Hydrothermal alteration and isotopic variations of igneous rocks in Barton Peninsula, King George Island, Antarctica

Abstract

O, H, S and Sr isotopes were investigated to characterize the nature of hydrothermal fluids and alteration processes in volcanic and intrusive rocks in Barton Peninsula, King George Island. The oxygen and hydrogen isotope compositions of altered basaltic andesite and granodiorite are similar to each other. The δ18O and δD values gradually decrease from altered basaltic andesite to altered dyke and quartz-veined volcaniclastic rock. The genetic linkage between hydrothermal fluid and granodiorite intrusion accounts for similar δ34S values among sulfide minerals, both in the granodiorite stock and in nearby altered basaltic andesite. Broadly positive correlations of δ18O-SiO2, δ18O-(87Sr/86Sr)i, and 87Sr/86Sr-SiO2 can be established by siliceous fluids enriched in 18O as well as 87Sr. In addition, a positive relationship between (87Sr/86Sr)i values and 1/Sr contents in basaltic andesites can result from mixing between fresh rocks and infiltrating fluids with different endmember 87Sr/86Sr values and Sr contents. The hyperbolic variation of (87Sr/86Sr)i-K/Rb suggests that meteoric water participated in hydrothermal activities via mixing with magmatic water, and the contribution of seawater was insignificant. All of the above results indicate that hydrothermal alteration was influenced by fluids related to granodiorite intrusion, and 18O- and 87Sr-rich meteoric water circulating in the upper crust significantly contributed to hydrothermal activities in Barton Peninsula.