Magmatic signatures and metasomatism in the high-pressure metamorphic Raspas Complex, Ecuador.

Halama, Ralf, John, Timm, Herms, Petra, Schenk, Volker, Garbe-Schönberg, Dieter and Hauff, Folkmar (2012) Magmatic signatures and metasomatism in the high-pressure metamorphic Raspas Complex, Ecuador. [Poster] In: The Lübeck Retreat, Collaborative Research SFB 574 Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters. , 23.05-25.05.2012, Lübeck . The Lübeck Retreat: final colloquium of SFB 574; May 23-25, 2012: program & abstracts. ; pp. 10-11 .

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Abstract

The Raspas Complex is a high-pressure (HP) metamorphic complex in southwest Ecuador,
representing an exhumed fragment of oceanic lithosphere. It is one of the few high-pressure
ophiolites that contain the whole lithological sequence of a typical subducting slab and is therefore ideally suited to investigate how the different parts of the slab and their element budgets behave during subduction. The complex is subdivided into the Raspas Formation, which comprises eclogites, blueschists and metasediments, and the El Toro Formation, which consists of eclogite-facies, serpentinized peridotites [1]. Peak P-T conditions for eclogites and garnet-chloritoid-kyanite metapelites are very similar at about 2 GPa and 550-600 °C, and the age of metamorphism is around 130 Ma [2, 3]. Two distinct types of eclogites can be distinguished based on petrography and trace element geochemistry. The first eclogite type has a geochemical signature similar to mid-ocean ridge basalts (MORB) [3, 4], whereas the second eclogite type is characterized by the presence of zoisite and the
relative enrichment in many incompatible trace elements (e.g. Th, U, Pb, LREE). Stable (Li, O) and radiogenic (Sr, Nd) isotopic compositions of the eclogites are used to constrain the effects of fluidinduced metasomatism in metabasaltic rocks at HP conditions and to identify fluid sources. For the MORB-type eclogites, Sr and O isotopic compositional differences on outcrop scale and Sr-Nd
isotopic trends typical for seafloor alteration suggest inheritance from variably altered oceanic crust [5]. Lithium isotope compositions point to Li isotopic fractionation by diffusion-related fluid-rock interaction, demonstrating the high sensitivity of the Li isotope system to metasomatic effects. The zoisite eclogites are thought to have formed by a fluid-induced high-pressure metasomatic overprint, and their Sr-Nd isotopic compositions indicate a metasedimentary fluid component [5]. Within the serpentinized ultramafic rocks, which show evidence for prograde HP dehydration, metamorphosed dikes contain pargasitic amphibole, vesuvianite and rare Ca-rich garnet. Although the primary magmatic mineral assemblage has been replaced, elevated Al2O3 and CaO contents and correlations between Ni and MgO are similar to pyroxenites from orogenic ultramafic massifs. Positive Eu anomalies and up to 10" chondritic REE abundances point towards an origin as plagioclasebearing ultramafic cumulates based on similarities to cumulate gabbros and plagioclase wehrlites from abyssal plateaus and ultramafic massifs [6].
References:
[1] Feininger, T. (1980): J. Petrol. 21:107-140. [2] Gabriele, P. et al. (2003): Eur. J. Mineral. 15:977-989. [3] John, T. et al.
(2010): Contrib. Mineral. Petrol. 159:265-284. [4] Halama, R. et al. (2010): Geochim. Cosmochim. Acta 74:1636-1652. [5]
Halama, R. et al. (2011): Chem. Geol. 281:151-166. [6] Borghini, G. et al. (2007): Lithos 94:210-229.

Document Type: Conference or Workshop Item (Poster)
Keywords: Geodynamics
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
OceanRep > SFB 574 > B6
OceanRep > SFB 574 > C2
OceanRep > SFB 574 > C1
OceanRep > SFB 574
Kiel University
Date Deposited: 10 Sep 2012 10:37
Last Modified: 21 Sep 2012 03:55
URI: http://oceanrep.geomar.de/id/eprint/15169

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