Structural observations

The immediate host-rocks to the intrusion are mainly mylonitic orthogneisses that are highly deformed at temperatures above ~450°C showing striped gneiss microstructures. Notably, this orthogneiss mylonite contains numerous high-temperature skarns of brown andraditic garnet with green hedenbergite and magnetite (Petrakakis et al., 2004). The skarns form either foliation-parallel layers or veins cross-cutting the mylonitic foliation. In general, both types of skarn occurrence record low-strain SSW-directed shear deformation forming syn-kinematically rotated flanking and boudinage structures. Locally, some less deformed types of this rock (protomylonites) still preserve magmatic relics like perthitic porhyroclastic K-feldspar and sharply zoned plagioclase. Besides both feldspars, these protomylonites may contain hornblende, biotite, primary white mica as well as tourmaline zircon and opaques. Chemical analyses of the mylonites hosting the granodiorite show that the protolith was a S-type granite (Iglseder et al. 2007, submitted) that has intruded the Serifos country rocks. Its intrusive nature may be best demonstrated at the scale of the geological map (Figure 2) by its contact relations with rocks occurring at different tectonostratigraphic levels. Numerous outcrops along the northern margin of the granodiorite show that the orthogneiss mylonites have been later cross-cut by the granodiorite intrusion. Thus, the HT-deformation and HT-skarn formation recorded by the orthogneiss mylonites predate the late Miocene granodiorite intrusion. Preliminary ion microprobe U-Pb dating of zircons in this rock yielded Late Eocene ages (D. Schneider, pers. comm.).

Hydrothermal solution, precipitation and alterations also occurred during and after the emplacement of the granodiorite and resulted in the formation of world famous gemstone quality crystals from Serifos (Prager, 1935; Vogt, 1991). Hydrothermal alteration of the granodiorite is recorded by numerous fractures along which post-magmatic fluid circulation caused leaching of the mafic components from the granodiorite. Such metasomatic activity is probably also responsible for the iron ore formation in the southwest parts of Serifos, where most of the relics of the historic mining activities can be found.

The higher structural levels are mainly exposed in the northern parts of Serifos. They are tectonically separated from the amphilolite and orthogneiss unit by steep N-dipping cataclastic normal fault zones (Figure 2). The greenschists typically consist of chlorite + white mica + epidote + calcite +/- actinolite +/- biotite. They are intimately interbedded within m-scale domains with impure marbles, marble meta-conglomerates and gneiss horizons. Locally, the greenschists bear multiphase-grown epidote poikiloblasts containing relics of glaucophane. Obviously, the Serifos greenschists have been affected by an earlier high-pressure metamorphic evolution (Salemink, 1985) that is typical for the Cycladic Blueschist Unit. Several fold generations are overprinted by folds with N-S oriented fold-axes and with upright axial planes. The resulting refold structures deform a pre-existing W-E trending stretching lineation.

Figure 2. Geological map of Serifos

Geological map of Serifos

Geological map of Serifos (modified after Petrakakis et al. 2007). Boxes labelled Day 1, 2 and 3 indicate selected traverses in Figures 3, 7 and 10 respectively.

Although the granodiorite intrusion clearly crosscuts the regional metamorphic fabric, the uppermost structural levels of the granodiorite together with its host rocks are strongly overprinted by greenschist-facies, ductile to cataclasitic conditions, low-angle shear zone networks that record a notably consistent stretching lineation associated with non-coaxial SSW directed shear (Grasemann et al. 2004). The spectacular brittle / ductile low-angle faults cut through the peninsula south of Meghálo Livádhi (Μεγάλο Λιβάδι) as well as east of Livádhi (Λιβάδι) and at Platís Yialós (Πλατύς Γιαλός) in the north, respectively. High-strain is localized during low-greenschist ductile deformation in ultra-mylonitic fine-grained marbles of up to a few metre thickness. A strong NNE-SSW stretching-component during non-coaxial shearing is recorded by abundant shear-zone parallel symmetrically boudinaged layers of schists. S-directed shearing and stretching is accompanied by WNW-ESE shortening and folding of the mylonites into upright folds with fold-axes parallel to the stretching lineation locally developing sheath folds. The folded low grade mylonites are cut by knife-sharp planes above which tens of centimetre thick multistage ultracataclasites record ongoing SSW directed shearing in the brittle / ductile and brittle regime. Similarly, the cataclasites are folded into upright folds with NNE-SSW striking fold-axes. Above the cataclastic fault core of the low-angle faults the rocks consisting mainly of marbles and schists are strongly overprinted by proto-cataclastic deformation and fluid alteration.

Some highly altered serpentinites and talc schists south of Meghálo Livádhi probably belong to the widespread metaophiolitic remnants reported from several other Cycladic islands (see Bröcker and Pidgeon 2007 for a recent review and new Late Jurassic protolith ages for metagabbroic rocks from Andros) However, no other clear evidences of rocks from a hanging wall unit of the low-angle normal faults have been observed.

Based upon the presence of magmatic microstructures and the intrusion of steeply-dipping dacitic to andesitic dykes along brittle fractures, the granodiorite is considered to be, intruded at high crustal levels (ca. 5-10 km depth) during a late stage of SSW-directed low-angle normal faulting. Some of the dykes are also deformed by the S-directed low-angle brittle / ductile shear zones. However, most of the dykes are undeformed and strictly follow a conjugate WNW-ESE-striking, high-angle normal fault system indicating roughly a NNE-SSW extension that post-dates the emplacement of the main granodiorite pluton. WNW-ESE-striking brittle conjugate high-angle normal faults can be observed throughout the island especially at Meghálo Livádhi, SE of Livádhi and at Kéndarchos (Κένταρχος), where strong weathering of the non-cohesive cataclasites influences the topographic evolution of the island.