Date of Award

Summer 8-15-2017

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Edward S. Grew

Second Committee Member

Alicia Cruz-Uribe

Third Committee Member

Martin Yates

Abstract

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. Early retrograde conditions are characterized by decompression (estimated to be 4.5 kbar by previous research) and partial melting. Monazite populations interpreted as representing this stage yield an age of 525 ± 3 Ma. Late retrograde metamorphism is characterized by further decompression and cooling and monazite populations interpreted as representing this stage yield an age of 512 ± 2 Ma. Dating of monazite in the pegmatites often gave contradictory results, however, the dates confirm that the analyzed pegmatites were emplaced synchronously with the Cambrian metamorphic event.

The results indicate that rocks presently exposed in the Larsemann Hills were buried to mid-crustal depths during the Neoproterozoic metamorphic event at 911 ± 9 Ma and were subsequently exhumed to a shallower depth. At 537 ± 6 Ma during peak metamorphic conditions the Larsemann Hills were buried to a depth of 20 – 24 km and were raised to a depth of around 10 km by 512 ± 2 Ma when the Indo-Antarctic and Australo-Antarctic cratons collided.

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