Non-mare silicic volcanism on the lunar farside at Compton-Belkovich

Non-basaltic volcanism is rare on the Moon. The best known examples occur on the lunar nearside in the compositionally evolved Procellarum KREEP terrane. However, there is an isolated thorium-rich area-the Compton-Belkovich thorium anomaly-on the lunar farside for which the origin is enigmatic. Here we use images from the Lunar Reconnaissance Orbiter Cameras, digital terrain models and spectral data from the Diviner lunar radiometer to assess the morphology and composition of this region. We identify a central feature, 25 by 35 km across, that is characterized by elevated topography and relatively high reflectance. The topography includes a series of domes that range from less than 1 km to more than 6 km across, some with steeply sloping sides. We interpret these as volcanic domes formed from viscous lava. We also observe arcuate to irregular circular depressions, which we suggest result from collapse associated with volcanism. We find that the volcanic feature is also enriched in silica or alkali-feldspar, indicative of compositionally evolved, rhyolitic volcanic materials. We suggest that the Compton-Belkovich thorium anomaly represents a rare occurrence of non-basaltic volcanism on the lunar farside. We conclude that compositionally evolved volcanism did occur far removed from the Procellarum KREEPÂ terrane.