NguyÔn v¨n thµnh¹, ng« phó an², phïng v¨n du©n²,
ph¹m huy häc¹, v­¬ng m¹nh s¬n³

Many research works have been touching upon the magmatic rocks in the Tßng B¸ area, but their forming mechanism and age still rest as remaining questions. In his Report on the geological mapping of the Tßng B¸ - B¾c Mª band (Hµ Giang Province) at 1:50,000 scale T¹ Hoµng Tinh [1971, Geological Archives] assigned the Tßng Kho¶ng Massif to the Lower Paleozoic intrusive complex, while the remaining part consisting of metamorphosed felsic-alkaline effusives and their tuffs interbedded with crystalline schists - to B¶n Vµn and Cao Vinh formations of Middle Devonian. Hoµng Xu©n T×nh [2] assigned granitoids of the Tµng Kho¶ng Massif to the Lower Paleozoic Tßng B¸ Complex, while felsic-alkaline effusives and their tuffs - to the Tßng B¸ Formation (O-S? tb). In the work on the revision of the Geological Map of the B¶o L¹c Sheet at 1:200,000 scale, the Tµng Kho¶ng Massif was assigned to the Middle Paleozoic Phia M¹ Complex, while effusives - to the Tßng B¸ Formation of Lower Devonian [3].

At present, basing on factual geological materials, petrographical composition, petro-geochemical characteristics, characteristics of ore-making elements, etc. we propose to establish the S«ng MiÖn Complex (g x p MZ? sm), including the S«ng MiÖn and Tµng Kho¶ng massifs together with many other small massifs, dykes, lenses and veins developed along the NW-SE trending faults and distributed in the confines of the S«ng G©m structural zone [4] with an area of about 3 km². This is a characteristic subvolcano-intrusive complex with the composition from potash feldspar granite, granosyenite, syenite, aplitic granite, trachyte and trachyrhyolite.

Granitoids of the S«ng MiÖn Complex are exposed in the S«ng MiÖn, Tµng Kho¶ng and Lµng Tµi massifs, developed in the distributive area of the B¶n KÑp Formation (D₁? bk) together with some

small massifs met in the B¶n MÞch, B¶n Cam and B¶n Khuæi Rào localities, developed in the distributive area of the B¾c Mª Formation (D₁? bm).

- The S«ng MiÖn Massif extends along the two sides of the MiÖn River on an area of about 15 km². It penetrates metamorphosed sediments of the upper part of the B¶n KÑp Formation (D₁? bk), that has been observed in the shore of the river. On the petrographic and textural sides the rocks of this massif are not completely homogeneous, but there is a distinct facial change from the centre towards the margin of the massif. In the centre the rock composition consist mainly of porphyritic potash feldspar granite and porphyritic granite; these rocks are of porphyritic texture with the aplitic or microgranular hypidiomorphic background. In the margin, where there is the contact with crystalline schist, trachyte occurs with the porphyritic texture with the characteristic trachytic background.

- The Tµng Kho¶ng Massif is situated in the upper course of the MiÖn River, in the northwest of the Tßng B¸ Valley with an area of about 16 km². In the studied sections one can find that granitoids of this massif penetrate metamorphosed sediments of the B¶n KÑp Formation (D₁? bk). On the petrographic and textural sides the rocks of this massif have also the facial change as described in the S«ng MiÖn one. It is worthy to note that, in the margin of both these massifs still fresh, not subjected to metamorphism yet, microveins or long-flame, wen-like bodies of trachyte or trachyrhyolite have been met, conformably penetrating crystalline schists of the B¶n KÑp Formation. This zone is usually developed with a thickness of about 5-10 m, sometimes up to 15 m. Previous geological data considered these rocks as metamorphosed agglomerate and tuffs of felsic-alkaline composition (!).

- Other small massifs are exposed in the B¶n MÞch, B¶n Cam and B¶n Khuæi Rµo areas, developed along NW-SE trending faults. They are usually lentiform, elongated with the length many times greater than the width. Granitoids of these blocks usually penetrate conformably or with a small angle the bedding surface of metamorphic sediments of the B¾c Mª Formation. On the petrographic and textural sides, they are similar to granitoids in the S«ng MiÖn and Tàng Kho¶ng Massifs.

The S«ng MiÖn Complex is composed of syenite, granosyenite, potash feldspar granite, trachyte and trachyrhyolite.

- Syenite and granosyenite are flesh pink coloured. Their texture is various: in the centre of large massifs it is porphyritic with the crystalline granulo-hypidiomorphic background. In lentiform bodies and in the margin of large massifs it is porphyritic with the aplitic background. The mineral composition consists mainly of potash feldspar (microcline), quartz, a little biotite, muscovite, apatite, zircon and magnetite. Granosyenite often contains quartz with the content of 5-10%, sometimes up to 15%.

- Potash feldspar granite is pinkish -light grey coloured. Its texture is various: in the centre of large massifs it is porphyritic with the microgranular hypidiomorphic background, while in the massif margin and lentiform bodies it is porphyritic with the granophyric or microaplitic background.

The mineral composition consists of potash feldspar, quartz, some acidic feldspar, biotite, muscovite, apatite, zircon and magnetite.

 - Trachyte and trachyrhyolite are pinkish light-grey coloured, of flowage structure, distributed in the margin of large massifs or as vein-like, pseudovein-like separate bodies. Trachyte is of porphyritic texture with trachytic background. Its mineral composition consists mainly of potash feldspar and a small amount of biotite, zircon and magnetite. In trachyrhyolite the phenocrysts consist mainly of potash feldspar, in its background appears quartz together with potash feldspar forming the felsitic texture.

The analytic results of silicate chemistry have been showing that the rocks of the S«ng MiÖn Complex consist mainly of syenite, quartz syenite and potash feldspar granite.

The SiO₂ content varies between 56.54 - 75.68%; the total alkali K₂O + Na₂O = 9.3%, with K₂O / Na₂O = 5.37, belonging to the potassic series.

Aiming to determine the age and origin of the S«ng MiÖn Granitoids the authors of the paper collected 2 sample collections: 1) 6 whole rock samples: 22° 58’ N; 105° 03’ E; 2) 5 whole rock samples: 22° 54’ N; 105° 06’ E.

Breaking the samples by the acidic mixture HF + HCl 6N (5:1); diluting the isotopes with the use of marker isotopes ⁸⁷Rb and ⁸⁴Sr (the rules of processing and fabricating of samples are presented in [4 & 5]).

The analysis of isotopic ratios and determination of Rb and Sr content by the method of isotope dilution was realized on the mass-spectrometer TSN 206 SA. The separation technique of Rb and Sr isotopes was realized on the mass-spectrometer with the Ta thread (size: 50 m m - 0.75 mm) in the following regime of burning flow: I(Rb) = 1.6 - 0.2 A and I(Sr) = 2.8 - 0.2 A.

The values of ⁸⁷Sr/ ⁸⁶Sr isotopic ratio, the ⁸⁷Rb/ ⁸⁶Sr content ratio and age of the collection of 6 granitoids (granosyenite) samples from the S«ng MiÖn Complex are given in the Table 1. The isotopic age determined by the Rb-Sr method is as follows: T = 173 - 14 Ma with the initial ⁸⁷Sr/ ⁸⁶Sr value: (⁸⁷Sr/ ⁸⁶Sr)₀ = 0.776 - 0.01 (Fig. 1).

Table 1. Result of determining the ⁸⁷Sr/ ⁸⁶Sr isotopic ratio, ⁸⁷Rb/ ⁸⁶Sr content ratio
and age of the collection of 6 granosyenite samples from the S«ng MiÖn Complex

The values of ⁸⁷Sr/ ⁸⁶Sr isotopic ratio, the ⁸⁷Rb/ ⁸⁶Sr content ratio and age of the collection of 5 granitoids (syenite) samples from the S«ng MiÖn Complex are given in the Table 2. The isotopic

age determined by the Rb-Sr method is as follows: T = 172 ± 15 Ma with the initial ⁸⁷Sr/ ⁸⁶Sr value: (⁸⁷Sr/ ⁸⁶Sr)₀ = 0.739 ± 0.015 (Fig. 2).

Table 2. Result of determining the ⁸⁷Sr/ ⁸⁶Sr isotopic ratio, ⁸⁷Rb/ ⁸⁶Sr content ratio
and age of the collection of 5 syenite samples from the S«ng MiÖn Complex

Figure 2. Isochronic line of the collection of 5 syenite samples from the S«ng MiÖn Complex dated as: T = 172 ± 15 Ma (1s ) with (⁸⁷Sr/ ⁸⁶Sr)₀ = 0.739 - 0.015.

 3. Discussion on the results

The Rb-Sr isotopic age of the two granitoid sample collections from the S«ng MiÖn Complex is as follows: 173 ± 14 Ma and 172 - 15 Ma, conforming with the result of S«ng MiÖn Complex dating by the K-Ar method in Japan as 176.2 - 3.8 Ma (TrÇn Ngäc Nam, analysis on potash feldspar from granite in the Research Institute for Natural Sciences, Okoyama University, 1999; oral communication).

So, the dating for the S«ng MiÖn Complex as Middle Jurassic seems to be reasonable.

The values of the initial ⁸⁷Sr/ ⁸⁶Sr isotopic ratio as 0.767 and 0.739 are greater than the normalized isotopic value of the rocks of crust origin (0.705); associated with geological materials and the general view of the studied area, we can consider that they are of remelted crust origin.

The S«ng MiÖn Complex has been dated as Middle Jurassic and considered as of remelted crust origin, that conform to geological materials and general view of the studied area.

On the occasion of the paper completion, the authors would like to express their deep thanks to the Directorates of the Centre of Geological Experiments and Analyses, of the Intergeo Division and colleagues of the Group of Mass-spectrometer, of the Geological Party No. I.3 for their help in creating many favourable conditions for the completion of the paper.

1. Faures G., 1986. Principles of isotope geology. Wiley and Sons, New York.

2. Hoµng Xu©n T×nh, 1976. On the Lower Devonian sediments in the B¶o L¹c Map sheet. B¶n ®å ®Þa chÊt, 30 : 20-27 Hµ Néi (in Vietnamese).

3. Hoµng Xu©n T×nh (editor), 2001. Geological and mineral resources map of ViÖt Nam at 1:200,000 scale: B¶o L¹c Sheet, with the Explanatory note. Dpt of Geol. & Min. Res. of ViÖt Nam, Hµ Néi (in Vietnamese).

4. TrÇn V¨n TrÞ (editor), 1977. Geology of ViÖt Nam: The North part. Sci. & Techn. Publ. House, Hµ Néi (in Vietnamese)