Hydrothermal dolomitisation of a deep‐water bioherm isolated in a non‐dolomitised intraplatform basin within the Norian Dolomia Principale (Southern Alps, northern Italy)

Abstract The Norian Dolomia Principale in the Southern Alps, northern Italy, is composed of fully dolomitised carbonate platforms, locally interrupted by intraplatform basins that are only partially or not affected by dolomitisation. We studied the intraplatform basinal limestone of the Calcare di Zorzino as well as the dolomitised deep‐water Monte Zenone bioherm, which—with an area of ~8 km 2 —is considered to be one of the largest bioconstructions in the Southern Alps, is isolated within the basinal deposits and is detached from the Dolomia Principale platform. We combine field and petrographic observations with thin section microscopy, X‐ray diffraction, cathodoluminescence and both conventional and clumped stable‐isotope analyses (δ 13 C, δ 18 O, ∆ 47 ) of several adjacent carbonate lithologies ranging from non‐ to fully dolomitised. Reordering models indicate that early diagenetic ∆ 47 signatures are preserved in bioherm dolomite, whereas Calcare di Zorzino micrite may have experienced both recrystallisation and thermal resetting during the thermal history of the Norian carbonate sequence. Resulting clumped‐isotope derived temperatures ( T (∆ 47 )) and calculated paleofluid compositions (δ 18 O Fluid ) of well‐preserved micrite, matrix and cement dolomite document that—in contrast to the Dolomia Principale platform—growth of the Monte Zenone bioherm on a tilted and drowned platform block was controlled by the syn‐tectonic and fault‐controlled ascent of fluids associated to the Late Triassic–Early Jurassic rifting phase. These deep and warm fluids fostered bioherm growth, with early diagenesis providing the hard substrate for colonisation of microbial communities as well as serpulids and molluscs, and the eventual dolomitisation of the entire bioherm body.