Identifier

etd-07102007-180009

Degree

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

Document Type

Dissertation

Abstract

Tourmalinites from a well-developed meta-evaporite sequence in the Neoproterozoic Duruchaus Formation of central Namibia contain tourmalines with chemical and textural features that are excellent recorders of their environment of formation. Unlike most other meta-evaporitic deposits, this tourmalinite locality exhibits only minor deformation. The tourmalinites form as finely laminated units ca. 1 m thick within a sequence of quartzites, mica schists, dolomitic marbles, and carbonate breccias. Well-preserved pseudomorphs are considered to replace a variety of evaporitic minerals. The meta-sedimentary rocks of the Duruchaus Formation have been interpreted as representing lacustrine, playa lake, fluvial, and non-marine evaporite deposition in an intracontinental rift that developed as a consequence of the initial breakup of the Rodinia supercontinent (~800-740 Ma). Three localities (Gurumanas West, Gurumanas Oos and Farm Stolzenfeld) contain distinct types of tourmalinites. The Gurumanas West locality contains two generations of dravitic tourmaline in the tourmalinites. The initial generation of tourmaline is complexly zoned and reflects the aluminous nature of the associated rock layers. The second generation tourmaline is magnesian (Na0.50-0.75Fe0.08-0.79Mg2.27-2.67Al5.86-6.17Si6O18 (BO3)3 (OH)3O) and developed after a reactive fluid entered some layers resulting in partial replacement of first generation tourmaline (Na0.77-0.93Fe0.60-1.13Mg1.98-2.47Al5.78-5.99Si6O18 (BO3)3 (OH)3O) with second generation tourmaline. The Gurumanas Oos locality has carbonate-bearing tourmalinites with tourmaline chemistries similar to Gurumanas West. There is a quartz-tourmaline vein that cuts the tourmalinites in which the vein tourmaline inherits the chemistry of the host tourmalinites. The carbonate minerals exhibit complex zoning and replacement textures consistent with influx of a reactive fluid. The tourmalinites from Stolzenfeld have a compositional signature (Fe3+-rich, Al-poor tourmaline) similar to those of other meta-evaporitic tourmalinites worldwide. There is not necessarily a single type of tourmaline mineral chemistry that indicates meta-evaporitic provenance. Application of calcite-dolomite, Ti-in-biotite and Zr-in-rutile geothermometers reveal the maximum temperature during metamorphism (~700ºC). Based on distinct boron isotope values for tourmalines, the origin of tourmalinites in Stolzenfeld is interpreted as being formed after a B-rich marine incursion into an early non-marine rift evaporite sequence whereas the tourmalinites in the Gurumanas area involved the interaction with the late-stage, fluid-rich mushes which play an important role in the thrusting and nappe formation.

Date

2007

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Darrell Henry

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