Abstract

Postorogenic magmatic suites are common to many orogens, in many cases apparently just postdating the cessation of deformation. They differ from preceding orogenic suites in that they have higher temperatures, more primitive isotopic signatures, and bimodal natures, and thus are compositionally similar to suites found in extensional regimes. We propose that thinning of the lithospheric mantle, which may be an automatic response to orogenic lithospheric thickening, is responsible for these magmatic suites. Mantle lithospheric thinning moves the asthenospheric-lithospheric thermal boundary higher in the lithospheric column, thereby raising the overall thermal budget of the orogen and the likelihood of basaltic magmatism. This removal of much of the dense, lithospheric mantle root of the orogen also invokes uplift capable of producing horizontal orogenic buoyancy forces that will oppose and potentially terminate deformation. Extreme fractionation of the magmas is promoted by high temperatures and a noncompressional or tensional lithospheric stress regime to produce accompanying felsic (A-type) magmas. This model explains why the cessation of deformation is coincident with high-temperature bimodal magmatism and inferred uplift, as well as the long-noted similarity and confusion between "postorogenic" and extensional magmatic suites. This arises because both reflect thinning of the lithospheric mantle, not because both reflect extensional tectonics. We suggest that these magmatic episodes may be important in transferring lithospheric mantle material and its compositional signatures into the crust.

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