Abstract:

The giant phenocryst adamellite is an important magmatism record of the Mongol Ocean plate in the late period of the Early Paleozoic Era. Their formation age and geochemical characteristics are significant to reveal the tectonic framework and evolution history of the Mongol Ocean plate at that period. The paper presents the zircon U-he U-Pb age of zircon of adamellite given is (454.3±3.8) Ma, belonging to the Late Ordovician. The geochemical data of granite show its high-K calc-alkaline nature. The granite is also characterized with high silicon, with SiO₂ being 69.82%-75.16%, Al₂O₃ 12.91%-14.49%, and Na₂O/K₂O 0.60%-0.77%, peraluminous, with aluminum saturation index being in the range of 1.11-1.22, and Mg^# index of 39-44. The total content of REE is 43.47×10^-6 -158.11×10^-6, δEu=0.26-0.49. The REE distribution pattern of adamellite is demonstrated by a significant negative Eu anomaly and a “swallow-type” shape. The spider diagrams of trace elements standardized by primitive mantle show apparent enrichment of Rb, Th, U, K, Zr, Hf, strong depletion of Ba, Sr, P, Ti and medium to slight depletion of Nb and Ta. The initial ¹⁴³Nd/¹⁴⁴Nd values are high (0.512 449-0.512 507), and the average value is 0.512 477. The ε_Nd(t) values are all positive (0.2-1.3). The characters of major elements, trace elements, and isotopes show that the giant phenocryst adamellite in the Southeast Mongolia was formed at an extensional geotectonic environment during the transitional period from syn-collision to post-orogenic tectonic systems. The extensional tectonics and the underplating of mantle-derived basic magmas are suggested to be the main dynamic mechanism to form the granite. Since the adamellite is the important magma record of the Mongol Ocean plate subduction in the early Paleozonic, the present research is helpful to reveal the tectonic evolution and the characters in the Palaeo-Mongolia Ocean plate area.

Key words: Mongolia, giant phenocryst adamellite, zircon U-Pb age, geochemistry, tectonic significance