Petrographic and Sr–Nd–Pb–Li isotope characteristics of a complex lamproite intrusion from the Saxo-Thuringian Zone: A unique example of peralkaline mantle-derived melt differentiation

Abstract

Variscan orogenic lamproites in the Bohemian Massif predominantly occur as 1 to 2 m wide and petrographically uniform dykes along the eastern borders of the Moldanubian and Saxo-Thuringian zones. Variscan orogenic lamproites were derived by preferential melting of subduction-related olivine-free metasomatic vein assemblages stabilised in the lithospheric mantle. These lamproitic melts may subsequently undergo extensive differentiation. In this study, we present the first combined petrographic and Sr–Nd–Pb–Li isotope characteristics of a complex lamproite exposed at ca 100 m long profile near Horní Rokytnice (Czech Republic) in the Saxo-Thuringian Zone. This lamproite is characterised by the primary mineral assemblage of K-amphibole + K-feldspar ± aegirine and quartz that petrographically varies from relatively primitive (fine-grained, mafic) to more differentiated (medium- to coarse-grained, felsic) pegmatitic lamproite domains. These domains may represent the product of crystallisation of immiscible liquids that had separated from the mafic melt. The primitive lamproite zone is characterised by the typomorphic minerals – baotite, benitoite, and henrymeyerite. The more differentiated pegmatitic domains are free of aegirine and show replacement of primary red-luminescent (Fe3+-rich) K-feldspar by blue-luminescent (Fe-poor) K-feldspar. Residual fluids rich in Ca, Ti, and HFSE in combination with the decreasing peralkalinity of the lamproite system resulted in the local formation of secondary zircon, titanite and quartz at the expense of the primary Ti–Ba–Zr–K lamproitic mineral assemblages. Lamproites from the Moldanubian and Saxo-Thuringian zones fall on separate mixing trends in the 87Sr/86Sr(t) - εNd(t) diagram, which indicates that the mantle beneath these two zones had been metasomatised by different crustal material. The scatter in the peralkalinity index vs. δ7Li diagram indicates that the Li isotope composition is not controlled by mixing of two end members metasome and ambient depleted mantle alone, but may also be affected by late-stage magmatic and hydrothermal processes. The compositionally zoned Horní Rokytnice dyke is special as the petrographically different types show a variation of about 4 δ-units in δ7Li due to dyke-internal processes, such as fractionation, which increases δ7Li in late-stage lamproitic melts, and post-emplacement interaction with fluids that reduced δ7Li in samples that have lost Li. Post-emplacement alteration also led to the disturbance in the Pb isotope systematics of the differentiated orogenic lamproite as indicated by variable over-correction of in situ radiogenic Pb ingrowth.