HR: 15:35h
AN: V43E-07    [Abstracts]
TI: Norwegian "External" Orthopyroxene Eclogites
AU: * Cuthbert, S
EM: simon.cuthbert@paisley.ac.uk
AF: University of Paisley, High Street, Paisley, PA1 2BE, United Kingdom
AB: Orthopyroxene (opx) eclogites have been recognised in the Norwegian Western Gneiss Complex (WGC) since the work of Eskola. They have played a key role in pressure-temperature evaluation, but their complexity has fueled controversy. "External" eclogites are those directly enclosed in gneiss and are distinguished from "internal" eclogites enclosed within mantle-derived peridotites. They are restricted to the UHP domains. Their petrography is complex, with abundant secondary amphibole. Garnets are idioblastic and prograde-zoned with inclusions of amphibole and cpx, but also platy and interlaminated with opx, with inclusions of graphite. Some bodies are pegmatitic. Opx eclogites were important in early thermobarometry in the WGC because they were the only parageneses capable of giving absolute P-T values. They yielded very high pressures (3-4GPa), and the platy garnets as exsolution lamellae suggested a prior high-T history, possibly at very high P. This supported a model of early mantle evolution and subsequent tectonic introduction into the crust. This was dependant on pairing low-Al opx cores with garnet, and on the origin of lamellar garnets. Opx rims adjacent to garnet tend to have higher Al, and these gave much lower pressuress (2GPa), and the precursor for grains showing garnet exsolution may, instead, have been low-P, high Al opx. This was consistent with a model of metamorphism in-situ in overthickened continental crust. Discovery of coesite and microdiamond confirmed the original high pressure values, and was further corroborated by thermobarometers using phengite and kyanite. Recent pressure estimates are in excess of 4GPa. Evidence for very high pressures in gneissic country-rocks led to models with wholesale insertion of continental crust into the mantle, but allowed for imbrication of low-P, HP and UHP nappes. Here again opx eclogites proved useful, as they were used to test the imbrication hypothesis, but there was found to be no PT contrast across a nappe contact, emphasising coherence of lithotectonic units after peak pressures were attained. Evidence for an early high T history in these opx eclogites has not been confirmed. Higher-Al opx rims appear to be a result of late amphibole growth and the low-Al opx cores do seem to be a primary UHP feature. WGC opx-eclogites have been recognised as a "lineage" distinct from a kyanite-bearing group. Opx and biotite are stabilised by high Mg, low Ca and low Al compared to normal basaltic compositions. Bulk compositions are picritic. Some may have originated from layers in mafic intrusions, but field and isotopic evidence suggest metasomatism involving interaction with continental crust. Pegmatitic character indicates crystallisation in the presence of fluids. Veins of glimmerite and garnetite with abundant zircon and apatite, and concentrations of magnesite all suggest interaction with K-rich, silicate and C-O-H enriched melts or fluids. Similar lithologies have been found in xenoliths from ultrapotassic volcanics in the Pamir above a subducted continental slab. Opx eclogites appear to be peculiar to the WGC and less well-known from other UHP terrains. Early workers noted similarities with opx eclogites from kimberlites; future work should again focus on parallels between opx eclogites and mantle xenoliths, and the insights to be gained on metasomatism, crust-mantle interactions and the fate of deeply subducted continental crust.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3651 Thermobarometry
DE: 3654 Ultra-high pressure metamorphism
DE: 3660 Metamorphic petrology
DE: 3690 Field relationships (1090, 8486)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting