摘要
在赣东北蛇绿岩带中的樟树墩地区新发现新元古代高镁安山岩,其LA-ICP-MS锆石U-Pb法年龄为794.8±6.0 Ma。它们的Si O2介于54.90%~58.45%,Al2O3介于15.31%~16.77%（平均小于16%）;Ca O为2.46%~6.73%,Fe O^T/Mg O变化于0.87~1.20,具有高Mg O（6.39%~8.76%）、高Mg^#（64~71）特点;富集轻稀土元素且具弱负Eu异常,明显富集LILE而亏损HFSE,Sr含量（普遍〈200×10^-6）和Sr/Y比值（4.11~7.29）均较低,具有典型高镁安山岩的地球化学特征,类似于日本Setouchi火山带的Sanukitoids。锆石εHf（t）值介于10.23~17.79之间,反映它们起源于亏损地幔。主、微量及同位素特征均表明,本区高镁安山岩是被俯冲板片释放的含水流体交代的地幔楔部分熔融的产物,形成于大洋岛弧（洋内弧）环境,指示赣东北地区在约800Ma前时仍存在洋壳俯冲,双溪坞弧尚未增生到扬子东南缘,暗示扬子和华夏两大陆块此时尚未碰撞拼合。
The newly discovered Neoproterozoic high-Mg andesites are located at Zhangshudun of the NE Jiangxi ophiolite belt and are dated at a zircon U-Pb age of 794.8 ＋ 6.0 Ma. They have variable SiO2 （54.90% 58.45% ）, CaO （2.46% -6.73% ）, relatively consistent Al2O3 （ 15.31% - 16.77%, mostly less than 16% ） and FeO^T/MgO values （0.87 - 1.20）, and show relatively high MgO contents （6.22 -8.47% ）, high Mg^#（64 71 ）, which clearly exhibits the diagnostic features of typical high-Mg andesites. The study samples are enriched in LREE and have slightly negative Eu anomalies. In spider diagram, they show enrichment of LILE （eg. Rb, Ba） while depleted in Nb and Sr elements. These andesites have relatively low Sr abundance and Sr/Y ratios （4.11 7.29）, similar to Sanukitoids of the Setouchi volcanic belt in Japan. The zircon Hf isotopes show positive initial epsilon Hf values ranging from ＋ 10.3 to ＋ 17.8. These geochemical data indicate Zhangshudun high-Mg andesites should derived from the dephed mantle wedge metasomatized by fluids released from the subducted slab, within an oceanic arc setting. Therefore, it is infen＇ed that the Shuangxiwu arc was away from the southeastern margin of the Yangtze Block, and the Yangtze and Cathaysia blocks had not collided at least at ca. 800Ma.
出处
《地质论评》
CAS
CSCD
北大核心
2016年第5期1185-1200,共16页
Geological Review
基金
中国地质调查局项目"华东地区地质系列图件编制与综合研究"(编号:1212011120120)
"钦杭东段关键地区区域地质调查"(编号:12120113065200)
"江南造山带东段新元古代沟弧盆体系演化与大陆生长研究"(编号:12120114039001)的成果~~
关键词
高镁安山岩
新元古代
樟树墩
赣东北蛇绿岩带
江南造山带
High-Mg andesite
Neoproterozoic
Zhangshudun
the NE Jiangxi ophiolite belt
Jiangnan orogen
