Abstract
This study aims at clarifying the relationship between the Cape Steno mélange, southern Andros, and the main tectonic units of the Attic-Cycladic Crystalline Belt. Jurassic protolith ages and geochemical characteristics indicate a Pelagonian affinity and point to a correlative relationship with the Tsiknias Ophiolite on Tinos Island. However, jadeitites and high-Si phengite in the gneisses clearly indicate a high-pressure metamorphic overprint that is unknown from the Tsiknias outcrop and other occurrences of the Upper Cycladic Unit. A correlation with the Cycladic Blueschist Unit (CBU) is an obvious assumption, but initially seemed difficult to reconcile with the Cretaceous protolith ages of meta-ophiolitic rocks from the CBU and distinct geochemical characteristics of associated jadeitites. The Jurassic ages of the Cape Steno rock suite either document a broader spectrum of source rocks than previously known from the CBU, or the existence of a distinct tectonic unit. We assume that the geological and tectono-metamorphic evolution of the Cape Steno occurrence is similar to that of the Makrotantalon Unit of NW Andros, which represents a Pelagonian subunit in the nappe stack of the CBU, with abundant slices of serpentinites, rare meta-gabbro and a metamorphic history comprising both Cretaceous and Eocene HP/LT episodes.
Highlights
The Attic-Cycladic Crystalline Belt (ACCB, Fig. 1a) in the central Aegean region comprises three major groups of tectonic units which record different geological and tectonometamorphic histories
This study focused at unravelling the status of the Cape Steno mélange within the structural architecture of the Cyclades
The Cape Steno occurrence is different to the HP/LT mélanges of the Cycladic Blueschist Unit (CBU) on the neighboring islands of Syros and Tinos (e.g. Dixon and Ridley 1987; Bröcker and Enders 2001; Bröcker and Keasling 2006; Bulle et al 2010; Gyomlai et al 2021)
Summary
The Attic-Cycladic Crystalline Belt (ACCB, Fig. 1a) in the central Aegean region comprises three major groups of tectonic units which record different geological and tectonometamorphic histories. From top-to-bottom, these groups are referred to as the Upper Cycladic Unit, the Cycladic Blueschist Unit and the Basal Unit (e.g. Dürr et al 1978; Dürr 1986; Papanikolaou 1987; Okrusch and Bröcker 1990; Ring et al 2010). The Upper Cycladic Unit (UCU) includes a heterogeneous sequence of unmetamorphosed Permian to Mesozoic sediments, ophiolites with mostly unknown protolith ages, greenschist-facies rocks with Cretaceous to Paleogene metamorphic ages, Late Cretaceous granitoids and amphibolite-facies rocks of the same age Okrusch and Bröcker 1990; Forster and Lister 2005; Ring et al 2010; Phillipon et al 2012; Flansburg et al 2019; Glodny and Ring 2021, and references therein). On Tinos, Evia and Samos, metamorphic rocks below the CBU were interpreted as para-authochthonous units, which are separated from the structurally higher sequences by thrust faults (Avigad and Garfunkel 1989; Ring et al 1999, 2001; Shaked et al 2000)
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