Eclogite
Eclogite (Template:IPAc-en) is a metamorphic rock containing garnet (almandine-pyrope) hosted in a matrix of sodium-rich pyroxene (omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisite, dolomite, corundum and, rarely, diamond. The chemistry of primary and accessory minerals is used to classify three types of eclogite (A, B, and C). The broad range of eclogitic compositions has led to a longstanding debate on the origin of eclogite xenoliths as subducted, altered oceanic crust.
The name eclogite is derived from the Ancient Greek word for 'choice' (Template:Wikt-lang, Template:Grc-transl), meaning 'chosen rock' on account of its perceived beauty. It was first named by René Just Haüy in 1822 in the second edition of his work Traité de minéralogie.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
OriginsEdit
Eclogites typically result from high to ultrahigh pressure metamorphism of mafic rock at low thermal gradients of <Template:Cvt as it is subducted to the lower crust to upper mantle depths in a subduction zone.<ref>Template:Cite journal</ref>
ClassificationEdit
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.<ref name=":13">Template:Cite journal</ref><ref name=":0">Template:Cite journal</ref> The classification distinguishes each group based on the jadeite content of clinopyroxene and pyrope in garnet.<ref name=":0" /> The rocks are gradationally less mafic (as defined by SiO2 and MgO) from group A to C, where the least mafic Group C contains higher alkali contents.<ref name=":02">Template:Cite journal</ref>
The transitional nature between groups A, B and C correlates with their mode of emplacement at the surface.<ref name=":0" /> Group A derive from cratonic regions of Earth's crust, brought to the surface as xenoliths from depths greater than 150 km during kimberlite eruptions.<ref name=":13"/><ref name=":0" /> Group B show strong compositional overlap with Group A, but are found as lenses or pods surrounded by peridotitic mantle material.<ref name=":0" /> Group C are commonly found between layers of mica or glaucophane schist, primarily exemplified by the New Caledonia tectonic block and off the coast of California.<ref name=":03">Template:Cite journal</ref>
Surface versus mantle originEdit
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 the gabbro to eclogite transition as a major driving force for subduction.<ref name=":12">Template:Cite journal</ref><ref name="O'Hara 69–133">Template:Cite journal</ref><ref>Template:Cite journal</ref>
Group A eclogite xenoliths remain the most enigmatic in terms of their origin due to metasomatic overprinting of their original composition.<ref>Template:Cite journal</ref><ref name=":13"/> Models proposing a primary surface origin as seafloor protoliths strongly rely on the wide range in oxygen isotope composition, which overlaps with obducted oceanic crust, such as the Ibra section of the Samail ophiolite.<ref name="onlinelibrary.wiley.com">Template:Cite journal</ref><ref>Template:Cite journal</ref> The variation found in some eclogite xenoliths at the Roberts Victor kimberlite pipe are a result of hydrothermal alteration of basalt on the seafloor.<ref name="onlinelibrary.wiley.com"/> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Other mechanisms proposed for the origin of Group A eclogite xenoliths rely on a cumulate model, where garnet and clinopyroxene bulk compositions derive from residues of partial melting within the mantle.<ref name="O'Hara 69–133"/> Support of this process is result of metasomatic overprinting of the original oxygen isotope composition, driving them back towards the mantle range.<ref>Template:Cite journal</ref>
Eclogite faciesEdit
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 containing lawsonite (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 of oceanic crust at depths between about Template:Cvt.<ref>Template:Cite journal</ref>
ImportanceEdit
Formation of igneous rocks from eclogiteEdit
Partial melting of eclogite has been modeled to produce tonalite-trondhjemite-granodiorite melts.<ref>Template:Cite journal</ref> Eclogite-derived melts may be common in the mantle, and contribute to volcanic regions where unusually large volumes of magma are erupted.<ref name="Foulger">Template:Cite book</ref> The eclogite melt may then react with enclosing peridotite to produce pyroxenite, which in turn melts to produce basalt.<ref>Template:Cite journal</ref>
DistributionEdit
Occurrences exist in western North America, including the southwest<ref>William Alexander Deer, R. A. Howie and J. Zussman (1997) Rock-forming Minerals, Geological Society, 668 pages Template:ISBN</ref> and the Franciscan Formation of the California Coast Ranges.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Transitional granulite-eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya.<ref>Template:Cite journal</ref> The oldest coesite-bearing eclogites are about 650 and 620 million years old and they are located in Brazil and Mali, respectively.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
ReferencesEdit
External linksEdit
Template:Sister project Template:Metamorphic Facies Template:Rock type Template:Authority control