  A radiative transfer model was used to reproduce several millions of OMEGA
(Observatoire pour la Min\'eralogie, l'Eau, les Glaces et l'Activit\'e) spectra
representative of igneous terrains of Mars. This task provided the modal
composition and grain sizes at a planetary scale. The lithology can be
summarized in five mineral maps at km-scale. We found that the low albedo
equatorial regions of the Martian surface (from 60{\deg}S to 30{\deg}N) are
globally dominated by plagioclase with average abundance ~50 vol% and pyroxenes
with total averaged abundance close to 40 vol%. An evolution of the
LCP/(LCP+HCP) ratio is observed with time at the global scale, suggesting an
evolution of the degree of partial melting throughout the Martian eras. Olivine
and Martian dust are minor components of the modelled terrains. The olivine
distribution is quite different from the other minerals because it is found on
localized areas with abundance reaching 20 vol%. A statistical approach, to
classify the pixels of the abundances maps, using k-means clustering,
highlighted seven distinct mineral assemblages on the surface. This
classification illustrates that diverse mineralogical units are found in the
Noachian and Hesperian terrains, which suggests the presence of various and
complex magmatic processes at a global scale during the two oldest eras. The
chemical composition was derived from the modal composition maps. The
OMEGA-derived chemical composition is quite consistent with several distinctive
geochemical characteristics previously considered as fingerprints of the
Martian surface. A major discrepancy is in regards to the Fe content that is
significantly smaller than soil and rock analyses from GRS and in situ
measurements. The discrepancy could be partly explained by the assumptions used
for the spectral modelling or could also indicate surface alteration rinds.
