Deformation-driven differentiation of granitic magma: the Stepninsk pluton of the Uralides, Russia

The wide compositional spectrum of the Variscan batholiths of the Urals, a continuum from gabbros (or diorites) to leucogranites, was produced by crystal fractionation, but the physical mechanisms involved in formation of the bodies remains obscure. To test whether syn-magmatic deformation was essential to enhance the efficiency of the process, we studied the Main Series of Stepninsk, a pluton emplaced into an active crustal-scale strike-slip shear zone. The Main Series (>95 vol.% of total granitoids) is high-K calc-alkaline, comprises rocks with SiO2 from 51 to 77 wt.%, and stands out because most major and many trace elements yield excellent linear or curvilinear correlations with silica. It includes deformed gabbrodiorites to monzogranites, and undeformed syenogranites to alaskites. Deformed and undeformed rocks are coeval (283±2 Ma). All rocks, irrespective of their silica content, have the same initial Sr and Nd isotope ratios (87Sr/86Sr283 Ma=0.70488±0.000131; ε(Nd)283 Ma=−0.79±0.49), and contain amphibole and biotite with the same compositions. Based on thermodynamic and trace-element fractionation simulations, we propose a model of deformation-driven filter-pressing differentiation consistent with these features. The Main Series is derived from a hydrous high-K granodioritic magma which intruded containing ∼0.3 of early-formed solids. These accumulated locally by flow differentiation to produce the gabbrodiorites. The crystallization continued until the fraction of solids was higher than ∼0.55, after which different magma batches were efficiently squeezed by differential stress coupled with the opening of tensile fractures in the shear zone. This process produced a range of residua and segregates, the composition of which depended on the fraction of early-formed solids, the fraction of solids present when squeezing occurred and, especially, the efficiency of melt segregation. The monzodiorites and quartz-monzodiorites represent efficiently squeezed residua, the granodiorites to monzogranites represent unfractionated or little fractionated magma batches, and the leucogranites represent melt segregates with a few entrained crystals of amphibole and biotite. We proposed that wide-spectrum fractionation of granite magmas mainly occurs when they crystallize under compressive regimes, and is caused by deformation-driven filter-pressing differentiation.