Abstract:

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.