Multi-episodic formation of baddeleyite and zircon in polymetamorphic anorthosite and rutile-bearing ilmenitite from the Chiapas Massif Complex, Mexico

Abstract

Massif-type anorthosite and comagmatic associations of rutile-bearing ilmenitite (RBI) and oxide-apatite-rich amphibolite (OARA) from the Chiapas Massif Complex (CMC) in southeastern Mexico display 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. 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 spatially associated with pristine to partly retrogressed Zr-bearing igneous and metamorphic minerals (e.g., ilmenite, rutile, högbomite and garnet), suggests 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. A high-grade metamorphic event at c. 950 Ma is likely responsible for the formation of coarse-grained rutile (~0.1–10 mm in diameter), srilankite, zircon and garnet with rutile inclusions as well as metamorphic högbomite surrounding Fe-Mg spinel. Zr-in-rutile minimum temperatures suggest >730°C for this event, which may correlate to rutile-forming granulite facies metamorphism in other Grenvillian-aged basement rocks in Mexico and northern South America. A younger generation of baddeleyite exsolution occurred during post-peak cooling of coarse-grained rutile, reflected in rimward Zr depletion and formation of discontinuous baddeleyite coronas. Baddeleyite around rutile was then transformed into zircon possibly during subsequent metamorphism at c. 920 or 620 Ma, resulting from syn-kinematic and contact metamorphism, respectively. Regional metamorphism at c. 450 and 250 Ma extensively overprinted the existing zircon population, especially during the Triassic event, as suggested by a significant presence of zircon with this age. Nearly pristine baddeleyite occurring interstitial to ilmenite yielded an isochron age of c. 232 Ma according to in situ U–Pb secondary ion mass spectrometry (SIMS), suggesting either formation during metamorphic peak conditions or post-peak cooling. Zircon with ages of c. 80–100 Ma in anorthosite is identified for the first time within the CMC and coincides with cooling ages of c. 100 Ma for coarse-grained rutile. This age is similar to those of rocks occurring ~200 km further to the east in Guatemala, which are also bounded to the Polochic fault system but overprinted by eclogite facies metamorphism. A high-pressure event in the southern CMC after 200 Ma, however, is presently unsupported. Although the abundance of rutile and ilmenite is unusually high in the CMC anorthosite assemblage compared with 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.