Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador
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ORIGINAL PAPER
Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador Timm John Æ Erik E. Scherer Æ Volker Schenk Æ Petra Herms Æ Ralf Halama Æ Dieter Garbe-Scho¨nberg
Received: 5 February 2009 / Accepted: 9 July 2009 / Published online: 30 July 2009 Ó Springer-Verlag 2009
Abstract The metamorphic Raspas Complex of southwest Ecuador consists of high-pressure mafic, ultramafic, and sedimentary rocks. The Lu–Hf ages of a blueschist, a metapelite, and an eclogite overlap at around 130 Ma and date high-pressure garnet growth. Peak metamorphic conditions in the eclogites reached 1.8 GPa at 600°C, corresponding to a maximum burial depth of *60 km. The geochemical signatures of the eclogites suggest that their protoliths were typical mid-ocean ridge basalts (MORB), whereas the blueschists exhibit seamount-like characteristics, and the eclogite-facies peridotites seem to represent depleted, MORB-source mantle. That these rocks were subjected to similar peak PT conditions contemporaneously suggests that they were subducted together as an essentially complete section within the slab. We suggest that this section became dismembered from the slab during burial at great depth—perhaps as a consequence of scraping off the seamounts. The spatially close association of
Communicated by J. Hoefs.
Electronic supplementary material The online version of this article (doi:10.1007/s00410-009-0427-0) contains supplementary material, which is available to authorized users. T. John Physics of Geological Processes (PGP), University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway T. John V. Schenk P. Herms R. Halama D. Garbe-Scho¨nberg Institut fu¨r Geowissenschaften and SFB 574, Universita¨t Kiel, Olshausenstr. 40, 24098 Kiel, Germany T. John (&) E. E. Scherer Institut fu¨r Mineralogie, Universita¨t Mu¨nster, Corrensstr. 24, 48149 Mu¨nster, Germany e-mail: [email protected]
MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments points to an exhumed high-pressure ophiolite sequence. Keywords Andes High-pressure ophiolite Seamount subduction Blueschist Eclogite Serpentinite Slab dismembering Earthquakes
Introduction Ophiolites are fragments of oceanic lithosphere that have been obducted onto continental crust during collisional events. Ophiolites sensu stricto comprise rocks of the uppermost lithospheric mantle, volcanic and intrusive rocks of the oceanic crust, and deep-sea sediments, all in their original structural positions (Anonymous 1972). In recent years, however, it has become increasingly evident that such a strict use of the term is no longer appropriate, given the complexities of structural features, chemical compositions, and tectonic settings of ophiolites (e.g., Dilek 2003). For example, Moores (1982) and Dilek (2003) have defined a Cordilleran- or Franciscan-type ophiolite in which dismembered parts of an oceanic lithosphere have been integrated into accretionary orogenic belts. They are tectonically intercala
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