ZIRCON U-PB ISOTOPE AGE OF SYENITE FROM PU SAM CÁP COMPLEX AND ITS GEOLOGICAL IMPLICATION

Nguyễn Thị Bích Thủy

General Department of Geology and Minerals of Vietnam

Corresponding author: thuynguyendcks@yahoo.com

Abstract: The Pu Sam Cáp complex consists of granosyenite, syenite, alkaline syenite and granite. These rocks exposed under the form of small bodies. The rocks are grey, light grey in color and have the porphyritic texture with phenocryst of K-feldspar. The main rock-forming minerals are K-feldspar, plagioclase, biotite and alkaline amphibole and pyroxene. Accessory phases are sphene, zircon and apatite. Zircon grains separated from two syenite samples collected in Chinh Sáng area have grain sizes ranging from 80 to 120 µm. Most of zircon grains have isometric and euhedral shape, display oscillatory zoning indicating magmatic origin. LA-ICP-MS zircon U-Pb analytical results from these two samples revealed emplacement age of the Pu Sam Cáp complex at ca. 35 Ma. The new geochronological data of magmatic rocks of the Pu Sam Cáp complex provide evidence for Middle Paleogene magmatism and formation of continental crust in the study area.

Keywords: Pu Sam Cáp syenite, U-Pb zircon age, Chinh Sáng, Northwest Vietnam

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