Because mountains and areas of high relief are fleeting, the only remnants of many ancient mountain belts (orogens) are the detritus shed from them and transported by rivers and glaciers to sedimentary basins, where the detritus is preserved as sandstones and conglomerates. Therefore sandstones, also known as clastic sediments, are a key tool in recognising past orogens and palæo-drainage networks (where rivers used to be), and can thereafter be used in palæo-geographic reconstructions to provide evidence as to the past arrangement of the continents.
This clastic record of ancient mountain-building events is commonly inspected through analysis of specific heavy mineral phases derived from orogens, which are transported to and preserved in sandstones and conglomerates. These minerals can record the date of their crystallization; they contain radiogenic “parent” elements like Uranium, Thorium or Potassium (K) that decay to “daughter” products such as Lead (Pb) or Argon. By measuring the ratios of the parent isotopes to the daughter isotopes we can calculate the age of individual crystals and therefore fingerprint ancient mountain-building events that stimulated the widespread crystallization of these critical mineral phases.
As different mineral phases are sensitive to different geological conditions; i.e. they only crystallize or “reset” their radiogenic ages at specific temperatures and pressures, or occur in only specific rock types; some minerals are more sensitive to certain kinds of mountain-building events than others. By-far the most widely used mineral for used for dating the components of sandstones is the mineral zircon (zirconium-silicate); zircon has proven useful for dating as it contains high amounts of U, which results in its precise dating, and also because it is physically and chemically robust, which means that it is commonly present in almost all sandstones.
Despite these attributes, there are aspects of zircon that make it a very poor recorder of magma-poor orogens, mountain building events that produce few igneous rocks – such as the Alps in Europe, and the southern Appalachians in North America; this stems from the fact that zircon only commonly forms at very high metamorphic temperatures (800-900°C+), or in felsic plutonic igneous rocks, such as granites. These attributes result in zircon being “blind” to magma-poor orogenesis (the process of creating mountain chains), and so in the southern Appalachians zircon does not detect the culminating phase of Appalachian orogenesis – the Alleghanian phase of 320 million years ago – which represents the collision of Laurentia (Palæozoic North America) and Africa forming the supercontinent Pangæa. A geologist relying solely on zircon U-Pb age data in Appalachian-derived sands could not therefore conclude that Pangæa had ever formed! This unsuitability of the zircon U-Pb system for dating these kinds of orogens means that we must search for alternative minerals to screen for these sorts of magma-poor mountain-building events in the geological record.
In our study, recently published in the GSA journal Geology, we examined the age of modern-river detritus shed by the southern Appalachians; specifically the ages of the minerals apatite (a calcium-phosphate mineral which is also found in teeth and bone) and rutile (titanium-dioxide), and compared our data to previously published zircon (and monazite, another dateable mineral) age-data from the same samples (see Hietpas et al., 2010).
Unlike zircon, apatite and rutile can crystallise in metamorphic rocks of medium-temperatures (c. 350-650°C) and should therefore be good candidates for the dating of magma-poor orogens. The river sampled was the French Broad River in North Carolina and Tennessee, which bisects the southern Appalachian Mountains and flows out to the west to eventually join the Mississippi.
From our figure (shown) it is easily seen that that the apatite and rutile ages are mostly far younger than the zircon ages. Zircon records very old ages that are not relevant to the most recent major geological events to have affected the region, and zircon is therefore of no great use in determining the sediment sources. In comparison, apatite and rutile fully record all phases of Appalachian mountain building, represented by age peaks at c.440, 370 and crucially the peak at 320 Ma, which represents the collision that produced Pangæa. We can therefore conclude from our results that when geologists are trying to determine the source of sediment shed from magma-poor orogens, far more useful and unique information can be obtained using apatite and rutile than is possible with zircon. Additionally our work may provoke a broader re-examination of older published detrital zircon U-Pb work to identify “hidden” orogens, which were previously overlooked by focusing too narrowly on a single mineral.
By Gary O’Sullivan, Ph.D. student, Trinity College Dublin