AGE CONSTRAINTS ON THE PETROGENESIS OF LAMPROPHYRE FROM SOUTH CENTRAL VIỆT NAM

TRẦN TUẤN ANH¹, TRẦN TRỌNG HÒA¹, A.E. IZOKH², A.S. BORISENKO², A.V. TRAVIN²

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

Abstract: For the first time, a detailed research on Ar-Ar age dating of different lamprophyre bodies from the South Central Việt Nam has been carried out. Recent Ar-Ar isotopic geochronological studies in Đắk Long, Đắk Ripen and Kon Fam lamprophyres yield the plateau ages of 228.6 ±2.6 ÷ 240.4±2.7 Ma 246.6±2.6 Ma , and 117.1±1.7 Ma, respectively. The presence of lamprophyres in South Central Việt Nam indicates two end-point stages, reflecting the latter tectono-magmatic activities of Indosinian and Yanshanian orogenies in South Central Việt Nam.

I. INTRODUCTION

The Southeast Asia comprises a complex assemblage of continental lithospheric blocks that were amalgamated during Late Paleozoic to Cenozoic times and were periodically disrupted by strike-slip faulting and the rotation of blocks due to ongoing collisions, plate reorganizations and plate interactions [10]. Any discussion on Paleozoic stratigraphic syntheses for the region needs to be relied upon this tectonic framework and underpinned by palinspastic reconstructions of continents, oceans and related magmatism.

In this paper, for the first time we present the results of a detailed Ar-Ar age dating data of samples of different lamprophyre bodies from the South Central Việt Nam. These new results allow us to have better understanding about: 1/ Ages and tectonic setting of lamprophyres; 2/ The relationship between studied lamprophyres and adjacent magmatism; and 3/ The magmatic and tectonic evolution of the South Central Việt Nam.

II. GEOLOGICAL SETTING AND PETROGRAPHICAL CHARACTERISTICS

Since their separation from Gondwana in Ordovician - Silurian [13] or Devonian [7, 16], South China and Indochina are considered to be already amalgamated along the Sông Mã Suture Zone in the Carboniferous (Metcalfe, 1988) or Late Permian to Triassic [3, 7, 8, 13, 17, 18] or Late Triasic to Jurassic [14, 15]. The Indochina Block component is bounded to the northeast by the Sông Mã Suture Zone and
to the west by the Uttaradit-Nan - Sra Kaeo - Bentong-Raub sutures in Thailand and Malaysia, respectively. The basement of the Indochina Block comprises the Precambrian Kon Tum Massif of granulite facies rocks. Early to Middle Paleozoic rocks are rare
and restricted in the marginal areas of the blocks [10].

The lamprophyre dikes crop out at Đắk Long, South Đắk Tô (Đắk Ripen) [4, 6, 11] (Fig. 1), Mang Xim [2, 12], Ba Bích, Ba Trang, An Quang [6], which are situated at mobile belts: Trà Bồng - Khâm Đức, Pleiwek - Ngọc Hồi sutures, located at the margin of ancient continental basement (Kan Nack - Ngọc Linh) or major faults: Pô Kô River, Ba Tơ - Giá Vực. A number of lamprophyre bodies are found in Kon Fam gold deposit (Kon Chrô), that have undergone intensive hydrothermal alterations.

Figure 1. Simplified geological map and sample localities of lamprophyre

The lamprophyre is formed as dikes, with 0.5 - ~5 m in width, 50-100 m in length, showing porphyritic texture with clinopyroxene and phlogopite as phenocrysts [19]. In some samples, amphibole occurs in phenocrystal phase. The groundmass is composed mainly of glass and/or small microlitic crystals of clinopyroxene, phlogopite, plagioclase and sanidine, among them feldspathoids play an important role (Fig. 2).

Geochemically, the above rocks range from basaltic to trachytic and rhyolitic in composition (Fig. 3). They show ultra-potassic or shoshonitic character, which is high in potassium, aluminium and silica, and low in magnesium contents [19]. K-Ar dating of Đắk Long potassic lamprophyre (DL-422/1) has been giving the age of 223.9 Ma (Late Triassic) [6]. In stratigraphical correlation, lamprophyre dykes do not penetrate through younger than Late Triassic geological formations.

III. SAMPLES AND ANALYTICAL METHOD

⁴⁰Ar/³⁹Ar step-heating experiments were performed for 5 mineral separates. Mineral separates were obtained by crushing and magnetic separation techniques, then by paper shaking and finally hand picking to remove all visible impurities. Pure mineral separates along with neutron flux monitor LP-6 biotite [1] were irradiated in the nuclear reactor at Tomsk. After irradiation, samples were degased by incremental heating procedure and argon isotopic component was measured by using a MICROMASS 5400 mass-spectrometer at Institute of Geology and Mineralogy - Siberian Branch of the RAS. Total gas, plateau and intercept ages were calculated. Age spectrum and isotope correlation diagram were plotted for evaluating possible gas content of each sample. Details of the analytical procedure and data process can be found in [20].

Phenocrysts of phlogopite in Đắk Long lamprophyre. Magnification x40. Nicol (+). Sample DLG-428/1	Đắk Ripen lamprophyre. Magnification x40. Nicol (-). Sample DL-129
Đắk Long lamprophyre. Magnification x40. Nicol (-). Sample DL-2563	Kon Fam lamprophyre. Magnification x40. Nicol (+). Sample DL-147

Figure 2. Petrographical characteristics of lamprophyres from South Central Việt Nam

IV. RESULTS

Argon isotope data and a summary of age calculations are listed in Table 1.

1. Đắk Ripen lamprophyre

Phlogopite from Đắk Ripen lamprophyre (DL-129) yields well-defined plateau over 80% of ³⁹Ar_K released with the age of 246.6±2.6 Ma (Fig. 2a). The plateau age is highly conformable with intercept age (246.0±1.5 Ma) obtained from least-square regression of data for their plateau steps. Their ⁴⁰Ar/³⁶Ar intercept values generally agree with the atmospheric composition (⁴⁰Ar/³⁶Ar = 295.5) (Fig. 2b). Apparently, Đắk Ripen lamprophyre intruded and then were emplaced without further disturbance after the intrusion.

2. Đắk Long lamprophyre

Phlogopite of sample DL-2563 yields a fairly flat age spectrum with a well-defined plateau over 90% of released ³⁹Ar_K . But, the plateau could be divided into 2 parts with age values of 228.6 ±2.6 Ma and 240.4±2.7 Ma (Fig. 2b). Its intercept age (236.4 ± 1.9Ma) obtained from least-square regression of data for the plateau step is perfectly conformable with its respective plateau age. ⁴⁰Ar/³⁶Ar intercept value of DL-2563 phlogopite also agrees with the atmospheric composition.

3. Kon Fam lamprophyre

The least altered phlogopite was found in the DL-147 specimen. The content of secondary minerals (chlorite and sericite) in this specimen is less than 10%. More altered and clarified variety of this mineral contains

Table 1. Argon isotope data of phlogopite from Đắk Long lamprophyre

Sample	T^o(^oC)	Age (Ma)	+-	^40Ar/^39Ar	+-	^38Ar/^39Ar	+-	^37Ar/^39Ar	+-	^36Ar/^39Ar	+-	Cummulative^39Ar
DL129 Biotite	550	48.62	2.99	96.39025	0.23	0.152062	0.00	0.85494	0.12	0.305303	0.00	2.151522933
DL129 Biotite	600	131.76	9.89	118.9379	0.87	0.154789	0.00	2.381371	0.35	0.344552	0.01	2.901994766
DL129 Biotite	700	232.71	3.17	60.13721	0.15	0.110399	0.00	0.006332	0.01	0.098218	0.00	6.878326918
DL129 Biotite	750	246.71	2.81	44.53482	0.04	0.083824	0.00	0.034376	0.03	0.038641	0.00	12.11422904
DL129 Biotite	800	245.28	2.86	39.76164	0.05	0.076012	0.00	0.045189	0.05	0.023183	0.00	17.68629064
DL129 Biotite	850	246.82	3.01	36.59244	0.05	0.067963	0.00	0.004443	0.00	0.011707	0.00	23.4666857
DL129 Biotite	950	250.53	2.73	35.87058	0.05	0.068743	0.00	0.051103	0.01	0.007459	0.00	40.83321381
DL129 Biotite	1000	249.26	2.71	35.07182	0.03	0.063839	0.00	0.013354	0.01	0.005376	0.00	56.21873871
DL129 Biotite	1050	244.53	2.65	35.00578	0.03	0.066509	0.00	0.058456	0.02	0.007451	0.00	79.98053324
DL129 Biotite	1100	242.33	2.63	34.90325	0.03	0.064732	0.00	0.002709	0.00	0.008174	0.00	98.94223828
DL129 Biotite	1150	234.26	3.75	62.57355	0.14	0.107129	0.00	0.64504	0.12	0.105717	0.00	100
Mean		246.60
Sample	T^o(^oC)	Age (Ma)	+-	^40Ar/^39Ar	+-	^38Ar/^39Ar	+-	^37Ar/^39Ar	+-	^36Ar/^39Ar	+-	Cummulative^39Ar
DL2563 Phl	500	225.53	30.99	121.5821	2.42	0.256302	0.01	0.573507	0.58	0.314622	0.02	0.207543024
DL2563 Phl	600	195.43	5.63	52.84624	0.21	0.12299	0.00	0.097879	0.10	0.095643	0.00	1.38090849
DL2563 Phl	650	241.15	5.03	46.55197	0.13	0.122726	0.00	0.008005	0.01	0.053545	0.00	2.907597767
DL2563 Phl	700	229.06	3.20	35.87301	0.04	0.116615	0.00	0.041057	0.04	0.02296	0.00	5.836982361
DL2563 Phl	750	231.13	2.99	34.71744	0.03	0.118368	0.00	0.079333	0.04	0.018099	0.00	11.13682236
DL2563 Phl	800	227.36	2.67	32.45849	0.03	0.098925	0.00	0.008592	0.01	0.012183	0.00	17.33397717
DL2563 Phl	850	228.37	2.64	31.4475	0.04	0.08236	0.00	0.090927	0.02	0.008298	0.00	23.17741907
DL2563 Phl	900	227.92	2.68	30.60312	0.03	0.079029	0.00	0.040377	0.03	0.005645	0.00	28.50030431
DL2563 Phl	950	228.62	2.62	30.22023	0.02	0.081399	0.00	0.055012	0.01	0.00403	0.00	37.41015802
DL2563 Phl	1000	233.27	2.68	30.60321	0.02	0.074892	0.00	0.043862	0.02	0.003195	0.00	50.23847557
DL2563 Phl	1050	238.52	2.72	31.00984	0.03	0.068528	0.00	0.005077	0.01	0.002161	0.00	75.96669989
DL2563 Phl	1100	241.64	2.77	31.3557	0.03	0.070064	0.00	0.007592	0.01	0.001895	0.00	94.25628165
DL2563 Phl	1150	241.18	2.88	32.47147	0.03	0.073919	0.00	0.169086	0.06	0.005885	0.00	100
Mean		228.64
Mean		240.40

Figure 3. Plot of Na₂O+K₂O - SiO₂ of lamprophyres of South Central Việt Nam in comparison with lamproites of NW Việt Nam (Trần Tuấn Anh et al., 2000)

secondary products in elevated amount up to 20 % (DL-272/2). Pseudomorphs of light micas after phlogopite contain sericite, muscovite and relic phlogopite (about 20 %).

The estimated Ar-Ar age of phlogopite specimen (DL-147) of weakly altered lamprophyre is 117.1±1,7 Ma (Borisenko et al., 2006, in press). The Ar-Ar age of the yellow mica aggregate from pseudomorphs after phlogopite is 115.6±0.6 Ma (Borisenko et al., 2006 in press).

V. DISCUSSION AND CONCLUSIONS

Along the Pô Kô fault zone, Ar-Ar data show a conformable range of the formation ages from 228 to 246 Ma, which correspond to Permian - Triassic. Đắk Ripen lamprophyres intruded and then were emplaced without further disturbance after the intrusion, whilst two stages were recorded in Đắk Long lamprophyre, that leads to the conclusion on a Middle Triassic tectono-thermal event affecting the Đắk Long lamprophyre.

The present results suggest that in the Middle Triassic (ca. 240-246 Ma), the Đắk Long and Đắk Ripen lamprophyres as a whole had cooled to below ~400-450^oC. The presented thermochronological data suggest that post-Triassic structural or thermal modification of the lamprophyre occurred locally (~ 228 Ma, Fig. 2b), probably representing either a later stage of Indosinian Orogeny or an activity of the Pô Kô fault system. The presence of post-collision lamprophyres on the Pô Kô fault system might indicate that the suspected suturing along the Pô Kô fault zone occurred probably before Late Triassic time.

Toward the east, the Kon Fam lamprophyre yields the age of 117.1±1.7 Ma, which corresponds to Early Cretaceous. Its occurrence recorded the latter post-orogenic phase that might be correlated with J-K magmatism.

Figure 4. Ar-Ar age dating of the lamprophyre from South Central Việt Nam

Thus, the presence of lamprophyres in South Central Việt Nam has been recorded in two different tectono-magmatism stages, reflecting the latest activities of Indosinian and Yanshanian magmatism in South Central Việt Nam.

This paper is completed with the support of Project “Research on the forming conditions and distribution rules of precious minerals in relation to magmatic activities of Central Việt Nam and Tây Nguyên regions” (DTDL-2003/07) and the basic research project N^o.70.78.06.

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