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

PHẠM TÍCH XUÂN¹, NGUYỄN HOÀNG^1,2, 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 intensive extension of territory (high degree of partial melting), in shallow mantle (low pressure). In contrast, the upper series 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 of distribution of late Cenozoic basalts: most of the eruptive centres are concentrated in the southern part of Central 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 the collision between the India and Eurasia plates. The most part of interpretations on the geodynamics in Southeast Asia during the neotectonic stage have been basing on the tectonic model extrusion suggested by Tapponnier P. [15, 16] and other hypotheses based on the above model. According to this model the Indochina Peninsula was extruded and glided southeastward along the Red River strike-slip. Former studies [13, 14] have been recognizing two tectonic phases on the Vietnamese territory from Paleogene to Present. The early phase is characterized by left-lateral shear along NW-SE trending faults parallelly with the Red River Fault in the field of subparallel compression strain. On the contrary, the late phase is characterized by right-lateral shear along the above faults and the east-west extension regime on submeridional faults. Cenozoic basalts in Việt Nam erupted mainly along submeridional extension faults or NE-SW normal faults, and were considered as closely related to the extensional regime of the late phase. In this paper the authors present the geochemical features of the studied basalts with the aim to clarify their origin and forming conditions, contributing in 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 South and 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. The earliest occurrence time of Late Cenozoic basalt eruption in Việt Nam, to date, has been recognized at the end of Early Miocene in Đà Lạt area (17.6 Ma). The youngest volcanism has been recognized in Xuân Lộc, Cồn Cỏ and Đức Trọng with the age respectively of 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 Middle Pleistocene, 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 of Cenozoic effusives. They are composed mainly of quartz tholeiite and olivine tholeiite. Basalts of the late phase are characterized by the type of central effusion leaving many volcanic structures, such as cones, maars … and usually have the eruptive character with such 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

Localities	Areas (km^2)	Thickness (m)	Age (Ma)	Localities	Areas (km^2))	Thickness (m)	Age (Ma)
Lũng Pô Hồ	1	100	?	Buôn Ma Thuột	3,800	260	8,9 - 1,63
Điện Biên Phủ	2	-	5,8	Pleiku	>4,000	> 440	7,4 - 1,59
Nghĩa Đàn	6	-	5,9	Quảng Ngãi	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 Long	> 6,000	250	9,1 - 4,6

II. METHOD OF ANALYSIS

Samples used in analysis consist of fresh rock samples selected from drill cores and surface exposures taken from almost all basalt groups described in [4, 8-10]. The components of main elements have been analyzed by XRF method on the Japanese measurer Rikagu RIX 2100 with the degree of precision of ± 5%. The components of trace elements have been analyzed by plasma-mass spectrum (ICP-MS) method on the measurer ELAN 6000 of the Perkin Elmer Company with the degree of precision of ± 2% for rare earth elements and ± 3% for other elements. The analyses have been realized at the Centre of Analysis of the Chungnam National University, Republic of Korea. The results of composition analysis of Late Cenozoic basalts in Việt Nam are presented in the 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 CIPW classification diagram (Fig. 2) the major part of basalts fall into the field of quartz tholeiite (QT) with components bearing converted quartz, 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; Undifferentiated basalts: 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)

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 Biên Phủ; 10) Lũng Pô Hồ; 11) Khorat basalts (in Zhou et al, 1997); SCB – East Sea basalts; NHRL – North hemisphere related line

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 the feature 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 Cenozoic basalts 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 late phase.

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

2. Composition of trace elements

On the diagram in the Figure 4, the curve demonstrating components of incompatible elements converted after primary mantle has the upward convex form, and similar to the oceanic island basalt type reflects the enrichment of incompatible elements. The major part of 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 field elements (HFSE) (for example, mean Nb usually over 60 ppm), but basalts of the two phases are both characterized by the low LILE/HFSE ratio (Fig. 6). The enrichment of light rare earth elements and the high LREE/HREE ratio of basalts of the late phase can reflect 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 the major 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 the Zr/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 fractionated crystallization. On the contrary, basalts of the late phase have the strongly oscillating MgO content, proving a higher degree of fractionated crystallization. The positive correlation between MgO and CaO (Fig. 3) shows that the separation phase consists mainly of 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 as Phủ Quỳ, Điện Biên and Lũng Pô Hồ. However, the effect of crustal contamination in basalts is, in general, not great. One can say that the enrichment of incompatible elements in basalts reflects the source peculiarity or level of partial melting more than the effect of fractionated crystallization. On the Figure 6d the major part of basalts falls into the E-MORB field, reflecting the enriched source peculiarity 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]. It is clear that the lower level of partial melting in greater depths will lead to the enrichment of alkaline components and incompatible elements, 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. On the 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 of about 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 the early phase was formed in the strong extension condition with the uplift of the asthenospere; as for the late phase it was formed in more limited extension condition. The features of major element and trace element components of the two early and late phases reflect rather 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 model of 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-MORB corresponding to the anomalous asthenosphere (A-MORB) characteristic for Southeast Asia and West Pacific marginal basins. It should suppose that the DUPAL anomaly in basalts of Việt Nam and other areas in the region are of endogenous source formed by the abrasion of the lithosphere of old cratons by protruded asthenosphere caused by the collision between the India and Eurasia plates [1, 4, 5]. At the same time, the EM2 can be added due to the interaction with the Phanerozoic basement matter and/or with sediments of small depth. In comparison with basalts of the East Sea, although some samples of basalts of Việt Nam fall into the composition field of East-Sea basalts, but the difference between them is very clear. The East-Sea basalts are close to the N-MORB type, 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 this basalt group can be formed from a common source. The polymorphism in composition of basalts is the results of the difference of physico-chemical conditions in the magma formation, the degree of partial melting or the inhomogeneity of the basement matter. The authors consider that the both eruptive phases were resulted from a process of lithosphere extension and the uplift of the asthenosphere, and belong to the late phase (16-0 Ma) of Cenozoic magmatism in the east of the India-Eurasia collision zone set forth 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 weaker extension condition with restricted osmosis of the lithosphere, forming magmatic chambers in greater depths, having the existing time longer, 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 the regional neotectonic activities. However, basalts can give us only the minimum age of the extension regime. The north-south compression regime can begin earlier (in Paleogene-Miocene) causing the east-west extension along submeridional faults, first of all in South Việt Nam, and reached its maximum in Pliocene (about 5-6 Ma). The east-west extension regime in the North can occur very later 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 extension regime is the cause of distributive characteristics of the Late Cenozoic basaltic eruption in Việt Nam: the eruption was animated in the South, 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 of the early phase formed extensive basaltic plateaux, including tholeiite and olivine tholeiite effusing along fractures, while the late phase consists mainly of the effusion of central type with the predominance of high-alkaline basalt and olivine tholeiite. Basalts of the early phase 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 phase usually 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 or the 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 in Việt Nam is close related to the east-west extension regime of the territory. This process, maybe, began earlier in the south part and became gradually later northward, at the same time the extension scale gradually decreased. The distributive features of volcanic activities 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) for creating good conditions in field investigation and collection of samples. Phạm Tích Xuân would like to thank the Korean Science and Engineering Fund (KOSEF) and Prof. Dr Lee Hyun Koo for support in the post-doctorship training course and realizing analyses in the 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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