Tectonic Provenance
Introduction
The tectonic setting of a sediment source area determines, to a large degree, what kind of minerals and rock fragments are shed into adjacent depositional basins. By analysing the framework composition of sandstones - specifically the proportions of quartz (Q), feldspar (F), and lithic (rock) fragments (L) - geologists can assign a deposit to one of three principal tectonic provenance categories. This approach, developed around the QFL triangular diagram, is one of the most widely applied tools in sedimentary basin analysis. It connects the composition of a sandstone directly to the plate tectonic setting of its source area.
Three principal tectonic provenance types have been identified: continental block provenances, recycled orogen provenances, and magmatic arc provenances. [1]
Continental Block Provenances
Continental block provenances are located within continental masses, which may be bordered by a passive continental margin on one side and an orogenic belt on the other. Source rocks consist of plutonic igneous, metamorphic, and sedimentary rocks but include few volcanic rocks. Sediment eroded from these sources typically consists of quartzose sand, feldspars with high potassium feldspar-to-plagioclase ratios, and metamorphic and sedimentary rock fragments. This sediment may be transported off the continent into adjacent marginal ocean basins or deposited in local basins within the continent. [1]
Recycled Orogen Provenances
Recycled orogen provenances are zones of plate convergence where collision of major plates creates uplifted source areas along the suture belt. Where two continental masses collide, source rocks are typically sedimentary and metamorphic rocks that were present along the continental margins before collision. Detritus consists of abundant sedimentary-metasedimentary rock fragments, moderate quartz, and a high quartz-to-feldspar ratio. Where a continental mass collides with a magmatic arc complex, uplifted source rocks may include deformed ultramafic rocks, basalts, oceanic rocks, and a variety of types such as greenstone (weakly metamorphosed basic igneous rock), chert, argillite (weakly metamorphosed shale), lithic sandstones, and limestones. Chert is a particularly abundant constituent of sediments derived from this provenance type. [1]
Magmatic Arc Provenances
Magmatic arc provenances are located in zones of plate convergence where sediment is eroded mainly from volcanic arc sources (volcanogenic highlands - undissected arcs). Volcaniclastic debris shed from these highlands consists largely of volcanic lithic fragments and plagioclase feldspars. Quartz and potassium feldspars are typically very sparse, except where the volcanic cover is dissected by erosion to expose underlying plutonic rocks (dissected arcs). Sediment shed from volcanic highlands may be transported to an adjacent trench or deposited in fore-arc and back-arc basins. [1]
The QFL Triangular Diagram
To distinguish sediment from the three tectonic provenance types, triangular composition diagrams are used that show framework proportions of monocrystalline quartz, polycrystalline quartz, potassium feldspars, plagioclase feldspars, and volcanic and sedimentary-metasedimentary rock fragments. By determining the compositions of sand-size grains in a sandstone and plotting them on these diagrams, the field in which most plotted points fall - for example, craton interior or recycled orogen - indicates the probable tectonic setting of the source rocks. [1]
The model is valid for average values of large data sets taken from large-scale sampling of various tectonic settings. Not every individual sand or sandstone plots where expected based on its tectonic setting. Compositional data must be generated using the Gazzi-Dickinson point-counting method to ensure consistent grain-size-corrected modal analysis. [1]
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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