Parent page gets reloaded when opening colorbox which renders partial returned from rails

Question

A colorbox is launched with:

$.colorbox({href:'appointments/admin_index'});

Which calls a controller that returns a partial ('admin_new'):

respond_to do |format|
  format.html { render :action => "admin_new" }
end

This works in that the colorbox renders the correct partial, however it also causes the main page to partially reload as well. If I change the format.html line to:

format.html { render :text => "my text" }

The colorbox opens properly and the main page remains as it should. So rendering the action is causing the problem -- causing the main page to partially reload. Any ideas?