Island arc-type bimodal magmatism in the eastern Tianshan Belt, Northwest China: Geochemistry, zircon U–Pb geochronology and implications for the Paleozoic crustal evolution in Central Asia
Island arc-type bimodal magmatism in the eastern Tianshan Belt, Northwest China: Geochemistry, zircon U–Pb geochronology and implications for the Paleozoic crustal evolution in Central Asia
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DOI：
10.1016/j.lithos.2012.10.006
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发表时间：
2013-05
期刊：
影响因子：
3.5
通讯作者：
Xijie Chen;Xijie Chen;L. Shu;M. Santosh;M. Santosh;Xixi Zhao
中科院分区：
文献类型：
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作者：
Xijie Chen;Xijie Chen;L. Shu;M. Santosh;M. Santosh;Xixi Zhao
东天山红山口地区(中国西北部)玄武岩和流纹岩的双峰式组合为研究边缘收敛背景下的岩浆成因和构造演化提供了重要的资料。在这项研究中，我们提供了玄武岩和流纹岩的岩石学、地球化学和锆石U-Pb年代学数据。玄武岩中的锆石结晶年龄为347.1±3.9 Ma，流纹岩中的锆石结晶年龄为344.4±1.9 Ma和345.0±2.5 Ma。玄武岩的主量元素和微量元素特征表明，高铝玄武岩(HAB)是软流圈地幔交代形成的源岩。镁铁质和长英质岩石均富含轻稀土元素(LREE)和大离子亲石元素(LILE)，亏损高场强元素(HFSE)。镁铁质岩石和长英质岩石的Rb-Sr和Sm-Nd同位素组成也具有广泛的均质性。玄武岩表现出明显的不相容元素和正εNd(345 Ma)(+5.87~+8.25)。我们设想了与俯冲有关的成因，解释了LILE的富集弧状熔体(富Th和LREE，亏损Nb，εNd(345 Ma)&Gt；+6)是通过亏损地幔源区的部分熔融而产生的，并有少量地壳成分的参与。流纹岩还具有高εNd值(345 Ma)(+6.35~+8.53)。因此，我们认为类似大洋中脊玄武岩(亏损端员地幔)的源岩与弧状岩浆的混合过程是基性岩的成岩作用，而流纹岩最好的解释是玄武岩母岩浆的分离结晶形成的。元素和同位素地球化学方面的一系列证据表明，红山口双峰组中的镁铁质岩石和长英质岩石之间存在密切的成因关系。我们认为，该杂岩本质上是双峰型的，反映了晚古生代准噶尔板块俯冲引起的弧后伸展岩浆作用的特征。
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.