GEOCHEMICAL SIGNIFICANCES OF PERMOTRIASSIC MAGMATISM OF NE VIETNAM Tran Trong Hoa1, Tran Tuan Anh1, Ngo Thi Phuong1, Poliakov G.V 2, Izokh A.E 2, , Lan C.Y 3, Chung S.L4, Lo C.H4 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 (TiO2 – 1.08 – 1.36 wt %) to high titanium (TiO2 – 2.22-3.39 wt.%), sodium rich medium to low alkalinity (Na2O/K2O – 1.4-8.5), relatively low in Al2O3 (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 87Sr/86Sr initial ratio in basalts and lherzolite (0.7066 – 0.709) are relatively high Dacite and rhyolite (SiO2 = 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%; K2O=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 87Sr/86Sr ratios is 0.723 , represents strongly crustal signature Mafic intrusions of adjacent Phu Ngu and Lo Gam structures are low Timedium-Mg gabbro, along with high-Ti, low-Mg monsogabbro Their alkalinity varied widely, especially in monsogabbro, K2O = 2.91-4.16 wt.% They are enriched in Rb, Th, U, LREE and depleted in Nb, Zr, Ta Their REE and spidergram patterns (Fig 3) are similar to Song Hien’s rocks, minor differences are sharply Eu anomaly Associated syenites are mainly of shoshonitic to potassic series (K2O = 6.72 – 11.39 wt.%) Similar REE and spidergram patterns are recorded Ininial 87 Sr/86Sr ratios of gabbro and syenites are homogenous: 0.708-0.710, 143Nd/144/Nd(T[250 Ma])=0,51198- 0,51213, (εNd(T)= -3,58 -6,45), and0,51188-0,51205 (εNd(T)= -5,19 -8,51), respectively Granite (251Ma) are garnet-bearing aluminous type They are characterized by medium- to high- alkaline, potassium rich, enriched in Rb, Ba, Th, U, Lree and depleted in Nb, Ta Their isotopic significances are: 143 87 Sr/86Sr – 0,7328-0.7388, Nd/144Nd – 0.5119-0.5121, (εNd(0) = (-11) – (-13) Geochemical and isotopic characteristics of the volcano-plutonic rocks of NE Vietnam represent they are both derived from magmas that were strongly contaminated by crustal and subduction – derived materials This feature are distinct to the Permian – Triassic magmatism of Song Da rift, Northwest Vietnam, that were related to Emeishan plume Fig 1: Chondrit normalized (Sun&Mc.Donough, 1986) paterns of PermianTriassic lerzolites (255±14 Ma), gabbro, basalts (266±13 Ma) and ryolites (227±1.6 Ma) in the Songhien depression – NE Vietnam Fig 2: PM-normalized (Sun&Mc.Donough, 1986) paterns of Permian-Triassic lerzolites, gabbro, basalts and ryolites in the Songhien depression (NE Vietnam) Fig.3 Chondrite- and PM- normalized (Sun&Mc.Donough, 1989) patterns of Permian-Triassic gabbro (245 Ma) and syenites (233 ma) in Lo Gam structure, NE Vietnam Reference: Tran Trong Hoa, Tran Tuan Anh, Ngo Thi Phuong, A.E Izokh, G.V Poliakov, P.A Balykin, Lan C-Y, Hoang Huu Thanh, Bui An Nien, Pham Thi Dung, 2004 Gabbro-Syenite Associations of East Bac Bo structures: Evidences of Intra-plate Magmatism? Jour of Geology, Series B, No23 P.12-25 Tran Trong Hoa, Tran Tuan Anh, Ngo Thi Phuong, Pham Thi Dung, Tran Viet Anh, 2004 Permian-Triassic Basalt-Ryolit associations in the Song Hien structure, NE Vietnam J of Sciences of the Earth 26 (4) pp 392-405 Tran Trong Hoa, Ngo Thi Phuong, Tran Tuan Anh, Hoang Huu Thanh, Vu Van Van, Tran Viet Anh, Poliakov G.V., Balykin P.A., Izokh A.E., Glotov A.I., Lan C-Y, Chung S-L, 2004 Permo-Triassic Bimodal Basalt-Ryolite Associations in Song Hien Depression, NE Vietnam IGCP 430 Continenntal Dynamics Workshop: Collision-Related Mantle Flow and Lithosphere Deformation Models May 23-31, Kunming , China Tran Trong Hoa, Tran Tuan Anh, Ngo Thi Phuong, Hoang Huu Thanh, Vu Van Van, Nguyen Viet Y, Tran Quoc Hung, Phan Luu Anh, Bui An Nien, Pham Thi Dung, Tran Viet Anh, Hoang Viet Hang, Tran Hong Lam, 2004 Magmatic Formations in Vietnam and Their Tectonic Implications IGCP 430 Continenntal Dynamics Workshop: Collision-Related Mantle Flow and Lithosphere Deformation Models May 23-31, Kunming, China ... to the Permian – Triassic magmatism of Song Da rift, Northwest Vietnam, that were related to Emeishan plume Fig 1: Chondrit normalized (Sun&Mc.Donough, 1986) paterns of PermianTriassic lerzolites... Songhien depression – NE Vietnam Fig 2: PM-normalized (Sun&Mc.Donough, 1986) paterns of Permian -Triassic lerzolites, gabbro, basalts and ryolites in the Songhien depression (NE Vietnam) Fig.3 Chondrite-... 0,7328-0.7388, Nd/144Nd – 0.5119-0.5121, (εNd(0) = (-11) – (-13) Geochemical and isotopic characteristics of the volcano-plutonic rocks of NE Vietnam represent they are both derived from magmas that