Hello,
I am not certain of the identity of your plant since I am not generally familiar with plants of Argentina. However, the plant looks like it could be Portulaca oleracea. This is a cosmopolitan weed that is found in many places worldwide including Argentina. If you need an exact identification you may try to ask someone who studies weeds at a local university.

Below is a link to a site with distribution and photos of this weed.  You can compare it to what you found and see if they look to be the same plant.
http://www.cabi.org/isc/datasheet/43609

thank you for using Ask an Expert.

The response is very usefull and the link provided has the needed information. Given the pictures of the link, plus others I found, I am pretty sure it is the plant you mention. Many thanks for the help! PS: Wikipedia states that it has many culinary uses plus significal nutritional benefits. (http://en.wikipedia.org/wiki/Portulaca_oleracea#Uses - 1/21/2015). And that it has a place in tradicional cuisine in many countries I am going to investigate further in order to verify it "usability" and eventually no longer treat it as a weed.

I'm glad the plant seems to be the Portulaca in the description I sent. Yes, I saw that it has potential culinary uses, but I would caution you against eating it unless you are absolutely sure of what it is just to be safe. Best of luck and thanks for using Ask an Expert!