Evidence for the generation of juvenile granitic crust during continental extension, Mineral Mountains Batholith, Utah

Abstract

Field, chemical and isotopic data from the Miocene Mineral Mountains batholith in southwest Utah are consistent with the batholith being derived through differentiation of material recently separated from the lithospheric mantle, with little involvement of pre-Oligocene crust. The batholith ranges in composition and texture from diabase and gabbro to high-silica rhyolite and granite and is distinctly calcalkaline in nature. Field evidence for anatexis of intermediate-composition Oligocene crust and magma mixing suggest that fractional melting and mixing were important processes during the evolution of the batholith. Major oxide and rare earth element data for the batholith are consistent with chemical evolution of the magma system being controlled by fractionation of hornblende, plagioclase and sphene (all of which occur in restitic portions of Miocene migmatites exposed in the field area) during partial melting, and mixing between gabbro and granite. Isotopic data indicate a lithospheric mantle source for mafic rocks in the study area and, on the basis of field data and their similarity in isotopic composition, granitic rocks are interpreted to be derived indirectly from the same source during Basin and Range extension. Evolution of the granites is hypothesized to involve a series of partial melting steps, one of which is exposed in the batholith, which refine mantle-derived gabbros into high-silica rocks. Thus the Mineral Mountains batholith represents juvenile granitic material added to the crust during extension. This raises the possibility that extension may be an important granitic crust-forming event. Furthermore, this suggests that pure-shear igneous inflation of the crust by the mantle can be an important mechanism during extensional deformation. Data presented here indicate that fractional melting of young mafic crust may be an important process in the evolution of isotopically homogeneous intrusive suites which span a broad compositional range. Furthermore, the data support the idea that lithospheric mantle in the Great Basin region may be Proterozoic in age.