TECTONOMETAMORPHIC EVOLUTION OF SE TINOS, CYCLADES, GREECE

The two main tectonic units of Tinos Island differ in their lithologic al contents and their metamorphic history. The present study investiga tes in some detail the tectono-metamorphic evolution and juxtaposition of these two main units on SE Tinos. The upper unit is dominated by o phiolite members which show evidence of an early low-pressure (oceanic ?) overprint. No high pressure relies are found in the upper unit, by contrast to the lower unit. The low-angle fault separating them, a nor mal fault in the interpretation of AVIGAD and GARFUNKEL (1991), remain s difficult to interpret on the basis of our field data. However, the two main units show identical mesoscopic structural features (coaxial folds, parallel axial planar schistosities), and their regional medium -pressure greenschist facies overprint is very similar. Based on this evidence, we suggest a joint evolution of the two tectonic units since the mid-Tertiary (Oligocene?). Hence, the two units appear to have ac ted as a coherent stack during the subsequent thrusting event that emp laced them on top of the low-grade metacarbonates exposed in the Panor mos window of northern Tinos. The timing of this emplacement remains u nclear but it is likely to predate the intrusion (at a depth of 7 +/- 2 km) of Miocene monogranites in southern Tinos. Their contact aureole affects both the lower and upper unit rocks. During the final stages of uplift the contact at the base of the upper unit was reactivated, d isplacing the hanging wall to the N by some 500 m. Greenschists of the lower unit (LU) and the upper unit (UU) are petrographically very sim ilar. In the contact zone between the two units, mylonitic types preva il, and the precise location of the tectonic boundary is difficult to map. Geochemical data provide a means to distinguish greenschists of t he LU from those of the UU. Furthermore, these data point to a range o f source materials for the UU metabasic rocks: Besides the common MORB -affinity more evolved ferro-gabbroid dikes occur, and a minor gabbro suite of boninitic character was identified, the significance of which is presently not understood.