闽北角闪辉长岩的地球化学特征及其地球动力学意义

GEOCHEMICAL CHARACTERISTICS OF BOJITE IN NORTHERN FUJIAN PROVINCE AND THEIR GEODYNAMIC SIGNIFICANCE

赤门角闪辉长岩是中国东南部晚白垩世末基性岩浆活动的产物,分布在福建省南平市的北部,总体呈近NS向展布。岩石属于低钾(拉斑)岩石系列,富A l、Na2O>K2O的特征,岩浆演化过程中经历了以橄榄石、辉石、斜长石和Ti-Fe氧化物的分离结晶作用。赤门角闪辉长岩具相对富集大离子亲石元素(LILE)和轻稀土元素(LREE)、亏损高场强元素(HFSE)的特性,不相容元素蛛网图显示出消减带岩石的地球化学性质,以Nb、Ta、Ti负异常为特征。微量元素地球化学研究表明,赤门角闪辉长岩浆在上升侵位过程中未发生地壳物质混染,来自不含石榴石的富集岩石圈地幔岩部分熔融的产物,地幔源区的演化与太平洋板块俯冲密切相关。赤门角闪辉长岩形成于大陆拉张带-陆内初始裂谷的过渡环境。综合晚中生代的基性岩类资料,与形成较早的沿海辉长岩对比研究,赤门角闪辉长岩是拉张向裂谷转换体制下形成的过渡岩石类型,标志初始裂谷活动的开始。

The Chimen bojite with a nearly NS strike, was a product of mafic magma activity in southeastern China at the end of Late Cretaceous. And it is spreaded in the northern part of Nanping City in Fujian Province. It belongs to low-K （tholeiite） rock series characterized by high Al contents and Na2O 〉 K2O, possibly resultant from fractional crystallization of olivine, clinopyroxene, plagioclase and titanium-iron oxide. These stocks are significantly enriched in LILE （such as K, Rb, Ba, Th , and U ） and LREE, depleted in HFSE （Nb, Ta, Zr, Hf）. In the primary man- tie-normalized spidergram, they show the geochemical features of subduction-zone rocks and negative Nb, Ta, Ti anomalies. The results of trace element geochemical study indicated that the bojite was not contaminated by crustal material during emplacement it was derived from partial melting of garnet-free enriched lithospheric mantle, and the evolution of the mantle source region is closely related with the subduction of the Pacific plate. The Chimen bojite was formed in the geodynamic setting between the crustal extension belt and the continental initial rift. In combination with the results of geological studies for the Late Mesozoic mafic igneous rocks in SE China, it is suggested that the Chimen bojite belongs to the transitional rock series in the transitional zone from the crustal extension belt to the continental initial rift, which may be used to trace rift start up when compared to the coastal gabbro.