UNRAVELING THE PROTEROZOIC LIP RECORD OF SOUTHERN AFRICA: AGE OF THE MASHONALAND IGNEOUS PROVINCE, ZIMBABWE, AND REGIONAL IMPLICATIONS

Proterozoic intraplate igneous provinces form a critical part of the global LIP record, but many examples still lack reliable constraints on their duration and extent. A case in point is the Mashonaland province in southern Africa, which comprises diabase sheets and dikes found over an area of ~150,000 km² in the Archean Zimbabwe craton. Sites from the diabases in the eastern half of this area have yielded a robust paleomagnetic pole that has been widely used in Precambrian paleomagnetic studies, but no precise age constraints on the timing of this igneous event exist. We report U-Pb baddeleyite crystallization ages of ~1.88 Ga for two representative diabase sheets in NE Zimbabwe, both of which have Mashonaland magnetization directions.

To the south, within the Kaapvaal craton, coeval intraplate magmatism is represented by a suite of diabase intrusions that occur over an area of at least 22,000 km² in the northern part of the craton and have baddeleyite ages of ~1.88-1.87 Ga. Paleomagnetic data indicate that the diabases are part of a larger province that includes flood lavas in the Soutpansberg Group, which unconformably overlies high-grade rocks in the Limpopo belt between the Zimbabwe and Kaapvaal cratons. A reasonable first-order interpretation is that the Mashonaland and northern Kaapvaal-Soutpansberg (NKS) mafic rocks belong to a single LIP emplaced across the region at ~1.88 Ga. However, the mean NKS and Mashonaland poles are 39° apart, suggesting significant post-1.88 Ga displacement between the Zimbabwe and Kaapvaal cratons, which must have occurred along or across the intervening Limpopo belt. Tilting either craton 28º about a horizontal axis superimposes the two poles, but there is no geological evidence for such tilting. The poles can also be superimposed by restoring ~2100 km of sinistral strike-slip motion along the Limpopo belt. If the latter model is valid, either the ~1.88 Ga igneous suites on both cratons are not directly related, in spite of their closely similar ages, or they may represent fragments of a more extensive LIP juxtaposed by later plate motion. This example illustrates the need to integrate geochronological and paleomagnetic studies when attempting to correlate segments of LIPs on different cratons.