Wirth, K.R., 1991, Processes of lithosphere evolution: Geochemistry and tectonics of mafic rocks in the Brooks Range and Yukon-Tanana region, Alaska: Ithaca, New York, Cornell University, Ph.D. dissertation, 367 p., illust., maps.
Mafic rocks provide a record of Mesozoic - Cenozoic evolution of lithosphere in the Alaskan cordillera. In the Brooks Range, a Late Jurassic - Early Cretaceous fold and thrust belt, the structurally highest thrust sheets consist of ultramafic to mafic plutonic and volcanic rocks. The plutonic rocks, which include tectonized dunite and harzburgite, layered dunite, pyroxenite, and gabbro, and late-stage intrusive diorite and plagiogranite, are similar to the basal sections of ophiolites and oceanic lithosphere. The ultramafic and mafic plutonic sequences are structurally overlain by mafic volcanic, and minor intrusive and sedimentary, rocks. 40Ar/39Ar analyses of amphibole from the plutonic and intrusive rocks indicate that they crystallized between 187 and 184 Ma ago. The ages of biotite, plagioclase, and potassium-feldspar constrain the thermal evolution of the plutonic rocks from the time of thrust-faulting (169-163 Ma), to the time of emplacement onto the margin of Arctic Alaska (~145 Ma), to the time of uplift (~56 Ma). Field and geochemical studies of the volcanic rocks in the Brooks Range establish that they are geologically and geochemically diverse. The combined field and geochemical data indicate that these rocks formed in various tectonic settings, including volcanic arc and intraplate oceanic settings. The diversity of volcanic rocks in the Brooks Range suggests that they are the remnants of 'anomalous' lithosphere from a large ocean basin that was closed during the Brookian orogeny. South of the Brooks Range, tholeiitic to alkaline volcanic rocks were erupted through the various terranes of central Alaska during the Late Cenozoic. Trace-element and isotope data from these volcanic rocks indicate that they are geochemically similar to intraplate basalts erupted on some ocean islands, yet the spatial and temporal relationships of the basalts preclude a mantle plume origin. Preliminary studies suggest that the late Cenozoic volcanic rocks of central Alaska are the result of small-degree melts of Pacific MORB-type asthenosphere and sub-continental lithosphere. The mechanism that caused the formation of these isolated volcanic eruptions over this large region was, most likely, lithosphere extension.
Theses and Dissertations
