Publication Date

12-2010

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Geosciences

Department

Geosciences

Major Advisor

Clyde J. Northrup, Ph.D.

Second Advisor

Mark D. Schmitz, Ph.D.

Abstract

Late Paleozoic to Mesozoic arc terranes comprising the Blue Mountains Province of northeastern Oregon and western Idaho are poorly understood in terms of the timing and duration of recorded magmatism and deformation. Intrusive basement rocks of the Wallowa and Olds Ferry arc terranes are investigated using a combination of highprecision U-Pb geochronology, radiogenic isotope and trace element geochemistry, and structural geology to help constrain their tectonomagmatic evolution and to provide a better understanding for the accretionary history of the Blue Mountains Province.

The Cougar Creek Complex is a compositionally diverse intrusive complex interpreted as the mid-crustal portion of the Wallowa arc terrane. New geochemical data and high-precision U-Pb zircon ages define two compositionally and temporally distinct cycles of magmatism, and two separate episodes of contractional deformation in the Wallowa arc. An apparent gap in igneous activity, contractional deformation, regional uplift and erosion of the arc, and a switch from predominantly calc-alkaline to tholeiitic magmatism is interpreted as a response to the subduction of a spreading ridge beneath the overriding Wallowa arc in the early Mesozoic. Late Triassic synmagmatic left-lateral mylonitic shear zones in the Cougar Creek Complex are interpreted to illustrate the structural development of subvertical, arc-parallel ductile shear zones in an active midcrustal arc-axis environment undergoing sinistral-oblique subduction.

New high-precision U-Pb ages for small multi-grain and single-grain analyses of neoblastic titanite from the Cougar Creek Complex directly constrain the timing and duration intra-arc strike-slip ductile shearing, and reveal the utility of this method for resolving the temporal and structural evolution of mid-crustal, synmagmatic deformational processes. Trace element characteristics of titanite, determined by laser ablation inductively coupled plasma and thermal ionization mass spectrometry, provide a geochemical means of distinguishing magmatic and neoblastic grains.

U-Pb ages for supracrustal and intrusive rocks from the inboard Olds Ferry arc terrane, combined with new trace element data, constrain three separate unconformitybounded volcano-plutonic sequences. Sr, Nd, and Pb isotopic data for intrusive rocks of the Olds Ferry arc indicate derivation from a more isotopically enriched source compared to those from the Wallowa arc, and establish a clear distinction between the two arc systems. This distinction strengthens current paleogeographic interpretations of a continental fringing Olds Ferry arc and an intra-oceanic Wallowa arc, and provides 1) essential constraints for developing a comprehensive tectonic model for the accretionary history of the Blue Mountains Province, and 2) a basis for comparison to other Cordilleran arcs. The Olds Ferry and Wallowa arc terranes are illustrated by a two-arc, doubly-vergent Molucca Sea-type collisional model. The Olds Ferry fringing arc is interpreted as the southward continuation of the Stikinia-Quesnellia arc system located in the southern Canadian Cordillera. The Wallowa arc terrane is correlated with the Wrangellia terrane of southeastern Alaska and western Yukon.

Isotopic analyses of Permo-Triassic intrusive igneous rocks from the Wallowa and Olds Ferry arc terranes provide new constraints on regional lithospheric architecture, with implications for models of Laramide crustal shortening and the provenance of Cenozoic magmatism. The isotopically intermediate character of the Olds Ferry fringing arc terrane is recognized as a primary early Mesozoic feature predating Cordilleran shortening. Isotopic profiles generated orthogonal to the Wallowa-Olds Ferry terrane boundary and the western Idaho shear zone show abrupt increases in 87Sr/86SrI compositions, and mark the transitions between three geochemically distinct lithospheric columns corresponding to: 1) relatively depleted intra-oceanic lithosphere of the Wallowa arc; 2) isotopically intermediate lithosphere of the Olds Ferry fringing arc; and 3) the truncated margin of the ancient and isotopically evolved North American continent. West to east spatial variability in the isotopic compositions of Neogene volcanic rocks may thus be explained by the partial melting of these three geochemically distinct lithospheric reservoirs, in contrast to models requiring offset of the North American cratonic margin. Combined Sr, Nd, and common Pb isotopic data also support interpretations that inherited arc-related mantle of the Wallowa and Olds Ferry terranes play a primary role in the petrogenesis of low-K, high-alumina olivine tholeiites from the northwestern Great Basin.

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