Two types of garnet-clinopyroxene-plagioclase metabasites in the Mala Fatra Mountains crystalline complex, Western Carpathians: Metamorphic evolution, P-T conditions, symplectitic and kelyphitic textures

In the pre-Upper Carboniferous Mala Fatra Mts. metamorphic complex in the W estern Carpathians, banded amphibolites and amphibole gneisses include Grt- Cpn-Hbl-Pl metabasites of two types. Type I comprises massive coarse-graine d Grt-Aug-Hbl-Pl-Qtz rocks without symplectites and kelyphites. The rocks c ompose boudins up to a few centimeters across in amphibolites. Their garnet shows prograde zoning with thin outer retrograde rims; the augite (in the matrix and in inclusions in garnet) contains no more than 4% Jd; the An con centration in the plagioclase is 49-87%; and the brown-green amphibole is c lassed with low-Na hornblende-tschermakite series. The type-I assemblages s uggest a one-stage prograde-retrograde cycle: according to Grt-Cpx thermome try, the temperature first increased from 630 to 705 degreesC from the core s to rims of progradely zoned garnet crystals and then decreased to 640 deg reesC during the exhumation, when the outermost retrograde rim grew. The ty pe-II metabasites are fine- to medium-grained boudins of Grt-Cpx-Pl-Qtz roc ks with a Hbl-Pl-Ilm +/- Qtz or Hbl-Aug (1-4% Jd)-Pl (20-405 An) +/- Qtz ma trix. In contrast to the type-I metabasites, these rocks contain patches of fine-grained Aug (1-4% Jd)-Pl-Hbl-Ttn symplectites, which were previously thought to be produced by the replacement of early omphacite. It was also h ypothesized that the type-II metabasites are recrystallized eclogites. Howe ver, it was definitely established that omphacite relies are totally absent from these rocks. The garnet has prograde zoning, sometimes with thin retr ograde rims, and contains inclusions of Hbl, Ep, and PI but never armored r elies of high-pressure minerals. According to Grt-Hbl thermometry, the temp erature increased from 600 to 699 degreesC from the cores to rims. This ind icates that the Aug-bearing symplectites and the analogous matrix were prod uced not by the recrystallization of eclogites but by the reaction Hbl + Cz o + Qtz --> Cpx + Pl + Grt (outer prograde rim) + Ttn + H2O, which marks th e transition from the epidote amphibolite to amphibolite facies at T-max si milar to 700 degreesC (Grt-Hbl thermometry) and P similar to 8-9 kbar (Grt- Hbl-Pl-Qtz barometry). Chemical reactions resulting in symplectites in the metabasites are examined as functions of the bulk-rock (Ca + Al)/(Mg + Fe) ratios and the X-Fe values. The very close compositions of minerals in the metabasites of types I and II, similar zoning of their garnets, and their i dentical P-T metamorphic parameters indicate that both rock types underwent a similar one-stage prograde-retrograde evolution from the epidote amphibo lite to high-pressure amphibolite facies and a subsequent retrogression. So me differences between the Ca and Al concentrations in the metabasites of t ypes I and II account for their textural dissimilarities and the morphology of Cpx crystals in them. Another type of reaction textures in the type-II metabasites is Pl-Hbl kelyphite rims (with inner plagioclase and outer horn blende parts) at contacts between garnet and Cpx-Hbl-Pl symplectite. The ri ms are demonstrated to have developed early during the exhumation stage due to the decomposition of the Grt-Aug assemblage by the reaction Grt + Cpx ( in symplectite)+ Na2O + H2O --> Pl + Hbl + Qtz in response to decompression and Na inflow at a temperature close to the metamorphic culmination.