Ar–Ar ages in phlogopites from marble-hosted ruby deposits in northern Vietnam: evidence for...

Ar–Ar ages in phlogopites from marble-hosted ruby deposits in

northern Vietnam: evidence for Cenozoic ruby formation

Virginie Garnier a,*, Gaston Giuliani b,1, Henri Maluski c,2, Daniel Ohnenstetter a,Trinh Phan Trong d, Vinh Hoang Quang d, Long Pham Van e,

Tich Vu Van c,f, Dietmar Schwarz g

aCRPG/CNRS, UPR 2300, BP 20, 54501 Vandouvre-les-Nancy, FrancebIRD and CRPG/CNRS, UPR 2300, BP 20, 54501 Vandouvre, France

cLaboratoire de Geochronologie, Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier,

Universite de Montpellier 2, Place Eugene Bataillon, 34095 Montpellier Cedex 05, FrancedInstitute of Geological Sciences, CNST, Nghia Do, Cau Giay, Hanoi, Viet nam

eVietnam National Gem and Gold Corporation, 91 Dinh Tien Hoang Street, Hanoi, Viet namfLaboratory of Geology, Vietnam National University, 90 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet nam

gGubelin Gemmological Laboratory, 102 Maihofstrasse, CH-6000 Lucerne 9, Switzerland

Received 6 June 2001; accepted 22 March 2002

Abstract

Ruby growth in phlogopite-bearing marbles has been indirectly dated using the 40Ar/39Ar laser stepwise heating technique

on purified syngenetic phlogopite and other micas from ruby deposits in Yen Bai, Luc Yen and Quy Chau mining districts, in

northern Vietnam. The principal results indicate the following. (1) Across the Red River shear zone, the phlogopites from the

Yen Bai deposits yielded Miocene cooling ages between 23.2 and 24.4 Ma identical to those previously published using the

same dating method on magmatic and metamorphic rocks from the Day Nui Con Voi range. (2) Luc Yen ruby deposits in the Lo

Gam zone, on the eastern flank of the Red River shear zone, yielded Oligocene cooling 40Ar/39Ar mica ages between 30.8 and

34.0 Ma. Regarding the age of ruby crystallisation itself, the most plausible hypothesis is that all rubies in both zones formed

during the period 40 to 35 Ma. Diachronism of cooling in adjacent zones leads to the conclusion that around 35 Ma, the ductile

deformation in the Lo Gam zone ended and the ruby-bearing marbles cooled rapidly while the high-temperature deformation

remained in the Red River shear zone, resulting in cooling through blocking temperature, some 15 Ma later. (3) The Quy Chau

ruby deposit is restricted to the Quy Chau shear zone that bounds the eastern part of the Oligocene–Miocene Bu Khang dome.

Phlogopite and biotite samples reveal Miocene cooling ages between 21 and 22.5 Ma which are minimum ages for ruby

formation. These ages could be linked with the end of the extension of the Bu Khang dome. Vietnamese ruby formation is

0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0009 -2541 (02 )00063 -3

* Corresponding author. Tel.: +33-3-83-59-42-42; fax: +33-3-83-51-17-98.

E-mail addresses: [email protected] (V. Garnier), [email protected] (G. Giuliani), [email protected]

(H. Maluski), [email protected] (D. Ohnenstetter), [email protected] (T. Phan Trong), [email protected] (V. Hoang Quang),

[email protected] (L. Pham Van), [email protected] (D. Schwarz).1 Fax: + 33-3-83-51-17-98.2 Fax: + 33-4-67-54-73-62.

www.elsevier.com/locate/chemgeo

Chemical Geology 188 (2002) 33–49

linked to Cenozoic tectonics resulting from continental collision between the Asian and Eurasian plates as for other marble-

hosted ruby deposits in Central and Southeast Asia.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Ruby deposits; Ar–Ar geochronology; Phlogopite; Shear zone; Yen Bai; Luc Yen; Quy Chau; Vietnam

1. Introduction

Ruby is the reddish variety of corundum which

results from the substitution of chromium for alumi-

nium in the crystal structure. Excellent quality rubies

purchased for their intense colour and high trans-

parency are found mainly in marbles from Central

and Southeast Asia. Ruby deposits hosted by marbles

occur in Tadjikistan, Afghanistan, Pakistan, Azad

Kashmir, Nepal, Myanmar, Northern Vietnam and

South China (Hughes, 1997). The chemical and phys-

ical processes governing ruby crystallisation in a host-

rock normally depleted in chromium and aluminium

are still debated, especially regarding the origin of the

mineralizing fluids (Bowersox et al., 2000; Garnier et

al., 2001) as well as the timing of ruby crystallisation

relative to metamorphic and magmatic events (Okrush

et al., 1976; Kammerling et al., 1994; Terekhov et al.,

1999). Ruby mineralisation in marbles indicates fluid

circulation and fluid–rock interaction (Giuliani et al.,

2000). It is also an indicator of specific tectonic and

metamorphic processes related to the formation of

thrusts and shear zones which in Vietnam were

generated during and after continental collision

between the Indian and Eurasian plates (Scharer et

al., 1990; Leloup et al., 1993, 1995; Phan Trong et al.,

1999; Fan, 2000; Pecher et al., 2001). Moreover, the

tectonic structures formed during the initial stages of

collision may be affected by later deformational

events. Therefore, indirect dating of ruby by K-bear-

ing mineral from its host-rock may contribute to a

better understanding of the timing of the different

phases of deformation and to constrain the ages of

fluid circulation in the host-structures. Dating of ruby

deposits contained in marbles has confirmed the

important role played by the continental India/Eurasia

collision for ruby formation in Central and Southeast

Asia. The ages of 16 Ma found in phlogopites from

the Nangimali ruby deposit in Kashmir record a

Neogene cooling linked to the extrusion of the Nanga

Parbat massif and a minimum Miocene formation age

for ruby (Pecher et al., 2001). Cooling ages as young

as 4.6 Ma were found for the Ruyil deposit in Nepal

located in foliated marbles on the border of the Main

Central Thrust (Garnier et al., 2001).

Vietnamese ruby deposits in the northern Indo-

china peninsula are a good example of ruby found

in structures resulting from Cenozoic tectonics. Phan

Trong et al. (1999) defined two distinct ruby districts:

(1) the Yen Bai and Luc Yen mining districts in two

different geological settings outside and within the

Red River shear zone in the Yen Bai Province (Fig. 1),

(2) the Quy Chau mining district, in the Vinh Province

located in the large metamorphic core complex of the

Bu Khang dome, at 200 km Southwest of the Red

River shear zone (Fig. 2). Several questions related to

these two ruby districts arise: Is there a temporal

relationship between them? Do they present a genetic

and dynamic relation? What are the relationships

between their genesis and regional tectonics? Is their

age of formation related to the main geodynamic

events, which acted upon their host rocks? To answer

these questions, we have performed 40Ar/39Ar step-

wise heating experiments on single grains of phlogo-

pite and muscovite extracted from the enclosing host-

rocks of ruby and for the first time on syngenetic

phlogopite crystals associated with ruby. The age data

for each ruby district are compared and discussed in

the light of previous radiometric data obtained on the

metamorphic and magmatic rocks of both tectonic

zones.

2. Regional geology

2.1. Geological settings of the Yen Bai and Luc Yen

mining districts

(1) The corundum deposits of Yen Bai occur within

the high-grade metamorphic gneisses forming the Day

Nui Con Voi range, which extends to the Southeast

from the Ailao Shan in Yunnan. This range is bounded

V. Garnier et al. / Chemical Geology 188 (2002) 33–4934

Fig. 1. Simplified geological map showing the major tectonic domains of the Red River shear zone (map modified after Phan Trong and Hoang

Quang, 1997). The main ruby deposits in the Day Nui Con Voi range and Lo Gam zone are shown as well as the location of samples and the

different 40Ar/39Ar ages from this study. Inset map of Vietnam shown with the locations of Figs. 1 and 2.

V. Garnier et al. / Chemical Geology 188 (2002) 33–49 35

Fig. 2. Geological map of the eastern part of the Bu Khang complex showing the major ruby and sapphire deposits of the Quy Chau mining

district (map modified after Phan, 1991; Geological Survey of Vietnam, 1995; Pham Van Long, unpublished data).


by lateral strike–slip faults forming the major geo-

logical discontinuity in East Asia known as the Ailao

Shan–Red River shear zone (Tapponnier et al., 1986,

1990). The timing of this structure has been mostly

determined in Ailao Shan and was constrained by more

than 20 U/Pb ages obtained on monazite, xenotime,

zircon and titanite from leucogranitic melts and gran-

itoid intrusions (Scharer et al., 1990, 1994; Leloup et

al., 1993, 1995; Zhang and Scharer, 1999) and more

than 100 40Ar/39Ar ages on the metamorphic and

plutonic rocks (Harrison et al., 1992, 1996; Leloup et

al., 1993, 2001; Wang et al., 1998). Felsic magmatism,

lasting from 33 to 22 Ma, was coeval with the high-

temperature tectonic activity of the left-lateral shear

since 35 Ma (Scharer et al., 1994). The 40Ar/39Ar data

showed cooling diachronism in the different ranges

from peak metamorphism in the amphibolite facies to

present-day conditions.

The Day Nui Con Voi range is composed of similar

high-grade metamorphic rocks with sillimanite–bio-

tite–garnet gneisses, micaschists with local alterna-

tion of marbles and amphibolites. The deformation

occurred under amphibolite facies conditions (pres-

sure = 4–6.5 kbar and temperature = 600–750 jC,Phan Trong et al., 1998; Leloup et al., 2001).40Ar/39Ar data indicate that the Day Nui Con Voi

experimented temperatures above 450 jC until 27 Ma,

then started cooling from above 350 jC to below 150

jC between 25 and 22 Ma (Leloup et al., 2001).

Unlike the Ailao Shan range, the Day Nui Con Voi

rocks do not show cooling diachronism.

In the Day Nui Con Voi, corundum occurs (a) in

garnet–sillimanite micaschists and gneisses which

contain leucosome and leucocratic dykes; (b) in

amphibolites converted by the effect of metasomatism

in biotite schists with some layers containing centi-

metre-sized corundum crystals (north of Tan Huong

mine, Fig. 1); (c) in large marble boudins intergrown

with gneiss, micaschist and amphibolite. These mar-

bles represent previous limestones intergrown with

mudstones, which were sheared and metamorphosed

during tectonic activity along the Red River shear

zone.

(2) The ruby deposits of Luc Yen are set in poorly

deformed marble units of Upper Proterozoic–Lower

Cambrian age in the eastern side of the Red River

shear zone: the Lo Gam zone. Ruby occurs as (a)

disseminated crystals within marbles with phlogopite,

dravite, margarite, pyrite, rutile and graphite (Bai Da

Lan, An Phu, Minh Tien, Nuoc Ngap, Luc Yen and

Khoan Thong mines); (b) veinlets associated with

calcite, dravite, pyrite, margarite and phlogopite (An

Phu mine); (c) fissures with graphite, pyrite, phlogo-

pite and margarite (Bai Da Lan mine).

Fig. 3. Scanning Electron Microscopy photomicrographs of phlogopite and ruby intergrowth. (A) Phlogopite (Ph) and apatite (Ap)-solid

inclusions trapped during the growth of the ruby (Ru) crystal (Bai Da Lan mine). (B) Detail of the phlogopite crystal from Fig. 4A. (C) Pyrite

(Py) and phlogopite-solid inclusions co-existing with the formation of primary fluid inclusion cavity in a ruby crystal (Minh Tien mine).


Only one 40Ar/39Ar age constrains up to now the

timing of the shear zone in the Lo Gam zone: a

phlogopite extracted from a ruby-bearing marble in

Luc Yen yielded an age of 33.5F 0.7 Ma (Leloup et

al., 2001).

2.2. Geological setting of the Quy Chau mining district

This area located 200 km south of the Red River

shear zone is occupied by the Bu Khang dome (Fig. 2).

It consists in a broad antiform of Paleozoic and Mes-

ozoic sedimentary and metasedimentary rocks over-

laying a core of micaschists, granitoids, paragneisses

and orthogneisses (Jolivet et al., 1999). The northeast-

ern part of the dome is limited by the major extensional

shear zone of Quy Chau which contains the corundum

deposits. 40Ar/39Ar analyses on micas yielded Oligo-

cene ages (33 to 23 Ma) for rocks from the cover and

the core of the dome, andMiocene ages (22 Ma) for the

syntectonic micas in the sheared rocks (Maluski et al.,

1997; Jolivet et al., 1999). Concordant U/Pb analyses

on monazite and zircon yielded Oligocene crystallisa-

tion ages (26 to 23 Ma) for the granitoid intrusions in

the dome (Nagy et al., 2000).

Rubies and sapphires have beenmined since 1987 in

the primary and placer deposits of Doi Ty, Doi San and

Mo Coi mines (Fig. 2). The mineralisation is contained

in the Quy Chau shear zone and occurs as: (a) dissemi-

Fig. 4. (A) Hand specimen photographs of ruby mineralisation from the Minh Tien mine (sample MTH3, Luc Yen district). Syngenetic growth

of phlogopite (Ph) and ruby (Ru) in a calcitic marble (Cc). (B) Crystallisation of corundum (C) in a biotite micaschist (Bi) within an amphibolite

(sample TH1-1A, near the Tan Huong ruby mine, Yen Bai district). (C) Biotite (Bi)-bearing pegmatite (Pg) of the Mo Coi drill core (sample

MC12, north of the Doi Ty quarry, Quy Chau). (D) Phlogopite metasomatism (Ph) developed at the contact between amphibolite (Am) and

marble (Cc). Sample MC21, Mo Coi drill core (Quy Chau).


Table 1

Argon isotopic data for analysed minerals

Steps 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar % Atm Cumulative % 39Ar AgeF 1SD

BDL4 phlogopite (J = 0.013428)

1 0.821 2.634 0.270 0.131 77.8 1.9 19.8F 29

2 1.295 1.455 0.440 0.370 43.0 8.4 31.1F1.8

3 1.292 2.048 0.305 0.068 60.5 17.3 31.0F 0.8

4 1.347 0.866 0.552 0.075 25.6 44.3 32.3F 0.4

5 1.257 0.357 0.711 0.053 10.5 97.2 30.2F 0.3

6 1.329 0.413 0.660 0.918 12.2 100 31.9F 20.2

Total age = 30.7F 1.2 Ma

KT3b muscovite (J = 0.013428)

1 21.2 0.76 0.036 0.000 22.4 0.0 452F 200

2 2.161 0.764 0.358 0.000 22.6 0.7 51.6F 7.2

3 1.251 0.804 0.608 0.117 23.7 5.4 30.1F1.1

4 1.314 0.214 0.712 0.184 6.30 11.1 31.6F 1.0

5 1.245 0.408 0.705 0.090 12.0 21.5 29.9F 0.5

6 1.304 0.252 0.709 0.266 7.40 25.1 31.3F 0.7

7 1.248 0.284 0.733 0.124 8.40 31.0 30.0F 0.5

8 1.313 0.152 0.727 0.075 4.50 38.8 31.5F 0.9

9 1.269 0.311 0.715 0.078 9.20 50.1 30.5F 0.4

10 1.293 0.215 0.723 0.020 6.30 89.4 31.1F 0.2

11 1.061 0.797 0.720 0.092 23.5 99.9 25.5F 0.6


MTH1 phlogopite (J = 0.013414)

1 3.352 0.017 0.296 0.000 0.5 0.2 79.4F 9.4

2 1.408 0.783 0.545 0.049 23.1 6.0 33.8F 0.6

3 1.257 0.229 0.741 0.014 6.8 11.9 30.2F 0.3

4 1.308 0.089 0.744 0.000 2.6 16.4 31.4F 0.5

5 1.294 0.063 0.758 0.007 1.8 47.1 31.0F 0.1

6 1.281 0.070 0.764 0.006 2.0 99.0 30.7F 0.1

7 0.774 1.632 0.668 0.840 48.2 99.6 18.6F 3.0

8 39.18 2.596 0.005 117.3 76.7 99.7 762F 108

9 0.611 1.947 0.694 0.965 57.5 100 14.7F 5.1

Total age = 31.0F 1 Ma

MTH3 phlogopite (J = 0.013414)

1 1.403 2.218 0.245 0.453 65.5 1.5 33.6F 4.7

2 1.401 1.137 0.473 0.019 33.6 14.2 33.6F 0.7

3 1.336 0.556 0.625 0.009 16.4 77.7 32.0F 0.2

4 1.299 0.218 0.719 0.000 6.4 88.3 31.2F 1.2

5 1.345 0.110 0.718 0.000 3.2 95.4 32.3F 1.1

6 1.383 0.008 0.720 0.000 0.2 100 33.2F 1.5

Total age = 32.2F 1 Ma

VIET8-1 phlogopite (J = 0.014273)

1 0.532 3.013 0.205 0.656 89.0 0.0 13.7F 26

2 1.891 1.506 0.293 0.202 44.5 0.3 48.1F 70

3 2.411 0.539 0.348 0.000 15.9 0.4 61.0F 11.4

4 1.709 0.986 0.414 0.000 29.1 0.7 43.5F 6.8

5 1.301 0.915 0.560 0.063 27.0 1.9 33.2F 1.4

6 1.344 0.440 0.646 0.000 13.0 3.8 34.3F 0.7

(continued on next page)


Table 1 (continued)


VIET8-1 phlogopite (J = 0.014273)

7 1.273 0.406 0.691 0.000 12.0 6.5 32.5F 0.5

8 1.296 0.306 0.701 0.028 9.0 10.3 33.1F 0.6

9 1.336 0.113 0.723 0.015 3.3 22.7 34.1F 0.2

10 1.329 0.056 0.739 0.004 1.6 99.2 33.9F 0.1

11 0.847 1.412 0.687 0.375 41.7 100 21.7F 2.7


TH1-1a biotite (J = 0.014273)

1 0.942 1.033 0.737 0.010 30.5 45.4 24.1F 0.4

2 0.969 0.920 0.751 0.016 27.1 77.5 24.8F 0.6

3 0.974 0.730 0.804 0.041 21.5 87.7 24.9F 1.6

4 0.886 0.541 0.947 0.052 15.9 92.4 22.7F 10

5 0.932 0.006 1.070 0.198 0.1 93.7 23.9F 1.6

6 0.988 0.217 0.946 0.000 6.4 99.9 25.3F 2.3


TH1-1 phlogopite (J = 0.013428)

1 1.804 2.77 0.099 0.534 82.1 0.1 43.2F 4.6

2 1.002 1.42 0.577 0.093 42.1 3.4 24.1F 0.9

3 1.052 0.46 0.820 0.104 13.6 4.7 25.3F 1.1

4 1.014 0.65 0.795 0.698 19.3 5.3 24.4F 1.3

5 0.960 0.92 0.757 0.076 27.2 7.9 23.1F 0.6

6 0.970 0.40 0.907 0.000 11.9 47.6 23.3F 0.1

7 0.966 0.23 0.963 0.000 6.9 84.1 23.3F 0.1

8 0.934 0.23 0.994 0.000 7.0 99.9 22.5F 0.2


QC-2a phlogopite (J = 0.013414)

1 1.892 2.96 0.067 0.069 87.4 0.2 45.2F 14.7

2 1.907 0.02 0.522 0.289 0.46 0.8 45.6F 7.1

3 0.866 1.08 0.787 0.222 31.8 2.9 20.8F 2.3

4 0.896 0.75 0.869 0.000 22.1 7.5 21.6F 1.1

5 0.921 0.38 0.965 0.000 11.1 18.7 22.1F 0.5

6 0.898 0.32 1.009 0.000 9.3 100 21.6F 0.1


QC-6 phlogopite (J = 0.013428)

1 8.203 2.03 0.049 1.482 60.0 0.0 188.5F 39

2 5.283 1.86 0.085 0.384 55.1 0.0 123.6F 47

3 3.671 1.59 0.144 1.885 47.0 0.0 86.8F 20.6

4 1.135 1.99 0.364 2.073 58.7 0.2 27.3F 9.1

5 0.945 1.68 0.533 0.049 49.6 3.3 22.8F 0.7

6 0.901 1.23 0.706 0.014 36.4 8.1 21.7F 0.3

7 0.918 0.19 1.029 0.002 5.5 99.9 22.1F 0.1


MC12 biotite (J = 0.014273)

1 0.860 1.945 0.494 0.012 57.5 44.1 22.0F 0.7

2 0.905 0.390 0.977 0.000 11.6 83.3 23.2F 0.5

3 0.872 0.537 0.963 0.203 15.9 89.9 22.3F 2.7


nated rubies in marbles associated with pyrite and

graphite, (b) in phlogopite-bearing skarns developed

at the contact between granitic pegmatites with marble

and amphibolite, resulting from the hydrothermal alter-

ation of fluids which circulated along the pegmatite

veins, (c) in biotite–garnet–sillimanite gneisses at the

contact with intrusive pegmatites.

3. Samples description

3.1. The ruby mines in the Lo Gam zone, Luc Yen

district

Sample BDL4 is a typical ruby-bearing coarse-

grained white marble located in the southern part of

Table 2

Summary of 40Ar/39Ar ages of analysed minerals from the ruby mining districts of Luc Yen (Lo Gam zone), Yen Bai (Red River shear zone–

Day Nui Con Voi range) and Quy Chau (Quy Chau shear zone)

Locality and mine Sample number Rock type Analysed mineral Plateau age

(Ma)F 1SD

LO GAM ZONE

Luc Yen ruby mining district

Bai Da Lan BDL4 marblea phlogopiteb 30.9F 0.9

Minh Tien MTH1 marble phlogopite 30.8F 1.0

Minh Tien MTH3 marblea phlogopiteb 32.2F 1.0

Minh Tien-An VIET8-1 marble phlogopite 33.8F 0.4

Phu road

Khoan Thong KT3b marble muscovite 30.8F 0.8

RED RIVER SHEAR ZONE

Yen Bai ruby mining district

Tan Huong TH1-1 marblea phlogopite 23.2F 0.6

Tan Huong TH1-1a schista biotiteb 24.4F 0.4

QUY CHAU SHEAR ZONE

Quy Chau ruby mining district

Doi Ty QC2a marblea phlogopite 21.6F 0.7

Doi San QC6 marblea phlogopite 22.1F 0.6

Mo Coi MC12 pegmatite biotite 22.5F 0.5

Mo Coi MC21 schist phlogopite 21.0F 0.7

a Ruby-bearing marble.b Syngenetic phlogopite-bearing ruby.

Table 1 (continued)


MC12 biotite (J = 0.014273)

4 0.891 0.678 0.897 0.250 20.1 95.0 22.8F 0.4

5 0.885 0.592 0.931 0.133 17.5 99.9 22.7F 4.8


MC21 phlogopite (J = 0.013414)

1 1.069 2.349 0.285 0.928 69.4 0.6 25.7F 23

2 0.852 0.586 0.970 0.067 17.3 18.4 20.5F 0.8

3 0.810 0.637 1.001 0.001 18.8 34.2 19.5F 0.1

4 0.898 0.232 1.036 0.000 6.9 92.6 21.6F 0.3

5 0.510 2.021 0.790 0.040 59.7 94.7 12.3F 6.7

6 0.759 0.864 0.980 0.000 25.5 100 18.3F 3.6



Fig. 5. 40Ar/39Ar age spectra of phlogopite and muscovite samples from the ruby deposits of the Luc Yen mining district in the Lo Gam zone.

Ruby mines: BDL—Bai Da Lan; KT—Khoan Thong, MTH—Minh Tien, VIET—road leading to the An Phu mine. Ar–Ar plateau ages are

shown and the arrows indicate the extent of the Ar–Ar plateau.


the Upper Proterozoic–Lower Cambrian formation

(Fig. 1). The marble is characterised by the alternation

of light and dark foliated bands. The light bands are

composed of calcite and dolomite; and the dark bands

are made of graphite, phlogopite, dravite, margarite,

pyrite, ruby and calcite with dolomite. Phlogopite

forms masses mixed with ruby and syngenetic crystals

of micas are observed in some rubies (Fig. 3).

Sample KT3b is a fine-grained grey marble from

the Khoan Thong mine composed of calcite and

dolomite (90%), muscovite (3%), chlorite (4%) and

graphite (3%). Muscovites appear as disseminated

flakes within the carbonates.

Sample MTH1 is a white medium-grained marble

from the Minh Tien mine. It is a typical white marble

composed of calcite and disseminated phlogopite,

with no ruby and graphite.

Sample MTH3 is a ruby-bearing marble from Minh

Tienmine (Fig. 4A). It consists of a coarse-grained lens,

20 cm longand5cm thick, oriented in the foliationplane

of the medium-grained phlogopite-bearingmarble. The

lens is characterisedbygrey calcite, phlogopite,margar-

ite, dravite and pink ruby. Phlogopite formed irregular

nests, 1–3 cm in length, containing rubies. Under the

microscope, ruby crystals show syngenetic phlogopite

trapped along growing zones.

Sample VIET8-1 is a phlogopite–graphite-bearing

marble, located 2 km south of the Minh Tien mine, on

the road going to An Phu ruby deposit (Fig. 1). It is a

white medium-grained marble composed of calcite

with disseminated flakes of phlogopite and graphite.

3.2. The ruby mines in the Red River shear zone, Yen

Bai district

Sample TH1-1 is a medium-grained marble from

the Tan Huong mine. The marble is composed of

calcite with disseminated flakes of phlogopite and

graphite.

Sample TH1-1A is a corundum-bearing biotite

micaschist from an amphibolite collected 2 km west

of the Tan Huong mine (Fig. 1). This rock formed as a

result of K-metasomatism induced by the circulation

of fluids in sheared fractures. Corundum and biotite

crystallisation is synchronous with K-metasomatism

(Fig. 4B).

3.3. The ruby mines in the Quy Chau shear zone

Sample QC2A is a foliated white marble collected

in the Doi Ty quarry at the contact with a granitic

pegmatite vein linked to a fine-grained biotite granite.

It is a phlogopite-bearing medium-grained marble

where ruby has been mined in different prospecting

pits.

Sample QC6 is a phlogopite and graphite-bearing

coarse-grained foliated marble from the Doi San

quarry. Phlogopite, graphite and spinel crystals are

Fig. 6. 40Ar/39Ar age spectra of biotite and phlogopite samples from the ruby and corundum mineralisation of the Tan Huong mine (TH) in the

Red River shear zone. Ar–Ar plateau ages are shown and the arrows indicate the extent of the Ar–Ar plateau.


found either in fissures parallel to the main foliation or

as disseminated flakes within the white marble.

Two samples were extracted from the Mo Coi

drilling MC-7: sample MC12 is a 1-m-thick biotite-

bearing deformed pegmatite vein cross-cutting a fine-

grained biotite granite. The pegmatite is coarse-

grained with quartz, K-feldspar, plagioclase and

coarse biotite up to 7 cm long (Fig. 4C); sample

MC21 is composed of foliated amphibolite and mar-

ble in contact with a fine-grained biotite granite. Each

contact zone, between the marble and the amphibolite

was underlined by fractures filled with phlogopite.

The K-metasomatism of the mafic rocks is occasion-

ally intense and results in phlogopite schists (Fig.

4D).

4. 40Ar/39Ar experimental technique

Several phlogopite, biotite and muscovite grains

were carefully separated by hand picking from mar-

bles, micaschists and pegmatites, and cleaned in ace-

tone in an ultrasonic bath. The mica concentrates were

analysed by X-ray diffraction and all the samples

showed pure mica spectra without traces of alteration.

For laser analyses, single grain samples are wrapped in

Fig. 7. 40Ar/39Ar age spectra for phlogopite and biotite samples from the Quy Chau mining district in the Quy Chau shear zone (Bu Khang

complex). Quarries mines: QC2A: Doy Ty; QC6: Doi San; MC12 and MC21 from the Mo Coi drill core MC-7. Ar–Ar plateau ages are shown

and the arrows indicate the extent of the Ar–Ar plateau.


pure Al-foil packets, loaded into an irradiation canister

together with age monitors. Irradiation with fast neu-

trons was carried out in the McMaster Reactor in

Ontario (Canada) for 70 h. Age monitors used in this

irradiation included 520.4F 1.7 Ma MMHb horn-

blende (Alexander et al., 1978) and 24.21F 0.32 Ma

HD-B1 biotite (Hess and Lippolt, 1994). Single grain40Ar/39Ar stepwise heating analysis was carried out

using a LEXEL 3500 continuous wavelength 6 W

argon-ion laser. Five argon isotopes were measured

using a MAP 215-50 mass spectrometer equipped with

a Nier source and a JOHNSTON MM1 electron multi-

plier at the University of Montpellier (France). Meas-

ured Ar isotopes were corrected for blanks, atmos-

pheric contamination, mass discrimination, irradiation

induced Ar isotopes and radioactive decay of 37Cl and39Ar. Age calculation was achieved using constants

recommended by Steiger and Jaeger (1977) and

McDougall and Harrison (1988). Reported errors are

1 sigma (1r) for plateau and total ages, which include

analytical uncertainties and age monitors. Errors are

calculated following McDougall and Harrison (1988).

The strict criteria of a plateau fraction are not definite

in this study because all spectra exhibit more than 88%

of released argon-forming plateau corresponding to

clustered ages, less than 5% difference.

5. Results

Argon isotopic results for analysed minerals are

given in Table 1 and a summary of Ar–Ar ages are

listed in Table 2. 40Ar/39Ar laser stepwise heating

analysis of individual phlogopite, muscovite and bio-

tite grains yielded uniform and remarkable flat Ar–Ar

age spectra (Figs. 5A–E, 6A–B, 7A–D). Each of the

11 age spectra do not indicate the presence of excess

argon component or any diffusive loss and yielded an

age plateau consisting of 88% to 100% of the total 39Ar

released.

6. Discussion

The calculated 40Ar/39Ar ages obtained from the

marble-hosted ruby deposits show some significant

differences. The geological significance and metallo-

genetic implications will be discussed in the light of

thermochronology and geodynamic timing for each

area.

6.1. Red River area

The Ar–Ar ages can be divided in two different

groups: a first group of Miocene ages (from 23.2F 0.6

to 24.4F 0.4 Ma) from Day Nui Con Voi samples, in

the high-grade metamorphic gneisses; a second group

of Oligocene ages (from 30.8F 1.0 to 33.8F 0.4 Ma)

from Lo Gam zone samples in the relatively unde-

formed Upper Proterozoic–Lower Cambrian marble

unit.

The distribution of the 40Ar/39Ar data in the different

ruby mines from the Yen Bai and Luc Yen mining

districts shows ages which are older in the eastern flank

of the shear (Fig. 1). The Oligocene ages recorded by

phlogopite in the ruby deposits from the Lo Gam zone

are in agreement with the single published Ar–Ar age

at 33.5F 0.7 Ma (Leloup et al., 2001). There is no

difference in age between syngenetic phlogopite asso-

ciated with ruby (30.9F 0.9 to 32.2F 1.0 Ma) and

phlogopite and muscovite disseminated in ruby-free

marbles (30.8F 1.0 to 33.8F 0.4 Ma). These ages

contrast with the apparent 23.2–24.4 Ma Ar–Ar ruby

mineralisation ages in the high-grade metamorphic

gneisses of the shear zone.

The fundamental issue of dating is to determine if

the ages correspond to mineral crystallisation, defor-

mation or cooling?

(1) Day Nui Con Voi range. The deformation

occurred under amphibolite metamorphic conditions

at temperatures between 600 and 750 jC (Leloup et

al., 2001). Furthermore, calcite–graphite isotopic

thermometry in the marbles from the Tan Huong

mine, indicates temperatures of 600–625 jC (Giuliani

et al., 1999). Calcite and graphite are associated with

phlogopite and ruby. As ruby crystallisation is coeval

with the formation of phlogopite, ruby formed above

600 jC. These temperatures are above the given

closure temperatures calculated by Dodson (1973)

and experimental diffusion parameters obtained by

Giletti (1974), Harrison et al. (1985) and Hames and

Bowring (1994), i.e., phlogopite (415F 40 jC), bio-tite (280–360 jC) and white mica (345–435 jC).Therefore, the 23.2–24.4 Ma phlogopite ages repre-

sent cooling ages for micas formed before this time

and consequently minimum ages for corundum for-


mation. These Miocene ages fit with previously pub-

lished Miocene cooling ages for the Red River shear

zone (Harrison et al., 1996; Wang et al., 1998; Leloup

et al., 2001; see Table 3).

(2) Lo Gam zone. The metamorphic conditions are

only constrained by carbon isotope thermometry

applied to coexisting calcite and graphite in ruby-

bearing marbles from throughout the Luc Yen mining

district (Giuliani et al., 1999). Temperatures are be-

tween 630 and 745 jC for the respective mines of Bai

Da Lan (675–700 jC), Minh Tien (630 jC), An Phu

(690 jC), Khoan Thong (651–694 jC) and Nuoc Ngap(745 jC). They are higher than the closure temperatures

of datedminerals. Consequently, the Ar–Ar ages found

for the phlogopites also represent cooling ages and so

far, a minimum age for ruby formation.

Evidence of cooling is found in the primary

fluid inclusions trapped by ruby where diaspore

was determined by Raman spectrometry occurring

as a non-visible film coating the wall of the whole

fluid inclusion (Giuliani et al., 2002). The precip-

itation of diaspore is a consequence of cooling and

is due to the presence of water in the fluid which

reacted with the host corundum at temperatures

lower than 400 jC, following the chemical equili-

brium: 2AlOOH X Al2O3 +H2O.

6.2. Bu Kang complex area

Ar–Ar phlogopite Miocene ages of 21.6F 0.7 and

22.1F 0.6 Ma in the ruby-bearing marbles from the

Doi Ty quarry are similar to those found in the

phlogopite-bearing mafic rocks (21.0F 0.7 Ma) and

the pegmatite (22.5F 0.5 Ma) from the Mo Coi drill

core. These ages agree with the previously published

Ar–Ar ages ranging from 21.4F 0.4 to 22.5F 0.4 Ma

(Maluski et al., 1997; Jolivet et al., 1999) for rocks

from the Quy Chau shear zone (Table 4).

Table 3

Summary of 40Ar/39Ar and U/Pb ages in the Red River shear zone (RRSZ) and the adjacent FanSiPan range and Lo Gam zone in northern

Vietnam

(1) Zhang and Scharer (1999), (2) Leloup et al. (2001), (3) Maluski et al. (2001), (4) Harrison et al. (1996).


Ruby is exclusively found within the shear zone

and so far directly linked to the extensional deforma-

tion of the dome which favoured the circulation of

fluids within themarbles and along the pegmatite veins,

after the intrusion of the granitoids at 26.0 to 23.7 Ma

(Nagy et al., 2000). Ages obtained on phlogopites

correspond to a minimum age for the ruby crystallisa-

tion.

7. Conclusions

(1) Ages of phlogopite crystals syngenetic or

coeval with rubies in the Red River area define two

phases of cooling: (a) the first one occurred during the

Oligocene (30.8 to 34 Ma) in the Lo Gam zone in the

eastern border of the shear zone. (b) The second

during the Miocene (23.2 to 24.4 Ma) is restricted

to the ruby deposits in the Red River shear zone.

(2) Two interpretations can be proposed to explain

the relative ages of ruby formation: (a) two Cenozoic

periods for ruby crystallisation, (b) only one period of

ruby formation around 35–40 Ma when ductile

deformation was active under peak metamorphic

temperature conditions in both zones. Diachronism

of cooling in both adjacent zones leads to the con-

clusion that around 35 Ma, the ductile deformation in

the Lo Gam zone ended and the ruby-bearing marbles

cooled rapidly while the high-temperature deforma-

tion remained in the Red River shear zone.

(3) The Ar–Ar phlogopite Miocene cooling ages

found for the Quy Chau ruby deposit overlap with

those previously obtained from Quy Chau shear zone

(Jolivet et al., 1999). These ages represent minimum

ages for ruby formation and may be linked with the

end of the extension of the Bu Khang dome.

(4) Vietnamese rubies in marbles are directly

linked with the tectonometamorphic activity of Cen-

ozoic structures resulting from the deformation of the

Asian plate under the thrust of India during the

collision. Systematic dating of ruby hosted in marbles

may provide a fundamental clue for the reconstruction

of the timing of the continental collision in Central

and Southeast Asia.

Table 4

Summary of U/Pb, 40Ar/39Ar and Rb/Sr ages in the Bu Khang complex

Bu Khang Complex

Locality Dating

method

Sample

number

Rock type Analysed

mineral

Interpreted

age (Ma)

References

COVER Ar/Ar VN235 marble phlogopite 35.6F 0.4 1

Ar/Ar VN966 marble muscovite 33.6F 0.5 1

Ar/Ar VN967 marble biotite 36.1F1.0 1

COMPLEX Ar/Ar VN9710 micaschist muscovite 23.3F 0.7 1

Ar/Ar VN9717 granite biotite 27.3F 0.5 1

Ar/Ar VN9705 gneiss biotite 26.4F 1.1 1

U/Pb VGS-32 granite monazite 26.0F 0.2 2

U/Pb VGS-33 granite monazite 23.7F 1.6 2

U/Pb VGS-34 granite zircon 23.7F 1.7 2

U/Pb VGS-35 granite allanite 23.7F 1.8 2

Rb/Sr VGS-32 granite biotite 19.8F 0.6 2

Rb/Sr VGS-33 granite biotite 19.6F 0.5 2

QUY CHAU SHEAR ZONE Ar/Ar VN228 granite biotite 21.4F 0.4 1

Ar/Ar VN230 gneiss biotite 22.1F1.0 1

Ar/Ar VN231 marble muscovite 22.4F 0.4 1

Ar/Ar VN9715 gneiss muscovite 22.3F 0.5 1

Ar/Ar VN9708 gneiss biotite 22.5F 0.4 1

Ar/Ar VN9709 orthogneiss biotite 24.5F 0.3 1

Ar/Ar QC2a marble (ruby) phlogopite 21.6F 0.7 this work

Ar/Ar QC6 marble (ruby) phlogopite 22.1F 0.6 this work

Ar/Ar MC12 pegmatite biotite 22.5F 0.5 this work

Ar/Ar MC21 micaschist phlogopite 21.0F 0.7 this work

(1) Jolivet et al. (1999); (2) Nagy et al. (2000) with ages given on 2-sigma confidence.


Acknowledgements

This study was supported by Institut de Recherche

pour le Developpement (IRD), CNRS (CRPG) and by

the Programme International de Cooperation Scienti-

fique between CNRS (INSU) and CNST (Hanoi,

Vietnam). Phan Trong Trinh and Hoang Quang Vinh

are grateful to the national Vietnamese programme of

basic research. Thanks also go to Dr. M. Topliss for

correcting the English. We are also grateful to J.A.

Wartho, R. Rudnick and an anonymous reviewer for

their helpful recommendations and comments for the

final version of the manuscript. Contribution CPPG

1585. [RR]

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