Petrografia e geochimica delle ooliti del Giurassico inferiore della Piattaforma di Trento

This study has the aim of petrographically and geochemically characterize the Lower Jurassic coated grains, especially ooids, of the Trento Platform (Southern Alps, Veneto, Trentino Alto Adige and Lombardia). A morphological and petrographical classification of the coated grains is here presented, that stems from that one proposed by Scudeler Baccelle (1976). Besides, this study is also aimed at identifying the original mineralogy of the ooids, and better date the shift between the Mississippian-Jurassic “aragonite sea” and the Jurassic-Cenozoic “calcite sea” of Sandberg (1983). Samples from sections and localities near the western margin of the platform were considered. Analytical techniques as optical microscopy, scanning electronic microscopy and cathodoluminescence microscopy were exploited, together with in situ geochemistry (LA-ICP-MS). Information about morphology, ultrastructure, luminescence and microporosity of the ooids were gained, as well as about the content of some major and trace elements. Along the Jurassic succession microoncoids, micritized ooids, radial ooids and ooids replaced by microsparite were recognized. These four types are made of low-Mg calcite, with a content of MgCO3 between 0.8 and 1.8 % mol. Manganese is always less than 30 ppm, except for radial ooids of the San Vigilio Oolite, in which Mn content can be greater than 100 ppm; these radial ooids are also characterized by a Mn/Fe ratio greater than 0.5. Silicon and aluminum are more concentrated in ooids replaced by microsparite. All the coated grains are enriched in iron, zinc and uranium with respect to the other rock components. Radial ooids belonging to San Vigilio Oolite are petrographically different from those found in the Calcari Grigi Group. Their cortex is made of rhombohedral sintaxial crystals, arranged in fibers, while the ooids belonging to the Calcari Grigi Group are made of elongated crystals. The radial ooids of the San Vigilio Oolite show a greater microporosity than any other kind of ooids and microoncoids of the Calcari Grigi Group. Although it wasn’t possible to chemically determine the original mineralogy of the coated grains, an hypothesis has been made based on petrographic characterization. It is here hypothesized that the shift between “calcite sea” and “aragonite sea” is at the Pliensbachian-Toarcian boundary. This hypothesis needs further data to be confirmed.