A STUDY OF LUECOBANDS IN THE WAX FACTORY LACCOLITH, BIG BEND RANCH STATE PARK, TX

The Wax Factory Laccolith (WFL) is located in Big Bend Ranch State Park, TX. The WFL is approximately 32 Ma in age, and is part of the Trans Pecos Magmatic Province (TPMP) which was formed between 48 and 17 Ma. The WFL is composed mostly of monzonite, but has numerous light colored bands (“luecobands”) of syenite with various shapes including irregular “blebs” up to 2 meters in size and horizontal bands approximately 10 cm thick. Prior studies of the laccolith lacked an adequate interpretation of the light colored bands. A previous hypothesis (Farmer, 2017) suggested that the bands were caused by alteration of the rock by local spring water. Scanning Electron Microscopy (SEM) and X-Ray Fluorescence (XRF) analysis conducted on the luecobands reveals that the syenitic luecobands have an overall similar mineralogy to the monzonite, but a distinct difference in mineral composition, contradicting the hypothesis by Farmer. The variation is concluded to have been caused by crystal fractionation. This hypothesis has been supported by major and trace element models. The models use a parent monzonite composition that evolves to the syenite composition with the removal of approximately 50% by mass of the observed minerals plagioclase, pyroxene, Fe-Ti oxides and apatite. In this model, the host monzonite magma formed originally and crystallized to a partially solid crystal framework. Subsequent evolved syenitic magma intruded into this framework in the form of the blebs and horizontal bands. This study contributes to our understanding of the petrogenetic history of the WFL and of our overall understanding of the TPMP.