ON THE ORIENTED RUTILE NEEDLES IN GARNET OF METAMORPHIC ROCKS: A RESULT OF SOLID-STATE EXSOLUTION?

Although oriented rutile needles in garnet have been reported from several ultrahigh-pressure (UHP) rocks and considered to be important UHP indicators, their crystallographic features including growth habit and lattice correspondences with garnet host have never been properly characterized. Here we report the detailed analytical electron microscopic (AEM) study on oriented rutile needles in garnet of two UHP eclogitic rocks from Sulu, one UHP diamondiferous gneiss from Erzgebirge, and one high-pressure (HP) felsic granulite from Bohemia. For all samples, AEM revealed that rutile needles are oriented along the <111> directions of garnet with the sides bounded by the {110} planes of garnet, and that the growth directions of most needles are close to the normal of {101} planes of rutile. Except for the above gross direction coincidence, no other specific crystallographic orientation relationships between rutile and garnet host were observed. The lack of specific crystallographic orientation relationships between rutile and garnet host, along with the absence of pyroxene phase(s) necessitated by the alleged exsolution reaction of rutile in garnet, do not justify a simple solid-state exsolution scenario for the formation of the rutile needles. It can, however, be rationalized by the cleaving of garnet host under high strain rates followed by rutile deposition healing, with Ti derived either from Ti-bearing garnet or from an external source. Under the case of an external Ti source, however, the amount of oriented rutile needles in garnet can not provide estimations of pressure conditions.