Late Permian to Early Triassic crustal evolution of the Kontum massif, central Vietnam: zircon U-Pb ages and geochemical and Nd-Hf isotopic composition of the Hai Van granitoid complex

摘要

The Hai Van granitoid complex constitutes an important part of the Indosinian batholith of the northern Kontum massif in central Vietnam. The Kontum massif is a key region for understanding the palaeo-Tethyan accretionary history and the crustal evolution of the Indochina terrane. In this study, zircon U-Pb ages and geochemical data on the Hai Van complex are reported to shed more light on these geological processes. Zircon U-Pb isotopic analyses of six representative samples by the LA-ICP-MS technique show that granitoids from this complex crystallized between 242 and 224Ma, indicating a prolonged period of magmatism in a late orogenic stage. Rocks from the Hai Van complex have S-type geochemical characteristics with high SiO2, Na2O+K2O, and Zr contents as well as typical S-type minerals such as cordierite and muscovite. Trace elements of the samples are depleted in Ba, Nb, P, Sr, and Ti and enriched in Rb, K, Th, U, and Pb. Negative Eu-anomalies indicate feldspar fractionation during magma crystallization. The Hai Van granitoids exhibit Nd-Hf isotopic features similar to Proterozoic rocks exposed in South China, with low initial e(Hf) and initial e(Nd) values and two-stage Hf model ages of 1.9-1.7Ga. It is suggested that during the palaeo-Tethyan orogeny, central Vietnam experienced strong early Mesozoic magmatism that originated from partial melting of crustal material similar in composition to the basement of the South China block.