THE MINERALOGY OF PHLOGOPITE FROM LAMPROPHYRES OF CENTRAL VIỆT NAM: IMPLICATION FOR THE MAGMA EVOLUTION.

TRẦN TUẤN ANH¹, MAI KIM VINH²,
TRẦN TRỌNG HÒA¹, NGÔ THỊ PHƯỢNG¹, TRẦN VIỆT ANH¹

¹Institute of Geological Sciences, NCNST, Hoàng Quốc Việt, Cầu Giấy, Hà Nội;
²Geological Mapping Division of the South, 200, Lý Chính Thắng Str., HCMC

Abstract: This paper presents new data on phlogopites from different lamprophyre dykes in Đăk Long, South Đăk Tô (Kon Tum); Măng Xim, Ba Bích, Ba Trang (Ba Tơ, Quảng Ngãi), and other localities at Central Việt Nam, and their implication to reconstruct the crystallization sequences of the lamprophyre. Three paragenesis of phlogopite are identified: 1/Phenocrystal phlogopite; 2/ Mantled phenocrystal phlogopite; and 3/ Groundmass phlogopite. Phlogopites of the lamprophyre of Central Việt Nam are Al-rich phlogopites, representing high-pressure crystallization condition of the rock. Groundmass, mantled phenocrysts are typically depleted in Cr, Mg, Si and enriched in Fe, Mn, and Ti, relatively enriched in Al in comparison with phenocrysts, and thus may be considered to be more evolved than the later. The presence of chlorine, instead of fluorine content in all phlogopite types show that they are not a typical phlogopite of lamproite.

Two stage of crystallization are identified: First stage characterized by low Ti, high Mg phenocrystal phlogopite. In this stage, some turbulance changes in composition of the magma are identified by mantled phenocrystal phlogopite, which have both normal and inverse zoning. The second stage is represented by groundmass phenocrystal phlogopites, which are crystallized in a shallow depth.

I. INTRODUCTION

Lamprophyres are potassic–ultrapotassic igneous rocks, found in all over continents. Despite their small volume and petrologic perculiarities, lamprophyre can potentially provide many informations on the nature and evolution of their mantle source formation.

Research on the mineral composition by electronprobe microanalyzer (EPMA) nowaday is one of the most advanced technique, by which with minimum cost we can re-establish the formation history of magma, the crystallization order of minerals in the rock, the origin and the forming condition of magmas such as: pressure, temperature and volatile regimes.

 Phlogopite is a water-bearing mineral phase, which can exist in a wide range of pressure and temperature. The presence of phlogopite in lamprophyres and peridotites represents metasomatism or contribution of metasomatic agents into the formation of the mantle-derived magma. This paper presents new data of phlogopite from different lamprophyre dykes in Đăk Long, South Đăk Tô (Kon Tum); Măng Xim, Ba Bích, Ba Trang (Ba Tơ, Quảng Ngãi), and other localities in Central Việt Nam, and their implication for reconstructing the crystallization order of lamprophyres.

 II. GEOLOGICAL SETTINGS AND PETROGRAPHY OF LAMPROPHYRES

In Late Permian - Early Triassic, the major tectonic phase occuring in Southeast Asia is the collision of Indochina, South China and Sibumasu plates followed by the closure of Paleo-Tethys. As their results, the post orogenic magmatism occurs on all over the previous consolidated Kon Tum Block (Nguyễn Xuân Bao et al., 2000, in Archives).

Lamprophyre magmatism is one of the post-orogenic magmatism, which crops out at Đăk Long, South Đăk Tô (Đỗ Văn Chi et al., 1998; Nguyễn Quang Lộc et al., 1998; Mai Kim Vinh et al., 2000, in Archives), Măng Xim, (Nguyễn Thành Tín et al., 1997, in Archives, [1]), Ba Bích, Ba Trang, An Quang [2]. These localities are situated at mobile belts, namely: Trà Bồng - Khâm Đức, Pleiweck - Ngọc Hồi sutures, or located at the margin of ancient continent (Kan Nack – Ngọc Linh) or major faults: Pô Kô River, Ba Tơ - Giá Vực. They are formed as dykes of 0.5 - ~5 m in width, 50-100 in length, showing porphyric texture with clinopyroxene and phlogopite as phenocrysts (Fig. 1). In some samples, amphibole occurs in the phenocrystal phase. Their groundmas is composed mainly of glass and/or small microlitic crystals of clinopyroxene, phlogopite, plagioclase and sanidine, among them feldspars play an important role (Fig. 1).

Geochemically, these rocks range from basaltic to trachytic and rhyolitic composition (Fig. 2). They show ultra-potassic or shoshonitic character, which are high in potassium, aluminium, and silica and low in magnesium contents. K-Ar dating age of the Đăk Long potassic lamprophyre (DL-422/1) gives the age of 223.9 Ma (Late Triassic) [2]. On the stratigraphic relation side, lamprophyre dykes do not penetrate geologic formations younger than Late Triassic.

III. SAMPLES AND ANALYTICAL METHOD

Representative samples were collected from Ba Tơ, Đăk Long, South Đăk Tô, and Trà Vằn. These samples are fresh, with LOI less than 3%. Their mineral composition was analyzed in polished thin sections, by the electron-microprobe CAMECA Kevex at the Institute of Petrography and Mineralogy, Siberian Branch of the Russian Academy of Sciences at an accelerating voltage of 15 keV, 20nA beam current. International natural mineral standards were used for calibration.

IV. MINERALOGY OF PHLOGOPITE IN LAMPROPHYRE

Phlogopite is a common mineral phase in all localities. It occurs in the following paragenetic associations:

1.      Phenocryst (Phl₁);

2.      Mantled phenocrystal phlogopite (Phlm)

3.      Groundmass (Phl₂);

Phenocrystal phlogopite is strongly pleochroic, from brownish to pale yellow and from pinkish hue yellow to reddish brown (Fig.1). The intensity of pleochroism increases with the increase of titanium content. The majority of phenocrysts are inclusion-free. They often have corroded and embayed margins, indication of resorption in the magmas that formed their host rocks.

Mantled phologopites (Phlm) are found mainly in Ba Tơ and Đăk Long lamprophyres. This feature proves that phlogopite appears to be not a liquidus phase throughout the entire crystallization interval of the magma, and the mineral may occur exclusively either as phenocrysts or groundmass plates [7]. The mantles often exhibit stronger pleochroism than the cores. Commonly, the core-mantle interface is not well defined and represents a zone of strong continuous zoning rather than discrete overgrowth. This phlogopite type probably results from magma mixing processes, in which crystals of relatively earlier evolved phlogopite are incorporated into relatively later evolved magmatic fluid.

The groundmass crystal phase (Phl₂) consists of poikilitic subhedral plates, with the size of about some hundreds micrometers in maximum sise. They are scattered in the groundmass, in which the zoning is not observed so far.

Chemical composition

The representative composition of phenocrystal, mantled phenocrystal and groundmass phlogopites are presented in Table 1, 2, 3. The significant features of mica in all lamprophyres are: 1/ high in Al₂O₃, varying from 12.20 to 16.97 wt.% (Table 1-3); 2/ relatively high in MgO (11.08-25.00 wt.%); and 3/ wide variation of titanium (0.57 - 5.25 wt%). On the classification diagram, all analyses are of phlogopitic nature, with the changing trend toward siderophyllite. Compared to phlogopite from lamprophyre and lamproite of Northwestern Việt Nam (Trần Tuấn Anh et al., 2000), phlogopite of Central Việt Nam is higher in Fe/(Fe+Mg) ratio (Fig.3). The most phlogopitic nature belongs to the phenocrystal phase, except phenocrysts from Ba Tơ, which have highest iron and AlIV content.

Phenocrystal phlogopite (Phl₁) of Đăk Long, South Đăk Tô and Trà Vằn has highest SiO₂ and MgO contents (SiO₂ = 35.16-41.62 wt%; MgO = 13.42-24.39 wt.%, Table 1, Fig.4), it is lowest in FeO*, MnO, and (FeO* = 3.67 – 8.50 wt.%; MnO

 

= 0.026 – 0.11 wt%; Table 1, Fig.4). Their chromium content varies in a wide range (Cr₂O₃ = 0.001 – 1.04 wt %, Table 1, Fig 4). Phenocrystal phologopite from Ba Tơ lamprophyre is characterized by highest content of titanium, iron and manganese (TiO₂ = 1.33 – 5.25 wt%, FeO* = 3.67 – 17.19, MnO = 0.12-0.26 wt.%), lowest in magnesium (MgO = 13.42 – 23.84 wt%, Table.1, Fig.4). There are no significant difference of Al₂O₃, CaO, Na₂O and Cl content between phenocrystal phlogopite and the other phlogopite types (Fig. 4).

Mantled phlogopite (Phlm) seems to be an intermediate variety between phenocrystal and groundmass phlogopites. In comparison to phenocrystal phlogopite, it has similar Ti, Al, Na and Cl contents, and is lower in Fe, Mn, Ca, higher in Cr, Mg, and K (Table 2, Fig.3). Among this group, Ba Tơ mantled phlogopite has highest titanium (TiO₂ = 4.63-5.01 wt%), iron (FeO* = 9.14-16.54wt%), sodium (Na₂O = 0.16-0.34 wt%), manganese (MnO = 0.09-0.30 wt%), and chlorine (Cl = 0.07-0.11 wt%) contents, but is lowest in magnesium (MgO = 11.08-16.72 wt%) and chromium (Cr₂O₃ <0.09 wt%) (Table. 2, Fig.3). In all phlogopite of lamprophyre, the zoning trend is not so well-defined as in those from lamproite of North Việt Nam, and it compositional differences between core and rims are not identical.

The groundmass phlogopite (Phl₂) is widespread in all lamprophyre localities, except in Ba Tơ lamprophyre. Compared to phenocrystal phlogopite, it is characterized by the lower content of magnesium, silica, chromium, and potassium (MgO = 16.95 - 20.06 wt%, SiO₂ = 36.32-39.65 wt%, Cr₂O₃ = <0.19 wt%, K₂O= 8.33-9.60 wt%), but higher content of iron, manganese (FeO = 8.62-14.16 wt%, MnO = 0.06-0.14 wt%) (Table 3, Fig. 3). Their calcium and sodium contents vary in a wide range (Fig.3). Especially, in groundmass phlogopite of South Đăk Tô lamprophyre, two different variation phlogopite types are recorded: titanium-high along with aluminium-low and titanium-low accompanied with aluminium-high types.

V. DISCUSSIONS AND CONCLUSIONS

In general, compositional significant features of phlogopite in lamprophyres of Central Việt Nam could be summarized as follows:

·         Phlogopite of lamprophyres of Central Việt Nam is lower in chromium, magnesium, calcium, sodium, but higher in iron, manganese contents in comparison to lamproite of Northwest Việt Nam.

·         Groundmass and mantled phenocryst phlogopites are typically depleted in Cr, Mg, Si and enriched in Fe, Mn, and Ti, relatively enriched in Al in comparison with phenocrystal phlogopite, and thus may be considered to be more evolved than the later.

·         Compositional evolution trends are typical with the decrease of Mg, slight decrease of Al and increase of Fe and the constancy or increase of Ti. This feature may be caused by the increase of Fe²⁺, reflecting the evolution toward titanian phlogopite-biotite solid solutions [7].

·         The presence of chlorine, instead of fluorine content in all phlogopite types shows that they are not a typical phlogopite of lamproite.

Solid solution in phlogopite of Central Việt Nam lamprophyres

The phlogopite from Central Việt Nam has sufficient Si and Al for filling the tetrahedral net. AlVI is present in all phlogopite varieties, that shows they are all aluminous phlogopites. In all phlogopites, Si and Al always fit the ideal value of six Si atoms /formular unit (Fig. 4), that could be explained that possibly these phlogopites represent the initial crystallization when the Al deficiency was less.

Hartman (1969) has proposed the existence of Ti⁴⁺ in the tetrahedral network, but experimental data from Robert [9] and Forbes and Flower [3] show that this possibility is remote. Consequently, it is considered that Ti exclusively occupies the octahedral net replacing divalent or trivalent cations. They proposed the following substitution schemes:

2Mgvi²⁺ Û Tivi⁴⁺ +  Forbes and Flower (1974)      (1)

Mgvi²⁺ + 2Siiv⁴⁺ = Tivi⁴⁺ + 2Aliv³⁺        Robert (1976)              (2)

Figure 5 shows that all phlogopites from Northwest Việt Nam mainly follow the (2) substitution rule, which explain the contrary variation of titanium and aluminium content.

Wagner and Velder [12] have proposed a substitution as follow:

Mgvi²⁺ + Siiv⁴⁺ = Tivi⁴⁺ + Mgiv²⁺          Wagner and Velder (1986)      (3)

The (3) substitution fits quite well with phlogopite of the lamprophyres of Central Việt Nam, that is demonstrated by inverse correlation (Fig.6) between Ti and (Si+Al+Cr). Such relationship has been found also in Smoky Butte phlogopite [6, 12] and in phlogopite form Murcia-Almeria and Sisco [12].

Phogopite crystallizing process in lamprophyres of the Central Việt Nam

The composition of phlogopite of the lamprophyre implies the crystallization from a MgO-TiO₂ rich, H₂O-Cl bearing magma. Experiments of different authors have been showing that phlogopite is stable over a very wide range of pressure and temperature, ranging from the upper crust to the upper mantle. Experiment study of Orlando and other [8] on a wadeite-phlogopite system at 2-3 Gpa has been showing that phlogopite is absent at 1300-1400^oC temperature. Sato et al. [11] concluded that phlogopite is stable at the depth shallower than 180-200 km in the subcontinental mantle and shallower than 200 km in the mantle wedge at subduction zones. Esperanca and Holloway [2], using a minette as starting composition, have been determining that between 10 and 20 kbar and 1070-1200^oC the TiO₂ content of phlogopite increases with the temperature and pressure decrease and oxygen activity increase . The highest titanium phlogopite (7.8 wt %) is formed at 1100^o C at 10 kbar with oxygen activity equivalent to this of the QFM buffer. Synthesized phlogopite contains 3.3-7.8 wt% TiO₂, 14.7–16.3 wt% Al₂O₃, and 6.2-11.2 wt% FeOT [2].

In conclusion, the mica from lamprophyre of Central Việt Nam consists of Al-rich phlogopite, representing high-pressure crystallization condition of the rocks. Two stages of crystallization are identified: the first is characterized by low Ti, high Mg phenocrystal phlogopite. In this stage, some disturbances in the composition of magma are identified by mantled phenocrystal phlogopite, which has both normal and inverse zonings. The second stage is represented by groundmass phenocrystal phlogopite, which is crystallized in a shallow depth. For further comprehensive interpretation, it needs to have more detailed research on other mineral phases of the lamprophyres. This paper is completed under the support of Project "Research on the forming conditions and distribution rules of precious minerals in relation to magmatic activities of Central Vi#t Nam and T#y Nguy#n regions" – DTDL-2003/07 and the Basic Research Project grant No 71.31.04.

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