MINERALOGY OF ROCK-FORMING MINERALS IN GABBRO-SYENITE INTRUSIONS OF CHỢ ĐỒN AREA, NORTHEAST VIỆT NAM

NGϡ THỊ PHƯỢNG, TRẦN TRỌNG HOÀ, TRẦN TUẤN ANH,
TRẦN VIỆT ANH, PHẠM THỊ DUNG

Institute of Geological Sciences, NCNST, Hoàng Quốc Việt, Cầu Giấy, Hà Nội

Abstract: In Chợ Đồn area, there are two gabbro-syenite associations with disctinct geochemical significance: Ti-low, alkali-low gabro-syenite of Khuổi Loỏng and alkaline gabbro - nepheline syenite association of Bằng Phúc type. Clinopyroxene, amphibole and bitotite in Ti-low, alkali- low gabbro and pyroxene syenite have features of minerals that were crystallized from a mantle-derived magma, which was significantly contaminated by crustal material. The primary magma of gabbro, alkaline pyroxenite and nepheline syenite of Bằng Phúc massif, according to geochemical significance, although derived from a mantle-derived magma too, but significantly contaminated not only by crustal material, but also by carbonate assimilation. The compositional variation of clinopyroxene, amphibole of syenite-gabbro-pyroxenite series is expressed by the increase of aluminium, iron and alkalinity, that is opposite to the normal trend. In that case, alkaline gabbro-syenite and nepheline syenite of Bằng Phúc massif are products of magmatic activities, which are related to post-orogenic extension. After Nguyễn Trung Chí et al., 2003, the forming age of Chợ Đồn nepheline syenite is c.a. 233 Ma, that could probably coincide to the relaxation between two collisions: Indosinian (Late Paleozoic – Early Mesozoic) and Yenshanian (Late Mesozoic) at the margin of South China Platform.

INTRODUCTION

Gabbro-syenite intrusive rocks are widespread all over the Lô-Gâm and Phú Ngữ structures of East Bắc Bộ folded system [4,7], belonging to 3 following types: titanium-low gabbro- syenite, titanium-high monzogabbro– monzodiorite-syenite, and alkaline gabbro (gabbro-pyroxenite) - nepheline syenite [Trần Trọng Hòa et al, 2004, this issue]. The first and second types are widespread in Lục Yên, Tích Cốc areas of Lô-Gâm Structure, whilst the last one includes the Bằng Phúc and Khuổi Loỏng massifs of Chợ Đồn area. The Khuổi Loỏng massifsis composed of titanium-low, alkali-low gabbro, while petrographical composition of the Bằng Phúc massif is of alkaline gabbros and nepheline syenites. However, in Bằng Phúc massif titanium- low, alkali-low gabbro and foid-free syenite are met. The mineralogical significance of pyroxene, amphibole and biotite of Lục Yên and Tích Cốc gabbro-syenite and their implications to the correlation of magmatic associations and petrogenesis are mentioned [4]. This paper presents new geochemical data of pyroxene, amphibole, biotite, nepheline in gabbro (gabbro-pyroxenite) and syenite of Khuổi Loỏng and Bằng Lũng massifs, Chợ Đồn area. Especially, for the first time, the significance of trace elements and rare earth elements are presented, providing valuable information on mineral characteristics, their formation and origin.

GEOLOGICAL SETTING AND PETROGRAPHICAL SIGNIFICANCES

The Khuổi Loỏng and Bằng Lũng massifs are small alkaline-intermediate intrusives, belonging to the Chợ Đồn Complex. The Bằng Phúc massif includes tianium-low, alkali-low gabbro and gabbrodiorite, pyroxene syenite, nepheline-biotite syenite (miaskite) and alkaline gabbro-pyroxenite. No intermediate variety between these rock types is recorded so far. The dark colour variety, which is correlative to alkaline pyroxenite of the Bằng Phúc massif is considered to be skarn or contaminated products [1]. The Khuổi Loỏng massif consists mainly of gabbro, with some gabbro-pyroxenite and gabbrodiorite. This massif is brecciated and chapped, containing fractured gabbro cemented by syenite and/or granosyenite materials.

Syenites at Chợ Đồn cross-cut Ordovician-Silurian sediments and penetrates gabbroid of Núi Chúa Complex and Late Triassic biotite granite of Phia Bioc Complex [1]. The Rb-Sr dating gives the age of 233 Ma to nepheline syenite of the Bằng Phúc massif [Nguyễn Trung Chí et al, 2003. Petrology and metalogeny of alkaline formations in North Việt Nam, in Archives].

Samples from Khuổi Loỏng massif (H-1545-1546-1548, P-545-550) are titanium- low, alkali-low pyroxene gabbro (sometimes altered to amphibole gabbro), gabbrodiorite, which are relatively similar to Lục Yên gabbroid. Samples from Bằng Phúc massif are gabbrodiorite (H-1646) similar to Khuoi Loong gabbrodiorite, monzonite (H-1535-1536), alkaline pyroxenite (H-1215-1538-1539), nepheline-biotite syenite (H- 1221-1216-1540...). Detailed geochemical significances of gabbroid and syenite are described in [Trần Trọng Hoà et al., 2004, this issue].

Titanium-low, alkali-low gabbro and gabbrodiorite are medium to fine-grained rock, characterized by Pl+Cpx+Amf+Bi mineral aggregate. Sometimes, clinopyroxene is replaced by amphibole at its margins. Biotite is small individual greenish brown plates. Potassic feldspar is met at the contact between gabbro and cross-cutting syenitic veins.

Monzodiorite and pyroxene syenite consist of plagioclase, potassic feldspar, clinopyroxene, some amphibole and biotite. Their mineral habits are similar to those of gabbro and gabbrodiorite.

Alkaline gabbro and pyroxenite are characterized by complex mineral aggregates: Cpx+Amf+ Bi+Ne+Ca+Sf+Ap. Clinopyroxene consists of common idiomorphic elongated prisms. Sometimes, they have zoning structure with thin margins, replaced by alkaline amphibole (pargasite). Idiomorphic amphibole is met, with dark green colour. Biotite exists in short dark green brown plates. Scarce nepheline consists of allomorphic crystal, intercalating potassic feldspar and pyroxene. Primary calcite is allomorphic plate, interpolating pyroxene and/or amphibole.

Nepheline-biotite syenite (miaskite) consists mainly of light coloured minerals: potassic feldspar, plagioclase, nepheline and biotite (<5%). Biotite consists of small reddish scabs, which express the enrichment of iron and titanium. Nepheline is small idiomorphic crystals, sometimes appearing to replace feldspar.

MINERALOGY OF CHỢ ĐỒN GABBRO-SYENITE

Major element of minerals were analyzed in polished thin sections by an electron-microprobe CAMECA Kevex at an accelerating voltage of 15 keV, 20 nA beam current; trace elements and rare earth elements were analyzed by LA-ICPMS at the United Institute of Geology, Geophysics and Mineralogy, SB of RAS. International natural mineral standards were used for calibration. Representative mineral composition is presented in Table 1-7.

Pyroxene is present in all rock types, except Bằng Phúc miaskite. The chemical composition of pyroxene varies widely, but mainly are diopside – hedenbergite. In alkaline pyroxenite and gabbro, pyroxene a is rich in wollastonite component (Table. 1, Fig. 1).

 Table 1. Representative composition of pyroxene from Chợ Đồn gabbro – syenite association

Note: - Bằng Phúc massif: Alkaline gabbro-pyroxenite - 1,2,5,6,10,13,14 (H-1215; H-1538; H-1616; P-543 ); Monzonite- 3,4,11,12 ( H-1535, H-1536, P-536); Alkaline gabbro-pyroxenite - 7,8,9 (H-1539, H-1614); - Khuổi Loáng massif: Gabbro - 15,16,17,18,19 (H-1545, H-1546, H-1548, P-545, P-550)

Table 2. Representative composition of amphibole from Chợ Đồn gabbro – syenite association

Sample	H1532/2	H1536/2	H1538/2	H1538-03	H1538-12	H1538-13	H1538-19	P536/2	P537/1	P540/2	H1215-10	H1215-11	H1614/1	P537/3
	1	2	3	4	5	6	7	8	9	10	11	12	13	14
SiO2	40.62	42.39	39.11	38.32	38.10	38.24	38.35	39.31	40.92	37.59	38.27	37.91	44.34	46.34
TiO2	1.96	1.26	1.57	1.23	1.06	1.08	1.22	3.05	3.10	1.87	1.90	1.86	2.20	1.19
Al2O3	9.48	8.84	16.47	17.14	16.70	16.72	16.84	12.56	11.51	14.58	15.69	15.64	9.07	7.16
FeO	27.54	23.92	18.55	15.11	15.09	14.97	15.02	20.64	19.30	22.82	20.15	20.13	19.31	19.97
Cr2O3				0.01	0.01	0.00	0.01				0.00	0.01
MnO				0.22	0.21	0.20	0.22				0.30	0.30
MgO	3.27	5.98	3.57	9.18	9.30	9.40	9.31	6.20	7.26	4.74	6.52	6.49	8.75	9.75
CaO	10.94	11.37	14.69	11.70	11.78	11.79	11.61	11.69	11.25	11.29	11.46	11.29	11.24	10.67
Na2O	1.71	1.50	1.50	1.82	1.91	1.90	1.90	1.40	1.72	1.86	2.04	1.97	1.42	1.24
K2O	1.64	1.75	1.95	2.93	2.95	2.93	2.90	2.24	1.36	2.61	2.56	2.55	0.97	0.72
Cl				0.02	0.02	0.02	0.01
Total	97.16	97.01	97.41	97.68	97.13	97.25	97.40	97.09	96.42	97.36	98.89	98.15	97.30	97.04
Si	6.51	6.67	6.31	5.80	5.81	5.82	5.82	6.16	6.35	5.94	5.85	5.83	6.71	6.93
Ti	0.24	0.15	0.19	0.14	0.12	0.12	0.14	0.36	0.36	0.22	0.22	0.22	0.25	0.13
Al	1.79	1.64	3.13	3.05	3.00	3.00	3.01	2.32	2.10	2.71	2.83	2.83	1.62	1.26
Fe3+	0.11	0.09	0.00	0.18	0.15	0.15	0.19	0.00	0.00	0.06	0.17	0.26	0.21	0.69
Fe2+	3.58	3.05	2.51	1.73	1.78	1.76	1.71	2.71	2.51	2.96	2.41	2.33	2.24	1.81
Mn	0.00	0.00	0.00	0.03	0.03	0.03	0.03	0.00	0.00	0.00	0.04	0.04	0.00	0.00
Mg	0.78	1.40	0.86	2.07	2.12	2.13	2.10	1.45	1.68	1.12	1.49	1.49	1.98	2.17
Ca	1.88	1.92	2.54	1.90	1.93	1.92	1.89	1.96	1.87	1.91	1.88	1.86	1.82	1.71
Na	0.53	0.46	0.47	0.53	0.57	0.56	0.56	0.43	0.52	0.57	0.61	0.59	0.42	0.36
K	0.34	0.35	0.40	0.57	0.57	0.57	0.56	0.45	0.27	0.53	0.50	0.50	0.19	0.14

- Note: Pyroxene syenite (amphibolization):1; Monzonite: 2,8,9,14, 16; Gabbro-pyroxenite: 10,15; Alkaline gabbro-pyroxenite: 3,4,5,6,7,11,12,13,

 

Table 3. Representative composition of biotite from Chợ Đồn gabbro – syenite association

 

 

  

 

Pyroxene in titanium-low, alkali-low gabbro (and pyroxenite) of Khuổi Loỏng massif and Bằng Phúc monzonite are diopside - Wo_44.57-51.04En_29.13-38.47Fs_{11.59-25.32 ,} characterized by titanium-low (TiO₂ = 0.02-0.26 wt%, sometimes up to 0.56 wt%), relatively low aluminium content (Al₂O₃ = 0.50-0.86%). However, diopside of Khuổi Loỏng gabbro is richest in enstatite, but lowest in ferrosilite component, whilst diopside of Bằng Phúc monzonite are lowest in magnesium, highest in iron content (Table 1). In Wo-En-Fs ternary plot (Fig.1), clinopyroxene from monzonite is close to hedenbergite field.

Clinopyroxene from alkaline gabbro (and pyroxenite) of Bằng Phúc are characterized by enrichment of wollastonite component (Wo_49.69-54.43En20_.20-29.13Fs_17.45-28.60), high in titanium (TiO₂ = 1.37-1.91 wt.%), and is similar to titano-augite. They are rich in aluminium content (Al₂O₃ = 5.20-9.89 wt.%), relatively high in sodium (Na₂O = 0.47-0.82 wt.%), and intermediate in magnesium content (MgO = 6.41-9.30 wt.%) between diopside of gabbro-pyroxenite and monzonite (Fig.2). In the binary plots of fCpx-Si/Al v# (Ca+Na)/Si/Al (Fig.1) pyroxene of alkaline gabbro is distributed close to the vertical axis or radical.

In general, pyroxene of titanium-low, alkali-low gabbro and monzonite are close to that from tholeiitic or subalkaline mafic-ultramafic association, whilst pyroxene of Bằng Phúc alkaline gabbro are similar to that of alkaline gabbro and nepheline syenite association of Dismal (Southern Victoria Land) [10].

Amphibole: The chemical compostition of amphibole from different rock types is relatively fluctuated, but can be distinguish into two major types: magnesio-hornblende and pargasite, in which magnesio-hornblende is typical in titanium-low gabbro and gabbro-pyroxenite, while pargasite is found only in Bằng Phúc alkaline gabbro.

Magnesio-hornblende in titanium-low gabbro mainly replaces diopside and hedenbergite. Their chemical significance (P-540/2) is silica-low, aluminum-high (Al₂O₃ = 14.56 wt.%), iron-high (FeO = 22.82 wt.%), magnesium-low (MgO = 4.74 wt.%) and relatively potassium-high (K₂O = 2.61 wt.%). Amphibole in monzonite is characterized by high titanium (TiO₂ = 1.28-3.05 wt.%), aluminium (Al₂O₃ = 7.16-12.56 wt.%), iron (FeO = 19.97-23.92%) and low potassium content (K₂O = 0.72-1.75 wt.%), in comparison to that of titanium-low gabbro (Table 2, Fig 3).

Table 4. Representative composition of nepheline from Chợ Đồn gabbro – syenite ssociation

Pargasite is found only in Bang Lung alkaline gabbro. It is low in titanium (TiO₂ = 1.02-1.90 wt.%), exceptionally high in aluminium (Al₂O₃ = 15.64-17.14 wt.%), medium in magnesium (MgO = 6.49-9.40 wt.%), relatively high in alkali, especially in potassium (Na₂O = 1.82-2.04 wt.%; K₂O = 2.55-2.95 wt.%). The potassium content is significantly higher than amphibole of titanium-low, alkali-low gabbro (Table 2, Fig 3).

Thus amphibole belongs to 2 types: 1/ low in silica, low in titanium, high in alkali, typical for gabbro and monzonite of tholeiitic and subalkaline series and 2/ high in aluminium, high in alkali, typical for nepheline-bearing amphibole of the Bằng Phúc massif.

Biotite belongs to 3 following types: 1/ high in titanium (TiO₂ = 4.85-5.10 wt.%), relatively high in iron (FeO = 23.82-24.03 wt.%), low in aluminium (Al₂O₃ = 13.99-14.22 wt.%) relatively low in magnesium (MgO = 7.75-8.19 wt.%). This type is typical for titanium-low gabbro and monzonite of Khuổi Loỏng and Bằng Phúc massifs; 2/ low in titanium (TiO₂ = 1.05-2.02 wt.%), high in aluminium (Al₂O₃ = 17.77-17.92 wt.%), in magnesium (MgO = 13.07-14.92 wt.%) in alkaline gabbro of Bằng Phúc Massif; and 3/ biotite of nepheline syenite (miaskite). They are fairly high in titanium (TiO₂ = 3.62 - 4.67wt.%), in aluminium (Al₂O₃ = 20.34-21.81wt.%), in iron (FeO = 25.59-26.00 wt.%), manganese (0.68-1.35 wt.%) but low in magnesium (MgO = 2.98-3.32 wt.%). Comparing the biotite in nepheline syenite of Bằng Phúc to that of Dusmal nepheline syenite, they are higher in Ti, Al, and Mg, but lower in Fe (Table 3, Fig.4).

Nepheline is present only in alkaline gabbro and nepheline-biotite syenite of the Bằng Phúc massif as well. The chemical composition of nepheline in gabbro-pyroxenite and nepheline syenite (Table 4) is stable, represented by Al₂O₃ = 33.60-34.10%, Na₂O = 15.42-15.82%, K₂O = 5.75-6.10%. These characteristics are similar to those of nepheline of typical alkaline intrusive over the world, especially in nepheline syenite of Dismal region (Southern Victoria, Antarctica).

Garnet occurs in alkaline gabbro-pyroxenite and monzonite. They have chemical composition of grossular (gross. = 81.13 - 98.97%), that is typical for feldspathoid-bearing alkaline magma (Table 5).

Feldspar: Plagioclase consist andesine and labrador (Ab_45.9-65.1An_33-53.4Or_0.5-2.1), up to albite (Ab-97.8) (in nepheline syenite). Potassium feldspar has Or_82.70Ab_16.90 , which is rich in albite in comparison to other alkaline rocks (Table 6).

Representative trace elements (Rb, Sr, Ba, Y, Zr, Nb, Hf, Ta, Th, U) and rare earth elements (REE) of pyroxene, garnet, calcite and apatite from gabbro and alkaline pyroxenite of Bằng Phúc massif (H-1538, H-1539) are listed in Table 2. The data of pyroxene in N-Q alkaline basalt is listed for

Table 5. Representative composition of garnet from Chợ Đồn gabbro–syenite association

Table 6. Representative composition of feldspar from Chợ Đồn gabbro–syenite association

Table 7. Representative trace element and REE composition of mineral from Chợ Đồn gabbro–syenite association

correlation. Chondrite- and PM-normalized spidergrams show that clinopyroxene of H-1538 (high Ti-Al) and H-1539 (low Ti-Al) samples share similar pattern of REE and trace element distribution (Fig. 5, Table 7): relatively rich in REE, slightly negative anomalies of Eu, whilst Ti-Al - high clinopyroxene has more significant negative anomaly. However, REE values and La/Sm, Ce/Yb and Nb/Ta ratios of Ti-high clinopyroxene (2.46, 7.5, and 30.5, respectively) are higher than Ti-Al - low one (1.4, 1.05, and 4.6, respectively). The REE pattern of Ti-Al - low clinopyroxene does not represent significant fractionation between LREE and HREE. The spidergram also shows the similarity pattern of both Cpx types, but having higher value and positive anomalies of Nb, Rb, and Zr. High concentration of REE in Ti-Al – high Cpx positive correlation to titanium and aluminium contents, especially the enrichment of LREE and HFSE represents metasomatism in the formation of the magma. However, it must be realize that REE concentration of Cpx of Bằng Phúc rocks are significantly lower than Cpx of N-Q alkaline basalts, especially the REE concentration of Ti-Al – low Cpx exceeds Chondrite in about 5-6 times.

REE and trace element patterns of calcite from H-1538 and H-1539 samples in general are similar to those of the above described pyroxenes: enrichment of LREE in comparison to HREE, slightly Eu negative anomaly (H-1538) and positive anomaly of Gd (H-1539). Calcite accompanying Ti-Al – low Cpx (depleted in REE) has lower concentration of REE than the one accompanying Ti-Al – high Cpx (Fig. 5, Table 7), and the total REE concentration of calcite is lower than that of Cpx. This significance implies that calcite is primary mineral of the magma; like pyroxene, it is not a REE-hosted mineral. The La/Sm, Ce/Yb and Nb/Ta ratios of calcite vary from 11.2 to 15.45, 10.2 to 18.1, and 17.1 to 70, respectively.

The REE and trace element significance of pyroxene and calcite shows that they are derived from the same source, and the formation of different rock types is caused by magmatic differentiation.

The REE and trace elements in apatite and garnet is only analyzed in sample H-1538 (Table 2), which show that apatite is the major source of REE and trace element enrichment of the hosted rock, with total REE value up to 9703 ppm., Sr (1101 ppm), Y (1260 ppm), Th (247 ppm), and U (43,4 ppm) but depleted in Zr (9.4 ppm), Nb (0.46 ppm), Ba (24 ppm), Ta (0.01 ppm). Negative anomalies of Zr, Ta on primitive mantle- normalized spidergram are significant (Fig.5). REE pattern of garnet is characterized by enrichment of LREE in comparing to HREE, along with negative anomaly of Eu (Fig.5) that is not typical for garnet, because normal garnet must be enriched in HREE in comparing to LREE.

DISCUSSION AND CONCLUSIONS

The petrochemical research allows to classify the alkaline mafic association of Chợ Đồn area into two following types: 1/ titanium-low, alkali-low gabbro and foid-free syenite; and 2/ alkaline gabbro (and pyroxenite) rocks (meltegite, malignite,..) and nepheline-biotite syenite (miaskite) [Trần Trọng Hoà et al, 2004, this issue]

The chemical composition of pyroxene amphibole and biotite of the first type implies that they are products of mantle- derived magma, which is contaminated by crustal material according to trace elements and isotopic significances. The composition of pyroxene from gabbro to monzonite shows the increase of iron, along with the decrease of magnesium, which means from diopside rich toward hedenbergite. This trend could probably represent the sequence of magma differentiation.

The mineral composition of pyroxene syenite and monzonite also reflects the subalkalinity, showing that they were crystallized from a crust-contaminated deep magma. The geochemical significance of gabbro and syenite is typical for subduction-related magma [Trần Trọng Hoà et al., this issue], representing by the negative anomalies of Nb, Ta, Zr, Hf, and LREE-rich pattern, similar to calc-alkaline rocks. The similar syenite is common for post-collision magma, due to the extension on continental arc.

The complex aggregation and chemical composition of minerals in nepheline- bearing alkaline rocks of Bằng Phúc area express their complicated origin and forming conditions. The presence of distinct pyroxene in alkaline gabbro-pyroxenite (high and low in Ti-Al) probably is the differentiation, rather than derived from distinct sources. Moreover, REE and trace element characteristics of clinopyroxene, calcite, apatite, and garnet clearly indicate the orthomagmatic-source. The enrichment of Wo component in Ti-Al – high Cpx, the presence of primary calcite and the popularity of sphene in melteigite, malignite as well are important evidences of the contamination-assimilation of carbonate by nepheline syenite at the secondary magmatic chamber. The combination of nepheline syenite and marble at Bằng Phúc is probably support to this conclusion. Research on geochemical significances, carbon and oxygen isotope of calcite in alkaline gabbro-syenite and surrounded graphite-bearing marble could help us to have a better understanding about the formation of the rocks.

In summary, in Chợ Đồn area, there are two gabbro-syenite associations with distinct geochemical significances: Ti-low, alkali-low gabbro-syenite of Khuổi Loỏng massif and alkaline gabbro - nepheline syenite association of Bằng Phúc massif. Clinopyroxene, amphibole and bitotite in Ti-low, alkali-low gabbro and pyroxene-syenite are typical for minerals that were crystallized from a mantle-derived magma, which was significantly contaminated by crustal material. The primary magma of gabbro, alkaline pyroxenite and nepheline syenite of Bằng Phúc massif, according to geochemical significance, was derived from a mantle-contaminated magma too, and significantly affected by carbonate assimilation. The compositional variation of clinopyroxene, amphibole of syenite-gabbro-pyroxenite series is the increase of aluminium, iron and alkalinity, that is opposite to the normal trend. In that case, alkaline gabbro-syenite and nepheline syenite of the Bằng Phúc massif are products of magmatic activities, which are related to post-orogenic extension. After Nguyễn Trung Chí et al., 2003, the forming age of nepheline syenite of Chợ Đồn is c.a. 233 Ma, that could probably coincide to the relaxation between two events: Indosinian (Late Paleozoic – Early Mesozoic) and Yenshanian (Late Mesozoic) at the margin of South China Platform.

Acknowledgement

The authors would like to thank Ph.Ds. Hoàng Hữu Thành, Bùi ấn Niên, especially Prof. Phan Tr­ờng Thị, Ph.D. Nguyễn Trung Chí for fieldwork at Bằng Phúc area, V. Shyrokikh and T. Petrova for sample preparation. This paper is completed with the support of Project "Intraplate magmatism and related mineralization of Việt Nam" and Fundamental Research Project No 71.19.04.

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