Magmatic architecture and basin evolution in the midst of a silicic flare-up: U-Pb zircon geochronology of volcanic deposits from two Early Permian, Collio-type basins of Southern Alps (Italy)

Abstract

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.