MINERALOGIC AND ISOTOPIC STUDIES OF OUR LADY OF THE LAKE INTRUSION, MONTANA

 

Our Lady of the Lake Intrusion is a composite intrusion located along the margin of the Cretaceous Tobacco Root Batholith in southeastern Montana. It is a tabular shaped body with an exposed area of around 2 sq. km in contact with Archean quartzofeldpathic gneisses. Rock types include pyroxenite, gabbro and diorite. Sedimentary structures like current bedding, trough cross bedding and graded are present in all units. Contacts between gabbro and diorite unit may be sharp, with fragments of gabbro included within diorite.

Pyroxenite contains cumulate pyroxene and interstitial plagioclase whereas the gabbro contains non-cumulate pyroxene, plagioclase, amphibole and biotite and the diorite contains non-cumulate amphibole and biotite. Alteration is pronounced in every unit.

The An content of plagioclase in pyroxenite unit ranges between 63 and 83, whereas the An content of plagioclase in gabbro and diorite units vary between 33 and 47 and between 31 and 34, respectively. The Mg# of pyroxene from the pyroxenite, gabbro and diorite units varies between 0.71 and 0.80. Harker diagrams show decreases in FeO, MgO and CaO and increases in Na₂O and Al₂O₃ with increasing SiO₂. TiO₂ and Y decrease with SiO₂ whereas Ba increases. Major element mass balance and trace element fractionation calculations suggest that the rock types may be related via fractionation crystallization. d¹⁸O values of pyroxene in pyroxenite and gabbro vary between 5.3 and 6.4‰, and are consistent with derivation from a mantle source with no crystal contamination. d³⁴S values of the pyroxenite vary between -0.2 and 0.7‰, and are consistent with this interpretation. However, d¹⁸O values of plagioclase from the diorite are slightly enriched and vary between 6.7 and 7.1‰. Sulfur isotopic values of the gabbro are also elevated with a range from 2.4 to 6.5‰, suggesting that gabbro and diorite have been contaminated with crustal material and may be genetically unrelated to the spatially associated pyroxenite.

The chemical and isotopic data suggest that local and selective volatile contamination, that did not perturbed major element composition, affected the gabbro and diorite units of our Lady of the Lake intrusion.