右江盆地晚侏罗世钾玄质高镁安山岩的厘定及其构造意义

The identification and implications of the Late Jurassic shoshonitic high-Mg andesite from the Youjiang basin

右江盆地在大地构造上处于特提斯和滨太平洋构造域的结合部位。本文报道了该盆地东部杨屯安山岩的岩相学、年代学和地球化学特征。杨屯安山岩具斑状结构和气孔状构造,斑晶主要为辉石,基质以针状斜长石微晶为主,含少量辉石。LA-ICP-MS锆石U-Pb定年结果表明杨屯安山岩的喷发时代为159.3±2.8Ma,即晚侏罗世。样品的Si O2含量为53.13%~55.71%,具高MgO(6.74%~8.85%)、Mg#值(63~72)、Cr(416×10^(-6)~565×10^(-6))和Ni(207×10^(-6)~246×10^(-6)),低Fe OT/MgO(0.83~1.23)等特征,与典型高镁安山岩的特征相似。所有样品具高K2O(3.39%~4.77%)和K2O/Na2O(1.31~2.33),同时也具钾玄质岩石的地球化学特征。样品强烈富集大离子亲石元素和轻稀土元素,而亏损高场强元素,具较高的Rb/Sr(0.20~0.46)和较低的Ba/Rb(7.17~9.30),富集的Sr-Nd同位素组成((87Sr/86Sr)i=0.70738~0.70739,εNd(t)=-3.6^-3.4)。元素和同位素特征表明杨屯钾玄质高镁安山岩是含金云母的富集岩石圈地幔部分熔融的产物,形成于板内伸展环境而不是岛弧环境。晚侏罗世杨屯钾玄质高镁安山岩的厘定,反映了该时期右江地区处于岩石圈伸展-减薄阶段。

The Youjiang basin is located in the joint area of the paleo-Tethys and paleo-Pacific dynamic domains. This paper reports petrographic, geochronological and geochemical data of the Yangtun andesite from the eastern Youjiang basin. The andesite was characterized by porphyrotopic texture. LA-ICP-MS zircon U-Pb dating results give an eruption age of ca. 159Ma for the Yangtun andesite. The andesitie samples are geochemically marked by high MgO （6. 74% -8. 85% ）, Cr （416 × 10^-6 -565 × 10^-6） and Ni （207 × 10^-6 -246× 10^-6） contents, high Mg# values （63 - 72） and low FeOT/MgO ratios （0. 83 - 1.23 ）, similar to those of typical high-magnesian andesite （HMA）. The samples meanwhile belong to the shoshonitic rocks with high K20 （3.39% -4. 77% ） and K2O/Na2O （1.31 -2. 33 ）. They are strongly enriched in large ion lithophile elements and depleted in high field strength elements, with high Rb/Sr （0. 20 -0. 46） and low Ba/Rb （ 7. 17 - 9. 30） ratios. Isotopically, their （ 87 Sr/86 Sr） ;ratios range from 0. 70738 to 0. 70739 and εNO （t） values from -3. 6 to -3.4. Geochemical and isotopical data indicate that the Yangtun HMA was derived from partial melting of the phlogopite-bearing lithospherie mantle. The Yangtun HMA was generated in the within-plate setting, reflective of lithospheric extension in the Youiiang area during the Late Jurassic.