Unveiling the Geochemical Signature of the Early Permian Orobic Basin (Laghi Gemelli Group; N Italy)

The sedimentary succession of the Laghi Gemelli Group (? Late Carboniferous-Early Permian) outcrops in the northern-central Southern Alpine domain (North Italy, Lombardy) and is composed of volcaniclastic, siliciclastic and subordinate carbonate sediments. The succession was deposited within a fault-bounded basin (Orobic Basin) formed under extensional-transtensional tectonics in the post-Variscan scenario. The stratigraphic succession that build up the Laghi Gemelli Group non-conformably overlies the Variscan Metamorphic Basement and consists of three lithostratigraphic units recording different basin filling phases: 1) alluvial systems dismantling metamorphic uplands (Conglomerato Basale), 2) caldera-centered acidic volcanic systems (Cabianca Volcanite Fm.), and 3) endorheic alluvial systems dismantling metamorphic and volcanic uplands (Pizzo del Diavolo Fm.). A prominent angular unconformity marks the top of the Laghi Gemelli Group, at whose top sits the? Middle-Late Permian Verrucano Lombardo alluvial system.

Excellent outcrop exposures in central Bergamasque Alps (Valgoglio, BG) have progressively fostered a substantial advancement in the understanding of the Lower Permian stratigraphy. However, their geochemical composition and variability – major and trace elements – have largely remained unexplored. Sediment chemistry can contribute to the identification of distinctive geochemical markers that can be employed for basin-scale correlations, a valuable approach in settings characterized by units with pronounced thickness and facies variations.

For the first time, 73 rock samples collected in the central sector of the Orobic Basin from outcrops of the Laghi Gemelli Group and lowermost Verrucano Lombardo have been analyzed at the University of Milan by using the LA-ICP-MS technique. Prior to the analysis, Pressed Powder Pellets (PPPs) were prepared. Although the data analysis didn’t reveal clear significant geochemical trends in both major and trace elements, several local changes are noteworthy. Where preserved below the unconformity with the Verrucano Lombardo, the upper part of the Pizzo del Diavolo Formation, displays an averagely low Na content and substantial increases in K, Li and Cs. No significant geochemical trend or variation has been detected across the unconformity into Verrucano Lombardo sediments. Future investigations may assess whether similar chemofacies occurs in other sectors of the Orobic Basin at a comparable stratigraphic position. Geochemical analysis has also revealed localized anomalies involving a narrow spectrum of elements. Four main patterns were identified: a) high U, Pb, Sb and Cu in altered tuffstones from the top of the Cabianca Volcanite; b) high Zn, Pb and Cd in lacustrine sediments of the lower Pizzo del Diavolo Formation; c) high B in volcaniclastics and mud-heterolithic deposits (Cabianca Volcanite and Pizzo del Diavolo Formation); d) As enrichment accompanied by increased Mo, In and Sb in mudstone facies of the lower Pizzo del Diavolo Formation.

Starting from these new geochemical analyses, future works can peer into the processes that led to such an anomalous concentration in volcano-sedimentary deposits that, except for uranium, have been poorly investigated in the past. Furthermore, broader sampling will reveal how and why some of these anomalies spread into the basin.