1 GEOCHEMICAL SIGNIFICANCES OF PERMO- TRIASSIC MAGMATISM OF NE VIETNAM Tran Trong Hoa 1 , Tran Tuan Anh 1 , Ngo Thi Phuong 1 , Poliakov G.V. 2 , Izokh A.E. 2 , , Lan C.Y. 3 , Chung S.L 4, Lo C.H 4 . 1- Institute of Geological Sciences – Vietnamese Academy of Sciences and Technology, Hanoi, Vietnam. 2- United Institute of Geology, Geophysics and Minerallogy – Siberian Branch - Russian Academy of Science, RUSSIA 3- Institute of Earth Sciences, Academia Sinica, Taipei, TAIWAN. 4- Department of Geosciences, Taiwan National University, Taipei, TAIWAN The Northeastern part of Vietnam is belonged to Southeastern margin of Yangtze Craton. The Permo-Triassic magmatism in the region are abundant and diversify. They are bimodal mafic-acidic volcanic-plutonic associations of Song Hien depression; aluminous granites and gabbro-syenite of Lo Gam and Phu Ngu structures. Ar-Ar dating of basalts , rhyolites, biotite granites, monsogabbero and nepheline syenites gave the ages: 266±13 Ma; 227±1.6 Ma; 250±1.1 Ma; 245 Ma, and 233 Ma, respectively. Basalts and andesitobasalts are belonged to tholeiitic series, with medium (TiO 2 – 1.08 – 1.36 wt. %) to high titanium (TiO 2 – 2.22-3.39 wt.%), sodium rich - medium to low alkalinity (Na 2 O/K 2 O – 1.4-8.5), relatively low in Al 2 O 3 (12.72-14.76 wt.%) and MgO (MgO – 4.63-8.33 wt.%). Both series are relatively enriched in Rb, Ba, Th, U and REE, and low in Sr, Nb, Ta, Zr. PM-normalized spidergram patterns (Fig. 2) are characterized by significant negative anomalies of Eu, Nb, Ta, and Zr, which are indications of subduction-involved magma. Associated rocks such as gabbro, lherzolite and picrite (MgO – 25.08-29.89 wt.%) also share same patterns. The 87 Sr/ 86 Sr initial ratio in basalts and lherzolite (0.7066 – 0.709) are relatively high. Dacite and rhyolite (SiO 2 = 62.44-74,54 wt.%) of Song Hien Depression are belonged to medium - high K – calc alkaline series (Total alkalinity: 5.85-7.84 wt%; K 2 O=4.68-5.21 wt%), relatively enriched in Rb, Sr, Th, Zr, and REE, depleted in Nb, Ta, P. REE (Fig. 1) and incompatible elements significances are similar to the above mafic rocks but have more sharply anomalies of Eu, Nb, and P. Initial 87 Sr/ 86 Sr ratios is 0.723 , represents strongly crustal signature.