Magma flow in shallow-level laccoliths and their feeder dykes (Elba island and Orciatico, Tuscany) revealed by AMS and structural data

The study of subvolcanic and shallow-level intrusions has the potential to contribute to the comprehension of the evolution of the continental crust, the increase of the petroleum prospectivity in sedimentary basins, the evaluation of the evolution and formation of ore deposits and of geothermal systems. Based on these premises, the aim of this study is to give a contribution to the understanding of feeding and growth mechanisms of shallow-level intrusions (less than 3-4 km deep) through the study of the emplacement dynamics of some well exposed intrusive bodies within the Miocene-Quaternary Tuscan Magmatic Province (TMP): the felsic San Martino and Portoferraio laccoliths (central and western Elba Island) and the lamproitic laccolith/sill of Orciatico in mainland Tuscany. The fabric patterns of late Miocene San Martino multilayer felsic laccolith and its two large subvertical feeder dykes (Elba Island, Tuscan Archipelago Italy) has been investigated to reveal details about the feeding and growth mechanisms. The fabric data are derived from field measurements of the attitudes of K-feldspar megacrysts (50 sites / 2500 measurements) and from anisotropy of magnetic susceptibility (AMS) determinations (150 sites/1500 cores). Magnetic mineralogy investigations outlined the paramagnetic nature of the AMS signal, carried by chloritized biotite with negligible contribution of ferromagnetic minerals. A good correlation between the AMS and K-feldspar is generally observed, suggesting that almost everywhere hydrothermal alteration did not affect significantly the orientation of the magnetic anisotropy. The AMS data of the main body has evidenced the presence of the of a central dike that fed the main body with a lateral spreading of the magma as a single propagating and inflating pulse where the particles arranged perpendicularly respect to the magma displacement direction. Felsic porphyritic multilayer laccoliths of the Elba Island Miocene igneous complex (Portoferraio and San Martino laccolith) offers also a wide range of examples of contact features, many of which constraint magma flow direction. In particular, a set of structure related to the deformation of external morphology of intrusive surfaces with formation of folds (waves, lobes and ropes) and solid-state stretching lineation have been defined. An AMS investigation of samples close to the contact with such features has pointed out a strong correlation between the magnetic fabric and these features showing that both are flow-related and can be considered good flow indicators but on a very local scale. The small Early Pliocene intrusion of Orciatico (Pisa, Italy), a lamproitic igneous body that was emplaced at very shallow depths (ca. 50 m) has been investigated to reveal details about the emplacement mechanism and the shift between vertical and sub-horizontal flow. The internal fabric of the laccolith has been studied by means of the anisotropy of magnetic susceptibility (AMS) technique (43 sites / 430 cores). The comparison of AMS results with the measured vesicle attitude (14 stations) strengthens his validity. The magnetic mineralogy, investigated with heating/cooling experiments, is dominated by paramagnetic phases (iron-rich phlogopite) with very minor ferromagnetic phases (ti-magnetite and ti-maghemite). Fabric data suggest that the intrusion was feeded laterally by the dike bordering the main body and that the growth is probably related to a simple inflation model after the propagation of a sill.