湘北益阳古火山岩的成因及其地质意义

Origin of the Yiyang paleovolcanic suite in northern Hunan, China, and its geological significance.

湖南益阳古火山岩分布于益阳市石咀塘和大渡口两地,出露面积分别约为1km2和16km2,主要由科马提岩类和拉斑玄武岩类组成。其中石咀塘古火山岩可进一步分为含橄榄石斑晶玄武质科马提岩和含单斜辉石斑晶玄武质科马提岩2种类型。含单斜辉石斑晶玄武质科马提岩鬣剌结构发育,尚发现典型的中空骸晶橄榄石,SiO2<53%,MgO>9%,K2O<0.9%,TiO2<0.7%,CaO/Al2O3>1。该地区火山岩总体属钙碱性系列,具有高MgO、富Cr、贫不相容元素、稀土元素总量较低、轻重稀土元素分异不明显等特点,表明火山岩主要来源于形成深度较大、部分熔融程度较高的亏损上地幔区。石咀塘火山岩形成于大洋构造环境,大渡口火山岩形成于岛弧构造环境,反应该地区经历了从洋壳形成到俯冲的构造演化过程。

The Yiyang paleovolcanic suite in Hunan is recognized at Shizuitang and Dadukou, Yiyang City, Hunan Province, covering an area of about 1 km2 and 16 km2 respectively. The volcanic suite consists predominantly of komatiites and tholeiites. Among other things, paleovolcanic rocks at Shizuitang may be subdivided into olivine phenocryst-bearing basaltic komatiite and clinopyroxene phenocryst-bearing basaltic komatiite. The latter is characterized by spinifex texture and typical hollow skeletal crystals of olivine, with SiO2 <53%, MgO>9%,TiO2 <0.7%, K2O<0.9% and CaO/Al2O3>1. The volcanic rocks in the area generally belong to the calc-alkaline series, exhibiting higher MgO and Cr, depletion in incompatible elements, low ΣREE and a low degree of HREE and LREE. These characteristics indicate that the volcanic rocks were derived by high degrees of partial melting of the deep depleted mantle. Volcanic rocks at Shizuitang formed in an oceanic environment and volcanic rocks at Dadukou formed in an island-arc environment, reflecting that this area underwent the tectonic evolution from the formation of oceanic crust to its subduction.