Comparative basaltic volcanism of two continental view backarcs: The Quaternary of northern Patagonia and the Silurian of southwestern New Brunswick

VAN WAGONER NA; HALLER MJ; MASSAFERRO GI; DOSTAL J; LEYBOURNE M; DADD K

Montreal

Congreso; GAC-MAC Annual Meeting; 2006

GAC-MAC

Quaternary basaltic volcanism occurs in a continental backarc position to the Andes in northern Patagonia, Argentina, and also occurred in the northern Appalachians during the Silurian. In both cases there is evidence that volcanism was relatively continuous for a long period of time and associated with a transpressional tectonic setting. This paper focuses on the basaltic volcanism of these areas and the similarities and differences in styles of volcanism and chemical characteristics. These studies are important to understanding the nature and controls on basaltic volcanism in back arc settings. Quaternary volcanic rocks of northern Patagonia form cinder cones, flood type basalt flows and tube-fed pahoehoe flows. The basalts are relatively homogeneous trachybasalts to basanites. Mg# ranges from 58-65. The basalts are LREE enriched (La/Sm = 4-6), and enriched in the LILE relative to NMORB, but with low Ba/La values of 13-17. Chemical variations are attributed to fractionation at a volcanic centre. There is no evidence for crustal contamination or prior metasomatism of the mantle related to subduction. The basalts were interpreted to form by 3-7% partial melting of an enriched mantle source. Basaltic volcanism of the Silurian Eastport Formation of the Northern Appalachians was interpreted to have occurred in a continental backarc. The basalts form similar morphologic features to those in northern Patagonia, with the addition of abundant phreatomagmatic volcanism forming maar deposits. In addition, the basalts are interbedded with felsic rocks, although the abundance of felsic volcanism decreases with time. In contrast with northern Patagonia, the basalts of the Eastport are subalkalic basalt to basaltic andesite. The Mg# ranges from 30 to70, and there is a trend toward more primitive compositions upward in the section. The basalts are LREE-enriched and are enriched in incompatible elements with respect to primitive mantle, with distinct Nb, Hf and Sr anomalies. The chemical characteristics of these rocks are interpreted to be the result of mantle melts modified by crustal contamination and mantle metasomatism from a previous subduction event. Most of the chemical variation can be explained by replenishment and fractionation of small magma chambers, The differences in the physical and chemical characteristics of these basalts from similar tectonic settings are considered in light of physical features such as crustal thickness, proximity to the subduction zone, slab geometry and position within the back arc, and processes such as rates of volcanism, storage and emption rates, and mechanisms of volcanism.