SYBILLE PIT FE-TI OXIDE BODY, LARAMIE ANORTHOSITE COMPLEX: MINERAL ASSEMBLAGES AND COMPOSITIONS

The ~1.4 Ga Laramie Anorthosite Complex (LAC) is present in the Laramie Mountain Range, ~50 km northeast of Laramie, Wyoming. There are multiple major igneous units within the LAC, including multiple anorthositic units, leucogabbros, and monzonitic intrusions. In addition to these major igneous phases, the LAC is host to multiple occurrences of iron-titanium (Fe-Ti) oxide-dominated bodies. The processes that form these Fe-Ti oxide bodies are still debated: leading hypotheses include cumulate formation of Fe-Ti oxide-rich layers, immiscible Fe-Ti oxide melt, or some mixture of these processes. These genetic hypotheses are unified in the interpretation that Fe-Ti oxide bodies are late-stage features within anorthosite complexes, which is supported by geologic observations. The Sybille Pit contains one such Fe-Ti oxide body in the LAC. Along with Fe-Ti oxides, apatite is a major constituent of certain portions of the Fe-Ti oxide-dominated bodies. In Fe-Ti oxide-dominated bodies, most REE will substitute into apatite preferentially over Fe-Ti oxide minerals. Therefore, apatite compositions from samples collected from Sybille Pit can help to document how incompatible elements were partitioned in late-stage magmatic processes. A variety of techniques, including electron microprobe, whole rock analysis, and petrographic investigation, were used to investigate the variability of apatite composition and trace element distribution within the Fe-Ti oxide bodies in this study. REE-spider diagrams of representative anorthositic host material show positive Eu anomalies due to the high plagioclase content, and Fe-Ti oxide ± apatite samples show negative Eu anomalies, as would be expected of a late-stage melt sourced from its host anorthosite.