The Eastern Pelagonian metamorphic core complex: insight from the 40Ar/39Ar dating of white micas

Schenker, Filippo and Forster, Marnie and Burg, Jean-Pierre (2012) The Eastern Pelagonian metamorphic core complex: insight from the 40Ar/39Ar dating of white micas. In: Abstract Volume, 10th Swiss Geoscience Meeting 10th Swiss Geoscience Meeting, Bern, Switzerland.

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The Pelagonian Zone in continental Greece constitutes a pre-alpine gneissic block trending approximately NNW-SSE between two domains with oceanic affinity: the Pindos in the west and the Vardar zone in the east. In the eastern Pelagonian zone we recognized a denuded metamorphic dome extending about 20 x 15 km with the long axis trending NNW-SSE. The main lithologies (gneiss, marbles and amphibolites) display a shallow-dipping foliation whose up-arch defines the structural dome. The metamorphic conditions decrease from upper-amphibolite in the core to greenschist metamorphic conditions in the flanks of the dome. Aligned micas and amphiboles and elongated quartz and feldspar grains define a prominent lineation trending SW-NE. Asymmetric structures in the XZ plane of finite strain record two regional senses of shear: (i) everywhere, top-to-the-SW sense of shear (direction: 252°±30; plunge: 8°±25) is associated with strain gradients from protomylonite to ultramylonite and recumbent, isoclinal and occasional sheath folds; (ii) top-to-the-E sense of shear (direction: 88°±24; plunge: 11°±12) is localized into narrow (0.1 to 100 m), low-angle shear zones on the eastern flanks of subdomes. The 40Ar/39Ar step-heating dating technique has been applied to micas from orthogneisses sampled from the core to the flanks of the dome. The micas have been separated with acoustical shockwaves produced in the SELFRAG apparatus, with the advantage to liberate morphologically intact grains. The liberated grains were sieved at different grain-sizes (between 100 and 300 μm) according to the microstructures recognized in thin-sections. Results show “plateau”-ages at ca. 100-120 Ma, 80 Ma and 50 Ma. The 100-160 μm fraction of white micas in mylonitic orthogneisses yielded slightly younger ages than the 160-300 μm fraction. This difference sets the discussion between neo-crystallization age of the smaller grain fraction and cooling age with multiple diffusion domains of the larger fraction. The latter hypothesis is tested with modeling of Ar-diffusion along Pressure-Temperature-time histories using the MacArgon program. These new ages are consistent with ages published on the southern Pelagonian, demonstrating major tectonic and metamorphic activity during early Cretaceous and Eocene times.

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