Geochemical evolution of Cenozoic-Cretaceous magmatism and its relation to tectonic setting, southwestern Idaho, U.S.A

Abstract

Magmatism in the western United States spanned a change in tectonic setting from Mesozoic and early Tertiary plate convergence to middle and late Tertiary crustal extension. This paper presents new major element, trace element, and isotopic (Sr, Nd, Pb) data on a diverse suite of Cretaceous to Neogene igneous rocks from the Owyhee area of southwestern Idaho to evaluate possible relationships between the evolving tectonic regime and temporal changes in igneous activity. The oldest studied rocks are Cretaceous granitic intrusives that probably formed by large-scale mixing of Precambrian crust with subduction-related magmas. Silicic Eocene tuffs are also rich in crustal components, but have isotopic compositions unlike the Cretaceous intrusives. These data require at least two crustal sources that may correspond to domains of significantly different age (Archean vs. Proterozoic). The oldest mafic lavas in the study area are Oligocene andesites and basalts compositionally similar to subduction-related magmas derived from asthenospheric mantle and erupted through thick continental crust. Direct crustal involvement during Oligocene time was limited to minor interaction with the mafic magmas. Miocene activity produced bimodal basalt-rhyolite suites and minor volumes of hybrid lavas. Compositions of Miocene basalts demonstrate the decline of subduction-related processes, and increased involvement of subcontinental lithospheric mantle as a magma source. Crustally-derived Miocene rhyolites have isotopic compositions similar to those of the Cretaceous granitic rocks but trace element abundances more typical of within-plate magmas. Several different source domains clearly contributed to the magmatism, including oceanic-type (asthenospheric) mantle variably modified by subduction, subcontinental lithospheric mantle, and continental crust. Temporal changes in the loci of magma production suggest an evolving thermal regime in the crust and mantle which can be viewed as a cycle of lithospheric heating (Cretaceous-Eocene), cooling (Oligocene), and reheating (Miocene) in response to the change from subduction-related to extension-related tectonism. Intracrustal melting during both convergence and extension is probably in response to invasion of mafic magma, but the Cretaceous and Early Tertiary compressional and strike-slip regimes induced efficient mixing of crustal and mantle-derived material. Late Cenozoic extension allowed magmas from these two sources (crust and mantle) to remain essentially separate. Paradoxically, the Middle Cenozoic transitional regime produced the best example of subduction-related mafic volcanism at a time when erupted volumes were minimal.