Origin of metabasites from upper tectonic unit of the Lavrion area (SE Attica, Greece): Geochemical implications for dual origin with distinct provenance of blueschist and greenschist's protoliths I. Baziotis ⁎, E. Mposkos Department of Geological Sciences, School of Mining and Metallurgical Engineering, National Technical University of Athens, Heroon Polytechniou 9 Str., 15773, Zografou, Athens, Greece abstract article info Article history: Received 19 March 2011 Accepted 15 July 2011 Available online 30 July 2011 Keywords: Geochemistry Metabasites Fractional crystallization Lavrion area The Lavrion area forms the westernmost part of the Attic–Cycladic crystalline belt (ACCB) and is built up by two tectonic units metamorphosed at HP/LT conditions. In the Upper Tectonic Unit metabasic rocks occur as greenschists and blueschists. Major and trace elements plotted against Mg# show a systematic increase in TiO 2 , Fe 2 O 3 *, Na 2 O, Zr, Y, V, La and Yb and a decrease in Al 2 O 3 , Ni and Cr with decreasing Mg#. Typically, the blueschists always exhibit a more evolved basaltic composition. The greenschists are characterized by LREE- depleted chondrite-normalized REE patterns and Zr/Nb values that range from 15 to 24. The blueschists are characterized by slightly LREE-enriched chondrite-normalized REE patterns and lower Zr/Nb ratios (6.6– 11.3). Both rock types share common geochemical features like flat HFSE patterns or slight positive Nb anomalies with La/Nb b 1. The protoliths of both greenschists and blueschists show that two different suites generated their protoliths and that their magmatic evolution at low pressures has also been different. The observed Zr/Nb vs. Ce/Y ratios span the compositions of greenschists and blueschists out on hyperbolae at an ideal spectrum from intermediate N-MORB to E-MORB. However, the REE patterns of the studied metabasites cannot be explained by fractional crystallization processes alone. We interpret that the protoliths of the Lavrion metabasites support a dual origin; the blueschist's protoliths are comparable to rift-related mildly alkaline basalts whereas the greenschist's protoliths probably formed as typical N-MORB at an oceanic spreading center or in an evolved back-arc basin. A mélange setting with two distinct provenances of the basic protoliths is the proposed genetic model, similar to that envisioned for other parts of the ACCB, e.g. the islands of Syros, Sifnos and Tinos. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The geochemistry of metabasites can provide useful information about the magmatic precursors and their geotectonic setting of for- mation. In case that isochemical metamorphism can be documented or reasonably be assumed, the chemical variability of metabasic rocks can be attributed to primary processes such as different degrees of partial melting, fractional crystallization or combination of assimila- tion and fractional crystallization (AFC). We are using major and trace elements data to evaluate the importance of the above-mentioned processes in the petrogenesis of the protoliths of the metabasic rocks from the Lavrion area (Attica, Greece). Lavrion belongs to the Attic–Cycladic Crystalline belt (ACCB), a well-studied polymetamorphic terrane within the Hellenides. Previ- ous research related to the ACCB has mainly been focused on the tectonometamorphic evolution of HP rocks and their exhumation. The ACCB is a typical case of a high-pressure belt in a back-arc tectonic environment, and detachments were described by various authors (Avigad and Garfunkel, 1989; Avigad et al., 1997; Ridley, 1984). According to Avigad and Garfunkel (1989) some of these detachments are related to post-orogenic crustal extension. Exhumation data come from K–Ar, Ar–Ar and Rb–Sr white mica geochronology (e.g. Bröcker et al., 1993, 2004, 2005). The prograde and retrograde metamorphic conditions are well constrained in terms of stable isotopes, conven- tional geothermobarometry and pseudosections (Parra et al., 2002; Schliestedt and Matthews, 1987; Schmädicke and Will, 2003; Trotet et al., 2001a,b). Only a few studies have examined the exposures of such rocks in the area of ACCB in sufficient detail to identify the geochemical characteristics of their protoliths (e.g. Arikas et al., 2001; Bröcker, 1991; Mocek, 2001; Seck et al., 1996). This study provides new geochemical data of the greenschists and blueschists from the Upper Tectonic Unit (UTU) of the Lavrion area in order to determine the source(s) and the tectonic setting of formation of their magmatic protoliths. Lithos 126 (2011) 161–173 ⁎ Corresponding author. E-mail address: baziotis@metal.ntua.gr (I. Baziotis). 0024-4937/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2011.07.014 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos