Petrography and zircon U-PB geochronology of the Cha Val quartz diorite, Quang Nam and its application

Abstract

Cha Val bodies are widely distributed in the South of the Truong Son belt, with composition mainly consisting of diorite, quartz diorites and granodiorites. The Cha Val quartz diorites are mainly characterized by plagioclase (40-45 %), hornblende (25-35 %), biotite (0-15 %), quartz (0-15 %), and K-feldspar (0-5 %). The presence of hornblende (25-35 %) is one of the specific evidences for the Itype granite. The zircon of quartz diorite sample V1116 was collected at Cha Val body, Quang Nam, with geographical coordinates (N15◦ 38' 14.1", E107◦ 33' 6.4") along National Highway 14D cutting through the Cha Val body. The cathodoluminescence image shows that the zircon grains from the Cha Val quartz diorites, Quang Nam are less broken and do not have surrounding metamorphic rims. They show euhedral grains in shape and oscillatory zoning structures, indicating typical magmatic origin. The Cha Val quartz diorites determined by the LA-ICP-MS U-Pb method on 20 zircon points display 206Pb/238U age of 229 Ma to 270 Ma and yield a weighted mean 206Pb/238U age 255 ± 4.7 Ma (n = 17, MSWD = 2.5). This age is considered to be the crystallization age of the Cha Val quartz diorites, corresponding to the Late Permian. The formation of the Cha Val quartz diorites is likely related to the convergence of the Indochina and South China blocks during the Permian-Triassic.