BH250-86
Title
Subject
Description
BH250-86 and BH250-86b are striking porphyritic volcanic rocks from the Bearpaw Mountains of Montana and represent an unusual example of silica-undersaturated alkaline magmatism. These rocks belong to the Bearpaw Green Porphyry, a classic alkaline volcanic suite noted for its distinctive mineralogy and evolved magmatic chemistry.
The hand samples display abundant light-colored rectangular to blocky phenocrysts set within a fine-grained greenish-gray groundmass. Many of the large crystals are pseudoleucites—pseudomorphic replacement textures formed after primary leucite crystals. In many alkaline volcanic systems, leucite becomes unstable during cooling or later alteration and is replaced by aggregates of nepheline and alkali feldspar while preserving the original crystal shape. These pseudoleucite textures are among the most diagnostic and visually distinctive features of the sample.
The mineral assemblage is characteristic of peralkaline to mildly metaluminous phonolitic rocks and includes:
Major Minerals
- Nepheline — a feldspathoid mineral indicating a silica-undersaturated magma.
- Aegirine — a sodium-rich clinopyroxene common in alkaline and peralkaline volcanic systems.
- Riebeckite — a sodic amphibole further supporting strongly alkaline magmatic conditions.
- Pseudo-leucite — alteration textures after original leucite phenocrysts, now replaced by nepheline and alkali feldspar.
Petrography
Thin sections reveal a complex volcanic groundmass composed of fine acicular and fibrous crystals surrounding larger phenocrysts. The groundmass commonly displays trachytic to pilotaxitic textures produced by aligned feldspathoid and feldspar microlites during lava flow and cooling.
Large pseudoleucite crystals preserve sharp crystal outlines despite extensive internal replacement. Under plane-polarized light, many of these crystals appear cloudy and altered, whereas crossed polars reveal intricate mosaics of fine crystalline replacement textures. Some phenocrysts contain fractures, reaction rims, and alteration patches filled with secondary minerals.
The groundmass contains abundant slender laths and needles of aegirine together with small sodic amphibole grains. The dark mafic phases are dispersed throughout the matrix and locally concentrated around larger pseudoleucite crystals. The abundance of sodium-rich mafic minerals strongly supports crystallization from an evolved alkaline magma enriched in Na, Fe, and volatile components.
Several photomicrographs show fine radiating fibrous textures surrounding opaque or altered mineral cores. These textures likely formed during late-stage devitrification, hydrothermal alteration, or replacement reactions associated with cooling of the volcanic rock.
Petrogenesis
The Bearpaw Green Porphyry formed from highly evolved alkaline magma generated under silica-undersaturated conditions. Such magmas are commonly associated with continental intraplate magmatism, low degrees of partial melting, and mantle sources enriched in alkalis and incompatible trace elements.
The presence of nepheline and pseudoleucite demonstrates that the magma crystallized in a system deficient in silica relative to feldspar stability. Instead of quartz and common feldspar assemblages typical of subalkaline volcanic rocks, the magma stabilized feldspathoids and sodium-rich ferromagnesian minerals.
The REE pattern for BH250-86 displays strong light rare earth element (LREE) enrichment and elevated incompatible element concentrations, consistent with an evolved alkaline magma derived from an enriched mantle source. The relatively smooth chondrite-normalized REE trend suggests fractional crystallization played an important role during magma evolution, while the absence of a pronounced Eu anomaly indicates limited plagioclase fractionation compared to strongly evolved felsic magmatic systems.
Educational Value
BH250-86 is an exceptional teaching sample for demonstrating:
- Silica-undersaturated igneous systems
- Feldspathoid mineralogy
- Peralkaline volcanic rocks
- Pseudomorphism and mineral replacement textures
- Alkaline magma evolution
- REE enrichment in evolved magmatic systems
- Volcanic groundmass textures in thin section
The sample provides students with an excellent opportunity to study mineralogical and geochemical evidence for unusual alkaline magmatism that differs substantially from more common calc-alkaline volcanic rocks.
Coverage
Nearby Geographic Feature: Bearpaw Mountains