Conditions of Formation and Crystallization Kinetics of Highly Oxidized Pseudotachylytes from the High Tatras (Slovakia) |
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| Authors: | PETRIK IGOR; NABELEK PETER I; JANAK MARIAN; PLASIENKA DUSAN |
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| Institution: | 1 GEOLOGICAL INSTITUTE OF THE SLOVAK ACADEMY OF SCIENCES, DÚBRAVSKÁ 9, PO BOX 106, 84005 BRATISLAVA, SLOVAKIA
2 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF MISSOURI–COLUMBIA, COLUMBIA, MO 65211, USA |
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| Abstract: | Tectonic activity during the Miocene exhumation of the Tatragranitoid basement resulted in frictional melting of granite.The activity marks the early stages of the faulting that isresponsible for uplift of the High Tatras. As indicated by pre-existingcataclasite metamorphic mineral assemblages, the ambient pressurewas about 250–300 MPa, corresponding to depths between10 and 12 km. The pseudotachylytes are Fe rich, highly oxidizedand crystalline. The matrix composition suggests disequilibriumpartial melting of a biotite-dominated assemblage. Oxygen isotopiccompositions of a pseudotachylyte sample and its constituentminerals show equilibration with the host granodiorite and allowfor the introduction of oxidizing external water rather thanoxidation as a result of the dissociation of free water liberatedduring melting. The kinetic information extracted from hematitecrystal-size distributions (CSDs) that are preserved in feldspathicmatrices shows that crystals in most places were accumulatedwhile the system was open. The melt was highly mobile and proneto strong differentiation. The hematite crystals reach a maximumof 26 vol. % (45 wt % Fe2O3). In rare places where the flowceased, the system became closed and produced distinct CSDs.The longest apparent crystallization times (90 s) are recordedmostly in pools in the central parts of the pseudotachylyteswhereas the shortest times (10 s) come from rims and tips offractures. The estimated hematite growth rate was about fiveorders of magnitude higher than that of ilmenite in lava lakes.Such extreme crystallization rates result from high undercoolingsassociated with high cooling rates. Very high cooling ratesare promoted by the extremely high surface/volume ratios ofthe pseudotachylyte sheets. KEY WORDS: pseudotachylyte; hematite; kinetics; High Tatra; Slovakia |
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