EBSD‐based criteria for coesite‐quartz transformation

Abstract

Ultra 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 (true1‐01) and (021).

Thermometry and Microstructural Analysis Imply Protracted Extensional Exhumation of the Tso Morari UHP Nappe, Northwestern Himalaya: Implications for Models of UHP Exhumation

“…Interpretations of the exhumation of the TMN have focused on the lack of evidence for transformation of the quartzo‐feldspathic Puga orthogneiss to eclogite facies mineral assemblages (especially coesite, omphacite, and garnet) and the proximity of low‐viscosity serpentinites (de Sigoyer et al, 2004; Epard & Steck, 2008; see, however, Bidgood et al, 2020, for discussion of microstructural evidence of former coesite). The Puga orthogneiss is the dominant exposed lithology in the TMN, and its relatively low bulk density (<3 g/cm 3 for granitic rocks, even with coesite) would have favored detachment from the subducting slab and facilitated upward movement.…”

“…The orthogneiss commonly contains boudins of eclogitic metabasite that are up to 3 m thick, which are interpreted to represent syn‐granitic mafic dikes (e.g., Epard & Steck, 2008). Thermobarometric studies of these eclogitized boudins have provided evidence for burial of the TMN to at least ~27 kbar (e.g., Bidgood et al, 2020; Pan et al, 2020; Sachan et al, 2004; St.‐Onge et al, 2013). Metasedimentary rocks are also present in the TMN, including intercalated metapelites and metapsammites.…”

Metamorphic diamond from the northeastern margin of Gondwana: Paradigm shifting implications for one of Earth’s largest orogens

“…Exsolution of rutile and apatite in garnet may form over a range of P-T conditions of >1 GPa and >600°C and have commonly been observed in HP/UHP rocks. Radial fractures around quartz grains included in garnets are generally regarded as evidence of former coesite (and hence, UHP conditions) ( 46 ), although no coesite has been identified from the Running River Metamorphics to date, so this interpretation remains speculative.…”

The emplacement, alteration, subduction and metamorphism of metagranites from the Tso Morari Complex, Ladakh Himalaya

“…The presence of palisade quartz microstructures in sample TMG06 is particularly noteworthy as this microstructure has been recently demonstrated to have formed during the coesite-quartz transition (Bidgood et al, 2021) and is a cryptic indicator of ultra-high-pressure metamorphism in felsic rocks.…”