Acta Musei Moraviae

Mineralogically remarkable lamproite occurrence in the Třebíč Pluton at Střítež near Třebíč

A new occurrence of a mafic dyke was recognised along the eastern termination of the Moldanubian zone at Střítež near Třebíč (western Moravia, Czech Republic), where crosscuts melanocratic amphibole-biotite granosyenite (durbachite) of the Třebíč Pluton. The studied rock is significant for both its exotic mineralogy and geochemistry, which allows us to classify it as a lamproite with a distinct affinity to a new high-silica lamproite variety (variety Šebkovice). It contains up to 20 vol. % of phenocrysts of Al-poor flogopite together with minor Al- , Na-poor diopside and an Al-rich phyllosilicate of the biotite-type with unusual atomic ratios of its major cations K : (Fe, Mg) : Al : Si ~ 1 : 2.5 : 2 : 2.5. The dyke matrix consists of anomalously Ti-rich (up to 5.13 wt. % TiO₂) potassic equivalents of sodic-calcic to sodic amphiboles (up to 25 vol. %), Fe-bearing microclines (~50 vol. %), minor fluorapatite (± cheralite?) and (K)-Ba-Ti accessory minerals replacing interstitial baotite in addition. Among (K)-Ba-Ti accessories, rare henrymeyerite, priderite (first occurrence in a Variscan Europe), benitoite and presumed bazirite were identified. The dyke has relatively high SiO₂ (58.5 wt. %) and MgO (6.0 wt. %) contents, along with very high contents of K₂O (9.7 wt. %), Ba (5 296 ppm), Zr (854 ppm) and extraordinary high LILE/HFSE ratios (Ba/Nb = 155; Rb/Nb = 8.6; Th/Nb = 2.2) and only low concentrations of total Fe (as Fe₂O₃) and CaO (both < 6 wt. %). Despite the fact that the rock contains priderite, a typomorphic mineral for several diamondiferous lamproites of an anorogenic-type confined to the stable continental settings, the Střítež lamproite belongs to the Variscan orogenic lamprophyre-cocite-lamproite association recently recognised in the eastern part of the Bohemian Massif.