We propose to take benefit of optical aperture synthesis arrays to resolve local magnetic structures and patchy stellar surfaces. This requires to be able to polarimetrically resolve magnetic lines and thus to add a spectro-polarimetric device at the combined focus of an interferometric array. Within this instrumental context, it becomes possible to map magnetic fields thanks to fringe visibility and phase measurements in circularly polarized light and to map abundance inhomogeneities thanks to "classical" interferometric measurements (i.e. without the polarimeter). This appears to be of great interest to better understand the key role of magnetism in atmosphere structuration, in ion migration across the stellar surface, in chemical stratification... In this talk we show how the interference fringe phase is the suitable observable for polarimetric measurements and for mapping patchy surfaces. We illustrate that on various typical cases of magnetic topology and abundance distribution of Chemically Peculiar (CP) stars. Finally we give some instrumental perspectives within the context of the optical interferometric arrays such as the VLTI.