Abstract:

The Chahayan pluton is located in the Eerguna block in the eastern Daxing’anling Mountains, and mainly consists of porphyritic biotitic syenogranite. The zircon grains separated from the syenogranite sample in the Chahayan pluton show generally euhedral form with fine oscillatory zoning on their CL images, and high Th/U ratios (0.32-1.12), which exhibit the features of magmatic zircons. LA-ICP-MS zircon U-Pb dating reveals that the Chahayan pluton emplaced at (236±1) Ma, belonging to the products of middle Triassic magmatism. Chahayan pluton displays massive structure without obvious deformation, and is characterized by enriched LILEs such as LREEs, Rb and K with negative Nb, Ta, P and Ti anomalies in geochemistry, which is similar to that of post-orogenic I-type granotoids, indicating the pluton probably formed in an extensional tectonic environment. Combined with previous studies in regional tectonics, the petrogenesis of Chahayan pluton is related to the lithosphere extension after the collision orogenic processes between the North China craton (NCC) and the Siberia plate. The Chahayan pluton has ε_Hf(t) values ranging from -3.94 to 2.19 with model ages from 1.1 to 1.5 Ga close to those of Early Paleozoic and Mesozoic granotoids in this region, indicating that the crustal accretion event of Eerguna block occurred mainly in Middle-and Neo-Proterozoic period.

Key words: Eerguna, Chahayan pluton, granite, U-Pb dating, Hf isotope, crustal growth