Geoscience Frontiers (Sep 2023)
Landsat 8 and Alos DEM geological mapping reveals the architecture of the giant Mesoproterozoic Kunene Complex anorthosite suite (Angola/Namibia)
Abstract
The quasi-monomineralic composition and huge spatial extent of massif-type anorthosites make detecting lithological regions and boundaries challenging. We use processed Landsat 8 OLI multispectral images and ALOS digital elevation models integrated with field and petrographic observations to characterize the architecture of the ≥17,000 km2 Mesoproterozoic Kunene Complex (KC) anorthosite suite in Angola and Namibia. Images of false colour composite bands 6, 4 and 1 and band ratios 4/2 (ferric minerals), 6/5 (ferrous minerals) and 6/7 (OH-bearing minerals), as well as assessment of the PCA and MNF matrices of eigenvectors and eigenvalues from Landsat 8 data using available spectral libraries, have substantially improved the interpretation of the Kunene Complex rock types and structures. The dataset shows that the reflectance signature of KC anorthositic rocks is primarily a function of the degree of metasomatism, which is most significant in olivine-poor rock types. The weathering intensity of the olivine-bearing anorthosite substrate is another control on the remote sensing signal.Our remote sensing and field-based approach has enabled us to divide the KC anorthosite suite into six distinct spectral and architecture domains and at least four distinct magmatic plutons when considering available high-precision geochronological data. The northernmost pluton (ca. 1380 Ma) of massive, olivine-bearing anorthosite shows distinct remote sensing signatures in the band ratio and MNF images marked by the dominance of OH-bearing and subordinate ferric minerals. The central pluton (1412–1400 Ma) is composed of NNE- to N-striking steeply dipping interlayered olivine-bearing and olivine-poor anorthosite, which correspond to ridges of dark-coloured, low albedo rocks with subordinate slightly oxidised ferrous mineral spectral signatures, and valleys of low albedo and OH-bearing mineral spectral signatures. A NNE-striking tectonic zone along a linear belt of KC granite gneiss separates the northern and central plutons. To the south is a NNW- to NNE-striking layered pluton (ca. 1390 Ma), marked by olivine-bearing anorthosite that forms ridges and metasomatized olivine-poor and pyroxene-bearing anorthosite that forms valleys. This domain is separated from the similarly layered and E–W-striking Zebra Lobe–Oryeheke anorthosite by a top-to-the north thrust zone. A small domain of ca. 1438 Ma olivine-bearing anorthosite sensu lato is situated to the SE of the main Kunene Complex.This work highlights the variability in layered and massive textures, the extent of metasomatism, and the importance of internal tectonic boundaries in the architecture of the Kunene Complex anorthosite suite. Our approach can be applied in other anorthositic terrains and large igneous bodies elsewhere on Earth, especially those where outcrop is poor, or access is inhibited.
