Conditions of formation and crystallization kinetics of highly oxidized pseudotachylytes from the High Tatras (Slovakia)

Abstract

Tectonic activity during the Miocene exhumation of the Tatra granitoid basement resulted in frictional melting of granite. The activity marks the early stages of the faulting that is responsible for uplift of the High Tatras. As indicated by pre-existing cataclasite metamorphic mineral assemblages, the ambient pressure was about 250-300 MPa, corresponding to depths between 10 and 12 km. The pseudotachylytes are Fe rich, highly oxidized and crystalline. The matrix composition suggests disequilibrium partial melting of a biotile-dominated assemblage. Oxygen isotopic compositions of a pseudotachylyte sample and its constituent minerals show equilbration with the host granodiorite and allow for the introduction of oxidizing external water rather than oxidation as a result of the dissociation of free water liberated during melting. The kinetic information extracted from hematite crystal-size distributions (CSDs) that are preserved in feldspathic matrices shows that crystals in most places were accumulated while the system was open. The melt was highly mobile and prone to strong differentiation. The hematite crystals reach a maximum of 26 vol. % (45 wt % Fe2O3). In rare places where the flow ceased, the system became closed and produced distinct CSDs. The longest apparent crystallization times (90 s) are recorded mostly in pools in the central parts of the pseudotachylytes whereas the shortest times (10 s) come from rims and tips of fractures. The estimated hematite growth rate was about five orders of magnitude higher than that of ilmenite in lava lakes. Such extreme crystallization rates results from high undercoolings associated with high cooling rates. Very high cooling rates are promoted by the extremely high surface/volume ratios of the pseudotachylyte sheets.