T139 Zircon and Beyond: Quantitative Methods for Characterizing Provenance in Modern and Ancient Sediment Routing Systems

Monday, 23 September 2019
1:30 PM - 5:30 PM
Room 225AB, North Building

EVALUATING SEDIMENT PROVENANCE AND VOLCANOTECTONIC EVOLUTION OF THE EAST AFRICAN RIFT SYSTEM WITH (U-TH)/HE AND U/PB LASER ABLATION DOUBLE DATING OF DETRITAL ZIRCONS FROM HOMININ SITES AND PALEOLAKES DRILLING PROJECT (HSPDP) DRILL CORES

Paper number: T139-13
Program: Technical Sessions
Time: 4:45 PM – 5:00 PM

Authors

Abstract

Drill cores collected by the Hominin Sites and Paleolakes Drilling Project (HSPDP) in Ethiopia and Kenya present an unusual opportunity to evaluate sediment provenance and landscape dynamics during critical intervals in Plio-Pleistocene hominin evolution, as well the volcanotectonic evolution of the East African Rift System (EARS). We collected 15 sand samples from the Chew Bahir, Northern Awash, and West Turkana drill cores for detrital thermochronology analyses as part of the HSPDP. We performed (U-Th)/He and U/Pb analyses on detrital zircons using single crystal laser ablation double dating (LADD) techniques. LADD analyses determine both the probable crystallization age of the zircon and the timing of its passage through the ca. 180˚C isotherm in its source region. Analyses of four Northern Awash samples yield zircon U/Pb dates younger than ~45 Ma, which are consistent with derivation from sources associated with plume-related volcanism in southern Ethiopia and Northern Kenya at 45–35 Ma, as well as Afar plume activity at ~32–23 Ma. Major modes in the U/Pb zircon apparent age population at ~7.8 Ma and ~3.4 Ma reflect initial rift-associated volcanism and active volcanism shortly prior to deposition of cored sedimentary deposits, respectively. Samples show a near to total lack of zircon U/Pb ages from ~22–12 Ma, implying a lull in volcanic activity in the source region. A subset of grains have concordant U/Pb and (U-Th)/He dates, indicating a brief time lag from eruption to deposition. Zircon (U-Th)/He ages range from crystallization age to slightly younger than the depositional age; the anomalously young dates imply limited hydrothermal alteration after deposition. One sample from West Turkana yields many zircons with dates similar to those from the Northern Awash core, but the West Turkana sample also sources a small population of Neoproterozoic zircons. Analyses on Chew Bahir zircon samples are in progress; this smaller basin is bounded by Precambrian granites and gneisses and should largely yield zircons sourced from crystalline bedrock, rather than Cenozoic volcanic rocks. Our results provide additional means to investigate the timing of volcanotectonic EARS activity and insight into the paleogeography of Plio-Pleistocene hominin landscapes.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-337771

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