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, INTRODUCTION 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 emplacement The mid-crustal granulite-facies problem in the mid-crust. In contrast, thermal modelling suggests that Granulite-facies terranes are rocks of the Earth’s lower suband superjacent magmatic accretion should not have exceeded and middle crust that equilibrated at high pressures (P ) 30 my in duration. This enigma is resolved by precise U–Pb zircon and temperatures (T ). Their petrology and geoSHRIMP dating of the major orthogneissic units of the region. chronology commonly preserve both prograde and retroThese data point to Kibaran crustal growth at 1220–1170 Ma, grade characteristics. Because these terranes reflect a which occurred on the margins of a Palaeoproterozoic (2000–1800 number of different crustal and tectonic processes, their Ma) continental nucleus. A later, distinct, orogenic episode, here origin is important in understanding the nature of contermed the Namaquan (time equivalent of the Grenvillian), involved tinental growth and crustal evolution. crustal thickening and magmatism at 1060–1030 Ma and was Granulite-facies rocks typically reflect P–T conditions responsible for, and coeval with, the peak of metamorphism. Lowof 6–9 kbar and 750–850°C and comprise anhydrous P granulite-facies metamorphism resulted from advective heating mineral assemblages that point to conditions of reduced and crustal thickening by magmatic accretion over a 30 my interval. water activity (Harley, 1989). In the lower crust some granulite-facies rocks are thought to be residues of partial melting that has moved melt and volatiles to higher crustal levels. Alternatively, granulite-facies rocks may form where mutually soluble CO2–H2O-rich fluids stream upwards through the crust, causing local reduction in volatile content and accompanying mineral phase changes along the fluid flow paths (Harley, 1989). In the