In Situ Dating of Multiple Events in Granulite-Facies Rocks of the Larsemann Hills, Prydz Bay, East Antarctica Using Electron Microprobe Analysis of Monazite

• Main Author: Spreitzer, Steven K
• Format: Text
• Language: unknown
• Published: DigitalCommons@UMaine 2017
• Subjects: chemical dating; exhumation rate; geochronology; petrology; thorium; yttrium; Geology; Tectonics and Structure; East Antarctica; Prydz Bay; Larsemann Hills; Søstrene; Antarc*; Antarctica
• Online Access: https://digitalcommons.library.umaine.edu/etd/2762; https://digitalcommons.library.umaine.edu/context/etd/article/3776/viewcontent/M_SpreitzerSteven_Final.pdf

The metamorphic rocks exposed along the southeast coast of Prydz Bay were affected by several metamorphic and plutonic events associated with collision in the Neoproterozoic and Cambrian. Critical units exposed in the Larsemann Hills and nearby ice-free areas include (1) basement Søstrene Orthogneiss, (2) cover sequence Brattstrand Paragneiss and (3) anatectic pegmatites intrusive into the Brattstrand Paragneiss. Zircon U-Pb data from previous studies yielded a maximum depositional age of 1023 ± 19 Ma for the Brattstrand Paragneiss, but only approximate ca. 900 Ma to ca. 1000 Ma and ca. 530 Ma ages for two metamorphic events. In order to constrain the timing of these two events, stages of these events, and anatexis resulting in pegmatites, monazite in the three units was dated in-situ with the electron microprobe. Entire thin sections were mapped with either the electron microprobe or SEM in order to locate monazite grains and to show the petrologic context of the monazite grains. Selected monazite grains were then mapped with the electron microprobe for U, Th, Y, and Pb to identify growth domains. Domains in a given sample having a similar chemical composition and occurring in grains sharing the same petrologic context are interpreted to constitute a single population that grew synchronously as a result of a specific metamorphic process. These populations were then dated and the dates of populations inferred to have formed as a result of the same stage of a metamorphic event were used to calculate the age of this stage. The results confirm that both the basement and cover sequence were affected by metamorphic events in the Neoproterozoic and Cambrian. Neoproterozoic metamorphism is dated at 911 ± 9 Ma (2σ uncertainties) which is consistent with previous studies. The spread in Cambrian ages for the Brattstrand Paragneiss is attributed to a clockwise P-T path. Monazite populations interpreted as representing peak conditions (estimated to be 6-7 kbar and 800-850˚C by previous research) yield an age of 537 ± 6 Ma. ...