SPECTRAL FEATURES OF THE KHANNESHIN CARBONATITE VOLCANO, AFGHANISTAN

Advanced Spaceborne Thermal and Reflection Radiometer (ASTER) data were used to identify and map carbonate rocks within the early Quaternary Khanneshin carbonatite volcano located in southern Afghanistan. The carbonatitic rocks are characterized by CO₃ absorption features near 11.2 micrometers and 2.31-2.33 micrometers. Some of the carbonatite flows at Khanneshin contain neodymium Rare- Earth-Element (REE) spectral absorption features in the 0.45 to 0.54 micrometer region measured from samples collected in the field on an Analytical Spectral Devices spectrometer.

Calcitic and ankeritic carbonatite were identified and mapped using the short wave infrared (SWIR) region of the ASTER data due to a slight shift of the CO3 absorption feature toward 2.26 micrometers (ASTER band 7) in the ankeritic carbonatite spectra. Spectral assessment using ASTER SWIR data suggests that the area is covered by extensive carbonatite flows that contain calcite, ankerite, and muscovite, though some areas mapped as ankeritic carbonatite on a pre-existing geologic map were not identified in the ASTER data. A contact aureole shown on the geologic map was defined using an ASTER false color composite image (R=6, G=3, B=1) and a logical operator byte image. The contact aureole rocks exhibit Fe²⁺, Al-OH, and CO3 spectral absorption features at 1.65, 2.2 and 2.33 micrometers, respectively, which suggest that the contact aureole rocks contain muscovite, epidote, and chlorite.

Visible through short-wave infrared spectral features were used to map: (1) laterally extensive calcitic carbonatite that covers most of the crater and areas northeast of the crater; (2) ankeritic carbonatite located southeast and north of the crater and some small deposits located within the crater; (3) agglomerate that primarily covers the inside rim of the crater and a small area west of the crater; (4) a crater rim that consists mostly of epidote-chlorite-muscovite-rich metamorphosed argillite and sandstone; and (5) iron (Fe³⁺) and muscovite-illite-rich rocks and iron-rich eolian sands surrounding the western part of the volcano. Thermal infrared spectral features were used to map laterally extensive carbonatitic and mafic rocks surrounded by quartz-rich eolian and fluvial reworked sediments.