Subducted seamounts in an eclogite-facies ophiolite sequence: slab dismembering due to local stress amplification

Ralf Halama

The metamorphic Raspas Complex of southwest Ecuador comprises blueschists, eclogites, and high-pressure ultramafic and sedimentary rocks. The Lu-Hf garnet-whole rock ages of a blueschist, a metapelite, and an eclogite cluster at ca. 130 Ma. Peak metamorphic conditions of the high-pressure rocks occurred at 1.8 GPa and 600 °C, which corresponds to a maximum burial depth of about 60 km. According to their geochemical signatures, the eclogites formed from typical mid-ocean ridge basalts (MORB), whereas the blueschists had seamount-like protoliths and the eclogite-facies peridotites apparently stem from depleted, MORB-source mantle. These rocks were contemporaneously subjected to similar peak PT conditions, suggesting that they were subducted together as a coherent section within the downgoing slab. We hypothesize that this section became dismembered from the slab during burial at great depth as the seamounts were being scraped off. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments is suggestive of an exhumed high-pressure ophiolite sequence.

Contrib Mineral Petrol (2010) 159:265–284 DOI 10.1007/s00410-009-0427-0 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-Schönberg Received: 5 February 2009 / Accepted: 9 July 2009 / Published online: 30 July 2009 ! 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-Schönberg Institut für Geowissenschaften and SFB 574, Universität Kiel, Olshausenstr. 40, 24098 Kiel, Germany T. John (&) ! E. E. Scherer Institut für Mineralogie, Universität Münster, Corrensstr. 24, 48149 Münster, Germany e-mail: timm.john@uni-muenster.de 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 intercalated with mélanges and high-pressure metamorphic rocks that are characteristic of subduction zones. In rare cases, high-pressure, low-temperature suites comprise depleted mantle, mafic crust, and sediments, all of oceanic origin. Even though it may not be known when these units were juxtaposed, i.e., during formation, during burial, or during exhumation, they may be considered to be high-pressure ophiolite sequences (e.g., Ernst 2003). Together with experimental constraints, geochemical studies of such ophiolites and mélange zones may reveal 123 266 extends southward into the El Oro Metamorphic Complex. The latter comprises various lithologically diverse terranes that have distinct metamorphic ages ranging from the Paleozoic to the Cretaceous, and these have been divided into smaller units by several E-W-trending strike-slip faults (Aspden et al. 1995). One of these units, the Raspas Complex, is bounded to the North by the La Palma—El Guayabo shear zone and to the South by a major tectonic contact with the migmatitic gneisses of the Tahuin Group (Fig. 1). The Raspas Complex comprises two formations, (1) the El Toro Formation, which consists of eclogite-facies peridotite (Gabriele et al. 2002) and (2) the Raspas Formation, which comprises eclogites, blueschists, and garnet–chloritoid–kyanite metapelites (Feininger 1978, 1980; Gabriele 2002; Gabriele et al. 2003). We observed no pillow- and flow structures or sheeted dikes in the Raspas formation. In situ contacts between eclogite and blueschist have not been found due to the extensive cover by jungle or farmland. In some places, zoisite-rich veins penetrate the eclogites. At one locality in the Quebrada Raspas, where the contact between eclogite and peridotite is exposed, fuchsite-rich rocks have formed (Feininger 1980). Because the eclogites and blueschists are not intercalated with the serpentinized peridotite, they are apparently not part of a tectonic mélange (cf. Aspden et al. 1995; Bosch et al. 2002; Gabriele 2002). The peridotites of the El Toro formation N Arenillas Migmatitic gneisses 5 km La Palma-El Guayabo Shea r zo one ar z She Eclogite-facies serpentinized peridotite ne Eclogite, metapelite, blueschist mainly ec, bs and mp Shea r zon e ec o s illa en Greenschists Ri Ar Rio Arenillas Tahuin Dam Lake Piedras Amphibolites Raspas Formation Quito ECUADOR El Toro Formation Cuenca PERU Arenillas-Panupali unit Area of main figure Piedras unit El Oro Metamorphic Complex Biron Terrane and Tahuin Group Raspas Complex COLOMBIA ras Migmatitic gneisses d Pie Rio 123 io The Andean range continues to form where the Nazca and Antarctic oceanic plates subduct under the South American continent and it is typical of chains that form at long-lived convergent margins. Even though protracted subduction has been postulated, high-pressure rocks are surprisingly rare in the Andes (Maruyama et al. 1996). The Ecuadorian continental margin belongs to the northern Andean segment (North of 5"S) and is separated from the central segment by a distinct change in the convergence angle at the Huancabamba Deflection. Subduction of oceanic crust beneath the continental margin started in the late Jurassic (Jaillard 1990; Aspden et al. 1995). The suture zone between the predominately oceanic terranes and the South American plate, which is related to the Jurassic–Cretaceous subduction and accretionary processes, is located at the western side of the Eastern Cordillera (Litherland et al. 1994). Aspden et al. (1995) have suggested that this suture R Geologic setting Ra sp as how the different parts of the oceanic lithosphere and their element budgets behave during subduction into the mantle. In the northern Andes, four occurrences of high-pressure mafic rocks have been recognized (Feininger 1980, 1982; Orrego et al. 1980; De Souza et al. 1984), the southernmost of which being the Raspas Complex in Ecuador. This is the only known eclogite occurrence in the Andes, and it has an oceanic origin (Maruyama et al. 1996; Bosch et al. 2002). The Raspas Complex has been grouped together with various metamorphic terranes into the ‘‘El Oro Metamorphic Complex’’ (Aspden et al. 1995). Recent studies have interpreted these terranes as a tectonic mélange whose elements were juxtaposed either in the late Jurassic to early Cretaceous within the deeper part of an accretionary prism (Aspden et al. 1995) or in the Cretaceous (or later) during exhumation (Bosch et al. 2002; Gabriele 2002). A limited set of geochemical data was interpreted to suggest that the eclogite-facies mafic rocks were derived from an oceanic plateau, whereas the lower grade mafic rocks, i.e., amphibolites, represent typical mid-ocean ridge basalt (MORB)-like oceanic crust (Arculus et al. 1999; Bosch et al. 2002). In the present study, however, we report detailed petrological, geochemical, and geochronological data of eclogites, blueschists, and associated high-pressure peridotites from the Raspas Complex that strongly suggest that a coherent section of oceanic lithosphere—including associated seamounts—was subducted and later exhumed as a high-pressure ophiolite suite. Considering its limited spatial extent, this sequence is therefore one of the few high-pressure ophiolites that contain the whole sequence of an ideal oceanic plate, with rocks from the mantle, oceanic crust, and sea sediments. Contrib Mineral Petrol (2010) 159:265–284 continental units oceanic units Fig. 1 Geological map showing the main structures and lithological units of the El Oro Metamorphic Complex, SW Ecuador (modified after Gabriele 2002). The Raspas Complex was sampled along the small Rio Raspas. The dotted line divides the eastern part of the river section, where blueschists (bs) and metapelites (mp) frequently occur along with eclogite, from the western part, where metapelites are less abundant and blueschist is very rare Contrib Mineral Petrol (2010) 159:265–284 have been serpentinized to varying degrees. Locally, they host mafic dikes that have eclogitic mineral assemblages (Gabriele 2002; Gabriele et al. 2002), indicating subduction depths similar to those of the associated eclogite-facies meta-igneous- and sedimentary rocks of the Raspas Formation. The mafic rocks from the Arenillas-Panupali and Piedras Units, located to the south of the Raspas Complex, have been overprinted at greenschist- and amphibolitefacies conditions, respectively. The greenschist-facies rocks were interpreted to be former blueschists (Gabriele 2002), whereas the amphibolite-facies rocks were considered to be prograde amphibolites (Aspden et al. 1995; Gabriele 2002). Together, these two units and the Raspas Complex represent pieces of oceanic lithosphere sandwiched between continentally derived units, namely the Biron Terrane to the North and the Tahuin Group to the South (Feininger 1987; Gabriele 2002) (Fig. 1). In the Raspas Complex, phengite K–Ar ages (132 ± 5 Ma; Feininger 1980) and Ar–Ar ages (123–129 Ma; Gabriele 2002) suggest an early Cretaceous age for cooling of the eclogite-facies rocks. Arculus et al. (1999) and Bosch et al. (2002) suggested that the eclogite-facies mafic rocks of the Raspas formation were originally part of an oceanic plateau, whereas the lower grade mafic rocks, i.e., amphibolites, represent oceanic crust with MORB affinities. This proposed bimodal subdivision led to a model in which an oceanic plateau entered the subduction zone and blocked further subduction, whereas the MORB-type rocks were never buried to great depth and thus belong to a different oceanic terrane (Bosch et al. 2002; Gabriele 2002). In accordance with this model, it was suggested that a causal link between oceanic plateau subduction and the subsequent blockage of subduction and the associated westward jump of the subduction zone might exist (Bosch et al. 2002). Analytical methods Elemental analyses of minerals (Table 1) were performed at Kiel University on a JEOL 8900R electron microprobe equipped with five wavelength-dispersive spectrometers. The instrument is typically operated with a 15-kV acceleration voltage and a 15-nA beam for mineral analyses, except for garnet (20 kV) and apatite (10 kV) analyses. Spot sizes were generally *1 lm in diameter except for mica and amphibole (5 lm), and apatite (10 lm). The matrix correction of the raw counts was performed using CITZAF (Armstrong 1995). Both natural and synthetic mineral standards were used for calibration. Estimation of ferric iron in garnet and omphacite are based on charge balance, assuming ideal stoichiometry. Whole rock major element contents were analyzed with a Philips PW1480 XRF spectrometer at Kiel University 267 (Table 2). Concentrations of 26 trace elements were determined by inductively coupled plasma mass spectrometry (ICP-MS) after HF–HNO3–HClO4 acid digestion of *100 mg of pulverized sample material in Teflon bombs at 180"C. Before analysis, the sample solutions were diluted 20-fold and spiked with 5 ng/ml indium and rhenium for internal standardization. The instrument was calibrated using aqueous multi-element calibration standards without further matrix matching. Measurements were done with an Agilent 7500cs ICP-MS instrument under standard operating conditions. The analytical results represent averages of three replicate measurements after subtraction of a laboratory reagent blank. Analytical quality was controlled by analyzing procedural blanks (‘‘Blank’’), sample duplicates, and international reference standards along with the sample series. Results for the international rock standards BHVO-2 and UB-N are reported in the results table (Table 2). Concentrations determined from duplicate digestions of reference standards and samples differed by less than 3% for most elements. Instrument stability was monitored by re-analyzing one sample every hour. Precision as calculated from four replicate analyses was better than 3% RSD for all elements except heavy rare earth element (HREE), Th, and U (better than 5% RSD). Results for W, Mo, and Sb are less precise, with analytical errors in the range of 10, 15, and 20% RSD, respectively. Further details of the sample preparation procedure and instrument calibration strategy can be found in Garbe-Schönberg (1993) and John et al. (2008). The Lu–Hf analyses were carried out at the Zentrallabor für Geochronologie in Münster (Table 3). Whole rocks (2–3 kg) were crushed with a steel jaw crusher, with one split of the resulting chips being powdered in an agate mill and another being ground to \355 lm grains with a disc mill. Mineral separates were prepared using a magnetic separator and hand picking. Where possible, whole garnet crystals were picked without further magnetic separation to preserve original proportions of core and rim material. To remove surface contamination, mineral separates were washed for 10 min in cold 1 M HCl, and then rinsed with deionized water. All samples were spiked with a mixed 180Hf–176Lu tracer before digestion with HF–HNO3–HClO4. Digestion of whole rock powders was accomplished in PFA vials placed in steel-jacketed Teflon bombs at 180"C. Mineral separates were digested in PFA vials on a hotplate using alternating HF–HNO3–HClO4 and 10 M HCl treatments. This avoids digesting some Hf-bearing inclusions (zircon and to a lesser extent rutile) that could contain inherited- or post-garnet growth Hf signatures and adversely affect the true garnet age (Scherer et al. 2000). Complete digestion of other inclusions, such as titanite, omphacite, and amphibole still occurs however. The effect of the latter phases on garnet ages is generally minor as compared to that of zircon and depends on the abundance of inclusions in the bulk garnet separate, their 123 268 123 Table 1 Representative mineral chemical data of eclogites and blueschists sampled within the Raspas Complex Sample rock type mineral location SEC 43-1 eclogite omphacite 3_prf2_p11 SEC 42-6 eclogite omphacite 1p16 SEC 17-3 blueschist omphacite G3a p3 SEC 43-1 eclogite garnet 1p1 SEC 42-6 eclogite garnet 3p4 SEC 17-3 blueschist garnet 3p25 SEC 43-1 eclogite phengite 5ap8 SEC 42-6 eclogite phengite 6p8 SEC 17-3 blueschist phengite G3a p1 SEC 43-1 eclogite amphibole 1p3 SEC 17-3 blueschist amphibole 1p1 SEC 13-4 eclogite clinozosite K_92 SEC 15-1 blueschist clinozosite Ad27_293 SiO2 55.16 54.33 49.60 38.87 38.42 38.42 49.30 49.25 49.60 43.38 57.19 38.89 38.42 Al2O3 9.99 10.25 27.20 21.65 21.54 21.19 28.50 27.83 27.20 15.34 11.16 30.05 26.09 TiO2 0.17 0.21 0.11 0.02 0.00 0.17 0.656 0.80 0.11 0.55 0.08 0.19 0.11 FeO[tot] 4.89 6.99 2.82 24.09 24.99 27.96 1.91 2.44 2.82 12.47 11.00 n.c. n.c. Fe2O3 n.c. n.c. n.c. n.c. n.c. n.c. n.c. n.c. n.c. 2.99 2.51 5.31 10.17 FeO n.c. n.c. n.c. n.c. n.c. n.c. n.c. n.c. n.c. 9.78 8.74 0.00 0.00 Cr2O3 n.d. n.d. 0.02 n.d. n.d. n.d. 0.00 0.00 0.00 0.04 0.00 0.04 0.07 MgO 8.77 7.62 6.64 5.03 5.01 2.76 2.89 2.98 3.10 10.54 9.45 0.16 0.05 MnO 0.03 0.01 0.06 0.40 0.40 0.69 0.02 0.03 0.03 0.00 0.07 0.00 0.23 CaO 14.62 13.74 11.58 10.63 8.87 9.69 0.00 0.00 0.02 8.59 0.90 23.47 23.05 6.07 6.30 7.32 0.00 0.00 0.00 1.10 1.03 0.40 4.20 6.57 0.03 0.01 0.00 0.00 0.00 0.00 0.00 0.00 10.30 10.42 10.99 0.75 0.02 0.00 0.00 Total 99.70 99.45 99.59 100.69 99.23 100.88 94.67 94.78 94.27 96.16 96.69 97.62 98.19 Si 1.976 1.963 1.998 3.001 3.009 3.013 3.316 3.322 3.369 6.405 7.930 5.977 6.016 Al 0.422 0.437 0.475 1.971 1.989 1.959 2.259 2.213 2.177 2.669 1.824 5.443 4.815 Ti 0.005 0.006 0.000 0.001 0.000 0.010 0.033 0.041 0.005 0.061 0.009 0.022 0.013 Cr 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.005 0.000 0.005 0.008 Fe2? 0.039 0.068 0.039 1.556 1.637 1.834 0.107 0.138 0.160 1.208 1.014 0.000 0.000 Fe3? 0.107 0.143 0.175 n.c. n.c. n.c. n.c. n.c. n.c. 0.332 0.262 0.614 1.198 Mg 0.468 0.410 0.355 0.579 0.585 0.323 0.290 0.300 0.314 2.320 1.953 0.037 0.011 Mn 0.001 0.000 0.002 0.026 0.027 0.046 0.001 0.000 0.002 0.000 0.008 0.000 0.030 Ca 0.561 0.532 0.446 0.879 0.744 0.814 0.000 0.135 0.001 1.359 0.134 3.865 3.867 Na 0.422 0.441 0.510 0.000 0.000 0.000 0.143 0.897 0.053 1.202 1.766 0.009 0.004 K 0.000 0.000 0.000 0.000 0.000 0.000 0.884 0.000 0.952 0.141 0.003 0.000 0.000 Total 4.000 4.000 4.000 8.013 7.991 7.999 7.034 7.048 7.033 15.702 14.903 15.974 15.962 Basis of O 6 6 6 11.5 11.5 11.5 11 11 11 23 23 26 26 XFe 0.19 0.26 0.33 0.27 0.32 0.34 0.34 0.34 Na ? K 1.03 1.03 1.01 Na/K 0.16 0.15 0.06 Na 0.56 0.00 K 0.14 0.00 0.73 0.74 0.85 XAlm 0.51 0.55 0.61 XGrs 0.29 0.25 0.27 XPrp 0.19 0.20 0.11 XSps 0.01 0.01 0.02 Q 0.57 0.55 0.49 Jd 0.39 0.38 0.47 Ae 0.04 0.07 0.04 Fe3?/FeTot 0.27 0.32 0.18 A-position n. c. not calculated, n.d. not determined Contrib Mineral Petrol (2010) 159:265–284 Na2O K2O Contrib Mineral Petrol (2010) 159:265–284 269 Table 2 Major and trace element compositions of eclogites, blueschists, and serpentinized peridotites Sample Rock-type Eastinga Northinga SEC 15-2 Blueschist 618202 9602278 SEC 16-1 Blueschist 619377 9602453 SEC 17-3 Blueschist 619522 9602542 SEC 42-6 Eclogite 618451 9601561 SEC 43-1 Eclogite 618458 9601634 SEC 43-3 Eclogite 618458 9601634 SEC 44-1 Eclogite 618578 9601854 SEC 46-1 Eclogite 618842 9602130 SEC 46-2 Eclogite 618842 9602130 SEC 47-1 Eclogite 618988 9602363 SEC 50-1 Eclogite 619587 9602502 SEC 26-3 Peridotite 617973 9600393 40.24 Major elements (wt%) SiO2 47.79 47.31 47.23 45.61 47.92 50.17 48.67 44.08 44.88 47.49 49.57 TiO2 1.95 2.39 2.25 2.26 1.79 1.86 1.91 1.87 2.00 1.94 1.88 0.04 Al2O3 14.80 18.15 17.09 14.54 14.67 13.89 14.84 14.71 13.49 15.07 13.62 1.62 Fe2O3 10.65 10.18 10.03 15.14 13.04 13.30 13.75 15.48 13.77 13.87 13.30 9.86 MnO 0.20 0.16 0.15 0.28 0.19 0.23 0.19 0.24 0.21 0.23 0.19 0.14 MgO 7.00 4.44 4.12 6.16 6.46 6.65 6.00 10.09 8.72 6.01 6.72 37.85 CaO 10.50 8.36 13.01 11.84 11.10 10.67 10.84 10.87 13.41 11.15 10.48 1.47 Na2O 3.04 3.29 2.40 3.26 3.09 3.02 3.02 1.69 2.30 2.77 3.04 0.04 K2O 0.27 1.59 0.51 0.15 0.53 0.07 0.48 0.16 0.11 0.51 0.07 0.01 P2O5 0.31 0.54 0.51 0.20 0.13 0.17 0.16 0.11 0.74 0.17 0.17 0.02 LOI 3.72 2.64 1.79 0.27 0.57 0.42 0.00 0.45 0.00 0.37 0.81 8.81 Total 100.23 99.05 99.09 99.71 99.49 100.45 99.86 99.75 99.63 99.58 99.85 100.10 0.84 Trace elements (ppm) Li 19.4 62.5 40.7 35.2 48.5 13.8 51.4 11.0 13.3 67.3 20.3 Sc 29.0 35.6 37.8 46.0 44.3 42.6 43.2 44.8 48.0 42.9 39.6 11.6 V 209 254 272 415 376 390 389 362 431 358 329 67.2 Crb 265 24 235 187 214 111 211 253 252 256 144 2,758 Co 37.2 31.5 44.5 50.2 45.7 40.1 45.0 51.9 37.9 45.4 38.4 126 Ni 146 112 141 80.0 77.5 53.0 75.9 71.2 59.1 74.2 47.0 2,276 Cu 48.2 46.9 45.6 78.5 92.6 35.3 52.8 79.0 137 69.3 32.9 16.2 Zn 77.8 113 82.6 136 229 106 218 280 277 143 57.4 53.0 Ga 16.3 25.2 24.5 18.7 19.2 18.2 18.4 13.1 10.9 19.0 16.3 1.52 Rb 2.21 28.2 7.11 4.38 15.3 1.06 13.1 0.53 0.42 12.7 0.54 0.032 Sr 269 329 376 76.7 92.5 77.0 66.8 31.6 36.8 103 84.9 2.28 Y 31.9 39.3 41.2 52.6 43.1 46.1 45.1 49.1 46.4 48.0 50.5 0.775 Zr 165 240 234 131 116 109 110 102 104 109 119 0.161 Nb 14.8 29.2 26.9 3.64 2.04 2.40 2.03 2.19 4.26 2.09 1.91 0.014 Mo 0.37 0.09 0.09 0.25 0.13 0.07 0.14 0.13 0.23 Sn 1.93 1.97 1.53 0.788 0.966 0.743 Sb 0.30 0.35 0.09 0.37 0.32 0.15 0.21 0.20 0.10 1.18 0.48 1.2 0.130 0.59 Cs 0.127 1.07 0.229 0.144 0.837 0.019 0.777 0.011 0.008 0.632 0.019 0.005 Ba 47.93 1,466 91.1 77.8 98.8 21.2 79.4 6.76 21.1 60.9 121 4.33 La 13.7 24.2 21.7 5.73 4.09 3.75 3.70 4.08 4.40 3.71 3.72 0.027 Ce 36.3 54.1 53.6 16.1 11.8 12.7 11.6 13.6 14.1 11.7 12.4 0.033 Pr 4.49 7.45 6.54 2.90 2.14 2.28 2.17 2.53 2.46 2.18 2.32 0.008 Nd 19.8 31.2 27.7 15.5 11.5 12.6 11.9 13.9 12.9 12.4 12.7 0.050 Sm 5.07 7.35 6.61 5.36 4.03 4.38 4.23 5.11 4.28 4.54 4.62 0.032 Eu 1.79 2.38 2.34 1.82 1.43 1.57 1.57 1.71 1.51 1.57 1.59 0.014 Gd 5.51 7.68 6.94 6.94 5.47 5.90 5.70 6.74 5.60 6.12 6.34 0.069 Tb 0.88 1.21 1.10 1.31 1.00 1.08 1.07 1.18 1.00 1.17 1.17 0.016 Dy 5.43 7.31 6.81 8.84 6.95 7.33 7.35 7.89 6.96 7.92 8.01 0.124 Ho 1.08 1.43 1.33 1.89 1.49 1.58 1.56 1.66 1.55 1.69 1.71 0.030 Er 2.94 3.93 3.64 5.40 4.26 4.59 4.48 4.74 4.52 4.83 4.89 0.091 Tm 0.425 0.566 0.525 0.800 0.628 0.688 0.668 0.713 0.679 0.712 0.727 0.016 Yb 2.80 3.76 3.44 5.36 4.16 4.60 4.49 4.83 4.54 4.80 4.89 0.119 Lu 0.413 0.553 0.510 0.788 0.600 0.674 0.654 0.723 0.678 0.709 0.730 0.021 Hf 3.82 5.23 5.13 3.68 3.01 2.85 2.89 2.86 2.91 3.13 3.25 0.014 Ta 0.928 1.73 1.60 0.239 0.126 0.141 0.129 0.140 0.183 0.146 0.142 0.000 W 0.26 0.17 0.52 0.40 0.17 0.45 Pb 1.28 2.44 2.39 2.97 3.81 1.50 4.06 0.646 0.566 0.30 2.90 0.738 0.03 Th 1.06 2.12 1.99 0.424 0.206 0.208 0.146 0.131 0.170 0.138 0.137 0.002 U 0.331 0.459 0.380 0.328 0.212 0.069 0.184 0.175 0.540 0.194 0.054 0.011 Nb/Zr 0.090 0.122 0.115 0.028 0.018 0.022 0.018 0.021 0.041 0.019 0.016 0.089 Hf/Yb 1.37 1.39 1.49 0.69 0.72 0.62 0.64 0.59 0.64 0.65 0.66 0.12 (La/Sm)N 1.70 2.07 2.07 0.67 0.64 0.54 0.55 0.50 0.65 0.51 0.51 0.53 Nb/La 1.08 1.21 1.24 0.64 0.50 0.64 0.55 0.54 0.97 0.56 0.51 0.53 0.279 123 270 Contrib Mineral Petrol (2010) 159:265–284 Table 2 continued Sample Rock-type Eastinga Northinga SEC 26-9 Peridotite 617973 9600393 Major elements (wt%) SiO2 41.66 TiO2 0.08 Al2O3 2.84 Fe2O3 8.67 MnO 0.12 MgO 37.22 CaO 2.76 Na2O 0.25 K2O 0.02 P2O5 0.01 LOI 6.98 Total 100.61 Trace elements (ppm) Li 3.96 Sc 16.2 V 80.7 Crb 2,514 Co 107 Ni 1,911 Cu 18.7 Zn 50.2 Ga 2.59 Rb 0.169 Sr 3.61 Y 2.15 Zr 0.772 Nb 0.007 Mo 0.21 Sn 0.162 Sb 0.23 Cs 0.131 Ba 4.57 La 0.008 Ce 0.033 Pr 0.015 Nd 0.157 Sm 0.123 Eu 0.051 Gd 0.228 Tb 0.049 Dy 0.366 Ho 0.083 Er 0.245 Tm 0.039 Yb 0.272 Lu 0.042 Hf 0.066 Ta 0.001 W 0.17 Pb 0.676 Th 0.001 U 0.000 Nb/Zr 0.009 Hf/Yb 0.24 (La/Sm)N 0.04 Nb/La 0.86 SEC 34-1 Peridotite 611704 9603766 SEC 35-2 Peridotite 611754 9603691 SEC 36-2 Peridotite 611786 9603642 SEC 40-1 Peridotite 610826 9604149 40.92 0.05 2.02 7.92 0.14 38.82 1.67 0.03 0.00 0.01 9.24 100.82 40.49 0.05 2.37 8.02 0.12 37.26 2.48 0.07 0.01 0.01 10.01 100.89 40.24 0.04 1.34 8.23 0.12 40.43 0.28 0.00 0.00 0.01 10.06 100.75 41.54 0.03 1.69 8.64 0.13 40.75 1.85 0.25 0.01 0.00 5.67 100.56 1.18 11.4 55.6 2,308 120 2,233 2.20 40.7 1.94 0.120 1.15 1.33 0.315 0.024 0.07 0.046 0.14 0.104 2.36 0.003 0.015 0.006 0.064 0.055 0.025 0.114 0.026 0.202 0.048 0.148 0.025 0.174 0.029 0.030 0.001 0.04 0.377 0.001 0.001 0.076 0.17 0.04 7.62 0.99 16.8 73.6 2,674 104 1,957 4.82 50.3 2.10 0.215 3.53 1.58 9.49 0.008 0.19 0.124 0.14 0.157 4.87 0.004 0.014 0.006 0.069 0.067 0.032 0.147 0.034 0.266 0.062 0.186 0.030 0.204 0.031 0.238 0.000 0.09 0.280 0.001 0.003 0.001 1.17 0.04 2.04 1.03 10.5 37.1 2,775 100 2,116 2.47 43.0 1.56 0.071 1.51 0.504 0.294 0.036 0.06 0.084 3.09 0.020 3.17 0.016 0.015 0.006 0.040 0.019 0.008 0.037 0.008 0.061 0.017 0.055 0.011 0.081 0.015 0.015 0.002 0.06 0.223 0.001 0.003 0.123 0.19 0.53 2.25 2.76 11.2 51.1 2,344 102 2,125 20.3 42.3 1.72 0.2 1.63 0.919 0.049 0.034 0.07 0.107 0.03 0.218 1.32 0.006 0.013 0.003 0.020 0.024 0.011 0.061 0.016 0.134 0.035 0.110 0.019 0.138 0.022 0.011 0.002 0.14 0.179 0.001 0.001 0.685 0.08 0.15 5.98 The La values in italics are most likely contaminated by blank UTM zone 17M b Cr is the only trace element measured by XRF, all others were measured by ICP-MS c Mo, Sn, Sb, and W only in 8 BHVO-2 and 3 UB-N standards measured a 123 BHVO-2 Basalt UB-N Serpentinite Basalt n = 15c Serpentinite n = 5c Mean SD RSD% Mean SD RSD% 4.52 32.4 323 279 44 118 127 105 21.1 8.98 400 24.6 169 17.2 4.5 1.7 0.1 0.098 135 15.0 36.8 5.37 24.4 6.06 2.04 6.10 0.936 5.23 0.959 2.41 0.319 1.94 0.270 4.40 1.09 0.30 1.58 1.19 0.414 0.102 2.26 1.56 1.15 0.2 1.6 11.9 21.5 1.7 4.9 4.8 4.1 0.5 0.2 5.8 0.8 7.1 0.3 0.5 0.1 0.2 0.006 2.6 0.3 0.9 0.1 0.3 0.09 0.05 0.1 0.01 0.1 0.02 0.03 0.007 0.08 0.01 0.1 0.02 0.04 0.09 0.03 0.01 4.7 4.9 3.7 7.7 3.9 4.2 3.8 3.9 2.5 2.7 1.4 3.4 4.2 1.9 10.1 2.8 21.3 5.7 1.9 1.7 2.3 1.8 1.2 1.4 2.5 2.3 1.5 2.4 2.1 1.4 2.3 3.9 3.6 2.7 1.4 14.6 5.6 2.7 2.4 28.4 12.4 66.5 2.79 0.2 3.3 9.8 1.5 5.0 97.8 1,783 23.1 77.8 2.59 3.32 7.74 2.53 3.64 0.056 0.50 0.265 0.33 10.7 28.3 0.309 0.772 0.117 0.590 0.211 0.080 0.301 0.059 0.409 0.091 0.274 0.041 0.282 0.044 0.126 0.016 17.62 12.1 0.062 0.058 0.015 0.45 0.92 0.18 1 43 0.9 1.1 0.07 0.1 0.2 0.1 0.1 0.01 0.01 1.5 2.4 3.9 1.4 2.8 2.8 2.9 4.1 3.1 10.5 1.3 0.08 0.3 2.1 0.01 0.03 0.003 0.009 0.003 0.002 0.006 0.001 0.006 0.002 0.004 0.001 0.006 0.001 0.005 0.002 2.83 0.7 0.004 0.003 24.4 2.9 7.4 3.5 3.8 2.9 1.5 1.6 3.0 1.9 1.5 1.5 1.7 1.5 2.3 2.2 2.7 3.6 11.7 16.1 6.0 6.8 4.8 Contrib Mineral Petrol (2010) 159:265–284 271 Table 3 Lu–Hf isotope data Sample, fraction Lu (ppm) Hf (ppm) 176 Lu/177Hf 2 RSD (%) 176 0.633 3.04 0.02960 0.26 Hf/177Hf 2 SE abs. 2 RSD (%) eHf(t) 0.283244 (6) 0.0045 ?16.6 0.283222 (6) 0.0045 Eclogite SEC 43-1 wr 0, bombed 3.04 wr 1, bombed 0.629 3.14 0.02842 0.25 0.283222 (6) 0.0046 cpx 0 0.0406 0.502 0.01149 0.25 0.283275 (8) 0.0062 0.283248 (8) 0.0062 cpx 1 0.0302 0.500 0.008577 0.25 0.283174 (5) 0.0041 [500 lm grains 0.863 0.322 0.3801 0.27 0.284151 (17) 0.0247 grt 0 1.92 0.110 2.487 0.27 0.289436 (19) 0.0141 0.502 grt 1 impure 2.04 0.158 1.837 0.26 0.287823 (16) 0.0116 grt 2 grt 3 whole grains 1.58 1.86 0.0324 0.125 6.943 2.118 0.27 0.25 0.300454 0.288339 (28) (12) 0.0195 0.0167 grt 4 whole grains 1.74 0.116 2.142 0.25 0.288433 (14) 0.0199 wr, bombed 0.536 5.23 0.01454 0.25 0.282997 (8) 0.0119 \250 lm grains 0.465 0.364 0.1816 0.25 0.283465 (24) 0.0342 ?15.9 Blueschist SEC 16-1 amp, px, & grt \500 lm 1.64 0.347 0.6722 0.29 0.284618 (21) 0.0295 grt 1 3.12 0.191 2.319 0.26 0.288389 (14) 0.0199 grt 2 3.17 0.191 2.359 0.26 0.288607 (15) 0.0207 7.76 0.009410 0.25 ?9.1 Metapelite SEC 47-4 wr, bombed 0.515 7.76 0.282356 (6) 0.0096 0.282353 (11) 0.0159 wr, hotplate 0.311 0.853 0.05184 0.26 0.282682 (15) 0.0215 grt 1 1.13 0.255 0.6307 0.27 0.284067 (7) 0.0111 grt 2 1.10 0.228 0.6867 0.27 0.284237 (9) 0.0127 -12.8 -5.2 Estimated uncertainties for 176Lu/177Hf values are based on the typical reproducibility of two to four analyses of single sample solutions and include error magnification for over- or under-spiking. For 176Hf/177Hf, the typical in-run uncertainties in the last digits are shown in parentheses. The relationship between in-run statistics (2 SE abs. = 2 times standard error, absolute) and the external reproducibility (2 RSD% = 2 standard deviations, reported as a percentage of the measured value) of our Hf standard solution analyzed at different concentrations was used to estimate the 2 RSD% uncertainties of samples that could only be analyzed once (see Bizzarro et al. 2003). Initial eHf values were calculated using the isochron ages and 176Hf/177Hf = 0.282785 and 176Lu/177Hf = 0.0336 for the CHUR reference (Bouvier et al. 2008) Hf concentration, and their age. Chemical separation procedures for Lu–Hf follow those of Münker et al. (2001), with an additional cation exchange step [a miniaturized version of column ‘A’ of Patchett and Tatsumoto (1980), but using AG50W-X12 resin] to remove all remaining traces of Lu and Yb from the Hf cuts. Lutetium and Hf were analyzed in static mode on a Micromass Isoprobe multi-collector ICP-MS, using 179Hf/177Hf = 0.7325 to apply an exponential law mass bias correction. For Lu isotope dilution measurements, the correction of the isobaric interference from 176Yb was performed using the linear relationship between ln(176Yb/171Yb) and ln(174Yb/171Yb) of Yb standards run interspersed with samples (e.g., Blichert-Toft et al. 2002; Barfod et al. 2003). Admixed Re was then used to apply an external mass bias correction to the Lu isotope ratio measurements (Scherer et al. 2001). Measured 176Hf/177Hf values are reported relative to 176Hf/177Hf = 0.282163 for the Münster Ames Hf standard, which is isotopically indistinguishable from the JMC-475 standard. Procedural blanks for Lu and Hf were B42 and B71 pg, respectively. Calculated ages and initial isotope compositions are based on k176Lu = 1.867 9 10-11/year (Scherer et al. 2001, 2003; Söderlund et al. 2004). The chondritic uniform reservoir (CHUR) parameters used for calculating initial eHf values are 176Hf/177Hf = 0.282785 and 176Lu/177Hf = 0.0336 (Bouvier et al. 2008). The resulting values are 0.4 epsilon units lower than those calculated using the CHUR parameters of Blichert-Toft and Albarède (1997). Petrology Sample descriptions Different varieties of eclogite, blueschist, and serpentinized peridotite occur in the Raspas Complex. The eclogites 123 272 contain variable amounts of barroisitic or katophoritic amphibole and are frequently phengite-bearing. Blueschists predominately contain amphibole, epidote, white mica, and garnet. The serpentinized peridotites are strongly lineated with a partly mylonitic texture. The main focus of our petrological work is related to the eclogites and blueschists because they have the highest potential to preserve information of their metamorphic evolution. The serpentinized peridotites will only be described briefly. Eclogites The eclogites are fine- to medium-grained rocks. Their prograde mineral assemblage is garnet ? omphacite ? rutile ? barroisitic or katophoritic amphibole ± quartz ± phengite ± carbonate. Accessory minerals include apatite, zircon, and in some samples, sulfides (pyrite, pyrrhotine, and chalcopyrite). Most eclogites are strongly deformed and partially recrystallized. In rare cases, they display almost monomineralic layers of garnet, omphacite, or amphibole. Garnet forms sub- to euhedral porphyroblasts (Fig. 2a), which typically contain inclusions of rutile and quartz, and sometimes inclusions of almost the entire prograde mineral assemblage. Subhedral omphacite grains up to 1 cm long mark the stretching lineation. Prograde subhedral amphibole occurs in textural equilibrium with omphacite (Fig. 2b). Whereas some eclogites only contain minor amounts of amphibole, others consist of garnet, amphibole, and rutile almost exclusively. Although the latter rocks are almost omphacite-free, we group them together with the eclogites on the basis of chemical similarities between phases common to both rock types and because they mainly represent layers of variable width within the eclogites. The amount of amphibole present in the eclogite seems to be mainly a function of fluid availability during the prograde metamorphic evolution. Phengite occurs in the eclogites either as large subhedral grains or interstitially (Fig. 2c). Late-stage titanite was observed in a few eclogites as rims around rutile or as interstitial, amoeboid grains. Intense retrogression to greenschist assemblages (albite, epidote, actinolite, carbonate, chlorite, and titanite) has only been observed in discordant veins within the eclogites and in the vicinity of these veins. Blueschists The fine- to medium-grained blueschists contain bluish amphibole with brownish green to bluish-green cores, garnet, clinozoisite, omphacite, paragonite, phengite, quartz, titanite, rutile, accessory apatite and zircon, and secondary carbonate and chlorite. The blueschists are usually strongly deformed with their constituent minerals being aligned with the foliation. White mica tends to occur 123 Contrib Mineral Petrol (2010) 159:265–284 as either large flakes, usually of paragonite composition, or as clusters of smaller grains comprising either paragonite or phengite. Omphacite is a less abundant phase, but in some subdomains it is in textural equilibrium with garnet and white mica. Clinozoisite and titanite inclusions are abundant in the cores of garnet porphyroblasts, whereas in garnet rims rutile inclusions are more abundant. Titanite also occurs in omphacite grains and brownish amphibole cores. The titanite and quartz inclusions in the garnet cores define an internal schistosity (si) that distinctly deviates from the external schistosity (se) (Fig. 2d). In other cases, garnet porphyroblasts show corrosive irregular grain boundaries in the fine-grained schistose matrix (Fig. 2e). Matrix rutile formed exclusively as a late prograde phase and is almost always rimmed by titanite. Blue amphibole formed around brownish or bluish-green amphibole (Fig. 2f) and in some cases it contains rutile inclusions. The blue amphibole rims were obviously grown in a rutilebearing matrix contemporaneously with the garnet rims, which also contain rutile inclusions. Locally, small omphacite inclusions in garnet rims were detected during microprobe work. Thus, the blueschist assemblage overprints an older high-pressure assemblage. Serpentinized peridotites The serpentinized peridotites are mostly schistose but some massive, very coarse-grained (cm-scale) types were found in which primary olivine, orthopyroxene, and clinopyroxene are preserved. The serpentinized rocks contain antigorite, chlorite, amphibole, spinel (chromite and magnetite), titanian clinohumite, carbonate, and chrysotile. The degree of serpentinization varies from almost pristine peridotite (B10%) up to serpentinite (95%). Antigorite is the main serpentine mineral and may constitute up to 90% of a rock. Minor chrysotile occurs along veins and fractures in olivine of coarse-grained massive peridotites. Less serpentinized rocks contain deformed, primary olivine, as well as recrystallized metamorphic olivine (Fig. 2g), orthopyroxene porphyroclasts (up to centimeter-size), and partially recrystallized clinopyroxene. At higher degrees of serpentinization, a mylonitic texture has developed, with strongly deformed and elongated ortho- and clinopyroxene porphyroclasts and aggregates within a fine-grained serpentine matrix. In less-sheared rock layers, metamorphic olivine is typically intergrown with large euhedral antigorite blades (Fig. 2g). Amphibole has partly replaced clinopyroxene, whereas chlorite has partially replaced orthopyroxene and olivine. Titanian clinohumite occurs at the margins of olivine and clinopyroxene clasts, and within recrystallized aggregates of these minerals (Fig. 2h). Anhedral carbonate grains occur either as vein fillings or as part of porphyroclast replacement assemblages. Contrib Mineral Petrol (2010) 159:265–284 273 Fig. 2 Microstructures of the eclogites, blueschists, and peridotites. a phengite-bearing eclogite with almost euhedral garnet porphyroblasts. b Eclogite with prograde amphibole in textural equilibrium to garnet and omphacite. c Eclogite displaying textural equilibrium among garnet, omphacite, and phengite. Note the rutile inclusions in garnet and angles of approximately 120" between the phases. d Typical textures of garnet porphyroblasts in blueschist. The inclusion trails comprise mainly titanite and quartz and are at an oblique angle to the dominant schistosity of the rock. e Blueschist with garnet porphyroblast showing irregular grain boundaries. f Typical blueschist textures. Note the formation of glaucophane around barroisite and the occurrence of phengite and clinozoisite in the matrix. g Serpentinized peridotite showing metamorphic olivine intergrown with large euhedral antigorite blades. h Microstructure of an extensively serpentinized peridotite with titanium clinohumite beside metamorphic olivine and antigorite. amph amphibole, ant antigorite, bar barroisite, chum titanium-clinohumite, czo clinozoisite, gln glaucophane, grt garnet, ol olivine, omp omphacite, phe phengite, rt rutile Mineral chemistry Omphacite Omphacite is rather uniform in composition within the eclogite and blueschist groups, but on average, blueschist omphacite tends to have slightly more of the jadeite (Jd) component than eclogite omphacite (Table 1; Supplementary Figure 1). Contents of Jd and Ae vary between 37 and 40 mol% and between 3 and 10 mol%, respectively. The XFe values range from 0.21 to 0.24. The highest Jd-content measured in an omphacite from an eclogite is 43 mol% Jd, at 4 mol% Ae and an XFe value of 0.22. Garnet Garnet porphyroblasts in the eclogites and blueschists show chemical growth zoning characterized by almandine- and sometimes spessartine-rich cores, and pyrope-rich rims (Table 1). The grossular (Grs) content typically decreases 123 274 from the core outwards, with an increase at the outermost rim (Supplementary Figure 2). The XFe value varies slightly from sample to sample, but generally decreases from core to rim with a final increase at the outermost rim. For eclogites, the range of core and prograde rim compositions is *Alm58–61Prp15–18Grs22–28Sps01–02, with an XFe value of 0.76–0.82, and *Alm52–54Prp20–24Grs23–26Sps\01, with an XFe value of 0.68–0.73, respectively. For the blueschists, core compositions are *Alm55–62Prp05–10Grs25–40Sps05–10, with an XFe value of 0.84–0.95, and prograde rim compositions are *Alm58–62Prp12–18Grs22–25Sps\02, with an XFe value of 0.76–0.82. White mica The white mica occurs either as phengite or paragonite. The blueschists contain both paragonite and phengite, whereas eclogites contain only phengite. The latter can be either zoned or unzoned with respect to Si content, which averages *3.29 Si atoms per formula unit (apfu) (Supplementary Figure 3; Table 1). Silicon contents of phengite cores and unzoned phengite vary from sample to sample and have ranges of 3.30–3.33 and 3.32–3.35 apfu, respectively. Rim compositions of zoned phengite grains range between 3.25 and 3.29 apfu. Average XFe values also differ from sample to sample and show an overall range from 0.29 to 0.35. Contents of Na and K range between 0.09 and 0.17 apfu, and between 0.85 and 0.91 apfu, respectively. No correlations between Na/K and Si content were observed. Paragonite is quite abundant in blueschists, where it occurs mainly as coarse-grained books. Paragonite is unzoned and its Si content ranges from 3.00 to 3.05 apfu for all samples. The paragonite component varies from 90 to 98 mol% and XFe values are in the range of 0.60–0.70. Contrib Mineral Petrol (2010) 159:265–284 amphiboles are enriched in Na and K as compared to the barroisite, with (Na ? K)A ranges of 0.7–0.8 and 0.40– 0.49, respectively. Rims can be of taramitic composition due to lower Si (6.2–6.3 apfu) and higher (Na ? K)A (0.9 apfu). The XFe values vary from rock to rock but are usually around 0.3 to 0.4, with slightly higher values at the rims. The late-stage amphibole composition ranges from magnesio-hornblende to actinolite. Epidote minerals Clinozoisite is the epidote-group mineral in all investigated samples. Clinozoisite in the eclogites has a low XEp of around 0.15, low Cr2O3 (*0.04 wt%), and low MnO (*0.05 wt%), whereas the TiO2 content is relatively high (up to 0.2 wt%). Clinozoisite in the blueschists may be zoned or unzoned. Zoned clinozoisite grains have average core compositions of XEp *0.28, *0.2 wt% Cr2O3, and *0.3 wt% MnO, and rim compositions of XEp *0.36, *0.04 wt% Cr2O3, and 0.02–0.1 wt% MnO. The cores usually have higher TiO2 concentrations than the rims. The XEp values and contents of Cr2O3, MnO, and TiO2 in clinozoisite vary only slightly within zoned grains and among grains in the same rock. Geothermobarometry Geothermobarometric estimates were made for the eclogites and blueschists (Fig. 3). Temperature estimates were obtained for both rock types by means of the Fe–Mg exchange garnet–clinopyroxene thermometer of Powell (1985), considering ferrous and ferric iron distribution, and the garnet–phengite thermometer of Green and Hellman Amphibole 123 ) /km oC 2.0 P [GPa] Sodic- to sodic–calcic amphiboles dominate in blueschists, where they are major phases. They can be classified as glaucophane (Table 1) and barroisite (nomenclature after Leake et al. 1997). Barrositic cores often have glaucophane rims (Fig. 2f). Glaucophane has Fe3?/(Fe3? ? AlVI) between 0.14 and 0.21 and XFe values between 0.32 and 0.36. Barroisite, which generally has higher Al and Ca and lower Si and Na contents than glaucophane, displays NaB = 0.8–1.0 in most samples. The XFe values range from 0.28 to 0.35 and are usually higher where barroisite coexists with glaucophane. In barroisite, (Na ? K)A typically ranges between 0.3 and 0.5, whereas there is no Na on the A site in glaucophane. Sodic–calcic amphiboles in the eclogites are either barroisitic or Mg-katophoritic in composition (Table 1), with the latter having higher Al and lower Si at similar Ca concentrations. The Mg-katophoritic 3.0 5 n( tio uc ld co d ub metapelites eclogites s eclogites serpentinites 1.0 ction bdu m su blueschists ] this study /km) o (13 C war 0 200 400 600 800 T[oC] Fig. 3 Peak pressure–temperature (P–T) estimates for all units of the Raspas Complex. The values for the blueschists and the eclogites obtained in this study are indicated by blue and green boxes. It is evident that all units experienced similar peak P–T conditions. The P– T range for metapelites (dark gray) is given in Gabriele et al. (2003). Data for other eclogites (medium gray) and serpentinites (light gray) are from Gabriele (2002) and Gabriele et al. (2002), respectively Contrib Mineral Petrol (2010) 159:265–284 (1982). The garnet–clinopyroxene calibrations yield a temperature range of 550–650"C for the blueschists at 1.4– 1.6 GPa (see below), whereas the temperatures obtained from the garnet–phengite thermometer are slightly lower (550–600"C). However, most (eight of ten) of the estimates obtained with both calibrations fall within 600 ± 20"C. These estimates are similar to—but span to lower values than—those calculated for the eclogites, which range between 600 and 700"C at 1.6–1.8 GPa (see below) when applying the garnet–clinopyroxene calibration, and 550– 680"C for the garnet–phengite calibration. Again, most temperature estimates with both calibrations cluster around 600"C (600 ± 30"C; 9 of 12). The absence of kyanite and the presence of paragonite in the blueschists provides a maximum H2O-dependent pressure of about 1.8–2.0 GPa at 500–700"C using the univariant reaction Pg = OmpJd50 ? Ky ? H2O (Holland 1979). An H2O-independent pressure estimate can be gained from the reaction Ab = Jd ? Qtz (Holland 1980). In the absence of albite, the maximum jadeite content in omphacite (43 mol%) indicates a minimum pressure of *1.3 GPa at 600"C. The garnet–omphacite–phengite equilibrium after Waters and Martin (1993) gives equilibrium pressures of 1.4–1.6 GPa at 600"C for the blueschists and 1.6–1.8 GPa at 600"C for the eclogites when using (1) omphacite having 43 mol% jadeite, (2) phengite having the average of highest measured Si contents, and (3) garnet rims with the highest XPrp and XGrs, as measured in subdomains that exhibit apparent textural equilibrium among all three minerals (e.g., Fig. 2c). Our pressure–temperature (P–T) estimates are in agreement with the P–T calculations of Gabriele (2002) who reported 600 ± 50"C at 1.8 GPa for the eclogites and Feininger (1980) who calculated temperatures of 580 ± 20"C for estimated minimum pressures of 1.3 ± 0.3 GPa. Very similar P–T estimates (*600 ± 50"C at 2.0 GPa) were obtained from phase equilibria considerations for garnet–chloritoid–kyanite metapelites of the Raspas Complex (Gabriele et al. 2003). According to Gabriele et al. (2002), relict omphacite–garnet–rutile associations in mafic dikes within the serpentinized peridotites additionally indicates eclogite-facies pressures for the serpentinized peridotites of the El Oro formation, for which maximum temperatures of about 550–650"C are constrained by the upper stability limit of the assemblage olivine ? diopside ? antigorite ? Ti-clinohumite (Engi and Lindsley 1980; Trommsdorff and Evans 1980; Trommsdorff et al. 1998; Gabriele et al. 2002). In some serpentinized peridotites of the western Alps, the tectonic relationships between eclogite and serpentinite (e.g., Scambelluri et al. 1991) are similar to those observed in the Raspas Complex. The serpentinized peridotites of ErroTobbio, for instance, host eclogitized mafic dikes, and therefore also reached HP conditions (Scambelluri et al. 275 1991). Given the close spatial association of the Raspas Complex peridotites with eclogites and blueschists (Fig. 1), the suggested eclogite-facies pressures (Gabriele et al. 2002) for the peridotites thus seem reasonable. The blueschists that have the peak assemblage omphacite ? garnet ? barroisite ? phengite clearly reached eclogitefacies conditions, but they did not to fully transform into eclogite. Because the differences in peak pressures and temperatures are negligible, we interpret this incomplete conversion to be due to bulk compositional differences between the protoliths of the blueschists and those of the eclogites. The constraints presented above indicate that the metamorphosed ultramafic, mafic, and sedimentary rocks were all subjected to similar peak P–T-conditions. Geochemistry Thirty-six representative samples (4 blueschists, 19 eclogites, including 4 amphibole-rich ones, and 9 serpentinized peridotites) were geochemically analyzed to constrain the tectonic setting in which their precursor rocks were formed. From these samples, three blueschists, eight eclogites, and six peridotites were selected for trace element analyses and the resulting data are shown together with the major elements in Table 2. The major element data of the remaining samples are shown in Supplementary Table 1. Because high- and low-temperature interaction with seawater could have significantly altered the major element budgets of the eclogite and blueschist protoliths, we used the method of Schumacher (1988) to scan for affected samples. Whereas two blueschist samples showed evidence for low-temperature alteration effects (concomitant Ca and Mg loss) typical of ocean floor basalts, no samples (blueschist or eclogite) were strongly affected by high-temperature alteration, in which Ca loss from the rocks is coupled with Mg gain (e.g., Humphris and Thompson 1978). Therefore, such high-temperature alteration, which leads to Mg enrichment and perhaps shifts the blueschist-to eclogite-transition to higher temperature (e.g., Barnicoat 1988), is probably not the main factor that prevented the Raspas blueschists from reacting to eclogite. Tholeiitic and calc-alkaline mafic rocks, as well as ultramafic rocks, may form in various tectonic settings, and trace elements may be used to distinguish among the different possibilities. In particular, high field strength elements (HFSE), which are considered to be relatively immobile in aqueous fluids and under conditions of hydrothermal alteration, seafloor weathering and up to medium grade metamorphic processes, are useful discriminators (e.g., Pearce and Cann 1973; Wood et al. 1979). For high-pressure metamorphic rocks, it has been demonstrated that in many cases fluids are involved in their 123 276 All eclogites have tholeiitic bulk compositions, with 46– 50 wt% SiO2 and Al2O3 contents between *13.5 and 15.0 wt%. They have high Fe and Mg contents with *12– 15 wt% Fe2O3 and *6–10 wt% MgO. The (La/Sm)N values of the eclogites are low (\0.7) and their HREE concentrations range between 20 and 25 times chondritic values and are thus similar to those of average normal MORB (N-MORB, Fig. 4a). Moreover, they display a slightly negative Eu anomaly. On a MORB-normalized trace element diagram, fluid immobile trace elements show patterns that are also similar to those of recent MORB (Fig. 4c). As expected, the fluid-mobile LILE scatter widely with up to ten times enrichment or depletion, and element ratios based on them show little or no systematic behavior. The eclogites display a wide range in Nb/La (with the majority lying between 0.5 and 0.7) but their low (La/Sm)N values are typical of MORB and preclude significant contributions from a subduction component (such as that observed in arc basalts) or crustal contamination (Fig. 5). The Nb/Zr values range from \0.02 to 0.04 at Hf/ Yb ratios clustering around 0.65, a combination that is suggestive of shallow melting in a depleted mantle source (Fig. 6). These characteristics, and values of Th/Yb between 0.02 and 0.09, Nb/Yb between 0.39 and 0.94, and TiO2/Yb between 0.38 and 0.44 (Fig. 7), strongly resemble those of average N-MORB. Blueschists Like the eclogites, blueschists have basaltic compositions, but they had an alkaline origin at least according to their trace element signatures. The blueschists have, if normalized to a volatile-free composition, higher Al2O3 and lower 123 sample/ chondrite blueschist eclogite OIB N-MORB 100 10 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu b sample / chondrite Eclogites a 1 0.1 0.01 serpentinized peridotites La Ce Pr 100 sample / MORB petrogenesis, where especially the highly fluid-mobile large ion lithophile elements (LILE) behave rather erratically (e.g., John et al. 2003). Therefore only relatively fluid-immobile trace elements such as the rare earth elements (REE) and the HFSE are useful for identifying the tectonic setting of the precursor rocks from high-grade metamorphic units (e.g., Möller et al. 1995; John et al. 2004; Miller et al. 2007; Jöns and Schenk 2008). We use mainly four geochemical indicators: (1) Nb/La to test for a subduction component, (2) (La/Sm)N, which is sensitive to mantle source characteristics and crustal contamination, (3) Nb/Zr, which is also sensitive to mantle source characteristics, and (4) Hf/Yb, which can be used to estimate melting depths (e.g., John et al. 2003, 2004). We further examine our results using the Th/Nb versus Nb/Yb and Ti/ Yb versus Nb/Yb relationships to evaluate crustal input and melting depth, respectively (see Pearce 2008). Contrib Mineral Petrol (2010) 159:265–284 Nd Sm Eu Gd Tb Dy Ho c Er Tm Yb Lu blueschist eclogite 10 1 0.1 0.01 Sr K Rb Ba Th Nb Ta La Ce Nd Zr Hf Sm Ti Tb Y Tm Yb fluid mobile fluid immobile Fig. 4 Chondrite-normalized REE variation diagrams (Boynton 1984) for: a eclogites and blueschists. Typical OIB (Sun and McDonough 1989) and N-MORB (Hofmann 1988) compositions are shown for comparison. b Serpentinized peridotites. The La values of two samples (SEC 26-3 and SEC 36-2) are not shown (see Table 2 for explanations). c Trace element variation plots, normalized to MORB (Hofmann 1988), for eclogites and blueschist. The most fluid mobile elements (LILE) are separated from the more fluid immobile elements (HFSE and REE), which are expected to remain essentially fluid-immobile in systems of low fluid flux Fe2O3 contents than the eclogites, with *15.5–19.0 wt% Al2O3 and *10–12 wt% Fe2O3. Chondrite-normalized REE patterns of the blueschists show a slight enrichment of the light REE (LREE) as compared to the HREE (Fig. 4a). Their Nb/La values, being greater than one, are too high to have been significantly influenced by a subduction component. The (La/Sm)N and Nb/Zr values of the blueschists, at [1 and [0.08, respectively, are similar to those of typical oceanic intraplate basalts (OIB, Figs. 5, 6), but HREE Contrib Mineral Petrol (2010) 159:265–284 277 1.6 10 1.4 seamounts ar ra y E-MORB ni c MORB ca OIB backarc basalts Eclogites 0.4 this study continental arc Blueschists oceanic arc 0.5 1.0 1.5 this study 2.0 2.5 B OI B- E-MORB Bosch et al. 0.2 OR Magmacrust interaction oceanic plateaus 0.6 M 0.8 Vo l 1 oceanic intraplate basalts Th/Yb Nb/La 1.0 0.1 3.0 Eclogites (La/Sm)N this study Bosch et al. N-MORB OIB Eclogites this study Bosch et al. 1.6 Blueschists this study 0.01 0.1 10 TiO2/Yb Fig. 5 Plot of Nb/La versus (La/Sm)N (after John et al. 2003) for the eclogites and blueschists of the Raspas Formation. The protoliths of the eclogites apparently formed in a mid-ocean-ridge environment, whereas the blueschists are compositionally distinct from MORB because their (La/Sm)N are too high, even for EMORB. According to their Nb/La and (La/Sm)N values, the blueschists seem to have originated in an intraplate seamount environment. The eclogite data of Bosch et al. (2002) are shown for comparison. Data for the oceanic plateaus are from Mahoney et al. (1993a, b), Tejada et al. (2002), and Ely and Neal (2003). The seamount data are from Batiza et al. (1989) and Niu and Batiza (1997). For data sources of the other fields, see John et al. (2003) 2.0 1 e East Hf/Yb 1.2 Ris Pacific b sea e dg -ri ion e m ct plu tera in unts N-MORB N RB 0.8 0.02 0.04 E-MORB Eclogites E this study Bosch et al. Blueschists primitive mantle depleted mantle source MORB array (shallow melting) oceanic plateaus MO 0.4 0.0 0.00 OIB 1 E-M Alk Th OIB array (deep melting) mo ORB 100 10 Nb/Yb Blueschists this study ar ra y ar c 1.2 0.0 0.0 Deep crustal recycling a this study 0.1 0.1 enriched mantle source 0.06 0.08 0.10 1 10 100 Nb/Yb 0.12 0.14 Nb/Zr Fig. 6 Plot of Hf/Yb versus Nb/Zr modified after John et al. (2004). Note the similarity between the eclogites and East Pacific Rise (EPR) MORB. In contrast, the blueschists follow an enriched, plumeinfluenced MORB trend that is defined by EPR seamounts (see text). EMORB would have lower Nb/Zr values, whereas an OIB-like source would be indicated by higher Hf/Yb values that are suggestive of residual garnet in the source. The eclogite data of Bosch et al. (2002) are plotted for comparison. Data for the oceanic plateaus are from Mahoney et al. (1993a, b), Tejada et al. (2002), and Ely and Neal (2003). The seamount data are from Batiza et al. (1989) and Niu and Batiza (1997), whereas the OIB data are from Frey et al. (1991). For data sources of the other fields, see John et al. (2004) Fig. 7 Plots of a Th/Yb versus Nb/Yb and b TiO2/Yb versus Nb/Yb, see Pearce (2008) for references. The compositions of both the eclogites and the blueschists follow the oceanic basalt array, with the protoliths of the eclogites apparently being derived from a MORBlike source and those of the blueschists displaying plume-ridge interaction with melting at shallow depths concentrations of these rocks, at 12–17 times chondritic values, are slightly higher than those of typical OIB (Fig. 4a). In addition, the (Ho/Lu)N values cluster around 1, whereas they are usually \1 in typical OIB (Frey et al. 1991; Sun and McDonough 1989). The Hf/Yb values are \1.5 and thus significantly lower than those of typical 123 278 OIB, which are usually above 2.0 (e.g., Frey et al. 1991) and may reach up to 3.6 (OIB of Sun and McDonough 1989). Additionally, the Th/Yb versus Nb/Yb relationship (0.38–0.58 vs. 5.3–7.8) places the blueschists on the MORB–OIB array of Pearce (2008), between enriched MORB and OIB (Fig. 7a). High Hf/Yb values suggest garnet in the residuum and thus deep melt source regions, whereas lower Hf/Yb values point to shallower melting sources, which is also shown by TiO2/Yb lower than 0.7 (Fig. 7b). Taken together, these distinct chemical features can be best explained by the mixing of a depleted mantle source (MORB-like) with an enriched one (OIB-like) and melting at shallow depths. Such a geodynamic setting can be found where seamounts form at or close to oceanic ridges (Batiza et al. 1989; Niu and Batiza 1997). In fact, the Nb/La versus (La/Sm)N and Hf/Yb versus Nb/Zr of the blueschists follow the trend of seamount-ridge variations observed at the East Pacific Rise (Figs. 5, 6). Contrib Mineral Petrol (2010) 159:265–284 a b Serpentinized peridotites The peridotites display a broad range of LOI from 5.7 to 11.9 wt%, reflecting variable degrees of serpentinization. They also have low Al2O3, CaO, and TiO2 contents (*1.3– 2.8, *0.3–2.8, and *0.03–0.08 wt%, respectively) and high MgO contents (*37–41 wt%). Concentrations of the transition metals, e.g., Cr (2,090–2,770 ppm) and Ni (2,080–2,370 ppm) are typical for abyssal peridotites that have 35–41 wt% MgO (Niu 2004). The chondrite-normalized REE patterns show a strong LREE depletion, particularly for La and Ce, which is also reflected in (La/ Sm)N \\ 1 and (Sm/Lu)N \ 1. The HREE concentrations range between 0.2 and 1.3 times chondritic values (Fig. 4b). Such patterns are consistent with modeled residues of a depleted mantle source after extraction of 5–10% melt produced by fractional melting, which is typical for the source of the basalts at mid-ocean ridges (e.g., Niu 2004). c Lu–Hf geochronology Whole rocks and mineral separates from an eclogite, a blueschist, and a metapelite were analyzed for Lu–Hf to determine whether the three lithologic types were subducted and metamorphosed contemporaneously or not. The resulting data are shown in Table 3, and isochrons are plotted in Fig. 8. Ten fractions from eclogite SEC 43-1 yielded a Lu–Hf age of 133.4 ± 2.1 Ma, albeit with a relatively high scatter as indicated by an MSWD of 18. The age of blueschist SEC 16-1, 126.4 ± 4.0 Ma (5 points, MSWD of 4.1), is consistent with that of eclogite 43-1. Depending on whether its bomb-digested- or hotplate- 123 Fig. 8 Lu–Hf isochrons of samples from the Raspas Formation digested whole rock fraction is used, the metapelite SEC 47-4 yields and age of either 147.9 ± 2.9 or 129.9 ± 5.6 Ma, respectively. The older ‘‘age’’ is most likely a spurious result caused by the dissolution of inherited zircon in the bomb digestion of whole rock powder. [This is analogous to the example shown in Figure 5a of Scherer Contrib Mineral Petrol (2010) 159:265–284 et al. (2000) where the Hf in inherited zircon grains did not isotopically equilibrate with the rest of the sample during later garnet growth.] In contrast, the hotplate digestion of whole rock chips (not powder) should have excluded most zircon, inherited or otherwise, as well as some rutile from the analysis because these minerals would not be digested. Therefore this analysis has probably not been significantly affected by an inherited zircon component, and we consider the age derived from regressing it with the two garnet fractions to represent the true crystallization age of the garnet in the metapelite. Whereas inherited zircon of substantially older age than the HP metamorphism might be expected to occur in metasedimentary rocks, zircon in eclogites and blueschists derived from juvenile oceanic crust would have crystallized between the age of that crust and the age of HP event or exhumation. In the case of blueschist SEC 16-1 and eclogite SEC 43-1, regressing only the data from hotplate-digestions (i.e., excluding the bombed whole rock fractions that might carry a zircon signature) does not reduce the scatter (i.e., MSWD values) or significantly change the ages or intercepts. From this, we infer that inherited zircon in these samples, if present, was either not much older than the HP event in which garnet grew or it only contributes a small part to the total Hf budget of these samples. The elevated scatter of the SEC 43-1 correlation, and to a lesser extent, that of SEC 16-1, casts some doubt on the significance of the stated age uncertainties. Well-equilibrated, high-temperature samples that have been dated using the same digestion methods at Münster yield a mean MSWD near 1, indicating that our estimated uncertainties in 176Lu/177Hf and 176Hf/177Hf are realistic, i.e., they account for the observed scatter on average. Thus, the excess scatter observed for the eclogite and blueschist isochrons is unlikely to be an artifact of the hotplate digestion procedure used here. Rather, we consider the likely cause to be variable proportions of different age components among the analyzed fractions. For example, variable ratios of older core- to younger rim material in each fraction could generate scatter, as would incomplete separation of younger retrograde minerals from the older, prograde mineral fractions. Nevertheless, the ages are broadly consistent, and we interpret them to demonstrate the essentially contemporaneous HP garnet growth at *130 Ma in all three rock types. Geodynamic interpretation The Raspas and the El Toro Formations, which define the metamorphic Raspas Complex, yield pressure and temperature estimates that are indicative of a metamorphic evolution within a subduction zone setting. The eclogites 279 and metapelites record the highest peak pressures, whereas blueschists likely reached slightly lower peak-pressures (Fig. 3). Peak-metamorphic temperatures for all of the rock types are rather similar, clustering around 600"C. This indicates a relatively warm subduction zone having a geothermal gradient of approximately 10–12"C/km on average for all lithologic units. However, the calculated temperatures are relatively high considering that pure glaucophane in blueschists (no Na in the A site) and the assemblage olivine–diopside–antigorite–Ti-clinohumite in the ultramafic rocks have not reacted out. The small variations in maximum pressure of about 0.2–0.3 GPa and essentially identical peak-temperatures point to a scenario in which different units were either (1) never really dismembered from each other or (2) juxtaposed during their exhumation but while still at great depth. However, the recorded difference in peak-pressures and thus burial depths are insignificant considering the errors on the individual pressure and temperature estimates (±50"C and 0.25 GPa see, e.g., Waters and Martin 1993). This, together with their similar ages may imply that these different rock units have remained close together during their entire metamorphic evolution (i.e., scenario 1 above). In addition, the bulk compositional difference between eclogites and blueschists, with the eclogites being the more Fe-rich of the two (Table 2), suggests that the observed peak mineral assemblage may be simply a function of the rock composition rather than of differences in peak metamorphic P–T conditions. A phengite K–Ar age of 132 ± 5 Ma (Feininger 1980) from a Raspas Complex metapelite and Ar–Ar phengite ages of 123.9 ± 1.4 and 127.1 ± 1.3 Ma (eclogites) and 123.4 ± 1.3 and 129.3 ± 1.3 Ma (metapelites) (Gabriele 2002; Bosch et al. 2002), are similar to the Lu–Hf garnet ages. The difference between the weighted means of Lu–Hf and Ar–Ar (K–Ar) ages is 6 ± 9 Ma. If the Lu–Hf ages are biased toward the time of peak temperature (*600"C), then cooling down to *400"C (e.g., Harrison et al. 2009) during exhumation occurred over a rather short time interval (0–15 Ma), # implying a cooling rate of *30þ1 $20 C=Ma. If, on the other hand, the Lu–Hf ages are biased toward the early, pre-peak T stage of garnet growth (e.g., Lapen et al. 2003), the implied cooling rate from 600"C would be even faster. Geochemical data show that the protoliths of all investigated rock types formed in an oceanic setting. For instance, the majority of the Raspas high-pressure metasediments are typically semipelagic or continentally derived, similar to sediments that are being deposited on active continental slopes today (Bosch et al. 2002; Gabriele et al. 2003). The strongly negative eHf(t) of the metapelite SEC 47-4 (-12.8 for the bomb-digested whole rock, Table 3) is consistent with this sample containing a component that was derived from ancient continental crust. 123 280 123 10 continental lithospheric mantle (median) sample / chondrite The eclogites’ basaltic precursors formed most likely at an oceanic ridge as indicated by their depleted trace element characteristics and the elevated eHf(t) of *?16 for sample SEC 43-1 (Fig. 4a, c; Table 3). In contrast, other types of basalt, such as OIB, usually have higher concentrations of LREE ([La/Sm]N C 1, Figs. 4a, 5) and lower concentrations of HREE, reflecting enriched mantle sources or smaller degrees of melting. Additionally, OIB would typically also have TiO2/Yb values above 0.7, which is not the case for the eclogites (Fig. 7b). Bosch et al. (2002) suggested that the Raspas eclogites represent a fragment of a subducted oceanic plateau, but basalts of oceanic plateaus usually have (La/Sm)N and Nb/La of around 1 (Mahoney et al. 1993a, b; Tejada et al. 2002; Ely and Neal 2003), and Nb/Zr between 0.05 and 0.07, which is similar to that of primitive mantle (Nb/Zr = 0.06, Fig. 6). By comparing the data of Bosch et al. (2002) with our new data and a more global data set (Figs. 5, 6, 7), it becomes evident that the trace element signatures of the Raspas eclogites are not compatible with an oceanic plateau setting. Instead we conclude that the eclogites have been formed from typical MORB. The chemical characteristics of the blueschists are clearly distinct from those of the eclogites. They are more alkaline rocks with higher trace element contents and trace element characteristics that are even distinct from enriched MORB (Figs. 5, 6, 7). The trace element data of the blueschists show many similarities with present-day seamounts, suggesting that the blueschists’ precursors were probably seamounts that formed close to—or at—an oceanic ridge. Hence it appears likely that the seamounts subducted together with the rest of the incoming plate. That the association of subducted MOR-basalts and seamounts represents a realistic scenario is documented at the Central American margin where seamounts having geochemical characteristics similar to those discussed here (Harpp and White 2001; Harpp et al. 2005) are located on the subducting Cocos plate close to the trench or have recently entered the subduction zone (von Huene et al. 2000). The association of peridotites with high-pressure mafic meta-igneous and meta-sedimentary rocks in the Raspas Complex is consistent with these units being a section of the subducted oceanic slab. On the other hand, HP and UHP metamorphic rocks are often associated with serpentinites that formed at the slab–mantle wedge interface (Guillot et al. 2001; Hattori and Guillot 2007). To clarify the relationship between the ultramafic and mafic rocks, it is therefore important to determine whether the Raspas peridotites represent subducted oceanic mantle or a part of the supra-subduction mantle. Serpentinites from the Cabo Ortegal Complex of northwestern Spain have been considered to be representative of typical mantle from the root of an arc, i.e., supra-subduction mantle (Moreno et al. Contrib Mineral Petrol (2010) 159:265–284 1 0.1 supra-subduction mantle abyssal mantle Raspas metaperidotites 0.01 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Fig. 9 Chondrite-normalized REE plot showing a comparison of the Raspas peridotites showing whole-rock data for abyssal mantle (peridotites from the Vulcan and Bullard fracture zones; Niu 2004), an exhumed mantle segment interpreted as supra-subduction zone mantle (serpentinites from the Cabo Ortegal ultramafic complex; Pereira et al. 2008), and continental lithospheric mantle (based on lherzolite and harzburgite xenoliths, McDonough 1990). The REE patterns of the Raspas peridotites overlap best with those of abyssal peridotites. Both show high degrees of LREE depletion. Note that the REE compositions of the Raspas peridotites and the abyssal peridotites correspond well to 5–30% batch melting of a depleted mantle source as modeled by Niu (2004) 2001; Pereira et al. 2008). The Finero peridotite massif (Ivrea Zone, Alps) is also interpreted to represent former subduction zone wedge mantle, but a strongly metasomatized type (Zanetti et al. 1999). For the Cabo Ortegal peridotites (Fig. 9), chondrite-normalized REE patterns are relatively flat and slightly U-shaped with a slight enrichment of LREE over HREE, similar to the median composition of the continental lithospheric mantle (McDonough 1990). The Finero peridotites have even higher REE concentrations and steeper chondrite-normalized patterns, reflecting the strong metasomatic trace-element enrichments. Complementary information about the composition of the sub-arc mantle wedge can be gained from mantle xenoliths in arc volcanoes, and we have compared our data from the Raspas peridotites with spinel peridotites from two key localities for mantle wedge xenoliths, the Kamchatka arc (Avachinsky volcano; Ishimaru et al. 2007; Halama et al. 2009) and Papua New Guinea (Grégoire et al. 2001). At both localities, the harzburgitic xenoliths are highly depleted in all REE and show U-shaped patterns that are similar in shape to those from Cabo Ortegal but with concentrations that are approximately one order of magnitude lower. These characteristics of supra-subduction mantle are opposite to what is observed in the Raspas rocks (Fig. 9). On the basis of this comparison, we conclude that the Raspas peridotites do not represent typical supra-subduction or continental lithospheric mantle. Finally, REE data Contrib Mineral Petrol (2010) 159:265–284 from bulk-rock abyssal peridotites show abundance variations of several orders of magnitude, suggesting complex enrichment processes (Niu 2004). Enrichment of LREE in abyssal peridotites has been interpreted to reflect postmelting refertilization in the thermal boundary layer beneath ridges (Niu 2004). For a comparison with the Raspas peridotite data, we have therefore chosen wholerock compositions of abyssal peridotites that show the smallest degree of LREE enrichment (e.g., Bullard and Vulcan fracture zones). The data of these abyssal peridotites show a significant overlap with the Raspas data (Fig. 9). The REE patterns of both the abyssal peridotites and the Raspas peridotites are very similar to patterns derived from various melting models as calculated by Niu (2004). In particular, 5–30% batch melting of a depleted mantle source can explain the REE patterns of the Raspas peridotites quite well. Additionally, the Raspas peridotites exhibit many chemical, metamorphic, and structural features that are similar to those of the Erro Tobbio peridotite (Italy), which has been interpreted to represent subducted hydrous oceanic mantle (Scambelluri et al. 2001). In particular, these two peridotite bodies have essentially identical REE patterns. We thus conclude that the Raspas peridotites once represented the mantle part of the oceanic lithosphere, which has been subducted together with oceanic crust, which is now present as HP eclogites and metasediments. Taking into account the petrological, geochemical, and geochronological evidence, the Raspas Complex represents all components of a complete oceanic lithosphere section. It is likely that they once have been components of a coherent package that was dismembered from the slab during subduction. Consequently we interpret the Raspas Complex as a high-pressure ophiolite. 281 incoming plate (Cloos 1992; von Huene et al. 2000; Mochizuki et al. 2008). At shallower depths (\40 km), seamount subduction may cause seismicity by acting as a hindrance at the slab–wedge interface (e.g., Mochizuki et al. 2008), and Cloos (1992) speculated that scraping off seamounts from the subducting plate might cause great earthquakes. However, some seamounts or portions thereof apparently remain attached to the downgoing slab, and the finding of high-pressure seamounts (Gao and Klemd 2003; John et al. 2004; van der Straaten et al. 2008; this study) indicates that they can be subducted to depths of at least 60 km. We propose that at such depths, stress induced by friction or shearing at the slab-wedge interface would eventually be high enough to scrape off the remaining parts of seamounts, which, in addition to potentially generating earthquakes, should result in a partial dismembering of the upper part of the slab. This process may allow fragments of the slab to be exhumed, either as individual eclogite and blueschist bodies or as larger, coherent packages such as the Raspas Complex. Conclusions (1) (2) Implications Blueschists or eclogites that have alkaline composition and are associated with tholeiitic MORB-type eclogites have been documented for paleo-subduction zones, e.g., in the Alps (Hermann 2002) and New Caledonia (Spandler et al. 2004). In some cases an even more specific seamount- or OIB-like trace element signature has been reported, e.g., in Zambia (John et al. 2004) and Tianshan (Gao and Klemd 2003; van der Straaten et al. 2008). The existence of seamounts that have been deeply subducted and then finally exhumed has some implications for how parts from the downgoing slab may become dismembered, which is a prerequisite to their eventual exhumation. Seamounts are rather prominent features on the oceanic plate (e.g., 20– 30 km across and 2–3 km high; von Huene 2008) and thus represent obstacles to subduction on the top of the (3) (4) In the Raspas Complex, eclogites, blueschists, metasediments, and peridotites all exhibit a similar P–T evolution that suggests a maximal burial depth of about 60 km, where the rocks were heated to about 600"C. These values imply a rather warm geothermal gradient of about 10–12"C/km at least in the crustaland uppermost mantle parts of the slab. The Lu–Hf ages of an eclogite, a blueschist, and a metapelite from the Raspas Complex are broadly similar and indicate that these samples were all subjected to prograde HP metamorphism at around 130 Ma. Cooling of the rocks down to *400"C was finished by about 123 Ma, suggesting a short time interval of \15 Ma between garnet growth and cooling below the closure temperature of Ar in phengite. The trace element signatures of the Raspas blueschists provide evidence for exhumation of subducted seamounts. Precursors of the eclogites were most likely MORB-type basalts and evidence for an oceanic plateau affinity, as formerly suggested, is absent from the sample suite investigated in this study. Additionally, the closely associated eclogitefacies peridotites display chemical signatures resembling those of depleted MORB-source mantle. The association of MORB-type eclogite, seamounttype blueschist, eclogite-facies serpentinized peridotite, and HP metasediments point to exhumed highpressure ophiolite sequence. Such sequences may 123 282 Contrib Mineral Petrol (2010) 159:265–284 provide clues that lead to a deeper understanding of how the different parts of a coherent slab behave during subduction and about the associated fluidinfiltration and metamorphic processes. Acknowledgments This study was financially supported by Deutsche Forschungsgemeinschaft (DFG) grants Sche 265/S1-2. 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Subducted seamounts in an eclogite-facies ophiolite sequence: slab dismembering due to local stress amplification

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