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

Chemical Geology 188 (2002) 33 – 49 www.elsevier.com/locate/chemgeo 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 Hoa`ng Quang d, Long Pham Van e, Tich Vu Van c,f, Dietmar Schwarz g a CRPG/CNRS, UPR 2300, BP 20, 54501 Vandouvre-le`s-Nancy, France 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 Guăbelin Gemmological Laboratory, 102 Maihofstrasse, CH-6000 Lucerne 9, Switzerland b Received 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 * 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) Fax: + 33-3-83-51-17-98 Fax: + 33-4-67-54-73-62 0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V All rights reserved PII: S 0 - ( ) 0 - 34 V Garnier et al / Chemical Geology 188 (2002) 33–49 linked to Cenozoic tectonics resulting from continental collision between the Asian and Eurasian plates as for other marblehosted 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 Introduction Ruby is the reddish variety of corundum which results from the substitution of chromium for aluminium in the crystal structure Excellent quality rubies purchased for their intense colour and high transparency 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 physical processes governing ruby crystallisation in a hostrock 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 (Schaărer 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-bearing 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 Indochina 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 stepwise heating experiments on single grains of phlogopite and muscovite extracted from the enclosing hostrocks 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 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 35 Fig 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 40Ar/39Ar ages from this study Inset map of Vietnam shown with the locations of Figs and 36 V Garnier et al / Chemical Geology 188 (2002) 33–49 Fig 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 by lateral strike – slip faults forming the major geological 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 granitoid intrusions (Schaărer et al., 1990, 1994; Leloup et al., 1993, 1995; Zhang and Schaărer, 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 hightemperature tectonic activity of the left-lateral shear since 35 Ma (Schaărer 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 –biotite –garnet gneisses, micaschists with local alternation of marbles and amphibolites The deformation occurred under amphibolite facies conditions (pressure = – 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, 37 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 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 centimetre-sized corundum crystals (north of Tan Huong mine, Fig 1); (c) in large marble boudins intergrown with gneiss, micaschist and amphibolite These marbles 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, phlogopite and margarite (Bai Da Lan mine) Fig 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) 38 V Garnier et al / Chemical Geology 188 (2002) 33–49 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 33.5 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 Mesozoic sedimentary and metasedimentary rocks overlaying 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 Oligocene 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 crystallisation 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 (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 39 Table Argon isotopic data for analysed minerals Steps 40 Ar*/39Ar 36 Ar/40Ar 39 Ar/40Ar 37 Ar/39Ar % Atm Cumulative % 39 Ar Age F 1SD BDL4 phlogopite (J = 0.013428) 0.821 2.634 1.295 1.455 1.292 2.048 1.347 0.866 1.257 0.357 1.329 0.413 0.270 0.440 0.305 0.552 0.711 0.660 0.131 0.370 0.068 0.075 0.053 0.918 77.8 43.0 60.5 25.6 10.5 12.2 1.9 8.4 17.3 44.3 97.2 100 Total age = 30.7 F 1.2 Ma 19.8 F 29 31.1 F 1.8 31.0 F 0.8 32.3 F 0.4 30.2 F 0.3 31.9 F 20.2 KT3b muscovite (J = 0.013428) 21.2 2.161 1.251 1.314 1.245 1.304 1.248 1.313 1.269 10 1.293 11 1.061 0.76 0.764 0.804 0.214 0.408 0.252 0.284 0.152 0.311 0.215 0.797 0.036 0.358 0.608 0.712 0.705 0.709 0.733 0.727 0.715 0.723 0.720 0.000 0.000 0.117 0.184 0.090 0.266 0.124 0.075 0.078 0.020 0.092 22.4 22.6 23.7 6.30 12.0 7.40 8.40 4.50 9.20 6.30 23.5 0.0 0.7 5.4 11.1 21.5 25.1 31.0 38.8 50.1 89.4 99.9 Total age = 30.6 F 0.8 Ma 452 F 200 51.6 F 7.2 30.1 F 1.1 31.6 F 1.0 29.9 F 0.5 31.3 F 0.7 30.0 F 0.5 31.5 F 0.9 30.5 F 0.4 31.1 F 0.2 25.5 F 0.6 MTH1 phlogopite (J = 0.013414) 3.352 0.017 1.408 0.783 1.257 0.229 1.308 0.089 1.294 0.063 1.281 0.070 0.774 1.632 39.18 2.596 0.611 1.947 0.296 0.545 0.741 0.744 0.758 0.764 0.668 0.005 0.694 0.000 0.049 0.014 0.000 0.007 0.006 0.840 117.3 0.965 0.5 23.1 6.8 2.6 1.8 2.0 48.2 76.7 57.5 0.2 6.0 11.9 16.4 47.1 99.0 99.6 99.7 100 Total age = 31.0 F Ma 79.4 F 9.4 33.8 F 0.6 30.2 F 0.3 31.4 F 0.5 31.0 F 0.1 30.7 F 0.1 18.6 F 3.0 762 F 108 14.7 F 5.1 MTH3 phlogopite (J = 0.013414) 1.403 2.218 1.401 1.137 1.336 0.556 1.299 0.218 1.345 0.110 1.383 0.008 0.245 0.473 0.625 0.719 0.718 0.720 0.453 0.019 0.009 0.000 0.000 0.000 65.5 33.6 16.4 6.4 3.2 0.2 1.5 14.2 77.7 88.3 95.4 100 Total age = 32.2 F Ma 33.6 F 4.7 33.6 F 0.7 32.0 F 0.2 31.2 F 1.2 32.3 F 1.1 33.2 F 1.5 VIET8-1 phlogopite (J = 0.014273) 0.532 3.013 1.891 1.506 2.411 0.539 1.709 0.986 1.301 0.915 1.344 0.440 0.205 0.293 0.348 0.414 0.560 0.646 0.656 0.202 0.000 0.000 0.063 0.000 89.0 44.5 15.9 29.1 27.0 13.0 0.0 0.3 0.4 0.7 1.9 3.8 13.7 F 26 48.1 F 70 61.0 F 11.4 43.5 F 6.8 33.2 F 1.4 34.3 F 0.7 (continued on next page) 40 V Garnier et al / Chemical Geology 188 (2002) 33–49 Table (continued) Steps 40 Ar*/39Ar 36 Ar/40Ar 39 Ar/40Ar 37 Ar/39Ar % Atm Cumulative % 39 Ar Age F 1SD VIET8-1 phlogopite (J = 0.014273) 1.273 0.406 1.296 0.306 1.336 0.113 10 1.329 0.056 11 0.847 1.412 0.691 0.701 0.723 0.739 0.687 0.000 0.028 0.015 0.004 0.375 12.0 9.0 3.3 1.6 41.7 6.5 10.3 22.7 99.2 100 Total age = 33.8 F 0.4 Ma 32.5 F 0.5 33.1 F 0.6 34.1 F 0.2 33.9 F 0.1 21.7 F 2.7 TH1-1a biotite (J = 0.014273) 0.942 0.969 0.974 0.886 0.932 0.988 0.737 0.751 0.804 0.947 1.070 0.946 0.010 0.016 0.041 0.052 0.198 0.000 30.5 27.1 21.5 15.9 0.1 6.4 45.4 77.5 87.7 92.4 93.7 99.9 Total age = 24.4 F 0.4 Ma 24.1 F 0.4 24.8 F 0.6 24.9 F 1.6 22.7 F 10 23.9 F 1.6 25.3 F 2.3 TH1-1 phlogopite (J = 0.013428) 1.804 2.77 1.002 1.42 1.052 0.46 1.014 0.65 0.960 0.92 0.970 0.40 0.966 0.23 0.934 0.23 0.099 0.577 0.820 0.795 0.757 0.907 0.963 0.994 0.534 0.093 0.104 0.698 0.076 0.000 0.000 0.000 82.1 42.1 13.6 19.3 27.2 11.9 6.9 7.0 0.1 3.4 4.7 5.3 7.9 47.6 84.1 99.9 Total age = 23.2 F 0.6 Ma 43.2 F 4.6 24.1 F 0.9 25.3 F 1.1 24.4 F 1.3 23.1 F 0.6 23.3 F 0.1 23.3 F 0.1 22.5 F 0.2 QC-2a phlogopite (J = 0.013414) 1.892 2.96 1.907 0.02 0.866 1.08 0.896 0.75 0.921 0.38 0.898 0.32 0.067 0.522 0.787 0.869 0.965 1.009 0.069 0.289 0.222 0.000 0.000 0.000 87.4 0.46 31.8 22.1 11.1 9.3 0.2 0.8 2.9 7.5 18.7 100 Total age = 21.8 F 0.7 Ma 45.2 F 14.7 45.6 F 7.1 20.8 F 2.3 21.6 F 1.1 22.1 F 0.5 21.6 F 0.1 QC-6 phlogopite (J = 0.013428) 8.203 2.03 5.283 1.86 3.671 1.59 1.135 1.99 0.945 1.68 0.901 1.23 0.918 0.19 0.049 0.085 0.144 0.364 0.533 0.706 1.029 1.482 0.384 1.885 2.073 0.049 0.014 0.002 60.0 55.1 47.0 58.7 49.6 36.4 5.5 0.0 188.5 F 39 0.0 123.6 F 47 0.0 86.8 F 20.6 0.2 27.3 F 9.1 3.3 22.8 F 0.7 8.1 21.7 F 0.3 99.9 22.1 F 0.1 Total age = 22.2 F 0.6 Ma MC12 biotite (J = 0.014273) 0.860 0.905 0.872 0.494 0.977 0.963 0.012 0.000 0.203 57.5 11.6 15.9 44.1 83.3 89.9 1.033 0.920 0.730 0.541 0.006 0.217 1.945 0.390 0.537 22.0 F 0.7 23.2 F 0.5 22.3 F 2.7 V Garnier et al / Chemical Geology 188 (2002) 33–49 41 Table (continued) 36 39 0.678 0.592 MC21 phlogopite (J = 0.013414) 1.069 2.349 0.852 0.586 0.810 0.637 0.898 0.232 0.510 2.021 0.759 0.864 Steps 40 Ar*/39Ar MC12 biotite (J = 0.014273) 0.891 0.885 Ar/40Ar Ar/40Ar Ar/39Ar % Atm Cumulative % 0.897 0.931 0.250 0.133 20.1 17.5 95.0 22.8 F 0.4 99.9 22.7 F 4.8 Total age = 22.5 F 0.5 Ma 0.285 0.970 1.001 1.036 0.790 0.980 0.928 0.067 0.001 0.000 0.040 0.000 69.4 17.3 18.8 6.9 59.7 25.5 0.6 18.4 34.2 92.6 94.7 100 Total age = 20.7 F 0.7 Ma 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 alteration of fluids which circulated along the pegmatite veins, (c) in biotite – garnet – sillimanite gneisses at the contact with intrusive pegmatites 37 39 Ar Age F 1SD 25.7 F 23 20.5 F 0.8 19.5 F 0.1 21.6 F 0.3 12.3 F 6.7 18.3 F 3.6 Samples description 3.1 The ruby mines in the Lo Gam zone, Luc Yen district Sample BDL4 is a typical ruby-bearing coarsegrained white marble located in the southern part of Table 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 Minh Tien Minh Tien Minh Tien-An Phu road Khoan Thong BDL4 MTH1 MTH3 VIET8-1 marblea marble marblea marble phlogopiteb phlogopite phlogopiteb phlogopite 30.9 F 0.9 30.8 F 1.0 32.2 F 1.0 33.8 F 0.4 KT3b marble muscovite 30.8 F 0.8 RED RIVER SHEAR ZONE Yen Bai ruby mining district Tan Huong Tan Huong TH1-1 TH1-1a marblea schista phlogopite biotiteb 23.2 F 0.6 24.4 F 0.4 QUY CHAU SHEAR ZONE Quy Chau ruby mining district Doi Ty Doi San Mo Coi Mo Coi QC2a QC6 MC12 MC21 marblea marblea pegmatite schist phlogopite phlogopite biotite phlogopite 21.6 F 0.7 22.1 F 0.6 22.5 F 0.5 21.0 F 0.7 a b Ruby-bearing marble Syngenetic phlogopite-bearing ruby 42 V Garnier et al / Chemical Geology 188 (2002) 33–49 Fig 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 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 Tien mine (Fig 4A) It consists of a coarse-grained lens, 20 cm long and cm thick, oriented in the foliation plane of the medium-grained phlogopite-bearing marble The lens is characterised by grey calcite, phlogopite, margarite, dravite and pink ruby Phlogopite formed irregular nests, 1– 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 km south of the Minh Tien mine, on the road going to An Phu ruby deposit (Fig 1) It is a 43 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 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 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 44 V Garnier et al / Chemical Geology 188 (2002) 33–49 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 biotitebearing deformed pegmatite vein cross-cutting a finegrained biotite granite The pegmatite is coarsegrained with quartz, K-feldspar, plagioclase and coarse biotite up to cm long (Fig 4C); sample MC21 is composed of foliated amphibolite and marble 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) 40 Ar/39Ar experimental technique Several phlogopite, biotite and muscovite grains were carefully separated by hand picking from marbles, micaschists and pegmatites, and cleaned in acetone 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 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 V Garnier et al / Chemical Geology 188 (2002) 33–49 pure Al-foil packets, loaded into an irradiation canister together with age monitors Irradiation with fast neutrons was carried out in the McMaster Reactor in Ontario (Canada) for 70 h Age monitors used in this irradiation included 520.4 F 1.7 Ma MMHb hornblende (Alexander et al., 1978) and 24.21 F 0.32 Ma HD-B1 biotite (Hess and Lippolt, 1994) Single grain 40 Ar/39Ar stepwise heating analysis was carried out using a LEXEL 3500 continuous wavelength 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 multiplier at the University of Montpellier (France) Measured Ar isotopes were corrected for blanks, atmospheric contamination, mass discrimination, irradiation induced Ar isotopes and radioactive decay of 37Cl and 39 Ar Age calculation was achieved using constants recommended by Steiger and Jaeăger (1977) and McDougall and Harrison (1988) Reported errors are 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 Results Argon isotopic results for analysed minerals are given in Table and a summary of Ar – Ar ages are listed in Table 40Ar/39Ar laser stepwise heating analysis of individual phlogopite, muscovite and biotite grains yielded uniform and remarkable flat Ar –Ar age spectra (Figs 5A – E, 6A –B, 7A – D) Each of the 11 age spectra 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 Discussion The calculated 40Ar/39Ar ages obtained from the marble-hosted ruby deposits show some significant differences The geological significance and metallogenetic implications will be discussed in the light of 45 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 23.2 F 0.6 to 24.4 F 0.4 Ma) from Day Nui Con Voi samples, in the high-grade metamorphic gneisses; a second group of Oligocene ages (from 30.8 F 1.0 to 33.8 F 0.4 Ma) from Lo Gam zone samples in the relatively undeformed 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 33.5 F 0.7 Ma (Leloup et al., 2001) There is no difference in age between syngenetic phlogopite associated with ruby (30.9 F 0.9 to 32.2 F 1.0 Ma) and phlogopite and muscovite disseminated in ruby-free marbles (30.8 F 1.0 to 33.8 F 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, deformation 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 (415 F 40 jC), biotite (280 –360 jC) and white mica (345 –435 jC) Therefore, the 23.2– 24.4 Ma phlogopite ages represent cooling ages for micas formed before this time and consequently minimum ages for corundum for- 46 V Garnier et al / Chemical Geology 188 (2002) 33–49 mation These Miocene ages fit with previously published 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 rubybearing marbles from throughout the Luc Yen mining district (Giuliani et al., 1999) Temperatures are between 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 dated minerals 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 Table Summary of Vietnam as a non-visible film coating the wall of the whole fluid inclusion (Giuliani et al., 2002) The precipitation 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 equilibrium: 2AlOOH X Al2O3 + H2O 6.2 Bu Kang complex area Ar – Ar phlogopite Miocene ages of 21.6 F 0.7 and 22.1 F 0.6 Ma in the ruby-bearing marbles from the Doi Ty quarry are similar to those found in the phlogopite-bearing mafic rocks (21.0 F 0.7 Ma) and the pegmatite (22.5 F 0.5 Ma) from the Mo Coi drill core These ages agree with the previously published Ar – Ar ages ranging from 21.4 F 0.4 to 22.5 F 0.4 Ma (Maluski et al., 1997; Jolivet et al., 1999) for rocks from the Quy Chau shear zone (Table 4) 40 Ar/39Ar and U/Pb ages in the Red River shear zone (RRSZ) and the adjacent FanSiPan range and Lo Gam zone in northern (1) Zhang and Schaărer (1999), (2) Leloup et al (2001), (3) Maluski et al (2001), (4) Harrison et al (1996) V Garnier et al / Chemical Geology 188 (2002) 33–49 Table Summary of U/Pb, 40 47 Ar/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 Ar/Ar Ar/Ar Ar/Ar Ar/Ar Ar/Ar U/Pb U/Pb U/Pb U/Pb Rb/Sr Rb/Sr Ar/Ar Ar/Ar Ar/Ar Ar/Ar Ar/Ar Ar/Ar Ar/Ar Ar/Ar Ar/Ar Ar/Ar VN235 VN966 VN967 VN9710 VN9717 VN9705 VGS-32 VGS-33 VGS-34 VGS-35 VGS-32 VGS-33 VN228 VN230 VN231 VN9715 VN9708 VN9709 QC2a QC6 MC12 MC21 marble marble marble micaschist granite gneiss granite granite granite granite granite granite granite gneiss marble gneiss gneiss orthogneiss marble (ruby) marble (ruby) pegmatite micaschist phlogopite muscovite biotite muscovite biotite biotite monazite monazite zircon allanite biotite biotite biotite biotite muscovite muscovite biotite biotite phlogopite phlogopite biotite phlogopite 35.6 F 0.4 33.6 F 0.5 36.1 F 1.0 23.3 F 0.7 27.3 F 0.5 26.4 F 1.1 26.0 F 0.2 23.7 F 1.6 23.7 F 1.7 23.7 F 1.8 19.8 F 0.6 19.6 F 0.5 21.4 F 0.4 22.1 F 1.0 22.4 F 0.4 22.3 F 0.5 22.5 F 0.4 24.5 F 0.3 21.6 F 0.7 22.1 F 0.6 22.5 F 0.5 21.0 F 0.7 1 1 1 2 2 2 1 1 1 this this this this COMPLEX QUY CHAU SHEAR ZONE work work work work (1) Jolivet et al (1999); (2) Nagy et al (2000) with ages given on 2-sigma confidence Ruby is exclusively found within the shear zone and so far directly linked to the extensional deformation of the dome which favoured the circulation of fluids within the marbles 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 crystallisation 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 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 (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 Cenozoic 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 48 V Garnier et al / Chemical Geology 188 (2002) 33–49 Acknowledgements This study was supported by Institut de Recherche pour le De´veloppement (IRD), CNRS (CRPG) and by the Programme International de Coope´ration Scientifique 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] References Alexander, E.C., Mickelson, G.M., Lanphere, M.A., 1978 MMHB1: a new 40Ar/39Ar dating standard U S Geol Surv Open-File Rep – 8, 78 – 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marbles has confirmed the important role played by the continental India/Eurasia... 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