Geological and thermochronological evolution of the lower crust of southern Africa

Geochronological, thermochronological and isotopic studies of kimberlite-borne crustal xenoliths have been used to elucidate the architecture and thermal evolution of the continental lithosphere of southern Africa. U-Pb accessory mineral geochronology of lower crustal xenolith assemblages illustrate the youth of granulite-facies metamorphism relative to the ancient stabilization of the craton, and demonstrate two distinct processes for their generation. Granulitic lower crust at the craton margins and in the bounding Proterozoic belts was generated during 1.1 to 1.0 Ga Namaqua-Natal orogenesis, in response to collisional crustal thickening. Ultra-high temperature granulites in the central cratonic lower crust were generated during dramatic advective perturbation of the lithosphere during 2.7 Ga Ventersdorp rifting and magmatism. Utilizing the U-Pb systematics of titanite, apatite and rutile- minerals with closure temperatures for Pb diffusion of 650 to 400ʻC- the thermal evolution of the lower crust and underlying lithosphere has been constrained. Thermal relaxation times for the lithosphere following tectonothermal perturbation indicated by these data (400-600 Ma) are consistent with predictions of simple conductive cooling models, however initial cooling rates in the lithosphere are slower than predicted. Closure of the U-Pb system in rutile, heralding the establishment of cratonic geotherms in the Proterozoic belts of southern Africa by 700 Ma, demands lithospheric thickness comparable to that beneath the Archean cratons.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2002

Author dc:creator

• Schmitz, Mark D. (Mark David), 1972-

Advisor dc:contributor.advisor

• Samuel A. Bowring.

• Earth, Atmospheric, and Planetary Sciences.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/58368

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/58368