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

Abstract

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 that sub- 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.