Partial melting due to breakdown of an epidote‐group mineral during exhumation of ultrahigh‐pressure eclogite: An example from the North‐East Greenland Caledonides

Abstract

In the North‐East Greenland Caledonides, P–T conditions and textures are consistent with partial melting of ultrahigh‐pressure (UHP) eclogite during exhumation. The eclogite contains a peak assemblage of garnet, omphacite, kyanite, coesite, rutile, and clinozoisite; in addition, phengite is inferred to have been present at peak conditions. An isochemical phase equilibrium diagram, along with garnet isopleths, constrains peak P–T conditions to be subsolidus at 3.4 GPa and 940°C. Zr‐in‐rutile thermometry on inclusions in garnet yields values of ~820°C at 3.4 GPa. In the eclogite, plagioclase may exhibit cuspate textures against surrounding omphacite and has low dihedral angles in plagioclase–clinopyroxene–garnet aggregates, features that are consistent with former melt–solid–solid boundaries and crystallized melt pockets. Graphic intergrowths of plagioclase and amphibole are present in the matrix. Small euhedral neoblasts of garnet against plagioclase are interpreted as formed from a peritectic reaction during partial melting. Polymineralic inclusions of albite+K‐feldspar and clinopyroxene+quartz±kyanite±plagioclase in large anhedral garnet display plagioclase cusps pointing into the host, which are interpreted as crystallized melt pockets. These textures, along with the mineral composition, suggest partial melting of the eclogite by reactions involving phengite and, to a large extent, an epidote‐group mineral. Calculated and experimentally determined phase relations from the literature reveal that partial melting occurred on the exhumation path, at pressures below the coesite to quartz transition. A calculated P–T phase diagram for a former melt‐bearing domain shows that the formation of the peritectic garnet rim occurred at 1.4 GPa and 900°C, with an assemblage of clinopyroxene, amphibole, and plagioclase equilibrated at 1.3 GPa and 720°C. Isochemical phase equilibrium modelling of a symplectite of clinopyroxene, plagioclase, and amphibole after omphacite, combined with the mineral composition, yields a P–T range at 1.0–1. 6 GPa, 680–1,000°C. The assemblage of amphibole and plagioclase is estimated to reach equilibrium at 717–732°C, calculated by amphibole–plagioclase thermometry for the former melt‐bearing domain and symplectite respectively. The results of this study demonstrate that partial melt formed in the UHP eclogite through breakdown of an epidote‐group mineral with minor involvement of phengite during exhumation from peak pressure; melt was subsequently crystallized on the cooling path.

The versatility of petrological modeling: Thermobarometry of high‐pressure metabasites from the Renge and Sanbagawa belts and phase evolution during warm subduction at Nankai

“…By working backward, the P-T conditions can be refined by; for example, comparing measured versus predicted mineral compositions and proportions within a calculated field of interest (Powell & Holland, 2008. Examples in the literature applying phase equilibrium modeling to do thermobarometrical calculations are vast (Baziotis et al, 2017;Cao et al, 2019;Dragovic et al, 2020;Groppo et al, 2015;Guevara et al, 2017;Gutiérrez-Aguilar et al, 2021;Hern andez-Uribe et al, 2019;Klonowska et al, 2017;Li et al, 2020;Massonne, 2015;Palin et al, 2012;Štípsk a & Powell, 2005;St-Onge et al, 2013;Wei et al, 2013;Weller et al, 2015; among many others). In the case of forward modeling, petrological calculations can be performed along a sequence of P-T points to determine paragenesis along a given P-T-X path.…”

Anatexis of the deeply subducted continental crust during exhumation: A case study of Rongcheng migmatite in the Sulu orogen

“…The generation of hydrous silicate melts by dehydration‐driven in situ anatexis has been increasingly recognized in UHP rocks (e.g., review paper of Zheng et al (2011)). Natural and experimental studies suggested that the most likely way of melting the deeply subducted continental crust is dehydration melting of hydrous phases (such as mica‐ and epidote‐group minerals, mainly phengitic muscovite) during a ‘hot’ exhumation (e.g., Auzanneau, Vielzeuf, & Schmidt, 2006; Cao et al, 2019; Hermann, 2002; Hermann et al, 2001; Massonne, 2009; Schmidt, Vielzeuf, & Auzanneau, 2004). Petrographic observations show that there are two types of biotite in the studied migmatite: relict biotite (Figure 2d) and new growth biotite (Figure 2b,c,e–g).…”

“…Anatectic process plays a significant role in the evolution of orogenic belts (e.g., Brown, Korhonen, & Siddoway, 2011; Xu & Zhang, 2017), because it strongly influences the thermal and rheological behaviours of orogenic crust (e.g., Whitney, Teyssier, Fayon, Hamilton, & Heizler, 2003). Anatexis of the ultrahigh pressure (UHP) metamorphic rocks has been widely reported in UHP metamorphic terranes, for example, the Sulu UHP terrane of China (e.g., Chen, Zheng, & Hu, 2013a, 2013b; Gao, Xu, Zhang, & Chen, 2018; Song, Xu, Zhang, Wang, & Liu, 2014a, 2014b; Wallis et al, 2005; Xu et al, 2012, 2013,b; Zeng, Liang, Asimow, Chen, & Chen, 2009; Zong et al, 2010), the Kokchetav UHP terrane of Kazakhstan (e.g., Dobretsov & Shatsky, 2004; Hermann, Rubatto, Korsakov, & Shatsky, 2001; Ragozin, Liou, Shatsky, & Sobolev, 2009), the North‐East Greenland Eclogite Province (e.g., Cao, Gilotti, Massonne, Ferrando, & Foster Jr, 2019; Lang & Gilotti, 2007, 2015), and the Western Gneiss Region of Norway (e.g., Labrousse, Jolivet, Agard, Hébert, & Andersen, 2002; Labrousse, Prouteau, & Ganzhorn, 2011). Thus, the nature and timing of the anatexis of the deeply subducted continental crust can help us to understand the tectono‐thermal evolution of the continental collision zones and the exhumation mechanism of UHP metamorphic rocks (e.g., Whitney et al, 2003).…”

Ubiquitous post‐peak zircon in an eclogite from the Kumdy‐Kol, Kokchetav UHP‐HP Massif (Kazakhstan): Significance of exhumation‐related zircon growth and modification in continental‐subduction settings

“…Many of the existing experimental data (Lambert & Wyllie, 1972;Liu, Jin, & Zhang, 2009;Skjerlie & Patino Douce, 2002) and some natural observations (e.g. Cao, Gilotti, & Massonne, 2020;Cao, Gilotti, Massonne, Ferrando, & Foster, 2019;Miyazaki et al, 2016;Nakamura & Hirajima, 2000;Xia, Zheng, & Zhou, 2008) advocate for partial melting of UHP mafic rocks during their exhumation through HP-MP conditions to crustal depths. Most of the studies linked the incipient melting to dehydration melting of phengite (Cao et al, 2020;Gao, Zheng, & Chen, 2012;Liu et al, 2009), whereas some recent research highlight potentially similar role of zoisite, epidote-group minerals or amphibole at least in some cases (Cao et al, 2019(Cao et al, , 2020.…”