SUMARRY

Petrogeochemical characteristics of the Bù Dình sub-volcanic granitoid massif

in Thanh Hóa, Viet Nam and its geological implication

Nguyễn Thị Xuân, Bùi Thế Anh, Đỗ Quốc Bình, Bùi Thanh Vân,

Tạ Đình Tùng, Nguyễn Đức Chính

The sub-volcanic granitoid of Bù Dình massif is located in Lang Chánh district, Thanh Hóa province, covering an area of nearly 50 km². The basic petrographic composition of the rock is porphyritic granite and coarse - medium to small grained granodiorite. In the rocks, the content of plagioclase mineral (mainly andezin) is equal or more than potassium feldspar. The rocks belong to medium K, calc-alkaline series and are dominant in sodium (K₂O/Na₂O <1). The concentrations of La, Ce, Sm and Nd elements are usually 20-70 times higher than those of the Chondrite. Furthermore, the rocks are characterized by medium fractionation between light - and heavy rare earth elements ((La /Yb)N = 12.8 -28.3). Bù Dình sub-volcanic granitoids have high SiO₂ content (>73%) and very low MgO with Mg# values of ca. 10 to 28, indicating the crustal origin of magma. Our new analytical results pointed out a similarity in petrography, mineralogy and geochemistry of the Bù Dình granitoids to those of the sub-volcanic granitioid of the Sông Mã complex and formed in the tectonic setting of plate collision.

Keywords: Red River Shear Zone, Tân Hương granite, zircon U-Pb isotope.