Tso Morari coesite eclogite: pseudosection predictions v. the preserved record and implications for tectonometamorphic models

Abstract

Ultrahigh-pressure eclogites of the Tso Morari area, NW Himalaya (Ladakh, India), have been intensively investigated petrographically and petrologically with surprisingly different results. Metamorphic subduction paths based on mineral isopleths in pressure-temperature pseudosections in some studies claim concave (to the temperature axis) pressure-temperature paths predicting significant Ca-Mg-Fe garnet growth in the lawsonite and glaucophane fields: a prediction at odds with abundant epidote/clinozoisite and sodic-calcic amphibole inclusions in garnet interiors more probable along a convex path. One study deduced strong heating still at high pressures and proposed a felsic diapir rising through the mantle wedge: an explanation strongly at odds with well-documented glaucophane cores to barroisite replacing matrix omphacite requiring a cold exhumation most likely back up the subduction channel. In addition, matrix magnesite rimmed by dolomite suggests pressures well into the coesite (if not diamond) stability field: something neglected in most studies. Despite the application of modern analytical and thermodynamic modelling tools, the peak conditions attained by Tso Morari ultrahigh-pressure rocks are often poorly deduced and at odds with simple observations. Is this problem perhaps hindering the reliable identification of new ultrahigh-pressure terranes?

EBSD‐based criteria for coesite‐quartz transformation

“…In the absence of metamorphic reactions at high pressures, such as in felsic lithologies (e.g. O’Brien, 2018), evidence for subduction to UHPs is entirely dependent on the identification of poorly preserved coesite and the qualitative identification of quartz microstructures after coesite (Chopin, 1984; Smith, 1984). This study defines a set of crystallographic criteria in quartz after coesite, which can be used as a quantitative indicator of UHP conditions.…”

“…The dominantly felsic composition of UHP continental terranes means that standard petrology‐based techniques for determining peak pressures and temperatures are often not possible (e.g. O’Brien, 2018). In such cases, evidence for subduction to UHPs is entirely dependent on the identification of coesite (Chopin, 1984; Smith, 1984).…”

“… Supporting Information 3. Summary of P–T results and proposed P–T paths for the Tso Morari eclogites after O’Brien (2018)…”

Thermodynamic modeling of high-grade metabasites: a case study using the Tso Morari UHP eclogite

“…Many previous studies have applied thermodynamic modeling programs to metabasites using the internally consistent thermodynamic database (e.g., Helgeson et al 1978;Berman 1988;Holland and Powell 1998) and various a-X relations (e.g., Berman 1990;Holland and Powell 1996;Meyre et al 1997;Dale et al 2000Dale et al , 2005Diener et al 2007;Green et al 2007;White et al 2007) to model phase equilibrium and estimate P-T conditions during prograde metamorphism (Konrad-Schmolke et al 2008;St-Onge et al 2013;Hernández-Uribe et al 2018, 2019. In some cases, studies modeling metabasites (Konrad-Schmolke et al 2008;St-Onge et al 2013;Imayama 2014;Hernández-Uribe et al 2019) used a-X relations that were developed originally for modeling felsic rocks (White et al 2007), omitted minor components (e.g., Mn, and Ti), or were not calibrated for ultra-high pressure (UHP) conditions.…”

Formation process of Al-rich calcium amphibole in quartz-bearing eclogites from The Sulu Belt, China

“…However, Si-poor and aluminous amphibole phases (~3.5-3.7 Al apfu and ~5.8-5.5 Si apfu) have been occasionally reported from retrograde eclogites in Alps (e.g., Mogessie et al, 1986;Messiga et al, 1991;Poli, 1991) and Himalaya (e.g., Palin et al, 2014;O'Brien, 2019). They sometimes occur as pseudomorphs after garnet also in coesite eclogites from several localities of the Sulu ultrahigh-pressure (UHP) metamorphic belt, China, such as Rongcheng, Yangkou, and Donghai areas (~3.5 Al apfu and ~5.5 Si apfu: Enami et al, 1993;Yang, 2004;Liu et al, 2009;Yamasaki et al, 2018).…”