subdivision and correlation of permian-triassic basaltoid associations in the s«ng ®µ structure (nw ViÖt Nam)

subdivision and correlation of permian-triassic basaltoid associations in the s«ng ®µ structure (nw ViÖt Nam)

trÇn träng hßa

Institute of Geology, NCNS&T, Hoµng Quèc ViÖt Rd, CÇu GiÊy, Hµ Néi

Abstract: Based on the content of TiO₂, (Na₂O+ K₂O) and MgO, the Permian-Triassic basaltoids in the S«ng §µ Structure have been subdivided into two series: high-titanium (and high-alkaline), and high-magnesian (low-titanium and low-alkaline relatively). The high-magnesian basalts in geochemical and mineralogical characteristics are closed to Phanerozoic komatiites. The high-titanium basalts may be divided into following associations: picrite-andesite-basalts, andesite-basalts and trachybasalt-trachyandesite-trachydacite. On the basis of the compositional comparison of above-mentioned basaltoid associations, the two alternative versions for grouping them are suggested.

INTRODUCTION

The S«ng §µ Structure (SDS) with its NW-SE trending fault systems plays a very important role in the Permian-Triassic geodynamic history and magmatic evolution in NW ViÖt Nam. The Permian-Triassic mafic-ultramafic magmatic formations distributed in its confines are reliable evidences for reconstructing the forming and developing history of the SDS, related to the destruction process of the southern margin of the East Asian Continent with the formation of many rift structures, among them the S«ng §µ Rift.

The products of Permian-Triassic magmatic activities in the SDS comprise volcanic and volcano-plutonic associations which are, by their nature, close to the formations of intracontinental rift [1, 2, 6-8]. These are basalt associations of the tholeiitic, subalkaline tholeiitic series which are of various composition, causing the difference in their subdivision. The main factor of this may be the detailed level of materials of each researcher strongly differs from one another.

In this paper, on the basis of synthesis and review of study results on Permian-Triassic basalts obtained during the 10 last years, the author would like to present his opinions on the problem of reasonable subdivision of Permian-Triassic basalts in the SDS.

I. Brief outlines of the subdivision of P-T basalts in the s«ng ®µ structure

The SDS in the recent understanding of many geoscientists includes some structural zones divided by Dovzhikov A. E. et al. (1965), namely: S«ng §µ, Ninh B×nh, S¬n La zones and a part of the Thanh Hãa Zone. In these above confines there are the following bands of basalts: CÈm Thñy (Thanh Hãa), Kim B«i - Viªn Nam (Hoà B×nh), Ba V× (Hà T©y), V¹n Yªn - B¾c Yªn, §Ìo ChÑn - Noong Xang, NËm Muéi - ThuËn Ch©u, S¬n La Pass (S¬n La), S×n Hå - Pa TÇn, and ThÌn Sin - Phong Thæ (Lai Ch©u) (Fig. 1).

Fig. 1. Distribution scheme of P-T volcanic and plutono-volcanic in the SDS and adjacent areas in Northwest ViÖt Nam. Compiled on the basis of materials of [6] with the use of materials from Gatinsky Yu. & §µo §×nh Thôc (1982), from Geological Maps of NW ViÖt Nam at 1:500,000 and 1:200,000 scales (Editors: Phan Cù TiÕn, 1977; §inh Minh Méng, 1978).

Note: 1) Quaternary sediments in the Hµ Néi Depression; 2) Cretaceous sediments; 3) Jurassic-Cretaceous volcanogeno-sedimentary formations in the Tó LÖ Depression; 4) Permian-Triassic terrigenous and carbonate sediments; 5-9) Permian-Triassic volcanic and plutono-volcanic formations: 5) High-magnesian volcanic association of basalt-komatiite composition in the NËm Muéi and T¹ Khoa areas; 6) Ultramafic rocks in the T¹ Khoa area; 7) Andesite-basalt volcanic associations; 8) Intrusive and subvolcanic formations of diabase-picrite and picritoid composition; 9) Trachybasalt-trachyandesite-trachydacite volcanic associations; 10) Paleozoic sedimentary and metamorphic formations; 11) Proterozoic formations; 12) Regional faults.

Distribution areas of Permian-Triassic basaltoids: I) S¬n La; II) T¹ Khoa; III) NËm So; IV) NËm Muéi; V) V¹n Yªn; VI) Ba V× - Kim B«i - Viªn Nam; VII) CÈm Thñy.

Structural zones: A) S«ng §µ; B) S«ng M·; C) Phan Si Pan; D) Tó LÖ; E) SÇm Na.

Since the end of the 70’ of the past century there have been 3 main following viewpoints on the subdivision of Permian-Triassic basaltoids in the SDS:

1) Unifying all basalts distributed in the above said areas in an unit named as CÈm Thñy Formation of Late Permian age [9], or as Viªn Nam Formation of Late Permian – Early Triassic age [1].

2) Subdividing them into 2 units: a) CÈm Thñy Formation of Late Permian age, including the basalts in CÈm Thñy and S¬n La Pass areas; b) Viªn Nam Formation of Late Permian – Early Triassic age, including the basaltoids in Kim B«i - Viªn Nam, Ba V×, V¹n Yªn - B¾c Yªn, §Ìo ChÑn - Noong Xang, S×n Hå - Pa TÇn and ThÌn Sin - Phong Thæ areas, i.e. almost all the basalt bands distributed in the former S«ng §µ, S¬n La and Ninh B×nh Zones [2, 4].

According to §ç §×nh To¸t [2] basaltoids of the CÈm Thñy Formation belong to the tholeiitic type, as for those of the Viªn Nam Formation – to the bimodal basalt-liparite formation, while §µo §×nh Thôc assigned all them to the alkaline olivine basalt and picritoid basalt formation.

As it will be exposed in the following part, the Viªn Nam Formation distributed in the NËm Muéi, §Ìo ChÑn and Noong Xang - T¹ Héc areas is composed of basaltoids basically different one from another, therefore its subdivision is modified as follows:

3) Establishing from Permian-Triassic basaltoids of the SDS 3 volcanic and plutono-volcanic associations [6, 7, 8] corresponding to 3 following formations:

a- Association of basalt-andesite (CÈm Thñy, S¬n La Pass areas), basalt-andesite-picrite (NËm So - Phong Thæ area) and basalt-rhyolite (Kim B«i, Ba V× - Viªn Nam areas) with the high-titanium, relatively high-alkaline (sodic) characteristics;

b- High-magnesian association of basalt-komatiite (NËm Muéi, §Ìo ChÑn - Noong Xang) with the titanium-low and alkali-low characteristics;

c- Bimodal subalkaline association of basalt (trachybasalt) - trachyandesite-trachydacite (NËm Muéi, V¹n Yªn - B¾c Yªn areas) with the more high-titanium and high-alkaline characteristics than the type a.

There still are two ways of subdivision of the "Viªn Nam" basaltoids seen in the Geological maps of the ThuËn Ch©u Sheet series (NguyÔn §×nh Hîp et al., 1995, in Archives) and of the Phong Thæ Sheet series at 1:50,000 scale (T« V¨n Thô et al., 1996, in Archives). According to these works the "Viªn Nam Formation" in the ThuËn Ch©u Sheet series (NËm Muéi area) was separated into 2 formations: NËm Muéi one (P₂-T₁ nm) including basalt-komatiite, and Viªn Nam Formation (T₁ vn) including contrast effusives; at that time in the Phong Thæ Sheet series basalts in the S×n Hå area including mainly high-magnesian basalt were assigned together with high-titanium basalt in the Phong Thæ area to the Viªn Nam Formation.

Reviewing the basis of the above classification, we can see that geologists dividing the basaltoids of the SDS into two CÈm Thñy and Viªn Nam formations have been basing on two geological evidences: the close spatial distribution of the CÈm Thñy basalts with terrigeno-carbonate beds of the Yªn DuyÖt Formation with the conformable relation of the Yªn Duyªt Formation upon basalts; while the Viªn Nam basalts have generally the conformable relation beneath terrigenous sediments of the T©n L¹c Formation (T₁o tl) (found in the Viªn Nam - §åi Bï area). At that time petrologists tend to divide the products of Permian-Triassic volcanic activities in the SDS on the basis of their material composition with the above said tripartite subdivision.

The harmonious combination of geological observations and the material composition is the highest requirement of the classification of geological units, but in the case of Permian-Triassic basaltoids of the SDS their material composition should be considered as the first standard of attention for classifying them into units (complex or formation), because they are the products of magmatic activities in a stage, although short (from the end of Late Permian to the beginning of Early Triassic) but forming magmatic bodies of various and complex composition.

ii. geological characteristics and material composition

The basic structural unit of volcanic complexes consists just of their componential rock associations, in which the dominant varieties determine the petrographic name of these complexes (associations). Basically, the main component of the Permian-Triassic mafic-ultramafic volcanic complexes of the SDS consists of basalts with various level of differentiation, therefore in this paper the author considers them as basic object of classification.

Based on the structure of the sections of the volcanite bands, the Permian-Triassic basaltoids of the SDS can be subdivided into, at least, 4 following association types of basalts and their differentiations:

1) The association including only compact basalt (aphanitic basalt) accompanied by some andesitobasalt or andesite. This association characterizes the effusive bands in the CÈm Thñy and S¬n La Pass areas. Basaltoids of this type are fairly simple in composition . They belong to the high-titanium (TiO₂ = 3.4%), relatively high-alkaline kind; of the sodic alkali type (Na₂O = 2.98-3.29%; K₂O = 1.03-1.8%) (Table 1). They are fairly rich in light rare earths, but poor in heavy rare earths; the ratio La/Yb and Ce/Yb is high (10-15, 28-35) [6, 8].

2) The association of more complicatedly differentiated: picrite -andesite- basalt, in which andesite and picrite are less in quantity but frequently met, besides melanocratic basalt varieties, such as picrobasalt or picrodolerite, are very widespread (with SiO₂ = 46.57%; MgO = 12-15%). In the SDS this association type occurs only in the NËm So - Phong Thæ volcanite band. The characteristics of picrobasalt and basalt, as well as andesitobasalt of this association type consist of high-titanium (TiO₂ = 2.3-3.3%), relatively high-alkali (Na₂O + K₂O = 3.1-4.5%, in which K₂O = 0.6-1.2% and P₂O₅ = 0.2-0.4%). It is to note that in melanocratic rocks (picrite with SiO₂ = 44.5%; MgO = 21%) the TiO₂ content is also high (2.09%) and P₂O₅ (0.26%) [8]. Picrobasalt and basalt in the NËm So - Phong Thæ area distinguish by their abundance in K, Rb, Sr, Nb, Zr, Y and rare earth elements. Their distribution characteristics are similar to those of basaltoids of the subalkaline tholeiitic series of continental rift or oceanic island types (Fig.2-5). The initial isotopic ratio ⁸⁷Sr/⁸⁶Sr in picrite: 0.7047 (unpublished data) shows that the source magmatic fluid is rather rich in Rb and Sr.

3) The association of high-magnesian, alkali-low and titanium-low basaltoids (komatiite basalt type) has complicatedly differentiated products from ultramafic (komatiite-peridotite with MgO = 30-35%), sub-ultramafic (komatiitic basalt with MgO = 18.5-20%) to mafic rocks (olivine basalt with MgO = 7.4-8.8%), even to leucobasalt (with MgO = 6.5%). In the section structure of this association the quantitative correlation between high-magnesian and medium-magnesian rocks is nearly equivalent. They are both titanium-low (TiO₂ = 0.6-1.6%), alkali-low (Na₂O + K₂O = 0.9-3.0%, especially low in K₂O = 0.03-0.8%). These rocks are fairly rich in Ni, Cr, poor in Rb, Sr, Nb, Zr and in LREE La, Ce, Nd and Sm (Table 1). The distribution characteristics of REE as well as of other incompatible elements are very complicated, both near to basalts of the alkali-calc series, and to basalts of the mid-oceanic range (Fig.2, 3, 6, 7). The initial isotopic ratio ⁸⁷Sr/⁸⁶Sr = 0.7035 of komatiite basalt shows that they were formed from an depleted mantle source. The distribution area of this basaltoid association is fairly large, occupying the central part of the SDS, including the NËm Muéi (ThuËn Ch©u), §Ìo ChÑn - Noong Xang (SW of T¹ Khoa Anticlinal) areas. According to the description of T« V¨n Thô and Bïi Minh T©m high-magnesian basalt of the described type is also widespread in the S×n Hå basalt band.

Table 1. Representative chemical composition (% of weight) and content of ore and rare elements (ppm) of Permian-Triassic basaltoids of the SDS

Samples	H 639	H 615	H 617	T 1500	B 6816	B 6817	G1448	B 67/89	T 1610	T 1648	P 181/89	16/86
No	1	2	3	4	5	6	7	8	9	10	11	12
SiO₂	46.27	46.07	50.09	54.29	46.31	48.47	43.7	50.74	42.28	48.47	47.85	50.33
TiO₂	2.31	2.35	2.69	3.26	3.38	3.41	0.58	0.65	0.7	1.58	2.64	4.04
Al₂O₃	9.04	14.02	11.36	10.33	14.09	13.6	10	12.81	12.99	14.64	15.03	13.18
Fe₂O₃	13.23	13.87	12.34	14.41	14.12	13.37	11.5	9.22	12.6	13.93	11.4	13.85
MnO	0.22	0.24	0.18	0.18	0.23	0.25	0.15	0.16	0.22	0.23	0.16	0.18
MgO	12.16	7.32	7.72	5.77	5.06	4.61	20.53	8.83	17.92	7.4	6.04	3.49
CaO	12.78	8.24	8.85	6.79	10.55	8.8	8.2	12.1	3.26	10.31	8.96	6.74
Na₂O	2.49	3.29	3.06	0.9	2.98	3.29	0.82	1.65	0.84	2.39	3.25	2.77
K₂O	0.63	1.25	0.82	0.57	1.03	1.81	0.08	0.97	0.03	0.79	1.07	3.24
P₂O₅	0.25	0.2	0.3	0.39	0.6	0.72	0.05	0.27	0.05	0.18	0.37	0.58
Ni	297	95	141	91	-	-	110	350	546	320	79	60
Co	67	59	59	41	-	-	71	40	73	54	30	37
Cu	87	163	79	118	-	-	900	18	13	30		51
Cr	848	125	438	263	-	-	2300	320	190	240	390	420
V	227	285	300	249	-	-	250	320	190	240	390	420
Rb	62	46	18	61	-	-	2.3	37	0.8	25.8	20	234
Sr	689	287	460	210	-	-	31	100	11.7	186	166	226
Nb	59	39	72	67	-	-	0.6	2.7	2.1	11.9	80	164
Zr	209	118	364	260	-	-	18	63	33.1	128.4	324	622
Y	22	26	34	45	-	-	10.3	22.2	21.6	33.3	37.5	73.6
La	33	20.4	31.3	43	27.4	37.8	1	8.5	1.3	13.6	24.8	59.3
Ce	68	43	64	86	58.9	78.5	3.2	18	3.7	29	52.3	112.2
Nd	36	24	34	47	35.2	45.2	2.9	10.6	3.5	17	30.6	58.3
Sm	74	5.6	7.8	10.5	9.6	12	1.1	2.9	1.4	4.5	8.2	14.3
Eu	2.2	1.8	2.3	3.5	2.9	3.4	0.4	0.6	0.4	1.4	2.5	3.9
Gd	5.4	4.9	6.6	8.4	8.7	11.6	1.5	3	2	5	8.9	11.6
Tb	0.8	0.8	1	1.3	1.3	1.7	0.3	0.5	0.4	0.8	1.4	1.7
Yb	1.7	1.8	2.2	2.6	2.3	2.3	1.4	2	1.8	3	2.9	3.4
Lu	0.2	0.2	0.3	0.3	0.3	0.4	0.2	0.3	0.3	0.4	0.4	0.4

Note: Picrite-andesite-Basalt association - NËm So area: 1) picrodolerite; 2) basalt; 3) andesitobasalt; Phong Thæ area: 4) andesitobasalt; S¬n La area: 5-6) basalt; Komatiite-basalt association - NËm Muéi area: 7) komatiitic basalt; 8) olivine basalt; §Ìo ChÑn area: 9) komatiitic basalt; 10) olivine basalt; Basalt-trachybasalt-trachyandesite association - NËm Muéi area: 11) basalt; 12) trachybasalt.

4) The association of basalt (trachybasalt) –trachyandesite-trachydacite is mainly distributed in the V¹n Yªn - B¾c Yªn, Kim B«i - Viªn Nam, Ba V× and NËm Muéi areas. At the ChiÒng Ngµm hydroelectric dam area (NËm Muéi) basaltoids and salic-subalkaline rocks of this association cover directly komatiitic basalt [3, 5]. The high-magnesian and titanium-low melanocratic basalt (SiO₂ = 45-50%; MgO = 10.4-17.8%; TiO₂ = 0.6-0.9%) (unpublished data) corresponding to komatiitic basalt and olivine basalt occurs also in the Hoµ B×nh hydroelectric dam area, but its relation with high-titanium and high-alkali basalt is not clearly determined yet. In the section of the V¹n Yªn - B¾c Yªn [7, 8] or Viªn Nam areas the occurrence of subalkaline intermediate-felsic rocks: trachyandesite, trachydacite has been recognized. In NËm Muéi they form effusive bands, while in the V¹n Yªn (Suèi Ch¸t Stream) volcanite field and Viªn Nam area trachyte and trachydacite have been usually met in the form of dykes or agglomerate bodies of the neck facies (7).

The characteristic feature in the composition of this association consists of high-titanium character (TiO₂ = 2.6-4.0%), very high-alkali (Na₂O + K₂O = 4.3-6.0%; K₂O = 1.1-3.2%), rich in Rb, Sr, Nb, Zr as well as REE. Their content and distribution characteristics are completely close to those of subalkaline basalts of intraplate magmatic activities (Fig.2-5). In comparison with above described high-titanium association, these basalts are richer in titanium, alkali and incompatible elements.

III. DISCUSSION AND CONCLUSIONS

From the above described Permian-Triassic basaltoid associations of the SDS, we can see that, based on main characteristics of their material composition they can be divided into 2 series: the high-titanium, high-alkali series corresponding to the subalkaline basalt, and the high-magnesian, titanium-low, alkali-low series corresponding to the tholeiite.

On the spatial distribution side high-titanium basaltoids usually occupy the marginal part of the SDS, as for the titanium-low basaltoids – in its central part [6, 7]. However, based on the observed geological relations – the stratigraphic relation with terrigenous sediments of the Yªn DuyÖt (P₂ yd) and T©n L¹c (T₁o tl) formations – these basaltoids can be subdivided into 2 association groups: the group formed before Yªn DuyÖt, and that formed after Yªn DuyÖt - before T©n L¹c. And this is the basis of the establishment of the 2 formations: CÈm Thñy (P₂) and Viªn Nam (P₂-T₁) [4]. Maybe, there is not any question for dicussion if in the Viªn Nam Formation there do not exist associations opposite in composition: alkali-low (potassic) tholeiite and subalkaline basalt, showing that their magmatic melts are different, at least there was a disparity in time for melting them. It is clear that these associations must be assigned to different formations. The relation between them [5] allows to determine that the komatiite basalt was formed earlier than the alkaline basalts of the bimodal series. And this is the basis for establishing 3 volcanic and plutono-volcanic association types: the CÈm Thñy (P₂), NËm Muéi - §Ìo ChÑn (P₂ - T₁), and Viªn Nam (T₁) types [6, 7] as above described. Basically, this subdivision scheme differs the bipartite scheme (CÈm Thñy and Viªn Nam) by separating the high-magnesian, titanium-low and alkali-low volcanites (komatiite basalt) of the Viªn Nam Formation from the association of subalkaline basaltoids of the contrast series. Up to present time, with the existing materials, this is the most satisfactory way if the geological mapping at 1:50,000 scale requires a detailed subdivision.

According to this subdivision, the association of the CÈm Thñy type includes basaltoids of the CÈm Thñy - S¬n La Band; of the NËm Muéi type - NËm Muéi, §Ìo ChÑn - Noong Xang and S×n Hå bands; of the Viªn Nam type - NËm So, B¾c Yªn - V¹n Yªn, Hoµ B×nh - Kim B«i and Ba V× - Viªn Nam bands. The assignment of the basalt-andesite-picrite association of the NËm So - Phong Thæ area - the boundary area between the S«ng §µ and the Phan Si Pan zones - to the Viªn Nam type is a reluctance, because examining various sides of their composition (as above stated) one can see


Fig. 2. Nb-Zr correlation in P-T basaltoids of the SDS. Symbols: 1) picrodolerite, basalt and andesitobasalt; 2) komatiitic - basalt and olivine basalt; 3) basalt, trachybasalt. I- Island arc and active continental margin; II- Basalt of oceanic island and intracontinental rift.		Fig. 3. Composition position of P-T basaltoids of the SDS on the Zr/Y-Zr diagram. Symbols as in the Fig. 2.

Fig. 4. Distribution characteristics of REE in high-titanium basaltoids. Symbols as in the Fig.2.	Fig. 5. PM- normalized spider diagram for basalts of HT-series. Symbols as in the Fig.2.

Fig. 6. Distribution characteristics of REE of komatiitic basalt and olivine basalt in the SDS.	Fig. 7. PM- normalized spider diagram for komatiitic basalt and olivine basalt in the SDS.

that between them there are many differences: if basaltoids of the NËm So - Phong Thæ area are composed mainly of mafic rocks with the inconsiderable role of intermediate and felsic rocks (andesite and dacite), those of the Viªn Nam association in the V¹n Yªn - B¾c Yªn, NËm Muéi (upper part of the NËm Muéi Section) and Viªn Nam - Kim B«i areas are frequent in intermediate and subalkaline felsic rocks. In addition, basalts of the NËm So - Phong Thæ area have lower alkalinity and titanium content than those of the V¹n Yªn, NËm Muéi, Viªn Nam ... areas. Besides, picrite of the basaltoid association in NËm So area is also high-titanium and high-alkali, while the melanocratic rocks (olivine basalt, picrobasalt?) in the Viªn Nam and Hoµ B×nh areas belong to the titanium-low and alkali-low kinds.

These differences, of course, can be explained by the local change in the magmatic chamber in an interval of hundreds kilometres. It is necessary to carry out the isotopic study for affirming the close relation between the NËm So and the Viªn Nam basaltoids.

It is not excluded that the NËm So - Phong Thæ basaltoids belong to another stage of magmatic activity (!). The author of this paper has been analyzing the isotopic age of dolerite basalt and picrite in NËm So by the Rb-Sr and Ar-Ar methods, but the results are not satisfactory by the very great error (278 ± 100 Ma.) of the analysis, while the analysis by Ar-Ar (whole rock sample - 136 ± ... and 124 ± ... Ma.), maybe, reflects the Cretaceous superimposed deformation phase in NW ViÖt Nam (unpublished data).

A simpler way of the subdivision of P-T basaltoid associations of the SDS is their unification into an unique complex of P₂ - T₁ age, which includes different associations according to their main characteristics (alkalinity, titanium content, magnesium content). The differences in rock paragenesis and some petrochemical characteristics related to the abundance level of content of titanium, alkali and other incompatible elements between the high-titanium and high-alkali basaltoid associations may be caused by local changes in different parts of the magmatic chamber. Regretably, up to now the isotopic study on the basaltoid associations of the SDS is still very restricted. The solitary data on the primary isotopic ratios of the NËm Muéi komatiite basalt (0.7035) and NËm So picrite (0.7047) are not enough for concluding that they were formed from different mantle source, but can reflect only their different levels of melting or different stages of melting from a mantle source [6]. This subdivision is even convinient for the regional correlation (of Indochina or SE Asia ...). The young basalts (N-Q) in South ViÖt Nam have been subdivided after this way into two series: tholeiite and alkaline olivine basalt. In this case the P-T basaltoids of the SDS can be named as "S«ng §µ Basalts" which has full geodynamic significance for the NW ViÖt Nam Region, such as in China international and Chinese geoscientists have been naming the Upper Permian basalts in Yunnan and Sichuan as Emeishan Basalts.

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