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Postorogenic shoshonitic rocks and their origin by melting underplated basalts: The Miocene of Limnos, Greece

¹ Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada
² Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada
³ Department of Geology, University of Patras, Patras 26110, Greece
⁴ Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada
⁵ Department of Geology, University of Patras, Patras 26110, Greece

View larger version (69K): [in this window] [in a new window]   Figure 1. Regional map showing distribution of the early Miocene shoshonitic volcanism of the northeastern Aegean and northwestern Anatolia, the surface trace of oceanic sutures that closed in the Cretaceous and Paleogene, and the main tectonic and volcanic features of the modern subduction zone (modified from Pe-Piper and Piper, 2002). AE—Agios Evstratios, I-A suture—Izmir–Ankara suture; PT—Patmos, SK—Skyros. Approximate line of section for Figure 12 is also shown.

View larger version (22K): [in this window] [in a new window]   Figure 2. Nomenclature of analyzed samples from Limnos in (A) the International Union of Geological Sciences (IUGS) system (Le Bas et al., 1996) and (B) the shoshonite diagram of Peccerillo and Taylor (1976). B also shows fields for Lower Miocene rocks of Samothraki (Vlahou et al., 2006), western Anatolia (Aldanmaz et al., 2000), and Lesbos (Pe-Piper and Piper, 1992).

View larger version (85K): [in this window] [in a new window]   Figure 3. Geological map of Limnos. Geological boundaries are from Innocenti et al. (1994), but assignment of rock units has been modified. Orientations of large elongate clasts on bedding planes of ignimbrite near Romanou are shown. At base, schematic cross section of Limnos shows inferred relationships between different volcanic units.

View larger version (156K): [in this window] [in a new window]   Figure 4. Backscattered electron images showing zoning in feldspar and clinopyrox-ene phenocrysts. (A) Sanidine (san, with wt% BaO) and plagioclase (plg, with wt% An), Agios Ioannis subunit. (B) Plagioclase (with wt% An), Agios Ioannis subunit. (C) Clinopyroxene (cpx, with wt% Cr2O3), Romanou Unit. (D) Clinopyroxene (with wt% Cr2O3), Fakos quartz monzonite subunit.

View larger version (24K): [in this window] [in a new window]   Figure 5. Composition of (A) feldspars, (B) amphiboles, and (C) clinopyroxenes from the volcanic rocks of Limnos. Abbreviations: Ab—albite; An—anorthite; En—enstatite; Fs—ferrosilite; Or—orthoclase; Wo—wollastonite.

View larger version (34K): [in this window] [in a new window]   Figure 6. Plots of major and selected trace elements against SiO2. Fresh samples only. Symbols are as in Figure 2. Well-developed trends are shown by dashed lines. LOI—loss on ignition.

View larger version (38K): [in this window] [in a new window]   Figure 7. Variations in selected elements normalized to pyrolite (McDonough and Sun, 1995). The most primitive (lowest SiO2, high MgO) sample from each unit is plotted. Also plotted are a representative Lesbos lamproite (LL639, G. Pe-Piper, 2007, personal commun.) and a basanite from Turkish Thrace (EA519: Aldanmaz et al., 2006).

View larger version (25K): [in this window] [in a new window]   Figure 8. Variations in rare earth elements (REE) normalized to C1 chondrite (McDonough and Sun, 1995). For comparison, the lamproite and basanite from Figure 7 and two representative Lower Miocene trachyandesites from each of Western Anatolia (Aldanmaz et al., 2000) and Samothraki (Vlahou et al., 2006) are also shown.

View larger version (35K): [in this window] [in a new window]   Figure 9. (A) Plot of Nd versus Sr isotopes for Limnos. Symbols are as in Figure 2. Plot also shows samples of Lower Miocene volcanic rocks from Lesbos (Pe-Piper and Piper, 2001), Samothraki (Vlahou et al., 2006), and western Anatolia (Altunkaynak and Dilek, 2006), and fields for some other groups of Aegean-area rocks (from Pe-Piper and Piper, 2002). (B–C) Plot of Pb isotope compositions of NE Aegean shoshonites from Samothraki and Lesbos together with fields for some other groups of Aegean-area rocks (from data compiled in Pe-Piper and Piper, 2002). Crustal growth curve is from Stacey and Kramers (1975).

View larger version (28K): [in this window] [in a new window]   Figure 10. Modeled rare earth element (REE) compositions derived from partial melting of enriched metabasalt and mixing with lamproite magma, also showing REE compositions of lamproite and basanite from Figure 7. Starting material (assumed metabasalt) is a 50:50 mix of Samos and Delos subduction-related diorites; batch partial melting uses mineral-melt partition coefficients for andesite from Rollinson (1993). For further explanation, see text.

View larger version (24K): [in this window] [in a new window]   Figure 11. (A) Variation in degree of light (L) and middle (M) rare earth element (REE) enrichment in Limnos rocks; symbols are as in Figure 2. Plot also shows fields for Lower Miocene volcanic rocks from Lesbos (gray tone, from Pe-Piper and Piper, 1992), Samothraki (Vlahou et al., 2006), and western Anatolia (Aldanmaz et al., 2000), lamproite from Lesbos (cf. Fig. 7), and average continental crust (avg. cont. crust, from Wedepohl, 1995). Selected high-La trachyandesite from other shoshonites: S—Sunlight volcano (Feeley and Cosca, 2003), A—central Andes (Kontak et al., 1986). (B) Modeled enrichment in LREE and MREE resulting from partial melting of enriched metabasalt (cf. Fig. 10), for various modal mineralogies (%) and proportions of partial melting (0.01, 0.1, 0.2, 0.5).

View larger version (46K): [in this window] [in a new window]   Figure 12. Tectonomagmatic model for the evolution of the trachyandesites of Limnos and adjacent areas of the northeastern Aegean shoshonite belt. Pl-out indicates pressure at which plagioclase is unstable. For explanation, see text. Schematic location is shown in Figure 1.