Eclogite from Norway with agarnet (red) andomphacite (greyish-green)groundmass. The sky-blue crystals arekyanite. Minor whitequartz is present, presumably from the recrystallization ofcoesite. A few gold-whitephengite patches can be seen at the top. A 23 millimetres (0.91 in) coin added for scale.
The nameeclogite is derived from theAncient Greek word for 'choice' (εκλογή,eklogḗ), meaning 'chosen rock' on account of its perceived beauty. It was first named byRené Just Haüy in 1822 in the second edition of his workTraité de minéralogie.[2]
Eclogites are defined as bi-mineralic, broadly basaltic rocks which have been classified into Groups A, B and C based on the chemistry of their primary mineral phases, garnet and clinopyroxene.[4][5] The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet.[5] The rocks are gradationally less mafic (as defined by SiO2 and MgO) from group A to C, where the least mafic Group C contains higheralkali contents.[6]
The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface.[5] Group A derive fromcratonic regions of Earth's crust, brought to the surface as xenoliths from depths greater than 150 km duringkimberlite eruptions.[4][5] Group B show strong compositional overlap with Group A, but are found as lenses or pods surrounded byperidotitic mantle material.[5] Group C are commonly found between layers ofmica orglaucophane schist, primarily exemplified by the New Caledonia tectonic block and off the coast of California.[7]
The broad range in composition has led a longstanding debate on the origin of eclogite xenoliths as either mantle or surface derived, where the latter is associated with thegabbro to eclogite transition as a major driving force forsubduction.[8][9][10]
Group A eclogite xenoliths remain the most enigmatic in terms of their origin due tometasomatic overprinting of their original composition.[11][4] Models proposing a primary surface origin as seafloorprotoliths strongly rely on the wide range inoxygen isotope composition, which overlaps with obducted oceanic crust, such as the Ibra section of theSamail ophiolite.[12][13] The variation found in some eclogite xenoliths at the Roberts Victor kimberlite pipe are a result ofhydrothermal alteration of basalt on the seafloor.[12] This process is attributed to both low- and high-temperature seawater exchange, resulting in large fractionations in oxygen isotope space relative to the upper mantle value typical of mid ocean ridge basalt glasses.[14][15] Other mechanisms proposed for the origin of Group A eclogite xenoliths rely on acumulate model, where garnet and clinopyroxene bulk compositions derive from residues ofpartial melting within the mantle.[9] Support of this process is result of metasomatic overprinting of the original oxygen isotope composition, driving them back towards the mantle range.[16]
This facies reflects metamorphism at high pressure (at or over 12kbar) and moderately high to very high temperatures. The pressures exceed those of greenschist, blueschist, amphibolite or granulite facies.
Eclogites containinglawsonite (a hydrous calcium-aluminium silicate) are rarely exposed at Earth's surface, although they are predicted from experiments and thermal models to form during normal subduction ofoceanic crust at depths between about 45–300 km (28–186 mi).[17]
Partial melting of eclogite has been modeled to producetonalite-trondhjemite-granodiorite melts.[18] Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted.[19] The eclogite melt may then react with enclosing peridotite to producepyroxenite, which in turn melts to produce basalt.[20]
Eclogite from Almenning, Norway. The red-brown mineral is garnet, greenomphacite and white quartz.
Occurrences exist in western North America, including the southwest[21] and theFranciscan Formation of theCalifornia Coast Ranges.[22] Transitionalgranulite-eclogite facies granitoid,felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of thePetermann Orogeny, central Australia. Coesite- and glaucophane-bearing eclogites have been found in the northwesternHimalaya.[23] The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located inBrazil andMali, respectively.[24][25]
^Wilke, Franziska D.H.; O'Brien, Patrick J.; Altenberger, Uwe; Konrad-Schmolke, Matthias; Khan, M. Ahmed (January 2010). "Multi-stage reaction history in different eclogite types from the Pakistan Himalaya and implications for exhumation processes".Lithos.114 (1–2):70–85.Bibcode:2010Litho.114...70W.doi:10.1016/j.lithos.2009.07.015.
^Santos, Ticiano José Saraiva; Amaral, Wagner Silva; Ancelmi, Matheus Fernando; Pitarello, Michele Zorzetti; Fuck, Reinhardt Adolfo; Dantas, Elton Luiz (2015). "U–Pb age of the coesite-bearing eclogite from NW Borborema Province, NE Brazil: Implications for western Gondwana assembly".Gondwana Research.28 (3):1183–1196.Bibcode:2015GondR..28.1183D.doi:10.1016/j.gr.2014.09.013.
