A COMPARATIVE STUDY OF LAMPROITIC LAVA PONDS IN THE LEUCITE HILLS, WY

The Leucite Hills qualify as Earth’s youngest lamproite volcanic field and consist of 22 separate exposures that erupted ~1-3 m.y. ago. Although they are best known for their unusual composition (ultrapotassic) and mineralogy (madupite, transitional madupite, and lamproite) the buttes and mesas of the Leucite Hills afford the unique opportunity to study the physical volcanology of lamproites. Several of the exposures occur as small lava flows that appear to have pooled within semi-circular explosion craters formed during the early phases of eruption. Structures in these small “lava ponds” such as vesicularity and jointing vary markedly from exposure to exposure. The goal of this study is to characterize, compare, and contrast the structures, textures, mineralogy, and crystallization history of lava ponds from the 3 rock varieties in the Leucite Hills. One of the most unusual exposures is Black Rock Butte (BRB). This lamproite has a concave upper surface, suggestive of lava drainback, and consists entirely of highly vesicular (37-52%) columnar-jointed rock. Another lamproite exposure at Cabin Butte is not nearly as vesicular (15-18%) and exhibits a crude concentric joint pattern. A transitional madupite at Hatchers Mesa also has a concave upper surface, contains 13-28% vesicles and is not jointed at all. Pilot Butte, a madupite, contains only 5-11% vesicles, and has no cooling joints either. In addition to being more abundant at BRB, vesicles are larger and more rounded than at the other exposures. Thickness seems to be a critical factor in the development of columnar joints in the Leucite Hills lava ponds, as only the thickest exposures (BRB ~40m and Cabin Butte ~35m) are jointed while the thinner exposures (Hatcher Mesa ~15m and Pilot Butte ~20m) are not. Black Rock Butte also has the highest phlogopite content (19%) of the rocks in this study, and unlike the other samples, most of the phlogopite occurs in the groundmass. A sudden vesiculation event in the BRB lava pond may be responsible for the abundant groundmass phlogopite and the finest groundmass grain size of all the exposures. Although the causes for the extraordinary vesicularity of BRB are not well understood, we hope that detailed petrographic analysis will provide further explanation for this unique combination of high vesicularity and columnar-joints.