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AGE AND SIGNIFICANCE OF RUBY-BEARING MARBLE FROM THE RED RIVER SHEAR ZONE, NORTHERN VIETNAM

Virginie Garnier^1,, Daniel Ohnenstetter¹, Gaston Giuliani², Henri Maluski³, Etienne Deloule⁴, Trinh Phan Trong⁵, Long Pham Van⁶ and Vinh Hoàng Quang⁷

¹ CRPG–CNRS, UPR 2300, 15, rue Notre-Dame des Pauvres, BP 20, F–54501 Vandœuvre-lès-Nancy, France
² IRD, Département ME, UR 154, LMTG – Toulouse, and CRPG–CNRS, UPR2300, 15, rue Notre-Dame des Pauvres, BP 20, F–54501 Vandœuvre-lès-Nancy, France
³ Laboratoire de Géochronologie, Institut des Sciences de la Terre, de l’Eau et de l’Espace de Montpellier, Université de Montpellier, 2, Place Eugène Bataillon, F–34095 Montpellier, France
⁴ CRPG–CNRS, UPR 2300, 15, rue Notre-Dame des Pauvres, BP 20, F–54501 Vandœuvre-lès-Nancy, France
⁵ Institute of Geological Sciences, CNST, Nghia Dô, Câu Giây, Hanoi, Vietnam
⁶ Vietnam National Gem and Gold Corporation, 91 Dinh Tien Hoang Street, Hanoi, Vietnam
⁷ Institute of Geological Sciences, CNST, Nghia Dô, Câu Giây, Hanoi, Vietnam

E-mail address: vgarnier{at}crpg.cnrs-nancy.fr

Marble-hosted ruby deposits occur in the Lo Gam tectonic zone along the northeastern flank of the Day Nui Con Voi Range in the Red River Shear Zone, in the northern part of Vietnam. Crystals of zircon included in ruby and spinel from Luc Yen and An Phu deposits were dated in situ by the U–Pb method with an ion microprobe. The patchy zoning of the zircon crystals and the wide range of ages (266–38 Ma) provide evidence for a complex metamorphic history, with at least two main thermal events: (1) Zircon included in spinel crystals probably crystallized during the Permian (256.6 ± 9.4 Ma), with a possible reopening of their U–Pb system in the early Triassic (231.7 ± 5.6 Ma). (2) Ruby formed at about 38 Ma when the Red River Shear Zone was the site of ductile deformation during the peak of metamorphism. The dating of zircon and phlogopite syngenetic with ruby documents the temporal relationship between high-temperature metamorphism and the cooling history of the Red River metamorphic belt. Ruby deposits hosted by marble sequences in central and southeastern Asia seem to be an excellent marker; they allow an interpretation of the timing of the activity of Cenozoic structures linked to the collision between Indian and Eurasian continents.

Keywords: ruby deposits, marble, zircon, in situ U–Pb dating, Cenozoic ages, ion-probe data, electron-microprobe data, cathodoluminescence, northern Vietnam.