Abstract

We have investigated petrogenetic characteristics of detrital opaque Fe-Ti oxide minerals (DOPQ) in sand samples collected from Holocene streams draining exclusively igneous and metamorphic source rocks in the Rocky Mountain region under a semi-arid climate, and from the Appalachian Mountains under a humid climate. Our sampling strategy allows us to compare and contrast the properties of detrital mineral grains not only from igneous and metamorphic source rocks, but also those from dissected-arc and recycled-orogen provenances. Reflected-light petrography including extensive modal analysis and electron probe microanalysis of polished grain mounts were the two principal methods of data gathering. Between 25% and 50% of all DOPQ show oxidation exsolution and/or other lamellar intergrowth textures, i.e., they are polymineralic grains. Lamellar intergrowth texture is present in about twice as many grains derived from igneous source rocks as from metamorphic source rocks. Lamellae thinner than 2 mu m are predominant in grains derived from dissected-arc provenance, whereas lamellae thicker than 10 mu m are predominant in grains derived from recycled-orogen provenance. Exsolution lamellae along {111} planes of magnetite (three directions) and presumably along {0001} planes of ilmenite (one direction) characterize dissected-are and recycled-orogen provenance, respectively. Detrital ilmenites derived from the igneous source rocks studied show a wide range of TiO 2 content with a mode around 47%, whereas those from the metamorphic source rocks show a tight cluster around 52% TiO 2 content. DOPQ derived from metamorphic source rocks contain < 0.5% MgO, whereas those from igneous rocks may contain up to nearly equal 1.2% MgO. Both MgO and MnO are low, 0.3% and 2.5%, respectively, in DOPQ derived from dissected-arc provenance, but may be relatively high, up to 1.2% and 8.0%, respectively, in grains derived from recycled-orogen provenance. Whereas no single character of DOPQ may be diagnostic of provenance, discriminant function analysis of the complete set of data show that collectively they are diagnostic in 95% to 100% of cases. We recommend the use of 1) presence of exsolution texture, 2) minimum lamellae widths and the number of crystallographic orientation of the lamellae, and 3) chemical composition including minor elements of DOPQ as important properties for provenance identification. This should supplement and refine existing criteria for provenance determination.

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