Search

The bimodal association of basalts and rhyolites in the Hongshankou area of the East Tianshan area (NW China) provides an important suite to investigate magma petrogenesis and tectonics evolution in a convergent margin setting. In this study we present petrological, geochemical and zircon U–Pb geochronological data from the basalts and rhyolites. Zircons from the basalts yield crystallization ages of 347.1±3.9Ma whereas those from the rhyolites are dated as 344.4±1.9 and 345.0±2.5Ma. The major and trace element signatures of basalts suggest high alumina basalt (HAB) as the source magma which was generated from metasomatised asthenosphere mantle. Both the mafic and felsic rocks are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), and are depleted in high field strength elements (HFSE). Broadly homogenous Rb–Sr and Sm–Nd isotopic compositions of mafic and felsic rocks are also characteristic. The basalts display a distinct enrichment in incompatible elements and positive εNd(345Ma) (from +5.87 to +8.25). We envisage a subduction-related origin to account for the LILE enrichment with arc-like melts (enriched in Th and LREE and depleted in Nb, with εNd(345Ma)>+6) produced through the partial melting of a depleted-mantle source, and the involvement of minor crustal component. The rhyolites also display high εNd(345Ma) (from +6.35 to +8.53). Consequently, we suggest a mixing process between a source similar to mid-ocean ridge basalt (depleted end-member mantle) and arc-like magmas for the petrogenesis of the mafic rocks, whereas the rhyolites are best interpreted to have formed by fractional crystallization from the parental magma of the basalts. Several lines of evidence from elemental and isotopic geochemistry suggest a close genetic relationship between the mafic and felsic rocks in the Hongshankou bimodal suites. We propose that the essentially bimodal character of this complex reflects the features of back-arc extensional magmatism, induced by the subduction of the Junggar plate during the Late Paleozoic. [ABSTRACT FROM AUTHOR]

Terra Nova, 23, 324-332, 2011 Abstract Fourteen samples of Precambrian bedrocks from the Kuluketage area, located in the north-eastern Tarim Craton, yield apatite fission track ages ranging from 166 to 82 Ma. These ages are far younger than their Precambrian age of stratigraphy or metamorphism, indicating that the samples experienced total annealing before Mesozoic exhumation. The samples were subdivided into three groups based on locations and ages. Multi-stage Mesozoic exhumation is linked to collisions of the Qiangtang, Lhasa and Kohistan terranes with the South Eurasian continental margin. The three cooling phases are also shown by stratigraphy and seismology. Considering modelling results with different constraint boxes, seismic profiles and the stratigraphic sequences of surrounding sedimentary basins, faint Cenozoic reheating and recooling were likely to take place in the Kuluketage area by northwards propagating deformation from India-Eurasia collision and continued convergence. [ABSTRACT FROM AUTHOR]

Abstract: The Kuruktag uplift is located to the north of the Tarim Block, NW China. At the western end of the Kuruktag uplift, mafic dykes crop out in the Korla area, which were rarely subjected to deformation and metamorphism in contrast to their metamorphic wall-rocks that were strongly deformed and metamorphosed to amphibolite facies. A SHRIMP U–Pb zircon age of 634Ma was obtained for a spessartite dyke, documenting the youngest known igneous activity associated with rifting in the Tarim Block during the Neoproterozoic. Most samples of the Korla mafic dykes show clear enrichments in Th, La, and variable depletions in Nb, Ta and Ti, except for samples 08T-14 and 08T-19 that display incompatible element distribution patterns similar to those of OIB without obvious depletions in Nb, Ta and Ti. The geochemical signatures suggest that the Korla mafic dykes were formed in an intra-plate setting and their primary magama was possibly produced by partial melting of a sub-continental lithospheric mantle that has been metasomatized by previous subduction processes, and then heated by a rising mantle plume. In combination with previous geochronological data of Neoproterozoic igneous rocks throughout the Tarim Block, at least three pulses of magmatic activity, from ca. 830 to 800Ma, from ca. 790 to 740Ma and from ca. 650 to 615Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block, implying that the breakup of the Rodinia supercontinent in the Tarim Block may have been a long-lasting process. [Copyright &y& Elsevier]