Discovery of an adakite-like pluton near Dongqiyishan (Beishan, NW China) — Its age and tectonic significance
Discovery of an adakite-like pluton near Dongqiyishan (Beishan, NW China) — Its age and tectonic significance
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DOI：
10.1016/j.lithos.2012.02.021
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发表时间：
2012-06
期刊：
影响因子：
3.5
通讯作者：
Wen Zhang;V. Pease;Tairan Wu;Rongguo Zheng;Jicheng Feng;Yuankai He;Hongling Luo;Cao Xu
中科院分区：
文献类型：
--
作者：
Wen Zhang;V. Pease;Tairan Wu;Rongguo Zheng;Jicheng Feng;Yuankai He;Hongling Luo;Cao Xu
东七一山花岗岩体出露于南中亚造山带中部北山东部地区。SIMS锆石U-Pb定年结果表明，东七一山岩体的侵位年龄为356 ± 2Ma。岩石地球化学特征为钙碱性，富SiO_2（> 65%）、Al_2O_3（大部分> 15%）、Na_2O/K_2O（1.3 - 3.9）和Sr（一般> 400ppm）。样品表现为Eu正-弱负异常（δ Eu = 0.81 - 2.32），重稀土、Y、Yb含量较低（Y <18ppm，Yb <1.9ppm），富集轻稀土、大离子亲石元素（Rb、Ba、Sr、U、K）、Zr、Hf，亏损高场强元素。它们的ε Nd（t）值较低（-2.9~+0.6），（87Sr/86Sr）比值较高（0.70581-0.70749）。总之，这些地球化学特征表明埃达克岩类的组成，反映了俯冲洋壳的熔融和较老的大陆材料的参与。同化和分离结晶（AFC）建模需要显着的地壳同化修改初始MORB衍生安山质熔体产生的混合花岗岩熔体，例如-东奇依山岩体。南部弧后盆地（以月牙山-西长井蛇绿带为代表）在早石炭世东七一山埃达克岩状岩体侵位于志留纪火山弧之前闭合。弧后盆地的闭合可能提供了东七一山岩体同位素特征中所见的大陆成分，并为AFC建模所需。后奥陶纪俯冲板片的压扁作用和弧后盆地闭合引起的地壳增厚，形成了俯冲板片与下地壳的紧密耦合，形成了东七一山类埃达克岩体。东奇义山岩体与向南俯冲（以小黄山蛇绿岩带为代表）一致，表明早石炭世古亚洲洋仍有洋壳生成。北山地区的埃达克岩类花岗质岩石可能比以前认为的更丰富。
The Dongqiyishan granitic pluton is exposed in the Eastern Beishan area, in the central part of the Southern Central Asia Orogenic Belt. SIMS U–Pb zircon dating indicates that the Dongqiyishan pluton was emplaced at 356±2Ma. Geochemically the pluton is calc-alkaline and characterized by high SiO2(>65%), Al2O3(mostly >15%), Na2O/K2O ratios (1.3–3.9) and Sr (generally >400ppm). Samples display positive to weakly negative Eu anomalies (δEu=0.81–2.32), low HREE, Y and Yb (Y<18ppm, Yb<1.9ppm), are enriched in LREE, LILE (Rb, Ba, Sr, U, and K), Zr, and Hf, and depleted in HFSE. They have relatively low εNd(t) values (–2.9–+0.6) and relatively high (87Sr/86Sr)iratios (0.70581–0.70749). Together, these geochemical traits suggest an adakite-like composition that reflects melting of subducted oceanic crust and the involvement of older continental material. Assimilation and fractional crystallization (AFC) modeling requires significant crustal assimilation to modify an initial MORB-derived andesitic melt to produce the hybrid granitic melt, e.g. — the Dongqiyishan pluton. A back-arc basin to the south (represented by the Yueyashan–Xichangjing ophiolitic belt) closed before the Early Carboniferous Dongqiyishan adakite-like pluton was emplaced into the Silurian volcanic arc. Closure of the back-arc basin potentially provides the continental component seen in the isotopic signature of the Dongqiyishan pluton and required by the AFC modeling. Post-Ordovician flattening of the subducting slab together with crustal thickening from closure of the back-arc basin, led to the formation of a tightly coupled subducting slab and lower crust which resulted in the formation of Dongqiyishan adakite-like pluton. The Dongqiyishan pluton is consistent with southward subduction (represented by the Xiaohuangshan ophiolitic belt) and indicates that oceanic crust was still being produced in the Paleo-Asian Ocean during the Early Carboniferous. Adakite-like granitic rocks may be more abundant than previously thought in the Beishan region.