SOME MATERIAL COMPOSITION CHARACTERISTICS OF THE MAFIC-ULTRAMAFIC INTRUSIONS IN CENTRAL VIỆT NAM, CENTRAL HIGHLANDS
AND NORTH VIỆT NAM

TRẦN QUỐC HÙNG, BÙI ẤN NIÊN, PHẠM THỊ DUNG, HOÀNG VIỆT HẰNG
Institute of Geological Sciences, Hoàng Quốc Việt Road, Cầu Giấy, Hà Nội.

INTRODUCTION

The mafic-ultramafic intrusions widespread in Central Việt Nam and Central Highlands have been studied in various extents during the 1:500.000 and 1:200.000 geological mapping projects and some other thematic researches. As results of such studies 3 main intrusive assemblages have been divided as follows: 1) Dunite-peridotite assemblage; 2) Gabbro-pyroxenite assemblage; 3) Gabbro-peridotite assemblage.

1. The dunite-peridotite assemblage consists of Hiệp Đức, Khe Mây, Làng Hồi ultramafic massifs extending commonly in sublatitudinal direction along the Hiệp Đức - Khe Mây - Làng Hồi strip; Đắk Sa, Đắk Nai massifs extending in sublongitudinal direction along the Đắk Sa - Đắk Gley - Plei Weck strip, of which the Plei Weck massif is mainly composed of pyroxenite which was altered by amphibolization.

In general, the ultramafic massifs in the study area are of small size and strongly altered by serpentinization and talcization.

2. The rocks of gabbro-pyroxenite assemblage are fairly widespread in many places and were formed in various periods. They include 2 varieties: titanium-low gabbro-pyroxenite and titanium-high gabbro-pyroxenite.

a. Titanium-low gabbro-pyroxenite forms small bodies, extending in the deformation direction of the country rocks in Re River, Cheo Reo, Kông Plông areas. The main rock types include pyroxenite, gabbro-pyroxenite, gabbro, gabbroamphibolite.

The titanium-low gabbro-pyroxenite intrusions occur also in areas with wide distribution of ultramafic rocks, such as: Tam Kỳ - Núi Vú - Hiệp Đức - Làng Hồi - Ph­ớc Sơn [Nguyễn Xuân Bao et al, 2001], Đăk Sa - Ngọc Hồi - Plei Weck. They are usually present in the form of small, elongated massifs and bodies and in conformity with the country rocks.

Besides, they are also widespread along a strip extending in sublatitudinal direction from Bà Nà to Bol Kol. On the 1: 500,000 and 1: 200,000 scale geological maps these intrusions are attributed to the Chà Vằn Complex.

A prominent feature of these intrusions is that in their chemical composition they have relatively low TiO₂ content (TiO₂<1%), and rather high magnesium and aluminium content.

b. Titanium-high gabbro-pyroxenite. These intrusions are in the form of dikes, widespread and highly concentrated in Phú Mỹ, Bồng Sơn and An Lão areas. In their petrographic composition they consist of gabbro, amphibole gabbro, gabbroamphibolite and amphibolite. The prominent characteristics in chemical composition of these rock types are high in iron, high in titanium (TiO₂>1%), low in aluminium and in magnesium content. These intrusions are attributed to Early Neoproterozoic.

Besides, titanium-high gabbro-pyroxenite of younger age, about Middle-Late Triassic occurs also in Phú Lộc, An Lợi, Bạch Mã areas. In petrographic composition the rocks consists of amphibole gabbro, gabbro, gabbro-pyroxenite. In chemical composition, they have high TiO₂ (over 1% to 2%), iron and phosphorus content, and low magnesium content.

3. The gabbro-peridotite assemblage including the Khe Dung, Chà Vằn, etc., massifs, widespread mainly in Giằng area, has more diverse petrographic composition ranging from plagioperidotite through melanogabbro, olivine gabbro, gabbronorite to amphibole gabbro. Its rocks are different from those of the gabbro-pyroxenite assemblage by high magnesium, moderate and low titanium (about 1%) content.

In brief, the previous study results have established the characteristics of geological structure, shape and size of the intrusive massifs and their relationship with the country rocks as well as their basic material composition, such as petrography, petrochemistry.

These results are valuable and important for further studies. However, besides the mineral composition of some massifs of Chà Vằn Complex, no evaluation in respect to the forming temperature, pressure and no justification on geodynamic conditions related to the mineralization process have been made.

On the basis of new analysis data and the results of previous studies by other researchers and the authors of this paper, we would like to contribute to the exchange of ideas and to the solution of the outstanding above mentioned problems.

Our studies have been concentrated on some mafic-ultramafic massifs widespread in the study area of the State level research project DTDL-2003/07, which include: Hiệp Đức massif in Hiệp Đức - Khe Mây - Làng Hồi strip of commonly sublatitudinal extending direction; Đăk Sa, Plei Weck massifs of Đăk Sa - Đăk Gley - Plei Weck strip of sublongitudinal extending direction, a gabbro massif near Đăk Nai village, Đăk Mon commune, Đăk Gley district that had previously been attributed to Núi Ngọc Complex.

I. GEOLOGICAL CHARACTERISTICS

In general the ultramafic massifs of the above zones are usually of very small size, elongated shape and associated with cherty sedimentary, mafic - intermediate effusive formations metamorphosed to various degrees. The geostructural characteristics and size of the massifs have been described rather adequately in the reports of the 1: 500,000 and 1:200,000 scale geological mapping projects, therefore we do not describe them here. These massifs have been all attributed by the previous authors- geologists of the Geological Division No 6 under the editorship of Nguyễn Xuân Bao - to the ophiolite assemblage.

All of these massifs have the common characteristics that at the inner contact zone the rocks are strongly altered by serpentinization, talcization, carbonatization, phlogopitization, etc.

In the area of Hai village, Quế Bình commune, Ph­ớc Sơn district, 300 m from the bridge in the direction to Khâm Đức, on the side of the newly built road occur ultramafic rocks exposed continuously on about 200 m. The ultramafic massif has a very clearly tectonic contact with the terrigenous sediments of A V­ơng Formation. Its extending direction coincides with the strike direction of the sedimentary country rocks. Along the contact zone the rock is crushed, crumbled in a chaotic manner, no indication of hot contact has been seen.

Continuing the field trip in the direction to Khâm Đức, at a distance of about 7-8 km, in the area of Làng Hồi occur again ultramafic rocks which are strongly serpentinized and compressed as in the area near Hiệp Đức bridge.

The rocks surrounding the ultramafic bodies in Plei Weck area, of which the typical one is the Plei Weck massif occurring on both sides of Pô Cô River, which now belongs to the reservoir of the Ya Ly hydropower plant, are composed of stratified chlorite schist and actinolite schist which are strongly crumbled. In these rocks there are many pockets and lenses of tremolite schist, which increase in amount with the height. Higher in the section there is amphibole schist penetrated by amphibole plagiogranitogneiss.

According to the description by Nguyễn Xuân Bao et al. [Geology and mineral resources of South Việt Nam, 1994, in Archives], in the country rocks in Plei Weck area are met also basalt and andesite effusive formations which are chaotically arranged; in some places they are in stratified form containing lenses of pyroxenite or dunite with chrome-magnetite.

II. PETROGRAPHIC CHARACTERISTICS

In the Hiệp Đức, Khe Mây, and Đăk Sa ultramafic massifs, the main petrographic components are serpentinized and talcized dunite and peridotite. The main mineral components are serpentinized and talcized olivine occupying about 90-95% in dunite and about 65-85% in peridotite, pyroxene occupying 5-7%; the rest consists of tremolite, chromite, magnetite, phlogopite. Especially, in some samples from Khe Mây massif occur small grains of colourless corundum regularly distributed in the rock.

Serpentinites are met in larger amount in the areas crossed by faults, near tectonic fractures and contact zones. The rocks are of yellowish green colour, banded structure, in mineral composition composed mainly of serpentine up to 60-90%; other minerals include calcite, tremolite, epidote, sulfides, chromite and magnetite. In many samples magnetite occupies up to 10-15%, in small grains regularly distributed in the rock.

In Plei Weck area the rocks are of relatively diverse in composition, consisting of amphibolized pyroxenite, gabbro, gabbrodiorite and diabase.

The pyroxenite here is medium-grained, amphibolized and talcized, composed mainly of amphibolized monoclinic pyroxene of large plate form, orthopyroxene of elongate, small-grained form, occupying 2-3% of the rock mass. Besides, phlogopite of small, elongate scale form, in some places epidote, saussurite and some acicular crystals of rutile are also met. Ore minerals are small-grained, irregularly disseminated, in some samples reaching 4-5%.

The pyroxenite is completely amphibolized, consisting of rather symmetrical platy hornblende crystals of various size. Besides, in the rock occur also tremolite, talc, some scales of muscovite, some aggregations of multicoloured epidote. Disseminated sulfide ore minerals occupy 2-3%.

Mafic rocks include melanogabbrodiorite and green small-grained diabase in 2 m thick dike form. They are met only on the north side of the Ya Ly hydropower reservoir.

In a small stream emptying into the reservoir we met amphibolized pyroxenite as above described, penetrated by gabbrodiorite, leucocratic diorite and diabase which are in the dike form.

In the area near Đăk Nai village, Đăk Mon commune, Đăk Glei district there is a gabbroid massif attributed by geologists of Geological Division No 6 to Núi Ngọc Complex. In this locality we found both gabbronorite and gabbrodiorite.

The amphibole gabbro is of medium to small grain size, melanocratic, containing amphibole up to 60-65%, plagioclase 30-35% in the form of small, isometric crystals with clear twinning. In the samples occurs also small amount of disseminated sulfide ore and epidote. The rocks are of oriented structure because the amphibolite is oriented in a fixed direction.

In some places the amphibole gabbro is more intensively altered. Here biotite, quartz and calcite are also met. Ore minerals occupy up to 5-7% and are usually associated with colour minerals. The colour minerals form sinuous strips arranged in a fixed direction.

The gabbronorite is of medium grain size, melanocratic, where colour minerals occupy up to 70%. The main mineral component is pyroxene, accompanied by plagioclase, biotite and some grains of quartz. Ore minerals are sparsely disseminated, occupying about 1%. Pyroxene is mainly diopside which is the main component of the rock, in the platy, isometric form, relatively fresh. Orthopyroxene occupies 5-7%, with the size greater than clinopyroxene. Plagioclase is of platy, twinned form, weakly sericitized, occupying from 25 to 30%. Biotite is of brown colour, occupying about 10% and is distributed rather regularly in the rock.

Amphibole gabbro is of medium to small, regular grain size, melanocratic, relatively fresh, weakly oriented. In mineral composition it is composed of amphibole, plagioclase, biotite and fine-grained disseminated ore minerals.

Amphibole occupies about 50%, with clear orientation, the crystals are usually of elongate form (1:3). Plagioclase occupies about 40% of the rock mass, in platy, twinned form, relatively fresh. Only some grains are sericitized. Biotite occupies about 7-8%, of red brown colour. Besides, small, well idiomorphic biotite crystals are also met encrusted in the amphibole.

The study gabbroid massifs study have much simpler petrographic composition than those of typical stratified types in the north, such as Núi Chúa, Khao Quế, etc. massifs. Here, we usually meet only 4-5 rock types, in the mean time the differentiated massifs of Núi Chúa type have the number of rock types by 3-4 times greater. This shows clearly their difference in chemical composition.

The gabbroid massifs of Núi Chúa types in the geological structures of North Việt Nam have high magnesium, low titanium, low alkali content and are characterized by the assemblage of elements Ni, Cu, Co and Pt [1-5, 8]. The gabbroids presented in this paper have quite different chemical composition, with usually low magnesium content, while their alkali and titanium content are higher. Ore elements, such as Ni, Co, Cu, are usually lower in content than those in the differentiated gabbroids of Núi Chúa type.

III. CHEMICAL COMPOSITION CHARACTERISTICS

The chemical composition of ultramafic rocks distributed in Central Việt Nam and Central Highlands is presented in Table 1. In this Table we present only the new analytical results of ultramafic rocks in the study area and those of the ultramafic rocks of Núi N­a massif for comparison.

The analytical results show that, although the composition of the ultramafic rocks of Đăk Sa, Khe Mây and Plei Weck massifs are different, they all have very low TiO₂ (0.036-0.014%) and alkali content and they are divided into two distinct rock groups: the ultramafic rocks of Đăk Sa and Khe Mây massif, and the sub-ultramafic rocks (Plei Weck massif mainly composed of pyroxenite, besides in some places at the margin of Khe Mây massif).

Table 1. Chemical composition of ultramafic rocks in Central Việt Nam, Central Highlands and Nói Na massif

Notes: 1, 2, 7, 8, 10, 13: Dunite; 5, 15: Serpentinized dunite; 3, 4, 8, 10, 11, 13, 14: Peridotite;

16: Serpentinized peridotite; 17, 18: Amphibolized pyroxenite.

The ultramafic rocks of Đăk Sa massif consist of strongly serpentinized dunite and peridotite with MgO content from 39.45% to 46.04%, TiO₂ from 0.035% to 0.139%, K₂O from 0.012% to 0.046%; Na₂O from 0.35% to 0.37% (Table 4). Besides, we also see that the alumina and silica content in these rocks is very low: Al₂O₃ from 0.38% to 1.82%; SiO₂ from 44.32% to 46.82%.

The peridotite of Khe Mây massif is basically similar in chemical composition to that Đăk Sa massif (Table 1), high in magnesium, low in titanium and alkali.

The sub-ultramafic rocks include pyroxenite and plagioperidotite, both strongly altered by serpentinization and amphibolization. They are the main components of Plei Weck massif. Similar to true ultramafic rocks, they are all characterized by very low titanium and alkali content: TiO₂ from 0.038% to 0.14%, K₂O from 0.12% to 0.16%. Other components, such as silica, alumina, calcium are all higher: SiO₂ from 49.01% to 57.46%, Al₂O₃ from 2.61% to 16.37%, CaO from 8.85% to 15.47%. The rocks have low magnesium content (MgO varies from 11.38% to 18.63%). Among the sub-ultramafic rocks the plagioperidotite of Khe Mây massif is different from pyroxenite of Plei Weck massif by drastically higher Al₂O₃ content (16.37%) and CaO, Na₂O being also higher (Table 1). This is also a problem to be noted in further investigations.

In general the ultramafic and sub-ultramafic rocks of Đăk Sa and Khe Mây massifs are in chemical composition and hydrothermal alteration quite similar to those widespread in Tạ Khoa area, Sơn La province.

In the three component diagram (CaO+Na₂O+K₂O) - (Fe₂O+FeO) - MgO (Fig. 1), the ultramafic rocks in the Central Highlands and Central Việt Nam lie both in the field of chromite mineralization potential (e. g. Núi N­a) and in that of therocks with copper- nickel mineralization potential. Besides, a comparison of the variation characteristics of oxides SiO₂, Al₂O₃, å FeO, CaO and Na₂O+K₂O in relation with MgO of the main rock types shows that the ultramafic rocks in Đăk Sa, Khe Mây massifs are quite similar to the dunite - harzburgite of Sông Mã zone and the oceanic crust [7].

Although they have similar characteristics as above mentioned, so far still no chromium or Cu-Ni ore deposit of economic values related with mafic- ultramafic intrusions in the Central Việt Nam and Central Highlands have been discovered like those in the North Việt Nam. Indications of disseminated pyrite and chalcopyrite mineralization has been only met in the contact zone of the Đăk Sa ultramafic massif.

The above mentioned geostructural characteristics of the ultramafic massifs in the study areas show that they are formed in small bodies and have been subjected to strong hydrothermal alteration. This proves that they have been subjected to more intensive and complicated effects of tectonic processes, and as a result they were compressed, crushed into many fragments before being emerged to their present position. In other words, these ultramafic bodies were emerged from the depth, wedging into the upper layers of the continental crust of the Central Việt Nam and Central Highlands.

The distribution characteristics of rare and rare earth elements in Table 2 shows that the light REE content is low. The Nb/Ta ratio is very high, varying from 50 to 137, characterizing the composition of the primary mantle. This proves that the original composition of the magma which formed the mafic - ultramafic rocks of Đăk Sa - Đăk Gley - Plei Weck strip basically was not contaminated by the elements from the earth crust.

The distribution of REE in Fig.2 shows that in general the distribution line of elements is a horizontal line, it is a distribution line characterizing the mafic-ultramafic rocks which are not much contaminated with the components of the Earth crust layer, rather similar to the variation line of the mafic-ultramafic rocks distributed in the ocean ridge. However, on the diagram we see that the content of lanthanum is slightly higher than normal value; this may be caused by the effect of the weathering process that occurred later, because the mafic-ultramafic rocks of the study areas have been strongly altered due to later tectonic activities. These are only preliminary remarks to confirm the magmatic origin and geodynamic settings related to the formation of the ultramafic intrusions in Plei Weck area, that need further additional studies.

IV. MINERAL COMPOSITION CHARACTERISTICS

On the basis of the microprobe analysis of some rock forming minerals such as clinopyroxene, amphibole and plagioclase (Tables 3, 4, 5), some remarks can be made as follows: on the illustration diagrams the content of amphibole, clinopyroxene, plagioclase is quite concentrated (Fig. 3a, 3b, 3c), reflecting a narrow range of variation in composition of the rock types. This proves that the study gabbroids are less differentiated than the stratified gabbroids of Núi Chúa type.

The results of microprobe analysis (Table 3, Fig. 3) show that clinopyroxene in the pyroxenite of Plei Weck massif consists of two varieties: diopside and augite. Diopside has MgO content from 15.39% to 16.78%, FeO from 4.20% to 4.85%, Al₂O₃ from 0.69% to 3.13%. The chemical composition of augite varies as follows: MgO from 17.52% to 20.78%,

Table 2. Results of analysis of rare and rare earth elements in pyroxenite (DL.809/1) and gabbro of Plei Weck massif

 

iron from 4.20% to 7.98%, calcium varies within a very narrow range from 12.62% to 13.02%. Differing from diopside, the Al₂O₃ content in augite varies within a wider range, from 1.40% to 5.01%. In general both augite and diopside have the typical characteristics that they have high in SiO₂, relatively high in Cr₂O₃ and richer in magnesium than the clinopyroxene in the gabbroids of Núi Chúa massif.

In the three component diagram of En-Wo-Fs, the points illustrating the composition of clinopyroxene, including diopside and augite are close to En-Wo, proving that they have high MgO and CaO, and low iron content.

According to Lindsley's method of temperature determination, diopside was crystallized in the temperature from 750^oC to 800^oC, while augite was crystallized at the temperature of about 1200^oC.

The results of studying thin sections show that the pyroxenites of Plei Weck massif do not contain olivine. From here it is possible to infer that the crystallization of the rock varieties of Plei Weck massif was started from about 1200^oC when crystallizing augite and terminated at about 750^oC to 800^oC.

The chemical composition of clinopyroxene in the gabbroids of Núi Chúa massif (Table 3, Fig.3a) shows that all of them are augite, but all the points illustrating the composition of augite lie close to the line separating augite and diopside.

All amphiboles in the gabbroids in the Central Việt Nam and Central Highlands, according to the results of microprobe analysis (Table 4, Fig.3b), are hornblende with high magnesium content.

Amphibole contains MgO (from 11.15% to 11.67%), CaO (from 11.54% to 11.70%) and Al₂O₃ varying within a relatively narrow range (from 9.22% to 9.73%). The iron content is from 15.75% to 16.03%, titanium varies within a wider range than the content of the above oxides (from 1.37% to 1.55%).

The amphibole composition diagram according to the results of microprobe analysis (Fig.3b) shows that the amphiboles in the gabbroids in Đăk Nai, Đăk Gley areas are rather uniform. The points illustrating their composition are nearly concentrated at one point in the magnesium hornblende field.

 

Table 3. Chemical composition of clinopyroxene in the ultramafic rocks of Plei Weck and Núi Chúa massifs

NC-1, NC-2, NC-3 clinopyroxene in gabbroid of Nói Chóa massif

The chemical composition characteristics of plagioclase in the gabbroids of Đăk Nai, Đăk Ley areas (Table 5, Fig. 3c) show that they have a rather stable composition. The variation in content of some main oxides is as follows: SiO₂ from 53.42% to 55.89%, Al₂O₃ from 27.91% to 29.16%, CaO from 10.60% to 11.73%, Na₂O from 5.06% to 5.65%, K₂O from 0.08% to 0.21%. On the diagram illustrating the Ab-Or-An composition they are concentrated nearly at one point pertaining to labrador (Fig. 3c). In comparison with the plagioclase of the gabbroids of Núi Chúa massif we can see a clear difference. The composition of plagioclase in Núi Chúa Complex varies within a very wide range, from labrador with An_60-65 to bitaunite with An_75-80 [3]. This once more proves that differentiation of the gabbroids of Núi Chúa complex is higher than that of the gabbroids widespread in the Central Việt Nam and Central Highlands.

DISCUSSION AND CONCLUSION

By analyzing and processing some results of recent studies on material composition of some mafic and ultramafic massifs widespread in the Central Việt Nam and Central Highlands compared with the results of previous studies on the mafic and ultramafic rocks widespread in the North Việt Nam, we have some preliminary remarks as follows:

1. Gabbroids widespread in the South have their petrographic, mineral and chemical compositions different from those of the gabbroids of Núi Chúa Complex. They usually do not contain olivine as in Phú Lộc, Đăk Nai, Bạch Mã etc, characterized by high titanium

Table 4. Chemical composition of amphibole in the ultramafic rocks of the Central Việt Nam and Central Highlands

Table 5. Chemical composition of plagioclase in the ultramafic rocks of Central Việt Nam and Central Highlands

Notes: Tables 1, 2, 3, 4 are the analytical results of the State level research project DTDL-2003/07- analyzed at the Institute of Geology, Novosibirsk Branch of the Russian Academy of Sciences.

content, 1-2%. The results of rare earth analysis of the gabbroids of Phú Lộc and Khe Dung massifs show that the contents of light REE are also higher, characterizing the rocks that assimilated the materials from the earth crust during their formation [6].

2. The above indications may be caused by the difference in composition of the earth crust in the Kon Tum Geoblock from that in the areas of wide distribution of the gabbroids of Núi Chúa Complex.

3. In mineralization, different from the gabbroids of Núi Chúa Complex which are potential in Cu-Ni, the gabbroids in the Central Việt Nam and Central Highlands have typical chemical composition in iron, titanium, vanadium and apatite in some areas such as: Phú Lộc, Nam Đông, Bạch Mã etc. Besides the typical characteristics with chromium prospect in areas such as Núi N­a, there are also some samples from the field with prospects in Cu-Ni. Besides, differing from the ultramafic rocks in the North Việt Nam, some ultramafic rocks samples of Khe Mây massif contain also disseminated small-grained corundum.

4. The geological characteristics, material composition including chemical composition of the main rock forming oxides, as well as the rare and rare earth elements, show that the ultramafic rocks widespread in the central part of the Central Highlands are similar to the ultramafic rocks of Núi N­a massif and the oceanic crust, i.e. they correspond with the composition of ophiolite formations. Besides, they have also a difference that some samples of ultramafic rocks in Khe Mây - Làng Hồi area contain small crystals of corundum, while none is found in the North.

5. The characteristics related to the shape and size of the massifs, as well as the diversity in mineralization, especially the presence of corundum prove that the ultramafic intrusions in the study area were formed in the geodynamic conditions which are different from the setting of in the formation of ultramafic massifs in the ophiolite assemblage in the North Việt Nam. However, in order to clarify the geodynamic settings as well as the causes of forming corundum in ultramafic rocks in the region, there must be more detailed studies.

This paper is finished under support of Project "Research on the forming conditions and distribution rules of precious minerals in relation to magmatic activities of Central and Tay Nguyen regions" – DTDL-2003/07 and the Fundamental Research Project No 71.10.04.

REFERENCES

1. Hoàng Hữu Thành, 1994. Mesozoic stratified massifs of peridotite - gabbro in the North of Vietnam. Doctor thesis, (in Russian).

2. Ngô Thị Phư­ợng et al., 1995. Minerals of the platinum group (Pt, Pd) in ultramafic intrusions of Sông Đà zone. J. Sci. of the Earth, 17/4 : 157-163. Hà Nội (in Vietnamese).

3. Poliakov G.V et al., 1996. Permian-Triassic mafic - ultramafic intrusions in North Vietnam. Sci. and Techn. Publ. House. Hà NộI, 172 p.

4. Trần Quốc Hùng et al., 1985. New data on mineralization and differentiation and stratification of Núi Chúa massif. Sci. Letter of Inst. of Sci., 2 : 54-59. Hà Nội

5. Trần Quốc Hùng, 1992. Mafic - ultramafic intrusions in North Vietnam. Dr of Science thesis (in Russian).

6. Trần Quốc Hùng, Bùi ấn Niên, 1998. Composition characteristics and forming mechanism of Khe Dung massif (North of Kon Tum uplift). J. of Earth Sciences, 2 : 39-45. Hà Nội (in Vietnamese).

7. Trần Quốc Hùng, 1999. Early Paleozoic ultramafic intrusions in North Central region of Vietnam. J. of Earth Sciences, 2 : 119-122. Hà Nội.

8. Trần Trọng Hoà et al., 1996. New results in the study on high magnesium-ultrapotassic rocks in Northewst Việt Nam. J. Sci. of the Earth, 18/3 : 159-170. Hà Nội.