Modeling the exhumation path of partially melted ultrahigh-pressure metapelites, North-East Greenland Caledonides

Abstract

Pseudosection modeling constrains the pressure-temperature (P-T) exhumation path of partially melted ultrahigh-pressure (UHP) metapelites exposed in the North-East Greenland UHP terrane. A robust peak P and T estimate of 3.6 GPa and 970°C based on mineral assemblages in nearby kyanite eclogites is the starting point for the P-T path. Although the peak assemblage for the metapelite is not preserved, the calculated modeled peak assemblage contained substantial clinopyroxene, garnet, phengite, K-feldspar and coesite with minor kyanite and rutile. Combining the pseudosection and observed textures, the decompression path crosses the coesite-quartz transition before reaching the dry phengite dehydration melting reaction where phengite is abruptly consumed. In the range of 2.5 to 2.2 GPa, clinopyroxene is completely consumed and garnet grows to its maximum volume and grossular content, matching the high grossular rims of relict megacrysts. Plagioclase joins the assemblage and the pseudosection predicts up to 12-13 vol.% melt in the supersolidus assemblage, which contained garnet, liquid, K-feldspar, plagioclase, kyanite, quartz and rutile. At this stage, the steep decompression path flattened out and became nearly isobaric. The melt crystallization assemblage that formed when the path crossed the solidus with decreasing temperature contains phengite, garnet, biotite, 2 feldspars, kyanite, quartz and rutile. Therefore, the path must have intersected the solidus at approximately 1.2 GPa, 825°C. The pseudosection predicts that garnet is consumed on the cooling path, but little evidence of late garnet consumption or other retrograde effects is observed. This may be due to partial melt loss from the rock. Isochemical PT-n and PT-X sections calculated along the P-T path display changes in mineral assemblage and composition that are consistent with preserved assemblages.

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

“…The P-T path for the kyanite eclogite ( Figure 12) displays a slightly different position and shape than the one deduced by Lang and Gilotti (2015) for metapelites from the UHP terrane. This might be caused by uncertainties in mineral analyses and the applied thermodynamic database and solution models (e.g., Palin, Weller, Waters, & Dyck, 2016).…”

“…Migmatitic gneiss with leucosome suggests that the UHP terrane was partially melted. Rare metapelite was partially melted via phengite breakdown on the exhumation path at HP conditions, indicated by phase equilibria (Lang & Gilotti, , ). The metapelite displays extensive evidence of partial melting, such as leucosomes with kyanite and garnet, polymineralic inclusions (consisting of kyanite, rutile, K‐feldspar, biotite, and plagioclase), polycrystalline quartz in restitic garnet, equigranular quartz, and euhedral garnet and kyanite crystals (Lang & Gilotti, ).…”

“…Water plays a crucial role in metamorphism, inducing partial melting and developing retrograde textures (e.g., Guiraud, Powell, & Rebay, ; Martin & Duchene, ; Spruzeniece, Piazolo, Daczko, Kilburn, & Putnis, ; Weinberg & Hasalova, ); therefore, the amount of water is critical to determining the position of solidus curves and mineral phase relations (e.g., Lang & Gilotti, ). A P–X (H 2 O) diagram was calculated at 900°C for the studied sample, with H 2 O content varying from 0 to 1 wt% (Figure ), normalized to a total of 100% for both ends of the horizontal axis.…”

“…In the lower-pressure region, theoretically-derived epidoteand amphibole-breakdown melting of metabasites are shown (Vielzeuf & Schmidt, 2001;VS01): zoisite is present at <790°C and melted at higher temperature at <2.5 GPa, while the grey area marks the multivariant field of amphibole+quartz breakdown. The exhumation path for the North-East Greenland UHP metapelites from Lang and Gilotti (2015) is marked as a thick, closely-spaced, dashed curve (LG15). Dashed-, and solid-arrowed curves are deduced metamorphic P-T paths of the studied UHP eclogite, with stars referring to estimated P-T conditions based on phase equilibria and empirical thermobarometry discussed below, a P-T path with slight cooling is more likely for exhumation.…”

Melting of eclogite facies sedimentary rocks in the Belomorian Eclogite Province, Russia

“…Partial melting of subducted crust affects the rheological properties of rocks and plays an important role in the formation, differentiation and exhumation of ultrahigh‐pressure (UHP) terranes (Hermann et al ., ; Zhong et al ., ; Labrousse et al ., , ; Chopin, ; Zheng et al ., ; Sizova et al ., ; Hermann et al ., ; Hermann & Rubatto, ; Lang & Gilotti, , ). Phengite is the common stable hydrous phase in UHP crustal rocks and it can be involved in fluid‐absent melting reactions during decompression (Hermann & Green, ; Schmidt et al ., ; Massonne, ; Hermann & Rubatto, ).…”