The genesis of Jitang duplex granites in east Tibet: Evidence from the zircon UPb age and rock geochemistry

Jitang duplex granites are located in the northern section of the Lancang River magma belt, and serve as an important window for studying the evolution process of Lancang River juncture zone. In this paper, the LA-ICP-MS zircon U-Pb chronology and rock geochemistry of biotite monzonitic granite and granitic diorite in Jitang duplex granites of Kakagong area along northern Lancang River were studied. The results show that the selected zircon samples exhibit obvious growth ring zone, the Th/U ratio is generally higher than 0.4, and the typical magma zircon has average weighted ²⁰⁶Pb/²³⁸U age of 222.8±1.5Ma (MSWD=1.60, n=16), 213.6±1.1Ma (MSWD=0.98, n=20) and 221.1±1.5Ma (MSWD=1.30, n=15), suggesting Late Triassic. The geochemical characteristics of rocks show that the Jitang biotite monzonitic granite and granitic diorite have relatively consistent content of main and trace elements, and their changing characteristics are also consistent. It is shown that these two kinds of rocks might have been derived from the same magma, and the Jitang duplex granites belong to the aluminum S-type granite, which has the same petrogeochemical properties as Lincang granite and Newcastle granite, and has a unified tectonic magmatic activity mode along the Lancang River granite belt. The formation of Jitang duplex granites was related to the thickening and warming of the crust and the deep melting effect related to the shearing and stretching period of lithosphere, with the closure time of Lancang Jiang Ocean was probably at about 273Ma.