Growth, annealing and recrystallization of zircon and preservation of monazite in high-grade metamorphism: conventional and in-situ U-Pb isotope, cathodoluminescence and microchemical evidence

Abstract

Zircon and monazite from granulite- to amphibolite-facies rocks of the Vosges mountains (central Variscan Belt, eastern France) were dated by ion-microprobe and conventional U-Pb techniques. Different granulites of igneous (so-called leptynites) and sedimentary origin (kinzigites) and their leucosomes were dated at 334.9 ± 3.6, 335.4 ± 3.6 and 336.7 ± 3.5 Ma (conventional age 335.4 ± 0.6 Ma). Subsequent growth stages of zircon were distinguished by secondary electron (SEM) and cathodoluminescence (CL) imaging: (1) subsolidus growth producing round anhedral morphologies and sector zoning; (2) appearance of an intergranular fluid or melt phase at incipient dehydration melting that first resulted in resorption of pre-existing zircons, followed by growth of acicular zircons or overgrowths on round zircons consisting of planar growth zoning; (3) advanced melting producing euhedral prismatic zircons with oscillatory zoning overgrowing the sector zones. Two further lithologies, the Kaysersberg granite and the Trois-Epis units, were both formerly considered as migmatites. The intrusion of the Kaysersberg granite was dated at 325.8 ± 4.8 Ma. The Trois-Epis unit was found to be the product of volume recrystallization of a former granulite, which occurred under amphibolite-facies conditions 327.9 ± 4.4 Ma ago. The amphibolite-facies overprint of the Trois-Epis zircons led to the complete rejuvenation of most of the zircon domains by annealing and replacement/recrystallization processes. Annealing is assumed to occur in strained lattice domains, which are possibly disturbed by high trace element contents and/or large differences in decay damage between adjacent growth zones. Investigation of cathodoluminescence structures reveals that the replacement occurs along curved chemical reaction fronts that proceed from the surface towards the interior of the zircon. The monazite U-Pb system still records the age of high-grade metamorphism at around 335 Ma. The chemical reagent responsible for the rejuvenation of zircon obviously left the monazite unaffected

Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?

“…dark-CL emission (U-rich), uniform in CL-appearance or sector zoned (e.g. Vavra et al 1996Vavra et al , 1999Schaltegger et al 1999;Rubatto and Gebauer 2000;Corfu et al 2003). This rim type is the most abundant and occurs in amphibolite facies samples as well as samples of restite or melanosome at granulite facies.…”

“…The enhanced CL-emission in this rim type may be related to a purification process (trace element rejection) in the zircon crystal structure (e.g. Schaltegger et al 1999;Vonlanthen et al 2012).…”

“…Roberts and Finger 1997;Vavra et al 1999;Schaltegger et al 1999;Hoskin and Black 2000;Rubatto et al 2001Rubatto et al , 2006Williams 2001;Rubatto 2002;Hermann and Rubatto 2003;Möller et al 2003;Tomaschek et al 2003;McFarlane et al 2005McFarlane et al , 2006Vonlanthen et al 2012;Vorhies et al 2013). Recent studies have improved our understanding of how to interpret metamorphic zircon in the context of the P-T evolution of rocks by modelling zircon growth during metamorphism (e.g.…”

“…In the absence of fluids, metamorphic zircon may form via solid-state (partial) recrystallisation (e.g. Schaltegger et al 1999;Hoskin and Black 2000;Tomaschek et al 2003;Rubatto et al 2006) or by the breakdown of Zr-rich minerals (Fraser et al 1997;Degeling et al 2001;Bingen et al 2004;Ewing et al 2013;Pape et al 2016). …”

“…Reddy et al 2006;Timms and Reddy 2009;Timms et al 2011), and annealing due to crystal lattice strain (e.g. Schaltegger et al 1999).…”