A BILLION-YEAR ODYSSEY: MULTI-EPISODIC FORMATION OF METAMORPHIC BADDELEYITE AND ZIRCON IN MAFIC ROCKS FROM THE CHIAPAS MASSIF COMPLEX, MEXICO

Although mafic igneous rocks rarely bear primary zircon, it commonly emerges as an accessory component after metamorphism. The Zr sources for the crystallization of neoformed zircon have been linked either to protolith-inherited zircon itself (protocryst) or to the release of Zr resulting from exsolution processes or breakdown reactions from magmatic or metamorphic Zr-bearing minerals (e.g., rutile, ilmenite, garnet). In massif-type anorthosites and comagmatic associations of rutile-bearing ilmenitite (RBI) from the Chiapas Massif Complex (CMC) in southeastern Mexico, a protracted billion-year accessory mineral record encompassing magmatic crystallization at c. 1.0 Ga to recent ductile shear deformation at c. 3.0 Ma is displayed. Multiple discrete zircon populations between these age end-members resulted from neoformation/recrystallization during local to regional metamorphism that affected the southeastern portion of the CMC. The ubiquitous presence of relict baddeleyite (ZrO₂), along with various zircon generations and textures spatially associated with pristine to partly retrogressed Zr-bearing igneous and metamorphic minerals (e.g., ilmenite, rutile, högbomite, garnet), suggest significant Zr diffusive re-equilibration (exsolution) during slow cooling and mineral breakdown followed by crystallization of baddeleyite. The subsequent transformation of baddeleyite into zircon was likely driven by reaction with Si-bearing fluids in several geochronologically identified metamorphic stages. Strikingly contrasting compositional signatures in coeval zircon from anorthosite (silicate-dominated) and comagmatic RBI (Ti-Fe-oxide-dominated) indicate a major role of fluids locally equilibrating with the rock matrix, as indicated by distinct zircon trace element and oxygen isotopic compositions. Although the abundance of rutile and ilmenite is unusually high in the CMC anorthosite assemblage compared to common igneous rocks, the reactions documented here nonetheless stress the importance of these phases for generating Zr-bearing accessory minerals over a wide range of metamorphic conditions.