DSpace at VNU: Cenozoic magmatism of Eastern Sea (South China Sea)

/N U Jo u rn a l of Science, E arth Sciences 23 (2007) 43-51 Cenozoic magmatism of Eastern Sea (South China Sea) Phan Truong Thi1, Vo Viet Van2'* 1College o f Science, VNU Hanoi University o f Technology, VNU Ho Chi M inh City Rcccived 07 Jan u a ry 2008; received ỉn rev iscd form 12 M arch 2008 A bstract In E astcrn Sea (South C h in a Soa), Jurassic - C retaceous m ag m atic intrusivcs are disco v cred , th e y vvere cu ttin g th e P re-C enozoic sed im en taries T he C cnozoic volcanic basalts are stro n g ly d istrib u te d to g eth cr vvith C en o zo ic scdim entarics T h e p a p e r conccrns only th e volcanic activities w ith d csc rip tio n on th cir geology, chem istry, gcochem istry, isotopic ages a n d their form ing m ech an ism In fact, th e m ain topic of thc p ap e r is d e a lt vvith the form ing m echanism of basalt in the d y n am ics of thc o pening of E astcm Sea in particular, and o í South C hina Sea in general Keyivords: C enozoic; M agm atism ; E astern Sea Generality on cenozoic geology of Eastem Sea The geom orphology and gencral d istribution picture of Eastern Sea crust types are presented in Fig The given d ata are received from the research of satellite im ages and of geophysic m easurem cnts The crusts are covercd bv C enozoic sedim entaries [2, 3] Their geom agnetic ages are also represented in Fig [2, 3] The geom agnetic anom alies lincs N.5 (16.5 M a) and N 13 (32 Ma) are rem arked, they also arc recognized as the tim ing of thc o pening and closing of Eastern Sea in his geodynam ics respectively The íollovvs are Late M iocene and Pliocene sedim entaries, covered all the territory of the South China Sea These results are obtained C o n tin en tal crust O ccanic crust E xtcnded Continental crust Fig T he g co m o rp h o lo g y and stru ctu re of crust ty p es of S outh C hina Sea by scientiíic cooperation betw een Hanoi U niversity and Paris VI U niversity [8, 9,12] * Corresponding author Tcl.: 84-913535203 E-maii: banganus@ yahoo.com 43 44 Phan Truông Thi, Vo Viet Van / VN U Ịoumaỉ of Sàence, Earth Sciences 24 (2008) 43-51 Petrography and Eastem Sea basalts petrochemistry of According to the research d ata of seismic m easurem ents on the Eastem Sea, the volcanic basalts m ainly cut ou t the sedim entaries of Oligocen - Pleistocen Geom orphological stud y of the bottom of the sea also m akes clear the strong relieí ch a c te r by a lot of u n d e rg ro u n d v o lcan ic m ountains After the stu d y of volcanic isotopic ages by m ethod K-Ar and by track íission of zircon grains, the ages of basalts varied m ainly from 13 Ma to 1924 year It is clear in Table that the age of basalt on the Re Island is 13 Ma, meanvvhile the age of the basalt in Con Co Island is only 3,200 years In the 1924 year, there w as a volcanic activity near N Trang City From the petrochem ical d ata (Tables and 2) it is clear that basalts of E astem Sea are belonged to tw o rock types: alkaline and tholeitic series The first one is characterized by the N e-norm atíve m ineral, and the second one - by the positive Q- norm ative and absent of N e-norm ative mineral Geologically, the basalt bodies m ainly cut up the sedim entaries of O ligocene and M idle M iocene ages, they have isotopic ages varied from 13 M a u p to day (1924) It m eans that they are m ore young than the closing tim ing of Eastern Sea (16.5 Ma) But aíter the drill core sam ples of C uu Long and N am Con Son basins, the intercalation of basalt layers am ong the sedim entaies also beeing seen (Fig [15]) A lthough that, m ainly the volcanic activity in Eastern Sea vvas strongly developed after the closing tim e of the spreading process of Eastern Sea Fig Schem a d istrib u tio n of geom agnetic an o m aly lin es [2, 3) Phan Truông Thi, Vo Viet Van / VNU Ịoumaì ofSáence, Earth Sciences 24 (2008) 43-51 45 T able T h e c h e m istry com p o sitio n s,iso to p ic ages of b asalt ío rm atio n s of th e E astern Sca (13) C on C o Ĩ8 Khe Sanh DBP AB OT OT OT 52.9 44.77 52.67 48 55.26 1.57 2.85 1.78 2.68 2.04 15.32 15.01 14.17 16.5 14.61 16 Sites X L II XLI -C en d re 1-C endre Kaw it-Is Re- Is Re-Is-2 QN Rock ty p c BA AB AB OT OT OT OT S i0 43.26 45.01 49.5 50.4 49.44 49.87 TiO j 2.76 2.03 2.34 1.9 2.67 22 A12O j 12.28 13.84 13.51 13.91 13.87 FeO* 12.15 12.78 11.67 11.52 11.42 10.93 0.43 11.51 10.22 10.83 9.66 M nO 0.2 0.15 0.16 0.15 0.16 0.14 0.14 0.22 0.12 0.15 8.16 M gO 12.96 9.22 7.86 8.18 8.5 7.1 7.45 11.21 4.17 8.62 5.36 C aO 10.99 9.86 9.13 9.13 8.18 8.93 10.12 5.67 9.15 6.15 N a20 2.87 2.76 3.27 313 313 3.11 3.28 3.22 4.92 3.09 k 2o 1.32 2.48 2.27 7.83 1.83 2.1 0.92 0.94 3.22 1.54 2.04 P 2O K20 / N a 20 0.94 0.56 0.42 0.38 0.38 0.57 0.29 0.72 0.54 0.32 0.46 0.81 0.61 0.58 0.53 0.704 0.28 0.201 0.65 0.458 0.68 Zr/Y 10.9 10 7.1 7.8 5.3 11 11 9.2 9.1 Z r/N b 236.6 251.8 262.3 164 100 263.2 4.06 200 5.98 Rb/Sr 0.17 0.08 0.06 0.03 0.05 0.07 0.08 0.01 0.22 Ba/Zr 3.64 3.29 2.3 3.25 2.24 3.9 3.35 2.93 1.75 n 8 1 Afio(Ma) 0.4-1.1 0.4-1.1 -0 7 0.4-1.2 12 7.1 0.35 1 * Sites : XL: X uan Loc; Q N : Q u a n g N gai; DBP : Dien Bicn Phu T ypcs: BA: ba/.anite; AB: A lkaline Basalt; OT: Tholeite basalt Fig Seism ic section of C uu Long b asin reAected th e "stratified layers" of volcanic b o d ies (black) in th e M idle M iocene sed im en tarie s [15] Phan Truong Thi, Vo Vỉet Van / VNU Ịoum aỉ o f Science, Earth Sciences 24 (2008) 43-5Ĩ 46 T ablc N o rm ativ e com position of so m e basalts o f E astern Sca S i0 T i0 a i 2o Fe20 FeO M nO M gO 51.11 173 16.01 50.00 52.91 2.43 13.98 - 46.95 2.36 15.87 - 49.71 210 16.22 3.74 16.23 - 3.40 16.70 - 9.92 012 10.78 0.13 5.60 11.09 0.15 7.00 8.21 0.14 6.97 10.80 0.11 6.03 10.15 0.14 3.06 10.11 0.19 6.12 9.00 10.86 9.18 3.12 0.72 0.24 97.81 0.97 - 3.08 3.57 0.51 0.30 99.73 2.11 9.52 3.28 2.19 0.62 98.9 - 3.02 4.05 C aO N a20 k 2o P2 O Total 5.50 4.77 8.42 312 0.70 97.03 2.01 0.75 97.48 Q c Or Ab An Lc 18.44 31.82 1.40 15.10 - 7.05 2.64 3.38 2.66 0.61 96.38 15.72 13.06 - 11.88 28.6 17.04 - 21.89 23.60 - Ne 4.63 - 0.24 4.26 26.40 25.07 - Di 8.10 - 16.49 — - 6.11 • 13.14 - 22.93 - 4.62 1.74 4.48 Wo Hy Ol 16.07 Mt 11 Ap Cc 3.29 1.62 1.41 48.9 46.73 1.11 98.21 12.48 30.21 12.95 18.07 24.38 - - 22.03 - 14.76 - 19.12 - 14.18 - 15.70 - 23.14 - 1.89 6.34 - - 8.45 7.87 1.74 164 1.62 2.66 0.56 3.99 0.65 710 2.42 6.46 1.35 26.06 28.94 - 5.25 13.18 T ablc T he chem ical com positions of stratiíie d layers of basaits am o n g th e sed im en tarie s of C u u Long B asin [15] SiCh TiCh AI2O Fc:Oy FeO M nO M gO CaO 45.18 1.45 17.67 2.28 6.27 0.14 50.38 0.95 47.26 1.68 47.67 49.34 47.47 17.70 3.04 15.38 4.08 8.08 1.67 17.42 1.88 16.59 1.26 16.92 3.53 6.36 9.76 3.14 8.15 0.14 0.30 6.45 2.69 2.10 6.67 3.67 5.29 0.66 4.30 - 10 49.00 1.32 47.39 1.19 48.56 15.90 2.18 6.27 17.16 10.94 - 50.16 1.32 15.68 5.18 4.92 0.16 9.00 0.10 0.17 0.12 5.87 7.38 2.11 5.86 8.87 2.09 2.45 4.06 5.22 9.61 3.34 0.79 2.45 4.41 0.11 5.84 5.90 5.22 0.10 0.82 0.16 021 0.16 0.16 1.33 16.44 8.47 - 6.85 6.03 0.11 5.63 7.81 N a :0 K2 O 4.75 1.38 3.09 1.42 P2O 0.32 0.39 0.11 0.29 s LOI 0.10 0.10 - 0.10 - 0.10 0.10 - 0.10 0.24 0.10 6.90 4.68 6.23 6.12 3.77 4.67 6.57 4.67 1.47 6.57 Total 99.32 99.60 99.49 99.49 99.50 99.41 99.39 99.50 99.54 99.50 - 6.77 4.78 3.02 0.61 7.84 0.27 Phan Truông Thi, Vo Viet Van / VNU Ịoumaỉ o f Science, Earth Sciences 24 (2008) 43-51 47 From Table 3, th e total a m o u n t o í (N a20 + K:0 ) is m ore than 5-6%, vvhat coníirm s that the alkality of rocks is high an d it is belonging to the alkaline basalt type Nc 01 Di Qz Hy Ậ Qwp