METAMORPHIC EVOLUTION AND FLUID COMPOSITION OF GARNET-CLINOPYROXENE AMPHIBOLITES FROM THE TATRA MOUNTAINS, WESTERN CARPATHIANS

Relies suggesting an early high-pressure eclogite facies stage have be en identified in the garnet and clinopyroxene-bearing amphibolites of the Tatra Mountains, in the Variscan basement of the Western Carpathia ns, Slovakia. In these rocks, primary omphacite (Cpx I) has been wholl y converted to symplectites of diopside (Cpx II) and plagioclase. Apar t from minor inclusions in the garnet cores, amphibole and plagioclase are secondary minerals, formed in the kelyphitic rims between garnet and clinopyroxene. Several generations of amphibole (pargasite, hornbl ende, cummingtonite, actinolite) are evidence of a transformation down to greenschist facies conditions. Thermobarometric calculations from mineral inclusions in the garnet cores yield 670-700 degrees C and 10- 15 kbar, recording the initial path from upper amphibolite to eclogite facies conditions. The attainment of the eclogite facies stability fi eld is inferred from the composition of a ''reconstructed'' omphacite (Jd36), implying a minimum peak-pressure of 15-16 kbar. Conditions of around 650 degrees C and 8-10 kbar record the post-eclogite breakdown and partial re-equilibration in the amphibolite facies region. The ear liest fluid inclusions contain high-density nitrogen-dominated, water- absent fluid. Younger are polyphase brines and two-phase H2O + N-2 + C H4 inclusions with signs of heterogeneous entrapment of coexisting gas -rich and water-rich immiscible phases. Pure nitrogen (+/- 0.5 mol.% C H4) is considered to have been the major component during the high-pre ssure metamorphism. In contrast, the later, aqueous inclusions are int erpreted to represent retrogression-related fluids, The brines have or iginated by leaching and re-entrapment of saline melt inclusions obser ved in tonalitic-trondhjemitic layers of the amphibolite enclosing the eclogitic relies. A similar mechanism is assumed for the origin of th e H2O + N-2 + CH4 inclusions. These represent relies of the primary, e clogite facies-related N, inclusions, re-equilibrated and re-entrapped during retrogression under increasing water activity. A possible sour ce of N-2 and Cl-rich fluids was primary amphibole, which decomposed d uring the prograde metamorphism from the amphibolite to the eclogite f acies. The metamorphic PT path is generally clockwise, reflecting a st eep increase in pressure and temperature during burial, followed by de compression and cooling, The retrograded eclogites occur in boudins wi thin an allochthonous unit of an inverted metamorphic sequence. Their exhumation was facilitated by tectonic transport along a ductile shear zone during the Variscan orogeny.