Tran Van Thanh, Pham Trung Hieu, Pham Minh, Do Van Nhuan & Nguyen Thi Bich Thuy
ABSTRACT
The Song Ma zone in Northwestern Vietnam is considered as a suture of the South China and Indochina blocks, and plays an important role in understanding the tectonic evolution in Southeast Asia and surrounding areas. Granitic rocks of the Muong Lat complex are distributed in the Song Ma suture, consisting mainly of monzogranite and granodiorite. The rocks are subalkaline in affinity with high K contents and characteristics of S-type granite. They have high SiO2, total Na2O + K2O, and Zr contents and high aluminium saturation index values (0.97 to 1.29 with an average of 1.14). All these geochemical signatures, together with high (87Sr/86Sr)i ratios (0.71391 to 0.74568), and low whole rock εNd(t) and zircon εHf(t) values (−13.1 to −9.4) suggested an origin from partial melting of crustal source rocks. Primitive mantle normalized trace element
contents exhibit positive anomalies of Rb, Th, U and Pb, but negative anomalies of Nb, Ta, and Ti, indicating a collision-related environment. The depleted Nd and Hf zircon model ages from investigated samples, in combination with inherited zircon ages, suggest the existence of Precambrian basement in the study region. S-type granites of the late Permian-early Triassic (the Muong Lat, Phia Bioc and Hai Van complexes) in Vietnam were formed by the subductioncollision of the South China and Indochina blocks.