Exhumation of high-pressure and ultrahigh-pressure eclogites in large orogens and associated petrological change during the process remain enigmatic problems. This dissertation examines eclogites from high-pressure (HP) and ultrahigh-pressure (UHP) terranes in the North-East Greenland Eclogite Province, aiming to decipher their metamorphic pressure-temperature (P-T) paths, evaluate spatial variation of P-T paths, and understand petrological changes during the exhumation.
Kyanite-bearing UHP eclogites from North-East Greenland contain a peak mineral assemblage of phengite, garnet, omphacite, kyanite, coesite, rutile and probably epidote-group minerals. Thermodynamic modeling with an XRF-derived bulk composition yielded a peak P-T condition of 3.4 GPa and 920 °C. Petrographic textures, such as graphic intergrowth of amphibole and plagioclase, cusps of plagioclase into garnet and quartz, and neoblasts of garnet indicate that the eclogites were partially melted through dehydration melting of phengite and epidote-group minerals. Since thermodynamic modeling could not yield a satisfactory solidus curve, experimental phase relations were considered in interpreting the melting process, and show a near isothermal decompression path across the epidote mineral melting curve. Additional thermodynamic modeling of a symplectite after omphacite, consisting of amphibole, plagioclase and clinopyroxene, yields a P-T condition of ~ 1.2 GPa and 800 °C. Thermodynamic modeling of a melt pocket yields a further P-T constraint of 1.4 GPa and 740 °C.
The HP zoisite eclogites from the Storstrømmen shear zone in the Sanddal area preserve partial melting textures both in garnet and in the matrix. The textures include multiphase solid inclusions of albite and K-feldspar in garnet, graphic intergrowth of amphibole and plagioclase, cuspate textures, and leucosome. Thermodynamic modeling combined with mineral composition and modes yielded an exhumation P-T path from subsolidus conditions at ~1.95 GPa and ~670 °C, to ~1.85 GPa and 715 °C at suprasolidus, to ~1.45 GPa and 640 °C. Paragonite, phengite, and amphibole were the major dehydration melted phases along the exhumation path.
The HP kyanite eclogite from the Danmarkshavn area contains disequilibrium textures developed during retrograde stages. Petrographic observation documents two groups of textures: a strongly zoned plagioclase (anorthite to andesine) enclosing a poorly developed symplectite of sapphirine + spinel + plagioclase after kyanite, and a less zoned plagioclase (labradorite to andesine) enclosing a fully developed symplectite after kyanite. Thermodynamic modeling of the bulk rock returns a peak P-T condition of 1.9 GPa and 840 °C. Thermodynamic modeling of a symplectite domain yields poor P-T constraints of 0.8 – 1.3 GPa and 700 – 900 °C. Modeling also indicates the plagioclase development would be richer in Ca during decompression while progressive replacement of kyanite induced the plagioclase rim to be less Ca-rich.
This study reveals that HP and UHP eclogites may experience partial melting on their exhumation path. Dehydration melting of hydrous minerals (e.g. phengite and zoisite) is the most plausible way in partially melt the eclogites, because of limited amounts of free fluid. The partial melting does not trigger exhumation of the eclogites, but may facilitate the exhumation process. The near-isothermal exhumation path for the UHP terranes suggests that it was initially exhumed through vertical extrusion. Lateral extrusion by the Storstrømmen and Germania Land shear zones is suggested to have further exhumed the HP and UHP rocks, which is analogous to the lateral escape tectonics in the Tibetan Plateau