Crustal vs. mantle contributions in the Erzgebirge/Krušné hory Mts. magmatism: Implications for generation of zoned, A-type silicic rocks in the late-Variscan Altenberg-Teplice Caldera, Central Europe

Abstract

Late-Variscan (S-, A-type) granites and rhyolitic ignimbrites/lavas (~325 Ma) of the Altenberg-Teplice Caldera (Erzgebirge/Krušné hory Mts., NW Bohemian Massif) were investigated by using mineral and whole-rock chemistry and Nd–Pb isotopic systematics. Focus was given to the normally zoned Teplice Rhyolite (TR) volcanic successions and, to a lesser extent, the younger comagmatic rapakivi granite porphyry to assess the involvement of magma mixing and to identify differentiation processes in the petrogenesis of A-type rocks. Mineralogical, geochemical, and isotope data disprove A-type magma evolution by closed-system fractional crystallization and favor crystal-liquid separation under complex open-system conditions. Although no compelling mineralogical or chemical evidence for basic-acid magma mixing was found in the caldera, all rocks have constant isotopic fingerprints (εNd(i) = −1.9 to −3.7, 207Pb/204Pb(i) = 15.53–15.65) and two-stage Nd model ages (TDM2 = 1.2–1.3 Ga) that represent intermediate ranges between crustal and mantle components of the Cadomian and Lower Paleozoic basement. Thus, the caldera rocks were derived from a heterogeneous source region without basic magma input. The A-type magmas were generated chiefly by biotite dehydration melting of metasediments and non-restitic granodioritic-tonalitic rocks, likely prompted by heat advection from basement exhumation. Contrasting degrees of fertility, together with the distribution of heterogeneous protoliths relative to the heat source, played a critical role in the production of nearly coeval S- and A-type magmas in the Erzgebirge. The shift from S- to A-type magmatism was then controlled by a change in protolith compositions during the late-orogenic regime. Origin of the rapakivi granite porphyry (An27–43, similar to the TR range) can be explained by self-mixing in the lowest zone of the chamber owing to ponding/recharge of hotter, isotopically similar granitic magma. This study reinforces the idea that generation of A-type rocks and zoned silicic systems does not necessarily require basic magma input despite heterogeneous isotopic fingerprints and presence of rapakivi-textured rocks. The concept of S- and A-type magmas requiring significantly different pressures, H2O contents, and protoliths in the source region does not conform to the Erzgebirge/Krušné hory Mts. magmatism. Similar pressures and water-absent conditions at the melting site suggest that mixed protoliths and their melting temperatures were the decisive variables in magma production.