The age, origin and emplacement of the Tsiknias Ophiolite, Tinos, Greece

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 degrees 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. Plain Language Summary Ophiolites (fragments of oceanic crust and mantle pushed onto the continents) are distributed throughout the Aegean and record the traces of relict subduction zones that were responsible for the closure of an earlier ocean to the Mediterranean (Tethys Ocean). In this work, the Tsiknias Ophiolite on Tinos, Greece, was investigated to improve our understanding of the geodynamic evolution of the Aegean. A new geological map of Tinos is presented and integrated with phase diagram modeling, geochemistry, and U-Pb dating of rocks from the ophiolite sequence and high-temperature metamorphic rocks beneath the ophiolite (Metamorphic Sole). The Tsiknias Ophiolite formed at 161.9 2.8 Ma above a subduction zone, comparable to ophiolites on mainland Greece. The underlying Metamorphic Sole rocks comprise of hot mafic rocks at the top and colder oceanic sedimentary rocks at the bottom. The hottest rocks represent oceanic crust that formed at 190 Ma and experienced heating and partial melting at 74 +/- 3.5 Ma. The Metamorphic Sole formed by heating and shearing in a protosubduction zone 90 Myr after the overlying ophiolite formed. Subduction continued until 52-46 Ma, when the Adriatic continental margin jammed this subduction zone marking the collision between Adria and Eurasia.