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Transcript
Geological mapping and geochronology of the Kaaimans
Group rocks along the coast south of George, Western Cape,
South Africa
Miller D.S1, de Wit M.J2, Armstrong R.A3
1. AEON (Africa Earth Observatory Network), and Department of Geoscience, Faculty of Science Nelson
Mandela Metropolitan University, South Africa, Email: [email protected]
2. AEON (Africa Earth Observatory Network), Faculty of Science Nelson Mandela Metropolitan University,
South Africa, Email: [email protected]
3. Research School of Earth Science, The Australian National University, Canberra 0200, A.C.T., Australia,
Email: [email protected]
ABSTRACT
Precambrian basement rocks of the Cape Fold Belt are not well understood with respect to their paleogeographic
relationship during the formation of the supercontinent Rodinia (middle to late Proterozoic) and its subsequent break up
into continental fragments that later amalgamated to form Gondwana (early Palaeozoic). A series of Pre-Cape inliers
outcrop along the western and south-eastern parts of South Africa, which represent tectonic windows that allow testing
of these different paleogeographic models. Metasediments and intrusives (syn-tectonic granite and mafic dykes) of one
of these terranes, the Kaaimans Group outcrop along the coast south of George. Previous work done on phase relations
and mineral compositions indicate a poly-metamorphic history associated with these rocks. On a regional scale they do
not exceed greenschist facies except where they were superimposed locally by low to medium grade contact
metamorphism related to the intrusion of syn-tectonic granite. Evidence from petrogenetic grids indicate temperatures
as low as 450°C and pressures between 2 and 3 kbar. Mafic meta-dykes are recrystallised to greenschist assemblages
and display the same structure as the enclosing metasediments. The dykes do not cut the granite suggesting their age be
bracketed by the onset of sedimentation of the Saldanian basin and intrusion of the syn-orogenic Pan-African granites.
These dykes contrast with a younger generation of mafic dykes that cut the granite near Blougat. U/Pb isotope analyses
from the intrusive (Rooiklip) granite, and detrital zircons associated with the upper metasedimentary sequence of the
Kaaimans Group in its present position were dated using the SHRIMP. The maximum age of deposition is 1020 Ma
after the youngest group of zircons in the metasediments and a minimum age constrained by the age of the intrusion of
the granite at 532 ±5 Ma. Concordant detrital zircon dates are prevalent between 1030-1290 Ma, with subordinate
groups at 1600 Ma, and 1800-2100 Ma. These data implies an important contribution of late-Neoproterozoic (Kibaran)
terranes to these sediments, possibly from the Namaqua-Natal Belt and indicate that the sediments are not likely
Neoproterozoic in age.
KEYWORDS: Cape Fold Belt, Kaaimans Group, Pan African Orogeny, Saldanian Belt
Inkaba yeAfrica : 9th Annual Workshop, Germany - 2012