碧口群不同岩片火山岩微量元素组成差异与古构造意义

Diversity of Trace Element Compositions of Volcanic in Different Slabs of Bikou Group: Implications for Tectonics

对碧口群内部大茅坪和筏子坝 2个岩片变火山岩微量元素的研究表明 ,二者在岩石组合、源区组成及产出的构造环境等方面均存在系统差异 .大茅坪岩片主要由偏碱性与非碱性的玄武质岩石组成 ,二者分别具有类似OIB和MORB的微量元素组成 ,起源于相对富集和相对亏损的地幔 ,并受到陆源物质不同程度的混染 .偏碱性与非碱性的玄武质岩石分别显示板内火山岩 (WPB)与洋脊玄武岩 (MORB)的属性 ,表明大茅坪岩片火山岩主要形成于裂谷 -有限洋盆环境 .筏子坝岩片由玄武质与中酸性火山岩组成 ,岩石化学显示明显的“双峰式”组成特征 ,二者分别具有类似MORB和壳源岩石的微量元素组成 ,为亏损地幔与壳源岩石部分熔融的产物 ,形成于大陆裂谷环境 .根据不同火山岩构造属性及组合特征 ,推断碧口群主要形成于陆内裂谷—有限洋盆构造环境 .

Bikou Group, a metamorphic terrene with a series of slabs, and formed in the mid-late Proterozoic era, has undergone multi-periods of complex metamorphisms and deformation. The composition of its major and trace elements of volcanic rocks shows there are obvious differences in source and tectonic settings between Damaoping and Faziba slabs. Damaoping slab, with alkalic and non-alkalic meta-volcanic rocks, possesses compositions similar to those of OIB-type and MORB-type source respectively. The systemic variations of the ratios between high incompatibility immobile elements and geochemical twins such as Nb/Ta indicate that derived from relative enrichment mantle for former and depleted mantle, they developed within plate tectonics and similar ridge setting respectively. Also the development of the alkalic rocks involved the participation of the crust material, so Damaoping slab volcanics may have been formed in rift or oceanic setting. Faziba slab with meta-basalt and medium-acid lava shows 'double mode' composition characteristics, in which the source composition of meta-basalt is similar to that of mid-ridge basalt. This may clarify that they were derived from relative depleted mantle. The medium-acid volcanic rocks, which are obviously different from meta-basalts, show typical crust source rock characteristics, so Faziba volcanic rocks are thought to have been formed in continental rift setting. According to what is discussed above, Bikou Group volcanics may have possibly developed in continental rift or limited ocean setting.