THE PETROGRAPHIC CHARACTERISTICS OF VOLCANIC ROCKS IN BA VI, HANOI

Abstract

Ba Vi mountain is located in the west of Hanoi, 50 km from Hanoi city, associated with the legend of Son Tinh - Thuy Tinh in the beliefs of the Vietnamese nation. Due to its advantageous location and natural conditions, the Ba Vi Mountain area has grabbed a lot of attention from tourists for sightseeing and tourism, as well as educational internship. Some research topics on the material composition of Ba Vi Mountain have been conducted to establish a heritage profile for the region, but they have only addressed the characteristics and origins of the volcanic eruption layers from an altitude of 1200 meters. To clarify the petrological characteristics and the origins of the volcanic rock formations that constitute Ba Vi Mountain, 14 petrological samples at altitudes from 120 - 200 m and 800 - 1200 m have been collected, were analyzed for their mineral composition using optical microscopy and their chemical composition using X-ray fluorescence (XRF) method. The results revealed that Ba Vi Mountain is a product of various stages of volcanic eruptions from mantle source magma, with high alkaline content (Na₂O + K₂O = 4 – 14 %) and belong to the basalte (trachybasalte), basalteandesite, trachydacite, rhyolite, and alkali basaltoid associations. The rocks have a massed or foliated structure, light gray to dark. They have fragmentary, tuff, small grains, or porphyritic textures with phenocrysts such as plagioclase and feldspar. The main mineral composition is plagioclase, K-feldspar, quartz, ore minerals are pyrite and secondary minerals are sericite, chlorite.