Metamorphic evolution and fluid composition of garnet-clinopyroxene amphibolites from the Tatra Mountains, Western Carpathians

Abstract

Relics suggesting an early high-pressure eclogite facies stage have been identified in the garnet and clinopyroxene-bearing amphibolites of the Tatra Mountains, in the Variscan basement of the Western Carpathians, Slovakia. In these rocks, primary omphacite (Cpx I) has been wholly converted to symplectites of diopside (Cpx II) and plagioclase. Apart 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, hornblende, cummingtonite, actinolite) are evidence of a transformation down to greenschist facies conditions. Thermobarometric calculations from mineral inclusions in the garnet cores yield 670–700 °C and 10–15 kbar, recording the initial path from upper amphibolite to eclogite facies conditions. The attainment of the eclogite facies stability field is inferred from the composition of a “reconstructed” omphacite (Jd36), implying a minimum peak-pressure of 15–16 kbar. Conditions of around 650 °C and 8–10 kbar record the post-eclogite breakdown and partial re-equilibration in the amphibolite facies region. The earliest fluid inclusions contain high-density nitrogen-dominated, water-absent fluid. Younger are polyphase brines and two-phase H 2O + N 2 + CH 4 inclusions with signs of heterogeneous entrapment of coexisting gas-rich and water-rich immiscible phases. Pure nitrogen (±0.5 mol.% CH 4) is considered to have been the major component during the high-pressure metamorphism. In contrast, the later, aqueous inclusions are interpreted to represent retrogression-related fluids. The brines have originated by leaching and re-entrapment of saline melt inclusions observed in tonalitic-trondhjemitic layers of the amphibolite enclosing the eclogitic relics. A similar mechanism is assumed for the origin of the H 2O + N 2 + CH 4 inclusions. These represent relics of the primary, eclogite facies-related N 2 inclusions, re-equilibrated and re-entrapped during retrogression under increasing water activity. A possible source of N 2 and Cl-rich fluids was primary amphibole, which decomposed during the prograde metamorphism from the amphibolite to the eclogite facies. The metamorphic PT path is generally clockwise, reflecting a steep increase in pressure and temperature during burial, followed by decompression and cooling. The retrograded eclogites occur in boudins within an allochthonous unit of an inverted metamorphic sequence. Their exhumation was facilitated by tectonic transport along a ductile shear zone during the Variscan orogeny.