 Rocks from Incapillo are rich in sodium and have high ratios of lanthanum and samarium to ytterbium and high ratios of barium to lanthanum , as well as high lead @-@ 206 to lead @-@ 204 and strontium @-@ 87 / strontium @-@ 86 ratios . These rare earth element patterns are similar to the Late Miocene Maricunga Belt rocks and contrast to early Miocene rocks . The changes occurred at the same time as the arc migrated eastward , terminating activity in the Maricunga Belt . The element ratios are pronouncedly arc @-@ like with some adakitic signatures . The rocks contain considerably more sodium and alumina than almost all Central Andes siliceous volcanic rocks . 
