ABSTRACT

The area that is located in the Tertiary Gampo basin is mainly composed of Cretaceous shale, Tertiary rhyolite, granite, and basalt in ascending order. The mineralogy, isotopic composition (δD and δ¹⁸O) of clay minerals, and chlorite geothermometry were performed to interpret the alteration history of these rocks. Clay minerals occurring in basalt have a limited range and are heavier in δ¹⁸O than the clay minerals in granite and rhyolite. Chlorite occurring in basalt has lower formation temperatures than chlorite in granite or rhyolite. Geochemical studies were carried out to identify the behaviors of major, trace and rare earth elements (REE) during alteration processes. Most major oxides such as SiO₂, Al₂O₃, Fe₂O₃, Na₂O, MgO, TiO₂, MnO, and P₂O₅ of basalt are relatively immobile in most altered zones. In contrast, these oxides in granite and rhyolite are relatively mobile, and show some irregular variations in the most altered zones. Some trace elements in basalt have less prominent variations than in granite and rhyolite. REE distributions of altered basalt and granite do not show prominent variations with increasing alteration degree, whereas rhyolite is enriched in REE and has a positive Eu anomaly. These results indicate that altered basalts that have experienced only low temperature alteration such as weathering, have indistinct variations of major oxides, some trace elements and REE according to alteration intensity. Granite and rhyolite that have experienced both hydrothermal alteration (low temperature) and weathering process are characterized by relatively prominent variations of major oxides, some trace elements and REE according to alteration intensity. The results coincide well with those of mineralogical studies, isotopic compositions of clay minerals, and chlorite geothermometry.