K-Ar dating of metamorphic rocks from Strandja massif, SE Bulgaria

摘要

The stratigraphic ages of the geological bodies that built up the Strandja diabase-phyllitoid complex cover a wide range - from Early Devonian to Middle Jurassic. The mineral paragenesies of the rocks are typical of the greenschist facies and include albite, chlorite, sericite (illite-muscovite), epidote, calcite. The temperature interval of 350-400°C is estimated by the chlorite geothermometer. The b0 values and the illite crystallinity (IC) values of illite/muscovite indicate high-temperature anchizonal-epizonal metamorphic conditions. K-Ar isotopic dating of illite/muscovite-rich < 2 im grain size fractions mainly from representative samples of phyllites and metadiabases leads to the following conlusions. Part of the K-Ar dates of the fractions < 2 urn, < 60 urn, > 60 urn indicate Jurassic (160-170 Ma) low-grade regional greenschist metamorphism. The K-Ar dates 110-150 Ma of the other part of the fractions < 2 u.m reflect the rejuvenation effect of the Upper-Cretaceous (80-100 Ma) regional tectonothermal (250-280°C) event related to the subduction of Tethyan oceanic lithosphere. The primary magmatic and metamorphic ages are Early Paleozoic (probably Early Devonian) based on a poorly defined whole rock Rb/Sr isochron from metadiabases. The K-Ar dates 246 - 309 Ma of some metadiabases confirm their Paleozoic age and indicate at least one Paleozoic low-grade or higher-grade metamorphic event. The 207Pb / 206Pb dates of single zircons from metagranites and metamorphic rocks from the central part of the Strandja massif (Turkey) could be regarded as an indicator for probable regional high-grade Hercynian metamorphism overprinted by Jurassic greenschist metamorphism.