We present OASIS observations obtained at the Canada-France-Hawaii Telescope for the SB ( rs ) c galaxy NGC 4900 .
About 800 spectra in the wavelength range 4700-5500 Å and 6270-7000 Å have been collected with a spatial resolution of \sim 50 pc .
This galaxy is part of a sample to study the stellar populations and their history in the central region of galaxies .
In this paper , we present our iterative technique developed to describe consistently the different stellar components seen through emission and absorption lines .
In NGC 4900 we find many young bursts of star formation distributed along the galaxy large scale bar on each side of the nucleus .
They represent nearly 40 per cent of the actual stellar mass in the field of view .
The age for these bursts ranges from 5.5 to 8 Myr with a metallicity near and above 2 Z _ { \odot } .
The extinction map gives E ( B - V ) values from 0.19 \pm 0.01 near the youngest bursts to 0.62 \pm 0.06 in a dusty internal bar perpendicular to the large scale bar .
The Mg _ { 2 } and Fe i absorption lines indicate the superposition of a background stellar population with an age between 100 Myr to 3 Gyr and a sub-solar metallicity on average .
We propose that all these episodes of star formation are the consequence of a secular evolution .
In this scenario , the galactic large scale bar plays an important role with respect to the recent bursts and the dusty nuclear bar observed .
The iterative technique allows us to improve the determination of the stellar population parameters , mainly an older age is obtained for the old component and more reliable stellar population masses are found .
A composite/transition type activity in the galaxy nucleus is also revealed with this technique .