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Constantin Balica, Babes-Bolyai University (Romania)
Ducea Mihai, University of Arizona (United States)
Costin Gelu, University of Arizona (United States)
Balintoni Ioan, Babes-Bolyai University (Romania)
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The basement of the Apuseni Mountains, Romania, consists of three Paleozoic terranes, comprising Early Ordovician protoliths, whose collision happened during Variscan Orogeny. They are called Somes, Biharia and Baia de Aries. During that time, Somes terrane was in the position of upper plate, Baia de Aries terrane was subductin and Biharia terrane was deformed along the suture. We are focused here on the Baia de Aries metamorphic sequence as the main body of Baia de Aries terrane. This metamorphic sequence has been described as a polymetamorphic medium grade entity described by staurolite, kyanite and sillimanite assemblage. Some big carbonate lenses also encapsulate eclogites-like lithons.
In order to understand the thermotectonic evolution of the Baia de Aries sequence, we collected from micaschist domain two garnet populations, separated by size reasons, through which we realized microprobe compositional profiles. We suspected that the two garnet populations to have different origins and metamorphic histories, assumptionj that was not supported by data. We mention a garnet population with 1-1.5 mm grain diameter and a second one exceeding 5 mm in diameter. All the garnets are zoned. From core-to-rim profiles Ca and Mn concentration decrease while Mg and Fe concentration increase. The decreasing trend is also specific for Fe/(Fe+Mg) ratio. These aspects suggest a frozen prograde thermotectonic evolution. Only the outermost rim, smaller than 28 microns, shows retrograde zoning with slight decreasing in Mg content while Mn and Ca slightly increase. This indicates a very rapid exhumation and minimal residence time at peak conditions.
In order to get the thermotectonic event age, we performed a Sm/Nd internal isochron, using whole rock and bulk garnet isotopic ratios. This gave an age of 325±24 Ma, with initial143Nd/144Nd of 0.51166±1.9e-0.5 and MSWD of 0.9. The age precision is condition by the close isotopic ratios between the three analyzed components. This age is Variscan and we infer that the main thermotectonic event affecting the Baia de Aries metamorphic sequence was Variscan too. A microprobe composition profile through eklogite-like rocks has a flat shape at MgO content around 7%, suggesting different behavior of garnets in micaschists and eclogites like rocks.
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