A Reactivated Back-Arc Source for CAMP Magma

John H. Puffer, Department of Geological Sciences, Rutgers University, Newark, NJ

Evidence is presented indicating that CAMP magma is the product of decompression induced eutectic melting of a back-arc source that had been dormant since Paleozoic magmatism associated with the assembly of Pangea.  The composition of CAMP basalt is unlike OIB, plume, hot-spot, N-MORB, or E-MORB magma types but resembles typical arc-related basalt and back-arc basin basalt in particular.  Similarities to arc and back-arc basalts include a distinct negative Nb anomaly and overlapping lines plotted on silicate earth normalized spider diagrams.  The uniform composition of CAMP basalt indicates that fractionation and contamination played only minor roles and that CAMP magma was quickly generated and extruded during the break-up of Pangea.  The composition of CAMP basalt also closely resembles Paleozoic volcanics that stratigraphicafly underlie Eastern North American CAMP flows including the Ordovician Ammonoosuc and Partridge Formations and the Silurian Newbury and Lieghton Formations that were extruded onto arcs and back-arc basins during the assembly of Pangea.  The enriched mantle involved in the Paleozoic magmatism remained trapped under the Laurentian - Gondwana suture until early Jurassic extentional tectonism forced renewed melting.  A new model is proposed that involves upward migration of undepleted asthenosphere displaced by the sinking of a detached oceanic lithospheric slab.  The undepleted asthenosphere was enriched in incompatible elements except Nb as it moved into a back﷓arc tectonic setting and fluxed with water before it underwent decompression melting.