SUMARRY

Petrogeochemical characteristics of the B Dnh sub-volcanic granitoid massif

in Thanh Ha, Viet Nam and its geological implication

Nguyễn Thị Xun, Bi Thế Anh, Đỗ Quốc Bnh, Bi Thanh Vn,

Tạ Đnh Tng, Nguyễn Đức Chnh

 

The sub-volcanic granitoid of B Dnh massif is located in Lang Chnh district, Thanh Ha 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). B Dnh 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 B Dnh granitoids to those of the sub-volcanic granitioid of the Sng M complex and formed in the tectonic setting of plate collision.

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