GEOLOGICA CARPATHICA, FEBRUARY 2009, 60, 1, 15—33 doi: 10.2478/v10096-009-0001-8 www.geologicacarpathica.sk The emplacement mode of Upper Cretaceous plutons from the southwestern part of the Sredna Gora Zone (Bulgaria): structural and AMS study NEVEN GEORGIEV 1* , BERNARD HENRY 2 , NELI JORDANOVA 3 , NIKOLAUS FROITZHEIM 4 , DIANA JORDANOVA 3 , ZIVKO IVANOV 1 and DIMO DIMOV 1 1 Department of Geology and Paleontology, Sofia University “St. Kliment Ohridski” 15 Tzar Osvoboditel bd., 1000 Sofia, Bulgaria; neven@gea.uni-sofia.bg 2 Paléomagnétisme, IPGP and CNRS, 4 av. de Neptune, 94107 Saint-Maur cedex, France 3 Geophysical Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev str., block 3, 1113 Sofia, Bulgaria 4 Steinmann-Institut, Universität Bonn, Nußallee 8, D-53115 Bonn, Germany (Manuscript received February 21, 2008; accepted in revised form June 12, 2008) Abstract: Several plutons located in the southwestern part of the Sredna Gora Zone – Bulgaria are examples of the Apuseni-Banat-Timok-Sredna Gora type of granites emplaced during Late Cretaceous (86—75 Ma) times. The studied intrusive bodies are spatially related to and deformed by the dextral Iskar-Yavoritsa shear zone. The deformation along the shear zone ceased at the time of emplacement of the undeformed Upper Cretaceous Gutsal pluton, which has in- truded the Iskar-Yavoritsa mylonites. A clear transition from magmatic foliation to high-, moderate- and low-tempera- ture superimposed foliation and lineation in the vicinity of the Iskar-Yavoritsa and related shear zones gives evidence for simultaneous tectonics and plutonism. Away from the shear zones, the granitoids appear macroscopically isotropic and were investigated using measurements of anisotropy of magnetic susceptibility at 113 stations. The studied samples show magnetic lineation and foliation, in agreement with the magmatic structures observed at a few sites. Typical features of the internal structure of the plutons are several sheet-like mafic bodies accompanied by swarms of mafic microgranular enclaves. Field observations indicate spatial relationships between mafic bodies and shear zones as well as mingling processes in the magma chamber which suggest simultaneous shearing and magma emplacement. Struc- tural investigations as well as anisotropy of magnetic susceptibility (AMS) data attest to the controlling role of the NW- SE trending Iskar-Yavoritsa shear zone and to the syntectonic emplacement of the plutons with deformation in both igneous rocks and their hosts. The tectonic situation may be explained by partitioning of oblique plate convergence into plate-boundary-normal thrusting in the Rhodopes and plate-boundary-parallel transcurrent shearing in the hinterland (Sredna Gora). Key words: Late Cretaceous, Sredna Gora – Bulgaria, tectonics, magma emplacement, strike-slip setting, AMS. Introduction The role of shear zones in the generation, ascent and emplace- ment of magma through the crust is a long-standing matter of debate (Castro 1987; Hutton 1988, 1997; Paterson & Fowler 1993; Acef et al. 2003; Rosenberg 2004). Numerous examples of granite emplacement along strike-slip fault systems in transpressional or transtensional tectonic settings (Tikoff & Teyssier 1992; Paterson & Fowler 1993; Roman-Berdiel et al. 1997; Steenken et al. 2000; Henry et al. 2004) suggest different possibilities to solve the “space problem”. Visible structures of the granitoids formed in the magmatic stage and superimposed solid-state foliations and lineations provide valuable informa- tion on magma emplacement kinematics and can help us to bet- ter understand the connection between tectonics and magmatism (Saint Blanquat & Tikoff 1997; Steenken et al. 2000). The latter is not a simple task, especially when the stud- ied intrusives are nearly isotropic and widely used structural methods are not applicable. In such cases the measurement of Anisotropy of Magnetic Susceptibility (AMS) is a standard pro- cedure (Tarling & Hrouda 1993). Determination of magnetic fabric is a quick and easy semi-quantitative method to provide information on the bulk fabric of the plutons (Henry 1980; Bouchez 1997; Saint Blanquat & Tikoff 1997). The results are often used to support structural data in constraining the syn- or post-tectonic emplacement of intrusions (Henry 1980; Bouchez 1997; Henry et al. 2004). On the other hand, some studies (Ferre et al. 1997) demon- strate that crustal-scale shear zones play a role of feeder channels and can provide space for chamber formation at different depths. These relationships can explain the frequent coexistence of magmas contrasting in composition (felsic and mafic) at the same crustal level (Michael 1991; Wiebe & Collins 1998). Interaction between contrasting magmas (mixing versus mingling processes) depends on parameters like volume, time, compositions, temperature, viscosity, etc. (Barbarin & Didier 1992). The Upper Cretaceous intrusive bodies located in the southwestern part of the Sredna Gora Zone, Bulgaria, crop out in close relationship with the regional dextral strike-slip Iskar-Yavoritsa shear zone (IYSZ). The aim of this study is to investigate the space- and time connection between plu- tons and shear zone using structural and AMS data, and to provide a model for emplacement of these granites.