Garnet–chloritoid–kyanite assemblages: eclogite facies indicators of subduction constraints in orogenic belts

Abstract

The assemblage garnet-chloritoid-kyanite is shown to be quite common in high-pressure eclogite facies metapelites from orogenic belts around the world, and occurs over a narrowly restricted range of temperature 550-600°C, between 20 and 25 kbar. This assemblage is favoured particularly by large Al 2 O 3 :K 2 O ratios allowing the development of kyanite in addition to garnet and chloritoid. Additionally, ferric iron and manganese also help stabilize chloritoid in this assemblage. Pseudosections for several bulk compositions illustrate these high-pressure assemblages, and a new thermodynamic model for white mica to include calcium and ferric iron was required to complete the calculations. It is extraordinary that so many orogenic eclogite facies rocks, both mafic eclogites sensu stricto as well as metapelites with the above assemblage, all yield temperatures within the range of 520-600°C and peak pressures 23±3 kbar. Subduction of oceanic crust and its entrained associated sedimentary material must involve the top of the slab, where mafic and pelitic rocks may easily coexist, passing through these P-T conditions, such that rocks, if they proceed to further depths, are generally not returned to the surface. This, together with the tightly constrained range in peak temperatures which such eclogites experience, suggests thermal weakening being a major control on the depths at which crustal material is decoupled from the downgoing slab.

Metamorphic pressure-temperature evolution of garnet-chloritoid schists from the Lake Zone, SW Mongolia

“…The mineral assemblage of kyanite -garnet -chloritoid is common in eclogite facies metapelites in several orogenic belts in the world (Smye et al, 2010). We examined the kyanitebearing garnet -chloritoid schists, and determined their metamorphic conditions, which differ significantly from those previously reported for the eclogites in the same complex (Štípská et al, 2010).…”

First-report on Mesozoic eclogite-facies metamorphism preceding Barrovian overprint from the western Rhodope (Chalkidiki, northern Greece)

“…Kyanite is predicted to be present with garnet + chloritoid at eclogite-facies conditions for Al-rich/Mg-poor bulk compositions but is restricted to the higher temperature for decreasing Al content (Wei and Powell, 2003). Indeed garnet + chloritoid assemblage (often co-existing with kyanite, phengite and rutile) has been found in eclogite-facies metapelites from the Alps (e.g., Smye et al, 2010), the Carpathians (e.g., Negulescu et al, 2009), Norway (e.g., Hacker et al, 2003) and Sardinia (Cruciani et al, 2013).…”

Using white mica40Ar/39Ar data as a tracer for fluid flow and permeability under high‐Pconditions: Tauern Window, Eastern Alps

“…, 1998a, 2001b). Peak metamorphic conditions of 2–2.6 GPa and 570–620 °C have been estimated by a variety of different methods (see Smye et al. , 2010 for a recent summary), followed by poorly constrained blueschist facies metamorphism at 0.7–0.9 GPa or 1.0–1.5 GPa and 450 °C (Holland, 1979; Zimmermann et al.…”

“…, 1989). For the VN, the reference model is based on crystallization of 0.5 mm radius muscovite at 600 °C and 1.3 GPa at 29 Ma, that is, that this nappe reached hotter and deeper metamorphic conditions than the overlying nappes at the time of their Barrovian overprint (Smye et al. , 2010).…”