TWO TYPES OF GARNET-CLINOPYROXENE-PLAGIOCLASE METABASITES IN THE MALá FATRA MOUNTAINS CRYSTALLINE COMPLEX, WESTERN CARPATHIANS: METAMORPHIC EVOLUTION, P-T CONDITIONS, SYMPLECTITIC AND KELYPHITIC TEXTURES

Abstract

In the pre-Upper Carboniferous Malá Fatra Mts. metamophic complex in the Western Carpathians, banded amphibolites and amphibole gneisses include Grt-Cpx-Hbl-Pl metabasites of two types. Type I comprises massive coarse-grained Grt-Aug-Hbl-Pl-Qtz rocks without symplectites and kelyphites. The rocks compose 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 concentration in the plagioclase is 49-87%; and the brown-green amphibole is classed with low-Na horn-blende-tschermakite series. The type-I assemblages suggest a one-stage prograde-retrograde cycle: according to Grt-Cpx thermometry, the temperature first increased from 630 to 705°C from the cores to rims of progradely zoned garnet crystals and then decreased to 640°C during the exhumation, when the outermost retrograde rim grew. The type-II metabasites are fine- to medium-grained boudins of Grl-Cpx-Pl-Qtz rocks with a Hbl-Pl-Ilm ± Qtz or Hbl-Aug (1-4% Jd)-Pl (20-40% An) ± Qtz matrix. In contrast to the type-I metabasites, these rocks contain patches of fine-grained Aug (1-4% Jd)-Pl-Hbl-Trn symplectites, which were previously thought to be produced by the replacement of early omphacite. It was also hypothesized that the type-II metabasites are recrystallized eclogites. However, it was definitely established that omphacite relics are totally absent from these rocks. The garnet has prograde zoning, sometimes with thin retrograde rims, and contains inclusions of Hbl, Ep, and Pl but never armored relics of high-pressure minerals. According to Grt-Hbl thermometry, the temperature increased from 600 to 699°C from the cores to rims. This indicates that the Aug-bearing symplectites and the analogous matrix were produced not by the recrystallization of eclogites but by the reaction Hbl + Czo + Qtz → Cpx + Pl +Grt (outer prograde rim) + Ttn + H2O, which marks the transition from the epidote amphibolite to amphibolite facies at Tmax ∼700°C (Grt-Hbl thermometry) and P ∼ 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 XFe values. The very close compositions of minerals in the metabasites of types I and II, similar zoning of their garnets, and their identical P-T metamorphic parameters indicate that both rock types underwent a similar one-stage prograde-retrograde evolution from the epidote amphibolite to high-pressure amphibolite facies and a subsequent retrogression. Some differences between the Ca and Al concentrations in the metabasites of types 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 hornblende parts) at contacts between garnet and Cpx-Hbl-Pl symplectite. The rims 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.