Pb age data shows that the rocks formed within the range of 2547-2504 Ma. The available Hf isotope data indicate that the majority of ca. 2.5 Ga zircons from the Dengfeng complex have high epsilon(Hf)(t) values close to the initial Hf isotope ratios of the contemporaneous depleted mantle. These data indicate that the rocks in the Dengfeng represent juvenile crust. The TTG gneisses in the Dengfeng complex display low Mg-# (41-48), MgO (<2 wt.%), Cr (6-14 ppm), Ni (9-22 ppm) contents and low Nb/Ta ratio (6-12), which are interpreted to have been produced by the partial melting of a flatly subducted slab. The metadiorites of the Dengfeng complex are typically characterized by high Mg-# (59-69), MgO (3.5-6.6 wt.%), Ni (82-130 ppm) and Cr (148-237 ppm) abundances, elevated Sr (1759-1927 ppm) and Ba (1742-2289 ppm) concentrations, and high LREE (La-N = 38-487). Such geochemical features are similar to Archean sanukitoids. A two-stage model is applied here to explain the genesis of metadiorites of Dengfeng complex: (1) firstly, the mantle is metasomatized either by melts or by aqueous fluids from a subducted slab; (2) subsequently, sanukitoid magmas were produced by partial melting of the hybridized mantle. Furthermore, the amphibolites of supracrustal rocks have a mixture of MORB- and arc-like geochemical affinities, suggesting the development of a back-arc in the southern NCC at ca. 2.5 Ga. The contemporary late Neoarchean TTGs, sanukitoids and MORB-back arc association may represent a late Neoarchean tectonic melange, implying a Neoarchean subduction-accretion process which would suggest that modern-style plate tectonics processes was probably initiated in the southern NCC by 2.5 Ga. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Dengfeng complex is located on the southern margin of the North China Craton (NCC) and belongs to the southern portion of the Trans-North China Orogen. This terrane is important to understand the formation and evolution of NCC during late Neoarchean (~2.5Ga). The Dengfeng complex is well exposed in the Junzhao region and comprises two distinct lithologic units: supracrustal assemblage and plutonic rocks. LA-ICPMS magmatic zircon U-Pb age data shows that the rocks formed within the range of 2547-2504Ma. The available Hf isotope data indicate that the majority of ca. 2.5Ga zircons from the Dengfeng complex have high I[micro].sub.Hf(t) values close to the initial Hf isotope ratios of the contemporaneous depleted mantle. These data indicate that the rocks in the Dengfeng represent juvenile crust. The TTG gneisses in the Dengfeng complex display low Mg.sup.# (41-48), MgO (<2wt.%), Cr (6-14ppm), Ni (9-22ppm) contents and low Nb/Ta ratio (6-12), which are interpreted to have been produced by the partial melting of a flatly subducted slab. The metadiorites of the Dengfeng complex are typically characterized by high Mg.sup.# (59-69), MgO (3.5-6.6wt.%), Ni (82-130ppm) and Cr (148-237ppm) abundances, elevated Sr (1759-1927ppm) and Ba (1742-2289ppm) concentrations, and high LREE (La.sub.N =38-487). Such geochemical features are similar to Archean sanukitoids. A two-stage model is applied here to explain the genesis of metadiorites of Dengfeng complex: (1) firstly, the mantle is metasomatized either by melts or by aqueous fluids from a subducted slab; (2) subsequently, sanukitoid magmas were produced by partial melting of the hybridized mantle. Furthermore, the amphibolites of supracrustal rocks have a mixture of MORB- and arc-like geochemical affinities, suggesting the development of a back-arc in the southern NCC at ca. 2.5Ga. The contemporary late Neoarchean TTGs, sanukitoids and MORB-back arc association may represent a late Neoarchean tectonic melange, implying a Neoarchean subduction-accretion process which would suggest that modern-style plate tectonics processes was probably initiated in the southern NCC by 2.5Ga.
The eastern part of the Central Tianshan Belt in China is characterized by abundant granitoids, which are not only indicators of the interaction between crust and mantle but also can be used for tracking the tectonic evolutionary history of the orogen. Four granitic plutons, muscovite granites (MG), biotite monzonitic granites (BMG), biotite granites (BG), and alkali granites (AG), have been recognized to have intruded in the Mishigou area, Central Tianshan Belt. The MG has pronounced a S-type affinity, which is strongly peraluminous with a high aluminum saturation index, and displays positive Rb, Th, U and LREE anomalies with a strong negative Ba, Nb, Sr, P. Ti and Eu anomalies. Therefore, the magma of the MG is considered to have been derived from melting of thickened continental crust, which is composed mainly of the sediments eroded from a pre-Silurian continental island-arc. The U-Pb zircon age of 424.5 +/- 2.6 Ma constrains the age of MG and crust thickening. The BG and BMG granitoids display I-type geochemical features and have affinities to subduction arcs. Both of them are characterized by calc-alkaline peraluminous granites with a significant LREE enrichment and a negative Eu anomaly, as well as a depletion in Ba, Nb, Ta, Sr, P and Ti and an enrichment in Rb, Th, U, Nd, Zr and Sm. These characteristics indicate that they were derived from a mixed magma source between lower continental crust and the input of components derived from the mantle wedge above the subduction zone. The LA-ICPMS zircon U-Pb ages of 411 +/- 4.7 Ma and 402 +/- 3.4 Ma represent the formation ages of the BG and BMG granitoids, respectively, which also constrain that the subduction of the South Tianshan oceanic crust also occurred during Early Devonian time. The AG shows a typical A-type granite affinity with depletion of Nb, Ta, Sr, P, Ti and Y. It is interpreted that AG was formed in a within-plate tectonic setting, and related to continental up-doming and rifting zones as a consequence of the extensional collapse after the collision of the Tianshan orogen. The U-Pb zircon age of 290 +/- 5.1 Ma represents the formation age of the AG, and the time of the extensional collapse and rifting. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The North China Craton (NCC) was subjected to an extensional regime after the Luliang movement at similar to 1.8 Ga and then was covered by an extensive Meso- to Neoproterozoic sedimentary succession, namely the Changcheng, Jixian and Qingbaikou Groups in ascending order. We report age spectra for detrital zircons and monazites, Hf isotopic systematics of detrital zircons, and whole-rock chemical and Nd isotopic compositions for sediments from the succession in the Ming Tombs area, Beijing, one of the typical Meso- to Neoproterozoic areas in the NCC. Detrital zircons of six sedimentary samples have two distinct age peaks at similar to 2.52 Ga and similar to 1.85 Ga. There are some detrital zircons at 2.4-2.0 Ga but none at 2.3 Ga and only a few >2.7 Ga. The detrital zircon age spectra change with time. Sediments in the lower succession (Changcheng Group) and in the upper successions (Jixian and Qinbaikou Groups) are dominated by significant detrital zircon populations of late Neoarchean and late Paleoproterozoic ages, respectively. The similar to 2.5 Ga detrital zircons of the Changcheng Group have epsilon(Hf)(2.5 Ga) values and t(DM)(Hf) model ages mainly ranging from -2 to +7 and 2.8 to 2.7 Ga, respectively. Detrital monazites of a sample from the Jixian Group exhibit a major age peak between 1.95 and 1.80 Ga with some data between 2.0 and 1.95 Ga. The sedimentary rocks of the Changcheng Group are characterized by high K2O contents (mostly 7.09-15.20%) and insignificant Eu anomalies (Eu/Eu* = 0.71-1.16). They have t(DM)(Nd) model ages ranging from 2.70 to 2.43 Ga, being older than the t(DM)(Nd) ages (2.11 and 1.99 Ga) of sedimentary samples from the Qingbaikou Group. Based on a comparison with ages for the early Precambrian (>1.8 Ga) basement of the NCC, it can be concluded that (1) the sediments of the Meso- to Neoproterozoic cover were undoubtedly derived from the NCC itself or once neighboring terranes; (2) variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late Paleoproterozoic rocks were exposed at the surface after the upper crustal late Neoarchean rocks had already been eroded. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Age-dating of detrital zircons from 22 samples collected along, and adjacent to, the Yarlung-Tsangpo suture zone, southern Tibet provides distinctive age-spectra that characterize important tectonostratigraphic units. Comparisons with data from Nepal, northern India and the Lhasa and Qiangtang terranes of central Tibet constrain possible sources of sediment, and the history of tectonic interactions. Sedimentary rocks in the Cretaceous-Paleogene Xigaze terrane exhibit strong Mesozoic detrital zircon peaks (120 and 170 Ma) together with considerable older inheritance in conglomeratic units. This forearc basin succession developed in association with a continental volcanic arc hinterland in response to Neotethyan subduction under the southern edge of the Eurasia. Conspicuous sediment/source hinterland mismatches suggest that plate convergence along this continental margin was oblique during the Late Cretaceous. The forearc region may have been translated >500 km dextrally from an original location nearer to Myanmar. Tethyan Himalayan sediments on the other side of the Yarlung-Tsangpo suture zone reveal similar older inheritance and although Cretaceous sediments formed 1000s of km and across at least one plate boundary from those in the Xigaze terrane they too contain an appreciable mid-Early Cretaceous (123 Ma) component. In this case it is attributed to volcanism associated with Gondwana breakup. Sedimentary overlap assemblages reveal interactions between colliding terranes. Paleocene Liuqu conglomerates contain a cryptic record of Late Jurassic and Cretaceous rock units that appear to have foundered during a Paleocene collision event prior the main India-Asia collision. Detrital zircons as young as 37 Ma from the upper Oligocene post-collisional Gangrinboche conglomerates indicate that subduction-related convergent margin magmatism continued through until at least Middle and probably Late Eocene along the southern margin of Eurasia (Lhasa terrane). Although the ages of detrital zircons in some units appear compatible with more than one potential source with care other geological relationships can be used to further constrain some linkages and eliminate others. The results document various ocean closure and collision events and when combined with other geological information this new dataset permits a more refined understanding of the time-space evolution of the Cenozoic India-Asia collision system. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Altai-Sayan Fold Belt epitomises how Phanerozoic island-arc continental crust contributed to the growth of the Palaeo-Eurasian continent. Its formation is dominated by the closure of the Palaeo-Asian Ocean (PAO) when island-arc systems and Gondwana-derived terranes accreted to the Siberian margin. Relics of the PAO related tectonic units and associated granitoids occur along the ophiolitic Charysh-Terekta-Ulagan-Sayan suture (CTUSs) between Gorny-Altai and Altai-Mongolia. Zircon LA-ICP-MS U/Pb dating of this igneous record constrains the multi-stage geodynamic PAO evolution. Primitive Kuznetsk-Altai island-arc crust formed at the Siberian margin during the Ediacaran-Early Cambrian (525-555 Ma). This island-arc matured during the Middle-Late Cambrian (similar to 510 Ma) and was consumed by PAO subduction in the Late Cambrian-Early Ordovician (480-490 Ma) forming the Siberian Early Caledonian accretion-collision belt. South of the CTUSs, granitic magmatism occurred within Gondwana-derived Altai-Mongolia during the Middle-Late Ordovician (450-470 Ma) Palaeo-Kazakhstan assembly and during a Silurian-Early Devonian (400-425 Ma) Andean-type collision. Middle-Late Devonian (360-395 Ma) granitoids were emplaced as a result of the collision of Altai-Mongolia with Siberia. During final Pangaea amalgamation, the suture was strike-slip reactivated (associated magmatism similar to 295 Ma). The youngest (220-255 Ma) sampled granitoids originated in an intra-plate setting. (C) 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Karamay area, situated in the eastern part of Western Junggar, Southern Altaids, contains an ophiolitic melange with ultramafic rocks, gabbro, basalt, chert and limestone, which show typical block-in-matrix structures, and coherent turbidites and tuffs. These lithological associations are interpreted as incoherent and coherent series formed in an accretionary complex. On the basis of detailed field mapping and analyses of the asymmetry of imbricate thrusts, duplexes, tilted structures, shear band cleavages, and the NW-verging inclined to overturned folds, we conclude that the overall movement in the accretionary complex was top-to-the-NW. The youngest tuff involved in the deformation contains detrital zircons that have a U-Pb age (LA-MC ICP-MS) of 308 +/- 7 Ma. Ar-39-Ar-40 resistance furnace step-heating of amphibole separates from a diorite dike, which cuts the folded and imbricated rocks in the accretionary prism, yielded a plateau age of 307 +/- 2 Ma. Consequently, the age of the deformation in the prism is tightly constrained at 307-308 Ma, implying that the deformation occurred in an extremely short time-span during SE-ward subduction. Combined contemporaneous occurrence of Baogutu adakite, high-Mg, Sr-enriched and Y-poor dioritic dikes, Miaoergou charnockite, and Maliya mafic rocks, we further suggest the accretionary complex was cut by near-trench volcanic rocks and plutons possibly due to interaction with a spreading ridge. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
There is a broad consensus that the North China Craton formed by the amalgamation of the Western and Eastern Blocks along the nearly N-S trending Trans-North China Orogen. However, it still remains controversial whether the Western and Eastern Blocks collided at similar to 2.5 Ga or similar to 1.85 Ga. U-Pb ages and Hf isotopic data of detrital zircons from foreland basins in the Trans-North China Orogen can place rigorous constraints on this controversial issue. One of such foreland basins is represented by the Hutuo Group in the Wutai Complex in the middle sector of the Trans-North China Orogen. The sequences of the Hutuo Group range from lower basal conglomerates and sandstones (Doucun Subgroup), through clastic sediments, dolomites and meta-basalts (Lower Dongye Subgroup), phyllites and dolomites (Upper Dongye Subgroup), to coarse-grained sandstones and conglomerates at the top (Guojiazhai Subgroup), most of which contain large amounts of detrital zircons. The detrital zircons from the Doucun, Lower Dongye, Upper Dongye and Guojiazhai Subgroups yield concordant (207)p/Pb-206 ages of 2.11-3.88 Ga, 2.01-2.84 Ga, 1.88-2.72 Ga and 1.92-2.65 Ga respectively, the majority with Neoarchean to Paleoproterozoic ages. The presence of similar to 1.88 Ga detrital zircons in the Upper Dongye and Guojiazhai Subgroups indicates that they deposited at some time after similar to 1.88 Ga. This is in accordance with collision between the Eastern and Western Blocks along the Trans-North China Orogen to form the North China Craton at similar to 1.85 Ga. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The left-lateral strike-slip shearing along the Ailao Shan-Red River (ASRR) shear zone in the Southeastern Tibet, China, has been widely advocated to be a result of the Indian-Eurasian plate collision and post-collisional processes. The Diancang Shan (DCS) massif, which occurs at the northwestern extension of the Ailao Shan massif, is a typical high-grade metamorphic complex aligned along the ASRR tectonic belt. Structural and microstructural analysis of the plutonic intrusions in the DCS revealed different types of granitic intrusions spatially confined to the shear zone and temporally related to the left-lateral shearing along the ASRR shear zone in the DCS massif. The combined structural and geochronological results of SHRIMP-II and LA-ICP-MS zircon U/Pb isotopic dating have revealed successive magmatic intrusions and crystallization related to the Oligo-Miocene shearing in the DCS massif. The pre-, early- and syn-kinematic emplacements are linked to regional high-temperature deformation (lower amphibolite facies) at relatively deep crustal levels. The zircon U/Pb geochronological results suggest that the left-lateral ductile shearing along the ASRR shear zone was initiated at ca. 31 Ma, culminated between ca. 27 and 21 Ma resulting in high-temperature metamorphic conditions and slowed down at ca. 20 Ma at relatively low-temperatures. Crown Copyright (C) 2010 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. All rights reserved.