LATE CRETACEOUS TO EARLY EOCENE LAMPROPHYRIC TEXTURAL AND PYROXENE MINERAL CHEMISTRY VARIATION FROM CALC-ALKALINE SUBDUCTION RELATED MAGMATISM AND THE CENTRAL MONTANA ALKALIC PROVINCE

Late Cretaceous and early Eocene lamprophyric rocks were sampled and analyzed to determine textural and pyroxene mineral chemistry variations from calc-alkaline subduction related magmatism and the Central Montana Alkalic Province (CMAP) of Montana. Porphyritic pyroxene phenocrysts display resorption during growth, spongy texture within the core and rims, and exhibit bizarre zonations. Biotite and phologopite within the groundmass and as large phenocrysts display different zonations and often bent or strained features. Cretaceous samples analyzed originate from Bull Mountain Range, just north of the Golden Sunlight Au-Ag-telluride deposit, and CMAP samples from the Highwood Mountains, the Crazy Mountains, and the Yogo sapphire lamprophyre dyke. Understanding the petrogenesis of these bizarre, potassium rich rocks could aid in field exploration of future economic deposits. There are three competing proposals and petrogenetic models tested in this study. The mantle metasomatism model after the work of Rock (1984) proposed that lamprophyres originate from low degrees of partial melting of volatile rich metasomatized mantle. The hybridization model by Prostka (1973) interprets lamprophyres to be the product of hybridization between syenitic and gabbroic magmas. The third model after Cogne (1962) surmises an origin from the mixing of mafic and felsic components through selective diffusional exchange. Petrographic textural features and analysis coupled with a mineral chemistry study with emphasis on pyroxenes aim to find which model is best suited for lamprophyric rocks from the CMAP. Zones of spongy texture within pyroxenes are thought to be the product of resorption and partial melting; these zones show a correlation with increased values of Mg#, NiO, and Cr₂O₃ across lamprophyric rocks from all areas of study. TiO₂ values behave irregularly and fluctuate from rim to core within the intermediate zones of grains alluding to possible mixing events during petrogenesis.

References

Cogne, 1962. Bulletin de la Societe Geologique de France. V(2): 141-156.

Prostka, 1973. GSA Bulletin. V 84 (2): 697-702.

Rock, 1984. Environmental Science Transactions of the Royal Society of Edinburgh. V 74 (2):193-227.