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We use 297 042 main sequence turn-off stars selected from the LSS-GAC todetermine the radial and vertical gradients of stellar metallicity of theGalactic disk in the anti-center direction. We determine ages of those turn-offstars by isochrone fitting and measure the temporal variations of metallicitygradients. Our results show that the gradients, both in the radial and verticaldirections, exhibit significant spatial and temporal variations. The radialgradients yielded by stars of oldest ages (>11 Gyr) are essentially zero at allheights from the disk midplane, while those given by younger stars are alwaysnegative. The vertical gradients deduced from stars of oldest ages (>11Gyr) arenegative and show only very weak variations with the Galactocentric distance inthe disk plane, $R$, while those yielded by younger stars show strongvariations with $R$. After being essentially flat at the earliest epochs ofdisk formation, the radial gradients steepen as age decreases, reaching amaxima (steepest) at age 7-8 Gyr, and then they flatten again. Similar temporaltrends are also found for the vertical gradients. We infer that the assemblageof the Milky Way disk may have experienced at least two distinct phases. Theearlier phase is probably related to a slow, pressure-supported collapse ofgas, when the gas settles down to the disk mainly in the vertical direction. Inthe later phase, there are significant radial flows of gas in the disk, and therate of gas inflow near the solar neighborhood reaches a maximum around alookback time of 7-8 Gyr. The transition of the two phases occurs around alookback time between 8 and 11 Gyr. The two phases may be responsible for theformation of the Milky Way thick and thin disks, respectively. And, as aconsequence, we recommend that stellar age is a natural, physical criterion todistinguish thin and thick disk stars. ... (abridged)

The evolution of stellar metallicity gradients of the Milky Way disk from LSS-GAC main sequence turn-off stars: a two-phase disk formation history?