ESASGD 2016

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Hanoi University of Mining and Geology

ISBN:978-604-76-1171-3

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INTERNA TIONAL CONFERENCES ON EARTH SCIENCES AND SUST AINABLE GEO-RESOURCES DEVELOPMENT

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ISBN: 978-604-76-1171-3

ESASGD 2016 Proceedings of

the ESASGD 2016

Hanoi, November 14, 2016

Session: Geology and Geo-resources (GAG)

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ESASGD 2016

GAG IPE (2016) 076–085

* Corresponding author, e-mail: [email protected], [email protected]

Conference Title: Geology and Geo-resources (GAG)

Evidence of crustal material involvement (source contamination) for the South Nam Xe carbonatites (Northwest Vietnam)

Nguyen Thi Thuy

^1,*

, Hideki Wada

²

, Nguyen Thi Hong Nu

³

1Hue University of Sciences, 77 Nguyen Hue st., Hue city, Vietnam

2Institute of Geosciences, Shizuoka University, Ohya 836, Suruga–ku, Shizuoka 422–8529, Japan

3Dong A University, 33 Xo Viet Nghe Tinh st., Danang city, Vietnam

Abstract

This work presents an investigation on C-O-Sr-Nd-Pb isotopic compositions and trace elements for the South Nam Xe carbonatites (Northwest Vietnam). Trace element composition of the carbonatites shows an enrichment in light rare earth element, Sr and Ba, accompanying with a depletion in high-field strength elements (Nb, Ta, Zr, Hf and Ti). Sr–Nd–Pb isotope data for the carbonatites exhibit relatively narrow ranges: ⁸⁷Sr/⁸⁶Sr(t) = 0.708026–0.708349; ¹⁴³Nd/¹⁴⁴Nd(t) = 0.512250–0.512273; εNd(t) = –6.35 to –6.80; ²⁰⁶Pb/²⁰⁴Pb(t) = 18.26–18.79; ²⁰⁷Pb/²⁰⁴Pb(t) = 15.62–15.66; ²⁰⁸Pb/²⁰⁴Pb(t) = 38.80–

40.18. Heavy oxygen isotopes range from 9.09-11.04 ‰ (in calciocarbonatites) to 12.01-13.26 ‰ (ferrocarbonatites) at relatively constant ¹³C values (δ¹³CV–PDB = –2.74 ‰ to –4.12 ‰). These isotopic compositions reveal that the carbonatites originated from a source contaminated by crustal materials mixing with enriched mantle sources prior to emplacement;

mantle metasomatism might not be the main cause of the distinctive trace element chemistry and the isotopic compositions.

Elevated δ¹⁸O values of the ferrocarbonatites accompanying with abundance of volatile-rich mineral assemblages such as fluorite, rare-earth-bearing fluorcarbonates, hydrous-thorite and sulfur minerals are attributed to hydrothermal subjection at low-temperatures.

Keywords: Crustal involvement, source contamination, carbonatite, South Nam Xe.

1. Introduction

There have many reports on Nam Xe carbonatites such as Vlasov et al. (1961), Thanh et al. (2002), Thanh (2004), Chi et al. (2008)... They all have concluded that the Nam Xe carbonatites include ferrocarbonatite (in north Nam Xe) and calciocarbonatite (in South Nam Xe), with hydrothermal/metasomatic and igneous origins, respectively. Other studies considered the carbonatites to be part of the high–K subvolcanic intrusions of the Pu Sam Cap complex (Huong, 1994; Chi et al., 2003; Chi et al., 2008), and to have a close relationship with Paleogene lamprophyres and lamproites that crop out in this region (Chi et al., 2008). However, the origin and evolution of the carbonatites and, in particular, the nature of their source(s) have not been firmly established, partly because of a lack of trace element and isotope data. This study, based on mineralogical and geochemical characteristics, particularly C–O–Sr–Nd–Pb isotope data of the South Nam Xe carbonatites and related rocks, provides a comprehensive overview on origin of the carbonatites.

2. Geological setting

South Nam Xe carbonatites are located in the northeastern part of the Song Da intracontinental rift in northwest Vietnam (Tien, 1977; Thang, 1986; Yem et al., 1995; Hoa, 2001; Anh et al., 2004; My et al., 2005).

The zone extends from the Vietnam–China border to Ninh Binh coastal area (Vietnam), over a region about 430 km long and 40–80 km wide (Fig. 1), and is bounded by the Song Ma suture to the southwest, the Ha Noi basin to the southeast, and by the Fansipan, Tu Le, and Ailao Shan–Red River Shear Zone to the west (Searle, 2006).

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The basement of the rift is heterogeneous, including Precambrian metamorphic rocks distributed along the southeastern margin of the Fansipan zone, and Lower to Middle Paleozoic formations and Carboniferous to Middle Permian shelf limestones, which are widely distributed in the Song Da basin and the northern Thanh Hoa area.

The South Nam Xe carbonatites including calciocarbonatites and ferrocarbonatites are exposed as dykes or lenses up to several tens of meters wide and tens to hundreds of meters long (Fig. 1) (Thanh et al., 2002; Chi et al., 2008), which intruded Permian limestone and Permian basalts. The dykes are located at the intersection of the Ban Man–Phong Tho Fault and the axis of a syncline developed in Permian sediments (Vlasov et al., 1961).

On the basis of field and age evidence, Thanh et al. (2002) and Chi et al. (2008) concluded that the carbonatites originated from the same parental magma with the silicate rocks. However, trace element compositions as well as Sr-, Nd- and Pb isotopic data of these rocks do not show any apparent linkage (Thuy et al., 2014).

Fig. 1. Geological map of the Song Da structure zone (revised after Leloup et al., 2001; My et al., 2005). Inset:

A - Sketch map showing location of the Song Da zone in northwest Vietnam (modified after Leloup et al., 2001;

Morley, 2007); B - Geological map of Nam Xe area, northwest Vietnam (modified after Chi et al., 2008).

3. Petrography and analytical methods

The four carbonatite samples are as follows: (1) NX315 is coarse-grained with a granular massive texture and contains calcite, pyroxene, biotite, amphibole, and accessory magnetite, apatite, and pyrite (Fig. 2a); (2) NX316 is foliated, fine- to medium-grained, and contains calcite, oriented biotite, fibrous amphibole, and minor magnetite and pyrite (Fig. 2b); (3) NX59 is gray and contains ankerite, calcite, coarse-grained feldspar, apatite, sulfates, and minor amounts of trace-element-bearing minerals (Fig. 2c); and (4) NX76 is pinkish due to the presence of rare earth carbonates and ankerite, and it also contains subordinate sulfates and calcite, and minor monazite, apatite, thorite, and sulfides (Fig. 2d).

Detailed mineralogical and geochemical analyses were performed on a suite of representative samples of carbonatites and related rocks including granite (NX55 and TS401, Fig. 2e), basalt (NX390.1, Fig. 2f), lamprophyre (CP378, Fig. 2g), and limestone (NX386 and NX394, Fig. 2h).

Trace element concentrations were determined by Perkin Elmer Elan DRCII inductively coupled plasma mass spectrometer (ICP–MS) at Kochi Core Center following Ishikawa et al. (2005).

Sr and Nd isotope ratios and Pb isotope ratios were determined using a Thermo Finnigan Trition thermal ionization mass spectrometer (TIMS) and a Thermo Finnigan Neptune multiple collector inductively coupled plasma mass spectrometer (MC–ICP–MS), respectively, at Kochi Core Center following Tanimizu and Ishikawa (2006).

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78 Nguyen Thi Thuy, Hideki Wada, Nguyen Thi Hong Nu/ESASGD 2016

C–O isotopic analysis of the carbonatites and carbonate country rocks was conducted on powder drilled from calcite in the samples. The powder was reacted with phosphoric acid at 60 °C to release CO2 (Wada et al., 1982).

Fig. 2. Hand-samples of the South Nam Xe carbonatites and related rocks: a- Coarse-grained calciocarbonatite (NX315); b - Foliated calciocarbonatite (NX316); c, d- Ferrocarbonatites: c- NX59, d- NX76; e- Pu Sam Cap

granite; f- Permian basalt; g- Coc Pia lamprophyre; h- Triassic limestone.

4. Results

4.1. Trace element chemistry

Rare earth and trace element concentrations of the South Nam Xe carbonatites and related rocks (Pu Sam Cap granite, Coc Pia lamprophyre, Triassic basalt, Permian and Triassic limestones) are summarized in Table 1.

The South Nam Xe carbonatites have very high REE concentrations (REE from 3,400 to 6,100 ppm in calciocarbonatites and from 43,200 to 163,900 ppm in ferrocarbonatites). These REE concentrations are clearly higher than the average REE concentrations of worldwide carbonatites (Woolley and Kempe, 1989). The analyzed calciocarbonatites also show similar light- and middle-REE (La–Ho) concentrations to calciocarbonatite worldwide (Woolley and Kempe, 1989), although the Er, Tm, Yb and Lu concentrations are somewhat higher; the ferrocarbonatites are highly enriched in light- and middle-REE relative to ferrocarbonatite worldwide (Woolley and Kempe, 1989).

The South Nam Xe carbonatites are highly enriched in Sr and Ba as compared with the granite, lamprophyre, basalt, and limestone. Concentrations of Sr (30,800–39,800 ppm in calciocarbonatites and 26,600–58,400 ppm in ferrocarbonatites) and Ba (6,700–11,100 ppm in calciocarbonatites and 30,000–82,500 ppm in ferrocarbonatites) are also distinctly higher than carbonatites worldwide (e.g. Woolley and Kempe, 1989;

Natarajan et al., 1994; Viladkar and Schidlowski, 2000; Gwalani et al., 2009). The ferrocarbonatites are also characterized by very high Th (1,110–2,150 ppm) and U (94–306 ppm) concentrations. HFSE (i.e. Nb, Ta, Zr, Hf, and Ti) concentrations in the carbonatites are depleted relative to Ba, Sr, and REE. The depletion is a common feature of many carbonatites (e.g. Dalton and Blundy, 2000; Rosatelli et al., 2000; Hou et al., 2006).

However, the Nb and Ta concentrations of the carbonatites are not significantly lower than those of the granite, lamprophyre, basalt, and limestone samples. Rb concentrations (5.5–92.6 ppm) of the South Nam Xe carbonatites are similar to those of other global examples of carbonatites (Woolley and Kempe, 1989). Zr, Hf, and Ti concentrations in the carbonatites are distinctly lower than all the other incompatible elements and also lower than the Zr, Hf, and Ti concentrations in most of the country rocks.

4.2. Sr–Nd–Pb isotope systematics

Although it is uncertain whether all the calciocarbonatites and ferrocarbonatites have the same age, an age correction based on the average isochron ages for NX76.1 of 30.8 Ma was applied to all the Sr–Nd–Pb isotope data. Sr–Nd–Pb isotope data for the carbonatites exhibit relatively narrow ranges: ⁸⁷Sr/⁸⁶Sr(t) = 0.708026–

0.708349; ¹⁴³Nd/¹⁴⁴Nd(t) = 0.512250–0.512273; εNd(t) = –6.35 to –6.80; ²⁰⁶Pb/²⁰⁴Pb(t) = 18.26–18.79; ²⁰⁷Pb/²⁰⁴Pb(t)

= 15.62–15.66; ²⁰⁸Pb/²⁰⁴Pb(t) = 38.80–40.18 (Table 2). Four carbonatite samples generally have higher ⁸⁷Sr/⁸⁶Sr(t)

and lower ¹⁴³Nd/¹⁴⁴Nd(t) ratios than the country rocks (Table 2), and are characterized by EMII-like isotope compositions (Fig. 3).

The Pb isotopic data of the carbonatites plot above the Northern Hemisphere Reference Line (NHRL, after Zindler and Hart, 1986) near to EMII composition (Fig. 4). Isotopic compositions of Paleogene carbonatites from the continental collision area of the Pakistan (Tilton et al., 1998) and Dalucao (Hou et al., 2006) are also plotted on the Sr–Nd, Sr–Pb, Nd–Pb and Pb isotope diagrams for comparison (Figs. 3-5). The carbonatites from Pakistan have distinctly lower ⁸⁷Sr/⁸⁶Sr(t) ratios than the South Nam Xe carbonatites and a wider range of

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206Pb/²⁰⁴Pb(t) that are attributed to Pb contamination during ascent of the carbonatitic magma (Tilton et al., 1998). On the other hand, the Dalucao carbonatites are characterized by variable and very low εNd(t) values, i.e. – 6.4 to –18.7, but have ⁸⁷Sr/⁸⁶Sr(t) ratios similar to the South Nam Xe carbonatites. However, the Dalucao carbonatites show marked positive Pb concentration anomalies and their high ²⁰⁷Pb/²⁰⁶Pb ratios due to a significant input of upper crustal material (Hou et al., 2006).

Table 1. Rare earth and trace element compositions (ppm) of carbonatites and related rocks from South Nam Xe.

Ele. JB3 NX315 NX316 NX59 NX76 NX55 TS401 NX390.1 CP378 NX386 NX394

Ti 8,400 110 860 210 28.8 814 1,760 17,400 3,700 3,100 19.4

Ni 36.6 7.76 8.39 24.8 15.9 15.3 1.55 63.2 257 20.9 8.51

Rb 13.7 5.50 92.6 39.9 20.2 67.7 88.2 60.3 210 38.9 0.17

Sr 390 39,900 30,800 26,600 58,400 1,100 5.90 1,100 1,200 3,200 350

Y 23.9 150 140 340 460 4.09 41.3 30.3 19.3 15.1 1.51

Zr 88.1 46.0 1.42 1.48 1.66 95.8 540 250 140 120 0.62

Nb 2.16 2.33 63.3 24.1 3.75 6.89 83.8 26.8 11.1 15.8 0.05

Ba 226 11,100 6,700 30,000 82,500 2,200 58.6 1,400 1,500 68.1 12.3

La 8.18 1,600 840 11,400 50,300 12.9 76.8 35.6 31.0 16.8 0.95

Ce 20.8 2,900 1,600 20,400 79,300 23.9 170 75.6 63.4 35.6 1.13

Pr 3.19 290 160 2,200 7,600 2.66 17.3 9.31 7.57 3.92 0.18

Nd 15.4 980 590 7,400 23,300 9.41 65.6 38.8 30.6 15.2 0.73

Sm 4.16 130 82.9 960 2,100 1.50 11.6 8.13 6.06 2.93 0.14

Eu 1.22 32.8 21.1 210 400 0.46 0.91 2.28 1.47 0.59 0.04

Gd 4.62 76.2 56.7 440 670 0.94 8.95 7.38 4.96 2.96 0.14

Tb 0.67 8.09 5.65 37.6 49.1 0.12 1.33 1.05 0.66 0.43 0.02

Dy 4.58 36.0 34.5 130 160 0.71 8.29 6.62 4.00 2.83 0.15

Ho 0.89 5.95 5.56 14.7 17.9 0.13 1.53 1.19 0.72 0.58 0.03

Er 2.58 12.4 13.8 23.7 25.5 0.39 4.61 3.20 2.10 1.66 0.07

Tm 0.37 1.47 1.60 2.24 1.75 0.07 0.64 0.45 0.29 0.24 0.01

Yb 2.39 9.03 9.83 12.9 11.2 0.46 4.37 2.73 1.90 1.54 0.07

Lu 0.36 1.21 1.25 1.40 1.05 0.08 0.65 0.39 0.28 0.22 0.01

Hf 2.62 0.92 0.15 0.50 0.74 2.83 13.6 6.26 3.60 2.93 0.02

Ta 0.12 0.51 4.88 0.55 0.22 2.77 8.37 1.93 1.16 1.05 0.002

Pb 4.75 61.6 63.1 21.4 29.3 29.3 6.72 6.54 27.3 2.96 0.65

Th 1.27 5.03 1.70 1,100 2,200 4.62 16.1 4.70 8.59 3.73 0.10

U 0.45 1.15 55.2 310 94.0 1.35 2.84 1.17 2.78 3.24 0.77

∑REE 69.4 6,100 3,400 43,200 163,900 53.7 370 190 160 85.5 3.67

Note: JB3–standard sample from Geological survey of Japan (GSJ); NX315–coarse-grained calciocarbonatite; NX316–foliated calciocarbonatite; NX59.1 and NX76.1–ferrocarbonatite; NX386–Trassic limestone; NX394–Permian limestone; NX55–syenite and TS401–granite from the Pu Sam Cap complex, CP378–lamprophyre from the Coc Pia complex; NX390.1–Permian basalt.

0,5120 0,5124 0,5128 0,5132 0,5136

0,701 0,703 0,705 0,707 0,709

143Nd/144Nd(t)

87Sr/⁸⁶Sr(t)

South Nam Xe carbonatite Granite

Lamprophyre Permian basalt Limestone Pakistan carbonatite Dalucao carbonatite

EMII BSE

t=30.8Ma

EMI DM

BSE

t=30.8Ma

EMI HIMU

Fig. 3. ¹⁴³Nd/¹⁴⁴Nd vs. ⁸⁷Sr/⁸⁶Sr isotope ratios from South Nam Xe carbonatites (solid circle), associated rocks and Paleogene carbonatites from Pakistan (Tilton et al., 1998) and Dalucao (Hou et al. 2006). Field of young carbonatites including African carbonatites is taken from Bell and Blenkinsop (1989), Bell and Tilton (2001). The positions of the mantle reservoirs identified by Zindler and Hart (1986): DM– depleted mantle, BSE– bulk silicate Earth, EMI and EMII– enriched mantle, HIMU– mantle with high U/Pb ratio.

4.3. C–O isotope systematics

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Eighteen samples of calcite from the carbonatites and surrounding Permian and Triassic limestones were analyzed for C–O isotopic compositions (Table 3). The South Nam Xe carbonatites are characterized by increasingly heavy oxygen isotopes at relatively constant ¹³C values (δ¹³CV–PDB = –2.74‰ to –4.12‰), whereas δ¹⁸OV–SMOW values exhibit greater variability and define two groups. The lower δ¹⁸O group (9.09–11.04‰) corresponds with the calciocarbonatites, whereas the higher δ¹⁸O group (12.01–13.26‰) corresponds with the ferrocarbonatites. C–O isotope ratios of the South Nam Xe carbonatites plot outside the primary magmatic carbonatite field proposed by Taylor et al. (1967) (Fig. 6). Models of C-O isotopic fractionation developed by Comin Chiaramonti et al. (2005) show that the isotopic compositions of the calciocarbonatites appear in the higher right field of the magmatic evolution, whereas those of the ferrocarbonatite are in the field of fluid- related hydrothermal recrystallization.

15.3 15.5 15.7

16 17 18 19 20 21 22

207Pb/204Pb(t)

(a) EMI

EMII BSE

DM

HIMU

37 38 39 40

16 17 18 19 20 21 22

208Pb/204Pb(t)

206Pb/²⁰⁴Pb(t) (b) ^DM

HIMU

EMI EMII

BSE

Fig. 4. Pb isotopic ratios from South Nam Xe carbonatites, related rocks and Paleogene carbonatites from Pakistan (Tilton et al., 1998) and Dalucao (Hou et al., 2006). DM, BSE, HIMU, EMI, EMII and NHRL as from Zindler and Hart (1986). Symbols as Fig. 3.

Table 2. Sr–Nd isotopic compositions of carbonatite and related rocks from South Nam Xe.

Sample ⁸⁷Sr/⁸⁶Sr(t) εSr(t) ¹⁴³Nd/¹⁴⁴Nd(t) εNd(t) 206Pb/²⁰⁴Pb(t) ²⁰⁷Pb/²⁰⁴Pb(t) ²⁰⁸Pb/²⁰⁴Pb(t)

NX315 0.708193 52.9 0.512265 -6.50 18.70 15.63 39.38

NX316 0.708349 55.1 0.512267 -6.46 18.79 15.63 39.38

NX59 0.708258 53.9 – – 18.26 15.62 38.80

NX76 0.708274 54.1 0.512250 -6.80 18.66 15.64 39.01

NX55 0.706643 30.9 0.512155 -8.65 18.66 15.63 38.97

TS410 0.740117 507 0.512631 0.64 19.33 15.63 39.88

NX390.1 0.707527 43.5 0.512490 -2.11 18.75 15.63 39.42

CP378 0.706043 22.4 0.512486 -2.19 18.60 15.61 38.67

NX386 0.707767 46.9 0.512423 -3.43 20.41 15.70 39.59

NX394 0.707456 42.5 0.512154 -8.66 21.59 15.78 39.02

t = 30.8 Ma; ¹⁴³Nd/¹⁴⁴NdCHUR(t) = 0.512598; ⁸⁷Sr/⁸⁶SrCHUR(t)= 0.704464

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0,702 0,704 0,706 0,708

16 18 20 22 24

87Sr/86Sr(t)

(a)

t=30.8 Ma

HIMU EMII

BSE

DM EMI

0,5116 0,5120 0,5124 0,5128 0,5132

16 18 20 22 24

143Nd/144Nd(t)

(b)

t=30.8 Ma

206Pb/²⁰⁴Pb(t) EMII EMI

DM

HIMU BSE

(b)

t=30.8 Ma

206Pb/²⁰⁴Pb(t) EMII EMI

DM

HIMU BSE

Fig. 5. ⁸⁷Sr/⁸⁶Sr vs. ²⁰⁶Pb/²⁰⁴Pb (a) and ¹⁴³Nd/¹⁴⁴Nd vs.²⁰⁶Pb/²⁰⁴Pb (b) diagrams for South Nam Xe carbonatites, related rocks, and Paleogene carbonatites from Pakistan (Tilton et al., 1998) and Dalucao (Hou et al., 2006). DM, BSE, HIMU, EMI, EMII fields from Zindler and Hart (1986). Symbols as Fig. 3.

Table 3. Carbon and oxygen isotopic compositions of carbonatites and limestones from South Nam Xe.

Sample δ¹³C (‰, V–PDB) δ¹⁸O (‰, V–SMOW)

NX315a –3.17 10.54

NX315b –3.16 11.04

NX315A46 –3.93 9.33

NX315A56 –3.55 9.40

NX315M9 –3.15 9.63

NX316a.1 –3.67 9.09

NX316b.1 –3.18 10.28

NX316b.2 –3.33 10.02

NX76.1.HH1 –2.95 12.01

NX76.1.HH3 –3.29 12.74

NX59.2c.1 –4.12 12.80

NX59.2c.2 –3.50 13.26

Permian limestone

NX394 3.77 9.89

NX394.2 3.19 5.81

Triassic limestone

NX382 0.18 24.17

NX383 0.69 22.52

NX385 3.68 27.87

NX387 1.96 24.77

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-8 -4 0 4 8

5 9 13 17 21 25

δ13C V-PDB(‰)

δ¹⁸O V-SMOW(‰)

Coarse-grained calciocarbonatite Foliated calciocarbonatite Ferrocarbonatite Permian limestone Triassic limestone

Primary carbonatites

Sedimentary contamination

Low-T alteration

Fig. 6. δ¹⁸O and δ¹³C plot for calcite from South Nam Xe carbonatites and limestones listed in Table 3. Arrows indicate the main processes responsible for the isotopic variations (after Deines ,1989). “Primary carbonatites” box is after Taylor et al. (1967).

5. Discussions

The South Nam Xe carbonatites are characterized by heavy C–O isotopes, negative εNd(t), and high ⁸⁷Sr/⁸⁶Sr(t)

ratios (Tables 2 and 3). In addition, the Pb isotopic data of the carbonatites plot above the NHRL (Zindler and Hart, 1986) near to EMII composition (Figs. 3-5). This isotopic signature indicates involvement by crustal materials, or mantle metasomatism, and/or hydrothermal alteration at shallow levels.

Crustal materials involvement or mantle metasomatism?

The crustal contribution might occur prior to or during crystallization of the carbonatites. The former is generally attributed to involvement of recycled carbon by subduction (Barker, 1996). In case of the latter, the elevated δ¹³C and δ¹⁸O values might be brought about through high-temperature hydrothermal exchange with, or by assimilation of high-δ¹³C and δ¹⁸O country rocks (Deines, 1989). However, the contamination occurred by assimilation of high-δ¹³C and δ¹⁸O country rocks during emplacement of the South Nam Xe carbonatites can be excluded because of following evidence:

(1) In general terms, interaction with old continental crust could account for the high ⁸⁷Sr/⁸⁶Sr(t) and low εNd(t)

nature of the samples. Nevertheless, the extremely high Sr–Nd concentrations of the South Nam Xe carbonatites (Table 1) mean that it is unlikely that their Sr–Nd isotope ratios (⁸⁷Sr/⁸⁶Sr(t) = 0.708193–0.708349, ¹⁴³Nd/¹⁴⁴Nd(t)

= 0.512250–0.512267) could be affected by country-rock assimilation during ascent of the carbonatites (e.g.

Bell and Blenkinsop ,1989; Natarajan et al., 1994). Additionally, although the Permian and Triassic limestones have elevated δ¹³C and δ¹⁸O values compared with the carbonatites (Table 3), the surrounding country rocks, including the limestone, have higher ¹⁴³Nd/¹⁴⁴Nd(t) and lower ⁸⁷Sr/⁸⁶Sr(t) ratios than the carbonatites (Table 2).

This implies that interaction with the country rocks did not modify the C–O–Sr–Nd isotopic signature of the carbonatites, and the isotopic ratios are thus likely to represent those of the mantle source.

(2) Vlasov et al. (1961) used the geological structure of the Nam Xe area, where Permian basalts partly cover Permian sediments, to infer that a large amount of carbonatitic melt was concentrated beneath the basaltic rocks.

This may have facilitated a sufficiently long magma residence time for the carbonatitic melt to undergo isotopic exchange and contamination with proximal Permian sediments. However, C–O isotope ratios of carbonatites sampled from localities where the Permian sediments are either overlain or not by Permian basalts have similar values. In addition, carbonatitic magma has a very low viscosity (η = 5 10^–2 P), low thermal diffusivity (η = 4

 10^–3 cm²/s) (Treiman and Schedl, 1983; Treiman, 1989), a short residence in the crust (Pyle et al., 1991), and low temperature. As such, it is improbable that the carbonatites have experienced any significant crustal contamination by country rock assimilation during uplift to the surface.

In summary, C-O-Sr-Nd isotopic compositions of the South Nam Xe carbonatites testify that they had been modified before crystallization, and reflect two distinct stages of the carbonatite evolution, i.e. magmatic stage and the subsequent hydrothermal subjection, as supported also by mineral associations and trace element abundances (Thuy et al., 2015).

Some mantle-derived rocks, including carbonatites, possessing high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd isotope ratios associated with LILE enrichment have been interpreted to be the result of mantle metasomatism (e.g.

Holm and Munksgaard, 1982; Eriksson, 1989). These rocks commonly include olivine, clinopyroxene, and some orthopyroxene as alteration products or relics of peridotite rocks. The mantle olivine and pyroxene are generally forsterite, diopside, and enstatite (e.g. Meen et al., 1989; Rudnick et al., 1993; Yaxley et al., 1998;

Martins et al., 2010). Metasomatism may also introduce minor but pervasive amounts of major HFSE hosts such as Ti-rich minerals, including Mg-ilmenite, rutile, and LREE-rich titanates (Jones, 1989). The Mg content of the rocks is also a useful indicator of mantle metasomatism. A number of experimental studies have shown that carbonatitic melts in equilibrium with lithospheric mantle peridotite are generally dolomitic in composition (e.g.

Sweeney, 1994; Yaxley and Green, 1996).

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Although the South Nam Xe carbonatites contain alkaline minerals that diagnostic of metasomatism as the principle accommodators for HFSE (e.g. amphibole and phlogopite; Jones, 1989; Arzamastsev et al., 2001), their TiO2 and MgO contents are fairly low (TiO2 = 0.03–0.13 wt.%, MgO = 0.65–4.72 wt.%) along with the presence of sodic pyroxene (Na2O = 14.05 wt.%, TiO2 = 0.41 wt.%, MgO = 1.36 wt.%, in average) and complete absence of olivine. Additionally, on the multi-element pattern diagram (Fig. 3.9a), the carbonatites illustrate no or negative Nb, Ta (with respect K and La), and Ti (with respect Sm, Tb) indicating a crustal source rather than mantle metasosmatism (cf. Srivastava and Sinha, 2007). Crustal contaminated samples essentially have negative Nb, Ta and Ti anomalies (e.g. Wilson, 1989; Saunders et al., 1992; Winter, 2001). Furthermore, not all metasomatized mantle rocks have low Nd and high Sr–O isotope ratios. For example, clinopyroxene- bearing metasomatized rocks in northern Tanzania have ⁸⁷Sr/⁸⁶Sr ratios of 0.7034–0.7035 (Rudnick et al., 1993);

carbonatites from Sung Valley (northeast India) have ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.70471–0.70489 and 0.51264–0.51266, respectively (Veena et al., 1998); and carbonate-rich melts from Santiago Island (Cape Verde) have ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.70318–0.70391 and 0.51261–0.51287, respectively (Martins et al., 2010). As such, it can be concluded that mantle metasomatism might not be the main cause of the distinctive trace element chemistry and isotopic composition of the South Nam Xe carbonatites.

Hydrothermal alteration at shallow levels?

The hydrothermal overprinted stage corresponds to the ferrocarbonatite, and could be explained by an involvement of meteoric water at low temperatures. Meteoric water in the latitue of study area (~ 22.3^oN) has negative δ¹⁸O values of about –3.2‰ using the GNIP database of the International Atomic Energy Agency , and if such waters were involved during secondary alteration, isotopic exchange must have occurred at temperatures

<200–250 °C in order to raise the δ¹⁸O of the carbonatites (Deines, 1989). A pervasive occurrence of the intermediate compositions of Ba-Sr sulfates and REE minerals in the South Nam Xe ferrocarbonatite, which mainly contains REE carbonate bands and plenty of volatile-bearing minerals indicates that these rocks were formed at low temperatures (< 200 °C). Furthermore, the high ⁸⁷Sr/⁸⁶Sr ratios of the carbonatites could be inherited, to some extent, from meteoric water (⁸⁷Sr/⁸⁶Sr = 0.710). Therefore, shallow-level hydrothermal alteration by meteoric water that infiltrated into the crust may have produced the high δ¹⁸O values (> 12‰) of the ferrocarbonatite (cf. Comin-Chiaramonti, 2005). A secondary process such as hydrothermal leaching could be also accounted for the depletion in HFSE coupled with relatively high δ¹⁸O values (Rosatelli et al., 2000).

The abundance of volatile-rich minerals such as REE carbonates, fluorapatite, fluorite, hydrous thorite, sulfates, and sulfides in the ferrocarbonatite requires involvement of a hydrothermal fluid derived from the carbonatite and represents low-temperature alteration of the calciocarbonatites after emplacement. This process did not impact on δ¹³C values, but it did result in elevated δ¹⁸O values (cf. Fig. 8, after Comin Chiaramonti, 2005).6.

Conclusions

C-O-Sr-Nd-Pb isotopic compositions and trace element concentrations of the South Nam Xe carbonatites indicate their derivation from a crustally influenced source prior to emplacement; mantle metasomatism might not be the main cause of the distinctive trace element chemistry and isotopic composition of the South Nam Xe carbonatites. The source of the carbonatites is characterized by high δ¹³C values of –2.74 to –4.12‰, negative εNd(t) (–6.46 to –6.80), high ⁸⁷Sr/⁸⁶Sr(t) (0.708193–0.708349), low ¹⁴³Nd/¹⁴⁴Nd(t) (0.512250–0.512267) and high Pb isotope ratios. The trace element chemistry of the South Nam Xe carbonatites is notable for its extreme LREE enrichment, Sr–Ba enrichment, and Nb–Ta–Zr–Hf–Ti depletion.

The increased delta-18O values and the enrichment of volatile minerals of the ferrocarbonatites compared with the calciocarbonatites reflect low-temperature interaction with meteoric water and hydrothermal fluids.

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Conference Title: Geology and Geo-resources (GAG)

Evidence of crustal material involvement (source contamination) for the South Nam Xe carbonatites (Northwest Vietnam)

Nguyen Thi Thuy

, Hideki Wada

, Nguyen Thi Hong Nu