Journal of Petrology, Volume 40, Issue 12: December 1999.

The History of Granulite-Facies Metamorphism and Crustal Growth from Single Zircon U-Pb Geochronology: Namaqualand, South Africa

L. J. ROBB¹, R. A. ARMSTRONG² AND D. J. WATERS³

¹DEPARTMENT OF GEOLOGY, UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG, SOUTH AFRICA
²RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
³DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF OXFORD, OXFORD OX1 3PR, UK

The Namaqualand Metamorphic Complex is a well-exposed, Mesoproterozoic, low-pressure, amphibolite-granulite-facies terrane flanking the Archaean Kaapvaal Craton of southern Africa. Previous isotopic dating in the region suggests an ~150 my period of prograde granulite-facies metamorphism and episodic granite emplacementin the mid-crust. In contrast, thermal modelling suggests thatsub- and superjacent magmatic accretion should not have exceeded 30 my in duration. This enigma is resolved by precise U-Pb zircon SHRIMP dating of the major orthogneissic units of the region. These data point to Kibaran crustal growth at 1220-1170 Ma, which occurred on the margins of a Palaeoproterozoic (2000-1800 Ma) continental nucleus. A later, distinct, orogenic episode, here termed the Namaquan (time equivalent of the Grenvillian), involved crustal thickening and magmatism at 1060-1030 Ma and was responsible for, and coeval with, the peak of metamorphism. Low-P granulite-facies metamorphism resulted from advective heating and crustal thickening by magmatic accretion over a 30 my interval.

Keywords: granulite;mid-crust;Namaqualand; U-Pb zircon

Pages 1747-1770

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