北部湾盆地福山凹陷古近纪辉长岩地球化学特征及构造意义

Geochemistry and Tectonic Implications of the Paleogene Gabbros in Fushan Sag,Beibu Gulf Basin

北部湾盆地福山凹陷古近系流沙港组二段泥岩中发育大面积巨厚辉长岩?地球化学分析表明,辉长岩具有中等TiO_2含量(2.15%~2.24%),较低MgO(5.77%~5.86%)、K_2O(1.18%~1.25%),P_2O_5(0.36%~0.39%)含量,且Na_2O>K_2O,指示其类似于板内拉斑玄武岩。辉长岩ΣREE为105.61×10^(–6)~111.26×10^(–6),富集LREE,(La/Yb)_N=8.74~9.64,具有右倾型稀土配分模式;与原始地幔相比,富集Rb、Ba、Th、U等大离子亲石元素以及高场强元素Nb、Ta、Zr、Hf,呈现板内碱性玄武岩特征,是地幔物质上涌与上覆薄而年轻的岩石圈地幔相互作用的产物?结合LA-ICP-MS锆石U-Pb测年结果显示,辉长岩形成时间介于37~32 Ma,代表福山凹陷古近系辉长岩的侵位年龄,是古近纪岩石圈持续伸展环境下的产物?这对探讨福山凹陷以及中国东南部古近纪构造活动及岩浆演化具有重要的意义。

Thick-layered gabbros were found widespread in the mudstone of the second member of the Paleogene Liushagang Formation in the Fushan Sag. The rocks are of sub-alkaline series with SiO_2 contents of 52.12%- 52.61%, MgO 5.77%- 5.86%, Fe_2O_3 11.06%- 11.49%, K_2O 1.18%- 1.25%, P_2O_5 0.36%- 0.39% and relatively high TiO_2 contents of 2.15%6- 2.24%. The gabbros have high ΣREE contents（105.61×10^（-6）- 111.26×10^（–6）） showing LREE-enriched type with（La/Yb）_N ratios of（8.74- 9.64）, and enrichment of large iron lithophile elements（including Rb, Ba, Th, U, Sr）, showing typical features of within plate basalt. The gabbros in the Fushan Sag were likely resulted from the interaction between the upwelling asthenosphere and the young and thin lithospheric mantle that formed by the predated underplating. LA-ICP-MS zircon U-Pb dating showed that the gabbros were crystallized at 37- 32 Ma as the products of the Paleogene lithosphere extension environment.