Abstract:

Neoarchean pelitic gneisses crop out in the Sidaolazihe formation of the Longgang Group, southern Jilin Province. Through systematical study on petrography, mineral chemistry and LA-ICP-MS zircon U-Pb chronology of these gneisses, this paper has discussed the metamorphic age, evolution, and its tectonic significance. Our results indicate that three distinct metamorphic stages are recorded in these rocks: 1) the prograde stage (M₁) with mineral assemblage of garnet + biotite + plagioclase + quartz + muscovite; 2) the peak granulite facies stage (M₂) with mineral assemblage of garnet + sillimanite + K-feldspar + biotite + plagioclase + quartz; and 3) the retrograde stage (M₃) with assemblage of kyanite + garnet + biotite + plagioclase + quartz + muscovite, which is characterized by the transformation from sillimanite into kyanite. Conventional geothermobarometries are used to estimate the metamorphic T and p conditions. The metamorphic conditions for M₁ stage are 468~515 ℃ and (3.8~4.3)×10⁵ kPa, 703~760 ℃ and (6.6~7.1)×10⁵ kPa for the M₂ stage, 552~591℃ and (5.5~6.0)×10⁵ kPa for the M₃ stage, which constitute a typical isobaric cooling (IBC)-type p-T-t path, suggesting that the metamorphism likely formed from underplating of the mantle-derived magmas under the interaction between mantle plume and lithosphere. Zircon U-Pb dating shows the granulite facies metamorphism occurred at 2 495 ~2 442 Ma, which likely represents the Neoarchean metamorphic thermal event.

Key words: Longgang Group, Sidaolazihe Formation, pelitic gneisses, metamorphism evolution, Eastern block of North China craton