Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Cordierite gneiss very rich in titano-hematite from Orlík near Humpolec
Abstract
The several km long magnetic anomaly E from Humpolec (ZEMÁNEK 1964) is probably caused by Ti-hematite, 1-2 % content of which in cordierite-rich paragneisses from the hill Orlík is documented. The portion of exsolved ilmenite in the hematite exceeds 35-40 %, which indicates a minimum crystallization temperature > 550-600 °C. Other major minerals include quartz, basic oligoclase, sillimanite, minor biotite and locally K-feldspar. Despite its anhedral forms, Ti-hematite is older than the silicates. The main evidence is as follows: 1. cordierite penetrates into Ti-hematite in cracks, 2. Ti-hematite contains numerous inclusions, among which paragonite and corundum occur, not found in the rock otherwise. There is also a minor hematite without detectable Ti, which is younger than the Ti-hematite. The lack of alteration of biotite and feldspar excludes a late import of Fe (+Ti) by a fluid. The Fe/(Fe + Mg) molar ratios of both biotite and cordierite are low (on average 0.45 and 0.29, respectively). The rock may have been formed by recrystallization of a very fine-grained to glassy matrix with Ti-hematite, which had been a product of an impact event; the shock-metamorphism may have also caused the loss of alkalies.
