Rocha de Ipanema

Garnet-bearing gneiss at Leblon near E end of Ipanema.

A bonus attraction for structural geologists visiting Rio is the gneisses and sheared granitic rocks that crop out several places within and near the city.

Sheared granite with large feldspar porphyroclasts, separated by shear bands. A classical S-C protomylonite. Click to enlarge!

Nice garnetiferous (kinzigitic) gneisses crop out at the E end of Ipanema (Leblon). These rocks must have seen considerable pressures and temperatures, perhaps someone can tell me more about this.

A relatively safe place to combine the beach and structural geology is the point between Ipanema and Copacabana. I stopped at the southern part of this point and found some very pretty S-C structures in the sheared granite indicating sinistral shear. Well worth seeing!

Enclaves (xenoliths) occur locally that at least vaguely suggest the amount of strain that these rocks have enjoyed.

The granitic rocks are Neoproterozoic and, according to the information I have, pre- to syn-collisional with respect to the orogeny that took place when Gondwana formed, in this particular area some 570 Ma.

Enclaves with their long axes along foliation. Hard to tell their original shape, but strain is probably somewhere between 1:5 and 1:10 I would guess (but I don’t really know).

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About Haakon Fossen

Professor of structural geology, University of Bergen. Author of book Structural Geology, published at Cambridge University Press
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3 Responses to Rocha de Ipanema

  1. Nicolas says:

    Hi,
    From what I see on the garnet-bearing gneiss picture, it looks more like a metatexite migmatite with garnet located in the leucosome. Morever some leucosomes seem to cross-cut the main foliation and could be dilatant structures. Based on the mineralogy the partial melting reaction might be :
    Biotite + plagioclase + aluminosilicate + quartz => melt + garnet ± K-feldspar

    • Nicolas says:

      In fact I have seen very similar textures but less sheared in Ecuador (El Oro metamorphic complex).
      here’s a picture:

      These migmatites have a metapelitic protolith, and peak P-T conditions were estimated at 6-7kbar and 750-800°C.
      I suspect that the Tmax in your garnet-bearing gneiss should be similar.
      Cheers,

      • Thanks Nicolas, your picture certainly matches the one from Rio! I only spent a few minutes at the Rio locality, and basically know nothing about it. What you write makes good sense. Good luck with your work in Equador!

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