Abstract

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.