Nowe dane o wieku i petrologii kenozoicznych bazaltoidów dolnośląskich

Abstract

New data on age and petrological properties of Lower Silesian Cenozoic basaltoids, SW Poland.Summary. Fifteen samples of basaltoid rocks have been analysed from the Lower Silesia, SW Poland, some of them from localities close to the Sudetic Marginal Fault. K–Ar datings have been made on whole rock samples, using the methodology applied by the Institute of Nuclear Research, Hungarian Academy of Sciences, Debrecen, Hungary. Most of the samples give ages ranging between 21–38 Ma, whereas that derived from a borehole in the Mokrzeszów Graben has been dated to 44 Ma. Another borehole sample (Jeżów Sudecki B–5), close to the Intrasudetic Fault, is of 59 Ma age. On the other hand, the supposedly „Quaternary” basaltoids from Dębowiec area fall into the interval of 29–30 Ma. The southeasternmost occurrences of the Lower Silesian basaltoids at Nowa Cerekiew display two generations of effusive activity: the older lava flows (26 Ma) are cut by plugs dated to 22 Ma. The Oligocene–Lower Miocene (26–33 Ma, 20–24 Ma) rocks represent alkali basalts and basanites. The alkali basalts consist of phenocrysts of olivine (chrysolite), altered to a different degree, and clinopyroxene (salite). In the matrix, clinopyroxene (Ti salite), olivine (hyalosiderite), plagioclases (labradorite–andesine), and opaque minerals (titanomagnetite, ilmenite) occur. The basanites contain phenocrysts of altered olivine (chrysolite) and clinopyroxene (salite). Their matrix consists of clinopyroxene (Ti–salites), olivine (hyalosiderite), nepheline, plagioclases (bytownite andesine), and opaque minerals. On the TAS diagram, samples of the studied basaltoids plot mainly in the fields of basalts and basanites/tephrites. The bimodality of some samples falling into two fields of basalts and basanites/nephelinites on the TiO2/Zr–Nb/y diagram results from their distinct geochemical signature.