Date of Award


Document Type


Degree Name

Master of Science (MS)



First Advisor

F.R. Karner


The purpose of this study was to map and study the petrology of the Cenozoic igneous rocks that crop out in the Lytle Creek area located in the southwestern Bear Lodge Mountains of northeastern Wyoming. Petro graphic and chemical data is used to interpret the Cenozoic phonolite trachyte rock association.

Cenozoic igneous activity in the Lytle Creek area appears to have been a response to the Laramide orogeny 80-40 m.y. B.P. The majority of the rocks were emplaced as laccoliths and sills at the Pahasapa Formation-Minnelusa Formation contact or at the Spearfish Formation Sundance Formation contact. Porphyritic volcanic textures are typical of the Cenozoic igenous rocks which include: calc-alkali phonolite, calc-alkali trachyte, trachyte, ferrohastingsite trachyte, ferrohasting site latite, and pseudoleucite trachyte porphyry. Sanidine and sodian ferroaugite are the major minerals present in the calc-alkali phonolite. calc-alkali trachyte and trachyte. Anorthoclase, ferrohastingsite, and oligoclase are the major minerals of the ferrohastingsite trachyte and ferrohastingsite latite. These rocks appear to be related to potassic alkali olivine basalt series rocks and when plotted on a Larsen diagram show a general parallel trend in enrichment of S102 , Al2O3, K2O, and Na2O and partial depletion of CaO, MgO, and total Fe-oxide. The scattering of points on the diagram suggest that the rocks do not represent a simple continuous liquid line of descent but may represent variable or interrupted crystal accumulation and/or contamination by granitic crustal rock. Syenitic and monzonitic xenoliths are present only in saturated to oversaturated trachyte and may represent granitic crustal material which has been depleted in quartz by metasomatic reaction with an undersaturated to saturated magma. Further study of the effects and the extent of assimilation of granitic crustal rock is needed to evaluate the role of contamination as a cause of igneous variation in the Cenozoic rocks of the Lytle Creek area.

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