Phase equilibria and geochronology of Triassic blueschists in the Bikou terrane and Mesozoic tectonic evolution of the northwestern margin of the Yangtze Block (SW China)

Abstract

The northwestern margin of the Yangtze Block is an important part of the northeastern-most margin of the Tibetan Plateau and is composed of several terranes. However, how and when many terranes or microplates accreted to the Yangtze Block remain to be resolved. Among these, the Bikou terrane consists of Archean to Paleoproterozoic basement and Neoproterozoic volcanic rocks. Blueschists occur as lenses along a ductile shear zone in the southern margin of the Bikou terrane. The age of blueschist facies metamorphism remains an unsolved issue but is important to understand the tectonic evolution. The mineral assemblage of the blueschists is sodic amphibole, epidote, chlorite, albite and quartz. The calculated pseudosection constrains the peak P-T conditions of the blueschist-facies metamorphism to be 5.2–6.5 kbar at 350–385 °C. The blueschists contain zircon grains with an age of 765.2 ± 11 Ma, representing the age of the protolith similar to mafic volcanic rocks in the Bikou terrane. Deformed granitic veins in the blueschists have been subjected to blueschist facies metamorphism and contain phengites with Si contents of 3.5–3.6 per formula unit (pfu). The deformed veins contain zircon grains with ages of 232.3 ± 6.8 Ma and 230.3 ± 4.3 Ma, indicating that blueschist metamorphism is younger than 230 Ma. The new data, combined with previous studies, suggest that the Bikou terrane was accreted to the Yangtze Block in the Late Triassic during the collision between the North China and South China blocks. The blueschist belt, which marks the Late Triassic accretionary boundary between the Bikou terrane and the Yangtze Block, merged with the Qinling-Dabie Orogen to the northeast.