THE EFFECT OF TRAPPED CRYSTALLIZING MELTS ON RUTILE AND CONSEQUENCES FOR ZR-IN-RUTILE THERMOMETRY

Zr-in-Rutile thermometry in granulitic terranes results in a large set of calculated temperatures and is of difficult interpretation.

We report an example of corundum and quartz bearing UHT granulite highly depleted in H₂O, SiO₂, K₂O and LILE supporting the decoupling of the rock in isolated microdomains during cooling from ~950 to ~650°C.

Corundum and quartz coexist in single thin sections but occur in texturally different domains: corundum is observed in grt + pl + spl + rt domains interpreted as the highly depleted counterpart, while quartz occurs in pl + rt ± kfs domains interpreted as crystallized trapped melts.

We recognize three groups of rutile: 1) pro-grade rutile preserving peak temperatures; 2) retro-grade rutile and 3) pro-grade rutile re-equilibrated during cooling.

1- In restitic domains we observe texturally homogeneous rutile (with minor inclusions) and rutiles with a net of oriented needles of Zr-rich phases; mineral inclusions (e.g. qtz + sil + zrn) and Zr-in-Rutile temperatures (comprised systematically between ~950°C and ~1050°C) support a pro-grade growth of rutile and UHT metamorphism respectively.

2- Crystallized melts are rich in homogeneous rutile with Zr-in-Rutile temperatures comprised between ~650 and ~950°C and chemically different (Zr vs Zr/Hf; Cr vs Nb; Nb, Ta, Nb/Ta, W and U) from rutiles observed in the restitic domains, these are interpreted as post-peak rutiles.

3- Exclusively at the intersection between crystallized melts and restitic domains it is possible to observe homogeneous rutiles with “low” Zr-in-Rutile temperatures (850-650°C) and surrounded by a thin rim of microzircons; these rutiles have inclusions and chemistry compatible with pro-grade rutiles in restitic domains and are interpreted as pro-grade rutiles re-equilibrated (in terms of Zr content) during cooling.

These observations highlight the importance of presence/absence of fluids during cooling for the preservation/re-equilibration of Zr content in rutile and provide a key for the interpretation of Zr-in-Rutile thermometry in granulites. In fact similar differences in rutile texture and chemistry are observed in less depleted granulites without, however, the clear spatial distribution observed in our sample.