ORIGIN OF THE COMPTON-BELKOVICH SILICIC VOLCANIC COMPLEX

The Compton-Belkovich Volcanic Complex (CBVC), located at 99.5°E and 61.1°N, has been previously recognized as a site of silicic (felsic) volcanism on the Moon [1]; however, its origin remains uncertain. On the basis of regional geologic relationships and remote sensing, we infer an origin by partial melting of a deep, fertile source in the lunar crust, followed by shallow emplacement of magma, inflation, fractional crystallization, pyroclastic eruption(s), and possibly late-stage silicate-liquid immiscibility.

The CBVC lies at the center of a thorium (Th) hotspot in Lunar Prospector gamma ray data [2], and Th is enriched in lunar silicic materials. LRO Diviner data indicate enrichment of SiO₂ in the CBVC [3]. The volcanic complex covers an area ~25x35 km, which includes a collapsed caldera (8x10 km), several prominent volcanic cones and domes (up to 6 km across at base), and numerous small domes (0.5-1.5 km across at base). Moon Mineralogy Mapper data reflect one of the highest indigenous water contents of nonpolar lunar geologic features, possibly associated with pyroclastic activity [4,5]. LROC Narrow Angle Camera (NAC) photometric data suggest that, in addition to areas of highly silicic (rhyolitic) compositions, some of the large domes and cones within the CBVC have intermediate (e.g., dacitic) compositions [6] that are consistent with a fractionated shallow intrusive body and perhaps a relatively high water content (for the Moon).

The enrichment in Th and SiO₂ within the CBVC occurs in isolation in this region of the Moon. Crater distributions indicate an origin at ~ 3.5 Ga [7], yet none of the large, nearby impact craters that occurred either before or after CBVC emplacement excavated Th- or Si-rich materials. Thus, we hypothesize that the “fertile” source material for CBVC volcanism originated deep within the crust. In this work, we test these observations and hypotheses with new NAC-based photometric and topographic data for the CBVC.

References: [1] Jolliff B. et al., 2011, Nat. Geosci. 4; [2] Lawrence D. et al., 2000, J. Geophys. Res. 105; [3] Glotch T. et al., 2010, Science 329; [4] Petro, N. et al., 2013, Lunar Planet. Sci. 44; [5] Bhattacharya S. et al., 2013, Current Science 105; [6] Watkins R. et al., 2017, Icarus 285; [7] Shirley K. et al., 2016, Icarus 273.