西藏大竹区蛇绿岩中的钠长花岗岩

The Albite Granite in the Ophiolite of the Dazhu Area,Tibet

钠长花岗岩(Albite granite)是以钠长石和石英为主要成分,含有少量暗色矿物的花岗岩类岩石。它以不含(或偶含)钾长石为特征,故区别于大陆的钠长花岗岩、奥长花岗岩和花斑岩等花岗岩类岩石。

The albite granite occurs at the top of the cumulates in Dazhu Area.It's a greyish white and fined-grained rock,consisting mainly of albite(An 1 and quartz with only minor amounts of ferromagnesium mineral and myrmekite.The albite granite can be distinguished from other granites by the absence of potash feldspar.It exhibits granitic texture and is characterized chemically by higher SiO_(2),Na_(2)O and extremely low K_(2)O.The REE pattern of the albite granite is depleted in LREE as those of the plagiogranite of Troodos.From the dunite at the bottom to the albite granite at the top of cumulates,the contents of SiO_(2),Na_(2)O,TiO_(2),increase and<FeO),MgO decrease gradually.The<FeO>/MgO ratio of cumulates also increase noticeably from 0.23 to 3.33.Meanwhile,the REE abundance of the albite granite is highest,being more than 18 times over those of chondrite,and that of the cumulus dunite is all lower than those of chondrite.All of these gradual variations of the cumulus sequence indicate the trend of the magmatic differentiation.Therefore.the albite granite represents the final differentiation product of a tholeitic magma.