Understanding Magmatic Timescales and Magma Dynamics in Proterozoic Anorthosites: a Geochronological Investigation of the Kunene Complex (Angola)

The Kunene Anorthosite Complex, located in south west Angola, is one of the largest massif-type anorthosite intrusions on Earth, with an areal extent of at least 18 000 km2. Previous studies considered the Complex to consist of a series of coalesced plutons. However, the ages and relative emplacement sequence of these plutons are unknown. Understanding the relative timing of the pluton emplacement is crucial for understanding how these enigmatic magmas form and how they rise through the crust. Here we present new high precision U-Pb ID-TIMS ages (n=10) on zircons and baddeleyites for many of the coalesced plutons across the 300-km-long anorthositic complex. These new geochronological results reveal subtle variations in crystallization age between the coalesced plutons. There is no gradual age progression between plutons, but distinct groupings of ages (Fig.1). Age clusters of 1379.8 ± 2 Ma (n=5) occur north of the Red Granite NE-SW-striking intrusions, whereas in the south there is an older age grouping of 1390.4 ± 2.3 (n=3). Two additional ages of 1400.5 ± 1.3 in the centre and 1438.4 ± 1.1 Ma in the south east have been obtained. These results indicate that the Kunene anorthosites were emplaced over 60 Ma and may suggest long-lived magmatic systems and/or slowly ascending plutons. We also find a link between pluton composition and age. In general, leuconoritic domains are older than the leucotroctolitic domains. This may imply that the first pulses of magma received a greater degree of contamination, forcing the broadly basaltic magma to produce orthopyroxene as the main mafic phase. The later pulses receive less contamination as they ascend through the already partially melted crust, producing olivine as the mafic phase and deforming the older domains. This study reiterates the multiphase petrogenesis of Proterozoic anorthosites and sheds light on the assembly of crystal-rich magmas as they ascend through the crust.