Is the Lower tectonic unit (Lower terrain) in the Rhodope a monolithic tectonic unit

The geological correlation of high grade metamorphic rocks across the Rhodope massif in Southern Bulgaria and Northern Greece has been controversial for decades. Two different models with contrasting approaches to correlation are employed today. According to the first model, the Rhodope massif is viewed as a stable crustal fragment of Precambrian age. Proponents of that model use a litostratigraphic approach to sub-divide and correlate metamorphic rocks across the massif. The second model considers the Rhodope massif a stack of tectonic plates that consists of two major tectonic units (a.k.a. the Upper and Lower terrains) separated by several mylonitic zones and “intermediate” tectonic units. Supporters of the latter model employ lithotectonic principles to subdivide and correlate metamorphic rocks across the massif.
   Both models, however, correlate massive marbles that crops out near the villages of Trigrad and Yagodina, i.e., in the southern section of the massif to marbles cropping out to the north near the town of Assenovgrad. According to the different classification schemes applied by the two models, marbles are either a part of the Dobrostan Formation (model 1) or the Asenitza lithotectonic unit (model 2).
   We use stable isotopic determinations and microscopically characteristic of marbles from three different localities to examine the application of this investigation for tectonic correlations.
   Locality 1 (L1) - Aseinitza lithotectonic unit. The investigated marbles are massive, poor calcite with a medium grained equigranular texture. Rare xenoblastic quartz and white mica (colorless phlogopite ?) grains are observed, too. The calcite grains contain numerous lamellar or intersecting deformation type III twins. Calcite MgO contents vary in a narrow range (0.68-0.81%), hence, the minerals can be classified as low-Mg calcites. Samples from
this location have nearly identical δ¹³C compositions, i.e., from +0.14 to +1.57 %o (average +0.66, n=4). The δ¹⁸Omarble values range from -5.50 to -6.40 ‰ (average -5.88, n=4), respectively.
   Locality 2 (L2) - Lower terrain. The samples comprise massive, poor calcite marbles. In contrast to the samples from L1, L2 samples have unequigranular texture. Relatively big calcite grains “float” in fine-grained recrystallized matrix. The large crystals have lobate outlines and core-mantle texture is a specific feature of these rocks. Wide lamellar and intersecting deformation twins are characteristic features of the big calcite grains. The MgO content of matrix calcite is 0.75-0.86%. L2 samples exhibit significant isotope variability with two samples having higher ¹³C and lower ¹⁸O compositions, respectively. As a result, the δ¹³C values of the samples vary from +2.16 to +4.21%o (average +3.18, n=4), while their δ¹⁸O vary from -2.40 to -10.60 (average -6.61, n=4), respectively. If previous data are considered, average δ¹³C and δ¹⁸O values are +1.96‰ and -1.98‰, respectively. The cause of the significantly different isotope values of the two L2 samples is likely related to localized water-rock interaction.
   Locality 3 (L3) - Asenitza lithotectonic unit. The samples were collected along the road to the Yagodina cave. These have massive structure and, in contrast to the L1 and L2 samples, contain dolomite. Texturally L3 samples are similar to L2, however, matrix calcite in L3 samples is slightly coarser. The large clasts in the L3 samples contain up to 3.30 % MgO, while matrix calcite MgO contents vary 0.77-1.67 % and are, thus, comparable to these of L1 and L2 calcites. The δ¹³C values range from +1.03 to 2.07‰ (average +1.67, n=5) and the δ¹⁸O from -2.00 ‰ to -4.30‰ (average -3.65‰, n=5). The δ¹³C and δ¹⁸O values of these samples do not vary significantly.
   Metamorphic P-T conditions in the marbles and the enclosing lithologies are estimated using calcite-dolomite solvus thermometry and other conventional thermometers. For chloritoid-bearing metapelites from the area of the village of Byala Cherkva (Asenitza unit) the results are: 330-360 ºC at 5 kbar (Bt-Ms thermometer) and 350-370 ºC at 5 kbar (Grt-Bt thermometer). The calcite-dolomite solvus thermometry of dolomite-bearing marbles from L3 yields 360-470 ºC at 5 kbar for the matrix calcite and 560-610 ºC at 5 kbar for the large calcite clasts. Those PT conditions are comparable to values determined from metapelites from the Lower terrain in northern Greece.
   Marble samples from the three different locations in the Rhodope massif exhibit significant mineralogical, textural and stable isotope differences. While L2 and L3 samples have similar textural characteristics and stable isotope compositions, these differ from the Asenitza unit (L1) samples. That indicates that marbles from the southern part of the Rhodope massif (i.e., the Yagodina and Trigrad area) are part from the Lower terrain (Lower tectonic unit) or Pangeon unit i.e. they present a tectonic window.