GIS DATABASE FOR TESTING OF THE IDEAS: (1) THAT ALKALINE IGNEOUS ROCKS AND CARBONATITES (ARCS) ERUPT IN RIFTS, (2) THAT THEIR DEFORMATION TO FORM DARCS IS CONCENTRATED IN SUTURE ZONES AND (3) THAT ARCS IN RIFTS MAY RESULT FROM DECOMPRESSION MELTING OF DARCS ON SUTURES IN THE UNDERLYING MANTLE LITHOSPHERE

Rifts in continents are characterized by sodium and carbonate rich igneous rocks. We are beginning work on a global database, elaborating on a published catalog of these alkaline rocks and carbonatites (ARCS) using a map of the ca. 700 intra-continental rifts of the world to estimate the generality of the association of ARCS with rifts throughout geologic time. Deformed alkaline rocks and carbonatites (DARCS) form a subset compositionally identical to ARCS but with gneissic structure. Gneissic deformation is associated with continental collision and sites of continental collision form sutures within continents. ARCS formed in continental rifts are preserved at continental margins when oceans open. When those oceans close the ARCS become deformed, developing gneissic structure and becoming DARCS. We are preparing a new map of the global distribution of suture zones and plan to test the idea that DARCS are concentrated in suture zones. ARCS associated with rifts have been recognized to have been erupted in discrete episodes over hundreds of millions of years. We can test the idea that this is because those rifts formed over older sutures within which DARCS had been carried by subduction to depths within the lithospheric mantle. Rifting episodes over DARCS at mantle depths generate new ARCS by decompression melting.