INVESTIGATING A RECENTLY DISCOVERED OLIVINE-GABBRO FROM THE IVREA VERBANO ZONE: NEW INSIGHTS INTO MAGMATIC DIFFERENTIATION IN THE LOWER CONTINENTAL CRUST

­­­­­­­Due to the inaccessibility of the lower continental crust, understanding magmatic processes that happen at that depth is challenging. The Ivrea Verbano Zone in northern Italy is an exceptionally preserved cross section of the lower continental crust. The Permian magmatic section of this continental crust sequence is dominated by the Mafic Complex, broadly subdivided into amphibole-gabbro, gabbro-norite, and diorite respectively from the bottom to the roof of this mafic pluton. Recently, we discovered an olivine-gabbro within the gabbro-norite enriched in garnet that displays exceptionally preserved magmatic texture capturing a wide crystallization sequence ranging from olivine, spinel and pyroxene to plagioclase, amphibole, biotite, apatite, and oxides. Along with 30 representative rocks collected from bottom to the top of the Mafic Complex, we conducted a combination of analytical inspections capturing: 1) major and trace element bulk rock compositions using X-ray fluorescence and LA-ICP-MS, and 2) microstructural analysis using optical microscope and scanning electron microscope, and 3) detailed mineral composition analysis of the olivine-gabbro using the electron probe micro-analyzer. The geochemical analysis shows that the olivine-gabbro appears to be high-Mg and high-Fe similar to amphibole-gabbros and low-Al, and low-Ca similar to basal gabbro-norites of the Mafic Complex, The bulk rock composition is consistent with the large modal proportions of pyroxene, olivine, amphibole, and spinel (80 vol.% ca.) with respect to plagioclase and garnet. From a microstructural standpoint, the rock texture unravels a sequence of crystallization that starts with a relatively primitive olivine (Mg# = 86%) and hercynitic spinel, followed by augite, amphibole, garnet, plagioclase, and accessory phases such as apatite, biotite, and ilmenite. This rock appears to be a relatively primitive link between amphibole-gabbro and basal gabbro-norite of the Mafic Complex, but emplaced at shallower pressure within the mafic pluton.