| Basaltic andesite,dacite and hydrotherm altered volcanic rock were got from Eastern Manus Basin by Remote Operated Vehicle.By analyzing the bulk major and trace elements,Sr-Nd-Pb isotope compositions and the plagioclase in situ components of these rocks,and conbining with previous data,we found that there was High-Mg andesite in Eastern Manus Basin ignored.We also studied the magma source of fresh volcanic rocks and elemental migration mechanism of altered rock in Eastern Manus Basin.Two pieces of basaltic andesite from Eastern Manus Basin have high Mg content.They have 5.3% and 5.54% of MgO,8.35% and 8.22% of FeOT,1.575 and 1.484 of FeOT/MgO,15.77% and 15.82% of Al2O3,9.22% and 9.28% of CaO,respectively.They are High-Mg andesite.The Eastern Manus Basin is an active back-arc basin with the subduction of the Pacific and Solomon Plates.The magma source of Eastern Manus Rifts(EMR)lavas has been debating.Some scholars think that the subduction materials come from the Pacific Plate,others think that the subduction materials are from the Solomen Plate.Strong enrichments of Pb and LILEs(K,Rb,Sr,Ba and U)and depletions of HFSEs(Nb,Th,Ta and Ti)and REEs in EMR lavas suggest that numerous subduction components have been injected into the mantle source of EMR magmas.The subduction components come from the Pacific and Solomon Plates based on the slightly continuous change in the 87Sr/86 Sr ratios along the Pacific Plate subducting direction and on the 238 U excesses of the EMR lavas,which reveal that components from the Pacific Plate have remained in the mantle beneath the Eastern Manus Basin for a long time and the Solomon subducting components have been added recently.High-Mg andesite was developed from peridotite;the isotope composition of peridotite was as same as which of the Manus MORB.The subducted components include submarine sediments and altered oceanic crust.Thus,the five end members probably include sediments and basalts from the Solomon Plate,sediments and basalts from the Pacific Plate,and Manus MORB.As all 5 members are taken into consideration,the ratio of Pacific basalts versus sediments is 86.5:13.5,of Solomon basalts versus sediments is 93.5: 6.5,of Pacific components versus Manus MORB is 8.2:91.8.If the content of Manus MORB remains stable,the ratio of Solomon compo-nents versus Pacific components is 9:1.We analyzed the bulk major,trace elements and the plagioclase in situ components of these rocks and carried out some comparative studies of altered rock and fresh rocks to explore the characteristics and influence factors of element migration of the hydrothermal altered rock in Eastern Manus Basin.As results,the type of hydrothermal sulfide of altered rock is copper-rich sulfide which mainly composed of chalcopyrite,sphalerite and ilmenite.Silicified alteration is the main alteration type of this rock as the SiO2 content is extremely high in the altered area.The content of major and trace elements of the altered rock may be the average value of the total content of basaltic andesite and dacite from the Eastern Manus Basin.The protolith of the altered rock may be andesite.During the process of hydrothermal alteration,mass of the altered rock were increased by 150%.The altered rock most enriched elements such as Cu,Zn,Ga,Sr,Cd,Ba and Pb,more enriched Ti,V,Mn,Co,Ni,Mo and U,less enriched Si and Fe,least enriched Sc,Cr,Nb,Ta,W and Bi.Altered rock most depleted elements such as Be and Ca,more depleted Li,Na,Mg,K,Rb,REE and Y,least depleted Zr,Cs and Th.Al,P and Hf are immobile elements.Influence factors of element migration of the hydrothermal altered rock may include mineralization,metasomatism(such as chloritization and silicification)and the activity of elements in fluids. |
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