Journal ofthe Geological Society, London, Vol. 148, 1991, pp. 749-758, 7 figs, 1 table. Printed in Northern Ireland The Oligocene Xanthi pluton, northern Greece: a granodiorite emplaced during regional extension I. KOUKOUVELAS' & G. PE-PIPER2 Department of Geology, University of Patras, 261 10 Patras, Greece Department of Geology, St Mary 'S University, Halifax, N.S. Canada B3H 3C3 Abstract: TheXanthi pluton is one of aseries of Oligocenesubduction-related granodiorites in northern Greece. Its emplacement was controlled by major faults. It is located on the ENE-trending Kavala-Komotini fault zone, which probably originated as a strike-slip fault. Geophysical data show that the pluton is laccolith-shaped, extending many kilometres south of the fault. Andesitic dykes several million yearsolder than the pluton indicate a NE-SW-directed extensional stress field. Minerallineations plunging tothe SW reflect continuedextension during cooling of thepluton. Subsequent jointing and dykes of aplite and lamprophyre reflect continued extension. Although this could result from extensional pull-apart at a bend during dextral strike-slip motion on the Kavala- Komotini fault, there is no evidence for subsequent strike-slip deformation. Furthermore, two similar laccolithic plutonsare unrelated to the Kavala-Komotini fault and the regional extent of thick Oligocene sediments suggests an extensional environment. During the extension that created a ramp spacealong the listric faultbounding the Xanthi basin, into which thegranite was intruded, the Kavala-Komotinifault acted as a transfer fault. Reactivation of earlyjoints occurred during late Oligocene-EarlyMiocene compression. Faults within thepluton parallel to the Kavala-Komotini faultaccompanied extension during Neogene basin formation. Emplacement of subduction-related magma into ramp space developed along listric faults, to produce laccolith-like plutons, may be a common feature of back-arc extension. The Oligocene Xanthi pluton (Christophidis 1977) intrudes consequence of local uplift and give anerroneous the southern margin of the Rhodope metamorphic complex impression of the subsurface abundance of granite. of northernGreece (Fig. 1).The southern edge of the The purpose of this paper is to describe field structural exposed pluton is marked by a major fault (the evidence for thenature of emplacement of the Xanthi Kavala-Komotini fault zone);to the south lies the E-W pluton. Thisinformation is used to evaluate the role of trending Komotini-Xanthi basin, with thick Neogene strike-slip and extensional tectonics in the emplacement of sediments. The magneticsignature of the pluton suggests the Xanthi pluton, in order to develop concepts applicable that it is a laccolith with a base at about 4.5 km (Maltezou & to other back-arc extensional areas. Brooks 1989). The laccolith is bounded to the southwest by the Avdhirahorst (Fig. l), in which Eocene-Oligocene volcanic and sedimentary rocks are exposed (Fig. 2). Rocks Tertiary igneous activity of similar age are exposed north of the Kavala-Komotini Rb-Sr ages obtained forthe Xanthi intrusion are fault zone, at the eastern end of the Xanthi pluton. Such 28.8 f0.7 Ma (in the western part) and 26.3 fO.l Ma (in Upper Eocene-Oligocene marine sediments are widespread theeastern part) with initial 87Sr/86Srratio of 0.7065 f in northeastern Greece and reach a thickness of over 2 km O.OOO1 (Kyriakopoulos 1987). Severalgranodiorite plutons in the Limnos well (Schroder 1986). of similar age to the Xanthi pluton outcrop in northeastern The Kavala-Komotini fault extends westwards to at least Greece. The Philippi pluton (28 Ma, Bitzios et al. 1981) and 25 km west of Kavala and eastwards to the Greek-Bulgarian the Myrodato pluton (Kouris 1978) have subcircular outlines boundary (Dimadis & Zachos 1986). Metamorphic rocks of and similar magnetic signature to the Xanthi pluton (Fig. 1). the Rhodope massif are found north and west of the fault; The Vathiremma pluton NW of Komotini lies just north of Rhodope metamorphicrocks, ophiolites of the circum the Kavala-Komotini fault, but is considerably deformed Rhodope zone and Neogene basins are found south andeast and is interpreted by Dimadis & Zachos (1986) as an older of the fault(Dimadis & Zachos 1986). Both the age and pluton. In Western Thrace (Del Moro et al. 1988), there is kinematics of fault are uncertain. It is likely that such a long an ENE-trendinglinear zone of small granitoidplutons linear fault zone originated by strike-slip motion. Since the (Maronia, Kassitera, Kirki, Leptokaria, Halasmata and Tris middle to upper Miocene, the fault has moved with a major Vrisses). These consist of quartz gabbros to granodiorites, dip-slip senseduring the Neogeneextension of northern with Rb-Sr isochron ages of 28-32 Ma. Greece. Maltezou & Brooks (1989) have shown from The Kavala pluton (Fischer 1964; Kokkinakis 1980; geophysical datathat severalplutons form laccolith-like Kyriakopoulos 1987) lies onthe trend of the Kavala- bodies beneath Tertiary basins, and suggest that there may Komotini fault. The pluton is mostly on the south side of the be a geneticbea link betweenextensional tectonics and fault, but the northeastern end of the pluton near Kavala emplacement of the plutons. The surface outcrops of appears to lie north of the fault, as if there is an offset in the plutons in such an extensional regime may be the fault in this region. The pluton consists of relatively 149 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/4/749/4891648/gsjgs.148.4.0749.pdf by guest on 26 September 2021 750 KOUKOUVELAS I. PE-PIPER & G. Fig. 1. Geological map of northeastern Greece, showing Rhodope Massif, prin- cipal faults, granite plutons, and Tertiary basins. Based on IGME 1 :500 OOO Geological map of Greece (Bornovas & Rondogianni-Tsiambaou 1983); Ne- ogene basins from Lalechos & Savoyat (1977); faults at sea from Martin(1987). Magnetic anomalies based on IGME t :+ -9 ** 1 : 50 OOO aeromagnetic maps of Rho- dope massif (ABEM 1967): contoured at Thasos Is. 100 nT, only +300 nT and greater contours shown. Fig. 2. Geology of Xanthi pluton and adjacent areas based in part on Christophidis (1977) and Eleftheriadis et al. (1984). Faults based on air photo- D' graph interpretation. Cross sections show nature of the margin of the pluton at selected locations. Magnetic anomalies based on ABEM (1967): contoured at 100 nT. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/148/4/749/4891648/gsjgs.148.4.0749.pdf by guest on 26 September 2021 STRUCTURE OF XANTHI PLUTON, GREECE 751 Table 1. Summary of structural events in the Xanthi pluton Andesitedykes, strike NWRegional NE-SW-directed extension Radiometric date 33.5 Ma (early Oligocene) Mineral lineationContinued extension duringintrusion Radiometric dates west,29 in Ma 26 Ma in east (late Oligocene) Early joints Continued extension during cooling Aplo-granites Intruded along early joints Aplite veins, lamprophyre Intruded along early joints, radial dykesand sills to margin of pluton,and along SE-striking subvertical planes associated with continued regional NE-SW-directed extension Faults mostlywith strike-Associated with regionalcompression: Regionally late Oligocene- slip motion many are reactivated early joint surfaces early Miocene Hydrothermal alteration Developed along early joint surfaces and ?Early Miocene faults associated with compression Conjugate faults, strike NW* Regional extension Steep-dipping faults, Parallelto Kavala-Komotini fault, associated Mid-Miocene and later strike Estrike or ENE with extension * These faults possibly predate the faults with strike-slip motion. leucocratic granodiorite; and it hasa different magnetic (Koukouvelas 1989). The age of this event is constrained by signature from the Xanthi pluton. The pluton cuts the early the youngest radiometric age for volcanism in the area of NNW-trending major folds in the Rhodope basement rocks, 23.6Ma (Eleftheriadis et al. 1984; Fytikas et al. 1984) and but ishighly deformed by structuresthat parallel the the lack of compressive deformation in mid-Miocene rocks in Kavala-Komotini fault. K-Ar datinghas yielded dates the Komotini-Xanthi basin. Neogene rocks are largely between 26.3 and 15.5 Ma, which are partly or entirely undeformed, but are affected by a series of syn-sedimentary cooling ages. Kokkinakis (1980) obtained highly discordant normal faults.Near Xanthi,these faults follow two main zircon ages (three of which were Cretaceous,one structuraldirections. One is thetrend of the Kavala- Hercynian). The pluton post-dates the main deformation of Komotini fault; the other is the NNW trend which bounds the Rhodope massif the age of this deformation is disputed, the Avdhira horst. There are two main hypotheses for the but is most probably Cretaceous (on the basis of ages of origin of thesestructural trends.They may represent syn-tectonic granites: Soldatos 1985) or Eocene (on the basis extensional faultsdeveloped in post-Eocenetime by of radiometricdating of metamorphic amphiboles:Liati extension of an overthickened nappe pile (Doutsos & 1986). In this case, the Kavala pluton may bea late Ferentinos 1984). Alternatively,they may represent Cretaceous or early Tertiary shear zone pluton. respectively dextralstrike slip and syntheticextensional Early Tertiary volcanic rocks are widespread in Thrace faults which experiencedpredominantly dip-slip motion and eastern Macedonia (Innocenti et al. 1984). Most during Neogene extension (Dewey & Sengor 1979; Martin radiometric age determinations