Tachylyte in Cenozoic basaltic lavas from the Czech Republic and Iceland: contrasting compositional trends

• Published in: Mineralogy and Petrology
• Main Authors: Ulrych, J. (Jaromír), Krmíček, L. (Lukáš), Teschner, C., Řanda, Z. (Zdeněk), Skála, R. (Roman), Jonášová, Š. (Šárka), Fediuk, F., Adamovič, J. (Jiří), Pokorný, R.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: basaltic glass; chemical composition; major genetic types; mineral composition; rift-related volcanites; Sr-Nd isotopes; Iceland
• Online Access: https://doi.org/10.1007/s00710-016-0483-x; http://hdl.handle.net/11104/0274663

Tachylytes in alkaline rift-related volcanic rocks were recognized in three geological positions: (i) irregular veinlets and fillings of cavities in host alkaline lava flows of the Kozákov volcano, Czech Republic, (ii) (sub)angular xenoliths in alkaline lava of the feeding channel of the Bukovec volcano, Czech Republic, and (iii) paleosurface of a tholeiitic lava flow from Hafrafell volcano, Iceland. The tachylyte from Kozákov displays phonotephrite to tephriphonolite whole-rock composition while that from Bukovec exhibits trachyandesite to tephriphonolite geochemistry. Host rocks at the two sites are of basanite composition. Both glass and host rock from Hafrafell are of tholeiitic basalt composition. The tachylyte from Kozákov, compared with the host rock, revealed a substantial enrichment in major elements such as SiO.sub.2./sub., Al.sub.2./sub.O.sub.3./sub., Na.sub.2./sub.O, K.sub.2./sub.O and H.sub.2./sub.O, associated with a depletion in MgO and Fe.sub.tot./sub. The contents of incompatible elements such as Rb, Sr, Ba, Nb, Zr, REE, Th and U in the tachylyte are higher than those in the host rocks. The tachylyte from Bukovec displays partly similar trends in major element contents but contrasting trends in the incompatible element contents. The similarity in chemical composition of the Hafrafell tachylyte paleosurface layer and parental tholeiitic basalt is characteristic for lava extrusions. The host/parent rocks and tachylytes have similar initial Sr–Nd characteristics testifying for their co-magmatic sources. The initial .epsilon.sub.Nd./sub. values of host/parent rocks and tachylytes from the Bohemian Massif (3.4 to 3.9) and those from Iceland (6.3) are interpreted as primary magma values. Only the tachylyte from Bukovec shows a different .epsilon.Nd value of -2.1, corresponding to a xenolith of primarily sedimentary/metamorphic origin.