The Late Cretaceous metamorphic rocks of the Akrotiri and Vari subunits on Tinos and Syros, Cyclades, Greece: field observations, geochemistry and geochronology

Abstract

The relationship of the Late Cretaceous amphibolite-facies Akrotiri and Vari subunits on the Greek islands of Tinos and Syros to similar occurrences in the Aegean is not fully understood, but a correlation with the Asterousia Crystalline Complex on Crete and corresponding rocks in the Upper Cycladic Unit on other islands of the archipelago is a plausible interpretation. There is currently no clear evidence that the Akrotiri subunit represents a fragment of the metamorphic sole of the nearby Tsiknias ophiolite, as there are differences in field appearance, geochemistry, metamorphic grade and retrograde overprint. Felsic rocks from amphibolite-gneiss sequences of the Akrotiri and Vari subunits are interpreted as reworked igneous rocks with minor admixture of terrigenous material. The Triassic U–Pb zircon age (c. 240 Ma) of such a gneiss from Syros indicates the magmatic crystallization age of the original source rocks. A similar interpretation is suggested for the studied Akrotiri gneiss, but is less clear due to the not fully clarified significance of the presumed Pb loss thought to be responsible for the considerable U–Pb age range of the dated zircons (c. 256–114 Ma). The Vari gneiss sensu strictu is closely associated with metadioritic rocks containing a Triassic zircon population (c. 238 Ma). The protolith is probably plutonic, but a mixture of volcanic and plutonic detritus cannot be excluded. Riebeckitic amphiboles occur in all rock types of the Akrotiri subunit, indicating late overprinting at elevated pressures, which is not known from similar occurrences in the southern Aegean.