Triassic magmatism on the transition from Variscan to Alpine cycles: evidence from U–Pb, Hf, and geochemistry of detrital minerals

Abstract

New data on the U–Pb geochronology and Hf-isotopic composition of detrital zircons, along with the geochemistry of garnets, chromian spinel and feldspars from Permian and Triassic volcano-sedimentary sequences in the Southern and Eastern Alps reveal similarities in the nature and tectonomagmatic setting of the Early Permian and the Middle Triassic magmatic events. Detrital zircon U–Pb geochronology displays magmatic activity at 290–280 Ma (Early Permian) and at 245–235 Ma (Middle Triassic). The two-stage Hf-depleted mantle model ages obtained from Permian and Triassic zircons cluster around 1.7–1.6 and 1.3–0.8 Ga. Epsilon Hafnium analysis on dated detrital zircons suggests mixing of mantle and crustal components into the parental magmas of Permian and Triassic magmatism, the latter showing less crustal contribution. The composition of detrital feldspars from the Permian and Triassic volcaniclastic sandstones reveals provenance from intermediate to acidic igneous rocks. Garnet in Middle and Late Triassic beds suggests the incorporation of material comparable to the mafic rocks of the Ivrea-Verbano Zone into the parental melts of the Triassic magmatism. Chromian spinel in syn-magmatic Carnian sandstones indicates an origin associated with MORB-type peridotites and therefore, corroborates a mantle contribution to the Middle Triassic magmatism. The integration of detrital zircon U–Pb ages, Hf isotopes and composition of detrital minerals suggests that continuous lithospheric stretching at the transition from the post-Variscan to the Alpine orogenic cycle, with periods of strike-slip movements, presumably led to an asymmetrical continental rifting process, in which the Middle Triassic magmatic episode represents the earliest stage of the Alpine Tethys oceanic domain.