Simultaneous resetting of the muscovite K‐Ar and monazite U‐Pb geochronometers: a story of fluids

Abstract

International audienceAlthough water is ubiquitous in the continental crust, its effect on geochronometers through mineral reequilibration is rarely taken into account. Herein, we present 40Ar/39Ar analyses on muscovite and U-Pb isotopic data on zircon and monazite from a Variscan syn-tectonic granite from western France. Both the K-Ar in the muscovite and U-Pb in the monazite isotopic systems were hydrothermally reset, whereas the U-Pb radiogenic system in most of the zircons was unaffected and dates the granite emplacement age. Titanium chemical maps obtained on muscovites from various dated samples display a spectacular overprinting of their magmatic zoning resulting from increasing fluid-rock interaction. These results reiterate the need to combine geochronological data with petrological, mineralogical and geochemical studies to accurately interpret ages obtained in this type of geodynamical settings

Tectonic record, magmatic history and hydrothermal alteration in the Hercynian Guérande leucogranite, Armorican Massif, France

“…Among them, the Questembert and Lizio granites ( Fig. 1), that have been dated at 316 ± 3 Ma (Tartèse et al, 2011b) and 316 ± 6 Ma (Tartèse et al, 2011a) respectively, were formed by the partial melting of Upper Proterozoic metasediments, and shared a similar magmatic history, marked by the fractionation of feldspar and biotite together with the zircon and monazite grains included in biotite (Tartèse and Boulvais, 2010).…”

“…Lithos 220-223 (2015) 1-22 They are present throughout the belt in the Bohemian Massif (Förster et al, 1999), in Cornwall (Willis-Richards and Jackson, 1989;Chen et al, 1993), in the Iberian Massif (Capdevila et al, 1973) as well as in the French Armorican and Central Massifs (La Roche et al, 1980;Lameyre, 1980;Bernard-Griffiths et al, 1985;Tartèse et al, 2011aTartèse et al, , 2011b.…”

Magmatic and hydrothermal behavior of uranium in syntectonic leucogranites: The uranium mineralization associated with the Hercynian Guérande granite (Armorican Massif, France)

“…1). Among them, the Lizio and Questembert granites, which were dated at 316 ± 6 Ma (Tartèse et al, 2011a) and 316 ± 3 Ma (Tartèse et al, 2011b), respectively, formed through the partial melting of metasediments (Tartèse and Boulvais, 2010). Some giant quartz veins are also associated with the SASZ.…”

Monazite stability and the maintenance of Th-U-total Pb ages during post-magmatic processes in granitoids and host metasedimentary rocks: A case study from the Sudetes (SW Poland)

“…Compositional al ter ations that may re sult in dis tur bance of the Th-U-Pb sys tem were pre vi ously doc u mented in na ture (Poitrasson et al, 1996(Poitrasson et al, , 2000Townsend et al, 2000;Petrík and Koneèný, 2009;TartÀse et al, 2011) and ex per i ments (Teufel and Hein rich, 1997;Seydoux-Guillaume et al, 2002;Harlov and Hether ing ton, 2010;Hether ing ton et al, 2010;Budzyñ et al, 2011Budzyñ et al, , 2015bHarlov et al, 2011;Wil liams et al, 2011). Ex per i ments at 900°C and 1000 MPa with monazite in the pres ence of an al kali-bear ing fluid Na 2 Si 2 O 5 + H 2 O re sulted in de ple tion of Pb or Th en richment .…”

“…The re-equil i bra tion induced by the in fil tra tion of flu ids into the min eral struc ture can in crease the vol ume dif fu sion and lead to ionic ex change within the de vel oped intracrystalline pores. Such compositional al terations, which af fect monazite dur ing post-mag matic, hy dro thermal and metasomatic pro cesses, have been rec og nized in var ious gra nitic rocks (e.g., Poitrasson et al, 1996Poitrasson et al, , 2000Townsend et al, 2000;Petrík and Koneèný, 2009;TartÀse et al, 2011). Ex per i men tal rep li ca tions of nat u ral pro cesses in dicate that the re-equil i bra tion of monazite via cou pled dis so lution-reprecipitation may sig nif i cantly dis turb the Th-U-Pb system un der low-to high-grade con di tions (Teufel and Hein rich, 1997;Seydoux-Guillaume et al, 2002;Harlov and Hether ington, 2010;Hether ing ton et al, 2010;Budzyñ et al, 2011Budzyñ et al, , 2015bHarlov et al, 2011), which can re sult in the com plete reset ting of the Th-U-Pb sys tem even at 450°C .…”