Tectonic setting of the Taltson magmatic zone at 1.9-2.0 Ga: a granitoid-based perspective

The Paleoproterozoic Taltson magmatic zone is one of the key tectonic features of western Laurentia. The existing tectonic model for the belt envisions its formation by subduction of oceanic crust beneath a continental margin, followed by direct collision between formerly separate crustal blocks. We tested this model by comparing the large geochemical and isotopic database available for Taltson magmatic zone granitoids with similar databases for Phanerozoic granitoid suites from different tectonic environments. The comparison reveals that the early granitoid suite of the Taltson magmatic zone, which had been ascribed to the subduction phase of orogenesis, lacks the mantle signature apparent in granitoids of Phanerozoic continental-margin arc settings. Instead, both early and late suites appear to have an intracrustal origin, similar to Mesozoic and Cenozoic granitoids of the Cordilleran interior of western North America, which formed in the distant hinterland of a convergent plate margin. In light of these findings, we propose an alternative tectonic model, which envisions formation of the Taltson magmatic zone in a plate-interior rather than a plate-margin setting. Modern-day examples of this setting are found in the mountain belts of central Asia, such as the Tian Shan, which are located many hundreds of kilometres away from the plate margin. The critical feature of these belts that make them an appealing analogue for the Taltson magmatic zone is that there is no subduction zone closely associated with their formation. Rather, magmatism occurs in response to thickening of crust in the continental interior.