DSpace at VNU: Ar-Ar ages in phlogopites from marble-hosted ruby deposits in northern Vietnam: evidence for Cenozoic ruby fon-nation

Ar–Ar ages in phlogopites from marble-hosted ruby deposits in northern Vietnam: evidence for Cenozoic ruby formation Virginie Garniera,*, Gaston Giulianib,1 , Henri Maluskic,2, Daniel Oh

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

Virginie Garniera,*, Gaston Giulianib,1

, Henri Maluskic,2, Daniel Ohnenstettera,

Tich Vu Vanc,f, Dietmar Schwarzg

a

CRPG/CNRS, UPR 2300, BP 20, 54501 Vandouvre-le`s-Nancy, France

b

IRD and CRPG/CNRS, UPR 2300, BP 20, 54501 Vandouvre, France

c

Laboratoire de Ge´ochronologie, Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier,

Universite´ de Montpellier 2, Place Euge`ne Bataillon, 34095 Montpellier Cedex 05, France

d Institute of Geological Sciences, CNST, Nghia Doˆ, Caˆu Giaˆy, Hanoi, Viet nam

e Vietnam National Gem and Gold Corporation, 91 Dinh Tien Hoang Street, Hanoi, Viet nam

f Laboratory of Geology, Vietnam National University, 90 Nguyen Trai Road, Thanh Xuan, Hanoi, Viet nam

g Gu¨belin 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 the40Ar/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 cooling40Ar/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: S 0 0 0 9 - 2 5 4 1 ( 0 2 ) 0 0 0 6 3 - 3

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

E-mail addresses: vgarnier@crpg.cnrs-nancy.fr (V Garnier), giuliani@crpg.cnrs-nancy.fr (G Giuliani), maluski@dstu.univ-montp2.fr (H Maluski), dohnen@crpg.cnrs-nancy.fr (D Ohnenstetter), pttrinh@ncst.ac.vn (T Phan Trong), pttrinh@ncst.ac.vn (V Hoa`ng Quang), vggc@fpt.vn (L Pham Van), gubelinlab@compuserve.com (D Schwarz).

1

Fax: + 33-3-83-51-17-98.

2

Fax: + 33-4-67-54-73-62.

www.elsevier.com/locate/chemgeo

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 (Scha¨rer et

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

1999; Fan, 2000; Peˆcher 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(Peˆcher 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) 33–49 34

Fig 1 Simplified geological map showing the major tectonic domains of the Red River shear zone (map modified after Phan Trong and Hoa`ng 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 40 Ar/ 39 Ar ages from this study Inset map of Vietnam shown with the locations of Figs 1 and 2

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).

V Garnier et al / Chemical Geology 188 (2002) 33–49 36

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(Scha¨rer et al., 1990, 1994; Leloup et

al., 1993, 1995; Zhang and Scha¨rer, 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(Scha¨rer et al., 1994) The40Ar/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 deformaalterna-tion

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)

40

Ar/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 one40Ar/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 33.5 F 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, and Miocene 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 been mined 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).

V Garnier et al / Chemical Geology 188 (2002) 33–49 38

Table 1

Argon isotopic data for analysed minerals

Steps 40 Ar*/ 39 Ar 36 Ar/ 40 Ar 39 Ar/ 40 Ar 37 Ar/ 39 Ar % Atm Cumulative % 39 Ar Age F 1SD BDL4 phlogopite (J = 0.013428)

Total age = 30.7 F 1.2 Ma KT3b muscovite (J = 0.013428)

Total age = 30.6 F 0.8 Ma MTH1 phlogopite (J = 0.013414)

Total age = 31.0 F 1 Ma MTH3 phlogopite (J = 0.013414)

Total age = 32.2 F 1 Ma VIET8-1 phlogopite (J = 0.014273)

(continued on next page)

Table 1 (continued)

Steps 40Ar*/39Ar 36Ar/40Ar 39Ar/40Ar 37Ar/39Ar % Atm Cumulative %39Ar Age F 1SD VIET8-1 phlogopite (J = 0.014273)

Total age = 33.8 F 0.4 Ma TH1-1a biotite (J = 0.014273)

Total age = 24.4 F 0.4 Ma TH1-1 phlogopite (J = 0.013428)

Total age = 23.2 F 0.6 Ma QC-2a phlogopite (J = 0.013414)

Total age = 21.8 F 0.7 Ma QC-6 phlogopite (J = 0.013428)

Total age = 22.2 F 0.6 Ma MC12 biotite (J = 0.014273)

V Garnier et al / Chemical Geology 188 (2002) 33–49 40

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 of40Ar/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)

(Ma) F 1SD

LO GAM ZONE

Luc Yen ruby mining district

Phu road

RED RIVER SHEAR ZONE

Yen Bai ruby mining district

QUY CHAU SHEAR ZONE

Quy Chau ruby mining district

a

Ruby-bearing marble.

b

Syngenetic phlogopite-bearing ruby.

Table 1 (continued)

Steps 40 Ar*/ 39 Ar 36 Ar/ 40 Ar 39 Ar/ 40 Ar 37 Ar/ 39 Ar % Atm Cumulative % 39 Ar Age F 1SD MC12 biotite (J = 0.014273)

Total age = 22.5 F 0.5 Ma MC21 phlogopite (J = 0.013414)

Total age = 20.7 F 0.7 Ma

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.

V Garnier et al / Chemical Geology 188 (2002) 33–49 42