Bouncing back from the deep

eclogite

Eclogite from Norway. Image by kevinzim via Flickr

Because the average density of the rocks making up the continental crust is about 2.7 t m-3 while that of the mantle is greater than 3.0 t m-3 it might seem as though continents cannot be subducted. Indeed, that was one of the first principles of plate tectonics, which would account for continental crust dating back to 4000 Ma, whereas there is no oceanic crust older than about 150 Ma. In the southern foothills of the Alps in Piemonte, Italy is a site which refutes the hypothesis in a stunning fashion. The minor ski resort of Monte Mucrone is backed by cliffs in what to all appearances is a common-or-garden granite: it even seems to contain phenocrysts of plagioclase feldspar. Microscopic examination of the megacrysts reveals them to be made up of a complex intergrowth between jadeite, a high-pressure sodic pyroxene, and quartz. This is exactly what should form if albite, the sodium-rich kind of plagioclase feldspar, if it descended to depths over 70 km below the surface, i.e. to mantle depths.

Monte Mucrone proves that continental materials can be subducted, but also reveals that these granites popped back up again when the forces of subduction were relieved at the end of the Alpine orogeny. Other examples have since turned up, but few so spectacular as continental rocks from Switzerland (Herwartz, D. et al. 2011. Tracing two orogenic cycles in one eclogite sample by Lu-Hf garnet chronometry. Nature Geoscience, v. 4, p. 178-183). The Adula nappe of the Swiss Lepontine Alps consists of granitoid gneisses and metasediments of continental affinities, associated with mafic and ultramafic metamorphic rocks. The mafic rocks include eclogites typical of high-pressure, low-temperature metamorphism characteristic of subduction. Their minerals record formation temperatures around 680°C at a depth of more than more than 80 km. Eclogites are beautiful green and red rocks containing high-pressure omphacite pyroxene and pyrope garnet. Garnets generally contain abundant rare-earth elements especially those with the highest atomic numbers. One of these is lutetium (Lu) that has a radioactive isotope 176Lu with a half-life of 3.78×1010 years to yield a daughter isotope of hafnium 176Hf; garnets can be dated using this method. Garnets are frequently zoned, and the Adula eclogites clearly show several generations of zonation. Zoning can form as metamorphic conditions change, so in itself is not unusual, but dating different generations is. The German team from the Universities of Bonn, Cologne and Münster found that the garnets defined two distinct isochrons, one of Variscan age of just over 330 Ma, the other Alpine around 38 Ma. Clearly the pre-Variscan crust (probably once part of the African continent) had been subducted twice but had wrested itself clear of the mantle’s clutches on both occasions, each time remaining more or less intact. One idea that stems from this coincidence is that the Variscan mountain belt that formed at the earlier subduction zone subsequently split at its high P – low T core, so that the eclogites lay at a new continental margin and could suffer the same extreme compression when new subduction began there.

It also turns out that the region in which  Monte Mucrone lies, the Sesia zone of the Western Alps, also records a double whammy of continental subduction, but a repetition that occurred during the early events of the  Alpine orogeny (Rubatto, D. et al. 2011. Yo-yo subduction recorded by accessory minerals in the Italian Western Alps. Nature Geoscience, v. 4, p. 338-342). The team of Australian, Swiss and Italian geologists focused on the P-T record preserved in zoned garnets, allanites and zircons and evidence for two generation of white micas in eclogites and blueschists. Backed by U-Pb dating of zircon and allanite zones, the authots uncovered two episodes of deep subduction separated by period of rapid exhumation over the period between 79 to 65 Ma ago. The double subduction took place while the African plate converged obliquely with Eurasia; a strike-slip configuration that probably resulted in large-scale switches from compression to extension.

See also: Bruekner, H.K. 2011. Double-dunk tectonics. Nature Geoscience, v. 4, p. 136-138

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