Although the Sibumasu terrane in Asia was previously considered to be composed of Phanerozoic rocks with Cambrian crystalline basement, no reliable or direct radiometric dating evidences of such crystalline basement was ever reported. Our new in-situ zircon U/Pb dating of the Khao Tao orthogneiss yields a concordant age of 501.5±7.5Ma (2σ), which provides the first robust evidence for the Cambrian crust in Upper Peninsula of Thailand. The zircon εHf(T) values range from +3.7 to -6.1 with model ages (TCDM) of 1244-1827Ma, suggests a mixed crust-mantle source. The chemical similarity and spatial continuity of the Khao Tao orthogneiss with other pre-Neotethys marginal Eurasian and Sibumasu granitoids indicate the linear paleogeographic association under a similar magmatic arc-related regime along the Gondwana India-Australia margin as part of the Pan-African Orogeny system.
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