Gem Sapphires and Zircons from Cenozoic Volcanism of the French Massif Central
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Gem sapphires and zircons from Cenozoic volcanism of the French Massif Central Etienne MEDARD1, Jean-Louis PAQUETTE1, Betrand DEVOUARD1, Julia RICCI2, Pierre BOIVIN1, Eloïse GAILLOU3, Jérôme ROCHAULT1, Manon HARDIAGON1 1Laboratoire Magmas et Volcans, Université Blaise Pascal / CNRS / IRD, Clermont-Ferrand, France ([email protected]) 2IDES, Université Paris Sud / CNRS, Orsay, France; 3Natural History Museum of the Los Angeles County, Los Angeles, California, USA Gem sapphire deposits are commonly associated with basaltic intraplate magmatism. Cenozoic magmatism in the French Massif Central historically provided occurrences of gem sapphires and zircons, episodically mined from the Middle Ages to the early XXth century. These gems are found in various types of deposits: alluvium or colluvium associated with basaltic volcanism (Riou Pezzouliou, Mont Coupet, Limagne); Introduction syenitic enclaves within trachyte (Menoyre); alluvial deposits (of still unknown primary origin) in the Mont-Dore. Exceptional recent finds in 2009 provided gem sapphires up to 5 carat cut stones, collected in placers associated with the Miocene volcanism of Limagne, but much larger stones were mined in the past. All sapphires show common features that reveal their magmatic origin: oscillatory zoning, inclusions of feldspars, zircons, ilmenite, and Nb-rich oxides. The occurrence of zircon inclusions in gem sapphires strongly suggests a common origin for sapphires and zircons, and allows dating of the sapphire deposits. The large number of zircon / sapphire occurences, and the presence of sapphire-bearing enclaves makes the French Massif Central the best place to investigate the origin of those gem minerals. 1. Volcanic sapphires from the Monts-Dore: 2. Timing of sapphires and zircons crystallization insights into sapphire parental melts. and evolution of the Western European Rift A number of sapphire / zircon deposits have been dated using the U-Pb method by Euhedral crystals of sapphires have been collected in a number of streams laser-ablation ICP-MS on gem zircons. Analysis of zircon inclusions in sapphires centered on the Mont-Dore stratovolcano. They are rich in inclusions with the indicates that sapphires and zircons from the same occurence are usually classical paragenesis but do not show any dissolution feature, suggesting that contemporaneous. Sapphire formation occurs at various times, from 19.9 ± 0.4 Ma in they were in equilibrium with the magma that brought them to the surface. the Limagne to 956 ± 11 ka in the Velay (see map). These ages are consistent with They contain abundant glassy melt inclusions, whose composition is indica- those of the local volcanism, and most of them indicate that gem sapphires and tive of the parental melt. Despite a slightly higher Al2O3 content (up to 23.7 zircons crystallized at depth in magma chambers less than a few 100000 years before wt%), melt compositions are consistent with late-stage differenciates of the the volcanism that brought them to the surface. This ages also indicate that zircon alcaline series of the Monts-Dore stratovolcano. The source of these sapphires 2 mm and sapphire formation is a continuous and ongoing process throughout the volcanic as not yet been identified, but they likely grew as phenocrysts from a trachytic history of the French Massif Central. A few single zircons even have preliminary / phonolitic magma, and were brought to the surface by an explosive eruption ages between 25 and 36 Ma, filling gaps in the magmatic history when no magma to explain their broad geographic distribution. Fig. 1. Gem-quality cut sapphires from a recent alluvial find can be observed at the surface nowadays. SAPPHIRE / ZIRCON DEPOSITS (ages in Ma) Bourgogne Menat Alluvial / colluvial from basaltic volcanoes Causse Forez QUAT. Alluvial from differenciated pyroclastites Bas Languedoc PLIOCENE Limagne Syenitic enclaves in trachyte Sioule Bourgogne Sillon Houiller Velay Oriental Cantal Aubrac Cézalier Coiron Devès/Bassin du Puy 2 mm Menat Monts-Dore Escandorgue MIOCENE Chaîne Bas Vivarais PALEOCENE EOCENE OLIGOCENE de la Sioule Chaîne des Puys Fig. 2. Euhedral, inclusion-rich sapphires from the Sioulot stream, Chaîne des Puys north of the Mont-Dore stratovolcano. Inclusions of zircon, ilmenite, Âge (Ma) 65 55 45 3525 20 15 10 5 0 feldspar and Nb-oxydes can be found, as well as glassy melt inclusions. Forez Sillon 2.65 ± 0.05 Fig. 5. Timing of volcanism in the French Massif Central (after Boivin et al. 2009). Yellow stars indicate the sapphire / zircon deposits that have been dated in this work houiller Comté (plus one data from Paquette and Mergoil 2009). See ages on the main map. melt inclusion Mont-Dore 19.9 ± 0.4 Fig. 3. Euhedral sapphire with 0.1919 Limagne triangular growth features and Massif Central Cézallier Phénocristaux Fig. 6. Example of U-Pb dates abundant mineral and melt 0.15 inclusions. Pb obtained on zircon crystals. Most 6.17 ± 0.05 206 zircons plot on an isochron at 20.3 Pb/ 207 Lower Intercept at 0.11 Ma, but a few older zircons can be 20.3 ± 0.5 Ma seen. They probably indicate older Velay-Oriental magmatic activity (Limagne area). 1 mm Cantal Devès 0.07 2.63 ± 0.04 32 28 24 20 238U/206Pb 16 0.957 ± 0.011 0.03 180 220 260 300 340 380 14 12 data-point error ellipses are 2 10 0.24 Pb Menoyre O (wt%) Aubrac 206 2 Pb/ enclave à zircon 8 207 O + K Bas-Vivarais 2 0.20 Na 6 Coiron Fig. 7. U-Pb dating of a zircon- bearing syenite from Menoyre 4 0.16 (Cantal). The syenites occur as 2 Lower Intercept at enclaves in a trachyte dated at 6.17 ± 0.05 Ma 0 0.12 Causses MSWD = 1.5 6.00 +/- 0.08 Ma, slightly youn- 35 40 45 50 55 60 65 70 75 80 85 ger than the syenite enclaves. SiO2 (wt%) 0.08 238U/206Pb 8 7 Fig. 4.Comparison between melt inclusions in sapphires (from the Sioulot 0.04 brook) and bulk rocks of the Monts-Dore alkaline series on a Harker TAS 700 800 900 1000 1100 diagram. Melt inclusionsare similar to the differenciated phonolites / trachytes of the silica-undersaturated alkaline series. AGE OF VOLCANISME Escandorgue Holocene Bas-Languedoc Fig. 8. Rough and cut pale Fig. 9. Alkali-syenite Pliocene enclave from Menoyre blue gem sapphires from the (Cantal) containing Upper Miocene Limagne volcanism. millimeter-sized crystals of sapphire and some Lower Miocene associated zircons. Paleocene Agde 2 mm 0 50 km 3. Sapphire-bearing enclaves from Menoyre: crystallization conditions of gem minerals. 4. Sapphire and zircon transport by alkali basalts The trachytic dome of Menoyre belongs to a series of differenciated Most of the sapphires (including all of the gem-quality sapphires) and zircons have volcanic edifices located directly on the Hercynian basement north of been brought to the surface by basaltic magmas. However, there is clear evidence of the Cantal stratovolcano. Quarries in the trachytic dome have produced disequilibrium textures, including dissolution features on the sapphire’s surface. a variety of dioritic to syenitic enclaves forming a complete plutonic Sapphires from basaltic terranes are also more rounded compared to the sapphires in differenciation series. The least differenciated terms are amphibole- syenites or trachytic magmas. This is clear evidence that basalts are transporting to diorite with a paragenesis identical to the trachyte phenocrysts. More the surface sapphires and zirconthat have previously crystallized from trachytic differenciated enclaves contain zircon and euhedral sapphires, with magmas. The short time interval between sapphire crystallization and basaltic volca- sometimes spinel and sillimanite. Observations suggest that the sap- nism suggests, however, that the two magmatisms are not completely unrelated. phire-bearing enclaves were produced by differenciation of a batch of Fig. 10. Dissolution features on sapphire crystals from colluvial deposits ni the trachytic magma (similar to that of the trachytic dome) which crystal- Velay volcanic province (Gaillou 2003) lized at depth a few 100000 years before the trachytic eruption. Preli- References a b minary thermobarometric investigation of the syenitic inclusions sug- Boivin P. et al. (2009) Volcanologie de la Chaîne des Puys, 5th edition, 42 pages + map. gest crystalization pressures of at least 500 MPa (eutectic crystalliza- Brousse R., Varet J. (1966) Les trachytes du Mont-Dore et du Cantal septentrional et tion of two alkali-felspars, Al-in hornblende barometry for quartz-un- Fig. 11 Cumulate texture in syenite leurs enclaves. Bulletin de la Société Géologique de France 7:8:246-262. 18 xenoliths from Menoyre (a) thin section Gaillou E. (2003) Les saphirs du Massif Central. Unpublished master thesis, Clermont- dersaturated systems) in a water-rich system. Low O in sapphires of an amphibole-clinopyroxene syenite Ferrand University (around 5 per mil, Gaillou et al. 2003) indicate little or no interaction (b) SEM image of a feldspar-rich area Paquette J.-L.et Mergoil-Daniel J. (2009) Origin and U–Pb dating of zircon-bearing with the crust. Sapphires and zircons are thus produced by crystalliza- in a sapphire-bearing syenite. nepheline syenite xenoliths preserved in basaltic tephra (Massif Central, France). Contribution to Minera- tion of a water-rich trachytic melt, derived either by fractionnal crystal- logy and Petrology 158: 245-562. lization of a basaltic mantle melt, or by direct low-degree melting of Ricci J. (2010) Caractérisation de saphirs gemmes provenant d’une nouvelle occurence the mantle. en Auvergne. Unpublished master thesis, Clermont-Ferrand University..