Formation, uplift and tectonic integration of a Periadriatic intrusive complex (Pohorje, Slovenia) as reflected in magnetic parameters and palaeomagnetic directions

Abstract

S U M M A R YThe Periadriatic lineament is a 700-km-long, first-order tectonic boundary in the Alps, along which several Tertiary intrusions are located that were emplaced while tectonic processes were active. The Pohorje pluton is one of the Periadriatic magmatic suites and comprises a pluton and a dacite stock that were emplaced at depths of about 10 and 2 km, respectively. Oriented samples were collected from 33 sites in the magmatic, and three in the surrounding metamorphic rocks and subjected them to standard palaeomagnetic and magnetic susceptibility anisotropy processing. There is extreme variation in susceptibility (although the magnetic mineral are always magnetite) and in its anisotropy within the pluton. Lowest values are in the mafic members (in the gabbro, the oldest and in some lamprophyres, the youngest member of the plutonic suite), while the tonalite (granodiorite) exhibits extreme variation, far exceeding those observed for other Periadriatic plutons. The 'abnormality' of the Pohorje pluton is attributed to the complex history that involves assimilation, fractional crystallization, stratification in the magma chamber, incorporation of wall rocks, variable degree of K-metasomatism and hydrothermal alteration. Despite the above-described heterogenities and sometimes very high degree of susceptibility anisotropy (much higher that in the metamorphic samples) in the acid member, the palaeomagnetic directions observed for the pluton define three clear-cut groups. The directions show no dependence on the degree of magnetic susceptibility anisotropy. In the order of crystallization, they exhibit decreasing degree of clockwise declination deviation from north. The dacite stock is characterized by counter-clockwise rotated declinations. As the Pohorje pluton was uplifted very fast within the right-lateral Periadriatic shear zone system, it is interpreted that the palaeomagnetic directions observed for the pluton are evidence for a clockwise rotation during exhumation. The palaeomagnetic directions for the dacite, which formed after the climax of the external forces, fits the regional pattern of counter-clockwise rotation observed in the Neogene sediments around the Pohorje pluton. This shows that by the time of the dacite intrusion, the Pohorje pluton was already incorporated into the 'block' that was driven by counter-clockwise rotated Adriatic microplate.

Reconstruction of early phase deformations by integrated magnetic and mesotectonic data evaluation

“…The formation is made of granodiorite, which suffered ductile deformation at an estimated temperature of 400 − 450 • C followed by brittle deformation after cooling [19]. This rock has a high susceptibility (≈ 10 −2 SI), extremely high degree of AMS (≈ 35% in average) and lineation (≈ 20% in average) [38]. In this case the classical method by V. Jelinek is a perfect procedure to determine the orientation of the AMS ellipsoid.…”

“…In some cases a general trend can be observed that is maintained throughout a stratigraphic sequence, especially in the fine grained clastics (mudstones). This trend can be attributed to weak tectonic deformation [35,14,38,39,40], especially when K3 is close to the bedding pole, i.e. the magnetic foliation is subparallel with the bedding plane.…”

The Northern Giudicarie and the Meran-Mauls fault (Alps, Northern Italy) in the light of new paleomagnetic and geochronological data from boudinaged Eo-/Oligocene tonalites

“…Since the intensity of natural remanent magnetisation in the tonalites was too weak, drill cores were taken mainly from mafic enclaves present in most of the tonalitic bodies. A similar sampling strategy has been used previously by Márton et al (2006) for the Pohorje pluton at the easternmost end of the PAL.…”

The bowing potential of granitic rocks: rock fabrics, thermal properties and residual strain

“…One sample (Cezlak granodiorite; SLO; Fig. 2a) originates from the Pohorje complex in N-Slovenia, near Oplotnica (for details see, e.g., Márton et al 2006). The other one is the Flossenbürg granite (FG, Fig.…”

“…In the central part of the Pohorje magmatic massive a *40 km long and 7 km wide calc alkaline igneous complex crops out that was emplaced into a polymetamorphic basement (Márton et al 2006). In the SE of this complex a grey granodiorite is quarried, which is characterized by cross cutting of light aplitic and pegmatitic veins of different generations.…”