Abstract

Tri Nang mafic - ultramafic intrusion is distributed mainly within the Tri Nang commune, Lang Chanh town, Thanh Hoa province. The intrusion has variety petrographical compositions from ultramafic to mafic rocks including plagioclase-bearing wehrlite and dunite, troctolite, dark to light-colored olivine-bearing and olivine free gabbro varieties, anorthosite, pyroxenite and pegmatoid. The samples used for analysis are gabbro olivine, which are main component of Tri Nang intrusion. Zircon separated from gabbro olivine was dated by U-Pb isotope using SHRIMP method. U-Pb isotopic analyses give age of 264±3,6 Ma, which is considered to be the age of intrusion. The newly determined age value of Tri Nang intrusion corresponds to Late Permian, much older than previously reported Late Triasic age and is very close to the recently published age of the Nui Chua complex (251±3.4 Ma) in the Northeast Viet Nam. It shows that the Tri Nang and Nui Chua complexes can be belong to the same Permian - Trias magma activity period in Vietnam. Therefore, in terms of age and some other characteristics, there is a basis to classify the Tri Nang and other intrusions of Tri Nang complex (the Tri Nang complex) in the Northwest region into Nui Chua complex. The newly obtained results provide valuable data for the study of history of regional geological development.