The geochemistry and geochronology of the barby formation in the sinclair area, Southern Namibia
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Date
2018
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Publisher
University of Namibia
Abstract
Volcanic and sedimentary rocks of the Sinclair Supergroup occur in the Konkiep
Terrane of Southern Namibia. Three volcanic and sedimentary cycles are recognised. The Barby Formation belongs to the second cycle which consists of the Kunjas. Formation, Barby Formation and the Guperas Formation. In this work, the volcanic rocks of the Barby Formation, a key unit in the Sinclair area are described and dated. The coeval Spes Bona Syenite and the Tiras Granite are also described and dated. The rock types in the Barby Formation are rhyolites, basaltic trachyandesites, trachybasalts and trachydacites as well as volcanoclastic rocks. The rocks are largely undeformed and partly altered by deuteric processes and also mildly metamorphosed. Based on geochemistry of the rock samples and previous work, the bulk of rock types are calcalkaline although some alkaline rocks are present. U-Pb dating and Lu-Hf isotopic microbeam analyses were made on zircon and baddeleyite grains from four samples using cathodoluminescent images. A Barby rhomb porphyry sample gives an U-Pb baddeleyite age of 1217 ±2 Ma and a Barby felsic tuff has an 207Pb/206Pb zircon age of 1214 ±5 Ma. The Spes Bona Syenite which intrudes the Barby formation has a U-Pb baddeleyite age of 1217 ±3 Ma and an indistinguishable Rb-Sr biotite isochron age of 1238 ±20 Ma, showing that there was no regional metamorphic event. Subduction zone
assemblages are generally made of high potassium basalts or calc-alkaline association. The Barby Formation records such assemblages. The Barby Formation reflects a subduction event which took place during the assembly of the Rodinia supercontinent. It is slightly younger at 1204 ±9 Ma and reflects the end of subduction-related volcanism due to the collision of Namaqua terranes with the Konkiep Terrane. The Spes Bona syenite and the Barby units have Lu-Hf crustal residence ages between 1682 and 1873 Ma suggesting the two units probably formed from a mixture of juvenile and older crustal material.
Description
A thesis submitted in fulfillment of the requirements for the Degree of Master of science in Geology
Keywords
Geochemistry, Geochronology, Sinclair, Volcanic and sedimentary rocks, Southern Namibia, Barby formation