THE LATE PERMIAN KOMATIITE-BASALT COMPLEX IN THE SÔNG ĐÀ RIFT, NORTHWESTERN VIỆT NAM

P.A. BALYKIN¹, G.V. POLYAKOV¹, E. HANSKI², R.J. WALKER³,
H. HUHMA⁴,TRẦN TRỌNG HOÀ⁵, NGÔ THỊ PHƯ­ỢNG⁵,
HOÀNG HŨU THÀNH⁵, TRẦN QUỐC HÙNG⁵, A.I. GLOTOV¹, T.E. PETROVA¹

1. Institute of Geology, SB of the RAS, 3 Koptyug Av., Novosibirsk, 630090, Russia.
2. Geological Survey of Finland, P.O. Box 77, FIN-96101, Rovaniemi, Finland.
3. Department of Geology, Univ. of Maryland, College Park, Maryland 20742, USA.
4. Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland.
5. Institute of Geological Sciences, NCNST, Hoàng Quốc Việt, Cầu Giấy, Hà Nội, Việt Nam.

 

Abstract: The Late Permian komatiite-basalt complex of the Sông Đà Rift, Northwestern Việt Nam, is represented in the axial part of this structure by NW-extended sheets and conjugate dikes and sills of komatiite-basalt composition as well as by small peridotite bodies. The bottom of the complex is dominated by komatiites alternated with komatiite basalts. Upward, they grade into komatiite and olivine basalts, which are overlain by plagiobasalts. The rocks of the complex are characterized by high - Na, low-alkali, extremely low-Ti and high-Mg trend with variations from high-Al komatiites to low-Al basalts (Polyakov et al., 1991). They are enriched in Mg, Al, Ni, Co, Cu, Cr and depleted in incompatible elements and display LREE depleted chondrite-normalized REE patterns with unfractionated HREE. The Rb-Sr age of the complex is 257 ± 24 Ma and the initial ⁸⁷Sr/⁸⁶Sr ratio - 0.70299 ± 3. The komatiites have high Os concentration up to 7.0 ppb and define an isochron with an age of 270 ± 21 Ma and an initial gOs = +0.02 ± 0.40. Initial gOs values for the komatiites range only from -0.7 to +0.6, indicating derivation from a source with a nearly uniform chondritic ¹⁸⁷Os/¹⁸⁸Os. The Os isotopic systematics of the komatiites show no effects of crustal contamination. In contrast, initial e _Nd values range from +3 to +8, reflecting varying but generally small degrees of contamination with Proterozoic sialic basement material. Associated low-Ti basalts have low initial e _Nd values (-0.8 to -7.5), high initial g _Os values (>15), flat or LREE-enriched REE patterns, and Nb-Ta depletion. These characteristics are also attributed to ariable extents of crustal contamination.

INTRODUCTION

The development of the southeastern margin of the East Asian continent in the Permian-Triassic was linked to intensive destructive processes of the continental crust [19, 26], which were expressed in the formation of NW-oriented rift structures. The coeval ultramafic-mafic complexes are spatially confined to these structures. In the Sông Đà Rift, they are represented by Late Permian picrite-andesite-basalt, diabase-picrite, and rhyolite-andesite-basalt complexes outcropped at the sides of this structure, and a komatiite-basalt complex in its axial part [5, 6, 32-35]. Below new data on the petrochemistry, geochemistry, Re-Os and Sm-Nd isotopic systematics of rocks from the Sông Đà Rift are given and its genesis is discussed.

GEOLOGIC SETTING, COMPOSITION AND AGE OF THE KOMATIITE-BASALT COMPLEX

The Sông Đà Rift komatiite - basalt complex in Northwestern Việt Nam has been regarded as part of the Emeishan large igneous province, which is thought to be generated in response to the upwelling of a mantle plume head beneath the Yangtze craton approximately at the time of the Permian-Triassic boundary [12]. This complex includes NW-trending sheets and conjugated dikes and sills of komatiite-basalt composition and of small peridotite bodies [6, 29, 32-34, 38]. It is most completely expressed in two areas: Nậm Muội and Tạ Khoa. Komatiites alternated with komatiite basalts are predominant in the bottom part of the volcanogenic sequence. Upward the section, they grade into komatiite and olivine basalt, which are overlain by plagiobasalt. The rocks of the complex are characterized chiefly by a porphyritic texture with phenocrysts of olivine and clinopyroxene. The olivine crystals are often acicurlar in form [32]. The olivine basalt and plagiobasalt of the Nậm Muội area contain acicular pyroxene crystals arranged to form a pyroxene spinifex texture.

The dike and sill bodies of the NW wing of the Tạ Khoa structure are made up of rocks of a similar composition. The dike bodies at the mouth of the Nậm Chim River have a zonal structure with variations in the rock composition from komatiite (in the centre) to olivine diabase (in the marginal parts of dikes). The peridotite bodies of the central part of the Tạ Khoa anticlinorium (Bản Phúc, Bản Sang, Bản Hoa, etc) as well as the Bản Mong dike situated 15 km west of the Bản Phúc Massif are made up of serpentinized dunite and wehrlite. Thus, it is most likely that the facies exposed in the central, heavily eroded part of the Tạ Khoa anticlinorium is a subvolcanic vent facies of the Sông Đà komatiite-basalt complex represented by ore-bearing dunite-wehrlite bodies.

The rocks of the complex have a high Na, low alkali, low Ti and high Mg trend with variations from high-Al komatiite to low-Al basalt [34]. They are enriched in Mg, Al, Ni, Co, Cu, Cr and depleted in Ti, Fe, Na, K, P, Cs, Rb, Ba, Sr, Nb, Ta, Nd, Hf, Zr, Y, REE [23, 24, 33] (Table 1). A primitive mantle-normalized trace element diagram reveals weakly fractionated pattern with a typical one- to ten-fold excess in the abundance relative to the primitive mantle. An exception is higher content of Cs, Rb, Th, U, La, and Ce in plagiobasalt and serpentinized peridotite. According to the behaviour of REE, the rocks of the complex are subdivided into four groups: 1) komatiite and komatiite basalt with low LREE/HREE and LREE and HREE content 1-7 and 5-9 times that in chondrites, respectively; 2) olivine basalt with flat REE pattern and 8-10 x chondritic REE abundance; 3) plagiobasalt enriched in LREE relative to chondrites by a factor of 20-40 coupled with a negative Eu-anomaly; 4) serpentinized peridotite from the Bản Phúc Massif and Nậm Chim and Bản Mong dikes showing weakly fractionated REE pattern with LREE and HREE levels 5-25 and 3-9 times chondrites, respectively, and also negative Eu anomalies (Bản Phúc). Comparison of the Sông Đà komatiite-basalt complex with some other komatiite-basalt complexes (Barberton, Munro Township, Belingwe, Gorgona, etc.) in petro- and geochemistry (including isotopic data) shows their similarity especially to rocks from the Gorgona Complex, Columbia [6, 23, 24, 32, 34].

 

Table 1. Variations in content of rare and rare-earth elements in rock groups from the Sông Đà komatiite-basalt complex (ppm)

Note: 1) Komatiite from extrusive sheets of the Nậm Muội and Tạ Khoa areas; 2 - komatiite basalt of the Nậm Muội and Tạ Khoa area; 3 - olivine basalt of the Nậm Muội area; 4 – plagiobasalt of the Nậm Muội area; 5 – serpentinized peridotites of the Nậm Chim and Bản Mong dikes of the Tạ Khoa area; 6 – serpentinized dunites of the Bản Phúc Massif of the Tạ Khoa area. Original data from Polyakov et al. [34] and by Hanski et al [24].

The rock-forming minerals of the rocks in the complex are olivine, plagioclase, and clinopyroxene, while the accessories are represented by Cr-spinel and ilmenite. There are also disseminated sulfides, sulfoarsenides, native copper, PGM and other ore minerals. The trends in mineral composition from dunite, peridotite, and komatiite to olivine basaltoid show a slight increase in total iron of olivine (from forsterite to chrysolite), a decrease in basicity of plagioclase (from bytownite to andesine), and higher content of Fe in clinopyroxene and Ti and Fe in Cr-spinel. In general, the mineralogical signature of the Sông Đà komatiite-basalt complex, making it close to the classical komatiite-basalt complexes, includes: 1) plagioclase with An_48-85 and low content of the orthoclase end-member and a high content of total iron (up to 1.7% å FeO); 2) olivine with Fo_78-93 and high Ni and Ca (NiO = 0.1-0.4 wt%, CaO = 0.02-0.6 wt%); 3) magnesian diopside-augite depleted in Ca, Ti, Na with "moderate" content of Cr and Al (En_36-53Fs_9-17Wo_36-48, TiO₂ = 0.2-1.56 wt%, Cr₂O₃ up to 0.3 wt %); 4) spinel depleted in Ti (TiO₂ = 0.3-0.6 wt%) and enriched in Cr (Cr₂O₃ = 34-51 wt%) and Al (Al₂O₃ = 17-31 wt%) [6, 24, 30, 32, 34, 38].

In the Tạ Khoa area, a Cu-Ni-Pt sulfide deposit is associated with the Bản Phúc dunite-wehrlite massif. Some compositionally similar ore occurrences have also been observed in other bodies (Bản Sang, B#n Mong, etc.) [21, 29, 36]. The Bản Phúc deposit includes two kinds of ores: veined bodies of massive ore at the southern exocontact of the massif and vein-impregnated ores at the bottom of the massif. The massive ores are high in Ni, while the impregnated ores are dominated by Cu. The noble metal content in the ores varies widely so that in places Pt reaches 1.6-3.5 ppm and Pd 0.44-1.49 ppm. The main sulfide minerals are pyrrhotite, pentlandite, and chalcopyrite. Subordinate minerals are violarite, heazlewoodite, galenite, sphalerite, stibnite, Ni arsenides, sulfoarsenides of Co and Ni, tellurides of Bi, Ag and Ni, complex Pb-Cu-Bi sulfides, gold, copper, and some PGM (sperrylite and michenerite). The presence of Cu-Ag-Pb-Bi sulfosalts as well as gold with an admixture of mercury, copper, and other metals including PGE makes these ores complex ones with many useful components. Compositional comparison of these ores with the ores hosted by Archean komatiites in Western Australia and picrite-dolerite massifs of the Norilsk district has revealed intermediate features of their composition and genesis [41].

A Late Permian age of the complex is inferred from a concordantly overlying carbonate-schist unit in the Nậm Muội area, containing Lower Triassic fauna and flora and from the fact that the intrusive bodies of this complex break through Carboniferous-Permian terrigene-sedimentary deposits (in the Nậm Muội area) and Devonian deposits containing Upper Devonian conodonts and foraminifers (in the Tạ Khoa area). The Rb-Sr age obtained using mineral separates from komatiite basalts of the Nậm Muội district is 257 ± 24 Ma [34, 35]. For the same rocks, the whole-rock Re-Os system yields an isochronic age of 270 ± 21 Ma [23, 24]

PARENTAL MELTS OF THE COMPLEX AND PT CONDITIONS OF THEIR GENERATION

According to experimental data by Ohtani E. [31], Al-depleted komatiite can be formed by a low degree of partial melting of mantle at the depths of 500-650 km and the Al-undepleted komatiite by a high degree of partial melting at the depth of less than 450 km. The experimental and theoretical modeling carried out by Ryabchikov and others [9, 20, 37] has shown that the primary melts of the Yilgarn type (Al-undepleted) komatiites could be formed by 50% batch melting of lherzolitic mantle at P = 35-37 kb and T = 1775-1825 ^oC. The Barberton-type komatiite magmas are generated under conditions of still more intensive melting of lherzolite (60-65%) at pressures of up to 50 kb and T = 1875-1975 ^oC. The same authors supposed that the fundamental difference between the picrite-basalt and komatiite-basalt complexes stems from the variations of the fluid regime: "dry" in producing parental melts of the komatiite-basalt complexes and higher H₂O potential in producing melts of the picrite-basalt and picrite-dolerite complexes. Our investigations have shown that this explanation can be extended. The preliminary calculations show that the parental melts for the komatiite-basalt complexes as compared with melts of the picrite-basalt and picrite-dolerite complexes were enriched in Al, Mg, Ca, Ni, Co, Cr, Yb, Lu and depleted in alkali, Ti, Fe, P, Rb, Sr, V, Nb, Ta, Zr, light REE [7]. In addition, a secular evolution trend has been revealed: a decrease in the normative hypersthene content in the parental magmas in the transition from the Precambrian komatiite-basalt complexes to Phanerozoic komatiite-basalt, picrite-basalt, and picrite-dolerite complexes.

Recalculation of chemical analyses of the rocks of the komatiite-basalt complexes to high-pressure mineral parageneses produces an olivine-pyroxene-garnet assemblage. The total contents of apatite, ilmenite, and phlogopite do not exceed 1-2 wt.% in them, whereas in the rocks of the picrite-basalt and picrite-dolerite complexes, their content is not less than 5-7 wt.%. This implies that, compared with the mantle source of the komatiite-basalt complexes, the mantle source of the picrite-basalt and picrite-dolerite complexes is enriched in fusible and volatile components. This is in agreement with the hypothesis of Arndt [3] according to which komatiite melts are formed by advanced melting of depleted mantle after melanobasaltoid magmas enriched in incompatible elements extracted from it. Such a succession of complex formation has been revealed in the Sông Đà Rift [6, 34]. The position of the composition of the petrographic groups on the Fo-Di-Py plot [14] shows that the parental melts of the komatiite-basalt complexes are close to the composition of komatiitic basalt, and of the picrite-basalt and picrite-dolerite complexes, melanocratic basalt [7].

The processes of crystallization of three chosen versions of parental melt composition (komatiite, komatiitic basalt, and olivine basalt) are numerically modelled with the use of the "Comagmat-3.3" program [2]. The purpose is to choose such a composition whose crystallization provides the maximum closeness between the modelled and actually established parageneses and succession of their formation. When calibrated with experimental data, the empirical geothermometers of "Comagmat 3.3" reproduce the liquidus temperature in 70% of the cases within 10 ^oC and the composition of the crystallizing phase with an accuracy of ca. 2% (2.8% for plagioclase). The evaluated composition of parental melts and physicochemical conditions of their generation are given in Table 2.

Thus, the composition of the parental melt of the komatiite-basalt complex of the Sông Đà Rift matches the composition of komatiitic basalt (1,3 - see Table 2) or komatiite [2] with MgO content of 17.0-23.2 wt.%. The high content of Al in clinopyroxene suggests that the crystallization of olivine and clinopyroxene phenocrysts began at pressures no less than 10 kbar [8]. Table 2 also presents estimation of P-T conditions at which the melts are produced, based on the Albarede equations of regression [1]

Table 2. Chemical composition of parental melts of the komatiite-basalt complex of the Sông Đà Rift estimated from extrusive sheets in the Nậm Muội (1), Tạ Khoa (2) area
and Nậm Chim dike (3) (in wt.% recalculated on a volatile-free basis)

Note: P₁ and P₂ are the pressures at which they were crystallized and molten out, respectively. T⁰ C are the temperatures at which melts were produced. 4 – average of 1-3.

Re-Os AND Sm-Nd ISOTOPES

Hanski et al. [23, 24] carried out Re-Os and Sm-Nd isotopic analyses of 14 whole-rock samples ranging in composition from high-Mg komatiite to low-Ti basalt. They obtained the following results:

Table 3. Nm-Nd data on Sông Đà Rift komatiitic and basaltic rocks

Errors in ¹⁴³Nd/¹⁴⁴Nd are 2s in last significant digits.

Osmium abundance in the Vietnamese komatiite is high falling in the range of 1.4 to 7.0 ppb, while one analyzed basaltic komatiite sample has a lower Os content of 1.2 ppb. The low-Ti basalt is significantly more depleted in Os with concentration of 0.12 ppb or less. Abundance of Re is relatively low for the whole Sông Đà suite, ranging from 0.07 to 0.96 ppb. The initial isotopic ¹⁸⁷Os/¹⁸⁸Os ratios of individual komatiite samples calculated at 250 Ma display a restricted range of 0.1243–0.1260 corresponding to gOs values of -0.7 to +0.6. The samples plot on an errorchron with an age of 270 ± 21 Ma and initial g _Os of +0.02 ± 0.40. The age is consistent with the previously reported Rb-Sr age of 257 ± 24 Ma [35] and also similar to the ages determined for the Emeishan province volcanism in China [28, 40]. The calculated initial g _Os value of +0.02 is identical to that of the contemporaneous chondritic reference. The two basaltic samples possess more radiogenic Os isotopic composition, with initial g _Os values between +18 and +58 suggesting significant crustal contamination.

The Nd isotopes show a large variation in the komatiitic samples. The initial e _Nd values at 250 Ma range from +3.2 to +8.0. The samples with the highest e _Nd values of ca. +8 are well within the range of the contemporaneous model depleted MORB mantle (DMM) (DePaolo, 1981) and attest for a long-term LREE depletion in the source mantle. The two analyzed low-Ti basalts have negative initial values of -0.8 and -7.5, which are consistent with their high g _Os values and evidently resulted from interaction of magma with continental crustal material. In summary, the isotopic analyses of the Sông Đà Rift rocks reveal a clear but variable crustal signature in their Nd isotopes, while the effect of crustal contamination is only reflected in the Os isotopes of the low-Mg rocks with the high-Mg komatiites retaining their mantle values, evidently due to the buffering effect of their high Os concentration.

DISCUSSION

Eruption of the Sông Đà suite was related to the formation of the Emeishan continental flood basalt (CFB) province and took place through continental crust. The effects of the underlying sialic basement is reflected in the chemical and isotopic data of the Sông Đà komatiite-basalt suite. Hanski et al. [24] have been calculating that the isotopic evolution of the Sông Đà suite can be explained by interaction of komatiitic magma with Proterozoic crust as represented by the granitic and granodioritic rocks of the Sông Hồng and Po Sen Complexes in North Việt Nam [27], whereas contamination with material from Archean crust would cause more drastic effects on the Nd isotopes. The low-Ti basalt with low e _Nd and high g _Os requires a higher degree of crustal assimilation (5 to 20%) and probably evolved from high-Mg parental magmas through assimilation-crystal fractionation processes.

Table 4. Rhenium and os isotopic and composition data for whole rock komatiites; Rhenium and Os abundance in ppb.

Sampleses	ReRe (ppbpb)	OsOs (ppbpb)	MgOgO	187OsOs/188OsOs	187ReRe/188OsOs	Timeme (GaGa)	Initialal 7/8	g OsOs
GG1436 Basaltlt	0.1188	0.0338	8.76	0.22075	17.156	0.25	0.14914	19.0
Duplicatete	0.0648	0.0330	8.76	0.23833	9.588	0.25	0.19831	58.2
PP73/89 Basaltlt	0.5149	0.1166	8.93	0.26805	21.659	0.25	0.17765	41.8
12/86 Komatiiticic basaltlt	0.8586	1.238	10.78	0.14043	3.348	0.25	0.12645	0.9
8/86 Komatiitete	0.3168	2.511	22.13	0.12764	0.6078	0.25	0.12511	-0.2
11/86 Komatiitete	0.1800	6.995	29.76	0.12549	0.1239	0.25	0.12498	-0.3
9/86 Komatiitete	0.1476	1.874	24.85	0.12769	0.3795	0.25	0.12610	0.6
BB6865 Komatiitete	0.5111	2.082	23.50	0.12995	1.1833	0.25	0.12501	-0.3
BB6887 Komatiitete	0.0421	4.075	30.94	0.12546	0.0498	0.25	0.12525	-0.1
BB6892 Komatiitete	0.4125	3.915	22.37	0.12752	0.5077	0.25	0.12540	0.1
GG1456 Komatiitete	0.3456	1.625	20.48	0.13005	1.0249	0.25	0.12577	0.4
46/89 Komatiitete	0.3378	1.366	21.64	0.12962	1.1920	0.25	0.12464	-0.5
GG1448 Komatiitete	0.0674	2.626	20.53	0.12550	0.1236	0.25	0.12498	-0.3
BB6889 Komatiitete	0.6265	2.8413	20.0	0.12980	1.0627	0.25	0.12501	-0.1
BB6891 Komatiitete	0.2102	2.5286	21.1	0.12668	0.4004	0.25	0.12487	-0.3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Uncertainties are ± 0.4% for ¹⁸⁷Re/¹⁸⁸Os and ± 0.2% for ¹⁸⁷Os/¹⁸⁸Os (2), except where shown
in parentheses. g _Os values are calculated for 270 Ma, assuming a chondritic ¹⁸⁷Os/¹⁸⁸Os ratio
at that time of 0.12509.

Despite the strong evidence of crustal contamination in the Sông Đà komatiite-basalt suite, one can conclude that the initial Os isotopic composition obtained from the Re-Os isochron is representative of the mantle source and the highest initial e _Nd value is also equal or close to the initial value in the mantle source region. The Sông Đà komatiites thus have tapped a mantle source that was characterized by a long-term depletion in incompatible elements including LREE, but had an essentially chondritic Re/Os ratio. Isotopically, the least-contaminated Sông Đà komatiites are very similar to some Archean komatiites from Canada and Australia [17, 18], Paleoproterozoic komatiites from northern Finland [22, Gangopadhyay et al., in prep.], and most of the Cretaceous Gorgona Island komatiites [39], all of which show positive initial e _Nd values and initial g _Os values close to zero. Thus, for most of Earth history, some primitive magmas have tapped similar depleted mantle sources.

In terms of the Nd and Os isotopes, the dominant source of the above mentioned komatiite suites seems quite similar to the depleted MORB mantle (DMM). However, this does not necessarily mean that the rocks were derived from an N-MORB source. We are faced here to the same problem as encountered by those who study depleted picrites and basalts in the Tertiary North Atlantic Igneous Province, for example. Based on geochemical and isotopic data, many authors have recently argued that the Icelandic basalts and picrites with a depleted incompatible trace element signature are derived from a source that is distinct from the MORB asthenosphere [e.g, 16]. However, so far consensus has rarely been reached as to whether plume-related oceanic magmas with depleted isotopic and geochemical signatures reflect intrinsic properties of a deep mantle plume source, represent entrained depleted upper mantle around the plume, or reflect effects of interaction with a spreading center. The situation with the supposedly plume-related continental flood basalts (CFB) is even more blurred, as the mantle source characteristics of magmas derived from sublithospheric sources may be masked by crustal assimilation or interaction with subcontinental lithospheric mantle (SCLM).

The most magnesian olivine (Fo₉₃) of the Sông Đà komatiites indicate a very MgO-rich (> 20 wt%) parental magma [24]. Hanski et al. [24] estimate that the parental magma of the most depleted komatiites had CeN/YbN of ca. 0.3. This means a very refractory mantle source, which most probably was also depleted in volatile constituents including H₂O. Evidence for this interpretation can be obtained from modern undegassed oceanic (intraplate) magmas, in which H₂O usually has an incompatibility similar to that of La or Ce (e.g., Danyushevsky et al, 2000). There is neither petrographical evidence for a notable H₂O content in the magmas that formed the subvolcanic bodies at the Sông Đà Rift. Dehydration melting of the DMM thus seems an unlikely mechanism in our case. Assuming that the Sông Đà komatiites are essentially dry, they represent high-T liquids whose generation requires mantle potential temperatures that exceed with more than 200 ^oC those calculated for primitive MORBs [25]. Such high temperatures are unlikely to be attained in SCLM by conductive heating [4]. This together with a chondritic Os isotopic composition of the Sông Đà komatiites allow us to exclude a SCLM origin for them despite their eruption in a continental setting. The generation of the Việt Namese komatiites is best explicable by adiabatic rising and melting of a deep, hot mantle plume with a large contribution from a refractory mantle plume component.

The high-Mg rocks and associated basalts in the Sông Đà Rift are exceptional and significant in many respects. The komatiites represent highly depleted magmas related to a large continental igneous province which contrasts with the evolved and enriched nature that is typical of most of the continental flood basalts. In fact, basalts with radiogenic isotopic signatures and incompatible trace element depletions equalling to those of contemporaneous N-MORB are exceptional in CFB provinces [e.g., 11]. The Sông Đà rocks provide an unique opportunity to evaluate the primary Os isotope composition of plume-related melts in a CFB setting. In addition, they offer additional evidence that 1) high-degree of partial melting could produce komatiitic liquids in a relative recent times, 2) depleted, N-MORB-like sources have been tapped by high-T, plume-related magmas from deep mantle through most of the geological time, and 3) a variety of parental magmas can be identified within a CFB province, similar to those found in oceanic plateaux. Finally, the association of high-Mg komatiites with low-Ti basalts provides a good opportunity to study the potential derivation of continental flood basalts from more primitive parental magmas.

CONCLUSIONS

The Sông Đà Rift komatiite–basalt complex in Northwestern Việt Nam are regarded as part of the Emeishan large igneous province, whose generation is thought to be related to the upwelling of a mantle plume head beneath the Yangtze craton ca. at the time of the Permian-Triassic boundary [12]. In the axial part of the Sông Đà Rift, this complex is represented by NW-trending sheets and conjugate dikes and sills of komatiite-basalt composition and by small peridotite bodies. The Tạ Khoa area comprises the Bản Phúc Pt-Cu-Ni deposit associated with a peridotite body. The lower part of the complex is dominated by komatiite alternated with komatiitic basalt. Upward, they are replaced by komatiite and olivine basalt which are overlain by plagiobasalt. The rocks of the complex are characterized by a low-alkali, low-Ti and high-Mg trend with variations from high-Al komatiite to low-Al basalt. In addition, they typically have high content of Mg, Al, Ni, Co, Cu, Cr and low content of Ti, Fe, Na, K, P, Cs, Rb, Ba, Sr, Nb, Ta, Nd, Hf, Zr, Y. The komatiite displays LREE-depleted chondrite-normalized REE patterns (CeN/YbN = 0.30-0.62) and have unfractionated HREE. The "Comagmat"-3.3-based numerical modelling suggests that the composition of the parental melts of this complex is similar to komatiitic basalt. The Rb-Sr age of the complex is 257 ± 24 Ma, while the age obtained from the Re-Os system is 270 ± 21 Ma.

The Sông Đà komatiite–basalt complex shares many features with the Cretaceous Gorgona Island komatiite–basalt complex, including the relatively high Al₂O₃ content, low concentration of incompatible elements, and low LREE/HREE ratio. The Nd isotopic and trace element systematics of the high-Mg rocks record variable but generally minor contamination with material from the Proterozoic sialic basement, while Os isotopes remained largely immune to such effects. Nevertheless, the original isotopic and chemical characteristics of the parental magma of the Sông Đà komatiitic rocks can be demonstrated to involve a strong depletion in incompatible elements (CeN/SmN < 0.3) and close to chondritic g _Os values and e _Nd values of > +8. This means that the mantle source had a long-term depletion in LREE and other incompatible elements, but essentially a chondritic Re/Os ratio. The Sông Đà komatiites thus belong to the rare examples of CFB province-related magmas retaining clear evidence for a strongly depleted chemical and isotopic component in their mantle source region. The komatiitic parental magma in the Sông Đà Rift is derived from the essentially anhydrous portion of a mantle plume head. The high MgO content of the parental magma required a deep source and upwelling of hot mantle with a potential temperature exceeding more than 200^oC the potential temperature inferred for the production of primitive MORB magma. Such high temperatures were likely achieved in a hot core zone of the starting mantle plume [10]. The Sông Đà komatiite is associated with low-Ti basaltic rocks (olivine basalt), which suffered from a higher degree of crustal contamination and were likely derived from higher-Mg magmas via ACF processes.

This work is supported by grants 02-05-65087 and 03-05-65088 from the Russian Foundation for Basic Research (to PAB and GVP) and by NSF grant 9909197 (to RJW), and Project "Intraplate magmatism and related mineralization of Việt Nam" (to TTH).

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