Field of Science

Fantastic Rocks and Where to Find Them – High Pressure Metamorphites

Only a small part of rocks are stable on earth´s surface, as widespread erosion and alteration shows. Magmatic rocks, like granite, crumble and minerals composing these rocks, like feldspar and mica, react with water to become soft clay. Similar things happen to metamorphic rocks, eroding under atmospheric conditions. Often metamorphites, formed deep within earth, quickly start to decay already during uplift. Especially unstable are metamorphic rocks formed under very high pressure (25-30 kbar) and temperature (650-700°C), conditions as found in earth´s crust in a depth ranging from 56 to 186miles (90-300km). One locality where such Ultra-High-Pressure (UHP) rocks are partially preserved are the Western Alps between Italy and France, in particular the Dora-Maira Massif.
 
Fig.1. Geological map of the Western Alps DM=Dora-Maira-Massif, by Phil Mair 85, source Wikipedia.

Here whiteschists, named after the white-greyish color, are embedded as large lenses in a thick layer of gneiss. The whiteschists are geologically speaking not true schist, as this term refers to low grade metamorphic rocks, but pyrope-quartzites. This rock is composed mostly of quartz with large garnets (pyrope) embedded within, also some other typical metamorphic  minerals, like kyanite (an aluminum bearing silicate), talc (a magnesium bearing silicate), phengite (a mica variety) and rutile (a titanium dioxide), can be found. 

Especially interesting is the presence of the high-pressure modification of quartz, called coesite, and the pink mineral ellenbergerite. Ellenbergerite is a complex magnesium-aluminum-titanium-silicate known only from the Dora-Maira-Massif and described for the first time as typical UHP mineral 30 years ago. The minerals are preserved as grains in large garnet crystals, the crystals acted like a pressure chamber preserving these unstable minerals. Both ellenbergerite, coesite and garnets formed when parts of the oceanic crust of the Penninic Ocean were subducted into a depth of at least 186miles. During the later uplift forming the Alps minerals like ellenbergerite and coesite adjusted to changes in pressure and temperature by decaying into other minerals, coesite becomes for example common quartz. However mineral grains entrapped in the larger garnets remained, like in a closed pressure chamber, under high pressure conditions and were also protected from circulating fluids. 

Fig.2. Outcrop of pyrope-quartzite. Some of the garnets embedded in the white quartz-phengite matrix are more than 10 inches in diameter, unfortunately very requested by mineral collectors only the casts remain here (geologist is sadly looking at the hole...)..

Fig.3. UHP rocks are rare even in the Dora-Maira Massif, as most decayed during uplift, adjusting to lower pressure and temperature. Evidence for medium-grade metamorphism during uplift is the presence of blueschists, here with blue glaucophane-amphibole and alterated garnets.

At the time of the discovery an unique find in the Alps, similar UHP rocks are nowadays described also from China and the German Erzgebirge.

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