Provenance Analysis
Introduction
The silicate mineralogy and rock-fragment composition of siliciclastic sedimentary rocks are fundamental properties that set them apart from other sedimentary rocks. Mineralogy is a particularly important property for studying the origin of siliciclastic sedimentary rocks because it provides almost the only available clue to the nature of vanished source areas - ancient mountain systems. [1]
The silicate mineralogy and rock-fragment composition of siliciclastic sedimentary rocks furnish important evidence of the lithology of source rocks. Rock fragments provide the most direct evidence: volcanic rock fragments indicate volcanic source rocks, metamorphic rock fragments indicate metamorphic source rocks, and so forth. Feldspars are also important indicators. Potassium feldspars suggest derivation mainly from alkaline plutonic igneous or metamorphic rocks, sodic plagioclase is derived principally from alkaline volcanic rocks, and calcic plagioclase comes mainly from basic volcanic rocks. [1]
Heavy Mineral Suites as Provenance Indicators
Heavy mineral suites are used for source-rock determination. Characteristic suites for different source lithologies include: [1]
- Alkaline igneous source: apatite, biotite, hornblende, monazite, rutile, titanite, pink tourmaline, and zircon [1]
- Basic igneous source: augite, chromite, diopside, hypersthene, ilmenite, magnetite, and olivine [1]
- Metamorphic source: andalusite, garnet, staurolite, topaz, kyanite, and sillimanite [1]
- Recycled sediment source: barite, iron ores, leucoxene, rounded tourmaline, and rounded zircon [1]
The trace element composition of individual heavy minerals also carries provenance information. The Ti and Fe content of ilmenite, for example, is a provenance indicator because its composition varies systematically with drainage basin source lithology. [1]
Quartz as a Provenance Indicator
Quartz grains carry provenance information in their optical properties. A high percentage of quartz grains with undulose extinction greater than 5°, combined with a high percentage of polycrystalline grains with more than three crystal units per grain, is typical of low-rank metamorphic source rocks. By contrast, nonundulose quartz and polycrystalline quartz containing fewer than three crystal units per grain indicate derivation from high-rank metamorphic or plutonic igneous source rocks. [1]
Quartz provenance can also be assessed using scanning electron microscope cathodoluminescence (SEM-CL) fabric analysis. Quartz grains from plutonic, volcanic, and metamorphic rocks display distinctively different cathodoluminescence patterns when excited by an electron beam, providing reliable provenance interpretation. [1]
Climate and Relief Indicators
Beyond source lithology, the weathering state of minerals provides evidence of climate and relief in the source area. Large, fresh, angular feldspars indicate a high-relief source area where grains were eroded rapidly before extensive weathering, or derivation from a very arid or extremely cold climate that retarded chemical weathering. Small, rounded, highly weathered feldspar grains indicate a source area of low relief and/or a warm, humid climate where chemical weathering was moderately intense. The absence of feldspars may indicate either that weathering was too intense for them to survive or that no feldspars were present in the source rocks. [1]
Such mineral-state analyses provide only tentative conclusions about climate and relief. They are also subject to misinterpretation because diagenetic alteration or destruction of source-rock minerals can mimic weathering effects. [1]
Tectonic Provenance
Geologists are also interested in the tectonic setting of source areas and associated depositional sites. Three principal tectonic provenance types have been identified: continental block provenances, recycled orogen provenances, and magmatic arc provenances. [1] Triangular composition diagrams plotting framework proportions of quartz, feldspars, and rock fragments are used to distinguish sediment derived from these settings. [1] See the dedicated page Tectonic Provenance for full treatment.
Detrital Thermochronology
A newer aspect of provenance analysis is estimation of the ages of single mineral grains - such as apatite and zircon - using radiometric techniques. Determining the ages of individual mineral grains provides the ages of the source rocks from which they were derived, making it possible to link grains to specific source areas of known ages. This technique is called detrital thermochronology. [1]
Related Topics
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References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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