On the origin of oriented rutile needles in garnet from UHP eclogites

Abstract

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. This paper presents a detailed analytical electron microscopic (AEM) study on evenly distributed oriented rutile needles in garnet of two eclogitic rocks from Sulu. Some garnet in one UHP diamondiferous quartzofeldspathic rock from the Saxonian Erzgebirge, and in one high-pressure (HP) felsic granulite from Bohemia also contain a few unevenly distributed oriented rutile needles. They have also been studied for the purpose of comparison. Despite different distribution patterns, AEM revealed that all rutile needles are oriented along the AE111ae directions of garnet with their lateral sides surrounded by the {110} planes of garnet, and that the growth directions of most needles are close to the normal of the {101} planes of rutile. No other specific crystallographic orientation relationships between rutile and garnet host were observed, and there is no pyroxene associated with rutile, as necessitated by the precipitation reaction of rutile in garnet as previously proposed. A simple solid-state precipitation scenario for the formation of the rutile needles in garnet in these two eclogitic rocks is not justified. Three alternative mechanisms are considered for the formation of oriented rutile needles: (i) the rutile needles may be inherited from precursor minerals; (ii) the rutile needles may be formed by a dissolutionreprecipitation mechanism; and (iii) the rutile needles may be formed by cleaving and healing of garnet with rutile deposition. None of these mechanisms can fully explain the observations, although the first one is less likely and the third one is preferred. This study presents an example where the presence of oriented/aligned inclusions in minerals does not necessarily imply a precipitation origin.

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

“…With respect to exsolution microtextures in UHP minerals of P-type eclogite, however, it is still controversial with regard to the interpretation of tectonic processes forming them [118][119][120][121] . Although some of them may indicate possible depths of over 200 km for the peridotite formation, it remains to be determined if they experienced crustal processes [6] .…”

Diverse P–T paths of the northern Dabie complex in central China and its reworking in the early Cretaceous

“…1). Although oriented exsolution textures have been commonly considered as evidence for peak UHP metamorphism Zhang and Liou, 1999;Ye et al, 2000b;Chopin, 2003;Zhang et al, 2003), they have recently come under scrutiny as to their suitability as unambiguous marker for peak UHP conditions (Page et al, 2005;Hwang et al, 2007;Yang and Powell, 2008;Zheng, 2008;Liou et al, 2009;Zhang et al, 2009). However, the UHP metamorphic microdiamonds in retro-eclogite relics from Raobazhai, Huangweihe and Baizhangyan (Xu et al, 2003 and UHP microdiamond inclusions in zircon in gneiss from Ta'erhe and in the eclogite relic from Luotian (Liu et al, 2007a,b) (Fig.…”

Pyroxenite xenoliths from Marsabit (Northern Kenya): evidence for different magmatic events in the lithospheric mantle and interaction between peridotite and pyroxenite

“…In cases where rutile is associated with pyroxene it may form by subsolidus exsolution from garnet (e.g., Hwang et al 2007, and references therein). Here, however, rutile also forms composite inclusions with Ti-pargasite (Fig.…”