  The distribution of stellar metallicities within and across galaxies is an
excellent relic of the chemical evolution across cosmic time. We present a
detailed analysis of spatially resolved stellar populations based on $>2.6$
million spatial bins from 7439 nearby galaxies in the SDSS-IV MaNGA survey. To
account for accurate inclination corrections, we derive an equation for
morphology dependent determination of galaxy inclinations. Our study goes
beyond the well-known global mass-metallicity relation and radial metallicity
gradients by providing a statistically sound exploration of local relations
between stellar metallicity $[Z/H]$, stellar surface mass density
$\Sigma_\star$ and galactocentric distance in the global mass-morphology plane.
We find a significant resolved mass density-metallicity relation $\rm
r\Sigma_\star ZR$ for galaxies of all types and masses above
$10^{9.8}\,\mathrm{M_\odot}$. Different radial distances make an important
contribution to the spread of the relation. Particularly, in low and
intermediate mass galaxies, we find that at fixed $\Sigma_\star$ metallicity
increases with radius independently of morphology. For high masses, this radial
dependence is only observed in high $\Sigma_\star$ regions of spiral galaxies.
This result calls for a driver of metallicity, in addition to $\Sigma_\star$
that promotes chemical enrichment in the outer parts of galaxies more strongly
than in the inner parts. We discuss gas accretion, outflows, recycling and
radial migration as possible scenarios.
