Early Mesozoic ferroan (A-type) and magnesian granitoids in eastern South China: Tracing the influence of flat-slab subduction at the western Pacific margin
Early Mesozoic ferroan (A-type) and magnesian granitoids in eastern South China: Tracing the influence of flat-slab subduction at the western Pacific margin
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DOI：
10.1016/j.lithos.2015.11.025
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发表时间：
2016
期刊：
影响因子：
3.5
通讯作者：
Kong-yang Zhu;Zheng‐Xiang Li;Xi‐sheng Xu;S. Wilde;Hanlin Chen
中科院分区：
文献类型：
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作者：
Kong-yang Zhu;Zheng‐Xiang Li;Xi‐sheng Xu;S. Wilde;Hanlin Chen
利用锆石U-Pb年龄、Hf同位素和全岩地球化学资料，对中国东南部晚三叠世-早侏罗世花岗岩类的成因及其构造环境进行了研究。浙江省宁波市225 Ma的丘王花岗岩和178 Ma的斜浦花岗岩的εHf（t）值分别为− 15.7 ~ − 9.9和− 17.6 ~ − 13.4。它们均为华南华夏地块古元古代基底中的钛铁矿系列花岗岩类。晚三叠世邱王花岗岩为强过铝质，具有高锆石饱和温度（850-900 °C），高Gd/Yb（2.7-4.8）和Ga/Al（3.7-4.6 × 10− 4）。相比之下，早侏罗世斜浦花岗岩是弱过铝质的，锆石饱和温度低（~ 670 °C），Gd/Yb低（1.0-1.1），Ga/Al高（2.4-2.6 × 10− 4）。丘王花岗岩的熔融温度和压力比年轻的斜浦花岗岩高，反映了碰撞构造到造山塌陷的构造体制的变化。区域研究表明，三叠纪沿西太平洋沿着的板块俯冲作用可能导致了相对靠近海沟地区的大陆地壳增厚和大陆岩石圈的交代-氧化作用，并在那里生成了科迪勒拉型镁质花岗岩类。然而，同生镁花岗岩类产生在加厚地壳远离海沟，那里的源岩具有均匀的中等氧逸度和交代氧化作用是微不足道的。华南沿海地区早中生代“A型”花岗岩类的形成与其形成的非造山构造环境无关，而与其形成的贫水、低熔融条件有关。
Zircon U–Pb ages, Hf isotopes and whole-rock geochemical data are used to constrain the petrogenesis of the Late Triassic–Early Jurassic granitoids in southeastern China and their tectonic settings. The 225 Ma Qiuwang and 178 Ma Xiepu granites at Ningbo, Zhejiang Province, have εHf(t) values of − 15.7 to − 9.9 and − 17.6 to − 13.4, respectively. They are both ilmenite-series granitoids derived from the Paleoproterozoic basement of the Cathaysia Block in South China. The Late Triassic Qiuwang granite is strongly peraluminous with high zircon saturation temperatures (850–900 °C), and high Gd/Yb (2.7–4.8) and Ga/Al ratios (3.7–4.6 × 10− 4). In contrast, the Early Jurassic Xiepu granite is weakly peraluminous with low zircon saturation temperatures (~ 670 °C), low Gd/Yb (1.0–1.1) and high Ga/Al ratios (2.4–2.6 × 10− 4). The Qiuwang granite records higher melting temperatures and pressures than the younger Xiepu granite, reflecting a change in the tectonic regime from collisional tectonics to orogenic collapse. A regional review indicates that Triassic flat-slab subduction along the western Pacific may have led to thickening of the continental crust and metasomatism-oxidation of the continental lithosphere in the regions relatively close to the trench, where Cordilleran-type magnesian granitoids were generated. However, contemporaneous magnesian granitoids were produced in the thickened crust away from the trench, where the source rocks have uniformly moderate oxygen fugacities and metasomatism-oxidation was insignificant. Such Early Mesozoic “A-type” granitoids in the coastal region of South China are related to water-deficient and reduced melting conditions rather than an anorogenic tectonic setting.