Some geodynamic and compositional constraints on "postorogenic' magmatism

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 are compositionally similar to suites found in extensional regimes. It is proposed that thinning of the lithospheric mantle is responsible for these magmatic suites. Mantle lithospheric thinning moves the asthenospheric-lithospheric thermal boundary higher in the lithospheric column. This removal of much of the dense, lithospheric mantle root of the orogen invokes uplift capable of producing horizontal orogenic buoyancy forces. 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. It is suggested that these magmatic episodes may be important in transferring lithospheric mantle material and its compositional signatures into the crust. -from Authors