Abstract
AbstractUltra high pressure (UHP) metamorphism observed in continental terranes implies that continental crust can subduct to ~40 kbar before exhuming to the surface. This process is one of the least understood and widely debated parts of the orogenic cycle. The dominantly felsic composition of UHP continental terranes means that many petrology‐based techniques for determining peak pressures and temperatures are often not possible. In such cases, the detection of UHP conditions depends on the preservation of coesite, a rarely preserved mineral in exhumed UHP terranes as it rapidly transforms to quartz on decompression. Consequently, the qualitative identification of palisade quartz microstructures that form during the retrograde transformation of coesite to quartz is often used to identify UHP terranes. In this study, we conduct electron backscatter diffraction and misorientation analysis of palisade quartz inclusions in the coesite‐bearing pyrope quartzite from the Dora Maira massif in the Alps, and matrix‐scale palisade quartz in the Polokongka La granite from Tso Morari in the Ladakh Himalaya, in order to quantitatively define crystallographic characteristics of quartz after coesite. The repeatability of our observations in two unrelated occurrences of UHP rocks supports our interpretation that the following features provide a systematic and predictable set of criteria to identify the coesite to quartz transition: (1) Quartz crystallographic orientations define spatially and texturally distinct subdomains of palisade quartz grains with ‘single crystal’ orientations defined by distinct c‐axis point maxima. (2) Adjacent subdomains are misorientated with respect to each other by a misorientation angle/axis of 90°/<a>. (3) Within each subdomain, palisade quartz grain boundaries commonly have intra‐ and inter‐granular misorientations of 60°/[0001], consistent with the dauphiné twin law. Our observations imply that the coesite‐quartz transformation is crystallographically controlled by the epitaxial nucleation of palisade quartz on the former coesite grain, specifically on potential coesite twin planes such as (01) and (021).
Highlights
Ultra high pressure (UHP) conditions have been proposed in continental terranes such as the Alps (Chopin, 1984), the Himalaya (Sachan et al, 2004), the Norwegian Caledonides (Smith, 1984) and the Dabie-Sulu region (Okay, Xu, & Sengor, 1989)
electron backscatter diffraction (EBSD), and misorientation analyses of palisade quartz surrounding a relict coesite inclusion in garnet from the Dora Maira Pyrope Quartzite (e.g. Lenze & Stöckhert, 2008; Schertl et al, 1991) and palisade quartz in the matrix of a granite from the Tso Morari complex, in order to investigate the crystallographic relationships of the coesite–quartz transformation
Based on the reoccurrence of systematic quartz crystallographic preferred orientations and grain boundary misorientations observed in both the Dora Maira and Tso Morari examples, we suggest that the nucleation of quartz during the coesite-to-quartz transformation is crystallographically controlled
Summary
Ultra high pressure (UHP) conditions have been proposed in continental terranes such as the Alps (Chopin, 1984), the Himalaya (Sachan et al, 2004), the Norwegian Caledonides (Smith, 1984) and the Dabie-Sulu region (Okay, Xu, & Sengor, 1989). Such occurrences continue to drive debate over the extent and mechanisms by which continental crust may be subducted and exhumed during continental collision With respect to our analysis of the Tso Morari granite, these microstructures support previous studies that argue for subduction of Indian continental crust to UHPs at onset of the Himalayan orogeny (O’Brien, 2006; O’Brien, Zotov, & Law, 1999; St-Onge, Rayner, Palin, Searle, & Waters, 2013)
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