Highly fractionated I-type granites in the northwestern part of Truong Son metallogenic belt, Laos-Vietnam: LA-ICP-MS zircon U-Pb age and its geological implications

The subduction age of the Song Ma suture zone has long been the hot spot for geological research in this region. The tectonic attributes of the granites from the Truong Son metallogenic belt in Laos-Vietnam zone are an important clue to the timing of the closure of the Song Ma Ocean. The present paper reports the LA-ICP-MS zircon U-Pb age of 261.34±0.69 Ma (MSWD=1.4) for the medium-to fine-grained granites from the northwestern part of Truong Son metallogenic belt, Laos-Vietnam zone. This age shows that the emplacement of the granites may be traced back to the Late Permian. The granites are characterized by high SiO₂(77.01%-78.07%), high K₂O (4.32%-4.57%), low Ca and Mg (CaO=0.32%-0.34%; MgO=0.04%-0.06%) and low P₂O₅, enrichment of Rb, U and Th, and depletion of Ba, Sr, P, Ti and Eu, suggesting the highly fractionated I-type granites. During the southward subduction of the Song Ma Ocean lithosphere, the mantle-derived magmas above the subduction zone firstly penetrated into the ancient crust giving rise to the generation of the juvenile crust, followed by the formation of the parent magmas caused by the mixed partial melting of the juvenile crust and ancient crust. Finally the highly fractional crystallization resulted in the formation of the granites in the study area.