云南金平县镁铁-超镁铁岩岩床群的地球化学特征

THE GEOCHEMICAL CONSTRAINTS ON MAFIC-ULTRAMAFIC SILL SWARMS FROM JINPING AREA,YUNNAN

金平地区的镁铁－超镁铁质岩岩床群侵入在晚泥盆世地层中。岩石类型包括粒玄岩、辉绿岩、辉长岩、橄榄辉玄岩和苦橄岩。岩床群的走向与区域构造方向一致，为北西向延伸。单个岩床的厚度在１－２００ｍ之间，较厚的岩床有明显的结晶分异作用。粒玄岩和辉绿岩含相当数量的钛铁矿，称为铁钛玄武岩。化学成分以强烈富Ｔｉ为特征，ＴｉＯ２含量平均达３．９％。Ｍｇ含量通常较低，是拉斑质或碱性玄武岩演化而来的。这种高Ｔｉ玄武岩还富Ｔｈ、Ｔａ、Ｎｂ，Ｔｈ／Ｔａ比值接近１，Ｚｒ／Ｎｂ比值较低，接近ＯＩＢ的特征，推测产于板内环境，是地幔柱部分熔融形成的岩浆。金平岩床群中苦橄岩和橄榄辉玄岩（ＭｇＯ含量在１５．１％—２３．３％之间）出露的比例相对较多，指示地幔柱部分熔融的温度较高，推测相当于地幔柱尾部的特征。

The mafic-ultramafic sill warms which intruded in later Devonian strate from Jinping area include dolerite、diabase、gabbro、ankaramite and picrite. The s ill swarms stretch in the north-west direction and are concordant with that of reg iona l tectonics. The thickness of sill varies from 1m to 200m. The crystallization d i fferentiation can be observed clearly in some thicker sills. The dolerite and di abase bearing lots of ilmenite are called Fe-Ti basalts and are characterized by high Ti content which the average TiO 2 abundance can go up to 3.9% in composi tion of dolerite and diabase and by low Mg′ number. The evidence of geochemistr y show that these two basalts are developed by tholeiite or alkaline basalt. In a ddition to high Ti, The dolerite and diabase also show high abundance of Th、Ta 、Nb. The value of Th/Ta is 1 approximately and Zr/Nb is low comparing to N-MOR B. So these two basalts show the feature of OIB and may be originated from partial melting of mantle plume in intralplate. The are more exposure of picrit e and anka ramite(15.1% 23.3% MgO) in Jinping than that around area suggests that the temp eratur e of partial melting of mantle plume be high comparatively and the source reserv oir of mafic-ultramafic sill warms in Jinping area may be in the tail of mantle plume.