RODINIA-GONDWANA SUPERCONTINENT CYCLE REFINED BY DETRITAL ZIRCONS FROM THE DAMARA OROGEN

The Neoproterozoic breakup of Rodinia and Cambrian accretion of Gondwana define one of the most significant supercontinent cycles in Earth history. The relationship between the Congo and Kalahari cratons throughout this cycle is unclear. These cratons may or may not have been adjacent in Rodinia as they are in Africa today, or one (or both) of the cratons may not have been part of Rodinia. If Congo and Kalahari were juxtaposed in Rodinia, it is also unclear if they remained adjacent until Gondwanan assembly, or if the cratons were separated by a large ocean basin prior to Gondwanan juxtaposition. These relations can be tested using detrital zircons in rocks included in the Damara-Lufilian-Zambezi orogenic system, the 570-530 Ma suture between the Congo and Kalahari cratons. This belt includes variably metamorphosed rift and passive margin sedimentary successions along with felsic and mafic volcanic rocks. Metasedimentary rocks from the former passive margins of Congo and Kalahari in the Damara belt yield a broad range of detrital zircon U-Pb ages consistent with known crustal components in the respective cratons, including Paleoproterozoic and Neoarchan ages, but dominated by grains <1400 Ma. The most abundant zircons along the Congo margin give ages of 1100-1000 and 800-600 Ma, whereas the most abundant zircons from the Kalahari samples range from 1350-1100 Ma. There are, however, distinct populations of Mesoproteroic and Neoproterozoic zircon from the strata that tie them to each margin. 1300-1250 Ma zircons in the Kalahari margin strata, are absent in the Congo strata. Zircons with ca. 1150 Ma ages are found in both groups, but an ~1050 Ma population from Congo is nearly absent from the Kalahari strata. Kalahari margin strata also lack zircons with ages of 900-600 Ma. After the Damara collision, the unique detrital zircon populations in the Neoproterozoic Congo strata were recycled into the Cambrian foreland basins (Nama Gp.) on the Kalahari side of the orogen. These data suggest that the Congo and Kalahari cratons were independent during Rodinian time, and not juxtaposed until accretion of Gondwana,. The highest proportion of zircon grains with primitive Lu-Hf isotopic values are Neoproterozoic, which suggests that most of the continental growth in the Damara Orogenic cycle took place during Neoproterozoic rifting.