西南极乔治王岛北海岸第三纪火山岩的岩石化学和地球化学研究

乔治王岛北海岸的第三纪火山岩包括菲尔德斯半岛组（ＦＰＧ）熔岩、Ａｄｍｉｒａｌｔｙ湾组（ＡＢＧ）岩颈和Ｗｅｇｇｅｒ峰组（ＷＰＧ）侵入岩。大多数中基性熔岩为斑状结构，斜长石和普通辉石是岩石中最主要的斑晶相。安山岩中尚含有少量斜方辉石，英安岩质熔岩则为细晶质结构。岩颈相岩石的物质组成与熔岩相似，但斑晶含量往往较少。岩石化学成分表明，所有岩石均富铝，为亚碱性火山岩，属于钙碱性岩石系列。岩石富集大离子亲石元素，轻稀土元素略为富集，稀土元素总含量落在钙碱性岩石的含量范围之内并基本随硅含量的增加而增加。岩石具有很低的８７Ｓｒ／８６Ｓｒ比值（０．７０３４左右）和较高的１４３Ｎｄ／１４４Ｎｄ比值（０．５１２７８－０．５１２８８），εＮｄ平均值在＋６左右。８７Ｓｒ／８６Ｓｒ比值与Ｓｉ、Ｋ、Ｒｂ、１／Ｓｒ等元素呈水平线性关系。在Ｃｅ／Ｙｂ－Ｃｅ和Ｔｈ／Ｔａ－Ｔｈ图解上所有岩石的投影点分布趋势相同，趋势线的斜率＞０。上述特征表明，乔治王岛北海岸第三纪火山岩均由上地幔物质直接生成而未受壳源物质的混染。推测在古太平洋板块向南极板块下俯冲的过程中，上地幔发生的不同程度部分熔融所生成的玄武岩岩浆从深部岩浆囊沿构造薄弱带穿过地壳直接喷出形成

PETROCHEMISTRY AND GEOCHEMISTRY OF TERTIARY VOLCANICS,NORTHERN COAST OF KING GEORGE ISLAND,WEST ANTARCTICA

Tertiary volcanic rocks located in Fildes Block and outcropped along the northern coast of King George Island(South Shetland Islands,West Antarctica) include basalt,basaltic andesite,andesite and dacite effusives(Fildes Peninsula Group,FPG),basaltic and andesitic plugs(Admiralty Bay Group,ABG).Intrusive rocks of Wegger Peak Group(WPG) in this study were collected from Barton Horst near North Foreland.Most of basic lavas are porphyritic with plagioclase and augite phenocrysts.Rare orthopyroxene is found in andesite and dacite is usually finely crystalline.The component of plugs is much similar to effusives although it contains less phenocrysts. The major and trace element contents of volcanic rocks were measured with XRF and INAA in China.As shown in majors and traces discrimination plots,all the effusives and intrusives which have higher Al 2O 3 content,are subalkaline rocks following a calc alkaline trend,although the WPG plutons have higher silicon and alkali content.The basalt has the lowest REE abundant and the andesite has the highest,that is,the REE abundant increases with increasing SiO 2 content of rocks.All chondrite normalized REE patterns are with light rare earth element enrichment and parallel to each other indicating calc alkaline characteristics and the affinity of magmatic generation.Comparing with primary mantle composition,the rocks are rich in LIL element.Also,the trace concentrations of rocks have the same varying patterns as shown by REE that indicate magma generation process. The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios of four samples of FPG and two ABG plugs are about 0.7034 and 0.5128 respectively,same as those from Fildes Peninsula,the south cost of King George Island and Cape Shirreff of Livingston Island.All plots of rocks distributed along the mantle array in 143 Nd/ 144 Nd vs 87 Sr/ 86 Sr isotope correlation diagram,near the Prevalent Mantle area and cumulate together with those from Fildes Peninsula and Livingston Island.The average ε Nd value of rocks is +6±.That implies these rocks did not undergo any crust contamination and might be derived from the same primary magma source of the volcanic rocks in Fildes Peninsula.There are almost no change in 87 Sr/ 86 Sr had been observed when SiO 2,K 2O,1/Sr,Rb,Th,Sm of the rocks increased.The distribution of all the rocks on Ce/Yb Ce and Th/Ta Th correlation diagrams follows same trend with the slope being positive,which strongly suppose the result mentioned above. It can be concluded that the volcanic rocks of FPG lavas and ABG plugs,as well as WPG granodiorites are all of calc alkaline characteristics,being part of the South Shetland magmatic arc.They might have been generated directly by partial melting of upper mantle,being associated with the same simple magma source that resulted from the Mesozoic to Cenozoic volcanism activities controlled by the subduction of proto Pacific oceanic crust beneath Antarctic Plate.