ULTRAMAFIC-MAFIC IGNEOUS FORMATIONS OF NORTH VIỆT NAM (REVIEW ON THE STUDY OF INSTITUTE OF GEOLOGY, SB RAS, AND INSTITUTE OF GEOLOGICAL SCIENCES, NCNST OF SRV, 1983-2003)

G. V. POLYAKOV¹, NGUYỄN TRỌNG YÊM², TRẦN TRỌNG HÒA²,
NGÔ THỊ PHƯỢNG², A. E. IZOKH¹, P. A. BALYKIN¹, A. I. GLOTOV¹,
TRẦN QUỐC HÙNG², HOÀNG HỮU THÀNH², BÙI ẤN NIÊN¹, TRẦN TUẤN ANH¹

¹Institute of Geology, Siberian Branch of the RAS; ²Institute of Geological Sciences, NCNST of SRV

Abstract: The article presents main results of more than 20 years cooperative studies between the Institute of Geology and Geophysics, SB RAS, Novosibirsk, and the Institute of Geological Sciences, NCNST, Việt Nam on ultramafic-mafic formations of Meso-Cenozoic structures of North Việt Nam. As a result, new important achievements, concerning the composition, evolution, and metallogeny of mantle-derived magmatism in the Meso-Cenozoic history of this region are obtained. One of the tremendous results of joint-studies on ultramafic-mafic formations in North Việt Nam is the discovery of Phanerozoic komatiites in Sông Đà zone. Another important of the cooperative research is the investigation ultrapotassic basic rocks, which is compositionally similar to lamproitic series. This special formation is a good indicator to clarify the geodynamic settings in relation to the Cenozoic collision between Indian and Eurasian plates, which led to the displacement along the Ailao Shan Red River Shear Zone.

Well-known fundamental studies of Soviet and Việt Nam geologists in the early 1960s, crowned with the monograph "Geology of North Việt Nam" [1], are the groundwork for research of magmatism of North Việt Nam. This monograph and other concurrent publications emphasized upon igneous formations of granitoid series, the most common on the territory of Việt Nam, because the most important (at that time) kinds of ore deposits are associated with them. This orientation long persisted in the subsequent works of Vietnamese petrologists. Less common ultramafic-mafic magmatism in structures of North Việt Nam received little attention, restricted to few works [1-5]. The most comprehensive information on ultramafic-mafic complexes of North Việt Nam is given in the monograph [1]. In this book, however, they are considered as part of intricate gabbro-granite and ultrabasic-gabbro-granite series rather than as petrologically independent derivates of mantle magmatism. Naturally, thorough petrological investigation into these formations requires analytical means, which were absent at that time.

A similar tendency is traceable in the history of petrological studies on the territory of Central Asia (southern Siberia and Mongolia). Traditionally, ultramafic-mafic magmatism of these regions received little attention. More efforts in this direction of research were made in Russia, including Siberia, at the end of last century. Researchers of the Institute of Geology and Geophysics, Novosibirsk, obtained principally new results on ultramafic-mafic magmatism of Central Asian mobile belt, which appeared to be quite appropriate for paleogeodynamic reconstructions and metallogenic forecasts in this region.

In 1983, petrologists of the Institute of Geology and Geophysics, Novosibirsk, and the Institute of Geological Sciences, Hanoi, initiated similar studies on ultramafic-mafic formations of Mesozoic-Cenozoic structures of North Việt Nam in the southern Indosinian mobile belt, one of largest structural elements of Southeastern Asia. As a result, new important information was obtained concerning the composition, evolution, and metallogeny of mantle magmatism in the Mesozoic-Cenozoic history of this region. Of the most important problems touched on, the following three are noteworthy:

1. First of all, it is interesting to study on geodynamic conditions of occurrence, petrology, and metallogeny of layered ultramafic-mafic massifs of northeastern Việt Nam, whose study is related to general problems of the formation of layered massifs of basic and ultrabasic composition in folded structures of Central Asian and Indosinian mobile belts.

Before the works in Việt Nam, Siberian petrologists carried out goal-oriented investigation into ultramafic-mafic formations of folded areas in southern Siberia and Mongolia and came to a principally new and important conclusion that the heterochronous structures of the Central Asian mobile belt contain typical layered ultramafic-mafic massifs, which were earlier supposed to be confined exclusively to ancient cratons and relatively rigid structures of early stabilization. The Siberian materials demonstrate a wide occurrence of layered massifs in folded areas. In some cases, they play the role of indicators of different geodynamic regimes related to rifting, spreading, subduction and collision. This approach is especially successful when applied to deeply eroded folded areas with poorly preserved complexes of volcanogenic rocks. In the Central Asian mobile belt, peridotite-gabbro layered massifs were used for this purpose, for their distinctly marked heterochronous subduction zones [6]. Under these conditions, indicator massifs are intrusive bodies with well expressed concentrically zonal structure, elements of magmatic layering and differentiation, macro- and micro-rhythmicity. Layered series, marginal and quenching facies are distinguished in them. The relationships of rocks in layered series and their compositions vary in different massifs depending on erosional section and composition of initial melt. The predominant rocks are dunites, wehrlites, lherzolites, troctolites, olivine gabbros, gabbronorites, and anorthosites. A leading mechanism of rock formation in such layered series is shown by model calculation to be crystallization differentiation of initial high-Al and high-Mg basic melt. Additional support comes from the composition of quenching facies, which also correspond to high-Mg basalts. According to specific chemistry (low contents of Ti, K, Sr, Ru, and other incompatible elements), pattern of REE distribution, and other geochemical characteristics, the rocks of these series are generally close to island-arc basalts [6].

The clustered mode of occurrence of layered intrusions inside mobile belts, their intimate conjugation in space and time with island-arc calc-alkaline basalt volcanic series indicate that, these intrusions form linear belts of magmatic generation of heterochronous paleosubduction zones. Thus, within complicate polycyclic fold belts, it is possible to reconstruct subduction zones and island arcs at different stages of their development.

A very important characteristic of ultramafic-mafic massifs is their metallogenic specialization. At present, platinum potential of layered massifs in folded areas is the most vividly debated issue. The related Pt-metal deposits belong to highly productive low-sulphide type accompanying well-known large ultramafic-mafic layered massifs of ancient shields and platforms.

In North Việt Nam, the considered type of layered ultramafic-mafic massifs include intrusions of the Núi Chúa Complex represented by olivinite-lherzolite-gabbronorite (Núi Chúa) and wehrlite-troctolite-gabbro (Khao Quế, Tri Năng, and Núi Yên Chu) massifs. They are of Triassic-Jurassic age. Petrological studies in 1983-2003 have determined in these plutons all main parameters of inner structure, composition, and ore-geochemical specialization of layered ultramafic-mafic massifs of folded areas. Massifs of these complexes are comprehensively characterized in numerous joint publications of Siberian and Vietnamese petrologists [7-17].

All plutons, especially olivinite-lherzolite-gabbronorite massifs, contain syngenetic impregnation and vein-disseminated mineralizations of Cu, Ni, and PGE sulphides. A wide set of PGE minerals have been first recognized in the sulphide ores of the Núi Chúa Massif: sperrylite, paolovite, michenerite, sobolevite, and Pd-bearing (up to 3.2% Pd) breithauptite. In general, the sulphide mineralization in layered peridotite-gabbro massifs of North Việt Nam is categorized as Cu-Ni with increased concentrations of Ag and PGE in ores. The largest massif of the complex, Núi Chúa, has properties of high-productive Pt-bearing plutons: sizes, structure, composition, pattern of distribution of sulphides and Pt-metal mineralization, especially, minerals of PGE etc. [15-20].

2. Another important result of joint petrological studies on ultramafic-mafic formations in North Việt Nam is that Phanerozoic komatiites have been found there. Their detailed description includes comparison with occurrences of this extremely interesting class of igneous rocks in other regions of the world. Study on derivates of mantle magmatism always proceeds from the assumption that komatiite series of high-Mg, low-Ti ultrabasic and basic rocks are pertinent to old, chiefly Archean, greenstone belts, whose formation is conjugated with the appearance of early convective flows in the mantle at that time. Predominant development of komatiites in the Precambrian epoch of the Earth's development is in agreement with other known tendencies in the evolution of mantle magmatism. In particular, statistical estimates of the mass of ultramafic-mafic magmatism in different epochs of the Earth's history show that, during the Archean, high-Mg ultrabasic and basic rocks were more widespread than in the Proterozoic and Phanerozoic. It is also established that from Archean to Phanerozoic, contents of Mg and Fe generally decrease in basaltoid and picritoids, with a stable Fe/(Fe+Mg). These tendencies are explained by two reasons: first, by the maturity degree of the Earth's lithospheric profile and, second, by a change in thermal regime with a total decrease in temperature background of the lithosphere in time. The mass melting of refractory high-Mg basic and ultrabasic magmas in the Archean and, as a result, the predominant occurrence of komatiite series during this epoch is indicative of high temperatures of zones of magmatic generation of the most ancient lithosphere.

The Phanerozoic komatiites recently found in some regions of the world complicate the above tendency and suggest a possibility of this kind of magmatism in later epochs of the Earth's history as well. At the same time, reliable examples of Phanerozoic komatiites are still quite rare. These are the Mesozoic komatiite-basalt association of the island of Gorgona in South Columbia, Late Paleozoic komatiites of the Mang Xim region in the Yunnan province, South China, and the Permian-Triassic complex of the Sông Đà rift, Northwestern Việt Nam, recognized by joint efforts of Siberian and Vietnamese geologists. Less reliable findings of the Phanerozoic high-Mg ultramafic-mafic associations, compositionally close to komatiite-basalt series, are known to be more widespread. They are described in eastern Russia (Taimyr, Chukotka, PenzhaAnadyr district, Kamchatka), on the Siberian Platform (Norilsk district), in Northern Mongolia (Eastern Hovsgol). However, these and above-mentioned more typical examples of the Phanerozoic komatiites need additional grounds and safer identification of the rocks of komatiite-basalt series.

There are still less reliable data on the specific composition of Phanerozoic komatiite-basalt associations as compared with Precambrian komatiite series. Petrochemical comparison of komatiites and basalts of the island of Gorgona with typical Archean komatiite-basalt series shows that the Phanerozoic associations were somewhat richer in Ti and poorer in Mg. At the same time, these comparisons need new examples of Phanerozoic komatiites and new data on their composition, including isotopic-geochemical characteristics. In particular, such material has been obtained for the axial part of the Sông Đà rift in northwestern Việt Nam, where Siberian and Vietnamese geologists have distinguished a Permian-Triassic volcanoplutonic complex of high-Mg ultrabasic and basic rocks, compositionally corresponding to komatiite-basalt series. We have comprehensively characterized this complex [21-30]. The obtained data permitted us to refer this complex to komatiite-basalt associations with olivine-clinopyroxene trend such as Munro Township in North America and Gorgona in Columbia. Their similarity with the Gorgona komatiite basalt complex is confirmed by isotope-geochemical characteristics [29]. The calculated data also show that the initial melts for this complex were close in composition to komatiite basalts [30].

The specific composition of the Phanerozoic komatiite-basalt associations must promote the clarification of the character of mantle magmatism in post-Precambrian time and the establishment, in Phanerozoic structures, of abnormally high-temperature zones or areas of magma generation which are close in thermal regime to Precambrian greenstone belts. This is a very important fundamental problem, which can be solved on the basis of purposeful study of komatiite-basalt associations in the Phanerozoic regions, including the territory of North Việt Nam.

In addition, this problem involves an extremely important implication, related to the possibility of finding large Cu-Ni and Pt-metal deposits, counterparts of the Precambrian greenstone belts, in Phanerozoic areas. Data on Vietnamese komatiites appear to be principally important also in this relation; the peridotites and dunites of the komatiite-basalt complex of the Tạ Khoa area are associated with the Bản Phúc Cu-Ni deposit and some smaller ore occurrences, with diverse Pt-metal mineralization revealed jointly by Siberian and Vietnamese geologists [18-20, 28]. These data have been permitting us to substantiate quite a promising Pt-Cu-Ni metallogenic province in the axial part of the Sông Đà rift, northwestern Việt Nam, related to the Permian-Triassic komatiite basalt complexes established in this region.

3. A very important present-day problem in the study on ultramafic mafic formations of mobile belts is to investigate occasional specific complexes of ultrapotassium basic rocks, compositionally similar to lamproitic series, to clarify the geodynamic conditions of occurrence of these complexes, to carry out their identification and comparative analysis, to get an insight into their petrology, and to estimate their diamondiferous potential. Lamproite occurrences known in separate regions of the world are of two types [31]: first, lamproites of ancient platforms subject to late rift activity (West Australian, Indian, North American, and Antarctic provinces) and, second, lamproites of the late-activity structures of regions of complete folding and boundary parts of mobile belts and platform blocks (West Mediterranean and Aldan provinces). Comparatively more common and better studied are lamproites of platform regions which host large diamond deposits in Western Australia. Low-Ti lamproites of Mediterranean type are less frequent and are poorly explored. In recent time, attention of petrologists is arrested by lamproite-like rocks that combine the features of lamproites of different types. In particular, these are cocites, high-K basic and ultrabasic rocks of North Việt Nam. Cocites were first recognized on the territory of North Việt Nam by Lacroix [32] in the early 1930s. He had described under this name specific leucite-bearing rocks. In the later inventory on North Việt Nam geology, they were included into the Paleogene Pu Sam Cáp volcanoplutonic complex, which also contained pyroclastic formations of trachytic series, as well as intrusive rocks enriched in alkali and potassium: syenites, monzonites, lamprophyres, shonkinites, absarokites, and tinguaites [1]. Recently, on the basis of additional examination of the same specimens from Lacroix's collection, Wagner and Velde [33] and then Mitchell and Bergman [34] have classified cocites as rocks that compositionally are similar to lamproites, but differ from them in some characteristics and, therefore, need additional studies. This extremely scarce information on cocites is actually devoid of data on geologic forms of their occurrence, and description of their petrochemical composition is extremely fragmentary. Joint efforts of Siberian and Vietnamese geologists are a success in new findings of cocites, reconstruction of geologic conditions of their formation, and obtaining of more complete and representative mineralogical-geochemical and isotope characteristics, permitting their comparison with lamproites from other regions of the world.

A specific set of the main rock-forming minerals of cocites, their petrochemical and isotope-geochemical characteristics as well as new geological information of conditions of their occurrence obtained by joint efforts of Siberian and Vietnamese geologists have been reported in some joint publications [35-41]. These data indicate that the cocites of North Việt Nam are referred to low-Ti lamproites of Mediterranean type and to the greatest extent they are close to the lamproites of Spain, North America, and Aldan province in Russia. They also indicate a complicate genesis of cocites, with possible participation of mixed magmas of different compositions but with a dominating role of lamproitic melts proper [40]. One of the still unsolved but topical problems concerning Vietnamese lamproites is the estimation of their diamondiferous potential requiring special goal-oriented studies of similar complexes not only on the territory of North but also of Central Việt Nam (Precambrian Kon Tum inliers and structures of its framing).

Recent investigations of geological institutions of the Siberian Branch of the RAS and NCNST of the SRV have, naturally, been involving with new issues very important for further study of Vietnamese geology and solving some fundamental problems of petrology. Of them, the following problems are of top priority:

1. Study on Early Precambrian ultramafic-mafic complexes, including layered massifs of corresponding age, clarification of geodynamic conditions of their formation, estimation of composition of initial magmas and primary substrata by isotope-geochemical methods.

2. Determination of the volume, age, and metallogenic specialization of ophiolite complexes; study on the character of occurrence of ultramafic-mafic associations in suture zones and collisional belts.

3. Correlation of Permian-Triassic magmatism of Việt Nam, China, and North Eurasia, clarification of causes of high yield of ultramafic-mafic magmatism at this stage relative to different kinds of mineralization, including Pt-metallic.

4. Problems of petrology and metallogeny of within-plate magmatism of Việt Nam, recognition and goal-oriented study of related alkaline-ultrabasic and carbonatite complexes and their minerallogeny.

5. Substantiation of young syn-kinematic ultrabasic-basic magmatism in the Sông Hồng strike-slip faulting zone as part of the Ailao Shan - Red River Zone.

6. Problems of relationship between magmatism and ore formation, recognition and purposeful study on the most productive ore-magmatic systems on the territory of Việt Nam (including Au-Hg and porphyry Cu-Mo mineralization widespread in Central and East Asia), physico-chemical simulation of conditions of their origin and development.

Some of these problems are discussed in papers of this issue, but their majority remain to be solved in the future studies, at a new step of cooperation between scientific staffs of Siberian and Vietnamese geological institutions.

This work was supported by grant 03-05-65088 from Russian Foundation for Basic Research and Project "Intraplate magmatism and related mineralization of Việt Nam" from Ministry of Sciences and Technology of Việt Nam.

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