GEOCHEMICAL AND PETROLOGICAL CHARACTERISTICS OF TRIASSIC GRANITES IN KONTUM MASSIF, CENTRAL VIETNAM

The Kontum Massif in central Vietnam is a major region of older sialic continental crust that has undergone extensive magmatic and metamorphic activity from the Late Paleozoic to Early Mesozoic with the most recent activity associated with the Indosinian Orogeny. During the early to middle Triassic, the Kontum Massif experienced its final phase of granitic magmatism, represented by the Hai Van, Van Canh, and Ba Na complexes. These complexes have high silica (SiO₂= 67.7-80.7 wt.%), alkali metal (Na₂O+K₂O= 5.95-9.0 wt.%), and relatively low aluminum (Al₂O₃ = 10.18-15.58 wt.%) and titanium (TiO₂=0.02-0.56 wt.%) concentrations. The rocks are LREE-enriched and have low T_Zr (~730^oC) and T_SiO2 (~680^oC) temperatures. Geochemically, they can be divided into (1) collision-related (229-251 Ma) granites and (2) post-collision (224-245 Ma) leucogranites. The collision-related granites consist of the Hai Van complex and the initial phase of the Van Canh complex and are peraluminous, magnesian, and calcic to alkali calcic. They are compositionally similar to plutons from the main portion of Corderillan batholiths. Furthermore, the collision-related granites classify as slab failure granites and imply that they represent a period of slab detachment during terminal phase of the Indosinian orogeny. The post-collision leucogranites consists of the Ba Na complex and the second phase of Van Canh complex. The rocks are peraluminous, ferroan, and calc-alkalic to alkalic-calcic. The leucogranites collected from Ba Na intrusion are were likely derived by partial melting of the upper crust (clay-poor sedimentary rocks), whereas the second phase of the Van Canh complex was derived from a non-sedimentary crustal source.