Geologic Setting And Xenoliths Of The Lodgepole Intrusive Area: Implications For The Northern Extent Of The Stillwater Complex, Montana

Abstract

Northerly dipping compositional layering in the central and western portions of the Stillwater Igneous Complex, the moderate northward slope of the Beartooth Front north of these outcrops, as well as gravity data (Bonini, 1981) provide structural evidence for the continuation at depth of the Complex north of the middle Cambrian unconformity along its northern boundary. The Lodgepole, Enos Mountain and Susie Peak plutons represent multiple intrusions of intermediate magmas at emplacement depths ranging from 2 km to near-surface conditions during late Cretaceous time. These intrusions lie respectively 8, 9, and 12 km north of the nearest outcrops of the Stillwater Complex. The Lodgepole Intrusion consists of an early dacite phase and a later diorite phase, the latter containing abundant xenoliths (up to 31 cm in diameter) in the area north of Clover Basin near its western margin. These xenoliths include foliated mafic amphibolites, gneisses, Paleozoic sediments, and cumulate textured basic rocks. Smaller xenoliths of similar lithologies are found in the Enos Mountain and Susie Peak intrusions. The cumulate textured xenoliths have magmatic textures, basic silica contents, and tholeiitic normative rnineralogies. The euhedral to subhedral, medium to coarse grained, tabular plagioclase in the xenolith sample suite has a total span of An62 to An86 (mole %) with variability in a single sample generally 1 to 5 mole % An. Minor primary-appearing augite is found in anorthositic specimens, but most of the primary mafic minerals have been altered to calcic amphibole and chlorite. The external habit and mineralogy of some mafic mineral domains suggest that they are pseudomorphic after primary pyroxenes and olivine. Based on interpretation of mineralogy and texture, cumulate xenoliths were classified as anorthosites, gabbros, norites, gabbronorites, troctolites, and altered ultramafic lithologies. One xenolith contained chromite, one contained graphite, and a few contained minor Fe and Cu sulfides. Mineral compositions and textures lead this author to conclude that the xenoliths were brought up from the underlying Stillwater Complex. Based on estimate of the fluid properties of the andesitic magma above its liquidus, ascent rates of greater than 1.7 m/second were needed to have raised the cumulate xenoliths. The lack of xenoliths with plagioclase compositions <An62, as well as structural space constraints imposed by the existence of non-cumulate basement lithologies north of the East Boulder Fault lead this author to question the existence of the thick differentiated Hidden Zone postulated by Hess (1960).