Lherzolite (Type Locality)

The name is derived from the Lherz Massif, an alpine peridotite complex (also known as orogenic lherzolite complex). A lherzolite is an ultrabasic igneous rock dominated by essential Olivine and clinopyroxene and orthopyroxene in equal proportions. Accessory minerals include plagioclase, spinel, garnet, ilmenite, chromite and magnetite. Lherzolites are a peridotite and the main component of the upper mantle. Their aluminous phases change with pressure, with plagioclase present at low pressures, spinel at intermediate pressure and garnet at high pressure.

The Lherz massif
The Lherz (or Lers) lherzolite massif, type-locality of the lherzolite, is the best known of several tens of mantle slices that were emplaced in the Northern Pyrenean Zone by the middle Cretaceous counterclockwise rotation of the Iberian plate with respect to Western Europe. The Pyrenean orogenic peridotites comprise about 40 distinct ultramafic bodies mainly composed of layered spinel lherzolites, with dimensions varying from a few m2 to 1 km2 in Lherz. The Pyrenean orogenic peridotites are distributed in seven groups: to the east, the Salvezines, Prades-Bestiac, Vicdessos-Lherz, and Castillon groups; to the west, The Tuc Desse-Arguenos, Avezac-Moncaut and Turon de Técouère-Col d’Urdach groups.

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Fig. 1: Distribution of the Pyrenean peridotite bodies along the North Pyrenean Fault. 1: Oligocene and post-Oligocene sediments; 2: Mesozoic and Eocene sediments; 3: Paleozoic basement; 4: area of LP-HT Pyrenean metamorphism; 5: lherzolite bodies; 6: granulites; 7: main thrusts external zones; 8: North Pyrenean Fault.



Exhumation of Pyrenean peridotites is usually ascribed to lithospheric thinning associated with successive opening and closing of elongated, asymmetrical pull-apart basins in response to the dominantly transcurrent motion of the Iberian plate relative to the European plate in the mid-Cretaceous. Most ultramafic massifs outcrop within carbonate rocks of Jurassic to Aptian age affected by a low pressure-high temperature metamorphism that characterizes the North Pyrenean Metamorphic Zone, and they are generally associated with crustal granulites. The North Pyrenean Metamorphic Zone is a narrow (0-5 km wide) band of Mesozoic carbonate sediments of Jurassic to Aptian age affected by a low pressure and high temperature metamorphism dated between 110 and 85 Ma, with a peak at 95 Ma. Emplacement of the ultramafic rocks in the crust has been dated at 105-110 Ma. In the Lherz massif, the last stage of exhumation of the peridotites is marked by extensive brecciation in the outer rim of the peridotite body and in the host limestones that show structures typical of sedimentary

The Lherz massif is predominantly composed of spinel lherzolites which are intermingled with metric to decametric bodies of highly refractory harzburgites in the upper part of the massif (Fig.2). Lherzolites contain numerous cm- to 10 cm scale spinel-websterite layers and some thicker (10-50 cm) tabular garnet pyroxenites. The harzburgites often contain mm- to cm-thick websterite layers near the contact with lherzolites. The entire massif is crosscut by a later generation of amphibole-bearing pyroxenite dykes and hornblendite veins, related to alkaline magmatism during the Cretaceous. The harzburgites generally occur as elongated bands a few meters to tens of meters-wide within the lherzolites. All harzburgites show a penetrative shallowly-dipping foliation. The contact between harzburgites and lherzolites is often sharp, steeply-dipping and marked by cm-scale websteritic layers that crosscut the harzburgite foliation.

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Fig.2: Geological map of the Lherz peridotite massif showing anastomosed fertile lherzolites enclosing irregularly-shaped refractory harzburgite bodies. From Le Roux, et. al. (2007).





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Panoramic view of the Lherz Massif. From (christian nicollet)






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Lherzolites with magmatic layering. Lherz Massif. From (christian nicollet)






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Lherzolite samples. Lherz Massif. From (christian nicollet)






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Lherzolite samples. Lherz Massif. From (christian nicollet)






Bibliography



• Le Roux, V., Bodinier, J. L., Tommasi, A., Alard, O., Dautria, J. M., Vauchez, A., & Riches, A. J. V. (2007). The Lherz spinel lherzolite: refertilized rather than pristine mantle. Earth and Planetary Science Letters, 259(3), 599-612.
• Clerc, C., Lagabrielle, Y., Neumaier, M., Reynaud, J. Y., & de Saint Blanquat, M. (2012). Exhumation of subcontinental mantle rocks: evidence from ultramafic-bearing clastic deposits nearby the Lherz peridotite body, French Pyrenees. Bulletin de la Société géologique de France, 183(5), 443-459.

Photo
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Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(6).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(4).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(5).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(6).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(3).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(7).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(8).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(10).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(11).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(12).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(13).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(14).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(17).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(19).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(20).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
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Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
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Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
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Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(28).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
lherz(29).jpg

Olivine (colorless), chromite (green-brown) and pyroxene (slightly beige) crystals. PPL image, 2x (Field of view = 7mm)
lherz(30).jpg

Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Olivine, chromite (isotropic) and pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Olivine (colorless) and pyroxene (slightly beige) crystals. PPL image, 10x (Field of view = 2mm)
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Olivine (colorless) and pyroxene (slightly beige) crystals. PPL image, 10x (Field of view = 2mm)
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Olivine (colorless) and pyroxene (slightly beige) crystals. PPL image, 10x (Field of view = 2mm)