Masters Theses

Date of Award

8-1993

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Claudia Mora

Committee Members

Larry Taylor, Ted Labotka

Abstract

The Boehls Butte Anorthosite (BBA), exposed in northern Idaho differs mineralogically, texturally, and geochemically from most Proterozoic massif anorthosites. As a result, a clear understanding of its origin and subsequent history has been prevented. Several origins have been proposed for this unique anorthosite, including metasomatism (Hietanen, 1963), partial melting of aluminous metasediments plus superimposed hydrothermal alteration/metasomatism (Nord, 1973) , subsolidus exsolution and shear differentiation (Juras, 1974), and igneous intrusion with assimilation and mass transfer at the contact (Goldberg, 1983) . Based on careful petrography and electron microprobe analyses, three varieties of BBA have been identified, including massive, sheared, and foliated varieties. In addition, plagioclase (+/-quartz) veins which cross-cut the anorthosite have also been identified. The most distinguishing mineralogical characteristic of massive BBA is andesine (An34-48) megacrysts (up to 20 mm) with anorthite (An82-98) inclusions. Sheared anorthosite is characterized by grain size reduction, with no change in composition relative to massive anorthosite. Alternating layers of anorthite laminae and medium andesine grains, as well as biotite and hornblende elongation, define the foliation in foliated BBA. Plagioclase (+/-quartz) veins, not inferred to be part of the anorthosite, are andesine or oligoclase + quartz in composition. Stable isotope analyses of plagioclase separates (andesine and anorthite) indicate that a complex meteoric-hydrothermal event affected the BBA after crystallization, and can be used to address previously proposed hypotheses for the origin of the unique mineralogy and texture of the BBA. A wide range of σ18O "whole-plagioclase" (wp) values occur in massive BBA, from -6.2 to 6.1o/oo (SMOW) . Sheared BBA retains low, less variable isotopic compositions of -5.8 to 1.7o/oo. Foliated BBA retains oxygen isotopic signatures of 5.0 to 6.0o/oo, consistent with most igneous anorthosites. Calculated oxygen isotope fractionations between andesine-anorthite at peak metamorphic temperatures (650-750°C; Grover et al, 1992) range from Δ18O = 0.5 to 0.6o/oo. These values are observed more frequently in sheared and foliated samples than in massive anorthosite samples. Petrographic and isotopic data are not consistent with any of the proposed hypotheses for the formation of the BBA. Exsolution and shear differentiation, proposed by Juras (1974), cannot be ruled out entirely. However, this hypothesis remains problematic. The size of the plagioclase intergrowths known to occur are on the micron scale (or less), whereas the intergrowths within the BBA are many orders of magnitude coarser (on the mm scale). Shear differentiation seems unlikely, as the shear strengths of andesine and anorthite are very similar. Leaching of Na and Si by a high temperature hydrothermal fluid, leaving a plagioclase phase that is enriched in Ca and Al, is an alternative hypothesis which might explain the occurence of andesine megacrysts containing anorthite inclusions and of anorthite-rich laminae. Anorthite is 18O- depleted relative to andesine, suggesting that the laminae may have been the focus of fluid flow. Some of the most 18O-depleted samples have apparent eguilibrium (M2) Δ18O values, suggesting that fluid-flow within these samples occurred just after the peak of metamorphism and while the rocks remained at elevated temperatures.

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