Mineral inclusions in pyrope from garnet peridotites, Kolín area, central Czech Republic

Abstract

Mineral inclusions were studied with emphasis on the relatively abundant acicular, preferentially oriented rutile in pyrope from the Kolin area. In contrast, rutile inclusions are rare or absent at two other, currently exploited pyrope deposits in northern Bohemia (Podsedice and Vestřev). The aim was also to obtain the first analytical information on variety of other mineral inclusions, some of which were previously indicated in the literature based on qualitative gemmological observation. Host garnet composition shows limited variation with Prp 67.9–71.6, Alm 13.3–18.9, Grs 2.0–7.5 and Uvr 2.6–9.6 mol. %. Electron microprobe study resulted in identification of the following mineral inclusions in pyrope: rutile, magnesian ilmenite, olivine, magnesite, enstatite, pargasite, phlogopite, barian phlogopite, spinel, and a probable crichtonite-series? mineral. Apatite, Ni-sulphide and chalcopyrite were identified based on qualitative spectra. Published Sm-Nd mineral ages point to the presence of two age groups of the Variscan garnet peridotites in the Moldanubian Zone and adjacent units in the Bohemian Massif, i.e. the “neo-Variscan” (~ 339 Ma), and the “palaeo-Variscan” (370–377 Ma). In the Kolin area, the Becvary body was dated at 377 ± 20 Ma. This raises a question, whether the “palaeo-Variscan” garnet peridotites of the Kolin area carry some mineralogical record of protracted annealing or repeated heating, which may have occurred between the two Variscan events. Information from garnet inclusions does not indicate for the Kolin peridotites a history of recycling of older peridotites with pre-Variscan crustal residence. Average TiO2 contents in pyrope from the Kolin area (0.37 wt. %) compare with 0.46 wt. % TiO2 in Vestřev pyropes and 0.52 wt. % TiO2 in Podsedice pyropes (northern Bohemia). This suggests that the relatively low Ti content in Kolin pyrope may be in part due to Ti unmixing in the form of oriented acicular rutile. The data from the present study point to the potential of mineralogical comparative studies on peridotites from individual HP units of the Bohemian Massif.