Radiogenic isotope geochemical and geochronological studies on the origins and break-up of Gondwana
Abstract
Dronning Maud Land (DML), Antarctica, and southern Mozambique were adjacent areas during the Gondwana supercontinent stage. We have studied rocks from both areas for Sm-Nd and Lu-Hf isotope compositions and dated them with single-zircon U-Pb or plagioclase Ar-Ar methods. Proterozoic rocks of our study are limited to DML and comprise A-type granites, mafic dikes, and igneous crustal xenoliths from a Jurassic basalt lava and from ultrapotassic dikes. Most of these rocks are of Grenvillean ages, (~1090-1010 Ma). Some of the xenolith zircons display Pan-African (700-500 Ma) overprinting. Also, an alkali feldspar granite xenolith has an age of 168 Ma, setting also the minimum age for the host lamproite. The xenoliths and a mafic dike also have inherited zircons, with ages up to 2.03 Ga. The Karoo Large Igneous Province (LIP) marks the onset of Gondwana break-up. We have dated several mafic dikes and a gabbro from Vestfjella, DML, and rhyolite beds from Mozambique, all belonging to the Karoo LIP. The majority of their ages cluster around 180 Ma, which corresponds with the emplacement age of voluminous flood basalts in both regions. The results also reveal prolonged emplacement of rhyolitic (182-172 Ma), and mafic (182-150 Ma) magmas. The geochemical compositions of the mafic dikes at different stages of magmatism are highly variable. The tectonic setting evolved from continental to oceanic during the time of their emplacement, but the geochemical variation does not comply with this transition. Both the Proterozoic and Jurassic mafic dikes are typified by initial ɛNd values close to that of the depleted mantle, indicating that they are juvenile additions to the continental crust. Also, all of the granitoid rocks and the rhyolites are more radiogenic than most of their country rocks, and have whole-rock ɛNd and zircon ɛHf values above chondritic. This suggests that the felsic magmatism does not represent partial melting of the surrounding older crust, but has a more juvenile source.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.V31D1995K
- Keywords:
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- 3640 MINERALOGY AND PETROLOGY / Igneous petrology;
- 9310 GEOGRAPHIC LOCATION / Antarctica;
- 9611 INFORMATION RELATED TO GEOLOGIC TIME / Jurassic;
- 9622 INFORMATION RELATED TO GEOLOGIC TIME / Proterozoic