This is a subject that is beating my mind latelly. I am working with
UHT granulites from Brazil. It is very commom in these rocks to see
rutile inclusions in Grt, rutile and sillimanite needles in Opx clevage,
and rutile needles in quartz are abundant too
What is the meaning of these oriented needles in Opx and Qtz?
As Opx has very high Al2O3 in these rocks (between 6 to 14 %) we
could imagine that Sil needles represent exsolution os Al2O3 during the
retrograde stage, as well as the Rut needles, what is not easy to prove
and probably it is not the case for Rut needles in Qtz. If we have a
mechanism that could take in account all these features I would guess
that melting of a Ti-biotite producing Sil, Rut, Opx and/or Grt + Melt
can be a good explanation. Rut and Sil grew together with Opx but
crystalographic oriented in the clivage planes and with random
distribution in Grt and Qtz.
It is not an entirely original idea of myself, as I have been
talking about these with some friends latelly, but I will keep their
names as I can share the guilt alone if it is not a good explanation
(but I think it is).