南极乔治王岛第三纪火山岩中斜长石成分特征及成因意义

COMPOSITIONAL FEATURES AND PETROGENETlC IMPLICATION OF PLAGIOCLASES FROM TERTIARY VOLCANICS IN KING GEORGE ISLAND, ANTARCTICA

乔治王岛第三纪火山岩含大量斜长石斑晶。斜长石的主元素、稀土和微量元素分析表明，它们主要为中长石—钙长石。斜长石种类与寄主岩之间有较好对应关系，即玄武安山岩和安山岩中以中长石和拉长石为主，而高铝玄武岩中为倍长石和钙长石；它们的ＲＥＥ、Ｌａ／Ｙｂ和δＥｕ等之间也有明显对应关系。但不同种类的斜长石之过渡金属元素特征相近，反映了过渡金属元素在斜长石中的含量和分异趋势主要受制于斜长石的晶体结构。综合研究表明，斜长石成分取决于其母岩浆成分，并反映了区域岩浆演化的趋势。

King George Island consists mainly of Tertiary volcanics which contain plentiful plagioclases. In the Admiralty Bay area(including Keller Peninsula,Ullman Spur and Point Hennequin) of central Island expose predominantly basaltic andesites and andesites, and in the Maxwell Bay area (including Barton and Weaver peninsulas) of southwestern Island occur mainly High Al basalts and basaltic andesites.Chemical analyses and electron microprobe analyses indicate that plagioclases are andesine labradorite (An41.47～61.26)for the Admiralty Bay area and mostly bytownite and anorthite.(An 72 .05～90.97)for the Maxwell Bay area,revealing an evident chemical correlation between plagioclases and their host volcanics.In addition,plagioclases and their specific host volcanics are also corresponding in ? 苮REE,La/Yb and δEu.On the other hand,various plagioclases have relatively similar feature of transitional metallic elements (such as Cr,Co,Ni,Zn),probably implying that contents and differentiation pattern of these elements are chiefly controlled by crystal structure rather than chemistry of plagioclase.In a word, synthetic studies show dependence of plagioclase compositions on those of their host volcanics. Considering that both the Admiralty Bay and the Maxwell Bay areas exist evolutional series from high Al basalts to dacites, but have different mineralogy,it is suggested that high Al basaltic magma in the Maxwell Bay area can be the parental magma of magmatic series in the Admiratly Bay area.