OPHIOLITE ULTRAMAFIC-MAFIC ASSOCIATIONS
IN THE NORTHERN STRUCTURE OF THE KON TUM BLOCK (CENTRAL VIỆT NAM)

A.E. IZOKH¹, TRẦN TRỌNG HÒA²,
NGÔ THỊ PHƯỢNG², TRẦN QUỐC HÙNG²

¹Institute of Geology and Mineralogy, SB RAS, Novosibirsk, Russia
²Institute of Geological Sciences, VAST, Hà Nội, Việt Nam

Abstract: Along the Tam Kỳ - Hiệp Đức suture zone, in the North of Kon Tum Block, some separated lenses and bodies of apodunite and apoharzburgite crop out. These hyperbasites are closely associated with black schists and thin-bedded marble that is not typical feature of ophiolitic formation in the open oceanic environment. The predomination of serpentinized harzbugite in mélange allows us to classify them to HOT-type ophiolite (according to A. Nicolas), that characterizes back-arc ophiolites. The peculiarities of geochemical significance of both hyperbasites and gabbroids represent the evidences of fragments of marginal sea ophiolites in the area.

INTRODUCTION

Pointing out of suture zones that divide different terranes plays an important role in tectonic and geodynamic formations. These zones are mainly marked by the outcrop of ophiolite complexes, their fragments and serpentinite melanges [2]. According to the investigations of the Central Asian fold belt, ophiolite associations formed in various geodynamic conditions, such as open-ocean, island-arc systems and oceanic plateaux, may be included in the composition of such zones [4, 12]. At the same time, layered gabbroids, that precede various granoids and are formed in different geodynamic environments including collisions and shifted collisions, are joined into ophiolite associations by mistake [9]. In this connection, important diagnostic features of suture zones are geological position of ultramafic-mafic associations and their geochemical characteristics.

I. GEOLOGICAL LOCATION OF ULTRAMAFIC AND MAFIC ROCKS IN THE NORTHERN PART OF THE KON TUM BLOCK

In the northern part of the Kon Tum Block in the Central Việt Nam in sublatitude area from the Khâm Đức till the Tam Kỳ townlets separate lenticular bodies of apodunite and apoharzburgite occur. Frequently, places of serpentinite mélange with pieces of metagabbroids and pyroxenites, and also separate bodies of well-deformed and metamorphic quartz diorites, tonalites and plagiogranites are seen.

Separate outcrops of serpentinites are seen in the Bãi Gõ gold deposits. They are located between black schists, quatzites and thin-bedded marbles. Rare intrusions of chromite substituted by magnetite are seen in serpentinites. In some places ultramafic rocks are too re-crystallized and a leafy system connected with the influence of granitoids is developed in them. In separate outcrops the areas composed of anthophyllites were formed due to pyroxenites and also metagabbroic amphibolites are found. In the latter the relics of gabbroic structures remain. The rocks underwent strong deformation and greenstone metamorphosis. The plagioclase is totally replaced by saussurite, and pyroxenite is substituted by actinolite.

Another area of ultramafic rocks and metagabbroid spreading is situated in the east from the Khâm Đức on the way to the Tam Kỳ Townlet. Here, in quarry where gravel for building is exploited a huge lens of very deformed apodunite serpentinites with boudins (2-3 m) of metagabbroids, metapyroxenites and rodingites, is exposed between schists. Sometimes, the areas with intrusions of chromate occur. Like in the previous place, gabbroids are characterized by strong catalysis and deformation, though the relics of magmatic structures remain.

Root outcrops of apodunite serpentinites are found in the area of Va Ha village in a roadside groove. Alongside the road to the Khâm Đức serpentinized ultramafic rocks, among which metapyroxenites, werlites and metagabbros encounter, are bared. Judging by rhythmic alternation of ultramafic rocks and pyroxenites, the fragment of dunite-werlite-clinopyroxenites-gabbroic linear complex that is a compound part of ophiolite association is encountered in this outcrop. In the same outcrop, linear bodies of metadolerites turned into chlorite schists are found. Sometimes massive dikes of metadolerites are encountered. These observations allow us to refer them to a dike complex of ophiolite association.

II. MINERALOGY AND PETROGRAPHY OF THE ROKCS

Ultramafic rocks are mainly represented by apodunite serpentinites. Sometimes, the relics of olivine remain in massive blocks in thin section. More often, even in this case olivine is fully serpentenised, but shadow structures of its substitution remain. In more pressed and deformed areas apart from serpentine chlorate appears. For apoharzburgite serpentinites lamellar pseudomorphs of serpentine on/upon/at orthopyroxenites (bastite) are observed. Chromespinellide make up a rare even intrusion and encounter both as inclusions in olive and as separate grains.

The composition of analyzed spinels is given in Table 1. In massive serpentinites we have stated the relics of chrome and magnesial alumochromate, that are substituted by magnetite during serpentinization. Such composition of spinellide is a characteristic feature of ophiolite hyperbasites, especially for the dunite-harzburgite association [15].

The rocks of the main composition are also much altered. In some places, they are turned into amphibolites and chlorite-actinolite schists. In some more undamaged places the primary gabbroic structure is remained. The absence of new plagioclase is the characteristic feature of the rocks of this association. There is also no olivine, but the existence of serpentine in some samples, the rock structure and their chemical composition enables us to suggest that they correspond to troctolite or olivine gabbro. Pyroxenites are also substituted by amphibole (actinolite or simple hornblende). In some samples brown amphibole, that probably coated pyroxene, occurs.

III. PETROCHEMICAL CHARACTERISTICS OF THE ROCKS

In Figure 1, the composition of ultrabasic and basic rocks is given. Ultrabasites are characterised by constant composition. Ultramafic rocks are characterized by high content of Mg at low concentrations of Ca and Al. Some variations of composition are caused by the increase of orthopyroxene part at the transition of apodunite serpentinites to apoharzburgite serpentinites. Ferruginousity of this rocks varies from 8 to 12 %, which is a characteristic feature of mantle hyperbasite associations. Ultramafic rocks of linear complex are characterized by higher values of hyperbasite ferruginousity (up to 15-18), and the cumulates of layered massifs have much greater values.

Judging by the chemical composition of metapyroxenites, they represented themselves olivine orthopyroxenites or orthopyroxenites. The evidences for this are low concentrations of alumina and calcium at high level of silica. The presence of such rocks is also the characteristic feature of mantle hyperbasite ophiolite complexes of NOT-type according to A. Nicolas [14].

According to their chemical composition, gabbroids are divided into two types. The first one is characterized by high concentration of alumina (up to 20% of Al₂O₃) at high concentration of magnesium. In petrochemical diagram, these rocks are put onto fractioning trend, olivine is the main plagioclase. Maybe, they correspond to troctolites. These rocks are characterized by low ferruginousity (F = 11-16). Another group of gabbroids corresponds to gabbro and gabbronorites by its composition. Its feature is moderate concentration of clay soil and higher ferruginousity (F = 15-30). Both groups of gabbro have low titanium concentration. Its increased concentrations are found only in amphibolite (up to 1.4% of TiO₂). We can assume that gabbroids are the fragments of linear complex of ophiolite association, whereas schists and amphibolites correspond to dike complexes or metamorphosed volcanites.

Figure 1. Composition of rocks in ophiolite association in the northern part
of the Kon Tum Block in MgO-Al₂O₃ co-ordinates

Table 1. Composition of chromespinellide and magnetites from apodunite serpentinites
of the Làng Hồi area

	Sample	FeO	NiO	MgO	Al2O3	TiO2	MnO	Cr2O3	Total
1	DLN1670a	25.19	0.15	10.02	25.21	0.10	0.65	37.49	98.81
2	DLN1670a	27.21	0.15	9.66	24.86	0.13	0.3	37.96	100.28
3	DLN1670	31.08	0.071	4.33	4.19	0.72	1.79	56.95	99.11
4	DLN1670	29.24	0.047	4.56	4.72	0.57	1.73	58.22	99.09
5	DLN1671	88.83	0.57	1.69	H.O.	0.09	0.281	5.04	96.49
6	DLN1671	88.92	0.62	1.47	0.02	0.06	0.351	5.03	96.48

Note: 1-2 - Mg - alumochromite; 3-4 - Chromite; 5-6 - Hematite.

IV. GEOCHEMICAL CHARACTERISTICS

Geochemical data of utrabasites and gabbroids of the examined association are given in Fig. 2, 3.

All these rocks are characterized by low concentration of REE. Metapyroxinites are characterized by a positive incline of the curve close to MORB. Weak positive anomaly of Eu, some enrichment in La in relation to Ce are seen, that shows the presence of plagioclase in pyroxenites. In contrast, gabbroids have negative distribution of curve inclines and strongly marked maximums of Eu and La, which cause the fractionation of plagioclase. Apodunite serpntinites are characterized by lower concentrations of REE and sudden minimum of Eu that is the evidence of their restite nature.

Figure 2. Composition of rocks in ophiolite association in the northern part
of the Kon Tum Block in MgO-CaO co-ordinates

In the mutielement diagram, the maximum in Sr is clearly seen both for pyroxenites and gabbroids, which can be connected with plagioclase fractionation. Minuses in Nb and Zr are also observed. Moreover, the enrichment in Rb, Ba and Th is marked. The indicated peculiarities of basites are characteristic for oversubductional ophiolites. The geochemistry of serpentinites attests this fact. They are characterized by sudden minimum in Hf, Nb and enrichment in Rb, Ba, U and Th.

V. FORMATION BELONGINGS

The peculiarities of geological formation of mineral, petrochemical composition and that of accessory chromespinelide allows us to point out an ophiolite association consisting of two complexes: mantle dunite-harzburgite complex and a dunite-wehrlite-pyroxenite-gabbro one (perhaps, with troctolites) in this region. Dunite-harzburgite association is typical for marginal sea ophiolites of different fold belts. Such massifs are widespread in Ural (Kempirsayskiy, Ray-Iz), Altai Saynskaya folded region, in Mongolia [15] and in North Việt Nam (Núi Nưa, Hà Giang).

Figure 3. The REE (rare earth elements) distribution in the rocks of ophiolite association
in the northern part of Kon Tum Block. Regulated for сhondrite С1 [3].

Figure 4. Multielement diagram of the rocks of ophiolite association
in the northern part of Kon Tum Block. Regulated for N-MORB [16]

Dunite-wehrlite-pyroxenite-gabbro massifs with similar features of composition are more widespread in the structure of the ophiolites of Central Asian fold belt. In the Eastern Mongolia, they constitute Late Riphean ophiolites of Shishkhid-Daribskaya zone and are found in Shishkhid massif [15] in Dzabkhanskaya zone (Dzhargalantuiskiy and Ontsaulinskiy massifs [1], in the structure of ophiolite association of the Daribi range [10-12], and also in Khantaishirinskaya association [18, 19]. Middle Mamakanskiy massif of Kelyano-Irokindinskaya zone in the Baikal-Muiskiy ophiolite belt is referred to this type [6]. Karashatskiy massif of Agardagskaya zone in Tuva in Altai-Sayanskyi folded region, Srednetersinskiy and Izykhskiy massifs in Kuznetskiy Altau [5], Duginskiy peridotite-gabbro massif in the Eastern Sayan [13] are also referred to this type. In all mentioned cases of this type the ophiolites are closely associated with island-arc magmatic complexes, which allows us to refer them to the fragments of marginal sea ocean crust.

CONCLUSION

The outcrops of hyperbasites in the northern part of the Kon Tum massif are closely associated with black schists and thin-layered marbles, that is not a characteristic feature of ophiolites in the open ocean. The predomination of apoharzsburite serpentinites in serpentinite melange allows us to refer them to ophiolites of NOT-type (according to A. Nicolas), that is more characteristic of back-arc ophiolites. The peculiarities of geochemical composition both hyperbasites and gabbroids are also the evidence that in this region the fragments of marginal sea ophiolites occur.

This work is supported by the Scientific School grant NS - 4933.2006.5 and by the project 70.87.06 of the Vietnam Basic Research Program.

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