Many, perhaps most, Mexicans, and South Americans generally, are of indian heritage (Aztec, Mayan, other) who bred with the Spanish. The original Spanish invaders were of European stock, some with Moorish influence no doubt, but not presenting physical characteristics consistent with what we see in Mexicans today.
I think it is safe to say that their "homeland", to the extent it can be traced going back, say, 500 years or more, is Central and South America. Not north of the Rio Grande.
"The original Spanish invaders were of European stock, some with Moorish influence no doubt, but not presenting physical characteristics consistent with what we see in Mexicans today."
Here is a link to a picture of a native "New Mexican." They think of themselves as Spaniards, since they were not Mexicans at any time. They were citizens of Spain - then they were US citizens. I would also point out the very "Moorish" appearance of the gentleman (who happens to be my husband.) The girl is my daughter. Please don't make broad assumptions about the appearance of those who lived in the Western United States prior to 1850.
http://myspace-485.vo.llnwd.net/00586/58/47/586307485_l.jpg
I've heard that the "historically correct" location of Aztlan was somewhere near present-day Puerto Vallarta. The Chicanos just conveniently moved in north to AZ, NM, and CA in the '60s.