Harrow and Hillingdon Geological Society

High Pressure Metamorphics

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Up the down escalator: the formation and exhumation of high-pressure rocks

Clare Warren

(Research Centre for Physical and Environmental Sciences, Open University)

High-pressure metamorphic rocks form in every continental collision belt and the rocks come back to the surface while material is still being dragged down. This presentation looked at what these rocks are, where, when and how do they form, how do they reach the surface and what can modelling tell us about the process.

Eclogites and ultrahigh-pressure eclogites form at depths of 100km and more. Horace Benedict de Saussure (1740-99) a Swiss aristocrat, Alpine traveller, physicist, geologist and botanist was one of the first to describe eclogites in Chamonix, in the Aosta Valley.

There was a big debate until the early 1900s as to whether they were magmatic or metamorphic. Friedrich Beale (1903) compared the molar volumes of gabbro and eclogite. Chmeically the two ricks are the same but eclogites are much denser – they are the high-pressure equivalents of gabbro. They are found with mica schists containing garnet and clinopyroxene (Stella, 1894; Franchi, 1900, 1902). Ringwood and Green (1966) carried out high-pressure experiments, which clearly demonstrated that eclogites with garnet and clinopyroxene were metamorphic rocks formed under high pressure.

Tectonic environment

Blueschists and eclogites form under a pressure of at least 12 kbars and temperatures up to 900 oC. Eclogites form during subduction, the only place you can keep the rocks cold enough for those pressures. Coesite was discovered in natural samples in Western Norway and the Alps in 1984. It is a high-pressure polymorph of quartz comprising spiral chains of 4-membered rings of tetrahedral. It forms quickly at pressures of 20kbars upwards but to be preserved it needs to be enclosed in a stronger mineral such as garnet or clinopyroxene. The coesite/quartz and jadeite/albite transitions are key indicators of high pressure and very high pressure.

Types of high-pressure rocks

High-pressure rocks are found in continental crust, oceanic crust and mantle peridotite. There are localities known in Canada, Brazil and Mali (at 620 million years), the Caledonides (400My), China (400-350My), Kazakhstan, the Alps (30My) the Himalayas (40-50My) and the youngest are in New Caledonia.

Key questions are

  • How deep does the crust subduct? – answerable with data;
  • How fast does it exhume? – answerable with data; and
  • How does it exhume? – answerable with models.

Determination of pressure/temperature conditions uses the mineralogical assemblage – garnet, clinopyroxene, kyanite, coesite , exchange equilibria for Fe and Mg in biotitre/garnet and net transfer equilibria, eg Na in plagioclase or clinopyroxene. The ultimate aim is to determine a pressure – temperature path. In Pakistan, peak metamorphism was at 3GPa.

Dating of peak metamorphism uses U-Pb techniques in zircon, monazite, rutile, titanite and baddeleyite on the assumption that there was no original lead in the crystal and that all the lead is derived from decay of uranium. Major minerals are linked to zircon by trace element compositions normalised to chondrite and it is possible to differentiate zircon rimws. For example, europium anomalies indicate the presence or absence of plagioclase and the jadeite/albite transition since it is necessary to remove plagioclase and turn it into clinopyroxene.

In the Himalayas, peak pressure is dated at 46.5My in allanite and 46.4My in zircon and exhumation from the mantle occurred at a rate of 2.5-8.0cm/year, ie similar rates to plate tectonic rates of movement. Similar rates have been found for the Alps but a little slower for Norway. At Dora Maira in the Alps, peak pressure was at 35.1My and it exhumed at 3.4cm/year then slowed to 1.6cm/year and finally to 0.5cm/year.

Modelling exhumation mechanisms

Mathematical models simulate taking the crust down and even high pressure versions are less dense than the mantle and the material is pushed, squeezed or pulled up to surface. Available mechanisms are buoyancy and tectonics and exhumation is probably the result of a combination of both.

A 2-dimensional thermal/mechanical finite element method numerical model has been used with a 2 x 10km grid and the lower part on a 10 x19km grid. Crust density is taken a 2,800, 2,850 and 2,900kg/m 3 and it is modelled as quartzite with a doleritic lower crust and olivine-rich mantle. The model has been run using 2 variables:

  • Margin strength (f) ranging from 5 to 0.1; and
  • Strain weakening (W) ranging from 1 to 100.

The results show

  • Run 1 with f=1 and W-5 found maximum pressure of 9kbars, temperature of 800 oC and exhumation at 5cm/year;
  • Run 2 with f=2 and W=100 found maximum pressure of 39kbars, temperature 720 oC and exhumation at 8.9cm/year;
  • Run 3 with f=0.1 and W=1 found maximum pressure at 28kbars, temperature at 630 oC and exhumation at 1.0cm/year.

With decreased initial strength, the crust separates from the subducting slab at shallower depth and slower exhumation, while increasing strain weakening results in separation at deeper levels and faster exhumation.

Subduction is favoured by a strong crust, high subduction velocity and a narrow channel while exhumation is favoured by a weak crust, low subduction velocity and a wide channel. Exhumation amounts depend on depth, pressure, width of channel, velocity and viscosity and may be linked to structure. High-pressure products are found right beneath the suture zone, with extension in the foreland probably formed by high-pressure rocks forcing themselves upwards. However, there is still a lot we don’t really know and the model results are dependent on the model inputs.

For subduction to 100km, material must stay attached to subducting slab (ie strong) but for eclogites and ultrahigh-pressure rocks it must detach and be buoyant (weal). Exhumation of high-pressure rocks requires buoyancy but it is not the only requirement, there also needs to be appropriate viscosity, subduction velocity and channel width. In practice, a combination of mechanisms may operate in different places at different times.

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