Crustal evolution of the northeastern Basin and Range [electronic resource] : insights from the structural and magmatic history of the Albion - Raft-River - Grouse-Creek metamorphic core complex
Keywords: Geological and Environmental Sciences.

The Albion -- Raft-River -- Grouse-Creek metamorphic core complex (ARG), located in the northeastern Basin and Range province, is one of the classic examples of metamorphic core complexes in the North American Cordillera. The ARG metamorphic core complex is an exceptional natural laboratory to study continental extension and its cause-and-effect relationship to magmatism, because the ARG has a rich Cenozoic magmatic history spanning more than 40 Myrs. The ARG also exposes parts of the Archean basement that once lay at depth beneath this region, providing constraints on the composition of the deep crust that underwent partial melting and contributed to Cenozoic magmatism during the extensional history of the region. This study utilizes zircon U-Pb geochronology, whole rock major and trace element geochemistry, whole rock tracer isotope compositions (Sr and Nd isotopes) and in situ zircon trace element compositions, oxygen and Hf-isotopic compositions to better understand the petrogenesis of Cenozoic igneous rocks exposed within and around the ARG metamorphic core complex. These isotopic data were modeled using constraints available on the isotopic composition of the deep crust in an effort to calculate the relative roles of crust versus mantle components in the Cenozoic magmas. Geologic mapping, cross-section restoration and re-interpretation of existing geophysical data for the region provided a context and temporal framework for magmatism with respect to the structural evolution of the region and the formation of the ARG metamorphic core complex. The results of this study indicate that Eocene-Oligocene (41-25 Ma) calc-alkaline magmatism is part of a regional magmatic event that is related to the foundering of the Laramide shallow slab that resulted in the intrusion of basalt in the lower crust. Earliest magmas (41-32 Ma) are present as shallow intrusions and eruptive equivalents and have isotopic compositions of 87Sr/86Sri = 0.7089 to 0.7110, [epsilon]Nd(i) = -16 to -24, [delta]18Ozr = 4.8 to 6.8 [per mil], and zircon [epsilon]Hf(i) = -15 to -32. Oligocene magmas (32-25 Ma) intruded into the middle crust, lack volcanic counterparts and have more evolved isotopic compositions of 87Sr/86Sri = 0.7111 to 0.7156, [epsilon]Nd(i) = -26.3 to -36.1, [delta]18Ozr = 4.7 to 6.1 [per mil], and zircon [epsilon]Hf(i) = -30 to -42. These 41-25 Ma magmas in the ARG record increasing amounts of crustal contamination through time, so that by Oligocene times (32-25 Ma), widespread crustal melting and crustal assimilation resulted in the diapiric rise of granite-core gneiss domes. These granite-cored gneiss domes equilibrated at depths of ~10-15 km, and are responsible for sillimanite grade metamorphism and the development of high-strain fabrics in their cover and wall rocks. It is inferred that this period of intrusion may have been accompanied by intense faulting and fracturing of the upper crust that may have resulted in circulation of meteoric fluids and hydrothermal alteration of the deeper parts of the crust. The granite-cored gneiss domes remained at depths of ~10-12 km until the Miocene (~14 Ma) when they were exhumed by Basin and Range extension. This exhumation is recorded by the sedimentary and volcanic deposits of Miocene syn-extensional basins developed east and west of the ARG. The Raft River Basin deposits were dated by U-Pb geochronology of zircon from sediments, tuffs and volcanic rocks span the interval from 13.5 to younger than 8.2 Ma. Miocene magmatism is represented by the 9.5-8.2 Ma bimodal Jim Sage volcanic suite, dated by zircon U-Pb geochronology. Isotopic compositions of the Miocene basalt ranges in 87Sr/86Sri = 0.7066-0.7075 and [epsilon]Nd(i) = -3.7, while the rhyolite lavas of the Jim Sage volcanic suite have 87Sr/86Sri = 0.7114-0.7135, [epsilon]Nd(i) = -6.7 to -7.1, [delta]18Ozr = -0.5 to 5.7[per mil] and zircon [epsilon]Hf(i) = -0.8 to -6.8. These results are consistent with the Jim Sage volcanic suite being genetically related to mantle-plume driven Snake River Plain magmatism. The silicic Miocene magmatism records ~40% assimilation of hydrothermally altered crust. Basaltic magmatism was responsible for Miocene heating and melting of the lower crust. Field relationships and geophysical data indicate that strata of the Miocene Raft River Basin were subsequently inverted/uplifted by normal faulting and rotated to steeper dips (up to 50o) by a series of intra-basin normal faults, active after the eruption of the 9.5-8.2 Ma Jim Sage volcanic suite. This faulting was driven by horizontal stretching accompanied by vertical crustal flow (doming) and reflects the increased mobility of the underlying continental crust. The basaltic input and heating implied by the lavas of the Jim Sage volcanic suite was the driver for the crustal mobility recorded by the post-depositional structural evolution of the Raft River basin.