geochemistry of late cenozoic basalts in việt nam and its tectonic significances

12/9/2015 GEOCHEMISTRY OF LATE CENOZOIC BASALTS IN VIỆT NAM AND ITS TECTONIC SIGNIFICANCES

http://www.idm.gov.vn/nguon_luc/Xuat_ban/2004/B24/b65.htm 1/11

GEOCHEMISTRY OF LATE CENOZOIC BASALTS IN VIỆT NAM AND ITS TECTONIC SIGNIFICANCES

PHẠM TÍCH XUÂN1, NGUYỄN HOÀNG1,2, LEE HYUN KOO3

1Institute of Geosciences, AS&T VN, Hoàng Quốc Việt, Hà Nội 2Institute of Geosciences, Geological Survey of Japan, Tsukuba, Japan

3Chungnam National University, Republic of Korea

Abstract: The Late Cenozoic basalts consist of two main series related to two eruptive episodes. The early

episode (bN1-N21) comprises voluminous tholeiitic and olivine basalts, erupted from extensional fissures. The

late episode (bN22-Q), formed by central eruptions, comprises mainly alkaline basalts. The lower series

basalts are characterized by high SiO2, low TiO2, FeO*, Na2O+K2O and P2O5, while the upper series basalts

have low SiO2, high TiO2, FeO*, Na2O+K2O and P2O5 and usually have higher contents of incompatible

elements. Diversity of compositions of basalts was due to the difference of melt generation conditions (P-T,fluid), degree of partial melting or heterogeneity of sources. The lower series were generated under intensiveextension of territory (high degree of partial melting), in shallow mantle (low pressure). In contrast, the upperseries were generated under limited extension of territory (low degree of partial melting), in deeper mantle(high pressure). The differentiation in intensity and appearance of W-E extension is the cause of feature ofdistribution of late Cenozoic basalts: most of the eruptive centres are concentrated in the southern part ofCentral Việt Nam, while the northern part comprises isolated single centres.

INTRODUCTION

Cenozoic basaltic eruption in Việt Nam is rather widespread and is a part of regional volcanic activities taking place after thecollision between the India and Eurasia plates. The most part of interpretations on the geodynamics in Southeast Asia during theneotectonic stage have been basing on the tectonic model extrusion suggested by Tapponnier P. [15, 16] and other hypotheses basedon the above model. According to this model the Indochina Peninsula was extruded and glided southeastward along the Red Riverstrike-slip. Former studies [13, 14] have been recognizing two tectonic phases on the Vietnamese territory from Paleogene toPresent. The early phase is characterized by left-lateral shear along NW-SE trending faults parallelly with the Red River Fault in thefield of subparallel compression strain. On the contrary, the late phase is characterized by right-lateral shear along the above faults andthe east-west extension regime on submeridional faults. Cenozoic basalts in Việt Nam erupted mainly along submeridional extensionfaults or NE-SW normal faults, and were considered as closely related to the extensional regime of the late phase. In this paper theauthors present the geochemical features of the studied basalts with the aim to clarify their origin and forming conditions, contributingin the study on the geodynamic settings during neotectonic stage in Việt Nam.

I. FEATURES OF LATE CENOZOIC BASALTIC ERUPTION IN VIỆT NAM

Late Cenozoic volcanic activities in Việt Nam have been leaving basaltic covers scatteredly distributed from the North to the Southand many areas in the East Việt Nam Sea, but concentrated mainly in the South Trung Bộ Plateaux. These basaltic covers have very

different areas from some km2, such as in Lũng Pô Hồ, Điện Biên, to thousand km2 in Phước Long, Pleiku, Buôn Ma Thuột, etc.

(Fig. 1). The total area of the basaltic covers approximates over 25,000 km2 with the thickness changing from 1-2 to ~500 m. Theearliest occurrence time of Late Cenozoic basalt eruption in Việt Nam, to date, has been recognized at the end of Early Miocene inĐà Lạt area (17.6 Ma). The youngest volcanism has been recognized in Xuân Lộc, Cồn Cỏ and Đức Trọng with the age respectivelyof 0.44, 0.4 and 0.37 Ma. One can say that the most part of young volcanism in Việt Nam ended in the first half of MiddlePleistocene, in except of the eruption in the Tro (Ash) Island in 1923 year.



Figure 1. Distributive schema of Late Cenozoic basalts in Việt Nam and adjacent areas. Absolute age of basalts is based on

[10].

Some features of the basaltic covers are presented in the Table 1. According to recent studies [5, 10], Late Cenozoic volcanic

activities have been subdivided into two main phases: early phase of Miocene - Early Pliocene age (bN1-N21) and, late phase of Late

Pliocene - Quaternary (bN22-Q). Corresponding to these two effusive phase there are two basalt groups. Basalts of the early phase

are characterized by the type of effusion along fractures forming large basaltic plateaux, basaltic flows and occupying the major part ofCenozoic effusives. They are composed mainly of quartz tholeiite and olivine tholeiite. Basalts of the late phase are characterized bythe type of central effusion leaving many volcanic structures, such as cones, maars … and usually have the eruptive character withsuch products as volcanic bombs, ashes, tuffs and pyroclastics. They are characterized by the predominance of high-alkaline basalts.Especially, in high-alkaline basalts of the late phase there usually are mantle xenoliths, such as spinel lherzolite, harzburgite, wehrlite,pyroxenite, etc. and large crystals of augite, olivine and plagioclase.

Table 1. Features of Late Cenozoic basaltic covers

LocalitiesAreas

(km2)

Thickness(m)

Age (Ma) LocalitiesAreas

(km2))

Thickness (m)

Age (Ma)

Lũng PôHồ

1 100 ? Buôn MaThuột

3,800 260 8,9 - 1,63

Điện BiênPhủ

2 - 5,8 Pleiku >4,000 > 440 7,4 - 1,59

Nghĩa Đàn 6 - 5,9 QuảngNgãi

5 - 16,8 - 1,68

Quảng Trị > 12 - 7,8 - 0,4 Xuân Lộc 2,400 > 130 11,58 - 0,44

Đà Lạt 2,500 300 17,6-0,37 Phú Quý 2,5 - (1923)

Phước > 6,000 250 9,1 - 4,6



Long

II. METHOD OF ANALYSIS

Samples used in analysis consist of fresh rock samples selected from drill cores and surface exposures taken from almost all basaltgroups described in [4, 8-10]. The components of main elements have been analyzed by XRF method on the Japanese measurerRikagu RIX 2100 with the degree of precision of ± 5%. The components of trace elements have been analyzed by plasma-massspectrum (ICP-MS) method on the measurer ELAN 6000 of the Perkin Elmer Company with the degree of precision of ± 2% forrare earth elements and ± 3% for other elements. The analyses have been realized at the Centre of Analysis of the ChungnamNational University, Republic of Korea. The results of composition analysis of Late Cenozoic basalts in Việt Nam are presented inthe Table 2.

III. RESULTS OF ANALYSIS

1. Major element components

The MgO content largely oscillates from –4.98% (Phước Long basalts) to 12.96% (Xuân Lộc basalts). On the CIPWclassification diagram (Fig. 2) the major part of basalts fall into the field of quartz tholeiite (QT) with components bearing convertedquartz, and olivine tholeiite (OT) with components bearing converted olivine and hypersthene; lesser there is alkaline basalt (AB)

bearing under 5% converted nepheline and, rarer, there is basanite bearing over 5% converted nepheline.

The correlation between rock forming oxides is presented in the diagram of Figure 3. SiO2 and MgO have rather clear negative

correlation, reflecting two groups of main elements: high- and low-SiO2 corresponding to basalts of the early and late phases (Fig.

3a). The TiO2 content of Late Cenozoic

Figure 2. Composition of Cenozoic basalts in Việt Nam on the CIPW diagram

(based on the Table 2 with the combination with data of Nguyễn Hoàng et al [4, 5])

Note: Basalts of the early phase: 1) Đà Lạt, 3) Pleiku, 5) Buôn Ma Thuột, 7) Xuân Lộc, 9) Phước Long, 13) Điện Biên, 14)Kông Plông, 15) Lũng Pô Hồ; Basalts of the late phase: 2) Đà Lạt, 4) Pleiku, 6) Buôn Ma Thuột, 8) Xuân Lộc; Undifferentiatedbasalts: 10) Phú Quý, 11) Quảng Ngãi, 12) Quảng Trị; QT – quartz tholeiite, OT – olivine tholeiite, AB – alkaline basalt.



Figure 3. Diagram of correlation between rock forming oxides and MgO in Cenozoic basalts in Việt Nam (Symbols seen in Fig. 2)

Table 2. Composition of major and trace elements in Cenozoic basalts in Việt Nam

Samples DL-I DL-I DL-I DL-II DL-II Plog Plog XL-I XL-II XL-II XL-II BMTI BMTII PL-I PL-I

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

SiO2 54.16 52.99 49.75 53.37 54.4 52.44 51.37 53.13 45.13 48.83 52.19 46.71 49.43 54.36 47.47

TiO2 1.61 1.87 2.3 1.75 1.54 1.54 1.82 1.86 2.32 2.64 1.66 1.97 2.15 1.5 2.93

Al2O3 15.08 13.93 14.13 16.69 14.69 14.35 14.8 14.39 12.37 14.27 14.67 13.81 15.26 14.91 13.66

FeO* 9.1 10.88 11.36 10.16 9.38 11.2 10.9 10.25 11.6 11.38 10.90 11.19 11.53 8.67 11.41

MnO 0.14 0.19 0.14 0.12 0.14 0.14 0.15 0.15 0.18 0.16 0.13 0.17 0.16 0.09 0.16

MgO 7.51 7.9 7.96 4.47 6.93 7.88 6.54 8.02 12.85 9.41 7.85 11.63 7.57 7.63 9.44

CaO 6.77 7.04 7.11 4.85 6.99 7.03 7.09 8.36 7.62 5.85 8.27 7.38 5.87 6.6 7.62

Na2O 2.36 2.23 1.68 4.26 2.22 1.99 2.31 3.01 3.18 2.45 3.12 2.67 3.73 1.94 2.17

K2O 1.59 1.3 1.82 3.23 1.09 1.03 1.01 1.16 2.87 3.27 1.10 2.34 3.76 1.23 1.7

P2O5 0.31 0.3 0.48 0.75 0.19 0.28 0.26 0.34 1.08 0.97 0.32 0.6 1.03 0.23 0.75

Tổng 98.63 98.64 96.73 99.65 97.58 97.88 96.25 100.67 99.19 99.24 100.21 98.47 100.5 97.15 97.29

Mg# 59.77 56.65 55.78 44.19 57.08 55.88 51.92 58.48 66.60 59.81 56.45 65.17 54.17 61.30 59.83

Cr 380 444 338 470 326 373 442 253 805 606 364 578 517 405 475

Ni 275 348 353 310 256 364 350 318 647 498 475 527 416 330 395

V 136 160 169 81 127 153 164 187 177 135 145 185 124 115 202

Cu 90 89 84 78 109 88 100 82 132 122 108 93 77 96 103

Zn 176 189 239 237 153 167 221 351 251 278 271 21 227 175 321

Rb 37 25 31 79 18 14 15 100 75 71 85 81 67 15 53

Sr 309 337 408 701 222 301 354 1371 831 842 837 637 949 418 665

Ba 428 335 416 761 193 383 236 635 703 691 697 572 587 263 684

Pb 10.5 10.7 11.0 9.9 13.0 11.8 15.0 12.0 14.2 9.8 15.8 10.1 16.7 13.0 12.0

U 1.1 1.2 0.9 2.0 0.5 1.1 0.6 2.0 2.8 2.0 2.2 1.5 2.2 0.6 2.0

Zr 110 120 161 275 83 92 126 375 278 296 318 146 251 100 271

Nb 23 27 39 68 14 22 16 105 79 79 82 59 78 15 67



Ta 1.4 1.7 2.4 5.0 0.9 1.3 1.1 6.6 5.2 5.0 5.4 3.5 4.4 0.9 6.5

La 19.4 21.4 2.8 53.5 11.6 15.8 15.0 76.0 78.4 74.0 74.5 41.5 67.9 15.3 53.2

Ce 37.0 38.3 55.6 98.3 22.1 30.2 31.4 146.4 145.2 141.0 141.0 74.5 125.8 30.5 105.9

Pr 4.6 4.5 6.9 11.6 2.7 3.8 4.2 17.6 16.7 17.0 16.5 8.7 14.6 3.9 13.0

Nd 14.3 12.9 18.9 31.1 7.9 10.9 13.0 48.9 42.8 44.0 44.1 23.8 37.4 11.4 34.5

Sm 3.6 3.8 5.0 7.8 2.7 3.3 3.8 10.8 8.8 10.0 9.7 5.7 8.2 3.4 8.3

Eu 1.4 1.5 1.8 2.9 1.1 1.4 1.5 3.7 2.9 3.0 3.3 2.1 2.8 1.3 2.8

Gd 4.8 5.3 6.2 9.3 3.9 4.3 5.2 13.0 10.8 12.0 11.7 6.8 9.5 4.3 9.5

Tb 0.9 1.1 1.1 1.6 0.8 0.9 1.1 2.1 1.7 2.0 1.9 1.2 1.5 0.8 1.7

Dy 3.5 4.2 4.1 5.0 3.2 3.3 4.1 6.8 5.5 6.0 6.3 4.2 5.0 3.0 5.5

Ho 0.9 1.1 1.0 1.0 0.9 0.9 1.1 1.6 1.3 2.0 1.5 1.1 1.1 0.8 1.3

Er 1.8 2.1 1.9 1.8 1.7 1.7 2.1 2.9 2.6 3.0 3.0 2.0 2.1 1.4 2.4

Yb 1.4 1.7 1.5 1.1 1.4 1.3 1.7 1.9 1.8 2.0 2.1 1.5 1.4 1.1 1.7

Lu 0.3 0.3 0.3 0.2 0.3 0.3 0.4 0.4 0.4 0.1 0.4 0.3 0.3 0.2 0.3

Y 21.5 26.5 2.4 25.0 21.2 20.8 26.4 36.0 31.1 36.0 36.8 25.7 27.1 18.3 31.2

Th 5.3 6.4 6.1 12.2 3.2 4.2 3.1 15.2 15.0 13.0 12.8 9.2 12.5 3.8 10.1

Hf 3.7 4.1 5.0 8.1 3.0 3.2 4.2 10.8 7.6 9.0 8.6 4.8 7.0 3.3 7.6

Table 2 (continued)

Samples PL-I PL-II PL-II PL-II PL-II PL-II PQ Re Qtri Qtri KPLg KPLg LPH LPH LPH

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

SiO2 52.66 50.94 49.89 51.06 50.58 50.60 50.73 52.33 52.56 49.59 52.97 51.57 50.81 51.84 51.68

TiO2 1.98 1.98 2.64 2.08 2.21 2.21 2.55 2.11 2.13 2.47 1.77 1.89 2.19 2.34 2.24

Al2O3 13.33 14.69 14.96 15.9 13.96 13.96 13.73 15.83 15.54 14.82 15.04 14.24 13.04 13.34 12.88

FeO* 10.81 10.15 11.16 10.68 11.60 11.60 10.86 10.09 9.45 10.55 9.77 10.00 9.78 9.73 9.49

MnO 0.14 0.16 0.15 0.22 0.13 0.13 0.14 0.13 0.2 0.15 0.14 0.13 0.16 0.14 0.14

MgO 8.07 7.91 8.07 4.94 6.94 6.94 8.12 7.59 5.7 8.83 8.05 7.45 9.21 8.97 9.49

CaO 8.45 6.04 6.73 5.12 8.50 8.50 7.13 7.05 6.33 7.4 7.01 8.82 7.45 6.24 6.99

Na2O 2.95 3.15 2.24 3.66 3.42 3.42 2.35 2.13 2.69 2.39 2.14 2.94 2 1.91 1.89

K2O 1.41 2.56 2.46 3.73 2.08 2.08 2.21 2.15 2.1 1.99 1.84 1.70 2.91 3.15 3.07

P2O5 0.31 0.6 0.69 0.76 0.52 0.82 0.61 0.59 0.51 0.55 0.38 0.37 0.63 0.59 0.56

Total 100.11 98.18 98.98 98.17 99.94 100.26 98.43 100 97.21 98.73 99.11 99.11 98.16 98.25 98.44

Mg# 57.33 58.38 56.55 45.43 51.85 51.85 57.37 57.52 52.05 60.10 59.73 57.28 62.90 62.40 64.29

Cr 484 742 364 422 593 272 480 554 383 373 471 282 630 462 631

Ni 400 513 370 325 485 379 434 377 316 276 375 318 416 379 466

V 184 129 178 99 149 181 160 143 166 159 135 153 175 173 183

Cu 91 90 112 89 85 89 82 105 72 89 111 75 127 113 106

Zn 302 242 242 261 331 286 257 223 216 245 209 218 248 241 247

Rb 48 173 165 107 94 54 18 51 49 37 46 44 263 92 85

Sr 675 738 603 1124 1034 570 666 508 501 663 449 392 509 493 515

Ba 512 851 723 820 860 750 538 457 529 509 562 484 868 589 777

Pb 11.1 19.0 13.2 14.2 13.0 9.8 10.2 13.9 16.7 10.4 15.5 13.8 21.0 10.9 18.0

U 2.0 2.5 1.4 4.0 3.3 1.6 1.5 2.3 0.8 1.3 1.5 1.0 1.4 1.0 0.9

Zr 258 297 214 436 362 205 191 206 157 135 135 142 189 188 187

Nb 61 81 60 119 116 58 44 49 40 38 26 28 53 55 54

Ta 4.1 5.7 3.9 8.0 7.0 3.6 2.9 1.8 2.4 2.7 1.7 2.0 3.7 3.6 3.2

La 50.9 87.0 44.2 95.8 95.1 44.3 41.3 45.3 33.3 29.9 25.1 26.1 41.7 37.8 36.0



Ce 99.9 130.4 85.3 170.7 175.5 85.1 76.5 85.8 62.7 56.2 49.2 51.1 79.7 70.8 70.1

Pr 12.4 15.5 10.2 18.8 20.3 10.2 9.7 10.2 7.5 6.8 6.1 6.3 9.9 9.3 9.1

Nd 34.2 38.7 28.1 46.1 51.8 27.5 27.0 26.9 21.1 20.2 18.2 17.7 29.5 26.8 26.2

Sm 7.9 8.2 6.7 9.6 11.1 6.5 6.8 6.2 5.5 4.7 4.5 4.5 7.0 6.9 6.8

Eu 2.7 3.0 2.5 3.4 3.8 2.4 2.4 2.1 1.9 1.8 1.6 1.7 2.3 2.3 2.3

Gd 9.4 10.1 8.0 11.4 13.4 8.0 8.3 7.7 6.6 5.6 5.7 5.7 7.9 7.8 7.8

Tb 1.6 1.7 1.4 1.8 2.1 1.4 1.4 1.3 1.2 1.0 1.1 1.1 1.3 1.4 1.4

Dy 5.4 5.8 4.7 5.8 6.4 4.7 5.1 4.4 4.1 3.3 3.9 4.0 4.6 4.7 4.7

Ho 1.3 1.4 1.1 1.3 1.4 1.1 1.3 1.1 1.0 0.8 1.0 1.0 1.2 1.1 1.2

Er 2.4 2.6 2.2 2.5 2.7 2.2 2.3 2.0 2.0 1.6 2.0 2.0 2.3 2.2 2.2

Yb 1.7 1.7 1.7 1.5 1.6 1.5 1.6 1.5 1.5 1.1 1.6 1.7 1.7 1.6 1.7

Lu 0.3 0.3 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.4 0.3 0.4 0.3 0.4

Y 30.5 39.1 28.2 31.9 34.5 27.6 34.1 25.7 25.0 18.3 23.2 25.2 25.7 27.0 28.0

Th 9.8 15.5 8.8 22.2 17.6 8.9 7.0 11.8 8.8 7.2 7.6 7.4 8.8 7.2 7.1

Hf 7.3 8.0 6.3 226.2 9.9 6.0 5.8 6.2 5.0 4.2 4.4 4.4 5.8 6.4 6.2

Note: DL-I, DL-II: Đà Lạt (early and late phase), Plog: Phước Long, XL-I, XL-II: Xuân Lộc (early and late phase), BMT-I,BMT-II: Buôn Ma Thuột (early and late phase), PL-I, PL-II: Pleiku (early and late phase), PQ: Phú Quý, Re: Re Island, Qtri:Quảng Trị, KPLg: Kông Plông, LPH: Lũng Pô Hồ (distribution seen in Fig. 1).

Figure 4. Features of composition of incompatible elements in Cenozoic basalts

in Việt Nam normalized by primary mantle.



Figure 5. Diagram of the relation between incompatible elements and MgO in Cenozoic basalts in Việt Nam. PL – Pleiku, BMT - Buôn Ma Thuột,

XL - Xuân Lộc, Plog - Phước Long (other symbols seen in Fig. 2)

Figure 6. Diagram of ratios between incompatible elements in Cenozoic N-MORB and E-MORB basalts in Việt Nam (symbols seen in Fig. 2 and 5)



Figure 7. Composition of isotopes of Cenozoic basalts in Việt Nam (according to [4, 5, 12])

basalts in Việt Nam oscillates in the interval of 1.5-3.2% and has negative correlation with SiO2. The high TiO2 content (over 2.7%)

characterizes the SiO2-low alkaline basalt of the late phase. The similar change in the section from TiO2-low to TiO2-high has been

usually seen in continental basalts, such as in Parana, Decan and Siberian trap [2]. Many authors have been considering that thefeature of TiO2 content change is related just to the magma source than to the easily contaminated fractionated crystallization process.

Similar to the mid-oceanic ridge basalts (MORB), continental basalts (CFB) and oceanic island basalts (OIB), the Late Cenozoicbasalts in Việt Nam have also clear negative correlation between SiO2 and FeO* (Fig. 3f). In general, high-alkaline basalts contain

higher content of TiO2 and FeO* than quartz tholeiite, and the olivine tholeiite has equivalent content of MgO.

MgO has common correlation with P2O5, and basalts of the late phase have quite richer P2O5 content than that of the early phase

(Fig. 3d). It is to note that basalts in Việt Nam have an extraordinary high H2O content, up to 5% [7], that once more shows the great

role of H2O in particular, and liquid in general, in the forming process of basaltic magmas, particularly in the late phase. Furthermore,

the richness of liquid of high-alkaline basalts is still expressed in the eruptive character and porous structure of basalts of the latephase.

The clearly positive correlation between MgO and CaO proves that the fractionated crystallization is mainly of plagioclase andclinopyroxene that conforms to the petrographic features of basalts [4, 11].

2. Composition of trace elements

Note: 1) Đà Lạt; 2) Pleiku; 3) Buôn Ma Thuột; 4) Xuân Lộc; 5) Phước Long; 6) Quảng Ngãi; 7) Quảng Trị; 8) Phú Quý; 9) Điện BiênPhủ; 10) Lũng Pô Hồ; 11) Khorat basalts (in Zhou et al, 1997); SCB – East Sea basalts; NHRL – North hemisphere related line



On the diagram in the Figure 4, the curve demonstrating components of incompatible elements converted after primary mantle hasthe upward convex form, and similar to the oceanic island basalt type reflects the enrichment of incompatible elements. The major partof samples of Vietnamese basalts has Ta and Nb positive anomaly. When the MgO content in the rocks increases (decrease of SiO2)

the content of incompatible elements also increases, furthermore basalts of the late phase usually have higher content of high force fieldelements (HFSE) (for example, mean Nb usually over 60 ppm), but basalts of the two phases are both characterized by the lowLILE/HFSE ratio (Fig. 6). The enrichment of light rare earth elements and the high LREE/HREE ratio of basalts of the late phase canreflect the difference or the inhomogeneity of the source. The Phước Long basalts express the Ba deficit in comparing with Rb and Th(Fig. 6), in contrast the Xuân Lộc and Đà Lạt basalts are rich in Rb, K and Ba in comparing with other groups. While basalts of themajor part of groups have the overlapping of Rb/Sr (0.02 - 0.11) and Zr/Ba (0.2 - 0.6) ratios, basalts from Phước Long have theZr/Ba ratio of up to over 2.8 and those from Xuân Lộc and Đà Lạt have the Rb/Sr ratio over 0.18 (Fig. 6).

IV. DISCUSSION

The stability of MgO content in basalts of the early phase proves that they were a little influenced by the process of fractionatedcrystallization. On the contrary, basalts of the late phase have the strongly oscillating MgO content, proving a higher degree offractionated crystallization. The positive correlation between MgO and CaO (Fig. 3) shows that the separation phase consists mainlyof plagioclase and clinopyroxene. However, the fractionated crystallization is inconsiderable. According to former studies [4, 5, 12],the considerable contamination of crustal materials has been observed in samples from isolated effusive areas in the North, such asPhủ Quỳ, Điện Biên and Lũng Pô Hồ. However, the effect of crustal contamination in basalts is, in general, not great. One can saythat the enrichment of incompatible elements in basalts reflects the source peculiarity or level of partial melting more than the effect offractionated crystallization. On the Figure 6d the major part of basalts falls into the E-MORB field, reflecting the enriched sourcepeculiarity of basalts.

Experimental studies have been showing that the composition of primary magmas depends on the source composition, pressure,melting temperature and level of partial melting. During the process of basaltic magma formation from peridotite mantle, the SiO2

content in the fluid depends on inverse proportion to pressure. When the pressure decreases, the SiO2 content increases, and in

contrast; at the same time, together with the increase of decompression process the level of partial melting also increases [3, 6, 7]. Itis clear that the lower level of partial melting in greater depths will lead to the enrichment of alkaline components and incompatibleelements, but to the SiO2 deficit (low in SiO2). On the contrary, the great level of partial melting in smaller depths should give the fluid

having higher SiO2 content, but lower content of alkali and incompatible elements. So, basalts of the early phase (SiO2-high, MgO-

and FeO-low) were formed in the conditions of relatively low melting pressure (small depth), but of high level of partial melting. Onthe contrary, basalts of the late phase (SiO2-low, MgO- and FeO-high) have a higher melting pressure (greater depth) and more

restricted level of partial melting. According to former calculations the basaltic magma of the early phase was formed in the depth ofabout 30-40 km, as for the late phase the forming depth can reach up to 60-70 km [5, 9]. One can say that the basaltic magma of theearly phase was formed in the strong extension condition with the uplift of the asthenospere; as for the late phase it was formed inmore limited extension condition. The features of major element and trace element components of the two early and late phases reflectrather clearly the above presented forming conditions of magmas.

According to [4, 5], after the isotopic components Cenozoic basalts in Việt Nam bear the DUPAL anomaly characterized by the

low 206Pb/204Pb ratio and the high 208Pb/204Pb one (Fig. 7). For explaining this anomaly the authors of this paper set forth the modelof isotopic mixing. According to this model, basaltic magma of the early phase is the mixing between enriched magma of type 2

(EM2) rich in 206Pb/204Pb with the source of normal mid-oceanic ridge basalt (N-MORB) having high K2O/P2O5 ratio and low

Rb/Sr and Ba/Nb ratios corresponding to the lithospheric mantle. On the contrary, the late phase has the low K2O/P2O5 ratio and

higher Rb/Sr and Ba/Nb ratios that is the mixing between enriched magma of type 1 (EM1) poor in 206Pb/204Pb with the N-MORBcorresponding to the anomalous asthenosphere (A-MORB) characteristic for Southeast Asia and West Pacific marginal basins. Itshould suppose that the DUPAL anomaly in basalts of Việt Nam and other areas in the region are of endogenous source formed bythe abrasion of the lithosphere of old cratons by protruded asthenosphere caused by the collision between the India and Eurasiaplates [1, 4, 5]. At the same time, the EM2 can be added due to the interaction with the Phanerozoic basement matter and/or withsediments of small depth. In comparison with basalts of the East Sea, although some samples of basalts of Việt Nam fall into thecomposition field of East-Sea basalts, but the difference between them is very clear. The East-Sea basalts are close to the N-MORBtype, while Vietnamese on-land basalts reflect clearly the enriched source.

Although basalts in Việt Nam include two groups having characteristic features, but recent studies [12] have been showing that thisbasalt group can be formed from a common source. The polymorphism in composition of basalts is the results of the difference ofphysico-chemical conditions in the magma formation, the degree of partial melting or the inhomogeneity of the basement matter. Theauthors consider that the both eruptive phases were resulted from a process of lithosphere extension and the uplift of theasthenosphere, and belong to the late phase (16-0 Ma) of Cenozoic magmatism in the east of the India-Eurasia collision zone setforth by Wang et al [17]. Basalts of the early phase were formed in the condition of strong extension with the uplift of the lithospere,causing the decompression melting of large scale in small depth. On the contrary, basalts of the late phase were formed in weakerextension condition with restricted osmosis of the lithosphere, forming magmatic chambers in greater depths, having the existing timelonger, giving products of higher differentiation.

Cenozoic basaltic eruption in Việt Nam has been considered as closely related to the extension regime in the late phase of theregional neotectonic activities. However, basalts can give us only the minimum age of the extension regime. The north-southcompression regime can begin earlier (in Paleogene-Miocene) causing the east-west extension along submeridional faults, first of all inSouth Việt Nam, and reached its maximum in Pliocene (about 5-6 Ma). The east-west extension regime in the North can occur verylater with lower intensity than in the South. Corresponding to this extension there was basaltic eruption, such as in Nghĩa Đàn, Điện



Biên, Lũng Pô Hồ, etc. but with much smaller scale. The differentiation in occurrence time as well as in intensity of the extensionregime is the cause of distributive characteristics of the Late Cenozoic basaltic eruption in Việt Nam: the eruption was animated in theSouth, but northward the eruption level decreased forming mainly isolated effusive areas.

V. CONCLUSIONS

The Late Cenozoic basalts is composed of two main groups corresponding to two eruptive phases: early and late ones. Basalts ofthe early phase formed extensive basaltic plateaux, including tholeiite and olivine tholeiite effusing along fractures, while the late phaseconsists mainly of the effusion of central type with the predominance of high-alkaline basalt and olivine tholeiite. Basalts of the earlyphase are characterized by high SiO2 content and low content of TiO2, FeO* and Na2O + K2O. On the contrary, basalts of the late

phase are characterized by low SiO2 content, but high content of MgO, TiO2, FeO*, P2O5 and specially high content of alkaline

elements. In Cenozoic basalts of Việt Nam there is the enrichment of incompatible trace elements, among them those of the late phaseusually have the content of incompatible elements higher than that of the early phase. Basalts of these two phases are close in source,in polymorphism of composition mainly due to the difference of physico-chemical conditions (P-T, volatiles), level of partial melting orthe inhomogeneity of the basement matter. The basaltic magma of the early phase was formed in the strong extension condition,causing the decompression melting in large scale (with the high level of partial melting) at small depth (low pressure). On the contrary,basalts of the late phase were formed in the more restricted extension condition and, therefore, the lithosphere has lower osmosis,forming magmatic chambers in greater depths (higher pressure). The process of occurrence and development of Cenozoic basalts inViệt Nam is close related to the east-west extension regime of the territory. This process, maybe, began earlier in the south part andbecame gradually later northward, at the same time the extension scale gradually decreased. The distributive features of volcanicactivities in time and space reflect the features of the above said extension regime.

The authors express their deep thanks to the Institute of Geosciences (Academy of Sciences and Technology of Việt Nam) forcreating good conditions in field investigation and collection of samples. Phạm Tích Xuân would like to thank the Korean Science andEngineering Fund (KOSEF) and Prof. Dr Lee Hyun Koo for support in the post-doctorship training course and realizing analyses inthe Korean Chungnam National University.

This work is realized in the framework of the Project 710602 (2002-2004) with the support of the Basic Research Program.

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geochemistry of late cenozoic basalts in việt nam and its tectonic significances

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