Time filter

Source Type

Song S.,Peking University | Niu Y.,Durham University | Niu Y.,Chinese Academy of Sciences | Su L.,Chinese University of Geosciences | And 2 more authors.
Geochimica et Cosmochimica Acta

Modern adakite or adakitic rocks are thought to result from partial melting of younger and thus warmer subducting ocean crust in subduction zones, with the melt interacting with or without mantle wedge peridotite during ascent, or from melting of thickened mafic lower crust. Here we show that adakitic (tonalitic-trondhjemitic) melts can also be produced by eclogite decompression during exhumation of subducted and metamorphosed oceanic/continental crust in response to continental collision, as exemplified by the adakitic rocks genetically associated with the early Paleozoic North Qaidam ultra-high pressure metamorphic (UHPM) belt on the northern margin of the Greater Tibetan Plateau. We present field evidence for partial melting of eclogite and its products, including adakitic melt, volumetrically significant plutons evolved from the melt, cumulate rocks precipitated from the melt, and associated granulitic residues. This "adakitic assemblage" records a clear progression from eclogite decompression and heating to partial melting, to melt fractionation and ascent/percolation in response to exhumation of the UHPM package. The garnetite and garnet-rich layers in the adakitic assemblage are of cumulate origin from the adakitic melt at high pressure, and accommodate much of the Nb-Ta-Ti. Zircon SHRIMP U-Pb dating shows that partial melting of the eclogite took place at ~435-410. Ma, which postdates the seafloor subduction (>440. Ma) and temporally overlaps the UHPM (~440-425. Ma). While the geological context and the timing of adakite melt formation we observe differ from the prevailing models, our observations and documentations demonstrate that eclogite melting during UHPM exhumation may be important in contributing to crustal growth. © 2014 Elsevier Ltd. Source

Luo Z.,Chinese University of Geosciences | Luo Z.,Northwest University, China | Zhang H.,Chinese University of Geosciences | Zhang H.,Northwest University, China | Diwu C.,Northwest University, China
Acta Petrologica Sinica

The Huai'an area of Northwest Hebei stands on the central-north of the North China Craton (NCC). Intermediat pyroxene-granulite in this area can provide important clues for the timing of regional metamorphism. The mineral assemblages indicate that it was undergone mid-pressure granulite metamorphism and overlapped by amphibolite facies retrograde. Zircon U-Pb, Lu-Hf isotope and trace elements compositions are analyzed. As a result, zircon cores with oscillatory zoning exhibit various 207Pb/206Pb ages in range of 2542-1902Ma (1σ. They show a positive relations with Th/U (0. 10-1. 92) ratios, σHREE (59. 1 × 10-6-452 × 10 -6) and εHf(t) values, indicating their different degrees of lattice recrystallization due to the postdated metamorphism. Zircon grains with the most concordant and old ages yield a mean 207Pb/ 206Pb age of 2471 ±18 Ma (2σ, MSWD =6.1). This age is interpreted to be the emplacement age of the protolith rocks. Their depleted Hf model ages of tDM and tDM c vary in range of 2550-2621Ma (peak at 2586Ma), and 2596-2716Ma (peak at 2665Ma), respectively. Whereas, they have εHf(t) values of 4. 1-6. 7, indicating that the protolith rocks is dominately derived from the Late Archean juvenile crust. The metamorphic over growth zircons occurring as metamorphic new-single grains or as zircon mantles overgrowth around the magmatic zircon cores, yield U-Pb ages varying from 1865Ma to 1782Ma, with relatively low Th/U (<0. 10), 176Lu/177Hf (<0. 0001), and HREE abundance. They yield a mean 207Pb/206Pb age of 1831 ±7Ma (2σ, MSWD = 2. 5). We interpret this age to be the age of granulite to amphibolite retrograde metamorphism. The Tiin-zircon temperatures record their formation temperatures in range of 683-714°C which are highly consistent with the temperatures from granulite to amphibolite retrograde metamorphism. Therefore, the isotopic ages of 1850-1800Ma might be representative of the stage of regional crust uplift postdating the Late Palaeoproterozoic cratonization. Source

Wang C.,Peking University | Song S.,Peking University | Song S.,Durham University | Niu Y.,Durham University | And 2 more authors.

We present the results of a geochemical and geochronological study for Late Triassic (230-220Ma) adakitic plutons within the Archean terrane of the eastern part of the North China Craton (NCC). These plutons show adakitic signatures with high Sr, Sr/Y, (La/Yb)N, and low Cr and Ni. The enriched Nd-Hf isotopic compositions (εNd(t)=-13.3 to -12.9; εHf(t)=-17.4 to -14.6) and old Nd (TDM2=2078-2037Ma) and Hf (TDM2=2366-2192Ma) isotope model ages suggest that the adakitic pluton may be derived from the underplated mafic lower crust of Paleoproterozoic age. The relatively low Cr and Ni contents and lower εNd(t) and εHf(t) values of the Taili adakitic plutons imply negligible input of mantle materials. Calculations of equilibrium mineral assemblages and modeling of trace element partition between melts and residual phases at different pressures confirm the interpretation that the petrogenesis of the Taili adakitic plutons is consistent with partial melting of the Paleoproterozoic mafic lower crust at 10-12kbar (36-43km) with a garnet granulite residue. Melting of the ancient mafic lower crust may be triggered by excess heating of the upwelling mantle in an extensional setting evoked by the contemporary subduction toward beneath the NCC from both north and south, which could serve as one possible mechanism for the destruction or lithospheric thinning of the NCC. Complex mantle-crust interaction through various mechanisms may have been responsible for the long-lived process of destruction or lithospheric thinning, which might have begun as early as in the late Triassic. © 2014 Elsevier B.V. Source

Song S.,Peking University | Song S.,Durham University | Niu Y.,Durham University | Wei C.,Peking University | And 2 more authors.

The Gongshan block near the Eastern Himalayan Syntaxis is a fault-bounded block at the northern tip of the triangle-shaped Indochina continent (NIC). Exposed in this block are late Paleozoic (Carboniferous to Permian) strata and a north-south belt of intermediate to felsic batholiths (i.e., Gaoligongshan magmatic belt). The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins (i.e., the so-called "Gaoligong Group"). U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64. Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at ~ 22. Ma. Leucosome and S-type granite of 22-53. Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block. An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic, Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks. These ages suggest that the NIC differs distinctly from the Indian continent, the Greater and Lesser Himalaya zones, and the Yangtze Craton, but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages. This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India-Asia continental collision in general. Furthermore, the high-grade metamorphism in the NIC indicates a strong crustal thickening (vs. strike-slip shearing) event during much of the Eocene to the Oligocene (~ 53-22. Ma) that has brought the Paleogene sediments to depths of greater than 25. km. Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntaxis may have led to the clockwise rotation, southeastward extrusion and extension of the southeastern part of the Indochina continent. © 2010 Elsevier B.V. Source

Song S.,Peking University | Niu Y.,Durham University | Niu Y.,CAS Qingdao Institute of Oceanology | Su L.,Chinese University of Geosciences | And 2 more authors.
Earth-Science Reviews

The North Qaidam ultra-high pressure metamorphic (UHPM) belt in the northern Tibetan Plateau records a complete history of the evolution of a continental orogen from prior seafloor subduction, to continental collision and subduction, and to the ultimate orogen collapse in the time period from the Neoproterozoic to the Paleozoic. Lithologies in this UHPM belt consist predominantly of felsic gneisses containing blocks of eclogite and peridotite.The 1120-900. Ma granitic and psammitic/pelitic gneisses compose the majority of the UHPM belt and is genetically associated with the previous orogenic cycle of Grenville-age, whereas protoliths of the HUPM eclogites are of both the 850-820. Ma continental flood basalts (CFBs) and the 540-500. Ma oceanic crust (ophiolite). The early stage of quartz-stable eclogite-facies metamorphism took place at ~. 445-473. Ma, the same age as that of the HP rocks in the North Qilian oceanic suture zone, representing the earliest subducting seafloor rocks exhumed and preserved. Coesite-bearing zircons from the metapelite and eclogite, diamond-bearing zircons from garnet peridotites constrain the UHP metamorphic age of ~. 438-420. Ma, which represents the timing of continental subduction at depths of 100-200. km, ~. 10-20. m.y. younger than the early stage of the Qilian seafloor subduction. Therefore, deep subduction of continental crust should be the continuation of oceanic subduction that is pulled down by the sinking oceanic lithosphere or pushed down by the overriding upper plate, which is an expected and inevitable consequence for the scenario of passive continental margins. Partial melting of subducted ocean crust might occur in response to continental subduction at ~. 435. Ma.The UHPM rocks started to exhume accompanied by mountain building and deposition of Early Devonian molasses in the North Qilian region at ~. 420. Ma. Decoupling of oceanic subduction zone and continent UHPM terranes may be attributed to the different exhumation path and mechanism between the subducted oceanic and continent crusts, or rollback of subduction zone. Decompression melting of UHP metamorphosed slab and continental crust during exhumation is responsible for the generation of adakitic melts and S-type granite. Mountain collapse and lithosphere extension happened in the period of ~. 400-360. Ma and formed diorite-granite intrusions in the UHPM belt, which marked the end of a complete orogenic cycle.This UHP metamorphic belt presents an example of multi-epoch tectonic recycles, represented by recombination of the Neoproterozoic Grenvillian orogenesis and the Early Paleozoic Caledonian orogenesis. © 2013 Elsevier B.V. Source

Discover hidden collaborations