GEOCHEMICAL ANALYSIS OF ULTRAMAFIC BODIES WITHIN THE CENTRAL RUBY RANGE, SOUTHWESTERN MONTANA: INTRUSIVE OR TECTONIC EMPLACEMENT?

The Ruby Range in SW Montana is one of several Archean-Early Proterozoic crystalline basement-cored uplifts. Rocks of the Ruby Range are part of the Montana Metasedimentary Terrane of the Wyoming Province. The Ruby Range can be divided into three principal lithotectonic suites which include from west to east the: Christian Ranch metasedimentary suite; the Dillon Granite Gneiss suite; and a Pre-Cherry Creek quartzofeldspathic gneiss suite. Several irregular podiform bodies of ultramafic rocks are found scattered within all the lithotectonic units of the Ruby Range and are the focus of this study.

Field work within the central portion of the Ruby Range, north of the Sweetwater Road, as identified at least three small ultramafic bodies that occur near or within the Christian Ranch sequence and the Dillon Granite Gneiss lithotectonic suites. The ultramafic rocks occur as 3-20 meter podiform bodies typically elongated parallel to the dominant regional foliation. They are defined by knobby weathered megacrystic pyroxene crystals up to 5 cm in size within a finer matrix of olivine and pyroxene grains. Previous work in the southern part of the Ruby Range, south of the Sweetwater Road, have documented significant migmatization associated with the ultramafic bodies. The ultramafic bodies in this study do not appear to be associated with any migmatization of the surrounding country rock and lack evidence of in situ melting. The podiform bodies are commonly surrounded by fine to coarse grained amphibolite/gabbroic rocks with foliations parallel to the regional fabric.

Geochemical results show SiO₂ contents between 45.4-50.5 wt.%, MgO values between 24.0-27.6 wt.%, and Mg# between 77.9-80.7. The ultramafic rocks from north of the Sweetwater Road all exhibit enriched LREE compared to chondrites and primitive mantle by 10-30 and 4-10 times, respectively. Two samples show a slight negative Eu anomaly. All the samples plot within the komatiite field on a FeO_t+TiO₂-Al₂O₃-MgO discrimination diagram. The preliminary results of this study indicate some similarities with ultramafic body occurrences in the more southern part of the Ruby Range and outcrops found in the Tobacco Roots Range, but also some differences.