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Abstract
The ophiolitic rocks of the Attic-Cycladic crystalline belt are considered of key importance for understanding the Mesozoic tectonic evolution of the Aegean region. Unresolved questions concern their tectono-stratigraphic relationships across the region. The mode of occurrence of the Cycladic ophiolites varies, as they appear as: (a) dismembered blocks (olistoliths) within the supra-detachment units of Paros and Naxos; (b) mélange formations in the upper structural unit of western Samos and Skyros; and (c) meta-ophiolitic mélange in the Cycladic Blueschist Unit (CBU) from central Samos. The trace element geochemistry and Pb-Sr-Nd isotopes of the mafic ophiolitic rocks indicate four geochemical groups: (a) the upper unit metabasites from Paros and western Samos (Kallithea) display an evolved basaltic composition (Mg# 40.2–59.6), with low Zr/Nb values (5–16) and high Ce/Y values (1.3 to 2.6) compared to MORB, indicating island-arc tholeiite affinities; (b) Naxos upper unit metabasalts show spider diagrams patterns indicating ocean island basalt (OIB-type) affinities; (c) Central Samos metagabbros (CBU) are primitive rocks with Back-Arc Basin basalt affinities; (d) the Skyros metadolerites and Tinos (Mt Tsiknias) and S. Evia (CBU) metagabbros, cluster as a separate geochemical group; they exhibit high MgO values (>10 wt %), very low TiO2 values (0.1–0.2 wt %), Y and Yb, and depleted trace element N-MORB normalized patterns, similar to volcanic rocks formed in modern oceanic fore-arc settings, such as boninites. A combination of the Pb- and Sr-isotopic compositions of Cycladic metabasites indicate that the Pb and Sr incorporated in the Cycladic ophiolites correspond to mixtures of magmatic fluids with seawater (206Pb/204Pb = 18.51–18.80; 207Pb/204Pb = 15.59–15.7; 208Pb/204Pb = 39.03–39.80 and initial 87Sr/86Sr80 = 0.705–0.707). Furthermore, peridotite relicts from Samos, Paros, and Naxos—irrespective of the structural unit—display chemical affinities of ocean floor peridotites formed in a supra-subduction zone. The characteristics of harzburgite relicts in Cycladic serpentinites and Skyros indicate a highly residual nature of the mantle source. Geochemical data from this study shed further light on the complex structure of the oceanic lithosphere from which the Cycladic ophiolites originated.
Highlights
Ophiolitic remnants are widespread in the Cyclades, southern Aegean, occurring in the upper portion of the tectono-stratigraphic stack (Figure 1) [1,2,3,4,5,6]
We examine in detail the chemical compositions of mafic and associated ultramafic rocks from the structurally upper unit of primitive basaltic rocks (Paros)-Naxos, Samos, and Skyros, as well as minor occurrences of metamafic rocks from the migmatitic gneiss dome of Naxos (Figure 2)
The petrographic, major and trace element and Sr-isotope data show that all the studied rocks underwent ocean floor alteration
Summary
Ophiolitic remnants are widespread in the Cyclades, southern Aegean, occurring in the upper portion of the tectono-stratigraphic stack (Figure 1) [1,2,3,4,5,6]. We make a comprehensive overview of published geochemical data of ophiolitic lithologies from typical ophiolite exposures of the upper structural unit (UU) of Tinos [15] and Ikaria [10], and compare them with the metamorphosed ophiolitic mélange units (CBU) from Tinos, southern Evia, Andros, Samos [4,15,28], and Naxos [29], in order to place the new data into a regional context With these new whole-rock geochemical data that include Sr-Nd-Pb isotopic data for metabasites and ultramafics, we aim to: (a) present a basic geochemical characterization from the ophiolite occurrences of different Aegean islands (Cycladic islands and Skyros); and (b) recognize distinguishing chemical characteristics in order to make regional correlations. Cycladic metamorphic belt showing the main structural units and the location of ophiolitic rocks, metamorphic belt showing the main structural units and the location of ophiolitic rocks, modified modified after [9]. after [9]
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