The Age, Origin, and Emplacement of the Tsiknias Ophiolite, Tinos, Greece

Abstract

The Tsiknias Ophiolite, exposed at the highest structural levels of Tinos, Greece, represents a thrust sheet of Tethyan oceanic crust and upper mantle emplaced onto the Attic‐Cycladic Massif. We present new field observations and a new geological map of Tinos, integrated with petrology, THERMOCALC phase diagram modeling, U–Pb geochronology and whole rock geochemistry, resulting in a tectonothermal model that describes the formation and emplacement of the Tsiknias Ophiolite and newly identified underlying metamorphic sole. The ophiolite comprises a succession of partially dismembered and structurally repeated ultramafic and gabbroic rocks that represent the Moho Transition Zone. A plagiogranite dated by U‐Pb zircon at 161.9 ± 2.8 Ma, reveals that the Tsiknias Ophiolite formed in a suprasubduction zone setting, comparable to the “East‐Vardar Ophiolites,” and was intruded by gabbros at 144.4 ± 5.6 Ma. Strongly sheared metamorphic sole rocks show a condensed and inverted metamorphic gradient, from partially anatectic amphibolites at P‐T conditions of ~8.5 kbar 850–600 °C, downstructural section to greenschist‐facies oceanic metasediments over ~250 m. Leucosomes generated by partial melting of the uppermost sole amphibolite, yielded a U‐Pb zircon protolith age of ~190 Ma and a high‐grade metamorphic‐anatectic age of 74.0 ± 3.5 Ma associated with ophiolite emplacement. The Tsiknias Ophiolite was therefore obducted ~90 Myr after it formed during initiation of a NE dipping intraoceanic subduction zone to the northeast of the Cyclades that coincides with Africa's plate motion changing from transcurrent to convergent. Continued subduction resulted in high‐pressure metamorphism of the Cycladic continental margin ~25 Myr later.

P–T–tevolution of the Cycladic Blueschist Unit in Western Anatolia/Turkey: Geodynamic implications for the Aegean region

“…Late Paleozoic ages (Vavassis, de Bono, Valloton, Stampfli, & Amelin, ) from the high‐grade hosting rocks of these intrusions (Most, ; Mposkos & Kostopoulos, ; Mposkos & Krohe, ) are assigned to a pre‐Alpine Variscan event. The Cyclades are regarded as the southeastern extension of the Pelagonian Zone (Anders, Reischmann, Kostopoulos, & Poller, ; Lamont et al, ). A number of granite intrusions dated at 300–315 Ma by U–Pb zircon, mid‐ to Late Carboniferous, clearly reveal the existence of a widespread Variscan crystalline basement (Engel & Reischmann, ; Reischmann, ; Tomaschek, Keiter, Kennedy, & Ballhaus, ).…”

“…The views placing the Pelagonian Zone–CB and Sakarya Zone to the northern margin of Gondwana (Lamont et al, ; Maffione & van Hinsbergen, ; van Hinsbergen et al, ) in Triassic contradict with their late Carboniferous tectonometamorphic and magmatic evolution associated with the Variscan evolution of the Central Europe, which is attributed to the closure of the Rheic/Rheno‐Hercynian Ocean and collision of the continental block separated from Gondwana during the early Paleozoic with the Laurussia (Okay & Topuz, ; Stampfli et al, ). Furthermore, the tectonic models (Lamont et al, ; Maffione & van Hinsbergen, ) assuming a similar paleogeographical position for Anatolides and Pelagonian/CBU to the northern margin of the Adria–Apulia–Tauride block cannot account for the E‐W lateral discontinuities of the Cretaceous to early Miocene tectonic zones occurring in Greece and western Anatolia.…”

Study of the chromite mineralization associated to ophiolites from Tinos Island, Attico-Cycladic Massif

“…The Tsiknias ophiolite complex in Tinos Island is a succession of partially dismembered and structurally repeated ultramafic and gabbroic rocks, whereas the upper crustal lithologies are not present (Lamont et al 2019). Vakondios (1997) described that the host rock of Tinos chromitites are mainly the serpentinized ultramafic rocks, the protolith of which consists of harzburgites and to a lesser degree dunites.…”

“…The Tinos ophiolite complex has been mapped by Melidonis (1980) and on a scale of 1:5000 by Vakondios (1997), whereas the age, the origin and the emplacement of the ophiolite has been recently studied by Lamont et al (2019). Tinos chromitite orebodies are exposed in Mount Tsiknias (Fig.…”

“…These rocks have been metamorphosed at eclogite to blueschist facies conditions (Brichau et al, 2010) at 53-40 Ma, and retrograded at greenschist to amphibolite facies at ≥ 25 and ~ 12 Ma (Bolhar et al, 2010). Lamont et al (2019) described that the Tsiknias ophiolite even though was formed at ~162 Ma, it was overthrusted at ~75 Ma. At the eastern part of Tinos, the entire tectonostratigraphy as well as the Tsiknias ophiolite is intruded by a ̴ 14 to 18 Ma monzogranite to granodiorite pluton (Brichau et al, 2007) and a boron-and fluorinerich leucogranite (Mastrakas and St. Seymour, 2000).…”