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Silicic magma flare-up episodes are characterized by the addition of large volumes of evolved magma (>65 wt% SiO2) to the continental crust in geologically short time intervals (106-107 years). Flare-up events are often associated with (trans-)extensional tectonics and contribute to crustal differentiation and critical metal mineralization. Related volcanic aerosol dispersion in the atmosphere can also trigger global environmental changes. During flare-up episodes, long-lived caldera complexes are thought to be primary eruptive sources at the Earth’s surface. However, a substantial proportion of the overall mobile magma can be trapped in extra-caldera dikes, fissures and monogenetic edifices controlled by the extensional stress regime.In the Southern Alps of Northern Italy, a post-Variscan magmatic flare-up is recorded in a ca. 400 km long array of largely undeformed magmatic bodies of Early Permian age (285-275 Ma; [1]), then located along the northern margin of Gondwana. In the Southern Alps this flare-up produced more than 5*104 km3 of rhyolitic volcanic and cogenetic intrusive rocks. Two major caldera complexes (Sesia Caldera; Ora Caldera) were capable of ejecting volumes >103 km3 of magma during individual catastrophic eruptive events. However, magmatic activity also resulted in numerous scattered volcanic centers with relatively small eruptions (0.1 – 1 km3 each) and punctuated by quiescent intervals.In this study we focus on two Early Permian fault-bounded basins, ca. 40 km apart, in the central Southern Alps: the Orobic Basin (Bergamo) and the Collio Basin (Brescia). The stratigraphic records of both basins preserve proximal and distal volcanic products and both successions terminate with erosional unconformities of Middle- to Late Permian age. New zircon LA-ICP-MS U-Pb ages indicate that the onset of explosive, rhyolitic magmatism was essentially coeval at ~284 Ma. The Collio Basin contains just a few ignimbrite sheets dispersed in an essentially (fluvio)-lacustrine sedimentary fill and recording a pulsated volcanic activity of nearly 5 Myr (youngest ignimbrite ~280 Ma). After an initial phase (1-2 Myr) of a similar pulsed nature, the Orobic Basin became the locus of extrusion of much larger volumes of rhyolitic magma (probably in excess of 100 km3) in less than 1 Myr (283-282 Ma). This was followed by a depositional style similar to the Collio but with a scarcer pyroclastic contribution.The contrasting volcanic record in these two basins, which share size and tectonic environment but not magmatic evolution, provides a striking example of magmatic architecture diversity in the midst of a silicic flare-up event. Further investigation into the timing (CA-ID-TIMS U-Pb geochronology) and compositional evolution (e.g., zircon d18O, eHf) of volcanic products in the Collio and Orobic basins is expected to provide a much better resolved comparison and open a window into the combined tectono-magmatic processes that ultimately regulate the size and frequency of catastrophic, caldera-forming eruptions in silicic flare-up provinces.[1] Schaltegger, U., & Brack, P. (2007). International Journal of Earth Sciences, 96(6), 1131-1151.

Tracing the origin and evolution of magmas on their pathway through the lithosphere is key to understanding the magmatic processes that eventually produce eruptions. For ancient magmatic provinces, isotope-geochemical tracers are powerful tools to probe the source regions and magma-crust interaction during ascent and storage.We present new hafnium isotopic compositions of ID-TIMS dated zircons to trace the evolution of the Middle Triassic magmatic province in the Southern Alps (northern Italy) at high temporal resolution [1]. Systematic changes in hafnium isotopic composition with time reveal a coherent temporal evolution from depleted mantle signatures towards crust-dominated signatures within less than four million years. This trend can be ascribed to progressive influence of a crustal source, incorporated into the reservoir from which these zircons crystallized. Towards the end of the magmatic episode, the εHf compositions abruptly revert within one-million-years back towards more juvenile compositions mainly recorded by the mafic to intermediate intrusive pulses (e.g. Monzoni and Predazzo), the effusive climax of basaltic lavas and the post-intrusive ash beds (e.g. Punta Grohmann) in the Dolomite region. We interpret the variation of Hf-isotopic signatures over time as a protracted contamination signal induced by interaction of the mantle-derived magmas with the lower crust.The dataset obtained in this study is further implemented into a two-component mixing model employing a range of potential crust and mantle endmember Hf isotope signatures and Hf concentrations which is directly translated into crustal melt/total melt (=sum of crustal and mantle-derived melt) ratios over time. Based on these observations we explored the thermal evolution and crustal melting as a function of time, lithology, water content and magma flux for a lower crustal magmatic system by numerical modelling. Dykes and sills of basaltic composition are incrementally emplaced at the mantle-crust boundary, which leads to changes in crustal over mantle melt ratios over time. Initial intrusions of basaltic dykes into the relatively cold lower crust cause only limited crustal melting and assimilation but ensuing magma injections into progressively hotter crust results in more extensive partial melting and assimilation of crustal material. Subsequent intrusions into the magmatic lower-crustal roots cannibalize previous intrusions with progressively less isotopic contrast due to dilution with mantle-derived magmas. This is potentially accompanied by an increase in magma flux, e.g. by delamination of dense lower crustal cumulates into the subcontinental lithospheric mantle.The observed trends in hafnium isotopic composition therefore do not necessarily require tectonic re-organizations or changes in mantle sources. Instead these trends may trace variations in mantle-crust interaction during thermally induced chemical maturation of the lower crustal magmatic roots progressively replacing ancient pelitic to mafic lower crustal lithologies by juvenile cumulates. [1] Storck, J.-C., Wotzlaw, J.-F., Karakas, O., Brack, P., Gerdes, A., Ulmer, P. Hafnium isotopic record of mantle-crust interaction in an evolving continental magmatic system, Earth and Planetary Science Letters, (in press).