EXAMPLES OF DEFORMED ALKALINE ROCKS AND CARBONATITES (DARCS) IN SUTURE ZONES AND AS SOURCES OF ALKALINE ROCKS AND CARBONATITES (ARCS) IN OVERLYING RIFTS FROM THE NORTHWESTERN CORDILLERAN CONTINENTAL MARGIN AND ELSEWHERE

In Africa DARCs lie in suture zones and ARCs lie in rifts above them (Geology 31, 391-394). Our explanation is:1. Alkaline igneous rocks erupt preferentially into intra-continental rifts. 2. Ocean-basin opening and closing carries the alkaline rocks of rifts to collision zones turning them into DARCS on sutures in crust and underlying mantle lithosphere. (3) Decompression melting in the mantle under new rifts above sutures melts DARCS to form ARCS. We report evidence of how these processes have contributed to the evolution of the NW Cordilleran margin where DARCS mark Neoproterozoic and early- to mid-Paleozoic rifted margins sutured in the Mesozoic, and where ARCS were later erupted into Cretaceous and Eocene rifts. Neoproterozoic rifting is recorded by extrusive and intrusive carbonatite complexes in the Monashee core complex, southern British Columbia, and mark the west-margin of rifted continental basement of unknown affinity. Gneissosity developed in response to Jurassic basin closure and suturing with Paleozoic to early Mesozoic arc crust to the west. Spatially associated Eocene ARCs (e.g. the Coreyell Syenite) record subsequent rifting within the Cordilleran hinterland. Ordovician-Silurian carbonatite complexes, lamproites and kimberlites within the western Rocky Mtns. are coeval with a sub-Devonian unconformity of regional extent, and indicate early Paleozoic rifting. Correlative DARCs known along the length of the Canadian Cordillera are weakly to moderately foliated, locally mylonitized, folded, and carried on and cut by thrust faults. These DARCs may define a cryptic suture separating a west-facing autochthonous continental margin from a more westerly, east-facing exotic continental margin that was sutured to North America during dextral transpression in the Late Cretaceous. Similar processes have operated elsewhere: In the Kola Peninsula where Mid-Paleozoic ARCs are concentrated among DARCs on Greenstone Belt sutures; in the Eastern Sayans where ARCs of rifts in the Siberian craton pass into DARCs of the Altaids; in Vitim(Baikalia) where Neogene ARC volcanics overlie DARCs of Mid-Paleozoic age, in Norway’s Caledonides where the Soroy DARC lies in a Silurian suture zone; and in the Aravallis of Rajasthan where DARCs occupy a Proterozoic suture.