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Abstract

We have investigated the metallicity distributions of mono-age stellar populations across the disc for 6 ≲ R ≲ 12 kpc and |Z| ≲ 2 kpc using samples selected from the main-sequence turn-off and subgiant (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 have found 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. The values of [α/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 as a consequence of significant radial migration. The radial distribution profiles of [α/Fe] show the opposite patterns of variations with age to those of [Fe/H]. We have also explored the impact of stellar mixing by epicycle motions (blurring) on the [Fe/H] and [α/Fe] distributions, and found that blurring changes mainly 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.