https://bereket-haileab.geology.sites.carleton.edu/files/original/cff9f14d06c3931e05a65336d5b6d7c9.jpg 9a0869126958d7be23ef61ffa4daacba https://bereket-haileab.geology.sites.carleton.edu/files/original/72d5a3e96164cae48a16af6032821ecf.jpg 2f4dd692406cf422087c2027a164047e Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Igneous Rocks Subject The topic of the resource This collection covers both volcanic and intrusive rocks, including rhyolites, granites, diorites, andesites, basalts, gabbros, diabases, pumices, obsidians, syenites, phonolites, peridotites, carbonatites, dacites, trachyte, dunites, xenoliths, ignimbrites, and pegmatites. Creator An entity primarily responsible for making the resource Bereket Haileab Source A related resource from which the described resource is derived Housed in Anderson Hall, Carleton College. Format The file format, physical medium, or dimensions of the resource Thin sections, thin section scans, hand samples, hand sample images, photomicrographs, chemical analyses (major elements, trace elements and REE analyses for selected samples). Thin section Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource BH250-86b Subject The topic of the resource Peralkaline phonolitic volcanic rock (phonolite) Bearpaw Green Porphyry Description An account of the resource 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 and BH250-86b are 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 The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant Location: Bearpaw Mountains, Montana, USA Nearby Geographic Feature: Bearpaw Mountains Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample 86b. Housed at Carleton College in Minnesota. Type The nature or genre of the resource Thin section Creator An entity primarily responsible for making the resource Bereket Haileab Relation A related resource <iframe width="100%" height="600" style="border: 0;" src="https://carleton.maps.arcgis.com/apps/instant/media/index.html?appid=99e70d695022407e93c457e907e4332f" frameborder="0" allowfullscreen="allowfullscreen">iFrames are not supported on this page.</iframe><br />View on ArcGIS Online <a href="https://carleton.maps.arcgis.com/home/item.html?id=99e70d695022407e93c457e907e4332f">here</a><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1id5fqrswQF7s9rUvreY_lT45HRewY7Bc/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/19kunVmCy4YD5_u48rj1On6m35ZNz6hlU/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1Jpgh59fPgMP-6l24QOUFr8UFt8n37JJQ/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1_mMW3QPKIwb5Ec6kqEXDwKQBNwVboQqI/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1qBBQ3rWVHjuZEkkkb-9iSCmZgwocrhfY/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1qvCotNORr-Wzm9nEcOf4VRHHFehS4R3y/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1IN6cRG9Ud9tUWA3gmBF_3S3Jl8WtJNhm/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1j8OsdaUiVSt-vFSmgTkzVMLT8VZWdXEW/preview" allow="autoplay"></iframe> Aegirine alkali-rich magma alkaline igneous rocks Alkaline magmatism alkaline volcanism Bearpaw Green Porphyry Bearpaw Mountains Bearpaw Mountains geology devitrification textures Feldspathoid Feldspathoid minerals Feldspathoid-bearing rock igneous leucite replacement mafic Microlites microlitic texture Montana geology Nepheline nepheline phonolite Nepheline-bearing rock Peralkaline igneous rock peralkaline phonolite peralkaline rocks phonolite phonolitic porphyry phonolitic rock pilotaxitic texture porphyritic texture porphyritic volcanic rock porphyry pseudo-leucite Pseudoleucite alteration Pseudoleucite texture replacement texture Riebeckite silica undersaturated silica-undersaturated silica-undersaturated magma Sodic amphibole Sodic pyroxene Trachytic Texture volcanic