Ph Lộc v H Tam Monzogabroid plutons during history of the Indosinian orogeny in Nam Trường Sơn

Trần Trọng Ha, Bi n Nin, Trần Tuấn Anh, Phạm Ngọc Cẩn, Phạm Thị Dung,

Ng Thị Phượng, Izokh A.E., Shelepaev R.A., Svetliskaya T.V., Nevolko P.A.

Studies of petrology, geochemistry, and LA-ICP-MS zircon U-Pb isotope age were carried out for the monzogabbroids in the northern (Ph Lộc massif) and eastern margin (H Tam block) of the Kon Tum terrane. The plutons are composed mainly of gabbro, gabbro - diorite and diorite, which have typical mineral assemblages of plagioclase, hornblende ( pyroxene), biotit, and K-feldspar. Accessory minerals are apatite and titanite, opaque minerals are ilmenite and magnetite. The characteristic chemical composition of the rocks are relatively high TiO2, Fe2O3, Na2O + K2O (K2O/Na2O >1), and P2O5, rich in Rb, Th, U, La, Ce, Pr, Nd, and Sm, but depleted in Ba, Nb, Zr, and Hf elements. The chondrite- anomalized rare earth and primitive mantle normalized trace element patterns, as well as the ratios of Rb/Sr, Zr/Y, Nb/Ta and Nb/U from monzogabbroids suggested the rocks were formed by partial melting of the lithosphere mantle with contamination of crustal material. The emplacement ages of monzogabbroid of Ph Lộc and H Tam plutons were determined by zircon U-Pb isotope are at 239.2 2.1 and 238.82.0 Ma. The formation of these intrusions was associated with the oceanic lithosphere extension process, following the Indosinian collision-orogeny at the eastern and southeastern margins of the Trường Sơn Fold Belt. Apart from the subduction of the South China block beneaths the Indochina Block, the impact of mantle plume could have played a role in providing heat to the melting of the lower part of the lithosphere to produce monzogabbroid as well as other alkaline magmatic rocks in the Nam Trường Sơn area.

Keywords: Monzogabroid, Nam Trường Sơn, Indosinia, Ph Lộc, H Tam plutons