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Yang L.-Q.,China University of Geosciences | Deng J.,China University of Geosciences | Dilek Y.,China University of Geosciences | Dilek Y.,Miami University Ohio | And 5 more authors.
Bulletin of the Geological Society of America | Year: 2015

We present structural field observations, in situ zircon U-Pb ages, Hf isotopic compositions, geochemistry, and Sr-Nd isotope data from the Puziba granitoid intrusions in the Qinling orogenic belt (China). These intrusions occur as ENE-WSW-oriented dike swarms in a highly deformed phyllite-matrix mélange along several major fault zones within the Mianlue suture zone. Granitoid rocks have a relatively wide range of SiO2 (69-77 wt%, volatile-free) and total alkali contents (Na2O + K2O = 2.76-4.83 wt%), and K2O/Na2O ratios >1, and they mostly fall in the granite-granodiorite fields in the total alkalis versus silica diagram. They display relative enrichments of light rare earth elements (LREEs) and relatively flat heavy rare earth elements patterns (HREEs) with weak negative Eu anomalies (δEu = 0.56-0.95) on chondrite-normalized rare earth element (REE) diagrams. They show enrichment in Th and Zr and depletion in high field strength elements (HFSEs; e.g., Nb, P, and Ti). They are enriched in large ion lithophile elements (LILE) and LREEs and have low zircon εHf(t) (-2.8 to 4.2) values, relatively high initial 87Sr/86Sr ratios (0.7107-0.7188), and variable Mg# (22-46). Their U-Pb zircon crystallization age is ca. 215 Ma, but they also show inherited zircon age clusters in the Paleoproterozoic, Mesoproterozoic, Neoproterozoic, and Paleozoic, with two prominent peaks at ca. 753 Ma and ca. 805 Ma. The high Rb and Ta elemental abundances, smooth chondrite-normalized REE patterns, and εNd(t) values (-5 to -3) of the Puziba dike rocks characterize them as K-rich, calcalkalinegranites. Their magmas were derived from partial melting of garnet-amphibole- bearing lower crust that was triggered by slab breakoff-induced asthenospheric upwelling in the aftermath of the oblique collision of the North and South China blocks during the Late Triassic. Emplacement of the ENE-WSW- oriented Puziba dike swarms was associated with a transtensional stress regime along/ across the collisional Mianlue suture zone. © 2015 Geological Society of America.

Yang L.-Q.,China University of Geosciences | Ji X.-Z.,China University of Geosciences | Santosh M.,China University of Geosciences | Li N.,China University of Geosciences | And 2 more authors.
Journal of Asian Earth Sciences | Year: 2015

The Mianlue suture marks the southern boundary of the Qinling-Dabie orogen and divides the South China and North China continental blocks. Here we present in-situ U-Pb geochronology and Lu-Hf isotopic data on detrital zircons in chert beds intercalated with continental detritus from the Sanhekou Group in Wenxian along this suture. Our data suggest that deposition of these sediments in the Mianlue suture zone occurred later than ca. 412. Ma. The data define four main age peaks with more than 25% of the detrital zircons yielding ages between 801 and 999 Ma with εHf(t) ranging from -26.9 to 12.2. The age data are comparable with those zircons in the Neoproterozoic granites and volcanic rocks in the Bikou Block which has an affinity to the South China Block. Zircons with ages from 2310 to 2595. Ma comprise 14.1% of the detrital population, and were mainly sourced from the South China Block as inferred from their mostly negative εHf(t) values compared to the North China Block. Zircons with ages ranging from 412 to 587. Ma constitute 12.6% and show markedly negative εHf(t) values ranging from -0.6 to -25.5 (the majority ranging from -4.4 to -19.6), correlating with Palaeozoic magmatic intrusions in the South China Block. Another age peak of 1548-1686 Ma with εHf(t) values varying between -6.5 and -14.8 is comparable to the zircons in magmatic intrusions of the South China Block. Our results suggest that the major source of zircons in the continental detritus mixed with chert within the Mianlue suture zone is Proterozoic and Palaeozoic magmatic intrusions in the South China Block. Combined with the geochemical features of the chert in Wenxian and other outcrops in the Mianlue suture zone, we infer that these sediments were deposited in the Mianlue sea basin adjacent to the continental margin, and that the opening of the Mianlue suture was initiated in the eastern section of the northern margin of South China Block, with subsequent migration to the western part, in a zipper-type opening of the continental rift from the east to the west of the Mianlue suture zone. © 2015 Elsevier Ltd.

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