  We have investigated the metallicity distributions of mono-age stellar
populations across the disc of $6 \lesssim R \lesssim 12$\,kpc and $|Z|
\lesssim 2$\,kpc using samples selected from the main-sequence turn-off and
sub-giant (MSTO-SG) stars targeted by the LAMOST Galactic Spectroscopic
surveys. Both the mean values and the profiles of the distributions exhibit
significant variations with age and position. We confirm that the oldest
($>11$\,Gyr) stars have nearly flat radial [Fe/H] gradients at all heights
above the disc but show negative vertical [Fe/H] gradients. For stars younger
than 11\,Gyr, the radial [Fe/H] gradients flatten with $|Z|$, while the
vertical [Fe/H] gradients flatten with $R$. Stars of 4--6\,Gyr exhibit steeper
negative radial [Fe/H] gradients than those of either younger or older ages.
Values of [$\alpha$/Fe] of mono-age stellar populations also show significant
radial and vertical gradients, with patterns varying with age. The [Fe/H]
distribution profiles of old ($>8$\,Gyr) stars vary little with $R$, while
those of younger stars exhibit strong radial variations, probably a consequence
of significant radial migration. The [$\alpha$/Fe] radial distribution profiles
show opposite patterns of variations with age compared to those of [Fe/H]. We
have also explored the impacts of stellar mixing by epicycle motions (blurring)
on the [Fe/H] and [$\alpha$/Fe] distributions, and found that blurring mainly
change the widths of the distribution profiles. Our results suggest that the
disc may have experienced a complex assemblage history, in which both the
"inside-out" and "upside-down" formation processes may have played an important
role.
