January 29, 2014 at 10:59 am

Geology Colloquium: Perspectives and Perils of Using U-Pb Zircon Geochronology to Constrain Stratigraphic Age, Jan. 31

The Department of Geological Sciences Colloquium Series presents Dr. Amy Weislogel on “Perspectives and perils of using U-Pb zircon geochronology to constrain stratigraphic age: lessons from the Permian-Triassic Karoo basin, South Africa” on Friday, Jan. 31, at 2 p.m. in Clippinger Laboratories, Room 205.

Amy Weislogel

Amy Weislogel

Weislogel is Assistant Professor in the Department of Geology and Geography at West Virginia University. She conducts research and teaches in the areas of sedimentology, stratigraphy and sedimentary petrology to elucidate paleoenvironmental, tectonic and climate processes. These pursuits can aid in energy exploration, carbon sequestration and reconstructing global change and Earth’s history, according to her website.

Abstract: The Karoo basin contains an important Carboniferous-Jurassic sedimentary record that chronicles the amalgamation of Pangea, subduction along the southern Panthallasan margin and later opening of the South Atlantic, Southern, and Indian oceans. It also preserves a robust record of the end-Permian extinction, Earth’s largest paleobiologic crisis. Fortuitously, this stratigraphic record is interleaved with numerous air-fall volcanic ashes likely derived from a continental magmatic system, part of which is currently exposed in the Choiyoi igneous province of South America. Our U-Pb geochronology of air-fall ash zircon from the Ecca and Beaufort Groups via SHRIMP analysis, along with LA-ICP-MS U-Pb analysis of detrital zircon from interbedded sandstones indicate that magmatic activity of this system likely persisted for at least 50 Ma. Particularly noteworthy is that uppermost Ecca Group detrital zircon signature exhibits few Precambrian ages and abundant Permian-Triassic ages; thus, by the start of marginal marine deposition, Karoo basin clastic influx was primarily sourced by some part of the Permian-Triassic Panthallassic magmatic system (i.e., the Choiyoi igneous province or its southern equivalent). Detrital zircon U-Pb ages suggest initial zircon formed within this magmatic system by ~300 Ma and that the apex of zircon crystallization occurred at ~265-275 Ma, after which, zircon crystallization decreased through time. We hypothesize that over the long duration of magmatic evolution, the melt began to become depleted with respect to zircon, such that by ~260 Ma, the melt became primarily undersaturated with respect to zircon. As a result, zircon was no longer a major accessory phase and ultimately after ~250 Ma was extruded only in scant trace amounts. This interpretation is also supported by U-Pb zircon geochronology on a few dozen Ecca and Beaufort Group ashes from across the Karoo basin. Individual ash ages were determined from statistically-robust, coherent populations of single U-Pb zircon ages. When interpreted in the context of recent ash U-Pb zircon geochronology results of others, the results depict a regional and repeatable occurrence of Permian ashes within the Triassic Beaufort Group that are consistently older than or within statistical error of ash ages found in the Ecca Group, despite the stratigraphic position of Ecca Group ashes 100’s to >1000 m downsection of the Beaufort Group ashes. Lacking a reasonable geological explanation for this inversion, it appears that zircon within Beaufort Group ashes are of recycled or xenocrystic origin and yield coherent U-Pb zircon age populations which are older than the ash depositional age. Thus, even when high analytical precision is attained and Pb-loss is considered, ash ages determined by U-Pb zircon geochronology may not reliably demarcate the record of the end-Permian extinction and associated environmental changes preserved within Karoo basin strata. Furthermore, any stratigraphic age constrained solely by U-Pb zircon age populations from one or even a few separate ash beds may in fact be spurious given the evolution of zircon saturation within the magmatic system that sourced the ash.

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