BH250-233
Title
Subject
Description
Minor Mineral: sphene, glass (devitrified glass)
Near Lead in the northern Black Hills, the phonolitic dike sample BH250-233 belongs to a suite of alkaline intrusive rocks emplaced during the Paleocene to Eocene. These dikes are much younger than the Precambrian metamorphic basement rocks they cut and were likely intruded during the later stages of the Laramide Orogeny, when regional uplift and crustal fracturing created pathways for magma ascent. Their presence records a phase of post-uplift magmatism in the Black Hills, when small volumes of unusual magma rose through deep fractures and solidified as narrow intrusive sheets.
Chemically, phonolite is a distinctive alkali-rich, silica-undersaturated igneous rock enriched in sodium and potassium relative to more common volcanic rocks. Typical compositions contain moderate silica (about 52–60 wt.% SiO₂), high Na₂O + K₂O, and relatively low calcium and magnesium. Because the magma lacked sufficient silica to form quartz, feldspathoid minerals crystallized instead. BH250-233, as a phonolitic rock, was likely enriched in trace elements such as barium, strontium, zirconium, niobium, and rare earth elements, reflecting derivation from evolved alkaline magma sources.
Mineralogically, BH250-233 is composed chiefly of alkali feldspar (commonly sanidine) together with nepheline, the key feldspathoid mineral diagnostic of phonolite. Dark minerals commonly include aegirine or aegirine-augite pyroxene, with minor biotite. Accessory minerals may include sodalite, nosean, titanite, apatite, and iron oxides. In hand sample, the rock is fine grained and gray to greenish-gray, while thin sections often reveal a feldspar-rich groundmass with scattered feldspar phenocrysts and slender green pyroxene crystals, textures characteristic of rapid cooling within a narrow dike intrusion.
Coverage
Nearby Geographic Feature: Black Hills, South Dakota
GPS Coordinates: 44.356578, -103.787958