MESOZOIC BIMODAL ALKALINE MAGMATISM
IN THE TÚ LỆ BASIN, NORTH VIỆT NAM: CONSTRAINTS
FROM GEOCHEMICAL AND ISOTOPIC SIGNIFICANCES

TRẦN TUẤN ANH¹, TRẦN TRỌNG HÒA¹, CHING-YING LAN²,
SUN-LIN CHUNG³, CHING-HUA LO³, PEI-LING WANG², STANLEY A. MERZMAN⁴

¹Institute of Geosciences,, ASTVN, Hà Nội, VIỆT NAM
²Institute of Earth Sciences, Academia Sinica, Taipei, TAIWAN, ROC
³Department of Earth Science, National Taiwan University, Taipei, TAIWAN, ROC
⁴Department of Geosciences, Franklin and Marshall College, PA, USA

Abstract: Mesozoic alkaline magmatism occurring in the Tú Lệ basin, North Việt Nam, resulted in several igneous complexes composed of different rocks. They are represented by the Suối Bé basalts, the Bản Hát gabbros, the Phu Sa Phìn syenites, the Văn Chấn rhyolites and the Ngòi Thia rhyolites, which overall show a bimodal chemical composition. Ar-Ar dating and stratigraphic data indicate that the magmatism clustered in two periods, i.e., the middle-late Jurassic (176 - 145 Ma) and the late Cretaceous - earliest Tertiary (80 - 60 Ma), respectively. The Suối Bé basalts, the Bản Hát gabbros, the Văn Chấn rhyolites and some of the Phu Sa Phìn syenites formed in the Jurassic stage, whilst the Ngòi Thia rhyolites and most of the Phu Sa Phìn syenites formed in the Cretaceous stage.

The mafic Jurassic magmas are silica-undersaturated (SiO₂ = 44 - 49 % wt) and sodium-rich, with low MgO (~7 - 3 % wt) but high TiO₂ (3.6 - 2.0 % wt). They exhibit various degrees of LREE-enrichment, with (La)_N = 79 - 290, 5.5 < (La/Yb)_N < 20 (chondrite-normalized) and without apparent Eu anomalies. On the other hand, the felsic magmas of Jurassic and Cretaceous ages show different geochemical features, with SiO₂ = 62 - 78 % wt, (Na₂O+K₂O) = 5.3 - 10.2 % wt, significant Eu anomalies (Eu/Eu* = 0.1 - 0.54), and enrichment in HFSE (Nb, Ta, Zr) and LILE (Rb, Th, U, K) along with pronounced depletions in Ba, Sr, P and Ti in the primitive mantle-normalized multi-element variation diagram. They are geochemically comparable to A-type granitoids. The mafic and felsic magmas have distinguishable Nd isotope ratios. In contrast to the Jurassic and Cretaceous felsic magmas that have uniform eNd(T) values (-1.5 to -2.8), the Jurassic mafic rocks are marked by more radiogenic and heterogenous eNd(T) values (-1.9 to -8.9), implying different magma sources and independent petrogenetic processes involved in generation of the Jurassic bimodal magmatism. Combining with relevant geological data from North Việt Nam and SW China, we propose an intraplate lithospheric extension setting to account for the Jurassic-Cretaceous magmatism whose generation postdated the continental collision between the Indochina and South China blocks in the early Triassic. Formed originally in the western margin of the South China block, the Tú Lệ basin and associated Mesozoic magmatic rocks were transported southeastward to the present location by the mid-Tertiary sinistral displacement of the Ailao Shan - Red River shear zone, related to the India-Eurasia collision.

I. INTRODUCTION

The Tú Lệ basin is an independent structure formed in Mesozoic time, lying between the Phan Si Pan Uplift and Sông Đà Depression [3, 16]. It is a trough filled by Jurassic - Cretaceous red beds, which underlie rhyolite, basalt, and trachyte - a so-called bimodal association. The basin has been considered as a continental rift, characterized by the high-K tholeiitic magmatism at the southwestern margin [10] or a hot spot that was developed from ancient mantle plume, with the formation of trachyte-rhyolite-basalt bimodal association [6]. So, what is the real nature of the Tú Lệ basin? Which mechanism is more fairly understood in explaining the magmatic characteristics and the tectonic implication? Is there any relation between the Indosinian Orogeny in Triassic time to the formation of Jurassic-Cretaceous basin? Does the Mesozoic volcanism in Tú Lệ share the similarity to that in Southeast China, which has been recorded an extensional tectonic regime?

In this paper we present the results of a detailed petrological and geochemical study on different rock types from the Tú Lệ basin. Our new results combined with those from Lan C.Y. et al [4] allow us to have better interpretation on: 1/ the volcanic and tectonic settings of the Tú Lệ basin; 2/ the relationship between the Tú Lệ basin and adjacent areas; 3/ the magmatic evolution of the basin as a whole.

II. GEOLOGICAL BACKGROUND

On the regional tectonic framework, the West Bắc Bộ (Northwestern part of Việt Nam) is composed of three major important tectonic structures, namely: 1/ Red River shear zone; 2/ Sông Mã suture zone; and 3/ Sông Đà intracontinental rift. The Red River shear zone has been considered as most influential zone in the geological evolution of North Việt Nam with the large-scale (~600 km) sinistral displacement acting during 27-22 Ma [1].

Figure 2. Classificasion of the rocks from Tú Lệ basin based on SiO₂ - Na₂O + K₂O
(Le Bas et al., 1986)

Figure 3a. Variation diagrams SiO₂ vs other major elements, illustrating the bimodality of the mafic and felsic rocks from Tú Lệ basin. Symbols as in Fig.2.

The Sông Mã belt has been considered as the suture zone between Indochina and South China blocks. It is characterized by the occurrence of metamorphosed mafic and ultramafic massifs, which are widespread as Lower Paleozoic greenschists of deep-sea origin and unconformably covered by Devonian red beds in many localities [2, 3]. The mafic-ultramafic rocks have been widely interpreted as ophiolitic fragments, derived from Paleo-Tethys after the collision of South China and Indochina blocks. Based on a detailed Ar-Ar dating study, Lepvrier et al [7] have shown that all rock members of the Sông Mã ophiolite share the same metamorphic age of ~245 Ma. This implies that the suture between Indochina and South China blocks took place in the earliest Triassic, related to the early phase of the Indosinian orogeny, with regional metamorphism and magmatism.

Figure 3b. Variation diagrams SiO₂ vs other trace elements, illustrating the bimodality of the mafic and felsic rocks from Tú Lệ basin. Symbols as in Fig.2.

The Sông Đà zone is a Permian- Triassic intracontinental rift system with the development of basalt-picrite-andesite, basalt-andesite or basalt-andesitodacite associations, which are similar to continental basalts in Southeast Asia and Emeishan (China) [13, 15].

The Tú Lệ basin was developed in the north of Sông Đà rift zone (Fig.1). It is a trough filled with Jurassic-Cretaceous red beds, underlying rhyolite, basalt and trachyte. In general, the magmatic activities of the Tú Lệ basin can be classified into three major stages: 1/ Early stage, corresponding to Văn Chấn Formation (J-K? vc) with the presence of trachytic (orthophyres) volcanites in the southwest; 2/ Middle stage, represented by from basalt to andesitobasalt volcanites in the middle of Suối Bé Formation (J-K? sb), and gabbros of Bản Hát Complex; and 3/ Late stage with the felsic volcanites of Ngòi Thia Formation, located in the north of Tú Lệ basin.

Analyzed samples

In this study, rock samples were collected from 2 magmatic complexes and 3 formations, namely: Phu Sa Phìn and Bản Hát complexes, Ngòi Thia, Văn Chấn and Suối Bé formations. The sampling localities are shown in Fig.1. The sample list, rock types and ages are listed in Table 1.

Table 1. Sample list, rock types and age of the rocks from Tú Lệ basin

Sample	Rock types	Formation	Age
H152	Rhyolite	Ngòi Thia
H187	-	-
H198	-	-
T929*	-	-	58.60±0.2
T962*	-	-	79.30±0.3
T985*	-	-	73.30±0.3
TLH-6	-	-
H178	Granite	Phu Sa Phìn
H182*	-	-	144.80±1.7
V188*	-	-	79.50±0.9
T931331	Basalt	Suối Bé	117.30±0.6
T931344	-	-	176.30±0.8
T931352	-	-	164.00±0.8
H154	Gabbrodiabase	Bản Hát
H155	-	-
H156	-	-
H158	-	-
H161	-	-
H167	-	-
TLH-1/1	Rhyolite	Văn Chấn
TLH-9	-	-
TLH-9/1	-	-
RR-34A	-	-
RR-34B	-	-
RR-38	-	-
RR-39	-	-

* Samples from Lan et al., 2000

The Văn Chấn Formation (J₃-K₁ vc) is the oldest one of the Tú Lệ basin. It is composed of trachyte and trachyrhyolite. It belongs to Jurassic-Cretaceous magmatism based on the correlation between the underlying Upper Triassic Suối Bàng Formation (T₃ n-r sb) and the overlying Cretaceous red beds of Yên Châu Formation (K₂ yc).

The Phu Sa Phìn Complex comprises alkaline granite, granosyenite and syenite that have been considered as corresponding to the volcanism of the Văn Chấn Formation. The Bản Hát Complex and Suối Bé Formation consist of mafic dykes and basaltic volcanic rock, respectively. They occur between two phases of rhyolitic volcanism. Pyroxenite, picrite and diabase of Nậm Chim Complex probably represent the residual phase of the volcanism. Our Ar-Ar age data gives an age of 79-144 Ma to Phu Sa Phìn granite, and 164-176 Ma to Suối Bé basalt.

Figure 4. Variation diagrams of Zr vs other trace elements, illustrating the bimodality of the mafic and felsic rocks from Tú Lệ basin. Symbols as in Fig.2.

The Ngòi Thia Formation (K nt) is composed of rhyolite and porphyritic rhyolite. It is the youngest volcanic formation of the Tú Lệ basin. Our unpublished Ar-Ar age data gives an age of 58.6 – 79 Ma.

III. ANALYTICAL METHODS

Samples are grinded in porcelain mortar. Major elements concentration is determined using X-ray fluorescence (XRF) technique on fused glass disks, other trace elements and rare earth elements were analyzed by ICP – MS at the Department of Geo-Sciences, National Taiwan University. The error on major element determination is <5%. The error for trace and rare earth elements is under 5%.

Nd and Sr isotopes are analyzed using a VG354 mass-spectrometer for Sr and a MAT 262 for Sm and Nd in the Institute of Earth Sciences, Academia Sinica, Taipei. The used analytical procedure was described by Lan et al. (1986) and Shen et al. (1993). In general, 10-55 mg of fine powder are dissolved using a mixture of HNO₃ and HF in closed teflon beakers maintained at temperature of 120^oC for two nights, then the aliquots are spiked for isotope analysis. Then, the sample solution is evaporated to dryness and converted to chloride. This procedure is repeated until careful observation of the sample solution under microscope confirms that total dissolution is achieved. Sr and REE are separated using AG50W-X8, 100 - 200 mesh cation-exchange resin column in a HCl medium. Subsequently, Sm and Nd are separated from other REE using AG50W-X4, 200 - 400 mesh cation-exchange resin column with 2 MLA (2-methyllactic acid) medium buffer at pH 4.44 under a pressure of 0.2 kg.cm^-2. Aqua regia is first used to digest the organic matter in the collected Sm and Nd fractions. It is followed by 30 min UV light exposure after adding one drop of 4 N HNO₃ in each of the collected Sm and Nd fractions before loading them onto individual filaments.

Sr is loaded on a Ta single filament while Nd and Sm are loaded on a Re single filament and analyzed as mono-oxide ions. Samples are oxidized at 1.5 amp in air for 1 min. Oxygen is introduced to the source chamber to enhance oxide emission. The isotopic compositions are measured in jumping multi-collection mode. The isotopic ratios were corrected for mass fractionation by normalizing to ⁸⁶Sr/⁸⁷Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, respectively. Values for the NBS987 Sr standard yield ⁸⁶Sr/⁸⁷Sr = 0.710240 with a long-term stability of 0.000038 (95% confidence level) and for La Jolla (UCSD) Nd standard, yield ¹⁴³Nd/¹⁴⁴Nd = 0.511867 with a long-term stability of 0.000028.

Table 2. Major (% wt) and trace (ppm) element concentrations of the rocks from Tú Lệ basin

Formation	Ngòi Thia							Phu Sa Phìn
Sample	H152	H187	H198	T929*	T962*	T985*	TLH-6	H178	H182*	V188*
Major elements (%wt) by XRF
SiO2	75.07	77.56	72.69	77.06	77.23	74.68	75.73	62.87	72.90	74.50
TiO2	0.24	0.22	0.31	0.25	0.22	0.23	0.28	1.00	0.33	0.34
Al2O3	11.90	12.26	12.25	11.21	11.02	12.33	11.58	16.06	12.77	11.88
Fe2O3	0.79	1.07	1.16	0.79	1.22	1.09	3.01	2.36	1.19	0.97
FeO	2.19	1.14	2.11	2.30	1.07	1.96		2.82	2.32	1.78
MnO	0.11	0.04	0.11	0.12	0.05	0.05	0.09	0.19	0.08	0.12
MgO	0.36	0.34	0.23	0.08	0.24	0.20	10.17	0.93	0.45	0.13
CaO	0.07	0.07	0.80	0.21	0.36	0.17	0.28	2.03	1.11	0.07
Na2O	1.39	1.84	3.91	1.10	2.63	3.07	3.29	5.21	3.65	2.63
K2O	5.43	3.43	4.05	5.32	4.12	4.78	4.57	4.97	4.21	6.27
P2O5	0.01	0.01	0.02	0.01	0.00	0.00	0.02	0.27	0.02	0.01
LOI	2.04	1.81	1.88	2.14	1.24	0.81		1.13	1.58	1.66
TOTAL	99.60	99.79	99.52	100.59	99.40	99.37	109.02	99.84	100.61	100.36
Trace elements (ppm) by XRF + ICP-MS
Sc	1	<1	2	1	1	1	17.45	8	3	2
V	5	<1	<2	<2	5	3		33	<2	6
Cr	7	4	6	23	8	11		7	23	4
Co	< 1	3.6	<1	2	4	1	2.398	5	<1	1
Ni	4	3	2	3	4	6		3	4	4
Cu	2	<1	1	3	1	1		5	2	16
Zn	58	74	40	180	42	170	74.99	164	54	295
Ga	28.8	31.2	32.8	23.1	34.1	30.5	31.83	26	28	21.4
Cs							0.199
Rb	190	168	139	176	209	97	143.3	98	108	202
Sr	9.2	9.4	14.6	15	13	18	13.75	196	26	7
Y	84	114	89	107	101	156	109.5	42	100	124
Zr	929	1023	818	1038	1002	975	916.7	320	877	1187
Nb	114.7	117.3	113.5	112.1	116.9	125.4	140.8	47.3	106.2	133.6
Ba	105	81	73	43	69	211	331.8	1245	277	91
La	167.13	191.63	149.66	207.60	88.70	185.7	110.7	62.4	120.7	166.7
Ce	287.33	326.48	272.73	419.80	190.60	321.8	228.3	128.7	248.9	327.8
Pr							24.64
Nd	204.24	185.65	202.62	243.30	121.80	209.40	97.35	78.7	134.90	160.60
Sm	23.00	27.55	24.26	28.80	16.90	29.70	18.7	10.6	20.40	21.40
Eu	0.82	1.23	1.39	0.72	0.76	1.67	0.968	2.95	2.07	1.20
Gd							16.02
Tb	2.37	2.99	2.78	3.17	2.44	3.76	2.907	1.20	2.78	3.60
Dy							18.04
Ho							3.706
Er							10.62
Tm							1.754
Yb	10.65	15.14	10.18	12.00	11.00	9.29	10.55	4.06	12.20	13.20
Lu	1.40	1.59	1.42	1.43	1.52	1.43	1.577	0.54	1.33	1.74
Hf	25.67	29.43	24.04	25.60	27.80	23.90	25.2	8.42	24.90	31.50
Ta	7.89	9.35	8.99	8.13	10.50	8.99	11.07	2.93	7.58	12.10
Pb	2	6	3	11	4	3	7.472	9	4	2
Th	24	27.7	23.1	29	23.4	28.4	26.15	7	24.6	38.3
U	4.1	6	3.6	5	5.8	5.3	6.34	2.8	4.3	7.7

Table 2. (continued)

Table 2. (continued)

Figure 5. Chondrite normalized (Sun & McDonough., 1989) REE distribution patterns of acidic and mafic rocks from Tú Lệ basin.	Figure 6. Primitive mantle normalized (Sun & McDonough., 1989) multielements distribution patterns of acidic and mafic rocks of Tu Le basin.

IV. GEOCHEMISTRY

1. Whole rock chemistry

Data on major, trace and rare earth elements for different rock types of the Tú Lệ basin are listed in Table 2. Geochemically, all samples are fairly distinguished into two series (Fig. 2): 1) the mafic series - MS (Bản Hát gabbrodiabase, Suối Bé basalt) mainly of basalt to tephrite basanite; they are silica-undersaturated (SiO₂ = 44 - 49 % wt) and sodium-rich, with low MgO (~7 - 3 % wt), but high TiO₂ (3.6 - 2.0 % wt); 2) the felsic series - FS (Ngòi Thia, Văn Chấn rhyolites, and Phu Sa Phìn granite) mainly of rhyolite, trachyte and trachydacite with SiO₂ = 62-78 % wt, (Na₂O+K₂O) = 5.3 - 10.2 % wt. The mafic–felsic bimodality of the Tú Lệ basin is definitely apparent (Fig. 3a, 3b), in which all samples fall into two distinct groups. The MS has more restricted range in silica content, while the FS shows a wider range (Fig. 2). On the contrary of silica content, the MS has wide variation of TiO₂, Fe₂O₃ and P₂O₅ contents. Except the similarity in aluminium content, they are significantly higher in other major elements (Fig. 3a). The FS are rich in elements that are incompatible with ferromagnesian minerals, such as Th, Zr, and most of the REE (La, Nd, Yb), whereas elements compatible with feldspar (Sr, Eu) or minor phase (Ti, P) are relative depleted. In the trace element correlation diagram (Fig. 4), the MF and FS show similar correlation, except the Ti-Zr correlation. In Ti-Zr correlation, two different trends are seen: in the MS, the Ti content varies widely, whereas Zr more constant; in the FS the Zr content varies, while Ti is more constant.

In spite of difference in formation ages, the felsic series share the similarity in REE concentration and REE distribution pattern (Fig. 5). They are rich in LREE and have flat pattern of HREE, relative to C₁ Chondrite [14] with 5.5 < (La/Yb)_N < 20, with strong negative Eu anomaly (Eu/Eu* = 0.1 –0.54). One exception is the sample H178, which doesn’t show Eu negative anomaly in its REE pattern. Its composition is more mafic than other FS samples, which probably represent no plagioclase fractionation of the source. The MS tends to have smoother patterns without significant Eu anomaly (Eu/Eu* = 0.9 –1.96).

Figure 7. (A) FeO*/MgO and (B) (K₂O+Na₂O)/CaO vs (Zr+Nb+Ce+Y) discrimination diagrams (Whalen et al., 1987) for acidic rocks of Tú Lệ basin.

In primitive mantle–normalized spider diagrams, the felsic rocks demonstrate similar shapes with enrichment in high-field strength elements (e.g. Nb, Ta, Zr), large-ion lithophile elements (Cs, Rb, Th, U, K) and pronounced depletion in Ba, Sr, P and Ti, while the MS show smooth spectra with no such significant negative anomalies, except Sr, Eu (Fig. 6), that are similar to oceanic island basalts (OIB) [14]. The sample H.178 shows the intermediate pattern between FS and MS samples, which are less depleted in Sr, P, and Ti, no depletion in Ba, and visible depletion of Th, Ta, and Nb. The FS show typical feature of A-type granite (Fig. 7) in the FeO*/MgO and (K₂O+Na₂O)/CaO versus (Zr+Nb+Ce+Y) discrimination diagrams [17]. Their ORG (Ocean Ridge Granite, [12]) - normalized trace element patterns shows generally similar shapes marked by variable enrichment in Rb and Th and depletion in Ba (Fig. 8), except the sample H.178, which shows similar pattern, but with Ba enrichment. It is more depleted in Hf and Zr that is similar to oceanic ridge granite.

Most of MS samples show features of intraplate basalts (Fig. 9a, b), but their high Th/Yb ratios are similar to basalts of continental margin arcs (Fig. 9c). Consistent with the MS, felsic series show the intraplate granite (WPG) characteristics (Fig. 10) in the Nb versus Y+Nb, and Nb versus Y tectonic classification diagram of Pearce et al. [12].

2. Istotopic composition

Isotopic data are listed in Table 3 and presented in Fig. 11. Isotopic composition of ⁸⁷Sr/⁸⁶Sr and εNd are different for mafic and felsic rocks. The MS shows a relative uniform low Sr isotope (⁸⁶Sr/⁸⁷Sr = 0.7061 to 0.71054), high variation of εNd (-8 to –1.9), and similar to those from continental basalts. Compared to other members of mafic rocks, the Bản Hát gabbro-diabase shows relatively higher variation in εNd (3.28 – 8.88), that is well correlative with the SiO₂ content. It may reflect that the crustal contamination was happened in different levels for the magma. The felsic rocks have high variation range of ⁸⁶Sr/⁸⁷Sr (0.7099 to 0.9101) and moderate one of εNd (-2.8 to -1.5), that is similar to the variation trend of those from marine sediments. It may either reflect the alteration or the contribution of fluid phase to the magma chamber. Such differences in isotopic composition between MS and FS could imply different magma sources and independent petrogenetic processes involved in generation of the Jurassic to Cretaceous bimodal magmatism. This interpretation is more apparent with T_DM calculation, which shows that the MS has older crustal residence age (1.07 to 2.08 Ga), in comparing with FS samples (0.65-0.97 Ga).

Figure 8. Ocean ridge granite (ORG, Pearce et al., 1984) normalized geochemical pattern of acidic rocks of Tú Lệ basin.	Figure 9. Tectonic discriminant diagrams for mafic rocks from Tú Lệ basin. A. Nb-Z-Y (after Meschede, 1986); B. Zr/Y vs Zr (after Pearce & Norry, 1979); C. Th/Y vs Ta/Yb (after Pearce, 1982).

V. INTERPRETATION AND DISCUSSION

1. Forming origin of mafic series (MS)

The above described relative steeply REE distribution pattern of the Bản Hát gabbro and Suối Bé basalts, combining with the high concentration of incompatible elements can be explained as a result of either partial melting of a source that was previously enriched in LREE and incompatible elements or low degree mantle melting, in which garnet was removed from partial melt. The melting process can happen under extensional regime like rifting. In Fig. 9, on most trace element diagrams for tectonic discrimination, the MS samples fall mainly in intraplate basalt field. In Fig. 9c, the samples fall in the field of continental margin arc, above the enriched end of the intraplate mantle array. This feature was also reported in mafic dikes from SE China, and was interpreted that the subcontinental mantle source of those dikes have been influenced by subduction-zone fluids or melts enriched in Th [8]. The transition from the relative flat pattern to the more steeply patterns and Suối Bé basalts may indicate a change in condition of melting (an increasing in melting depth associated with a decreasing of temperature) or a change in composition of the source.

Figure 10. Rb vs Y+Nb and Y vs Nb discrimination for acidic rocks from Tú Lệ basin (Pearce et al., 1984). ORG- ocean ridge granite; Syn-COLG - syn-collision granite; VAG - volcanic arc granite; WPG - within - plate granite.

2. The felsic series (FS)

In comparing to the mafic series, two individual processes can be identified in the felsic rocks: fractional crystallization and contamination. The extremely low concentration of trace elements could show the role of fractionation: Sr and Eu are depleted by the extraction of plagioclases, Ba by K-feldspar, P by apatite and Ti by Fe-Ti oxides. The steep distribution of LREE and flat one of HREE of the FS imply that they are evolved through feldspar fractionation. These samples have extremely low content not only of Sr, Eu but also of Ba, P, Ti, indicating that they underwent radical fractional crystallization. The Rb hight content in comparing to Sr could be explained by the abundance of mica phase, which is one of the major rock-forming mineral phases in felsic rocks. Sr large isotope variations of the FS in comparing to MS could be either caused by the contribution of the marine sediments, related metasomatism fluid or crustal contamination.

3. Crustal contamination and sequence of volcanism

The significant signatures of crustal contamination consist of negative anomalies of Ta, Nb, high La/Nb accompanied by high Th/Nb. The MS has high variation of La/Nb ratios (0.74 – 2.14), slight Ta, Nb anomaly, and relative medium Th/Nb ratio (0.04 – 0.31), which could be considered as an indication of crustal component contaminating in different levels the MS of different complexes. In particular, the basalt shows wider variation of La/Nb (1.4 – 2.14) and Th/Nb (0.04 – 0.31) than the diabase (La/Nb = 1.18 – 1.30; Th/Nb = 0.22 – 0.31). The FS show more restricted variation range of La/Nb (0.76 – 1.85), and Th/Nb (0.19 – 0.36) ratios, which are close to those of diabase of the MS.

The Ar-Ar age data and T_DM are shown in Table 3. The MS were formed between two cycles of felsic magmatism, but show the crustal residence age older than the oldest rhyolite volcanism (Văn Chấn Formation). Within 100 Ma of interval in formation age, these two cycles of felsic magmatism have the same crustal residence age, sharing similarities in geochemical and isotopic significances. It could be explained that the basaltic magma were intruded into the continental lithosphere, being interacted and contaminated by the crustal component and

Figure 11.⁸⁷Sr/⁸⁶Sr(i) vs ¹⁴³Nd/¹⁴⁴Nd of the rocks from Tú Lệ basin. Symbol as in Fig.2. Initial ratios are calculated as in Table 3.

fractionated to form the oldest felsic magma, then erupted. This stage is characterized by less alteration type of Sr isotope of older felsic rock (Fig. 11). The youngest felsic magmatism could be either the latest fractionated products of magmatism or another interaction of previous mafic magma with crustal derived fluid, characterized by highly Sr isotopic concentrations of Phu Sa Phìn and Ngòi Thia rocks (Fig. 11).

4. Tectonic environment of the Tú Lệ basin

The abundance of Mesozoic magmatism in the Tú Lệ basin reflects a regional extensional regime, which covered the whole South China block. On the basis of above shown geochemical and isotopic data, a rough generalization about the tectonic environment during Mesozoic are made as follows: the Indosinian orogeny was ended at the end of Triassic period, marking by the extermination of the oceanic-continental subduction at the Sông Mã suture zone, followed by the continental-continental collision between the Indochina and South China blocks. The formation of Sông Đà rift, which was interpreted as a result of mantle plume in Permian – Triassic [1] led to the formation of the Tú Lệ basin. As a result of the mantle plume formation, the asthenosphere went upward through the narrow rift and cross-cut the overriding lithosphere. The process created the thermal perturbation leading to the melting of the metasomatized overlying lithosphere, producing basaltic magmatism represented by the Suối Bé basalts and Bản Hát gabbro. These balsaltic magmas interacted with surrounding continental crust, and fractionated to form granitic magmatism of Ngòi Thia and Văn Chấn formations in the crust. The collected geological data from North Việt Nam and SW China have been showing an intraplate lithospheric extension setting for the Jurassic-Cretaceous magmatism whose generation followed the continental collision between the Indochina and South China blocks in the early Triassic. Formed originally in the western margin of the South China block, the Tú Lệ basin with its Mesozoic magmatic rocks moved southeastward to the present position by the mid-Tertiary sinistral displacement of the Ailao Shan - Red River shear zone, related to the India-Asia collision.

VI. CONCLUSION

1. The magmatism in the Tú Lệ basin shows the bimodality and can be classified into three major stages. The early and late stages are characterized by felsic magmas, while the middle stage is represented by basalt to andesitobasalt volcanites.

2. Geochemically and isotopically, the mafic rocks show the similarity to continental basalts, which were formed by partial melting of a mantle lithospheric component, which was contaminated in different level by the crustal component. The felsic rocks show the anorogenic (A-type) nature, which was formed probably by AFC process from the mafic magma.

3. The rocks of MS were formed between two cycles of felsic magmatism, but showed the crustal residence age older than the oldest rhyolite volcanism (Văn Chấn Formation). Within a 100 Ma interval of formation age, two cycles of felsic magmatism are of the same crustal residence age, sharing similarities in geochemical and isotopic significances.

4. Combining with relevant geological data from North Việt Nam and SW China, we propose an intraplate lithospheric extension setting for the Jurassic-Cretaceous magmatism whose generation followed the continental collision between the Indochina and South China blocks in the early Triassic. Formed originally in the western margin of the South China block, the Tú Lệ basin and its Mesozoic magmatic rocks moved southeastward to the present position by the mid-Tertiary sinistral displacement of the Ailao Shan - Red River shear zone, related to the India-Asia collision.

This paper is finished with supports of the Basic Research Project N^o.71.31.04 and Project “Intraplate magmatism and related minerallization of Vietnam”.

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