Year of Award
Doctor of Philosophy (PhD)
Department or School/College
Department of Geology
University of Montana
Anorthosite bodies ranging in outcrop area from less than ten square feet to more than one square mile occur in the metamorphic terrain bordering the idaho batholith on the east. The majority occur within a nine square mile area cut by bass creek canyon. All of the anorthosites occur concordantly within a quartzo-feldspathic gneiss containing a few per cent of sillimanite and designated as the sillimanite gneiss.
In addition to the anorthosites, there are numerous quartz monzonitic and some tonalitic pods within the sillimanite gneiss. The majority of the observed anorthosite-quartz monzonitic gneiss contacts are sharp and irregular. There is an abundance of quartz monzonitic gneiss mixed in with the anorthosites. Portions of the anorthositic bodies are tonalitic, and the anorthosite-tonalite contacts are gradational. Pods of amphibolite accompanied by tonalitic segregations occur within the sillimanite gneiss, and amphibolitic layers are found within the larger anorthosites. Miarolitic albite veins are abundant in the largest anorthosite which is exposed in bass creek canyon.
An average anorthosite specimen consists of 3% quartz, 93% labradorite, 2% biotite, and 2% chlorite (from the alteration of biotite). The compositional range of plagioclase within a given specimen of anorthosite is great, from an37 to an65 for instance, and zoning is in most cases pronounced and normal or normal oscillatory. The thermal state of plagioclase from both anorthosites and surrounding gneisses is typical of that for metamorphic rocks. Potash feldspar found in the quartz monzonitic gneiss and sillimanite gneiss is generally monoclinic and perthitic.
An anatectic origin is proposed for the anorthosites. Metamorphism of shales of the belt supergroup, possibly the prichard formation, produced the sillimanite gneiss. Further thermal metamorphism resulted in the partial melting of this unit with the consequent formation of a quartz monzonitic melt and an anorthositic residuum. The quartz monzonitic material was expelled from the anorthositic residuum and is now found in the quartz monzonitic bodies located adjacent to most of the an0rthosites. Partial melting of a shale at lower temperatures or of material of more acidic composition was responsible for the formation of the tonalitic pods within the sillimanite gneiss and anorthosite. These tonalites represent the residuum of this process. Albite veins within the anorthosites are thought to be the result of late remobilization of albite from the surrounding plagioclase.
Berg, Richard B., "Petrology of Anorthosite Bodies, Bitterroot Range, Ravalli County, Montana" (1964). Graduate Student Theses, Dissertations, & Professional Papers. 10953.
© Copyright 1964 Richard B. Berg