What are rare earths and why do they cluster near alkaline magmatism?
"Rare earth" metals consist of Scandium, Yttrium, and 15 other metals of the so-called "Lanthanide" series toward the bottom of the Periodic Table of elements. Basically, these are the chemicals that we didn't study in chemistry class at school.

Like other metals, they have two electrons in the outer shell, but unlike "metallic" chemicals such as sodium or magnesium, they have a special "metal" shell for electrons. They are located at the "left" (relative to the carbon family) or akaline end of the Periodic Table, which is why they are found near alkaline magnatism, in alkaline "compounds."

This is in contrast to the more common chemicals, which are typically found as "oxides," or "sulfides," or possibly in combination with fluorine or chlorine. These common chemical compounds are on the right, or acid side of the periodic table.