How can I use “getarchives_where” to get monthly archives for static pages?

wp_get_archives already have post_type="post" by default and to replace it try this:

add_filter('getarchives_where','my_archives_filter');
function my_archives_filter($where_clause) {
    str_replace( "post_type="post"" , "post_type="page"" , $where );
}

in sidebar:

add_filter('getarchives_where','my_archives_filter');               
wp_get_archives('type=yearly');
remove_filter ('getarchives_where','my_archives_filter');