Abstract:

As the old basement in northern Great Xing’an Range, the Jiageda Formation has attracted more and more attention in recent years. The different views advised by previous researchers on tectonic setting of the Jiageda Formation have significantly restricted the correct understanding of natures and geological evolution processes of the basement in this area. We conducted systematic studies on rock association, metamorphic-deformational fabric, geochemical characteristics and tectonic setting of ‘Jiageda Formation’ in the Badaguan area of E’erguna, Inner Mongolia. Research results indicate that the ‘Jiageda Formation’ in the study area is a Yanshanian intrusion that underwent tectonic metamorphism and deformation, instead of the Precambrian strata. The rock of this intrusion are actually biotite monzo-blastomylonite, granodioritic blastomylonite, biotite amphibole plagioclase mylonitic gneiss. These rocks are characterized by high K, Al and low Ti, and shown as high-K calc alkaline series. They are high in total REE and LREE contents, and show obvious fractionation of LREE from HREE with negative Eu anomaly to some extents. Such geochemical features as enrichment of the LILE and depletion of the HSFE suggest that their parent magma was contaminated by the crustal material. This suite of intrusion formed under the tectonic setting from compression to extension during the formation and evolution of the Mongolia-Okhotsk orogenic belt from the Late Triassic to Late Jurassic period. The intense activity of the Derbugan fault during the Early Cretaceous period resulted in the low amphibolite facies metamorphism and deformation of these rocks.

Key words: Jiageda Formation, metamorphism and deformation, protolith, geochemistry, Badaguan, E’erguna region